THERMAL ENERGY STORAGE PERFORMANCE METRICS AND USE IN THERMAL ENERGY STORAGE DESIGN Zhiwen Ma, Greg Glatzmaier, Craig Turchi, and Mike Wagner National Renewable Energy Laboratory 1617 Cole Blvd Golden, CO, USA Email: zhiwen.ma@nrel.gov ABSTRACT This paper presents thermal energy storage (TES) modeling approach and performance evaluation. We are developing modeling tools that will allow evaluation of the performance of a TES system that integrates into a concentrating solar power (CSP) plant. The overall performance, including round trip efficiency, for a thermal energy storage system is highly dependent on the operating parameters and operation strategy of the complete power plant. We attempt to develop a general method based on efficiency metrics and a CSP integration scheme to facilitate various TES designs, and to be able to evaluate their performance in CSP plants. The paper will discuss TES performance metrics in terms of three efficiencies: first-law efficiency, second-law efficiency, and storage effectiveness. The paper uses the design metrics to size TES as an example to illustrate derivation of the efficiency values and application of the three efficiencies. 1. CSP THERMAL ENERGY STORAGE Concentrating solar power plants with thermal energy storage capability provide utility-scale, dispatchable electricity to the power grid. TES allows electricity to be generated consistently at times when sunlight is not available, including momentary cloud transients, which otherwise disrupt electricity generation and cause widely varying power output [1]. For longer time scales, TES allows CSP plants to generate electricity well into the evening hours when electricity is highly valued, making the power plant more cost effective. TES also allows greater use of the turbine and other power-block components. These features provide an economic incentive for the addition of TES. Dispatchable delivery means power is reliably available when it is needed to meet the utility load demand. In addition to enhancing CSP dispatchability, TES enables increased deployment of renewable generations in general by adding flexibility to a grid for photovoltaic and wind power systems. 1.1 Types of Thermal Energy Storage Figure 1 lists a variety of TES options for CSP plants [2]. They fall into three general categories: sensible, latent, and thermochemical storage. Current thermal energy storage technology uses two-tank salt system. Two-tank storage system has hot and cold tanks that store the liquid salts separately. This system is used because the components associated with molten-salt handling pumps, valves, tanks, and heat exchangers have demonstrated reliable operation at commercial scale within their capable temperatures [3 4]. With a direct system, salt flows from cold tank to the solar receiver, is heated, and enters the hot tank. During power generation hot salt flows to the steam generator and returns to the cold tank. In an indirect system the hot salt discharges through a heat exchanger to heat the HTF and then enters the cold tank. Charging occurs with the opposite flows. TES options other than two-tank molten-salt storage systems including thermocline, phase change material, and thermochemical storage systems as shown in Figure 1 are also investigated broadly for potential low TES cost and high performance [5 9]. Those storage methods have certain advantages of cost, energy density, meanwhile, with challenges in performance or system configuration. To accelerate the adoption of advanced storage technology, an effective comparison methodology may be useful to identify research focus. 1
The outlet HTF temperature from the power cycle is calculated by considering the HTF mass flow rate, the heat input, the inlet temperature, and the specific heat of the HTF. The specific heat is assumed to vary linearly over the range of the inlet and outlet temperatures. Thus, an average specific heat value is used and is recalculated throughout the simulation.,, (2), Figure 1. Thermal energy storage options for CSP technologies. Thermal energy storage for CSP plants has been implemented in some parabolic trough and power tower plants to provide consistent electric generation irrespective of weather conditions or solar availability. Facing the various possibilities of TES choices, developing a uniform and general assessment method will help concept screening and setting development direction [10 15]. The method will not only be important to TES design itself, but also assist better understanding of incorporating a TES in CSP and the interaction with other parts of the CSP components, namely, solar field and power block. 1.2 Consideration of Thermal Energy Storage Integration with CSP When storage system integrates in CSP plant, it interacts with both solar field and power block. The solar field configuration and operating conditions determine the HTF exit temperature from the solar receiver, and consequently, the storage charging temperature, T h, charging. Mature trough technology uses a relatively expensive organic fluid as the HTF in the solar field and a molten salt fluid in the thermal storage tank. This indirect storage configuration requires a heat exchanger for transferring energy into and out of storage, as shown in Figure 3. This heat exchanger reduces the performance of the storage system and adds cost to the plant. Heat input needed by the power block is determined by simply thermodynamic power cycle efficiency.: (1) To measure different types of thermal storage performance, a uniform set of performance metrics will be valuable. From the modeling work performed for thermocline and two-tank system, three efficiencies can be used to define the thermal storage size and performance; they are first law efficiency for thermal energy losses, second law efficiency of thermal energy degradation, and storage system effectiveness, which indicates the usable capacity for the storage volume. The current approach for understanding TES performance is to integrate a detailed TES model into a CSP system model to simulate whole plant operation. The approach is very useful in whole plant operation and performance prediction with well designed and understood storage technologies. However, the complete plant simulation can be tedious and uncertain when screening a new storage concept. Use of efficiency metrics and a simplified integration relationship with the solar field and power block may provide a quick assessment of TES performance and design options. 2. GENERAL PERFORMANCE METRICS FOR THERMAL ENERGY STORAGE The three efficiencies can be specific to the storage design and system. The specific definition of TES metrics in terms of storage tank effectiveness, the first-law thermal efficiency, and the second-law energy conversion efficiency may also vary according to storage type. We try to start from general definition and discuss applicable cases. The effectiveness of the storage tank is the amount of heat discharged with respect to the total thermal energy stored when tank is fully charged, i.e., can the system be completely discharged or is some residual necessary? The effectiveness of storage tank definition is sometimes also called the discharge efficiency (Yang and Garimella, [16]) or the storage fraction. The storage effectiveness accounts for usable storage out of gross TES media load. For instance, in a two-tank storage system, approximately 20% of the hot or cold tank storage fluids often remain in 2
the tank and cannot be used due to the pump head need and requirement for the pump to be submerged in the liquid. For thermocline TES, it is determined by the usable portion of the stored fluid outside of the mixed-temperature thermocline region. The effectiveness of storage can be written as Eq. (3): first-law efficiency is generally quite high in the range of 93%-99%, with the highly effective thermal insulation applied. Q loss, top (3) Therefore, storage tank and storage media need to be oversized to accommodate the unusable residual thermal energy storage capacity. The first law efficiency is essentially the heat loss during charge, discharge, and holding. It reflects the round-trip efficiency of the energy in and energy out. The first-law efficiency of TES systems, η TES, I, can be defined as the ratio of the energy extracted from the storage to the energy stored in it. It is write in the form of Eq. (4):, where mc p is the total heat capacity of the storage medium; and T, T c are the hot and cold salt temperatures, respectively, of the storage during discharging. T h is the maximum temperature at the end of the charging period. Heat loss of the TES tank consists of convection to the environment and conduction to the foundation as shown in Figure 2, and sum up in Eq. (5). The total loss is integrated over the storage time in Eq. (6).,, (4) (5), (6) where 2 stands for perimeter for a round tank, and the heat losses from the tank side are integrated along the tank height. The thermal storage tank and flow loop can be well insulated, and the heat losses to the environment between the end of discharging and the beginning of the charging periods are usually very low. For large TES systems, the Figure 2. TES tank heat loss paths. The second law efficiency measures availability (exergy) conservation for the stored energy. The second-law energy conversion efficiency measures the energy quality degradation due to both the difference in charging and discharging temperatures and thermal losses [17]. It is a measure of the conservation of exergy through the storage, i.e., the ability to generate the same amount of power from TES as from the original energy used to charge the system. Exergy transfer through a system is given as Eq. (7): (7) where ΔG is exergy change of the storage fluid, ΔS is its entropy change and T amb is the ambient temperature as reference point, which is usually assumed to be 298 K (25 C) as a reference point. TES round trip efficiency based on exergy is written in Eq. (8):, (8) where subscripts c and d indicate TES charging and discharging processes, respectively. For a storage system, η TES,II, is also impacted by the presence of a heat exchanger between storage salt and HTF, for instance, the oil-salt heat exchanger in a trough plant. Heat exchanger exergy losses can be expressed as Eq. (9): Q loss, foundation Q loss, env (9) 3
t 0 is the starting time for charging or discharging, and T i and To are the temperatures flowing in and out of the heat exchanger. The heat exchanger derate factor is equal to the ratio of the realized temperature difference on the storage side of the heat exchanger to the solar field temperature difference (Wagner [18], 2011).,,,, (10) Eq. (10) can convert the solar field HTF temperature to TES temperature for indirect TES. The HTF coming from solar field can be oil or salt, depending on the plant design. The effect of temperature difference in charging and discharging process on thermal conversion efficiency can be simply calculated from Carnot cycle efficiency ratio. The drop of power efficiency by temperature drop can estimated from:, 1, 1 (10), wehre T env,ref is the ambient referennce temperature that is often 25 C. 3. TANK AND STORAGE MATERIAL SIZING RELATION TO PERFORMANCE METRICS Often early stage storage screening will look into cost closely to explore its economic benefits. The starting point for economic analyses will estimate the storage size and storage media needed. Idealized storage sizing without consider performance usually leads to over optimistic results that not reflect usability and losses. In this case, we try using the efficiency metrics for storage sizing purpose. The desired (usable) thermal storage capacity is the multiplication of design-point power cycle thermal load and required storage hours, which is rated with power cycle efficiency, ηp, together with second-law efficiency due to the difference in charging and discharging temperature, and can be expressed as Eq. (11): (11), In order to use the TES capacity rating to size the physical dimensions of the storage system, the heat capacity, density, and distribution of all active storage materials must be calculated. In the example of a thermocline system using filler material, Eqs. (12) and (13) can be used to calculate the amount of material that will be needed for storage fluid and filler. If we derive an average heat capacity from storage fluid and filler properties and void fraction, the average heat capacity of storage material expressed as:, 1, (12) The void fraction of the thermocline, є, is the ratio of fluid volume to the total tank volume: (13) The void fraction has a range from 0 to 1, where є=0 indicates a system entirely composed of filler material, or a pure solid material, and є=1 indicates an all fluid system. Then the tank volume needed to store amount of energy, E, is:, Δ (14),,, where, is the thermocline storage tank volume. If the heat exchanger derate factor, f hx, is known, then the TES temperature difference can be correlated to the solar field temperature. The selection of material will affect TES performance, cost, and compatibility with the storage liquid. If the TES system incorporates sensible and latent storage, the stored energy equation must be generalized as the sum of sensible subcooling, latent phase change, and sensible superheating:,, (15) where m i is the mass and C p, i is the specific heat. T h and T c represent the hot and cold temperature levels between which the storage operates, while T f represents the phase change temperature. The difference (T h T c ) is referred to as the storage temperature range. The amount of latent heat stored is represented by the second term, which depends on the mass (m) and latent heat of fusion (ΔH f ) of phase change material i. For a system without phase change material, the latent heat term in Eq. (15) can be dropped. 4
4. CONCLUSIONS This This work investigates the potential to use three efficiencies as adequate TES performance metrics for TES screening and preliminary design purpose. The paper has described those efficiencies in terms of first-law, secondlaw efficiencies, and storage effectiveness (or storage fraction). The first-law efficiency reflects heat losses by the storage tank. The second-law efficiency indicates potential energy conversion degradation through TES charging/discharging processes. Storage effectiveness gives the fraction of usable storage volume. The paper gives an example of using the efficiency metrics to size storage. Future work will study TES integration and interaction with solar field and power block to evaluate TES behavior and performance in a CSP system. 5. NOMENCLATURES A Tank Cross-Section Area, [m 2 ] Bi Biot number, hl/k [-] c Heat capacity, [J/kg K] d s Filler particle size, [m] E, Energy, [J/kg] f def Fraction of defocus, [-] h f Enthalpy per unit mass, [J/kg] h Convection heat transfer coefficient, [W/m 2 K] k Thermal conductivity of the solid fill [W/m K] l Characteristic length of the solid fill [m] Mass flow rate, [kg/s] Thermal load demand, [W] Available heat rate, [W] T Temperature, [K] u m Thermocline moving speed, nominal velocity of liquid, [m/s] V Volume, [m 3 ] Greek Letter η Efficiency, [-] ρ Density, [kg/m 3 ] ε Packed bed void fraction, [-] ν Fluid kinematic viscosity, [m 2 /s] Subscripts: chg Charge dis Discharge def Defocus f Fluid htf Heat Transfer Fluid pb Power Block s Solid sf Solar Field tes Thermal Energy Storage Superscript: Average value 6. ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory. The authors thank the inputs on thermocline design from Desikan Bharathan at NREL. 7. REFERENCES [1] Z. Ma, G. Glatzmaier, and C. Kutscher, Thermal Energy Storage and Its Potential Applications in Solar Thermal Power Plants and Electricity Storage, ASME ES2011, Washington D.C., August 2011. [2] G. Glatzmaier, New Concepts and Materials for Thermal Energy Storage and Heat-Transfer Fluids, Technical Report NREL/TP- 5500-52134, DE-AC36-08GO28308, May 20, 2011. [3] B.D. Kelly, U. Herrmann, and D.W. Kearney, Evaluation and performance modeling for integrated solar combined cycles systems and thermal storage system, Final report on contract RAR-9-29442-05, National Renewable Energy Laboratory, 2000. [4] H. Price, D. Brosseau, D. Kearney, and B. Kelly, DOE Advanced Thermal Energy Storage Development Plan for Parabolic Trough Technology, NREL Milestone Report, January, 2007. [5] D. Bharathan and G. Glatzmaier, Progress in Thermal Energy Storage Modeling, Proceedings of the ASME 2009 3rd International Conference of Energy Sustainability, ES2008, San Francisco, CA, 2008. [6] D. Bharathan, Thermal Storage Modeling, NREL Milestone Report, 2010. [7] J. T. Van Lew, P. Li,C. L. Chan, W. Karaki, and J. Stephens, Analysis of Heat Storage and Delivery of a Thermocline Tank Having Solid Filler Material, Journal of Solar Energy Engineering, ASME, MAY 2011, Vol. 133. [8] J. E. Pacheco, S. K. Showalter, and W. J. Kolb, Development of a molten-salt thermocline thermal storage system for parabolic trough plants, J. Solar Energy Engineering, v124, pp153-159, 2002. [9] R. Muren, D. Arias, D. Chapman, L. Erickson, A. Gavilan, Coupled transient system analysis: a new method of passive thermal energy storage modeling for high temperature concentrated solar power systems, Proceedings of ESFuelCell2011, ASME Energy Sustainability Fuel Cell 2011, August, 2011, Washington DC, USA. [10] G. J. Kolb and V. Hassani, Performance Analysis of Thermocline Energy Storage proposed for the 1MW 5
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