Progress in Energy and Combustion Science

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1 Progress in Energy and Combustion Science 39 (2013) 285e319 Contents lists available at SciVerse ScienceDirect Progress in Energy and Combustion Science journal homepage: Review Thermal energy storage technologies and systems for concentrating solar power plants Sarada Kuravi 1, Jamie Trahan, D. Yogi Goswami *, Muhammad M. Rahman, Elias K. Stefanakos Clean Energy Research Center, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA article info abstract Article history: Received 20 March 2012 Accepted 15 February 2013 Available online 22 March 2013 Keywords: Concentrating solar power plants Storage system design High temperature thermal energy storage Economic analysis Thermal storage media Efficiency analysis This paper presents a review of thermal energy storage system design methodologies and the factors to be considered at different hierarchical levels for concentrating solar power (CSP) plants. Thermal energy storage forms a key component of a power plant for improvement of its dispatchability. Though there have been many reviews of storage media, there are not many that focus on storage system design along with its integration into the power plant. This paper discusses the thermal energy storage system designs presented in the literature along with thermal and exergy efficiency analyses of various thermal energy storage systems integrated into the power plant. Economic aspects of these systems and the relevant publications in literature are also summarized in this effort. Ó 2013 Elsevier Ltd. All rights reserved. Contents 1. Introduction Plant level design considerations Concentrating solar power (CSP) plant systems Integration of thermal energy storage Design considerations with storage Key conclusions Component level considerations Storage media selection Storage types Stability Material characterization Compatibility of materials Storage tank components Heat transfer fluids Cost Maturity of storage technology System level considerations Types of TES systems Active storage Passive storage Combined systems Theoretical analysis of TES systems Active two-tank systems Steam accumulators * Corresponding author. Tel.: þ ; fax: þ address: [email protected] (D.Y. Goswami). 1 Present address: Dept. of Mechanical & Aerospace Engineering, Florida Institute of Technology, Melbourne, FL, USA /$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.

2 286 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Systems with extended/embedded heat transfer structures Packed bed systems Energy and exergy efficiencies of TES systems Exergetic efficiency of TES systems Optimization procedure for TES tanks Thermoeconomic analysis Life cycle assessment Case study Key conclusions Development activities in thermal energy storage for CSP Conclusions Acknowledgements References Introduction Concentrating solar thermal power, more commonly referred to as CSP, is unique among renewable energy generators because even though it is variable, like solar photovoltaics and wind, it can easily be coupled with thermal energy storage (TES) as well as conventional fuels, making it highly dispatchable. A multitude of advancements have taken place in recent years in an effort to make CSP more cost effective. Ongoing research efforts are in the areas of reflector and collector design and materials, heat absorption and transport, power production and thermal storage. The availability of storage capacity is expected to play an important role in the adoption of CSP plants by electrical utilities. By coupling TES with a CSP plant, the thermal energy can be stored for later use to drive a heat engine. TES has several advantages when compared to mechanical or chemical storage technologies. TES generally has lower capital costs as compared to other storage technologies, as well as very high operating efficiencies. A TES prototype system that was incorporated into the Solar Two project in Daggett, California demonstrated a round-trip efficiency greater than 97% [1e3] which was defined as the ratio of the energy discharged to the energy stored in the TES system. The thermal storage system in the above references was a two-tank system designed to deliver thermal energy at fullrated duty of the steam generator for three hours at the defined hot and cold salt temperatures of 565 C and 288 C, respectively. So after the first cycle, the salt from the cold tank is heated from 288 C to the hot tank temperature of 565 C during charging, and the process is reversed during discharging. The only losses are to the ambient through the insulation, which can be limited based on the amount of insulation, which is the reason for the very high roundtrip efficiencies reported above. A discussion on the definitions of TES system efficiencies is provided in Section 4. A thermal energy storage system mainly consists of three parts, the storage medium, heat transfer mechanism and containment system. The thermal energy storage medium stores the thermal energy either in the form of sensible heat, latent heat of fusion or vaporization, or in the form of reversible chemical reactions. Today, sensible heat materials in the form of synthetic oil and molten salt are the most widely used storage materials in large-scale CSP systems (see Table 2), while systems that utilize latent heat, thermochemical, and other sensible heat materials are still being developed. The purpose of the energy transfer mechanism is to supply or extract heat from the storage medium. The containment system holds the storage medium as well as the energy transfer equipment and insulates the system from the surroundings. Depending on the type of storage, there are several requirements that must be considered to ensure optimal storage dynamics and longevity. These requirements have been identified as [1]: 1. High energy density in the storage material 2. Good heat transfer between the heat transfer fluid (HTF) and the storage medium 3. Mechanical and chemical stability of the storage material 4. Chemical compatibility between HTF, heat exchanger, and storage medium 5. Complete reversibility for a large number of charging/discharging cycles 6. Low thermal losses 7. Low cost 8. Low environmental impact. Thermal energy storage systems must be designed to meet certain criteria, which are dependent on the type, size and design of a CSP plant. These criteria can be summarized as follows [1]: 1. Nominal temperature and specific enthalpy drop in the load (discharge and conversion side) 2. Maximum load 3. Operational strategy, and 4. Integration into the plant. All these requirements and criteria have to be considered when deciding on the type and design of the thermal energy storage system. This paper focuses on reviewing different thermal energy storage concepts available in the literature that are being or can be used for CSP plants. For a practical CSP plant design with storage, plant level strategy and design considerations come first, followed by selection of the storage material and design of components incorporating the storage materials, and design of the system consisting of storage tanks, heat exchangers, piping and pumps, respectively, that meet the requirements of the power plant. The overall TES system design includes considerations of efficiency, space and cost. Therefore, this TES review is based on the above considerations as follows: 1. Plant level e This level of design focuses on the overall CSP plant requirements, design strategy to meet the plant requirements and integration of the TES system into the power plant as well as its ability to be compatible with the other units/systems of the power plant. In a CSP plant, development of a design and operational strategy to meet the plant requirements, for example, long-term vs. short-term storage, number of hours of storage, charge/discharge rates and how storage is integrated

3 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Table 1 Description and status of CSP technologies. Parabolic trough Solar tower Linear Fresnel Dish-Stirling Maturity of technology Commercially proven Pilot Plants, commercial Pilot projects Demonstration projects projects under construction Key technology providers Abengoa Solar, Abengoa Solar, Novatec Solar, Areva Sener Group, Acciona, Siemens, NextEra, ACS, SAMCA, etc. BrightSource Energy, esolar, SolarReserve, Torresol, SunBorne Energy Technology development risk Low Medium Medium Medium Operating temperature 290e e e e750 of solar field ( C) Plant peak efficiency (%) 14e20 23e35 a w18 w30 Annual solar-to-electricity 11e16 7e e25 efficiency (net) (%) Annual capacity factor (%) 25e28 (no TES) 55 (10 h TES) 22e24 25e28 29e43 (7 h TES) Collector concentration 70e80 suns >1000 suns >60 suns (depends on secondary reflector) >1300 suns Receiver/absorber Storage system Grid stability Absorber attached to collector, moves with collector, complex design Indirect 2-tank molten salt at 380 C(DT ¼ 100 C) or Direct 2-tank molten salt at 550 C(DT ¼ 300 C) Medium to high (TES or hybridization) External surface or cavity receiver, fixed Direct 2-tank molten salt at 550 C(DT ¼ 300 C) Fixed absorber, no evacuation, secondary reflector Short-term pressurized steam storage (<10 min) High (large TES) Medium (back-up firing possible) Low Absorber attached to collector, moves with collector No storage, chemical storage under development Cycle Superheated steam Rankine Superheated steam Rankine Saturated steam Rankine Stirling Steam conditions ( C/bar) 380 to 540/ /100 to /50 n.a. Water requirement (m 3 /MWh) 3 (wet cooling) 2e3 (wet cooling) 3 (wet cooling) 0.05e0.1 (mirror washing) 0.3 (dry cooling) 0.25 (dry cooling) 0.2 (dry cooling) Suitability for air cooling Low to good Good Low Best Storage with molten salt Commercially available Commercially available Possible, but not proven Possible, but not proven a Upper limit for solar tower with combined cycle turbine. Adapted from Ref. [8] and updated. with the solar collection system on one side and the power block on the other side must be considered. This level of design considerations comes before any system or component level design can be started. Therefore, in this paper, plant level considerations are reviewed first, followed by component level and system level considerations. 2. Component level e This level of design pertains to the selection of the basic components that are used for forming the thermal energy storage system, including storage material, type of contact and heat transfer between the storage material and the HTF, and any heat transfer enhancements to be incorporated. 3. System level e The TES system design focuses on the integration of components such as storage tanks, pumps and heat exchangers, and controls for charging and discharging operations, reducing system level losses, improving efficiency, and system costs. Fig. 1 shows a diagram to illustrate these levels. A number of recent review papers discuss TES systems including experiences on both commercial and research scales [1,4e7] but do not address the hierarchy of TES design nor do they emphasize the interdependent nature of each level. This paper reviews the current options available at each of these levels and discusses the requirements that must be considered at each level for designing a thermal storage unit for integration into a CSP power plant. A plant level decision to include thermal energy storage in a CSP plant includes the considerations of the loads, mismatch between the loads and the available resource, operational strategy, space availability for storage and the increased size of the solar field, increased capital costs and their impact on the Levelized Cost of Energy (LCOE), available tariffs and Return on Investment (ROI). These considerations determine the capacity of the needed TES system, which is defined by the amount of energy that can be discharged in kwh th (thermal) and the maximum rate at which energy may be discharged (kw th ). The plant level considerations including the needed temperature and energy transfer rates for the power block, and potential temperatures and rates of energy transfer from the solar field help determine the type of storage (sensible heat, latent heat, thermochemical) which then leads to the selection of the storage material. The storage material selection is based on characteristics such as energy density, charge and discharge rates, cyclic life, toxicity, stability, compatibility between the storage medium and the heat transfer fluid, cost and availability. For designing the storage unit, both the first law and second law principles must be considered. For integration of a storage unit in a solar power plant, the solar field design and power block must be considered. 2. Plant level design considerations 2.1. Concentrating solar power (CSP) plant systems CSP plants concentrate direct solar radiation to heat a fluid (normally called the heat transfer fluid or HTF) and produce steam (or vapor of another working fluid). The working fluid runs an engine (steam turbine, Stirling engine, etc.) connected to a generator, producing electricity. There are four major types of CSP technologies: parabolic troughs, central receiver towers (also known as power towers), parabolic dish-stirling engine systems, and linear Fresnel reflectors, all of which can be integrated with thermal storage although a dish system would require a special design. Fig. 2 shows some details of the components of the three main parts of a CSP plant e the solar, field, thermal storage and the power block. Table 1 provides a comparison of the major features of the different CSP technologies.

4 288 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 Table 2 Operational solar thermal facilities with thermal energy storage systems. Project Type Storage medium Nominal temperature ( C) Storage concept Plant capacity Storage capacity Ref. SSPS-DCS test facility Almeria, Spain Nevada Solar One Nevada, USA Holaniku at Keahole Point Hawaii, USA Planta Solar-10 Sevilla, Spain Planta Solar-20 Sevilla, Spain La Florida Badajoz, Spain Andasol-1 Granada, Spain Andasol-2 Granada, Spain Extresol-1 Badajoz, Spain Manchasol-1 Ciudad Real, Spain Manchasol-2 Ciudad Real, Spain La Dehesa Badajoz, Spain Puerto Errado 1 Murcia, Spain Archimede Sicily, Italy Torresol Gemasolar Seville, Spain Dahan Beijing, China Cold Hot Parabolic trough Santotherm tank thermocline 1.2 MWth 5 MWht [18,19] Parabolic trough Dowtherm A Oversized field piping 64 MWe 0.5 h [20,21] Parabolic trough Water n.a. a 200 Indirect storage 2 MWth, 500 kwe 2h [22,23] Central receiver Pressurized water Steam accumulator 11 MWe 50 min/20 MWht [20,24e26] Central receiver Pressurized water n.a. 250e300 Steam accumulator 20 MWe 50 min [20] Parabolic trough Molten solar salt b tank Indirect 50 MWe 7.5 h [20,27] Parabolic trough Molten solar salt tank indirect 50 MWe 7.5 h/1010 MWht [20,27,28] Parabolic trough Molten solar salt tank indirect 50 MWe 7.5 h/1010 MWht [20,27,28] Parabolic trough Molten solar salt tank indirect 50 MWe 7.5 h/1010 MWht [20,27] Parabolic trough Molten solar salt tank indirect 50 MWe 7.5 h [20,27] Parabolic trough Molten solar salt tank indirect 50 MWe 7.5 h [20,27] Parabolic trough Molten solar salt tank indirect 50 MWe 7.5 h [20,27] Linear Fresnel Saturated steam n.a. 270 Steam accumulator 1.4 MWe n.a. [20] Parabolic trough Molten solar salt tank direct 5 MWe 8 h/100 MWht [20] Central receiver Molten solar salt tank direct 17 MWe 15 h [27] Central receiver Saturated steam/oil Combined steam accumulator/concrete 1 MWe 1 MWht [29,30] a n.a. ¼ not available. b Molten solar salt ¼ 60% sodium nitrate/40% potassium nitrate Integration of thermal energy storage Thermal energy storage systems reduce the mismatch between energy supply by the sun and energy demand [1]. Depending on variation in solar insolation throughout the day and year, as well as the electricity demand, TES systems can be integrated to perform one or more of the following functions: 1. Buffering e Cloud cover or periods of inclement weather force the turbineegenerator into a transient mode thus reducing the turbine efficiency due to start-up losses. Though heat transfer fluids (HTFs) have some thermal inertia to help the plant rideout short cloudy periods [9], experiences with large-scale facilities have shown that it may not be sufficient to prevent a turbine shut-down [10]. Addition of a small capacity Fig. 1. Thermal energy storage design considerations at each level.

5 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Fig. 2. Main parts of a CSP plant and their components. storage system can help mitigate short fluctuations in solar insolation [1]. 2. Delivery period displacement or extension e Peak power demand may not coincide with peak solar insolation. A TES system can improve dispatchability of a plant by collecting energy from the solar field during off-peak hours such as in the morning, then discharging energy during peak hours of demand which may provide higher electricity tariffs [1]. 3. Improving annual capacity factor e A power plant s capacity factor is a performance parameter that compares the net electricity delivered by the plant to the energy that it could have produced under continuous full-power operation during the same time period. Since solar energy is only available during a fraction of the day, a solar power plant without any storage has a very low capacity factor. A TES system can allow a plant to run when the sun is not available and if large enough, the plant can operate for 24 h. As shown in Table 1, seven hours of storage can increase the capacity factor, or solar fraction, from a typical 25 to 28% to as high as 43%. Storage systems intended for this use would necessitate a larger solar field than required for plants without storage [1]. A TES system must work in conjunction with both the solar field and the power block. Detailed models of these three subsystems are often integrated in a simulation in order to predict and improve the operation strategy and performance of CSP plants [11e13]. Operation strategies can be used to increase the thermal efficiency, thereby increasing the electrical output of the plant, which improves its economic value [14]. To make investment decisions regarding CSP and whether to integrate thermal storage, it is necessary to evaluate the tradeoffs between the annual electrical output, additional capital costs, LCOE, and time-of-use value of the electricity [13,15]. Thus it is important to study both the technical and economic aspects of CSP. A CSP system uses conventional thermodynamic power cycles and, as such, the power block efficiency depends primarily upon the operating temperature. The amount of electricity produced by a plant depends upon annual direct normal insolation (MWh/m 2 ) available to the plant, annual solar field collection efficiency, thermal losses, and net power cycle efficiency along with the storage efficiency when used [13,16]. The efficiency of a solar collector field is defined as the ratio of the useful thermal energy collected to the incident solar energy. The power cycle efficiency is the ratio of the net power out to the heat input. The TES unit efficiency is defined in several ways depending on the storage type and will be discussed in Section 4, which deals with the design considerations of a TES unit. The amount of heat, _ Q, needed to run the power block depends on the power cycle efficiency, h cycle, [12]: _Q ¼ _W h cycle : (1) The nominal inlet and outlet temperatures of the solar field, which depend upon the CSP technology and type of HTF in use, and the mass flow rate of the HTF determine the heat supplied by the solar field [12]: _Q ¼ _m HTF c HTF;ave THTF;out T HTF;in : (2) The specific heat is assumed to vary linearly with the temperature of the HTF, hence an average value can be used [12]. The mass flow rate of the HTF is adjusted throughout the day to accommodate the operating state of the plant, e.g. standby, preheat, and normal operation [16], and to ensure that the fluid stays within the design temperature range. The outlet HTF temperature from the solar field also defines the charging temperature of the storage system. One of the primary requirements for selection of an appropriate storage system is the nominal HTF operating temperature of the solar field. Additional requirements include the maximum electrical output required by the plant, the desired mode of operation for the storage system, the manner in which the storage system will integrate with the solar field and power block, and the solar field size [17]. To demonstrate, Fig. 3 presents the schematic of a current state-of-the-art molten salt two-tank TES system integrated into a parabolic trough power plant with the conventional heat transfer fluid [2]. The thermal energy storage unit is charged by taking hot oil heat transfer fluid (HTF) (with a nominal design temperature of 393 C) from the solar field and running it through oil-to-salt heat exchangers. Molten salt at 292 C is taken from the cold storage tank and flows countercurrently through the heat exchangers to obtain a maximum storage temperature of 385 C. The heated fluid is stored in the hot storage tank. When the stored energy is needed, the system simply operates in reverse to heat the HTF, which generates steam to run the power plant. In this scenario, the operating temperature range of the storage system is between 292 C and 385 C and the storage system relies on forced convection to transfer heat between the HTF and a liquid medium. The storage fluid must have a solidification temperature that is safely below the lowest operation temperature of the storage system and a decomposition temperature that is safely above the highest temperature of the storage system. The size of the solar field must be increased so that it can charge the

6 290 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 Fig. 3. Schematic diagram of a parabolic trough solar power plant with a two-tank molten salt storage [2]. storage system while providing thermal energy to the power block. The number of hours that the storage system can provide depends upon the storage system temperature range, the amount of material present in the storage system as well as the thermal properties of the material, and the energy requirement of the power block, based on Eq. (1). In an effort to evaluate the performance and tradeoffs between cost, performance, and economic parameters of a parabolic trough plant that uses the two-tank TES design, Price and Kearney [13] presented a computer simulation model and compared the annual performance of a 50 MW e parabolic trough-based power plant with 6 h of thermal storage and without thermal storage. The study revealed that addition of a TES system would result in the following: 1. By taking into account the efficiencies of all plant components, TES can improve the annual solar-to-electric efficiency (13.2% vs 12.4%), though it slightly lowers the steam cycle efficiency (37.5% vs 37.9%) due to the lower steam temperatures that occur while TES is in use. The two main reasons for the increased annual efficiency were (a) much reduced need to dump energy during very high insolation periods and (b) lower turbine start-up losses due to buffering of the intermittent periods. 2. The annual LCOE is reduced by 10% because of the higher capacity factor. 3. Slightly increases the thermal losses in the receiver because of the higher HTF return temperature to the solar field. 4. Introduces two types of losses: thermal losses from the TES storage system, and losses when the storage system is full and the power plant cannot accept more energy because it is already at maximum load. 5. Requires a larger solar field compared to the one without storage. 6. Increases the capital cost but produces more energy resulting in a lower cost of electricity. 7. Electric parasitics are slightly lower with TES because of the higher annual generation and lower percentage of off-line parasitic consumption. 8. Turbine start-up becomes a smaller fraction of total energy use since it is operated for more hours with fewer starts. Solar energy is a variable resource that requires substantial amounts of storage to create a reliable and consistent supply of electricity. The benefits of adding any amount of storage must be weighed with costs. Table 2 provides a description of the existing solar thermal power plants that have integrated storage Design considerations with storage The presence of TES may place additional design considerations on the power plant and optimization must be carried out in view of this. For example, in the case of a parabolic trough power plant, the maximum temperature of the HTF will be the result of the temperature from the solar field. During night time, when the system is being discharged, the highest temperature of the HTF will not be the same as that at day time and may result in lower heat transfer rates and thus generation of less steam at lower temperatures. To overcome this, the steam pressure can be reduced to decrease the water saturation temperature resulting in increased heat transfer and steam generation rates. While estimating the efficiency of the cycle, these changes must be taken into account. Once the TES system is designed at the system level, design optimization techniques are employed for simulating plant performance. As with any design, it is important to first define the basic parameters of the plant such as turbine inlet temperature and pressure, solar field outlet temperature, solar insolation and the size of the power plant. Several models/software are available for optimizing the power system with storage. Sizing of the TES system must be done in such a way that the plant can operate continuously with no waste of energy. The design will be based on the estimates of peak extraction requirements of all the places under consideration, integrated over the operating cycle. Different ways have been adopted by researchers for optimization of the power plant with thermal energy storage. A report from Sandia National Laboratories [31] provides information on the various software programs used for simulating the different components of CSP plants. Table 3 gives a list of the software codes used for simulating the various components.

7 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Table 3 Simulation tools for designing concentrating solar power plants [31]. Power plant type Components Software/Codes used Power tower Optical design and performance of heliostat ASAP [37], DELSOL [38e40], HELIOS, MIRVAL [39,41], SOLTRACE [42], Stress Analysis Codes such as ANSYS Ò [43] and CosmosWorks Ò [44], HFLCAL [45] Central receiver performance CAVITY a [39], DRAC a and TOPAZ a [39,46,47], FLUENT [48], RADSOLVER a [39], Thermal Stress Analysis Codes such as ANSYS Ò [43] and CosmosWorks Ò [44] Heat transfer fluid (HTF) transport, FLUENT [48], SAM [32,49], SOLERGY [39,50], TRNSYS [51] exchange, and storage Power cycle GATECYCLE [52], IPSEPRO [53], STEAM PRO [54], Ebsilon [45] Total system performance DELSOL [38,39,55], SAM [32,49], SOLERGY [39,50], TRNSYS [51] Linear concentrator systems Solar collectors ASAP [56], CIRCE [57,58], FLUENT [48], SOLTRACE [42], TROUGH HELIOS a (troughs and linear reflectors) Dish receiver AAETES [59], FLUENT [48] Power cycle GATECYCLE [52], IPSEPRO [53], STEAM PRO [54] Total system performance EXCELERGY a, SAM [32,49], SOLERGY [39,50], TRNSYS [51] Dish engine systems Dish solar collector ASAP [56], CIRCE [57,58], SOLTRACE [42] Heat transfer fluid (HTF) transport, FLUENT [48], SAM [32,49], SOLERGY [39,50], TRNSYS [51] exchange, and storage Dish field system performance Dish Field System Model [60] (an Excel-based model) a Program is no longer maintained. The Solar Advisor Model, SAM [32], is a modeling software package developed by the National Renewable Energy Laboratory. SAM can model the hourly output of photovoltaic (PV), concentrating PV (CPV) and CSP plants, for many locations. For a CSP plant, SAM models the solar field, TES unit, and power block on an hourly basis to provide the annual performance. This code can use default values or user-defined values to calculate the energy production, capital costs and the LCOE. SAM also has some capabilities to optimize a CSP plant with TES in order to compare the LCOE from a CSP plant to conventional power generators. Gilman et al. [32] provide a detailed description of SAM and its capabilities. TRNSYS (Transient System Simulation) is a sequential-modular transient simulation program developed at the Solar Energy Laboratory of the University of Wisconsin [33]. This publicly available source code is written in FORTRAN. TRNSYS has a large set of power plant components that can be used as a detailed design tool for hourly simulations of a solar energy system over an entire year. The component models which are either empirical or analytical, describe the component performance with algebraic and/or differential equations. A model of the Solar Energy Generating System (SEGS) VI plant was developed by Lippke [34] using EASY simulation software. The results suggest that the highest allowed HTF temperature is optimum for a summer day. The solar thermal electric component library, STEC, is organized by the international organization Solar PACES. A model of SEGS VI was available that utilized STEC components; however the complex model had convergence issues. Numerous private parabolic trough power plant models exist, such as PCTroughÔ by Solar Millennium, but they are not accessible in the public domain. Patnode [35] performed a detailed simulation of SEGS VI using the Engineering Equation Solver, EES, and TRNSYS. The annual performance comparison of the 50 MW e Andasol-like trough plants that employ either a 2-tank or a single tank molten salt thermal storage system [36] was carried out using TRNSYS software. Though a number of programs are commercially available, research and industrial groups are also developing simulations as a tool for modeling the performance of power plant systems and subsystems. A simulation model that reproduces the performance of parabolic trough solar thermal power plants with a thermal storage unit is presented in Llorente Garcia et al. [11]. The aim of this model is to facilitate the prediction of the electricity output of these plants during the various stages of their planning, design, construction and operation. Model results for a 50 MW e power plant are presented and compared to real data from an equivalent power plant currently operated by the ACS Industrial Group in Spain. A mathematicalestatistical model of hybrid solarefossil power cycles was developed in Adinberg [61], which is based on the energy balance equations and historical hourly data of direct normal irradiance and load profiles available in the literature. The performance characteristics of thermal storage with solar fractions from 0.2 (no storage applied) to 1.0 (pure solar operation of a power plant) were studied. From computations performed, it was suggested that for base load operations, an extremely large storage capacity equivalent to nearly a thousand full load operating hours should be available to a power plant to achieve continuous electricity production using only solar energy (solar fraction equal to 1.0) during an annual operating cycle. Arahal et al. [62] developed a model of a thermal storage tank to simulate the storage tank at the PSA (Platforma Solar de Almeria) CSP test facility and compared it with the experimental data from the facility. The model uses the Simultaneous Perturbation Stochastic Approximation (SPSA) technique to adjust the parameters of a serial grey-box model structure. The model simulates the performance of the system and tests various hierarchical control schemes proposed by the authors. Dynamic simulation results for a two-tank direct thermal energy storage system used in a parabolic trough concentrated solar power system are presented by Powell and Edgar [63]. The presence of the storage system, its interaction with the other components of the plant, and how it can be leveraged to control power output, in addition to the collector outlet temperature, was emphasized. Adding a storage system increases the solar share of the power plant by as much as 47% for a base load thermal power output of 1 MW. This reduces the supplementary fuel requirement by as much as 43%. A survey by Madaeni et al. [64] analyzes the economics of a parabolic trough power plant with TES using a general modeling framework that can be used to assess the benefits and value of CSP and TES. Operational performance was studied using the mixedinteger programming (MIP) model. This model uses the characteristics of the CSP plant (e.g., location, power block efficiency, TES efficiency, parasitic load of components), weather conditions (including solar insolation and ambient temperature), and size of the CSP plant as fixed.

8 Key conclusions S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 Q ¼ mc p DT (3) The presence of a TES system in the plant will affect the final performance of the plant both thermally and economically. Several important design constraints must be met in order for the TES system to be able to be integrated into the power plant. Also, depending upon the components used in the power plant and the thermodynamic requirements, the efficiency of the power plant varies. For higher efficiency, the power plants must be able to go to higher temperatures and the TES system must be able to store energy at the required temperatures and be compatible with other component thermodynamic considerations. 3. Component level considerations The performance of a TES system depends on the type of components and their performance capability. At the component level, storage material selection, containment material, heat transfer fluid and the inter-compatibility between the three are the major design considerations Storage media selection The fundamental subcomponent of any thermal energy storage system is the medium chosen to store energy. This material must be judiciously chosen such that its cost, material properties, reactivity, and thermal as well as chemical stability allow for a robust system that can withstand thousands of thermal cycles. These criteria have an overarching role in material selection, heavily restricting the inventory of materials that can be used for storage. The thermophysical properties important in the selection of the storage materials are density, specific heat capacity, thermal conductivity, coefficient of thermal expansion and cycling stability, as well as availability, costs, and production methods Storage types TES systems are classified as sensible heat storage, latent heat storage and thermochemical storage. Sensible heat storage (SHS) systems achieve storage by raising the temperature of a medium (usually a solid or a liquid), therefore the sensible storage materials undergo no change in phase over the temperature range of the storage process. Sensible heat storage in a material depends strongly on its heat capacity, rc p, which determines the energy density and the thermal diffusivity, k/rc p, which determines the rate at which that heat can be released and extracted. The amount of energy stored is given by the following equation: where, Q is the energy stored, m is the mass of the storage medium, c p is the specific heat of the material and DT is the temperature change during the process. All of the currently installed TES systems in utility-scale solar thermal electric plants store energy using sensible heat. These state-of-the-art systems employ molten salts in an indirect two-tank design. The benefit of this design is that heat transfer during both charging and discharging occurs through forced convection; therefore heat transfer is not a severely limiting factor for the system. Additional materials including concrete, rock, sand and metal can be found in Table 4. Though sensible heat storage systems for CSP plants have experienced pronounced development due to the ease of heat transfer and simplicity of the storage system, a number of research efforts are also focusing on techniques that take advantage of latent heat to store thermal energy. These systems utilize materials that change phase at a temperature that falls within the upper and lower limit of the solar field. In doing so they exploit the latent heat, or enthalpy, associated with phase transition. Phase change phenomena vary from solidesolid, liquidevapor and solideliquid transitions, however the latter is typically used due to its low volumetric expansion compared to the liquidegas transition and due to its high latent heat compared to the solidesolid transition. The energy stored in mass m for a solideliquid transition in a phase change material (PCM) is Q ¼ m c ps ðt m T s Þþh þ c pl ðt l T m Þ : (4) where c ps and c pl are the average specific heats in the solid and liquid phases respectively, h is the enthalpy of phase change, T m is the melting temperature, T s is the temperature of the solid and T l is the temperature of the liquid. Latent heat storage is a nearly isothermal process that can provide significantly enhanced storage quantities when compared to sensible storage systems of the same temperature range. Isothermal storage is an important characteristic because solar field inlet and exit temperatures are limited due to constraints in the HTF, solar field equipment and Rankine cycle [69]. Being that the storage capacity of a latent heat system is governed not just by the specific heat of the material but also by the enthalpy of phase change, this can potentially enable a smaller, more efficient lower cost alternative to sensible heat thermal storage systems. A few potential latent heat storage materials are presented in Table 5. The thermochemical storage system is the least investigated storage technology though it can potentially store more energy than sensible or latent heat systems due to the heat of reaction [70]. Table 4 Published data on potential sensible heat storage materials [1,80]. T cold ( C) T hot ( C) Material Thermal conductivity (W/m k) Density (kg/m 3 ) Average specific heat capacity c p (kj/kg K) Volumetric specific heat capacity (kwh th /m 3 ) Sanderockeoil Solid Reinforced concrete Solid Cast iron Solid NaCl Solid Cast steel Solid Silica fire bricks Solid Magnesia fire bricks Solid Synthetic oil Liquid Nitrite salts Liquid Liquid sodium Liquid Silicone oil Liquid Lithium liquid salt Liquid Dowtherm A at 155 C Liquid Therminol Liquid Type of medium

9 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Table 5 Published data on potential latent heat storage materials [76,77]. T melt ( C) Material Latent heat of fusion (J/g) 307 NaNO mol% NaOHe16.2% 290 NaCle6.6% Na 2 CO mol% LiCle6.4% BaCl 2 e39.4% KCl KNO wt% Zne48% Mg mol% LiCle42% KCl KOH mol% MgCl 2 e21.6% KCle33% NaCl wt% Zne4% Al wt% Na 2 CO 3 e35% K 2 CO 3 eli 2 CO wt% Ale35% Mge6% Zn wt% NaCle52% MgCl wt% KCle64% MgCl wt% Na 2 CO 3 e44% Li 2 CO wt% NaCle67% CaCl LiBr wt% LiFe44% NaF 2 e10% MgF Al MgCl Thermal conductivity (W/m K) These systems rely on heat from the solar field to drive reversible chemical reactions, thus the storage medium must have the ability to completely dissociate in the temperature range of the solar field. In this storage concept the reaction in the forward direction is endothermic while the reverse reaction is exothermic [71]. The amount of heat stored in a chemical reaction depends on the heat of reaction and the extent of conversion given by Q ¼ a r mdh (5) where, a r is the fraction reacted and DH is the heat of reaction per unit mass, m. Thermochemical storage is deemed advantageous over sensible heat storage because there is the potential for the chemical products of the dissociation reaction to be stored indefinitely at ambient temperature thus reducing thermal losses. In addition, the exothermic process occurs at a constant temperature if the heat is removed at a rate that would prevent self-heating [72]. Unfortunately these systems suffer from the impediments that are commonly exhibited by other systems including limitations in heat transfer, cycling stability, reversibility and cost. In addition thermochemical systems may experience losses due to storage of gases and they can be restricted by reaction kinetics [73]. Materials that can potentially be used for thermochemical storage are presented in Table 6. A number of publications cite various high temperature materials that can be used for the respective storage systems [7,74e77]. The storage material should possess high energy density which depends on the thermal properties of the materials. As demonstrated in Table 4 through 6, these sources present a few of the thermophysical properties that are important for material selection and system design. Thermal conductivity, for example, affects the dynamics of heat transfer and thus the performance of the system. Laing et al. [78] demonstrate this by providing the length of tube required for transferring heat from an HTF to concrete storage material for various thermal conductivities of concrete. They show that by increasing the thermal conductivity of the storage material, heat transfer is enhanced, allowing for greater distance between the heat transfer tubes and thus fewer tubes are required. For a 950 MWh th system, only a slight increase in thermal conductivity from 1 W/m K to 1.8 W/m K reduced the total required tube length by 46%. Buschle et al. [79] demonstrate the effect of latent heat of fusion and thermal conductivity for phase change materials. Using an analytical technique, they determined the required thickness of PCM around a heat transfer tube for two PCMs of different latent heat of fusion (i.e. 100 kj/kg and 300 kj/kg). For the same outer tube diameter and wall thickness, the PCM with the higher latent heat requires a thickness of 9 mm, whereas the other PCM requires a thickness of 20 mm to store the same amount of energy. They also found that improvements in solidification times are greatly enhanced when the thermal conductivity increases to 5 W/m K. Above this value, the improvements are not significant [79]. In materials where the thermal conductivity is very low, high conductivity materials such as graphite, metal fibers, or metal/ ceramic matrices can also be added for enhancement of the thermal conductivity. In such cases, the effective thermal conductivity must be used for thermal analysis. The heat transfer in a PCM can be increased by the addition of high thermal conductivity materials [87]. Due to its superior properties d high thermal conductivity, good process ability, and chemical inertness d graphite has distinct advantages for this purpose. Depending on the requirements of the respective application, various routes to combine PCM and graphite are used. An overview of actual and potential applications of PCM/ graphite heat storage systems focusing in the storage of solar heat for high temperature applications, such as process heat generation and solar thermal power plants, is presented in Ref. [88]. Reviews of materials used for enhancing the thermal conductivity of PCMs for different applications are provided in Refs. [89e91]. Another technique to enhance the heat transfer capability of PCMs is by macro- or microencapsulation of PCMs. There are many advantages of encapsulated PCMs, such as increasing the heat transfer area, reducing the PCM reactivity with the outside environment and controlling the changes in the storage material volume as phase change occurs. Macroencapsulated PCMs can be used in packed bed systems. The most often used microencapsulation methods are [92]: Physical methods: (i) pan coating, (ii) air-suspension coating, Table 6 Thermochemical storage media. Compound Temperature ( C) Reaction DH (kj/mol) DH (GJ/m 3 ) Manganese oxide [73] 530 (at 1 bar of reactant) MnO 2 þ DH 4 0.5Mn 2 O 3 þ 0.25O 2 42 Calcium hydroxide [73] 505 (at 1 bar of reactant) Ca(OH) 2 þ DH 4 CaO þ H 2 O 112 Calcium carbonate [73] 896 (at 1 bar of reactant) CaCO 3 þ DH 4 CaO þ CO Magnesium hydride [81] 250e500 MgH 2 þ DH 4 Mg þ H 2 75 Ammonia [82] 400e500 NH 3 þ DH 4 1/2N 2 þ 3/2H 2 67 Methane/Water [83] 500e1000 CH 4 þ H 2 O 4 CO þ 3H 2 n.a. Magnesium oxide [84] 250e400 MgO þ H 2 O 4 Mg(OH) Iron carbonate [85] 180 FeCO 3 4 FeO þ CO Metal hydride [83] 200e300 Metal xh 2 4 metal yh 2 þ (x y)h 2 4 Methanolationedemethanolation [86] 200e250 CH 3 OH 4 CO þ 2H 2 n.a

10 294 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 Table 7 Materials that have been evaluated for use as storage media in TES systems in terms of cycle life and stability. Material Storage type Number of cycles/hours tested Temperature, C Notes/Conclusions High temperature concrete Sensible 370 cycles 200e400 Suitable for use as storage material. [97] 60 wt% NaNO 3 /40% KNO 3 (solar salt) Sensible 30,000 h 290e565 Impurities in salt (98% pure) required [10,98] pretreatment to remove toxic NOx. Composition changed over time and melting point slightly decreased but these changes did not have an effect on salt s performance. Proven to be suitable for use as storage medium. Taconite (iron ore pellets) Sensible a 550 Should maintain integrity as filler material [99] for years in nitrate salts Taconite (iron ore pellets) Sensible a 350 thermal cycles 290e400 Porous pellets held together fairly well after [98,100] cycling. Acceptable for use as filler material in molten salt. Barium carbonate (mineral name e witherite) Sensible a 10 h 400 Reacted with Ca(NO 3 ) 2 during 10 h exposure [98] to Hitec XL nitrate salt b. Barium sulfate (mineral name e barite) Sensible a 10 h 400 Reacted with Ca(NO 3 ) 2 during 10 h exposure [98] to Hitec XL b. Aluminum oxyhydroxide (mineral name e bauxite) Sensible a 1000 h 400 Crumbled after exposure to Hitec XL nitrate salt b [98] Iron titanate (mineral name e illmenite) Sensible a 1000 h 400 Crumbled after exposure to Hitec XL nitrate salt b [98] Calcium carbonate (mineral name e KS limestone) Sensible a 1000 h 400 Crumbled after exposure to Hitec XL nitrate salt b [98] Calcium sulfate (mineral name e anhydrite) Sensible a 1000 h 400 Experienced significant weight loss after [98] exposure to Hitec XL nitrate salt b Silicon carbide (mineral name e carborundum) Sensible a 1000 h 400 Gained weight after exposure to Hitec XL [98] nitrate salt, most likely due to porosity. Silicon dioxide (mineral name e quartzite) Sensible a 553 thermal cycles 290e400 No compatibility issues with Hitec XL. [98] Only slight changes in color. Held up well and suitable for use as filler material. Apatite Sensible a 1000 h 400 No compatibility issues with Hitec XL at [98] tested temperature Calcium carbonate (mineral name e marble) Sensible a 350 thermal cycles 290e400 After thermal cycling, material softened and [98] fared poorly. Unacceptable as a filler material. Hydrated calcium carbonate Sensible a 365 thermal cycles 290e400 After thermal cycling, material became soft [98] (mineral name e NM limestone) and looked like mud. Unacceptable as a filler material. Aluminum oxide (mineral name ecorundum) Sensible a 1000 h 400 No compatibility issues with Hitec XL at [98] tested temperature Scheelite Sensible a 1000 h 400 No compatibility issues with Hitec XL at [98] tested temperature Tin oxide (mineral name e cassiterite) Sensible a 1000 h 400 No compatibility issues with Hitec XL at [98] tested temperature 16 wt% Ca(NO 3 ) 2 /34% NaNO 3 /50% KNO 3 mixture Sensible >72 h 531 Reagent grade salts heated in air. Minor [101] amount of CaCO 3 and nitrites detected due to decomposition. Water did not affect chemical stability. Stable up to 500 C. 30 wt% Ca(NO 3 ) 2 /24% NaNO 3 /46% KNO 3 mixture Sensible >72 h 504 Reagent grade salts heated in air. Minor [101] amount of CaCO 3 and nitrites detected due to decomposition. Water did not affect chemical stability. Salt stable up to 480 C. 42 wt% Ca(NO 3 ) 2 /15% NaNO 3 /43% KNO 3 mixture Sensible >72 h 501 Reagent grade salts heated in air. Minor [101] amount of CaCO 3 and nitrites detected due to decomposition. Water did not affect chemical stability. Salt stable up to 460 C. 12 wt% LiNO 3 /18% NaNO 3 /70% KNO 3 mixture Sensible >72 h 601 Reagent grade salts heated in oxygen. Minor [101] amount of oxides detected due to decomposition. Salt stable up to 550 Cin absence of atmospheric CO wt% LiNO 3 /28 % NaNO 3 /52% KNO 3 mixture Sensible >72 h 600 Reagent grade salts heated in oxygen. Minor [101] amount of oxides detected due to decomposition. Salt stable up to 550 Cin absence of atmospheric CO wt% LiNO 3 /33% NaNO 3 /40% KNO 3 mixture Sensible >72 h 600 Reagent grade salts heated in oxygen. Minor [101] amount of oxides detected due to decomposition. Salt stable up to 550 Cin absence of atmospheric CO wt% LiNO 3 /18% NaNO 3 /52% KNO 3 eutectic Sensible >72 h 600 Reagent grade salts heated in oxygen. Minor [101] amount of oxides detected due to decomposition. Salt stable up to 550 Cin absence of atmospheric CO 2. Aluminum Latent 130 cycles 570e % pure Al thermally cycled in Inconel X-750 container. Al alloyed with container material, preventing melting of Al; therefore cannot be used as storage medium if contained in Inconel or stainless steel alloys. [102] Ref.

11 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Table 7 (continued ) Material Storage type Number of cycles/hours tested Temperature, C Notes/Conclusions 60 wt% Al/34% Mg/6% Zn alloy Latent 1000 thermal cycles 25e550 Latent heat decreased by 10.98% and melting point decreased by 5.3 C after 1000 cycles. Overall good thermal stability 50 mol% NaNO 3 /KNO 3 Latent >100 thermal cycles 175e different samples were cycled more than 100 times at rate of 5 C/min. Neither subcooling nor thermochemical instability was observed. Sodium nitrate Latent 2600 h % purity material tested isothermally. Small amounts of Nitrite formed at 350 C. Overall thermally stable. LiKCO 3 (intermediate compound of 35 wt% Li 2 CO 3 /65% K 2 CO 3 mixture) Latent 5650 h/129 cycles 430e535 Cycled in 316SS vessel. Salt showed high degree of stability. Very slight decrease in melting point and no change in composition due to decomposition or chemical rxn. Very thin stable oxide layer formed on container mol% NaNO 3 e81.5% NaOH Latent 1000 cycles 230e300 Industrial grade material which generally contains trace amounts of NaCl was tested. Observed a small peak that was attributed to formation of ternary eutectic with NaCl. Authors concluded that little change occurred after cycling and material would be good for storage. Li 2 CO 3 Latent 408 h/13 cycles 676e776 Tested in a welded container of 316SS. Small salt leakage through weld due to oxidation and carburization of canister. Recycled in CO 2 cover gas which reduced this effect. Na 2 CO 3 Latent 288 h/21 cycles 808e908 Tested in welded 316SS container. Slight leakage around thermocouple fittings. Corrosion layer and subscale carburization observed on heat exchanger tube due to salt interactions wt% BaCO 3 e47.8% Na 2 CO 3 Latent 984 h/36 cycles 636e736 Observed melting pt. around 712e717 C instead of 686 C published value. Salt showed stable performance and good compatibility with containment material 81.3 wt% Na 2 CO 3 e18.7% K 2 CO 3 Latent 1032 h/38 cycles 737e797 The system was stable during operation. Salt melted incongruently, resulting in slush-type behavior, indicated by lack of columnar crystals on heat exchanger and by radial temp distribution. MgH 2 /Mg Thermochemical 700 cycles 475e522 Tested under pressure of 115e130 bar. Particles of Mg agglomerate and fused together at tested temps due to high vapor pressure and low melting point of Mg. Resulted in particles not completely reacting with hydrogen. Ref. [103] [104] [105] [106] [107] [108] [108] [108] [108] [109] a For use as thermocline filler material. b Hitec XL nitrate salt is a commercially available salt containing 42 wt% Ca(NO 3 ) 2 /15% NaNO 3 /45% KNO 3. (iii) centrifugal extrusion, (iv) vibrational nozzle, (v) spray drying and congealing (vi) fluid bed coating. Chemical methods: (i) interfacial polymerization, (ii) in situ polymerization, (iii) matrix polymerization (iv) solvent evaporation. However, the applicability of these methods for high temperature materials still needs to be investigated. Electroplating has also been used for encapsulation of metal PCMs with nickel, for storing and reusing high temperature waste heat from industrial applications [93] Stability As previously mentioned, the reactivity, thermal stability and chemical stability of a material are important criteria for media selection, therefore these properties must be well established. Unfortunately, the evaluation of these criteria can take considerable time due to the cyclic nature and long life expectancy of thermal storage systems. In an effort to evaluate the thermal stability of high temperature concrete, the German Aerospace Center, DLR, conducted a series of tests on concrete modules that use tube registers to increase heat transfer [94]. Concrete is a favorable sensible heat storage material because it is low in cost, high in strength, easy to handle and readily available [95]. DLR s first test module resulted in damage caused by excessive vapor pressure during start-up due to insufficient permeability within the material. After improving the concrete s mixture to allow for greater permeability, DLR, in conjunction with Ed. Züblin AG Construction Company, constructed a large-scale test module that underwent cyclic testing to simulate

12 296 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 charging and discharging under various temperatures. More than 370 thermal cycles were performed over 23 months of operation in the temperature range between 200 C and 400 C with successful results. Additional tests were extended to temperatures up to 500 C for several thousand hours to investigate the impact of temperature on the material stability and strength of the concrete. After a number of years under research and evaluation, final results indicated that the high temperature concrete is suitable for use as a sensible heat storage material up to 500 C [96,97]. Though the high temperature concrete tests proved the material as a feasible option for storage, a number of materials remain unevaluated for their practical use as storage media. Table 7 provides material tests that have been conducted beyond thermophysical properties. Nitrate salts are used extensively in both sensible and latent heat systems due to their cost, favorable thermal properties, low vapor pressure and low chemical reactivity; therefore they have been studied at length [2]. Important to a thermocline unit is whether the filler material is compatible with the liquid storage medium/htf. With respect to latent heat storage systems, the heat of fusion of the storage material must not degrade after thousands of thermal cycles and the material cannot decompose upon heating. Compatibility with the container materials is critical for all types of storage media and is further discussed in Subsection Material characterization Characterization of thermal energy storage material properties has been an extensive effort however a number of materials have yet to be characterized. Calculated values rather than experimentally verified values for thermophysical properties are often found in the literature. This is not only a problem because incorrect values make it difficult to perform an accurate cost analysis, but also because the materials used on an industrial scale could be of lower purity and hence the values of the thermophysical properties will be further reduced. The discrepancy between calculated and measured values of latent heat is particularly large for mixtures and eutectics that do not exhibit ideal-mixing behavior. Some examples of measured versus theoretical thermophysical properties are presented in Table 8. Characterization of thermal properties such as heat capacity, latent heat, melting point, and subcooling for phase change materials is generally carried out using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). During evaluations, compatibility or stability of the equipment crucibles with the storage material is very important. For example, in the presence of inorganic salts that are corrosive in nature, using aluminum oxide crucibles will aid in getting more accurate results, compared to using the low-carbon 316 stainless steel (SS316L) or aluminum crucibles typical to these materials [115] Compatibility of materials Once the storage medium is selected, it is important to identify the materials that will be used for the remaining components of the system including the storage tank, heat transfer fluid and, if used, encapsulation as well as heat transfer enhancement materials. In some cases, for example in a sensible heat packed bed system, encapsulation or containment of the material may not be needed as the storage medium can be directly in contact with the HTF. However, in latent heat storage systems PCMs must be contained from the HTF to avoid any problems during the phase change process. The containment material must be compatible with the storage material, stable and should not hinder heat transfer between the storage medium and the heat transfer fluid. PCM containment should meet the requirements of strength, flexibility, corrosion resistance and thermal stability; act as a barrier to protect the PCM from harmful interactions with the environment; provide sufficient surface for heat transfer and provide structural stability and easy handling. One of the main issues with material selection is corrosion. Corrosion decreases the life cycle of the system and changes the thermal performance of the system. Another important issue is the thermal stability of the containment system during repeated thermal cycling, which generally aggravates high temperature oxidation. Organic encapsulation materials such as polymers may also have problems with out-gassing. Data for high temperature salt corrosion with thermal cycling expected in a CSP plant are scarce. Most of the available data is for diluted salt hydrates, typically used in the chemical industry [116,117]. Porisini [118] tested the corrosion of four commercially available salt hydrates used as PCMs with melting points between 15 C and 32 C on stainless steel, carbon steel, aluminum alloys and copper in 1988 and found stainless steel to be the most stable. Cabeza et al. [119e122] studied the corrosion resistance of five common metals in contact with molten salt hydrates in an Table 8 Materials that have been characterized for their thermal storage capability. Material Measured Theoretical Latent heat (kj/kg) T melt (⁰C) Purity of material tested Ref. Latent heat (kj/kg) T melt (⁰C) 29 mol% Zn/71% Mg eutectic alloy Commercially pure [110] 247e464 e [110] 37.5 mol% Mg/62.5% Al eutectic alloy Commercially pure [110] 458e477 e [110] 17.5 mol% Cu/82.5% Al eutectic alloy Commercially pure [110] 359e380 e [110] 13 mol% Si/87% Al eutectic alloy Commercially pure [110] 571 e [110] 17 mol% Cu/16.2% Mg/Al Commercially pure [110] 400e406 e [110] ternary eutectic alloy 12.6 mol% Cu/5.1% Mg/Al Commercially pure [110] 449e549 e [110] ternary eutectic alloy 59 mol% LiCl/41% KCl Reagent grade, [111] 242e272, [76,111] vacuum dried for 6 h. KNO Analytical reagent grade, [111] 95.2e [111,112] vacuum dried for 16 h mol% LiCl/41.3% KCl % pure KCl/99% pure LiCl [113] [112] 95.5 wt% KNO 3 e4.5% KCl >99% [114] [114] wt% KNO 3 e7.44% KCle11.87% KBr 76.6 N/A a >99% [114] [114] wt% NaCle32.29% KCle32.9% LiCl >99% [114] [114] 60 wt% MgCl 2 e20.4% KCle19.6% NaCl >99% [114] [114] a N/A e this mixture melted over a range of temperatures and did not form a eutectic. Ref.

13 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e immersion corrosion test. Heine et al. [123e125] studied the corrosion performance of six molten salts (chlorides, nitrates and fluorides) melting between 235 C and 857 C vs. four different steels used at these temperatures. Other studies about corrosion in steel concluded that the impurities typically contained in commercial grades of alkali nitrates have relatively small effects on corrosion of stainless and carbon steels in molten salts prepared from these constituents [126e128]. Bradshaw and Goods [133,134] reported on the resistance of stainless steels (used to make the hot tank in a two-tank storage system) to corrosion during thermal cycling in mixtures of nitrate molten salts. In applications where exposure to nitrate salts can be limited to 400 C or less, the use of carbon steels may be considered. From tests carried out so far, it is found that using carbon steel is acceptable when operating with nitrate salts at temperatures below 400 C. For higher temperatures, it is useful to use stainless steel for tank materials or other components of the system. Table 9 shows some of the containment systems used with high temperature salts Storage tank components Once the tank material is identified, the remaining components of the tank system can be selected based on temperature ranges. The insulation material must be able to insulate the tank from the surroundings and minimize losses. Fig. 4 shows the different components of a 2-tank thermal storage system used at the Solar Two central receiver demonstration project [135,136]. Starting from the bottom and moving up, the tank foundation consists of a concrete slab, an insulating concrete foundation, foam glass insulation, insulating fire bricks and a steel slip plate. The perimeter of the foundation is somewhat different, consisting of a ring wall of fire bricks to support the large loads from the walls and the roof Heat transfer fluids Heat transfer fluids must be compatible with the containment materials, storage media and be able to operate in the required temperature range. As mentioned earlier, organic oil-based HTFs, air, water and molten salts have been used for transferring heat in CSP plants. The organic oil-based HTFs currently tend to break down at high temperatures (around 400 C). Inorganic salts maintain stability at high temperatures, but solidify easily at temperatures as high as 230 C. Therefore plants that operate at high temperature are able to use air, steam or molten salts as heat transfer fluids. Currently, the research in this area focuses on developing advanced HTFs that can operate at temperatures ranging from 80 Cupto 500 C. Research developments include [137]: Creation of standard eutectic salts that are less corrosive by the combination of two or more salts that can freeze at the same temperature Developing nano-scale encapsulated structures of 50e500 atoms that can then be suspended in fluids and be used for both TES and HTF, and can withstand repeated cycling between heating and cooling. These heat transfer fluids, when used with other advanced technologies, could significantly decrease the solar electricity cost to as little as US$0.05e0.07/kWh [137]. Lower costs would make solar thermal electricity competitive with gas and coal Cost Fig. 4. Storage tank foundation [135]. As can be seen in Fig. 5, the greatest cost of the state-of-the-art 2-tank system is due to the storage material. The cost of materials is very important, as the current motivation that drives research and funding opportunities is to store energy at low cost, thus reducing the LCOE of CSP. In the case of high temperature storage systems in which many inorganic salts are proposed as the storage medium, it is difficult to get a true bulk cost of the material for a preliminary assessment. Unlike metals and petroleum products which are sold through international exchanges, industrial salts are produced on a regional level and producers compete directly with each other and set the price [138]. Prices also vary between different salts and between purity levels for the same salt. In addition to material Utilizing nano-particle-based fluids as HTF and TES to modify the melting point and heat transfer capability Table 9 Container materials used for high temperature salts. Salt used Container material Operating Reference temperatures KNO 3 ; KNO 3 /KCl; NaNO 3 AISI 1015 (PCM) AISI T min ¼ 270 C; [129] K01200 (Tank) T max ¼ 350 C KNO 3 /NaNO 3 /Ca(NO 3 ) and 316SS and 570 C for SS; [126] binary and ternary mix A36 carbon steel 316 C for C steel KNO 3 /NaNO 3 /Ca(NO 3 ) 2 316SS 450 C and 500 C [130] MgCl 2 /KCl 316SS and high 850 C for 100 h [131] nickel alloys Fluoride salt eutectics Inconel C for 20,000 to 30,000 h [132] Fig. 5. TES cost breakdown for 2-tank indirect sensible heat storage [8].

14 298 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 costs, delivery charges, energy to melt the salt, if used in molten form, and handling costs need to be considered [139]. Fig. 5 also demonstrates that large reductions can occur by minimizing the number of tanks and heat exchangers used in the system. For systems that require heat transfer enhancement mechanisms, the associated costs must be included in the analysis Maturity of storage technology In 2011, Tamme [140] provided the maturity status of thermal energy storage systems. As can be seen in Fig. 6, the least mature technologies have the greatest potential to reduce the amount of material required for storage. A significant amount of research efforts are currently focusing on reducing the cost of CSP plants by developing storage systems that utilize materials with high energy density or are lower in cost than the current state-of-the-art systems. Major efforts are underway in the U.S. and Europe due to large-scale funding opportunities from their national energy departments [17]. In FY 2011 for example, the U.S. Department of Energy awarded 15 grants totaling $37.3 million USD for thermal energy storage projects through the Advanced Research Projects Agency-Energy (ARPA-E) [141]. The goal of the funding program is to develop revolutionary, cost-effective ways to store thermal energy. The storage concepts under investigation span across all three categories: sensible, latent, and thermochemical heat storage. In 2012, within the framework of the European Energy Research Alliance, the European Union awarded V8.65 billion to a solar energy project titled Optimization of a Thermal Energy Storage system with integrated Steam Generator (OPTS). Multiple European research institutions will work together over a three-year period to develop a single tank, stratified molten salt storage system that can store heat up to 550 C for large-scale CSP plants [142]. However, rather than reducing the cost by using different storage materials, this project focuses on the development of a new system design that will integrate the steam generator with the storage system using the same molten salt that is used in the 2-tank system. Different system level design concepts are discussed below, in Section System level considerations From the previous sections it is evident that TES system level efficiency and performance are very important in determining the efficiency of the power plant. The assembly of the selected components plays a major role in achieving the targeted storage density, charging and discharging capabilities and cost requirements of the TES system. At the system level, several factors determine the thermal behavior and hence the energy and exergy efficiency of the unit. The key requirements for developing a thermal energy storage system are shown in Table 10 [143]: Table 10 Key requirements for developing TES systems for CSP plants. 1 High energy density capability of storage material 2 Efficient heat transfer between the storage material and HTF provided by properly designed heat exchange equipment. 3 Fast response to load changes in the discharge mode 4 Low chemical activity of storage material and HTF towards the materials of construction 5 Good chemical stability of storage material/htf and temperature reversibility in a large number of thermal charge/discharge cycles comparable to a lifespan of the power plant, 30 years 6 High thermal efficiency and low parasitic electric power for the system 7 Low potential contamination of the environment caused by an accidental spill of large amounts of chemicals used in the TES system 8 Low cost of storage material, taking into account the embodied energy (carbon) 9 Ease of operation and low operational and maintenance costs 10 Feasibility of scaling up TES designs to provide at least 10 full load operation hours for large-scale solar power plants of 50 MW electrical generation capacity and larger 4.1. Types of TES systems TES systems can be categorized as active or passive types. When the storage medium is a fluid and is able to flow between the tanks, the systems are referred to as active type systems. If the storage medium is also used as the heat transfer fluid, the system is referred to as direct-active system. An additional heat exchanger is needed when the storage fluid and heat transfer fluid are different and the unit is referred to as indirect-active type. In cases where the storage medium is solid, the HTF passes through the storage material only for charging and discharging. Such system is called a passive type. Fig. 7 shows the various TES system configurations classified as active and passive storage systems. Each of these types has their own advantages and disadvantages compared to other types of systems. Depending on the storage material chosen, any configuration must be able to store the required amount of energy within the duration of application and should be economical. In terms of thermal performance, it is critical that the storage system has high energy density and has adequate charging and discharging rate capability. For example, in the case of active thermal storage systems, liquids are used as storage materials compared to gases since liquids possess high energy density and thermal conductivity compared to gases. However, using a liquid that is stable in the operating temperature of the system may not be economical. Similarly, if a cheap solid material with high energy density is chosen, the system may not possess high thermal diffusivity to charge and discharge the energy efficiently. Hence, high thermal conductive materials are embedded for enhancing heat transfer. In the following sections, the various active and passive types of systems are first described, followed by a discussion on the theoretical analysis of these systems. Fig. 6. Maturity status of storage technology [140]. Fig. 7. Categorization of TES systems presented in this review.

15 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Active storage In an active storage system, the storage material itself circulates between the heat exchangers for energy transfer. The employed heat exchangers can also be solar receivers or steam generators [144] Two-tank systems. Liquid storage media that are used for high temperature CSP systems are usually molten salts or synthetic oils. A two-tank direct storage (a system in which storage material also acts as heat transfer fluid) was used in early parabolic trough power plants (such as Solar Electric Generating System I) [145] and at the Solar Two power tower in California [146]. Several parabolic trough power plants under development and in operation in Spain and the U.S. use the indirect two-tank thermal energy storage concept (see Table 2). The plants will use organic oil as the heat transfer fluid and molten salt as the storage fluid. Currently, the 2- tank storage system is the most commercially mature technology and used in industry while other types vary in their level of maturity Thermocline systems. Liquids can also be used in a single tank system where a hot fluid such as synthetic oil is pumped into the top of a tank during the charging mode, gradually displacing a colder fluid. A thermal gradient is created within the system and is ideally stabilized and preserved by buoyancy effects. Such a system has been coined thermocline storage due to the thermal gradient that develops within the system. The hot fluid remains at the top and the cold fluid remains at the bottom, however, in these systems, it is difficult to separate the hot fluid from the cold fluid. An ideal thermocline is shown in Fig. 8, reproduced from Ref. [134], has an imaginary vertically movable perfect thermal insulation layer that protects the mixing of hot and cold fluids. The insulation layer can move depending on whether a hot fluid or cold fluid is being stored in the system. Typically, a filler material is inserted in the system. The presence of the filler material aids in maintaining the gradient and reduces natural convection within the liquid. The filler in these systems acts as the primary storage material, therefore systems using filler materials are categorized as passive systems and will be discussed later Steam accumulators. In addition to the use of synthetic oils and molten salts in active type storage, water can be used as the storage medium in systems called steam accumulators. In these systems, charging takes place when superheated steam or saturated water enters a pressurized storage tank that initially contains saturated steam and saturated water. If the system is being charged with superheated steam, the temperature and pressure of the water in the tank increases, thus changing the saturation state of the initial mass [148,149]. On the other hand, if saturated liquid is used to charge the system, pressure and temperature remain constant yet the mass in the volume is increased [149]. The discharging process takes place by reducing the pressure in the storage tank. This results in the production of saturated steam that decreases in pressure as the discharging process proceeds. If superheated steam is desired, a secondary storage system is needed to increase the temperature of the steam. Steam accumulators are well-suited for direct steam generation (DSG) CSP plants in which steam is produced directly in the solar field and then used in the power block to produce power. Steam that is produced in excess of what is needed by the turbine can be diverted to the steam accumulator. These systems can also act as both the storage system and the phase separator in DSG plants that are run in recirculation mode [149]. In this mode, saturated steam is separated from saturated liquid in a separator drum and then sent through another collector to get superheated. As can be seen in the schematic in Fig. 9, rather than using two distinct pieces of equipment, wet steam leaves the solar field and enters the combined steam accumulator/separator where the phases are separated and the pressure remains constant. Since these systems require pressurized and hence expensive storage tanks, and also possess low volumetric energy densities (volumetric storage capacity for water is 20e30 kwh/m 3 compared to approximately 100 kwh/m 3 for PCMs), they are useful when low thermal storage capacity is needed as is the case for buffer storage [149]. In order to increase the storage capacity, it has been proposed that latent heat storage is used in conjunction with the steam accumulator by either placing PCMs directly in the steam accumulator [149] or external to the steam accumulator [79] They can also be used in the indirect storage configuration, in which a separate heat transfer fluid such as synthetic oil enters a heat exchanger that is in contact with the mass in the steam accumulator Passive storage In passive storage systems, the HTF carries energy received from the energy source to the storage medium during charging and receives energy from the storage system when discharging (these systems are also called regenerators). The arrangement for the HTF to flow through the storage medium is a major parameter that Fig. 8. An active type ideal thermocline (left) and a passive type thermocline tank having a filler material (right) [147]. Fig. 9. Integration of a steam accumulator in a direct steam generation plant. The steam accumulator in this design also serves as the phase separator.

16 300 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 Fig. 10. Schematic of typical LHTS units: (a) flat-plate; (b) shell and tube with internal flow; (c) shell and tube with parallel flow; (d) shell and tube with cross flow; (e) sphere packed bed [150]. dictates heat transfer in the unit. When the heat transfer fluid is a liquid and the heat capacity of the solid in the storage system is not negligible, the system is called a dual storage system. Passive storage systems may utilize inexpensive solids such as rocks, sand or concrete for sensible heat storage materials, or phase change materials for storing thermal energy. Heat transfer can be more problematic in passive type systems since the storage medium is in solid phase rather than liquid phase. Several heat transfer mechanisms exist, varying from embedding tubes or pipes in the storage medium to using a packed bed type storage unit where the fluid directly flows through the storage medium with no additional heat exchanger required. If the storage medium is a solideliquid phase change material, design considerations must also include the separation of the liquid medium from the heat transfer fluid. Types of containment studied are bulk storage in tank heat exchangers, macroencapsulation and microencapsulation. Fig. 10 shows the various schematics of containment used in solideliquid phase change thermal energy storage systems [150]. Fig. 11 shows implementation of a concrete storage test module being implemented by the German Aerospace Center (DLR) for use in Direct Steam Generation [97]. The test module incorporates tube registers that are used to transport and distribute the HTF through the module while sustaining fluid pressure Systems with embedded or enhanced heat transfer structures. Embedded structures used for improving heat transfer have been studied by many researchers at both low temperature and high temperature. However, as mentioned in the Component level considerations section, if high thermal conductivity materials are used, they must be compatible with both the storage material and heat transfer fluid. For high temperature sensible heat storage, concrete systems with an integrated tubular heat exchanger have been used in Laing et al. [94]. The heated transfer medium passes through pipes embedded in the storage concrete to heat it. To discharge thermal energy, the cold transfer fluid flows through the concrete in the reverse direction and is heated up. A tubular heat exchanger is integrated into the storage material for efficient heat transfer. The heat exchanger demands a significant share of the investment costs. The technology is scalable and the costs come down significantly, especially for smaller plants (around 10 MW). Single unit and modular charging/discharging concrete storage concepts are investigated in Refs. [94,96,151]. A horizontal heat transfer structure (e.g., graphite or aluminum) is proposed to be placed between the layers of precast concrete (Fig. 12). The heat transfer structure surrounds the tubes for good contact and subsequently high transfer of heat. The feasibility of using different fin materials made either from graphite or aluminum with the sandwich concept was studied by Steinmann et al. [152] for cost-effective latent heat storage systems. Because of its high efficiency and relatively smaller volume, the shell and tube heat storage unit is widely used in latent thermal energy storage systems. In this heat storage unit, the phase change material (PCM) fills the annular shell space around the tube, while the heat transfer fluid flows within the tube and exchanges heat with the PCM. This type of latent heat thermal energy storage (LHTES) system has been studied by several authors using numerical and experimental methods [153]. Instead of using high conductivity materials, heat transfer can also be improved by using a high conductivity heat exchanging mechanism. The use of embedded heat pipes (HPs) or thermosyphons between a PCM and the HTF as a means of enhancing the thermal energy transport between them has been explored in the literature [154e157]. The working principle of a heat pipe may be found in Faghri [158]. Heat pipes have high effective thermal conductivities, can be tuned to operate passively in specific temperature ranges, and can be fabricated in a variety of shapes. A concept of a TES system, called RHTS (reflux heat transfer storage), was developed for CSP parabolic trough collectors where the solar-heated synthetic oil temperature can reach about 400 C and is presented in Refs. [61,159] (Fig. 13). It is based on the Fig. 11. Prefabricated tube registers being installed in a concrete storage test module at DLR [97].

17 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Fig. 12. Test module (without thermal insulation) for concrete thermal storage with embedded structures [96]. combined phase change effects of melting and vaporization occurring in the storage system simultaneously. A HTF within the system is used to transfer heat between the storage material (could be latent or sensible heat storage material) and the external heat exchangers by utilizing the effects of boiling evaporation and reflux condensation. In lab-scale tests, the overall thermal conductivity of an RHTS apparatus was determined to be about 500 W/m-K which is comparable to the performance of thermosyphons Packed bed systems. Packed bed systems consist of storage material elements in various shapes and sizes and a heat transfer fluid that flows between these elements for transferring heat to the storage material. Since the heat transfer fluid is in direct contact with the storage material, heat transfer coefficients can be large. These systems can maintain the thermal gradient when very low conductive materials such as rocks are used. Thermoclines with filler materials can be characterized as packed bed systems. Most packed bed systems are single tank systems that act as a thermocline. Using a solid storage medium and only needing one tank reduces the cost of this system relative to active two-tank systems. The Solar One central receiver pilot plant used a thermocline storage system with a eutectic mixture of sodium nitrate and potassium nitrate as the storage medium along with quartzite and silica sand as the low cost filler material [146]. It was found that at temperatures of 400 C, the cost of a thermocline heat storage system is 2/3 the cost of a two-tank system, both using the same molten salts. The details for molten salt storage where the tank is filled with nominally 1-inch-size quartzite rocks and 1/4-inch sand particles were studied by Bharathan and Glatzmaier [160]. These filler materials are envisioned to occupy 75% of the tank s volume. A recent report from Abengoa Solar, Inc. [161] analyzed and compared supercritical receiver designs using a packed bed thermocline and 2-tank systems for thermal storage. The 2-tank thermal storage system consists of a cold salt tank, operating at a temperature of 288 C, and a hot salt tank, operating at a temperature of 565 C. The thermocline system consists of a vertical vessel, which is filled with a granular ceramic material, such as quartzite. Their study suggested that it is not feasible to use supercritical steam as the heat transfer fluid in a thermocline system but it can work by using supercritical CO 2. Also, the report suggests that two-tank nitrate salt thermal storage systems are strongly preferred over the thermocline systems when using supercritical heat transport fluids. A solid 2-tank thermal energy storage concept for operation in power towers, using quartz sand as a storage medium, is presented by Warerkar et al. [162]. Sand is a low cost storage material, which can reduce the cost of the system depending on the success of the airesand heat exchanger, which is under development. Warekar et al. [144] describe the simulations results and some experimental measurements of the heat exchanger, and discuss the scale-up options. A schematic of the proposed system is demonstrated in Fig. 14. A high temperature thermal storage system using a packed bed of rocks for air-based central receiver CSP plants was modeled and validated by Hanchen et al. [163]. They validated their simulations with experiments that were performed with magnesium silicate rock as the storage material at 800 K. According to their study, the pumping power for a particle size of more than 10 mm was less than 1% of the power produced. Furnas [164] conducted an experimental study of heat transfer from air to a bed of iron ore pellets at temperatures up to 750 Cas well as studies at lower temperatures using limestone and coal as the storage media. He concluded that the coefficient of heat transfer varies linearly with gas velocity. Bradshaw et al. [165] studied heat transfer between air and nitrogen in packed beds using aluminum, steel and hematite spherical pellets with a maximum temperature of 800 C. Modeling and experimental studies were performed on a packed bed for high temperature energy storage using zirconium oxide pellets as the storage material [166]. The maximum temperature for the bed was about 1000 C, with flu gas as the charging fluid and ambient air as the discharging fluid Combined systems Though the state-of-the-art in TES makes use of a single type of system, i.e. 2-tank sensible heat storage, the use of multiple types of Fig. 13. Diagram of the RHTS concept [61,159]. Fig. 14. Sand storage concept for solar power towers [162].

18 302 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 systems has many benefits. In a traditional superheated steam Rankine cycle power block it is desirable to minimize the temperature difference between the storage medium and working fluid in order to reduce energy losses [167]. A typical Tes diagram for charging and discharging of steam in a power plant is provided in Fig. 15. As can be seen in the figure, the thermal match between the storage system and working fluid are maximized when steam production, which is an isothermal process, is coupled with an isothermal storage process. Being that latent heat storage is isothermal, it is deemed advantageous to use this type of system for the evaporation of steam. Since the preheating and superheating stages are sensible heat processes, they benefit when coupled with sensible heat storage systems (Fig. 16). A three-part storage system demonstrated in Fig. 17 is proposed by Laing et al. [168], where a phase change material is deployed for two-phase evaporation in a DSG plant, while concrete storage is used for storing sensible heat, i.e., for preheating of water and superheating of steam. Laboratory test results of a PCM test module with 140 kg NaNO 3, applying the sandwich concept for enhancement of heat transfer, are presented, proving the expected capacity and power density. A two-stage TES system will be used in the 1 MW Dahan DSG power tower plant in China. The low temperature stage consists of a steam accumulator that will store saturated steam at 2.35 MPa, C. During discharging, the saturated steam will be converted to superheated steam via a two-tank indirect storage system which uses oil as the storage medium [29]. A thermal analysis of a two-stage thermal energy storage system was carried out in which concrete is used in the high temperature heat storage stage and steam accumulator is used in the low temperature stage [148]. The results show that the performance of the high temperature concrete storage unit depends greatly on the concrete thermal conductivity and the distance between the neighboring steel tubes Theoretical analysis of TES systems For designing TES systems for any application, heat transfer and hydraulic analysis is very important in determining the optimum configuration. The behavior of any TES system depends upon the Fig. 16. Simplified schematic of integration of combined sensible/latent heat storage system for direct steam generation [167]. subcomponents and their assembly/interaction with each other. It is well known that the thermal storage system should be able to store the heat at the highest temperature required in the system and be discharged at the lowest allowable temperature in the system. The sections below provide the methodologies commonly used for the heat transfer analysis of TES systems Active two-tank systems The principal elements for a two-tank thermal energy storage system are the material inventory, HTF, heat exchangers and the storage tanks, apart from the storage material circulation pumps. During charging, the amount of heat stored in the fluid depends on the heat supplied by the solar field. Selection of the storage material depends upon several factors including the plant s operating temperatures. The heat exchangers must be designed with very small approach temperatures to minimize the performance penalty of the power cycle during thermal storage discharging and to maintain reasonable heat transfer fluid supply temperatures to the collector field during thermal storage charging. The design should also consider the resulting differential pressure in the heat exchangers between the HTF and the storage fluid. Shell and tube heat exchangers are usually considered for these applications, in which the high-pressure HTF is placed on the tube side and the storage fluid flows on the shell side [2]. Analytical thermal resistance models are used to design the heat exchanger. The overall heat transfer coefficient of a shell and tube heat exchanger is given as [169]: 1 UA ¼ 1 þ 1 þ R00 fi þ R00 fo þ ln ðd o=d i Þ : (6) h i A i h o A o A i A o 2pkL where h is the heat transfer coefficient, A is the surface area, R 00 fi is the fouling factor, D is the tube diameter and subscripts i and o represents the inner and outer tubes, respectively. This equation can be used for design conditions. However, due to the variability of the solar energy source, off-design conditions often occur and in this case, the overall heat transfer coefficient is approximated by [170]: UA UA ref ¼ _m _m ref! 0:8 : (7) Fig. 15. Tes diagram for steam production in a power plant corresponding to Fig. 16 ( line represents the storage process) [167]. The storage tank should be able to insulate the hot or cold tank from ambient, store the required amount of liquid and it must withstand the thermal expansion and contraction associated with

19 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Fig. 17. Overview of a three-part thermal energy storage system for DSG combining sensible and latent heat storage [168]. thousands of heat cycles. The size and shape of the tank will be optimized based on the thermal and flow analyses. The tanks used for high temperature CSP parabolic power plants are generally made of different grades of steel. Once the tank material is selected, the structural analysis of the tank can reveal whether the tank can withstand the appropriate number of cyclic charging and discharging. The analysis of two-tank systems can be found in Refs. [2,170,171] Steam accumulators Steam accumulators are dynamic 2-phase systems that benefit from years of practical experience as a result of their use in fossil fuel fired power plants [149]. The Ruth s storage design is commonly seen in CSP applications. This variable-pressure system (Fig. 18) is characterized by a vessel that is filled with saturated hot water and a small cushion of steam during the charging mode. During discharging, the saturated steam is directly removed from the top of the tank, thus reducing the pressure in the vessel, which Fig. 18. A variable-pressure (Ruth s) steam accumulator [79]. The charging nozzle was invented by Ruth to turn the flow of incoming steam upwards. causes water in the tank to flash. The gradual decrease in steam pressure and temperature demand design and operational strategies that meet the needs of the turbine cycle. Bai and Xu [148] discuss the design of a steam accumulator that will use a secondary sensible heat storage system to superheat the steam that exits the accumulator, for use in China s first 1 MW demonstration power tower CSP plant. The discharging process is discussed in detail and it is assumed that in the accumulator, the steam and water are always in equilibrium and therefore have the same saturated temperature and pressure, and the pressure variation depends on the output demands. The governing equations of mass and energy conservation respectively are: r s1 ðv V 1 Þþ r w1 V 1 ¼ r s2 ðv V 2 Þþ r w2 V 2 þ G Ds : (8) r s1 H s1 ðv V 1 Þþ r w1 H w1 V 1 ¼ r s2 ðv V 2 ÞH s2 þ r w2 V 2 H w2 ZDs þ HðsÞ GðsÞds: _ where s represents steam, w represents water, V is the volume, H is the enthalpy, _ G is the mass flow rate, G is the mass, s is time, 1 represents the initial state, and 2 represents the final state of the steam accumulator. Two distinct modeling methods are presented: a) the accumulator is operated by controlling the steam pressure, and b) the accumulator is operated by controlling the steam mass flow rate. In the former instance, the variation of pressure with time is used as an input to the model and the unknowns are V 2 and G Ds. For the steam mass flow rate model, the mass flow rate is known and assumed constant, and V 2 and p 2 must be solved for. O brien and Pye [172] modeled a Ruth s steam accumulator under the assumptions that a) complete heat of evaporation is provided by the liquid phase, b) the specific heat capacity of liquid water and the specific heat of vaporization are approximated by the average value for saturated liquid water at an average pressure p m ¼ (p start þ p end )/2, and c) the change in liquid mass m liquid during depressurization is neglected. When the fluid is in the 2-phase liquid/vapor region, rearrangement of the conservation of energy equation results in 0 (9)

20 304 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 T sat ðp end Þ¼ T sat ðp start Þþ dmðh uþ Q loss m vessel Cp vessel : (10) Once the saturation temperature is determined, the pressure change and all remaining fluid properties can be found since the total volume is constant [172]. In order to run the model, steam was injected into the vessel at constant temperature during charging, and the system was discharged until a minimum pressure was reached Systems with extended/embedded heat transfer structures Fins or extended surfaces are used to provide additional heat transfer surface in thermal systems. These surfaces can be in the form of cylindrical tubes, rods, plates etc. which can enhance the heat transfer area between the HTF and the storage material. Fig. 19 shows an example of a cylindrical TES system with HTF flowing inside the tubes arranged in a modular fashion. The storage material surrounds the tubes resulting in a significant augmentation in the interfacial surface area for heat transfer between the HTF and itself. In the case of PCMs, the fins also help protect them from the HTF by encapsulating them. Since the PCMs change their phase, it is important that PCMs are encapsulated. PCMs are typically placed in long thin heat pipes [154], cylindrical containers [173,174] or rectangular containers [175e177]. The role of different configurations of fins in enhancing the performance has been studied extensively by various researchers. The system is modeled as a heat exchanger with the fluid on one side and the storage medium on the other side. The problem is considered as one-, two- or three-dimensional based on the complexity of the system. For example, the temperature profile inside a tube at any height can be obtained by solving the energy equation in cylindrical coordinates in a twodimensional plane as given by: r vh vt ¼ 1 v kr vt r vr vr þ 1 v r vq k r vt vq (11) where H equals C p *T in case of sensible heat storage material; for a PCM, H is the enthalpy of the material in either solid (H s ) or liquid (H f ) phase and is equal to H f eh s during phase change. Parameters that are typically studied include the size of the tubes, distance between the tubes, thickness of the extended surfaces, height of the TES container, mass flow rate of HTF and pressure drop in the system. The most intensely analyzed LHTES unit is the shell and tube system, accounting for more than 70% [178]. This is probably due to the fact that most engineering systems employ cylindrical pipes and heat loss from the shell and tube system is minimal. A number Fig. 19. TES system with embedded tubes for increased heat transfer area. of studies have been conducted to analyze the overall thermal behavior of latent heat thermal energy storage systems [179e184]. A review of numerical studies on heat transfer in LHTES systems and their materials can be found in Refs. [185e188]. The analysis of heat transfer problems in phase change processes is complex due to the fact that the solideliquid boundary moves depending on the speed at which the latent heat is absorbed or lost at the boundary. The position of the boundary is unknown and forms part of the solution [189]. Designing of PCM systems requires a good understanding of the fundamental heat transfer processes involved in accurately predicting the thermal performance of the PCM system. Various correlations have been proposed relating thermal performance with dimensionless numbers, based on both experimental and numerical investigations conducted. Table 11 gives the common dimensionless numbers used to model the performance of LHTES systems and their significance [178]. Table 12 shows some of the correlations derived using dimensionless numbers. A high temperature latent thermal energy storage system containing multiple heat pipes between a heat transfer fluid and the PCM was analyzed using a thermal network model by Shabgard et al. [156]. The heat pipes can transfer heat between the HTF and the PCM with evaporation and condensation of the heat pipe working fluid occurring at the ends of the heat pipes (Fig. 20). Two storage configurations were considered (Fig. 21); one with the PCM surrounding a tube that conveys the heat transfer fluid, and the second with the PCM contained within a tube over which the heat transfer fluid flows. Both heat pipe effectiveness and generalized heat pipe effectiveness are introduced to quantify the augmentation associated with incorporating HPs during the melting and solidification processes. It is demonstrated that adding heat pipes enhances thermal performance, which is quantified in terms of dimensionless heat pipe effectiveness. Nithyanandam and Pitchumani [157] further analyzed the system by using a thermal resistance network model to describe the system during the charging and discharging processes (Fig. 22). They combined the physical model with a numerical optimization scheme to determine the optimum design of the system for maximizing the energy transferred, effectiveness, and the energy transfer rate during the charging and discharging processes individually as well as based on combined charging and discharging considerations. In general, it was found that an increase in the HTF mass flow rate, the module length and the tube radius reduced the effectiveness of the heat pipes, while an increase in the length of the condenser section, the length of the evaporator section and the vapor core radius enhanced its effectiveness. If there are any changes in the properties of the storage material due to the presence of embedded structures, the average or effective properties must be used. For example, since most inorganic PCMs have low thermal conductivity, apart from encapsulating them, high conductivity material particles, fibers, etc. are mixed to improve their overall thermal conductivity Packed bed systems In a packed bed system, the bed consists of storage materials such as rocks, ores or encapsulated PCMs, a container and the flow of heat transfer fluid through the voids in the bed. Most of the experiments performed in a packed bed are focused on finding the heat transfer correlations for different configurations and shapes of the particles that can be used for the thermal analysis of the system. In case of packed bed flow, the fluid flow phenomenon and the inter- as well as intraparticle heat conduction effects make the heat transfer mechanism complex. The expected heat transfer modes in a packed bed can be [193]: (1) convective heat transfer from the walls of the packed bed tank to the fluid; (2) convective heat

21 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Table 11 Definitions of some of the common dimensionless parameters employed in the analysis of phase change problems [178]. Dimensionless parameters Formula Nomenclature Significance Biot number, Bi hl/k h e heat transfer coefficient l e characteristic length k e thermal conductivity Ratio of conductive resistance to convective resistance. Determines the uniformity of temperature in the solid. Nusselt number, Nu hd/k d e characteristic length Ratio of the conductive thermal resistance to convective thermal resistance. Determines the ratio of actual heat transferred by a moving fluid to the heat transfer that would occur by conduction. Stefan number, Ste C p,l DT/l C p,l e specific heat capacity Ratio of thermal capacity of the melted solid to the latent heat. DT e temperature difference l e latent heat of melting Dimensionless time or Fourier number, Fo kt/(rc p l 2 ) t e time r e density Characterizes heat flux into a body or system. Rayleigh number, Ra gbdtl 3 /ay g e gravity b e coefficient of volume expansion a e thermal diffusivity y e kinematic viscosity Determines the onset of natural convection. Below a critical value, heat transfer is primarily conduction. The critical value is determined by the geometry and boundary conditions. Prandtl number, Pr y/a Approximates the ratio of momentum diffusivity to thermal diffusivity. Low Pr means effective heat conduction with dominant thermal diffusivity. High Pr means effective heat convection with dominant momentum diffusivity. Reynolds number, Re vd/y v e velocity Ratio of inertial forces to viscous forces. Determines whether flow is laminar or turbulent. Grashof number, Gr gbdtl 3 /y 2 Approximates the ratio of buoyancy force to the viscous force. transfer from the particles to the fluid flowing through the bed; (3) conduction heat transfer from the walls of the bed to the particles constituting the bed; (4) conduction heat transfer between the individual particles in the bed; (5) radiant heat transfer; (6) heat transfer by mixing of the fluid and (7) heat transfer from the container to the surroundings. Heat transfer coefficient and pressure drop are the two parameters that are commonly used to express the performance of packed beds. The total heat transfer coefficient generally incorporates convection between the fluid and the bed elements, conduction between the bed elements, bed to wall conduction, and the fluid to wall convection. Most of the experimental correlations express the heat transfer coefficient only as a function of the Reynolds number. Therefore, the applicability of these correlations is also limited to particular bed elements used in developing them. The following correlations are provided from literature as examples for estimating the performance parameters. The heat transfer coefficient can be estimated using the correlation given in Ref. [194] for a bed of randomly packed spheres as: h ¼ 2:0 c p G o Re o Pr þ 2:031 Reo 1=2 0 < Re o < 10000: (12) Pr 1=2 where, G o ¼ _m=a o, and Re o ¼ D s G o /m. The pressure drop can be estimated using the equation provided in Ref. [195] as: L rv 2 Dp ¼ f D s g c 0 1 x A: (13) x 3 where, x is the average void fraction of the packed bed and f is the friction factor given by f ¼ Að1 xþ=re o þ B; A and B are constants provided depending to smooth or rough particles [195]. Table 12 A list of some correlations provided by various researchers under different study conditions. Correlation Interpretation of correlation Ref. V/V o ¼ b(t t c ) Linear function of the melted volume of PCM with time. [190] V/V o ¼ 4.73Fo Ste Ra Melted volume fraction for the entire duration of test runs in terms of Fo, Ste and Ra. [190] Nu ¼ 0:0219Ra 0:387 Pr 0:019 ðh=dþ 0:062 Time averaged Nusselt number for the entire melting process. [190] t f ¼ L 2 2að1 þ wþste f1 þð0:25 þ Total heat input per unit surface area Q and average heat flux at r ¼ l over the [191] 0:17w0:7 ÞSteg duration of the melting process t f. q ¼ 2KDT f1 þð0:121 þ 0:0424wÞSte ð0:7645 0:2022wÞ g L k eqðsþ ¼0:228FRaðsÞ 1=4 1 dðsþ 1=4 d Equivalent thermal conductivity and dimensionless melting rate. [183] Z s AðsÞ½1 ln AðsÞŠ ¼ 1 4 k eqðsþds 0 Z 1 With M ¼ HeeC, instantaneous melting rate, instantaneous heating rate and [192] MðtÞ ¼ 1 St m 0 ðy; tþdy instantaneous cooling. 0 Z 1 HeðtÞ ¼ 2 dy d 0 Z 1 C t ¼ 2B dy l 0

22 306 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 Fig. 20. Heat transfer between an HTF and a PCM utilizing HPs [156]. The effective thermal conductivity of the packed bed (also used to express heat transfer rates in packed beds) is an averaged parameter dependent both on the thermal properties of the bed and on the flow rate of the heat transfer fluid [196]. Several models are available to simulate heat transfer inside packed bed flow and heat transfer characteristics [197e219]. A model for a packed bed latent heat thermal energy storage system using spherical capsules is developed by Regin et al. [220] to predict the thermal behavior of the system. The study investigates the effects of heat transfer fluid inlet temperature, mass flow rate, phase change temperature range and the radius of the capsule on the dynamic response of a packed bed latent heat thermal energy storage system using spherical capsules for both charging and discharging modes. The application of the method of characteristics for numerically predicting the charging or discharging process in a packed bed thermocline storage tank is presented in Ref. [147]. A numerical model was developed and validated with experiments using ZrO 2 pellets at high temperatures around 900 C by Jalalzadeh-Azar et al. [221]. Both convection and radiation are included in the model of the total heat transfer between the gas and the pellets. They found that thermal radiation and intraparticle conduction do not play a major role in the overall heat transfer in the packed bed sensible heat storage. The thermomechanical behavior of a high temperature packed bed was analyzed in Ref. [23]. For a review of analytical studies as well as experimental investigations of packed bed thermal storage systems, please see Singh et al. [222]. When a thermocline contains a filler material, the filler not only acts as a storage medium, but also helps maintain the thermal gradient. Different studies exist in determining the heat transfer performance of liquid thermocline systems. Schumann [223] in 1927 presented a set of energy conservation equations governing the fluid flow through a porous medium. These equations have been widely adopted in the analysis of thermocline heat storage with a solid filler material. rcp f ε vt f vs ¼ mcp A f vt f vy þ h v T b T f : (14) rcp b ð1 εþ vt b vs ¼ h v T f T b : (15) where, f subscripted variables denote fluid properties, b denote bed properties and ε is the porosity of the bed. The above set can be solved using discretization techniques or finite element and finite volume methods. While designing a thermocline with loose filler materials, it should be noted that the material may exert added hoop stresses in Fig. 21. Modules used for analysis in Ref. [156]; PCM is KNO 3, HTF is Therminol.

23 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Fig. 22. Schematic illustration of (a) the flow configurations in Module 1 and Module 2 of an LTES system, (b) a heat pipe cross-section identifying the internal geometry, (c) the thermal resistance network of LTES, and (d) the thermal elements for radial conduction [157]. the tank that are likely to be unmanageable in the long run because of thermal cycling. Another concern about thermocline storage is the phenomenon of thermal ratcheting due to a difference in the thermal expansion of the tank wall and filler material. During repeated thermal cycling, the tank expands upon heating, allowing the packed bed particles to settle to lower levels in the tank. As the tank cools during the discharge process, the thermal stresses in the wall are converted to mechanical stresses as the tank material meets the resistance of the larger volume of filler that has settled and which can no longer move up due to gravity. Recently, Flueckiger et al. [224] studied the potential of failure of a composite tank wall by thermal ratcheting. The effect of thermal ratcheting can be reduced by increasing the thickness of the shell wall, putting insulation between the filler material and the shell wall or by simply allowing more heat losses through the wall Energy and exergy efficiencies of TES systems Once the TES heat transfer mechanism is chosen, system performance must be analyzed. The storage unit has to fulfill demands concerning the energy transferred during the charging and discharging process. Usually, a higher heat transfer rate is related to an increase of thermodynamic irreversibilities, thus reducing the capacity of the system. There are many different but valid definitions of the energy efficiency of TES systems. For example, TES energy efficiency can be defined as: h th ¼ Energy recovered and remaining in a TES : (17) Energy input and originally in the TES Both definitions are reasonable but can yield different values in some circumstances. Many investigators carried out calculations of efficiency or a similar parameter employed for different applications. These investigations have focused on either the charging and discharging processes separately or the complete working cycle of the system depending on the application. As a reference, Seeniraj et al. [225] investigated the thermal performance of a shell and tube LHTS unit employed for space-based power generation. The thermal performance was studied using a quantity which is the ratio between the total heat stored and the maximum latent heat that can be stored. Hamid [226] calculated the storage efficiency of LHTS used for a solar water heater. The efficiency is expressed as a ratio between the latent heat stored in the PCM and the total solar radiation. Higher mass flow rates of the heat transfer fluid (HTF) and greater number of tubes used in the heat exchanger resulted in maximum storage efficiency. Hanchen et al. [163] investigated the efficiency of a high temperature packed bed system. The charging, discharging and overall efficiencies of the systems are defined as follows: h th;charging ¼ Energy stored ðenergy input þ Pumping EnergyÞ : (18) h th ¼ Energy recovered : (16) Energy nput which describes the fraction of the input and pumping energy required to charge the storage tank. A low value of h th,charging

24 308 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 indicates ineffective heat transfer or energy lost by the hot fluid leaving the tank, i.e., a tank charged with energy which can no longer be stored. The discharging efficiency can be defined as: Energy extracted h th;discharging ¼ ðenergy stored þ Pumping EnergyÞ : (19) which describes the ratio of the energy extracted from the storage tank relative to the energy stored and the pumping energy required for extraction. A third parameter is the overall thermal efficiency Exergy input ½Exergy recovered þ Exergy lossš Exergy consumption ¼ Exergy accumulation ð21þ where the quantities in square brackets represent total exergy output across system boundaries. Exergy consumption is due to irreversibilities, hence consumption is proportional to entropy generation [234]. Each exergy term is the sum of kinetic, potential, physical and chemical components of the exergy of matter h th;overall ¼ Energy extracted Energy input þ Pumping Energy charging þ Pumping Energy discharging : (20) and describes the ratio of the recovered energy for a single charge/ discharge cycle to the input and pumping energy Exergetic efficiency of TES systems While most TES efficiency definitions are based on energy, an analysis, which is solely dependent on the first law of thermodynamics, is inadequate as a measure of energy storage because it neither considers the availability or quality of energy, the effect of time duration through which heat is supplied, nor does it account for the temperature of the surroundings. The second law of thermodynamics allows for a more detailed and meaningful method to determine the efficiency of TES systems [227] because it introduces the concept of irreversibilities, which reduce the amount of useful energy that is available to perform work. Exergy is a term that is used to describe this useful energy and by definition it represents the maximum theoretical quantity of work that can be extracted from a system as it comes into equilibrium with its environment [228]. Once a system reaches equilibrium, it no longer has the potential to perform work and if irreversibilities are present while the system moves towards equilibrium, exergy is destroyed during the process. Therefore, unlike energy which is always conserved, exergy can be destroyed or consumed. As an example of how an energy analysis and exergy analysis differ, Bejan [229] demonstrates with a sensible heat storage system that uses gas as the heat transfer medium, that though an increase in charging time results in an increase in a system s ability to store energy, the same trend does not apply to the system s ability to store exergy. Rather, there is an optimal charging time in which irreversibilities of a system are minimized and beyond this, the exergy content of the gas stream is destroyed. Thus the use of exergy analysis is very important in developing a good understanding of the thermodynamic behavior of thermal energy storage systems because it clearly takes into account the loss of useful energy that is available to do work in storage applications. It more correctly reflects the thermodynamic and economic value of the storage operation, and is recognized as a powerful tool for efficiency evaluations of TES and other systems [83,230e232]. Other examples of irreversibility sources for storage systems include heat transfer across a finite temperature difference between the hot stream and colder storage material during charging, heat transfer across a finite temperature difference between exhausted HTF and the surrounding environment, mixing of different temperature storage fluids, and the frictional pressure drop that occurs due to the flow of HTF [229,233]. To perform an exergy analysis on a system, the general exergy balance for a process can be written as [227]: and must be appropriately applied for the given system under investigation [235]. Extending Eq. (21) to a TES system, Erek (2009) provides a simplified equation for the exergy balance of a process as Ex HTF; in þ Ex Qgain Ex HTF;out Ex consumed ¼ DEx system;t : (22) The quantity Ex Qgain denotes exergy associated with heat transfer, DEx system,t denotes exergy accumulation in the system, and Ex HTF,in and Ex HTF,out are due to flow exergy. Flow exergy, exergy transfer with heat loss, and exergy accumulation can be calculated respectively as follows [234]: Ex HTF ¼ Ex HTF; out Ex HTF; in ¼ _m Z t t ¼ 0 ðε out ε in Þdt: (23) where ε denotes specific flow exergy, [(h h o ) T o (s s o )] Ex Qgain ¼ Z t t ¼ 0 1 T o qdt (24) T where Ex Qgain is positive and represents stored exergy when the rate of energy transferred by the HTF to the storage material is assigned to q, and Ex Qgain is negative and represents exergy lost from the system to the environment when the rate of energy lost to the environment, q loss, is assigned to q [236],and DEx system;t ¼ Ex sys;t Ex sys;i ZZZ ¼ r½ðuðtþ u i Þ T o ðsðtþ s i ÞŠdV: (25) It is to be noted that in an exergy analysis, total exergy losses due to irreversibilities in the system can be calculated but the individual causes for exergy consumption cannot be identified [236]. In order to obtain information on when and where irreversibilities occur, an entropy analysis is more appropriate. A detailed description of the various approaches to the second law analysis can be found in Ref. [235]. For an evaluation of the overall storage process, including charging, storing and discharging, the total exergy balance can be expressed as: ε c ½ε d þ ε l Š I ¼ DX: (26)

25 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e where, ε c denotes exergy transfer due to heat input during charging, ε d represents exergy transfer associated with heat recovered during discharging, ε l represents exergy transfer associated with heat lost during the entire process, I denotes exergy consumption due to irreversibilities, and DX represents the accumulation of exergy in the TES [227]. Similar to energy efficiency, exergy efficiency is a ratio of products to inputs, and since there are various ways to express these values, numerous expressions for exergetic efficiency exist [227]. Jegadheeswaran et al. [236] illustrates several possible TES efficiency definitions for latent heat storage systems, which are provided in Table 13, and are expressed in terms of the charging period and discharging period separately or for the complete cycle. It is demonstrated that some authors take pumping power into account when defining input, while others may exclude it. There are also variations in the way that one defines how the exergy rate is supplied by the HTF, e.g. some authors only take into account the exergy that is supplied by using the difference between the inlet and outlet temperatures of the HTF while it is also feasible to include the maximum possible exergy input by using the difference between the inlet temperature of the HTF and the environment temperature. Table 14 includes examples of expressions used for stored and supplied exergy for sensible heat, latent heat, and thermochemical storage materials. In order to generate an appropriate formula for exergy efficiency, sound judgment should be utilized and a clear understanding of the system under evaluation should exist. Once the exergy terms are evaluated, the system can be optimized in such a way that the exergy efficiency of the system is at its maximum value. One way to improve the exergetic efficiency of the storage process is to reduce irreversibilities associated with the temperature gap that exists between the inlet heat transfer stream and the storage medium, as well as the temperature gap that exists between the outlet stream and the ambient [229]. This can be accomplished by implementing storage units that are cascaded in series. As can be seen in Fig. 23, successive placement of storage units can result in monotonically decreasing the temperature of the storage units in the direction of flow. This concept reduces the temperature gaps of the stream/storage material and the exhaust/ ambient. Cascaded latent heat storage (CLHS) systems are one possible TES alternative, which is marked by a minimum of necessary storage material. The use of a cascade of multiple phase change materials (PCM) shall ensure the optimal utilization of the storage material. A theoretical analysis based on a simplified optimization model of a CLHS system is presented by Aceves et al. [240]. They have suggested the overall exergetic efficiency of a system containing CLHS is given by: 4 ov ¼ t d Td;out 1 ln T d;out : (27) Tc;in 1 ln T c;in where T d,out (x) is the exit temperature of the discharging process at x ¼ 0, t d is the time length of the discharging process which can be found by optimizing the maximum exergetic efficiency, and T c,in (x) is the inlet temperature during the charging process. For single charging and discharging processes it is shown that a CLHS system yields exergetic advantages if operated in counter flow with respect to charging/discharging and if sufficient good heat transfer can be realized. Energy and exergy analysis of a thermal energy storage system employing multiple PCMs was developed by Domanski and Fellah [241]. Relative merits of thermal exergy storage systems using two, three and five PCMs are investigated. It is shown that the exergy efficiency can be significantly improved using multiple PCMs compared with a single PCM in a system. The CLHS packed bed type system for low temperatures was examined both experimentally and numerically [242e244]. Their work was limited to a heat storage module with vertical cylinders filled with PCM and also limited to the use of air as the heat transfer fluid. A numerical, one-dimensional model of a packed bed type CLHS is validated against experimental results in Watanabe et al. [245]. The study proves enhanced charge and discharge rates when operated as a CLHS with three PCM segments compared to an LHS with only one PCM. The reason is more uniform temperature distribution between the heat transfer fluid and the PCM. But this study also indicates sensitivities in the variation of the heat transfer fluid inlet temperature and the flow rate to the positive effect of a CLHS system. The same sensitivities are discussed in Gong and Mujumdar [246] by using a one-dimensional numerical model of a CLHS. If the inlet temperature or the flow rate of the heat transfer fluid or the heat input is disadvantageously adjusted, a CLHS system s performance can turn out to be worse compared to a noncascaded LHS system. If the operating parameters are properly set, a CLHS system yields a reduced variation of the heat transfer outlet temperature, which helps the connected thermodynamic process. Experimental and numerical results from the investigation of cascaded latent heat storage at high temperatures, with alkali salts like NaNO 3, KNO 3 and a mixture of KNO 3 /KCl, are presented by Michels and Pitz-Paal [129]. The melting temperature of these salts Table 13 Definitions of exergetic efficiency [236]. Efficiency Expression Description Charging (j char ) 1. Ex stored Ex HTF Ex 2. _ stored _Ex HTF _Ex 3. stored _Ex HTF þ Pump / ) work _Ex 4. stored _Ex HTF;init Presents the exergy efficiency defined as the amount of exergy stored out of total exergy supplied during charging Presents time-wise variation of exergy efficiency Takes into account the pumping power Presents the maximum possible exergy stored Ex Discharging (j dis ) 1. HTF Ex PCM; init _Ex 2. HTF _Ex PCM Presents the exergy efficiency defined as the amount of exergy recovered out of total exergy stored in the storage medium Presents the maximum possible exergy recovered Overall (j overall ) 1. Ex recovered Ex supplied Presents the total amount of exergy recovered out of total exergy supplied 2. j char j dis e Charging/discharging/overall 1. 1 N s Presents the quantity of exergy destroyed

26 310 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 Table 14 Examples of exergy expressions used in exergy efficiency calculations. Process Expression Description Ref. Exergy stored To a Rate of exergy stored by phase change material (PCM). [236] _m HTF C HTF ðt HTF;out T HTF;in Þ 1 T PCM To To Rate of exergy stored by PCM. Considers heat gain from surroundings. [236] _m HTF c HTF ðt HTF;out T HTF;in Þ 1 þ Q T gain 1 PCM T PCM ½DG f þ P ðnε chne ÞŠprod; tot ½DG f þ P ðnε chne ÞŠreact; tot b Exergy stored in reaction of thermochemical material. c [237] Z L Tðt; ZÞ Tðt; zþ Exergy stored in thermocline sensible heat material. [238] rvct 0 1 ln dz d 0 T 0 T 0 Z Exergy supplied during t THTF;in Exergy supplied by HTF for packed bed PCM storage system. [239] _mc charging p T HTF;in T o T oln dt 0 T o p _m 0 h ln 1 þ Dph p t s þ _m 0 c ln 1 þ Dpc e Minimum fan work for packed bed PCM system [239] t ðrf Þ 0 p 0 ðrf Þ 0 p R 0 (charging and discharging). a T o denotes reference temperature. T PCM denotes melting temperature of PCM. b The first term represents the total exergy of products and the second term represents total exergy of reactants. DG f denotes the Gibbs energy of formation on a molar basis, n denotes the amount of element per amount of element whose standard chemical exergy is being evaluated (in kmol/kmol), and ε chne denotes the standard chemical exergy of element on a molar basis. c Physical exergy components were neglected. d For a single medium stratified storage system, V represents tank volume, C represents the heat capacity of storage material, z represents vertical coordinate, and L represents the height of the tank. e h refers to hot fluid stream, c refers to cold fluid stream, S refers to storage period, R refers to removal period. lies between 306 C and 335 C(Fig. 24). The experiments were conducted with vertical shell and tube type heat exchanger devices under realistic operation parameters. The test facility as shown in Fig. 25 uses a synthetic heat transfer oil (similar to the SEGS plants), an electrical heater for the charging period and a cooling tower for the discharging period. The oil temperatures were varied between 290 C and 350 C. The experimental results were used in the Dymola/Modelica numerical model to simulate different CLHS configurations. The outcome of this work shows that the design of CLHS for this temperature range is complex and the low thermal conductivity of available PCMs is an obstacle which must be overcome to make full use of this promising storage technology. Proposals for CLHS applicable to thermal oil parabolic trough systems can be found in Refs. [247e249]. All of these proposals combine salts or mixtures of salts with an increasing melting temperature, T m, to be used as a TES for different kinds of power plants. From the analyses for cascaded latent heat systems, the following conclusions can be drawn: 1. Using a number of PCMs will aid in increasing the exergetic efficiency of the system. 2. The system temperatures will be more uniform. 3. The melting temperature difference between the multiple PCMs plays an important role in the performance of the CLHS unit. Therefore, choosing an appropriate number of PCMs is very important for the performance improvement of the CLHS unit. T hot,in T 0 Storage Material Stream Fig. 23. Schematic representation of a series of sensible heat storage units during charging. Adapted from Ref. [229]. An energetic and exergetic analysis of modular storage operation concepts, including a cost assessment of the modular TES system concepts was performed by Laing et al. [78]. The results of the analyses show a significant potential for economic optimization Optimization procedure for TES tanks The heat transfer analyses mentioned above will aid in determining the thermal variation and HTF exit temperatures from a given size TES system. However, the size of the tanks is necessary in order for the TES system to be integrated into the power plant. This will depend upon the storage capacity required for the TES system, the operation time, operating temperatures and the thermal performance itself. A simple methodology that can be used is shown in Fig. 26. Apart from the thermal analysis that can provide the temperature gradients and heat losses, structural analysis is also crucial in designing the system, as this will determine the optimum safe dimensions of the tank. A few considerations that must be included in the analysis during the designing of TES tanks are: 1. Resultant stress on the tanks due to the thermal gradients in the system 2. Resultant pressure inside the tank due to the presence of storage media 3. Allocation of space for thermal expansion of the heat transfer fluid or storage media 4. Thermal expansion of the solid media in case of a thermocline with filler material or packed bed systems 5. Thermal stress within the encapsulated shell, if encapsulated PCMs are used as storage medium 6. Size and shape of the tank 7. Maximum allowable height of the tanks based on the structural analysis and manufacturability. A two-tank molten salt heat storage system in the shape of a below-grade cone that uses an innovative high-performance concrete for structures and foundation is analyzed by Salomoni et al. [250]. Concrete durability in terms of prolonged thermal loads and the interaction between the hot tank and the surrounding environment (ground) is considered and assessed using a finite element (FE) model. The developed FE model simulates the whole domain, and a fixed heat source of 100 C is assigned to the internal concrete surface. The development of the thermal and hygral fronts within

27 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Fig. 24. Proposal of a cascaded latent heat storage with 5 PCM according to Ref. [247]. the tank thickness is analyzed and results discussed for long-term scenarios Thermoeconomic analysis From the previous discussion it is evident that the TES systems must have high exergetic efficiency and low cost. The cost of a TES system mainly depends upon the following items: The storage material itself The heat exchanger for charging and discharging the system The cost for the space and/or enclosure for the TES. As part of a study [251] to evaluate the application of solar energy to the commercial area, Atomics International concluded that the cost (in 1976 dollars) of sensible heat storage with oil as heat transfer fluid could be approximated by: Storage cost ¼ Tank costð$þþoil costð$þ ¼ 352 vol; ft 3 0:515 þ oil cost;$=ft 3 vol; ft 3 (28) This relationship is felt to be valid for storage systems in the range of 4.2 m 3 (150 ft 3 )e42,000 m 3 (150,000 ft 3 ). The capital cost of the tank must be corrected for inflation in order to use the equation with current oil costs. A detailed study of thermal storage costs and detailed design study was performed by Nexant for a two-tank, molten salt storage system [252]. Several researchers have used different optimization techniques for identifying the viable TES configuration. Rovira et al. [253] simulated two system configurations: (a) a double thermal energy storage (DTS) with different functionalities for each storage, and (b) the subdivision of the solar collector field (SSF) into specialized sectors for solar thermal power (Fig. 27). The results show that, if compared to the reference plant, both configurations may raise the annual electricity generation (up to 1.7% for the DTS case and 3.9% for the SSF case). Pistocchini and Motta [254] performed the economic potential assessment of an innovative hybrid-cooling system for steam condensation in concentrated solar power plants. The system consists of an air-cooled condenser (ACC) working in parallel to a latent heat storage system with phase change material (PCM). The purpose of the hybrid system is to store some of the latent heat of steam condensation during the turbine operation and reject it at night, in order to shift a share of the cooling work and exploit the high diurnal temperature range of desert areas. The system s energy and economic performance are assessed by the parametric analysis of a theoretical case study, referred to an existing solar power plant and based on historical meteorological data. An evaluation was carried out to investigate the feasibility of utilizing a molten salt as the heat transfer fluid (HTF) and for thermal storage in a parabolic trough solar field to improve the system performance and to reduce the levelized electricity cost Fig. 25. Simplified flow diagram and test module used in Ref. [129].

28 312 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 ratcheting could potentially lead to the failure of the thermocline tank. If this issue is solved, the thermocline design holds a great promise for reducing the capital cost of the thermal energy storage systems and reducing the LCOE of parabolic trough power plants. An economic evaluation of a latent heat thermal energy storage (LHTES) system for large-scale concentrating solar power (CSP) applications was performed in Ref. [257]. The concept of embedding gravity-assisted wickless heat pipes (thermosyphons) within a commercial-scale LHTES system is explored through use of a thermal network model. The size and cost of the LHTES system is estimated and compared with a two-tank sensible heat thermal energy storage (SHTES) system. The results suggest that LHTES with embedded thermosyphons is economically competitive with current SHTES technology, with the potential to reduce capital costs by at least 15%. A CSP system with integral storage has been presented by Slocum et al. [258] where hillside mounted heliostats direct sunlight into a volumetric absorption molten salt receiver either near the base of the hill where the sunlight can directly penetrate the salt, or at the top of the hill where the light is redirected off the lid of the receiver before it enters the salt. Using the NREL Solar Advisor program, the system is estimated to realize cost-competitive levelized production costs of electricity Life cycle assessment Fig. 26. Schematic of the tank sizing procedure (generalized using the procedure used in Van Lew et al. [147]). [255,256]. A study was performed by Sandia National Laboratories to compare the annual performance of 50 MW e Andasol-like trough plants that employ either a 2-tank or a thermocline-type molten salt thermal storage system [36]. It was found that thermal A life cycle assessment (LCA), also known as life cycle analysis, is a technique to assess environmental impacts associated with all the stages of a product s life (i.e., from raw material extraction through materials processing, manufacture, distribution, use, repair and maintenance, and disposal or recycling). LCAs can help avoid a narrow outlook on environmental concerns [259]: Compiling an inventory of relevant energy and material inputs and environmental releases; Evaluating the potential impacts associated with identified inputs and releases; Interpreting the results to help you make a more informed decision. Fig. 27. Solar thermal power plant with DTS and subdivided solar field (SSF) [253].

29 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Giuliano et al. [262] analyzed selected solar-hybrid power plants for operation in base load as well as mid load regarding supply security (dispatchable power due to hybridization with fossil fuel) and low CO 2 emissions (due to integration of thermal energy storage). The power plants were modeled with different sizes of solar fields and different storage capacities and analyzed on an annual basis. The results were compared to each other and to a conventional fossil-fired combined cycle in terms of technical, economical, and ecological figures. The results of this study showed that in comparison to a conventional fossil-fired combined cycle, the potential to reduce the CO 2 emissions is high for solarethermal power plants operated in base load, especially with large solar fields and high storage capacities. The life cycle environmental impacts of a two-tank, indirect molten salt TES system and an indirect thermocline are analyzed by Heath et al. [263]. It was found that the reduction of salt inventory associated with a thermocline design reduces both storage cost and life cycle greenhouse gas emissions. Lechon et al. [264] performed the LCA assessment of a two-tank, indirect molten salt TES system on a parabolic trough CSP plant and found that the TES component can account for approximately 40% of the plant s non-operational GHG emissions Case study The case study of a 600 MWh two-tank thermal storage system for a parabolic trough power plant presented by Gabrielli and Zamparelli [171] is discussed here. The parabolic trough solar thermal power plant studied is made up of the following main components: (i) a parabolic trough solar field, where the cold molten salt mixture of potassium and sodium nitrates, is heated to 550 C (ii) a high temperature atmospheric storage tank, where the hot molten salt is collected downstream from the solar field (iii) a molten salt-water/steam heat exchanger, where the thermal fluid is cooled to 290 C in the process of superheating the steam and (iv) a low temperature storage tank, where the cold thermal fluid is collected downstream from the heat exchanger. Fig. 28. Optimization of tank design procedure [171]. This methodology is used by researchers to analyze the environmental impact of thermal energy storage systems during their lifetime. LCA for three different TES systems for solar power plants is developed to analyze and compare the energy savings related to the stored energy of the different systems and their environmental impact produced during the manufacturing and operation phase of each storage system by Oro et al. [260]. Some hypothetical scenarios are studied using LCA methodology to point out the differences between each TES system. Burkhardt et al. [261] evaluated the environmental impacts of a hypothetical 103 MW, parabolic trough, wet-cooled concentrating solar power (CSP) plant in the U.S. Southwest with 6.3 h of thermal energy storage by means of a hybrid life cycle assessment. Life cycle greenhouse gas emissions, cumulative energy demand, and water consumption associated with the manufacture, construction, operation, dismantling, and disposal of the power plant are evaluated and disaggregated by major systems and components. The power plant has a nominal thermal power of 60 MWth, functions continuously over a period of 10 h a day, requires 600 MWh th thermal capacity of the storage system, which corresponds to about 5500 tons of molten salt. It is assumed that the salt is completely drained from one tank and pumped to the other depending on charging or discharging. The tanks are assumed to be made of carbon steel for reducing the cost of the storage system. A design constraint for the maximum tank height, H,is fixed based on the maximum allowable shaft length of 12 m. The iterative optimization procedure used for designing the tank is shown in Fig. 28. The following iterative steps for optimizing the tank design are described: (i) defining the structural characteristics of the storage tank and its constituent elements, (ii) calculating the thermal profiles within the tank s interior as well as the unsteady analysis of molten salt cooling, (iii) evaluating the stresses in the vessel plates via the finite element method (FEM) and selecting the most suitable shape for the roof, and (iv) evaluating the total investment cost. After deciding upon the type of construction that is able to guarantee efficient, safe storage of the high temperature molten

30 314 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 salt mixture, the salt storage chamber is initially assigned a height and diameter. Then, the thickness of each protective layer, as well as the choice of thermal insulating materials, is defined. According to the study, the optimum configuration is the configuration for which both the tanks can store the required amount of energy at the intended temperatures, the thickness of the tanks withstand the maximum stress that might result due to the thermal gradient during charging and discharging and finally the defined configuration minimizes the cost. For evaluating the thermal profiles of the tank, resistance analysis was used. If the amount of heat losses from the insulation is acceptable, the next step is to perform the unsteady thermal analysis of cooling of the salts, when there is no solar input. This step would ensure that the molten salts do not solidify or do not cool below the required temperature of the plant, when the system is in a standby condition. Once the tank dimensions, insulation, and thermal requirements are acceptable, the next step in the optimization process is to perform the structural analysis of the tanks. This was done using finite element methods using the SAP2000 software. During this analysis, the shape of the roof and container are considered in the parametric study. The constraints in this step are the resultant stress (when the tanks are pressurized, the stress must be below the limit) and the vacuum load (to avoid buckling). The final step in the optimization process is the economic analysis. The total inventory costs include the costs for construction and installation of the two tanks, molten salt and the overall costs of the actual operations associated with the heat loss environment. Based on the analysis they found the optimum design of the tank as 11 m high (owing to the shaft constraints) and 22.4 m diameter carbon steel tank with internal insulation as shown in Table Key conclusions The system level heat transfer mechanism and the exergy efficiency of a TES system impacts the plant level efficiency, while the cost of the system impacts the overall plant cost and energy output. Several thermal models can be used based on the storage mechanism, and the tank design must be done such that the systems possess the highest exergetic efficiency. Cascaded systems are one option in latent heat thermal energy storage systems that can help in achieving the highest exergetic efficiency. Thermoeconomic analysis is very important in determining the optimum TES configuration and the plant costs. The cost analysis of the system must consider all the components of the TES system. Table 15 TES system parameters for a 600 MWh th system for a parabolic trough power plant from Ref. [171]. Internally insulated carbon steel storage tank Diameter, m H, (m Q total, kw Weight of steel, tons Thickness of the lateral insulation material, mm Roof insulating thickness, mm Foamglas Ò thickness, mm No. of brick_foundation 2 2 No. of brick_vessel 1 0 (radially mounted) No. of brick bottom 5 0 TIC, MV Cost of steel, MV Cost of flexible liner, kv Stainless steel storage tank 5. Development activities in thermal energy storage for CSP Though the above-mentioned technologies have been under research for many years, only synthetic oil, molten salt and steam accumulator systems are currently being used in CSP. Remaining technologies are not mature enough to be used in large-scale power plants. However, some of the technologies under development, for example encapsulated PCMs, may become commercial in the very near future [265]. The U.S. Department of Energy s (DOE) funding efforts reflect the current and near term development activities in CSP. The Department partners with a number of companies and academic institutions to provide financial and technical support for projects that focus on reducing technology and manufacturing costs of CSP components and systems. These development activities include projects related to the development of molten salt heat transfer fluids, molten salt pumps, thermal storage systems utilizing thermochemical heat storage, sensible heat storage and latent heat storage, as well as a number of research initiatives that try to integrate novel storage techniques to improve heat transfer in the storage medium [265]. 6. Conclusions An overview of thermal energy storage technologies applicable to concentrating solar power plants is presented based on a comprehensive review of the literature. The storage concepts and the state-of-the-art design methodologies used by various researchers are discussed. Though all three basic types of storage media (sensible, latent, thermochemical) have the potential to make solar power plants viable, more research is still needed to improve the thermal performance and economics of these systems. Current research efforts in the area of thermal storage focus on developing new technologies that can reduce the cost from the present LCOE of thermal storage of 5 /kwh to 1 /kwh by 2020 [266], with the current trend moving towards higher temperatures. Recent developments are utilizing nano-based technologies and advanced manufacturing methods for achieving this goal. Researchers are emphasizing exergetic efficiencies in the design of storage systems. A combination of different types of storage can be a solution to achieve the required efficiency of power plants at low cost. Acknowledgements This work was done as part of projects funded by USDOE (DE-EE ) and the Florida Energy Systems Consortium (FESC). References [1] Herrmann U, Kearney DW. Survey of thermal energy storage for parabolic trough power plants. Journal of Solar Energy Engineering 2002;124(2): 145e52. [2] Herrmann U, Kelly B, Price H. Two-tank molten salt storage for parabolic trough solar power plants. Energy 2004;29(5e6):883e93. [3] Pacheco JE, Gilbert R. Overview of recent results of the solar two test and evaluations program, renewable and advanced energy systems for the 21st century. In: Proc. of 1999 ASME int. solar energy conf [4] Cabeza LF, Sole C, Castell A, Oro E, Gil A. Review of solar thermal storage techniques and associated heat transfer technologies. Proceedings of the IEEE 2012;100(2):525e38. [5] Fernández-García A, Zarza E, Valenzuela L, Pérez M. Parabolic-trough solar collectors and their applications. Renewable and Sustainable Energy Reviews 2010;14(7):1695e721. [6] Barlev D, Vidu R, Stroeve P. Innovation in concentrated solar power. Solar Energy Materials and Solar Cells 2011;95(10):2703e25. [7] Gil A, Medrano M, Martorell I, Lázaro A, Dolado P, Zalba B, et al. State of the art on high temperature thermal energy storage for power generation.

31 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e Part 1 e concepts, materials and modellization. Renewable and Sustainable Energy Reviews 2010;14(1):31e55. [8] International Renewable Energy Agency (IRENA). Renewable energy technologies: cost analysis series e concentrating solar power [9] Sioshansi R, Denholm P. The value of concentrating solar power and thermal energy storage. NREL-TP-6A [10] Reilly HE, Kolb WJ. Evaluation of molten salt power tower technology based on the experience of solar two. SAND Sandia National Laboratories; [11] Llorente Garcia I, Ãlvarez JL, Blanco D. Performance model for parabolic trough solar thermal power plants with thermal storage: comparison to operating plant data. Solar Energy 2011;85(10):2443e60. [12] Ma Z, Glatzmaier G, Turchi C, Wagner M. Thermal energy storage performance metrics and use in thermal energy storage design. Colorado: ASES World Renewable Energy Forum Denver; [13] Price H, Kearney D. Reducing the cost of energy from parabolic trough solar power plants. In: International solar energy conference Hawaii Island, Hawaii, [14] Wittmann M, Eck M, Pitz-Paal R, Muller-Steinhagen H. Methodology for optimized operation strategies of solar thermal power plants with integrated heat storage. Solar Energy 2011;85:653e9. [15] Cavallaro F. Multi-criteria decision aid to assess concentrated solar thermal technologies. Renewable Energy 2009;34:1678e85. [16] Zavoico AB. Nexant: solar power tower design basis document. Sandia National Laboratories, SAND [17] Electric Power Research Institute (EPRI). Program on technology innovation: evaluation of concentrating solar thermal energy storage systems. Palo Alto, CA, [18] Plataforma Solar de Almeria (PSA). Chapter 1: PSA solar thermal technology 1997 milestones and international coorperation in solar power development. PSA; [19] Plataforma Solar de Almeria, Euro-Energy.Net, Available from: euro-energy.net/infrastructures/25.html. [20] Concentrating solar power projects. National Renewable Energy Laboratory. Available: cfm; [21] Mehos MA. Part 3: task 1: solar thermal electric systems. Solar PACES; [22] Stancich R. Weekly intelligence brief: September 13eSeptember 20. CSP Today. Available: September 20, [23] Dreiigacker V, Müller-Steinhagen H, Zunft S. Thermo-mechanical analysis of packed beds for large-scale storage of high temperature heat. Heat and Mass Transfer 2010;46(10):1199. [24] NREL. Concentrating solar power projects. National Renewable Energy Laboratory. Available: [25] Tamme R. Development of storage systems for SP plants. Brussels [26] Solucar. 10 MW solar thermal power plant for Southern Spain. Final report [27] Dunn RI, Hearps PJ, Wright MN. Molten-salt power towers: newly commercial concentrating solar storage. Proceedings of the IEEE 2012;100(2): 504e15. [28] Sener. Andasol 1 & 2: 50 MW, 7 h molten salt heat storage. National Renewable Energy Laboratory, NREL trough technology workshop, Available: storage.pdf. [29] Xu E, Yu Q, Wang Z, Yang C. Modeling and simulation of 1 MW DAHAN solar thermal power tower plant. Renewable Energy 2011;36:848e57. [30] ADB. People s Republic of China: concentrating solar thermal power development. American Development Bank; [31] Ho CK. Software and codes for analysis of concentrating solar power technologies. SAND Albuquerque, NM: Sandia National Laboratories; [32] Gilman P, Blair N, Mehos M, Christensen C, Janzou S. Solar Advisor Model user guide for version 2.0. NREL/TP [33] Klein SA, Beckman WA, Duffie JA. TRNYSYS e a transient simulation program. ASHRAE Transactions 1976;82(1):623e33. [34] Lippke F. Simulation of the part-load behavior of a 30 MWe SEGS plant. SAND Albuquerque, NM: Sandia National Laboratories; [35] Patnode A. Simulation and performance evaluation of parabolic trough solar power plants. Madison: University of Wisconsin; [36] Kolb GJ. Evaluation of annual performance of 2-tank and thermocline thermal storage systems for trough plants. Journal of Solar Energy Engineering 2011;133(3):031023e5. [37] ASAP Software. Breault Research Organization. Available: breault.com/software/asap.php; [38] Dellin TA, Fish MJ. User s manual for DELSOL: a computer code for calculating the optical performance, field layout and optimal system design for solar central receiver plants. SAND Livermore, CA: Sandia National Laboratories; [39] Falcone PK. A handbook for solar central receiver design. SAND Livermore, CA: Sandia National Laboratories; C-9. [40] Kistler BL. A user s manual for DELSOL3: a computer code for calculating the optical performance and optimal system design for solar thermal central receiver plants. Livermore, CA: Sandia National Laboratories; SAND [41] Leary PL, Hankins JD. User s guide for MIRVAL e a computer code for modeling the optical behavior of reflecting solar concentrators. Livermore, CA: Sandia National Laboratories; SAND [42] NREL. Parabolic trough technology models and software tools. Available: [43] ANSYS I. ANSYS. Available: [44] Solidworks. Available: [45] Schmitz M, Schwarzbozl P, Buck R, Pitz-Paal R. Assessment of the potential improvement due to multiple apertures in central receiver systems with secondary concentrators. Solar Energy 2006;80:111e20. [46] Winters WS. DRAC e a user friendly computer code for modeling transient thermohydraulic phenomena in solar receiver tubing. Livermore, CA: Sandia National Laboratores; SAND [47] Winters WS. TOPAZ e the transient one-dimensional pipe flow analyzer: an update on code improvements and increased capabilities. Livermore, CA: Sandia National Laboratores; SAND [48] ANSYS I. Fluid dynamics solutions. Available: Products/SimulationþTechnology/FluidþDynamics; [49] System Advisor Model (SAM). National Renewable Energy Laboratory. Available: [50] Stoddard MC, Faas SE, Chiang CJ, Dirks JA. SOLERGY e a computer code for calculating the annual energy from central receiver power plants. Livermore, CA: Sandia National Laboratories; SAND [51] TRNSYS. A transient systems simulation program. Available: wisc.edu/trnsys/; [52] GE Energy. Generation solutions. Available: solutions/generation_solutions.jsp; [53] GmbH S. IPSEpro. Available: english/ipsepro.php; [54] Thermoflow I. STREAM PRO. Available: ConvSteamCycle_STP.htm; [55] Vittitoe CN, Biggs F. User s guide to HELIOS: part 1, introduction and code input. A computer program for modeling the optical behavior of reflecting concentrators. Albuquerque, NM: Sandia National Laboratories; SAND [56] Organization BR. Solar energy items. Available: multimedia-gallery.php?cattypeid¼2&catid¼89&mode¼category; [57] Ratzel AC, Boughton BD. CIRCE.001: a computer code for analysis of pointfocus concentrators with flat targets. Albuquerque: Sandia National Laboratories; SAND [58] Romero VJ. CIRCE2/DEKGEN2: a software package for facilitated optical analysis of 3-D distributed solar energy concentrators e theory and user manual. Albuquerque, NM: Sandia National Laboratories; SAND [59] Hogan Jr. RE. AEETES e a solar reflux receiver thermal performance numerical model. In: Proceedings of the 1992 ASME international solar energy conference, Maui, HI; [60] Igo J, Andraka CE. Solar dish field system model for spacing optimization. In: Proceedings of the energy sustainability conference. Long Beach, CA; p. 981e87. [61] Adinberg R. Simulation analysis of thermal storage for concentrating solar power. Applied Thermal Engineering 2011;31(16):3588e94. [62] Arahal MR, Cirre CM, Berenguel M. Serial grey-box model of a stratified thermal tank for hierarchical control of a solar plant. Solar Energy 2008;82(5):441e51. [63] Powell KM, Edgar TF. Modeling and control of a solar thermal power plant with thermal energy storage. Chemical Engineering Science 2012;71(0):138e45. [64] Madaeni SH, Sioshansi R, Denholm P. How thermal energy storage enhances the economic viability of concentrating solar power. Proceedings of the IEEE 2011;PP(99):1e13. [69] Tamme R, Laing D, Steinmann WD. Advanced thermal energy storage technology for parabolic trough. Journal of Solar Energy Engineering, Transactions of the ASME 2004;126(2):794e800. [70] Glatzmaier G. Summary report for concentrating solar power thermal storage workshop. NREL; NREL/TP [71] Brown D, Lamarche J, Spanner G. Chemical energy storage system for solar electric generating system (SEGS) solar thermal power plant. Journal of Solar Energy Engineering 1992;114:212e8. [72] Harries DN, Paskevicius M, Sheppard DA, Price TEC, Buckley CE. Concentrating solar thermal heat storage using metal hydrides. Proceedings of the IEEE 2012;PP(99):1e11. [73] Schaube F, Worner A, Tamme R. High temperature thermochemical heat storage for concentrated solar power using gasesolid reactions. Journal of Solar Energy Engineering 2011;133(3):031006e7. [74] Bhatt VD, Gohil K, Mishra A. Thermal energy storage capacity of some phase changing materials and ionic liquids. International Journal of ChemTech Research 2010;2(3):1771e9. [75] GargHP, MullickSC, BhargavaAK. Solar thermal energystorage. Springer; [76] Kenisarin MM. High-temperature phase change materials for thermal energy storage. Renewable and Sustainable Energy Reviews 2010;14:955e70. [77] Zalba B, Marin JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied Thermal Engineering 2003;23(3):251e83. [78] Laing D, Steinmann WD, FiB M, Tamme R, Brand T, Bahl C. Solid media thermal storage development and analysis of modular storage operation

32 316 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 concepts for parabolic trough power plants. Journal of Solar Energy Engineering, Transactions of the ASME 2008;130: e5. [79] Buschle J, Steinmann WD, Tamme R. Analysis of steam storage systems using Modelica. The Modelica Association; p. 235e42. [80] Geyer MA, Winter CJ. Thermal storage for solar power plants. Solar Power Plants 1991;7:110e5. [81] Felderhoff M, Bogdanovic B. High temperature metal hydrides as heat storage materials for solar and related applications. International Journal of Molecular Sciences 2009;10(1):325e44. [82] Lovegrove K, Luzzi A, Kreetz H. A solar-driven ammonia-based thermochemical energy storage system. Solar Energy 1999;67(4e6):309e16. [83] Hahne E. Thermal energy storage: some views on some problems. In: Proc. 8th int heat transfer conf p. 279e92. [84] Kato Y. Personal notes on solar chemical storage chapter [85] Visscher K. SolarCombi systems, heading for 100% solar fraction. In: ECN presentation given at VSK Utrecht [86] Shiizaki S, Nagashimga I, Iwata K, Hosoda T, Kameyama H. Development of plate fin reactor for heat recovery system using methanol decomposition. In: Proceedings of Terrastock 2000, 8th international conference on thermal energy storage [87] Wu J, Li J, Xu X, Yang L, Wu J, Zhao F, et al. Molten salts/ceramic-foam matrix composites by melt infiltration method as energy storage material. Journal of Wuhan UniversityofTechnologyeMaterials Science Edition 2009;24(4):651e3. [88] Schmitt R, Rommler M, Steinmann WD, Tamme R. High performance PCMgraphite heat storage systems for solar process heat. ASME Conference Proceedings 2009;2009(48906):567e71. [89] Fan L, Khodadadi JM. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renewable and Sustainable Energy Reviews 2011;15(1):24e46. [90] Jegadheeswaran S, Pohekar SD. Performance enhancement in latent heat thermal storage system: a review. Renewable and Sustainable Energy Reviews 2009;13(9):2225e44. [91] Liu M, Saman W, Bruno F. Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems. Renewable and Sustainable Energy Reviews 2012;16(4):2118e32. [92] Tyagi VV, Kaushik SC, Tyagi SK, Akiyama T. Development of phase change materials based microencapsulated technology for buildings: a review. Renewable and Sustainable Energy Reviews 2011;15:1373e91. [93] Maruoka N, Sato K, Yagi J, Akiyama T. Development of PCM for recovering high temperature waste heat and utilization for producing hydrogen by reforming reaction of methane. ISIJ International 2002;42(2):215e9. [94] Laing D, Lehmann D, FiB M, Bahl C. Test results of concrete thermal energy storage for parabolic trough power plants. Journal of Solar Energy Engineering 2009;131: e6. [95] Laing D, Steinmann WD, Tamme R, Richter C. Solid media thermal storage for parabolic trough power plants. Solar Energy 2006;80(10):1283e9. [96] Laing D, Bahl C, Bauer T, Fiss M, Breidenbach N, Hempel M. High-temperature solid-media thermal energy storage for solar thermal power plants. Proceedings of the IEEE 2012;PP(99):1e9. [97] Laing D, Bahl C, Bauer T, Lehmann D, Steinmann WD. Thermal energy storage for direct steam generation. Solar Energy 2011; e33. [98] Pacheco JE, Showalter S, Kolb W. Development of a molten salt thermocline thermal storage system for parabolic trough plants. Journal of Solar Energy Engineering 2002;124:7. [99] Burolla VP, Bartel JJ. High temperature compatibility of nitrate salts, granite rock and pelletized iron ore. SAND p. 32. [100] Laurent SS. Thermocline thermal storage test for large-scale solar thermal power plants. Sandia National Laboratory; SAND C. [101] Bradshaw RW, Meeker DD. High-temperature stability of ternary nitrate molten salts for solar thermal energy systems. Solar Energy Materials 1990;21:51e60. [102] Leiby CC, Ryan TG. Thermophysical properties of thermal energy storage materials e aluminum. Air Force Cambridge Research Labs; [103] Sun JQ, Zhang RY, Liu ZP, Lu GH. Thermal reliability test of Ale34% Mge6% Zn alloy as latent heat storage material and corrosion of metal with respect to thermal cycling. Energy Conversion and Management 2007;48:619e24. [104] Lopez J, Acem Z, Palomo Del Barrio E. KNO 3 /NaNO 3 e graphite materials for thermal energy storage at high temperature: part II. e phase transition properties. Applied Thermal Engineering 2010;30(13):1586e93. [105] Bauer T, Laing D, Kroner U, Tamme R. Sodium nitrate for high temperature latent heat storage. In: The 11th international conference on thermal energy storage. Effstock Stockholm, Sweden; [106] Maru HC, Dullea JF, Kardas A, Paul L. Molten salt thermal energy storage systems. Chicago, NASA: Institute of Gas Technology; CR [107] Takahashi Y, Kamimoto M, Abe Y, Sakamoto R, Kanari K, Ozawa T. Investigation of latent heat thermal energy storage materials: V. Thermoanalytical evaluation of binary eutectic mixtures and compounds of NAOH with NaNO 3 OR NaNO 2. Thermochimica Acta 1988;123:233e45. [108] Petri R, Claar T, Marianowski L. Evaluation of molten carbonates as latent heat thermal energy storage materials. In: 14th intersociety energy conversion conference p. 487e93. [109] Reiser A, Bogdanovic B, Schlichte K. The application of Mg-based metal-hydrides as heat energy storage systems. International Journal of Hydrogen Energy 2000;25:425e30. [110] Birchneall C. Heat storage in eutectic alloys. Metallurgical and Materials Transaction. Physical Metallurgy and Materials Science 1980;11(8):1415e20. [111] Clark RP. Heats of fusion and heat capacities of Lithium Chloride-Potassium Chloride eutectic and potassium nitrate. Journal of Chemical and Engineering Data 1973;18(1):67e70. [112] Janz GJ, Allen CB, Bansal NP, Murphy RM, Tomkins RPY. Physical properties data compilations relevant to energy storage II. In: Molten salts: data on single and multi-component salt systems. NIST; NSRDS-NBS 61 part II. [113] Korin E, Soifer L. Thermal analysis of the system KCleLiCl by differential scanning calorimetry. Journal of Thermal Analysis 1997;50:347e54. [114] Glatzmaier GC, Gomez J, Ortega J, Starace A, Turchi C. High temperature phase change materials for thermal energy storage applications. Granada, Spain: SolarPaces; [115] Gomez JC. High-temperature phase change materials (PCM) candidates for thermal energy storage (TES) applications. National Renewable Energy Laboratory; NREL/TP [116] Heine D. The chemical compatibility of construction materials with latent heat storage materials. In: Proceedings of the international conference on energy storage [117] Pettit FS. Molten salts, corrosion tests and standards, application and interpretation [118] Porisini FC. Salt hydrates used for latent heat storage: corrosion of metals and reliability of thermal performance. Solar Energy 1988;41(2):193e7. [119] Cabeza LF, Badia F, Illa J, Roca J, Mehling H, Ziegler F. Corrosion experiments on salt hydrates used as phase change materials in cold storage. In: IEA ECES IA annex 17 Kick-off workshop [120] Cabeza LF, Illa J, Roca J, Badia F, Mehling H, Hiebler S, et al. Immersion corrosion tests on metal-salt hydrate pairs used for latent heat storage in the 32 to 36 C temperature range. Werkstoffe und Korrosion 2001;52(2):140e6. [121] Cabeza LF, Illa J, Roca J, Badia F, Mehling H, Hiebler S, et al. Middle term immersion corrosion tests on metalsalt hydrate pairs used for latent heat storage in the 32 to 36 C temperature range. Werkstoffe und Korrosion 2001;52(10):748e54. [122] Cabeza LF, Mehling H, Hiebler S. Immersion corrosion tests on metal-salt hydrate pairs used for latent heat storage in the 48 to 58 C temperature range. Materials and Corrosion [123] Heine D, Groll M, Heess F. Physical and chemical properties of phase change materials for application in solar tower and solar farm plants. Electric Power Research Institute (report) EPRI EA p. 1187e91. [124] Heine D, Heess F, Groll M. Physical and chemical properties of latent heat storage materials. Heizen mit Sonne; p. 536e45. [125] Heine D, Heess F, Groll M. Investigation of the corrosion and melting/ freezing behavior of high temperature latent storage materials. In: Proceedings of the intersociety energy conversion engineering conference. 1980: p. 459e66. [126] Goods SH, Bradshaw RW. Corrosion of stainless steels and carbon steel by molten mixtures of commercial nitrate salts. Journal of Materials Engineering and Performance 2004;13(1):78e87. [127] Goods SH, Bradshaw RW, Prairie MR, Chavez JM. Corrosion of stainless and carbon steels in molten mixtures of industrial nitrates. SAND [128] Prairie MR, Dunkin SR, Chavez JM, Bradshaw RW, Goods SH. Isothermal corrosion testing of steels in molten nitrate salts. In: ASME-JSES-JSME international solar energy conference p. 587e94. [129] Michels H, Pitz-Paal R. Cascaded latent heat storage for parabolic trough solar power plants. Solar Energy 2007;81(6):829e37. [130] Brousseau D, Hlava P, Kelly M. Testing thermocline filler materials and molten-salt heat transfer fluids for thermal energy storage systems used in parabolic trough solar power plants. Sandia National Laboratory; SAND [131] Williams DF. Assessment of candidate molten salt coolants for the NGNP/NHI heat-transfer loop. Oak Ridge National Laboratory; DE-AC05-00OR [132] Fathollahnejad H, Tsao BH, Ponnappan R, Jacobson D. Post-test corrosion analysis of high-temperature thermal energy storage capsules. Journal of Materials Engineering and Performance 1993;2:125e34. [133] Bradshaw RW, Goods SH. Corrosion of alloys and metals molten nitrates Alberbeque, NM, SAND [134] Bradshaw RW, Goods SH. Corrosion resistance of stainless steels during thermal cycling in alkali nitrate molten salts. SAND [135] Kelly B, Kearney D. Thermal storage commercial plant design study for a 2-tank indirect molten salt system. National Renewable Energy Laboratory; NREL/SR [136] Tyner C, Kolb G, Prairie M, Weinrebe G, Valverde A, Sanchez E. Solar power tower development: recent experiences. SAND C [137] Skumanich A. CSP: developments in heat transfer and storage materials. Available: csp-developments-in-heat-transfer-and-storage-materials/; [138] Kogel JE, Trivedi NC, Barger JM, Krukowski ST. Industrial minerals and rocks: commodities, markets, and uses. 7th ed. Littleton, Colorado: Society for Mining, Metallurgy, and Exploration, Inc.; [139] Kelly B, Kearney D. Thermal storage commercial plant design study for a 2- tank indirect molten salt system. National Renewable Energy Laboratory; Golden, Colorado, NREL/SR [140] Tamme R. Thermal energy storage for industrial applications. In: Presentation: storage for industrial applications IEA workshop

33 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e [141] ARPA-E. Heats thermal energy storage program sheet. Available: HEATS_ProgramSheet_Final.pdf; [142] Craik D. Thermal storage: OPTS to halve costs. Available: com/technology/thermal-storage-opts-halve-costs-0.csp Today;2012. [143] Adinberg R. Report on the methodology to characterize various types of thermal storage systems. Deliverable [144] GmbH PSI. Survey of thermal storage for parabolic trough power plants. Golden, CO: National Renewable Energy Laboratory; NREL/SR [145] Kearney D, Kroizer I, Miller CE, Steele D. Design and preliminary performance of solar electric generating Station I at Daggett, California. Montreal, Que, Can p. 1424e28. [146] Pacheco JE. Final test and evaluation results from the solar two project [147] Van Lew JT, Li P, Chan CL, Karaki W, Stephens J. Analysis of heat storage and delivery of a thermocline tank having solid filler material. Journal of Solar Energy Engineering, Transactions of the ASME 2011;133(2): e10. [148] Bai F, Xu C. Performance analysis of a two-stage thermal energy storage system using concrete and steam accumulator. Applied Thermal Engineering 2011;31(14e15):2764e71. [149] Steinmann WD, Eck M. Buffer storage for direct steam generation. Solar Energy 2006;80:1277e82. [150] Yanbing K, Yinping Z, Yi J, Yingxin Z. A general model for analyzing the thermal characteristics of a class of latent heat thermal energy storage systems. Journal of Solar Energy Engineering 1999;121(4):185e93. [151] Tamme R. Concrete storage: update on the German concrete TES program. In: Workshop on thermal storage for trough power systems [152] Steinmann W-D, Laing D, Tamme R. Development of PCM storage for process heat and power generation. Journal of Solar Energy Engineering 2009;131(4):041009e14. [153] Fang M, Chen G. Effects of different multiple PCMs on the performance of a latent thermal energy storage system. Applied Thermal Engineering 2007;27(5e6):994e1000. [154] Horbaniuc B, Dumitrascu G, Popescu A. Mathematical models for the study of solidification within a longitudinally finned heat pipe latent heat thermal storage system. Energy Conversion and Management 1999;40(15e16):1765e74. [155] Tardy F, Sami SM. Thermal analysis of heat pipes during thermal storage. Applied Thermal Engineering 2009;29(2e3):329e33. [156] Shabgard H, Bergman TL, Sharifi N, Faghri A. High temperature latent heat thermal energy storage using heat pipes. International Journal of Heat and Mass Transfer 2010;53(15e16):2979e88. [157] Nithyanandam K, Pitchumani R. Analysis and optimization of a latent thermal energy storage system with embedded heat pipes. International Journal of Heat and Mass Transfer 2011;54(21e22):4596e610. [158] Faghri A. Heat pipe science and technology. New York: Taylor & Francis; [159] Adinberg R, Zvegilsky D, Epstein M. Heat transfer efficient thermal energy storage for steam generation. Energy Conversion and Management 2010;51(1):9e15. [160] Bharathan D, Glatzmaier GC. Progress in thermal energy storage modeling. ASME Conference Proceedings 2009;2009(48906):597e603. [161] Kelly B. Advanced thermal storage for central receivers with supercritical coolants. Abengoa Solar Inc.; DE-FG36-08GO [162] Warerkar S, Schmitz S, Goettsche J, Hoffschmidt B, Reissel M, Tamme R. Airsand heat exchanger for high-temperature storage. Journal of Solar Energy Engineering 2011;133(2):021010e7. [163] Hänchen M, Brückner S, Steinfeld A. High-temperature thermal storage using a packed bed of rocks e heat transfer analysis and experimental validation. Applied Thermal Engineering 2011;31(10):1798e806. [164] Furnas CG. Heat transfer from a gas stream to bed of broken solids. Industrial & Engineering Chemistry 1930;22(1):26e31. [165] Bradshaw AV, Johnson A, McLachlan NH, Chiu YT. Heat transfer between air and nitrogen and packed beds of non-reacting solids. Chemical Engineering Research and Design 1970;48. [166] Nsofor EC, Adebiyi GA. Measurements of the gas-particle convective heat transfer coefficient in a packed bed for hightemperature energy storage. Experimental Thermal and Fluid Science 2001;24(1e2):1e9. [167] Tamme R, Bauer T, Buschle J, Laing D, Muller-Steinhagen H, Steinmann WD. Latent heat storage above 120 C for applications in the industrial process heat sector and solar power generation. International Journal of Energy Research 2008;32:264e71. [168] Laing D, Bauer T, Lehmann D, Bahl C. Development of a thermal energy storage system for parabolic trough power plants with direct steam generation. Journal of Solar Energy Engineering 2010;132(2): [169] Incropera FP, Dewitt DP, Bergman TL, Lavine AS. Fundamentals of heat and mass transfer. John Wiley & Sons, Inc.; [170] Kopp J, Boehm RF. Comparison of two-tank indirect thermal storage designs for solar parabolic trough power plants. ASME Conference Proceedings 2009;2009(48906):683e8. [171] Gabbrielli R, Zamparelli C. Optimal design of a molten salt thermal storage tank for parabolic trough solar power plants. Journal of Solar Energy Engineering 2009;131(4):041001e10. [172] O Brien P, Pye J. Exergetic analysis of a steam-flashing thermal storage system. In: Proceedings of Solar2010, 48th AuSES conference. Canberra, Australia; [173] Agyenim F, Eames P, Smyth M. A comparison of heat transfer enhancement in a medium temperature thermal energy storage heat exchanger using fins. Solar Energy 2009;83(9):1509e20. [174] Papanicolaou E, Belessiotis V. Transient natural convection in a cylindrical enclosure at high Rayleigh numbers. International Journal of Heat and Mass Transfer 2002;45(7):1425e44. [175] Silva PD, Gonçalves LC, Pires L. Transient behaviour of a latent-heat thermalenergy store: numerical and experimental studies. Applied Energy 2002;73(1):83e98. [176] Zivkovic B, Fujii I. An analysis of isothermal phase change of phase change material within rectangular and cylindrical containers. Solar Energy 2001;70(1):51e61. [177] Kota K, Chow L, Leland Q. Laminar film condensation driven latent thermal energy storage in rectangular containers. International Journal of Heat and Mass Transfer 2012;55(4):1208e17. [178] Agyenim F, Hewitt N, Eames P, Smyth M. A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS). Renewable and Sustainable Energy Reviews 2009;14(2):615e28. [179] Brousseau P, Lacroix M. Numerical simulation of a multi-layer latent heat thermal energy storage system. International Journal of Energy Research 1998;22(1):1e15. [180] Esen M, Ayhan T. Development of a model compatible with solar assisted cylindrical energy storage tank and variation of stored energy with time for different phase change materials. Energy Conversion and Management 1996;37(12):1775e85. [181] Esen M. Thermal performance of a solar-aided latent heat store used for space heating by heat pump. Solar Energy 2000;69(1):15e25. [182] Gong Z-X, Mujumdar AS. Finite-element analysis of cyclic heat transfer in a shell-and-tube latent heat energy storage exchanger. Applied Thermal Engineering 1997;17(6):583e91. [183] Hirata T, Nishida K. An analysis of heat transfer using equivalent thermal conductivity of liquid phase during melting inside an isothermally heated horizontal cylinder. International Journal of Heat and Mass Transfer 1989;32(9):1663e70. [184] Kuravi S, Trahan J, Rahman M, Goswami DY, Stefanakos E. Analysis of transient heat transfer in a thermal energy storage module. In: ASME 2010 international mechanical engineering congress & exposition IMECE2010. Vancouver; [185] Dutil Y, Rousse DR, Salah NB, Lassue S, Zalewski L. A review on phase-change materials: mathematical modeling and simulations. Renewable and Sustainable Energy Reviews 2011;15(1):112e30. [186] Regin AF, Solanki SC, Saini JS. Heat transfer characteristics of thermal energy storage system using PCM capsules: a review. Renewable and Sustainable Energy Reviews 2008;12(9):2438e58. [187] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renewable and Sustainable Energy Reviews 2009;13(2):318e45. [188] Verma P, Varun, Singal SK. Review of mathematical modeling on latent heat thermal energy storage systems using phase-change material. Renewable and Sustainable Energy Reviews 2008;12(4):999e1031. [189] Li H, Hsieh CK, Goswami DY. Conjugate heat transfer analysis of fluid flow in a phase-change energy storage unit. International Journal of Numerical Methods for Heat and Fluid Flow 1996;6(3):77e90. [190] Wang Y, Amiri A, Vafai K. An experimental investigation of the melting process in a rectangular enclosure. International Journal of Heat and Mass Transfer 1999;42:3659e72. [191] Solomon AD. Melt time and heat flux for a simple PCM body. Solar Energy 1979;22:251e7. [192] Zhang Z, Bejan A. The problem of time-dependent natural convection melting with conduction in the solid. International Journal of Heat and Mass Transfer 1989;32(12):2447e57. [193] Balakrishnan AR, Pei DCT. Heat transfer in gas-solid packed bed systems. 3. Overall heat transfer rates in adiabatic beds. Industrial & Engineering Chemistry Process Design and Development 1979;18(1):47e50. [194] Beasley DE, Clark JA. Transient response of a packed bed for thermal energy storage. International Journal of Heat and Mass Transfer 1984;27(9):1659e69. [195] Ergun S. Fluid flow through packed columns. Chemical Engineering Progress 1952;48:89. [196] Borkink JGH, Westerterp KR. Influence of tube and particle diameter on heat transport in packed beds. AIChE Journal 1992;38(5):703e15. [197] Ahn BJ, Zoulalian A, Smith JM. Axial dispersion in packed beds with large wall effect. AIChE Journal 1986;32(1):170e4. [198] Aris R, Amundson NR. Some remarks on longitudinal mixing or diffusion in fixed beds. AIChE Journal 1957;3(3):280e2. [199] Babcock RE, Green DW, Perry RH. Longitudinal dispersion mechanisms in packed beds. AIChE Journal 1966;12(5):922e7. [200] Chao R, Hoelscher HE. Simultaneous axial dispersion and adsorption in a packed bed. AIChE Journal 1966;12:271e8. [201] Clement K, Jorgensen SB. Experimental investigation of axial and radial thermal dispersion in a packed bed. Chemical Engineering Science 1983;38(6):835e42.

34 318 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e319 [202] Coutier JP, Farber EA. Two applications of a numerical approach of heat transfer process within rock beds. Solar Energy 1982;29(6):451e62. [203] Epstein N. Correction factor for axial mixing in the packed beds. Canadian Journal of Chemical Engineering 1958;36:210e2. [204] Gomezplata J. Axial dispersion coefficients in packed beds at low Reynolds number. Canadian Journal of Chemical Engineering 1969;47(4):353e9. [205] Gunn DJ. Axial and radial dispersion in fixed beds. Chemical Engineering Science 1987;42(2):363e73. [206] Gunn DJ, De Souza JFC. Heat transfer and axial dispersion in packed beds. Chemical Engineering Science 1974;29(6):1363e71. [207] Howell JR, Bannerot RB, Vliet GC. Solar thermal energy systems. Mc-Graw Hill; [208] Hsiang TCS, Haynes Jr HW. Axial dispersion in small diameter beds of large, spherical particles. Chemical Engineering Science 1977;32(6):678e81. [209] Hughes PJ, Klein SA, Close DJ. Packed bed thermal storage models for solar air heating and cooling systems. Journal of Heat Transfer 1976;98(Series C(2)):336e8. [210] Jeffreson CP. Prediction of breakthrough curves in packed beds e 1, 2. AIChE Journal 1972;18(2):409e20. [211] Jeffreson CP. Prediction of break through curves in packed beds e experimental evidence for axial dispersion and intraparticle effects. AIChE Journal 1972;18:416e20. [212] Levec J, Carbonell RG. Longitudinal and lateral thermal dispersion in packed beds. Part I: theory. AIChE Journal 1985;31(4):581e90. [213] Lof GOG, Hawley RW. Unsteadyestate heat transfer between air and loose solids. Industrial & Engineering Chemistry 1948;40(6):1061e70. [214] Riaz M. Analytical solutions for single- and two-phase models of packedbed thermal storage systems. Journal of Heat Transfer 1977;99(Series C(3)):489e92. [215] Saez AE, McCoy BJ. Transient analysis of packed-bed thermal storage systems. International Journal of Heat and Mass Transfer 1983;26(1): 49e54. [216] Sagara K, Nakahara N. Thermal performance and pressure drop of rock beds with large storage materials. Solar Energy 1991;47(3):157e63. [217] Singh R, Saini RP, Saini JS. Nusselt number and friction factor correlations for packed bed solar energy storage system having large sized elements of different shapes. Solar Energy 2006;80(7):760e71. [218] Sodha MS, Sawhney RL, Verma R, Bansal NK. Effect of finite thermal conductivity on the thermal performance of a storage medium. Building and Environment 1986;21(3e4):189e94. [219] Vortmeyer D, Schaefer RJ. Equivalence of one- and two-phase models for heat transfer processes in packed beds: one dimensional theory. Chemical Engineering Science 1974;29(2):485e91. [220] Regin AF, Solanki SC, Saini JS. An analysis of a packed bed latent heat thermal energy storage system using PCM capsules: numerical investigation. Renewable Energy 2009;34(7):1765e73. [221] Jalalzadeh-Azar AA, Steele WG, Adebiyi GA. Heat transfer in a hightemperature packed bed thermal energy storage system e roles of radiation and intraparticle conduction. Journal of Energy Resources Technology 1996;118(1):50e7. [222] Singh H, Saini RP, Saini JS. A review on packed bed solar energy storage systems. Renewable and Sustainable Energy Reviews 2010;14(3):1059e69. [223] Schumann TEW. Heat transfer: a liquid flowing through a porous prism. Journal of the Franklin Institute 1929;208(3):405e16. [224] Flueckiger S, Yang Z, Garimella SV. An integrated thermal and mechanical investigation of molten-salt thermocline energy storage. Applied Energy 2011;88(6):2098e105. [225] Seeniraj RV, Velraj R, Lakshmi Narasimhan N. Thermal analysis of a finnedtube LHTS module for a solar dynamic power system. Heat and Mass Transfer 2002;38(4):409e17. [226] El Qarnia H. Numerical analysis of a coupled solar collector latent heat storage unit using various phase change materials for heating the water. Energy Conversion and Management 2009;50(2):247e54. [227] Rosen M. Appropriate thermodynamic performance measures for closed systems for thermal energy storage. Journal of Solar Energy Engineering 1992;114:100e5. [228] Bejan A. Fundamentals of exergy analysis, entropy generation minimization, and the generation of flow architecture. International Journal of Energy Research 2002;26:545e65. [229] Bejan A. Entropy generation minimization: the method of thermodynamic optimization of finite-size systems and finite-time processes. New York: CRC Press; [230] Bejan A. Entropy generation through heat and fluid flow. New York: Wiley; [231] Goswami DY, Kreith F, Kreider JF. Principles of solar engineering. Taylor and Francis; [232] Krane RJ. A Second Law analysis of the optimum design and operation of thermal energy storage systems. International Journal of Heat and Mass Transfer 1987;30(1):43e57. [233] Rosen M, Dincer I. Exergy methods for assessing and comparing thermal storage systems. International Journal of Energy Research 2003;27: 415e30. [234] Erek A, Dincer I. A new approach to energy and exergy analyses of latent heat storage unit. Heat Transfer Engineering 2009;30(6):506e15. [235] Rosen M. Second-law analysis: approaches and implications. International Journal of Energy Research 1999;23:415e29. [236] Jegadheeswaran S, Pohekar SD, Kousksou T. Exergy based performance evaluation of latent heat thermal storage system: a review. Renewable and Sustainable Energy Reviews 2010;14(9):2580e95. [237] Abedin A, Rosen M. Assessment of a closed thermochemical energy storage using energy and exergy methods. Applied Energy 2012;93:18e23. [238] Jack M, Wrobel J. Thermodynamic optimization of a stratified thermal storage device. Applied Thermal Engineering 2009;29:2344e9. [239] Adebiyi G. A second-law study on packed bed energy storage systems utilizing phase-change materials. Journal of Solar Energy Engineering 1991;113:146e56. [240] Aceves SM, Nakamura H, Reistad GM, Martinez-Frias J. Optimization of a class of latent thermal energy storage systems with multiple phase-change materials. Journal of Solar Energy Engineering, Transactions of the ASME 1998;120(1):14e9. [241] Domanski R, Fellah G. Exergy analysis for the evaluation of a thermal storage system employing PCMS with different melting temperatures. Applied Thermal Engineering 1996;16(11):907e19. [242] Farid MM, Chen XD. Domestic electrical space heating with heat storage. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 1999;213(2):83e92. [243] Farid MM, Husian RM. An electrical storage heater using the phase-change method of heat storage. Energy Conversion and Management 1990;30(3): 219e30. [244] Farid MM, Kanzawa A. Thermal performance of a heat storage module using PCM s with different melting temperatures: mathematical modeling. Journal of Solar Energy Engineering, Transactions of the ASME 1989;111(2): 152e7. [245] Watanabe T, Kikuchi H, Kanzawa A. Enhancement of charging and discharging rates in a latent heat storage system by use of PCM with different melting temperatures. Heat Recovery Systems and CHP 1993;13(1):57e66. [246] Gong Z-X, Mujumdar AS. New solar receiver thermal store for spacebased activities using multiple composite phase-change materials. Journal of Solar Energy Engineering, Transactions of the ASME 1995;117(3): 215e20. [247] Dinter F, Geyer M, Tamme R. Thermal energy storage for commercial applications. Springer Verlag; [248] Haslett RA, Kosson RL, Ferrara AA, Roukis JG. Thermal energy storage heat exchanger design. ASME Aerospace Division. In: Proceedings of conference on environmental systems. San Diego; [249] Steiner D, Heine D, Nonnenmacher A. Studie über thermische Energiespeicher für den Temperaturbereich 200 C bis 500 C. Germany: Study of the German Research Ministry; BMFT-FB-T. p. 82e105. [250] Salomoni VA, Majorana CE, Giannuzzi GM, Miliozzi A. Thermal-fluid flow within innovative heat storage concrete systems for solar power plants. International Journal of Numerical Methods for Heat and Fluid Flow 2008;18(7e8):969e99. [251] Boobar MG, McFarland BL, Nalbandian SJ, Willcox WW, French EP, Smith KE. Commercial applications of solar total energy systems second quarterly report. Canoga Park, CA, report no. Al-ERDA [252] Nextant. Thermal storage oil-to-salt heat exchanger design and safety analysis. San Francisco: National Renewable Energy Laboratory; [253] Rovira A, Montes MJ, Valdes M, Martinez-Val JM. Energy management in solar thermal power plants with double thermal storage system and subdivided solar field. Applied Energy 2011;88(11):4055e66. [254] Pistocchini L, Motta M. Feasibility study of an innovative dry-cooling system with phase-change material storage for concentrated solar power multi-mw size power plant. Journal of Solar Energy Engineering 2011;133(3):031010e8. [255] Kearney D, Herrmann U, Nava P, Kelly B, Mahoney R, Pacheco J, et al. Assessment of a molten salt heat transfer fluid in a parabolic trough solar field. Journal of Solar Energy Engineering, Transactions of the ASME 2003;125(2):170e6. [256] Kearney D, Kelly B, Herrmann U, Cable R, Pacheco J, Mahoney R, et al. Engineering aspects of a molten salt heat transfer fluid in a trough solar field. Energy 2004;29(5e6):861e70. [257] Robak CW, Bergman TL, Faghri A. Economic evaluation of latent heat thermal energy storage using embedded thermosyphons for concentrating solar power applications. Solar Energy 2011;85(10):2461e73. [258] Slocum AH, Codd DS, Buongiorno J, Forsberg C, McKrell T, Nave J-C, et al. Concentrated solar power on demand. Solar Energy 2011;85(7):1519e29. [259] Scientific Applications International Corporation (SAIC). Life cycle assessment: principles and practice. U.S. Environmental Protection Agency; Contract no. 68-C [260] Oro E, Gil A, de Gracia A, Boer D, Cabeza LF. Comparative life cycle assessment of thermal energy storage systems for solar power plants. Renewable Energy 2012;44:166e73. [261] Burkhardt Iii JJ, Heath G, Turchi C. Life cycle assessment of a model parabolic trough concentrating solar power plant with thermal energy storage. ASME Conference Proceedings 2010;2010(43956):599e608. [262] Giuliano S, Buck R, Eguiguren S. Analysis of solar-thermal power plants with thermal energy storage and solar-hybrid operation strategy. Journal of Solar Energy Engineering 2011;133(3):

35 S. Kuravi et al. / Progress in Energy and Combustion Science 39 (2013) 285e [263] Heath G, Turchi C, Decker T, Burkhardt J, Kutscher C. Life cycle assessment of thermal energy storage: two-tank indirect and thermocline. ASME Conference Proceedings 2009;2009(48906):689e90. [264] Lechon Y, de la Rua C, Saez R. Life cycle environmental impacts of electricity production by solar thermal power plants in Spain. Journal of Solar Energy Engineering 2008;130(2):021012e7. [265] US Department Of Energy. Concentrating solar power industry projects. Available: [266] US DOE Sunshot Team. Revitalizing American competitiveness in solar technologies. US Department of Energy; Available: energy.gov/solar/pdfs/sssummit2012_plenary_ramesh.pdf.