Storing electricity from renewable energy sources. High temperature latent heat storage using a metal based phase change material

Transcription

1 Storing electricity from renewable energy sources High temperature latent heat storage using a metal based phase change material Energinet.dk, project no

2 Table of contents 1. Project details 3 2. Executive summary 5 3. Project results WP2: Literature survey Storing high temperature heat: Classification and design criteria WP3-4: Storage design and specifications, environmental and safety issues Numerical model Material properties Boundary conditions Results Conclusion WP5: Business potential, fields of application and system cost analysis Preconditions The analyzed configurations Conclusions on business potential Case 1: Aluminium heat storage installed parallel to a power plant boiler Case 2: Aluminium heat storage connected to a solar power plant Case 3: Aluminium heat storage supplied with electricity from wind turbines Utilization of project results Project conclusion and perspective Bibliography 29 Appendix A Cost estimate of the aluminium phase change energy storage unit 30 Appendix B Simulation reports 31 2

3 1. Project details Project title Storing electricity from renewable energy sources Project identification Energinet.dk, project no Name of the programme which has funded the project ForskEL (ForskVE, ForskNG or ForskEL) Name and address of the enterprises/institution responsible for the project Danish Technological Institute Teknologiparken, Kongsvang Allé 29 DK-8000 Aarhus C Tel.: CVR (central business register) Date for submission This report investigates the technical and economic feasibility of storing heat as the phase change energy of aluminium (liquid/solid transition), i.e. as thermal energy at a high temperature. When a surplus of electrical or high temperature heat is available, the energy can be delivered into the storage system and thereby melt a quantity of aluminium at a constant temperature of approximately 660 C. The opposite process will take place in case of discharging, i.e. the energy is removed from the system and will therefore cause the aluminium to solidify. In the latter case, energy is used to convert thermal energy to electricity and possibly to district heating. The temperature level makes it possible to use the already proven and optimized steam based power plant technology for the power production at discharge. Even incorporation into existing power plants is possible resulting in an improved economy of the complete system. The project is based on a literature survey and a feasibility study, and the results from this project creates the basis for further studies regarding metal based latent heat storage technologies. The project results are compiled from literature surveys, expert interviews, simulations and considerations carried out by the project group. In general, the project findings can be divided into a part, Track 1 in Figure 1, describing the technological possibilities and identifying the main challenges (section 3.1 and 3.2) and a part, Track 2 in Figure 1, outlining the feasible technology applications from an economical point of view (section 3.3). The lastmentioned part estimates the value of the concerned energy storage technology in the current and future energy markets, which are two completely different scenarios concerning both the amount of available (renewable) energy for storage as well as the politically imposed taxes, subsidies, etc. In general, the Danish energy market discriminates most energy storage facilities due to, among other things, favourable biofuel subsidies making for instance wood chips a very attractive alternative to storing energy. Studies from this project indicate that maintaining a large pile of wood chips (e.g. as Verdo A/S does) is the most cost effective energy storage compared to all known energy storage technologies. However, the interesting perspective is how this turns out in the years to come with much more highly fluctuating renewable energy in the grid and a changed basis of taxation. Consequently, the yield of any energy storage 3

4 technology is very difficult to estimate, however, we anticipate that the demand for such technologies will grow due to, as mentioned before, a considerable increase in the consumed amount of renewable energy. Regarding the technological challenges, the results of this project point to the fact, that it is indeed possible to build a unit capable of containing a large volume of aluminium able to withstand several charge/discharge cycles. This project plan includes a number of work packages (Figure 1). The outcome of these work packages (WP2-6) are summarized in section 3. Figure 1: Project plan 4

5 2. Executive summary This report investigates the technical and economic feasibility of storing heat as the phase change energy of melting aluminium, i.e. as thermal energy at a high temperature. When a surplus of electrical or high temperature energy is available, the energy can be delivered into the storage system and thereby melt a quantity of aluminium at constant temperature at approximately 660 C. The opposite process will take place in case of discharging, i.e. the energy is removed from the system and will therefore cause the aluminium to solidify. In the latter case, energy is used to convert thermal energy to electricity and possibly to district heating. The temperature level makes it possible to use the already proven and optimized steam based power plant technology for the power production at discharge. Even incorporation into existing power plants is possible resulting in a better economic of the complete system. The investigation has not shown any fundamental issues with respect to realization of a high temperature phase change material (PCM) storage system based on melting/solidification of metal, but a number of engineering challenges need to be addressed and solved. The investigation has not shown fundamental issues with respect to realization of a high temperature phase change material (PCM) storage system based on melting/solidification of metal. However, the engineering challenges need to be addressed such as the mechanical stress due to expansion or thermal shock, insulation, material selection and corrosion, heat exchanger design, control, etc. Based on the assumptions set up for the analysis in Case 2 (see Table 1) the technology yields the lowest power cost. However, the results are very dependent on constraints such as tax on and cost of fuels and power transmission. 5

6 3. Project results A storage system utilized for superheating steam has been designed and analyzed in details showing that the system can deliver a very stable steam temperature. The feasibility of three cases has been analyzed and compared to competing solutions: 1. Storage parallel to the boiler in a conventional steam turbine based power plant 2. Storage for a high temperature concentrated solar power plant 3. Storage of excess wind power The results presented in Table 1 are very dependent on constraints such as tax on and cost of fuels and power transmission. For Case 1 the storage is not cost-effective today as the stored energy can be considered as an alternative fuel for the boiler. Instead biomass fuel is used in the boiler due to a very low price. The results for Case 2 and 3 are summarized in the table below showing the additional cost prices for including a storage technology. Here the aluminium heat storage was found to be the economically feasible solution. Additional cost per kwh [DKK/kWh] Aluminium latent heat storage Case 2: Storage connected to a solar power plant District heating is of no value Case 3: Storage supplied with excess electricity from wind farms District heating is of no value District heating is valuable Batteries > Diesel generators > 1.5 > 1.5 > 1.5 Table 1: Summarized cost estimates of various storage technologies. 3.1 WP2: Literature survey Referring to the project application [1], the objective of this WP is a review of the existing material and technologies concerning high temperature heat storage. In addition, this WP also concerns theoretical calculations and the involvement of experts in order to develop the energy storage concept as far as possible without conducting any experimental work. Consequently, this WP is primarily focused on the technical aspects of the overall concept, which is the storage of high temperature heat by letting a suitable material undergo a phase transition. In order to simplify the system design the energy storage medium must meet several requirements. Furthermore, the complete energy storage system must feature components for charging and discharging, as well as technology for data acquisition and system control. In the following section these above-mentioned challenges will be addressed by referring to relevant existing studies. WP3 and WP4 take this a step further and describe how a high temperature heat storage system could be designed based on the literature review and solutions provided by the project group. 6

7 3.1.1 Storing high temperature heat: Classification and design criteria When constructing a heat storage system several issues have to be taken into consideration. Firstly, the field of application and the character of the selected storage system have to be clarified. Some of the key storage parameters are capacity, energy density, charge/discharge rate and system costs. However, also environmental and safety issues have to be considered. In general, the heat storage can be divided into three different units [2]: The storage medium itself, a heat exchange system for charge/discharge and the storage system encapsulation. Within this categorization the most essential features to obtain when construction a heat storage are: high thermal conductance and energy density of the storage medium; good mechanical and chemical stability of the storage medium during several charge/discharge cycles; an efficient system for heat exchange; compatibility of the materials used for the storage medium, the heat exchanger and the system encapsulation; and low energy dissipation obtained by efficient insulation and an intelligent system design. Figure 2: Illustration of the five main challenges to address when constructing a thermal energy storage. Systems for charge/discharging, highly effective insulation to minimize energy dispersion, a container material able to withstand extreme thermal and mechanical stress and a suitable PCM material. Moreover, several parameters related to the overall field of application have to be considered as well, such as the total storage capacity, possible operation temperatures and how to integrate the storage system in the existing energy system. In the following section the above mentioned issues will be addressed by pointing out pros and cons regarding the choice of storage system. The intended use for the storage system is divided into two applications: electricity to electricity (combined heat and power plant) and electricity to heat (district heating). 7

8 The storage medium: Sensible or latent heat storage Primarily there are three methods of storing heat in a material as shown in Figure 3: By increasing the material temperature (sensible heat), by a phase change (latent heat). The storage material temperature dependence on the energy (heat) input is illustrated in Figure 4. The third method is chemical energy storage which Figure 3 is not covered by this report. Figure 3: Classification of energy storage materials [3]. For practical reasons, most often the solid liquid phase transition is utilized for storing latent heat. Generally, latent heat storage systems often have a higher energy density compared to sensible systems. Additionally, owing to the constant (and potentially high) discharge temperature, latent heat storage systems are highly energy efficient when used in connection with e.g. a heat engine for power production. However, the development of the PCM, the PCM container and the heat exchanger are facing some big challenges, especially regarding high temperature applications. Some of these challenges, and possible solutions, will be illustrated in the following section. Figure 4: Graph illustrating heat storage material temperature as a function of the added amount of heat On a general level, high temperature heat storage systems are classified as active or passive storage systems [2]. In passive systems the thermal storage medium itself does not circulate, the heat is instead transferred from the storage medium (e.g. a PCM) to a heat transfer fluid. On the other hand, an active heat storage also utilizes the storage medium itself as the heat transfer fluid. Active storage systems are typically applied for solar heat storage (see Figure 5), where molten salt is circulated in the system and used for driving a heat engine to produce electricity. 8

9 Figure 5: Molten salt is circulated in the system and consequently used as thermal storage material as well as heat transfer fluid. Adapted from [22]. For passive systems one material is used for thermal storage and another for heat transfer to the heat engine. This places certain demands on the storage material, the heat exchange system and the storage material encapsulation including the insulation. However, latent heat storage using a phase change material (PCM) has some essential advantages compared to sensible storage. These advantages, and the most substantial challenges in the establishment of a latent heat storage, will be discussed in the following sections Phase change material characteristics Latent heat storage systems most often utilize the enthalpy of fusion since other phase transitions (e.g. solid <-> gas, or solid <-> solid) are much more difficult to control, and furthermore are associated with a much lower heat capacity, e.g. see table 5 in [4]. Moreover, the phase change material (PCM) has to comply with a wealth of criteria, for instance the phase transition temperature has to match the specific application, also in order to optimize the heat capacity. In addition, the PCM has to meet a long list of technical requirements to ensure an efficient and cost-effective storage system [5]: A phase transition temperature at the desired operating temperature (conventional steam turbines operate at an input steam temperature around 500 C which makes aluminium PCM a suitable candidate with a solid <-> liquid phase transition at 660 C). High latent heat capacity in order to reduce the storage size. Congruent melting which is a fully reversible melting process. Some PCMs comprising of more complex constituents tend to decrease in heat capacity over time, because the PCM decomposes at high temperatures. Excellent heat conduction to avoid temperature gradients across the PCM volume and to facilitate a simple and efficient heat exchange system. The storage charge/discharge rate depends directly on the thermal conductivity. Minimal heat expansion close to the phase transition temperature in order to simplify the PCM encapsulation. 9

10 Sub-cooling must be avoided in order to control the storage discharge process precisely. Chemical compatibility of the PCM with the encapsulation to avoid corrosion. Toxic and inflammable PCMs should be avoided. The PCM must be inexpensive and abundant. Price (DKK/kJ storage capacity). Owing to the low energy density, phase transition temperature and poor thermal conductance, this report will not describe the PCMs consisting of organic and inorganic salts. Consequently, in the following section, metal based PCMs will be described, since these materials are relevant candidates for use as PCMs compared with the conventional salt based PCMs PCMs consisting of pure metals and metal alloys Only a few studies of the use of metals and alloys as PCMs are available [6], even though the thermo physical properties of metal based PCMs in general are superior to the salt based PCMs. However, studies suggest that aluminium and aluminium alloys are best suited as a PCM material. This is primarily due to the high heat capacity and the relatively low price of aluminium, resulting in a low price per stored energy unit [7], [8]. As illustrated in Figure 6, aluminium has a high heat capacity and a phase transition occurring in a practicable temperature range. Figure 6: Melting entropy as a function of melting temperature of different pure metals [7]. Studies of metal based PCMs often focus on various alloys, since most thermo physical properties can be adjusted by varying the alloy composition. For instance if silicium is added to an aluminium PCM, the heat capacity will increase significantly (see Figure 7) [9]. In addition, experimental studies of metal and alloy based PCMs indicate that the thermo physical properties are very stable through numerous melting cycles, and especially that the storage capacity and operating temperature remain stable [10]. 10

11 Figure 7: Melting temperature and latent heat capacity for various alloys [11]. Pure aluminium has a latent heat capacity of approximately 400 J/g. Figure 8 sums up why aluminium is preferred as a PCM, both when compared with other metals, and also when compared with e.g. salt based PCMs. In general, salts have a much lower heat capacity and also an extremely poor heat conductance ( times lower than metals), which makes the charge/discharge process very time consuming. The aluminium/silicium PCM is characterized by a high capacity, good thermal conductance, useful operating temperature and a low price compared to other metals with the same properties. 3 Cirkel-areal proportionalt til den latente varmekapacitet / J/g log(k) / Varmeledningsevne / W/m/K Zn (112) KNO3-NaNO3 (94) Al (397) AlSi12 (560) Cu (209) Ca (213) KNO3 (266) Tm / Smeltetemperatur / C Figure 8: Melting temperature as a function of the thermal conductance and the heat capacity (proportional to the circle area). 11

12 Encapsulating the PCM and an efficient heat exchange system for storage charge/discharge When designing the PCM container various problems have to be solved relating to e.g.: Charging (melting) the PCM (probably by resistive heating). Discharging by heat exchange with fluid in the secondary side (e.g. water or steam). Insulation in order to avoid heat dissipation. System to handle thermal expansion which induces mechanical stress. Chemical stability of the container (a ceramic material is probably needed to ensure compatibility of the container with the high temperature liquid metal). Several studies describe how to design such latent heat storages depending on the application area [9], [12], [13]. Containers made of e.g. steel seem to contaminate the PCM, and as mentioned above, corrosion is also a problem. However, studies show that ceramic materials are compatible and stable when used in combination with corrosive materials at very high temperatures [9]. Figure 9: High temperature heat storage with phase change material (31 kg Al/Si alloy, melting temperature 580 C, dimensions 59*75*17 cm 3 ). (1, 4, 7): Thermal insulation, (2): ceramic container, (5): Resistive heating. From [9]. The study described in [9] describes the construction of a high temperature latent heat storage designed for domestic heating (shown in Figure 9). The system utilizes a metal PCM (31 kg aluminium/silicium alloy) encapsulated in a ceramic material which, according to the authors, completely hinders corrosion. Surrounding the ceramic encapsulation, is a layer of steel (to prevent thermal radiation heat dissipation) and some ordinary insulation materials (to prevent conductive and convective heat dissipation). In order to charge the storage, the PCM is melted using two heating elements which are releasing 1500 W of power for 3.5 hours. During the discharge, heat is slowly released to the surroundings in order to maintain a comfortable indoor temperature for approximately 16 hours. This study illustrates several issues regarding the concept of PCM energy storage: Metals and alloys have a sufficiently high heat conductance is order to avoid temperature gradients. Consequently, charge and discharge of PCM systems are fast, and complicated heat exchange designs are not required. 12

13 Ceramic materials are useful as PCM containers, since no corrosion has been observed during the test period. Heat dissipation can be avoided by employing a metal shield for radiant heat insulation surrounded by more conventional materials with a low heat conductance Conclusion The literature survey presented in this section points to the fact, that a latent heat storage based on metals and alloys has several advantages compared to storage systems utilizing salt based PCMs. Especially in combination with steam production, aluminium has a suitable phase change temperature which is relevant in connection with e.g. a steam turbine system at combined heat and power plants. Moreover, this study shows that storage systems using metal based PCMs facilitates a much more simple design due to the excellent thermal heat conductance of the PCM. 3.2 WP3-4: Storage design and specifications, environmental and safety issues The outcome of WP3 and WP4 is outlined in this section in the form of ideas and sketches for use in the realization of a high temperature heat storage system using aluminium as PCM. The PCM is charged (liquefied) by electricity and discharged (solidified) by a heat exchange with water, driving a steam turbine for electricity production. The fundamental concept regarding the thermal energy storage application is illustrated in Figure 10, where the storage system is inserted instead of a conventional boiler. However, the high temperature thermal storage has various other potential applications as explained in the introduction, section 3. Figure 10: Illustration of the aluminium heat storage and the interplay with a conventional CHP system. Making use of this implementation, the heat storage becomes analogous to a boiler. It was decided to design and analyse the storage system in connection with a steam superheater due to the high risk of thermal shock when using the storage for direct steam production. The solutions presented in this section are extracts from a report prepared by Aalborg CSP A/S [14]. The report treats a numerical model for simulation of the discharge of an aluminium heat storage as sketched in Figure

14 3.2.1 Numerical model The numerical model used for this simulation is based on a numerical model (Mills, 2009), which is based on Resistance-Capacitance Formulation. This model is adjusted to include a steel pipe with at steam flow and to contain a phase change simulation in the aluminium Geometry The geometry of the model is based on a steel tube with an aluminium tube around it, as shown in Figure 11. Inside the steel tube a flow of dry steam flows. Heat from the charged hot aluminium is transferred through the tube into the steam, thereby increasing its temperature. This causes the aluminium to cool down. Figure 11: The modelled domain is a fictive aluminium tube surrounding a steel tube with a steam flow. The amount of aluminium in the model is the volume of the aluminium tube deducting the volume of the steel tube. This is sufficient to calculate the output of the storage. To model the heat transfer in the aluminium and steel, the domain is divided into a number of small cells. The model is a 2D model, where the two axes are radially from the steel tube surface and axially along the steel tube. The radial cells are made as rings, as the heat transfer is expected to be equal around the tube as shown in Figure 12. Figure 12: Radial cells for calculating heat transfer through aluminium Similarly the model is divided axially into a number of cells. Clearly the number of cells is determining the accuracy of the model. The more cells, the more accurate the model is. However, as this is a transient model, the number of calculations, and thereby the calculations become too time consuming. Therefore, the number of cells is selected to be 20 both radially and axially. As the tube is much longer than the diameter, the cells are very long and thin. This has, however, no significance on the accuracy of the model. 14

15 The calculation of the model is for one tube only, which gives a small mass flow. The total mass flow is increased by adding tubes. The steel tube is only modelled as a thermal resistance between the aluminium and the steam Material properties The storage model contains a steel tube and a PCM made purely from aluminium The aluminium PCM The aluminium for the storage is pure aluminium with the following properties [15]. Melting point: 660 C Density: 2702 kg/m³ Heat conductivity: W/mK (found by extrapolation) Specific heat capacity: J/kgK (found by extrapolation) Latent heat: 321 kj/kg Steel tube The steam at the Verdo plant is approximately 110 barg. Therefore, a design pressure of 121 barg is selected. As the storage temperature is around 660 C, a design temperature of 700 C is selected. The high pressure and temperature cause the material to be stainless steel, which unfortunately has a lower conductivity than carbon steel. The selected steel is 347H, which has the following properties [16]: Conductivity=21.4 W/mK. Due to the nature of the model, all other steel properties are obsolete Boundary conditions The aluminium storage is expected to be implemented in the Verdo plant which is a CHP with two boiler lines, generating 31.5 kg/s steam at 110 barg and 525 C each. The plant has three superheaters. To avoid problems with cooling of the boiler tubes, the storage is expected to be inserted after the last superheater, thereby the storage is supposed to perform as a fourth superheater add-on unit Boundary conditions for storage discharge simulation A number of conditions must be determined in order to make the simulation. This is described below. Steam Steam outlet temperature: 525 C Steam inlet temperature: 490 C (Two scenarios simulated) 500 C Steam pressure 111 bara The steam pressure before the turbine is 110 bara. To account for the pressure loss in the storage, the inlet steam pressure is 111 bara. 15

16 Geometry The tube thickness depends on the outer diameter. In order to withstand desired design pressure and temperature, a calculation according to ASME I is made. Tube outer diameter [mm] Tube thickness [mm] As the stainless steel tube has a low heat conductivity, it is required to have a tube as thin as possible. This, however, increases the steam velocity in the tube with a lower outlet temperature and an increased pressure loss as a consequence. To account for this, the mass flow is decreased. Tube length is selected in order to achieve an outlet temperature of 525 C. Aluminium tube is maintained at 150 mm. This means, that storage capacity is increasing with a smaller steam tube diameter Results A number of scenarios are calculated. Each calculation prints out a report as shown in Figure

17 Figure 13: Aluminium heat storage simulation report This report (sketched in Figure 13) shows the inlet conditions, the steam outlet temperature and the temperature distribution in the storage after the set time duration. Appendix B shows reports from the calculated scenarios. All scenarios are simulation with 1 hour of discharge (3600 seconds), except the two last, which have been simulated for 11 hours (39600 seconds) and 24 hours (86400 seconds). An important thing to notice in all calculations, except two specific simulation scenarios, is the very constant steam outlet temperature. As long as the storage is at melting temperature, the steam outlet temperature remains constant. Only in two specific simulation scenarios, where 11 and 24 hours of discharge are simulated, the steam outlet temperature starts to drop after approximately 10 hours. The temperature distribution in Figure 13 shows the temperature in the modelled domain. Here it should be taken into account that the radial cells (horizontal) are only covering m, while the axial cells (vertical) are covering 50 m. Most of the cells show a temperature of 17

18 600 C while the colour plot indicates a difference in temperature. This temperature difference is, however, very small. As it appears from the graph in Figure 13, the amount of energy discharged is only at 3%. This indicates that - with the given dimensions - the storage can support a discharge in 29 hours. This can also be seen from the amount of energy discharged at kj while the latent energy capacity is kj. The aluminium mass is 1404 kg. The capacity is different for each simulated scenario. Table 2: Results steam inlet temperature 490 C, outlet temperature 525 C Table 3: Results steam inlet temperature 500 C, outlet temperature 525 C Boundary conditions changed In the above sections a number of different scenarios are simulated. For each scenario the model is adjusted in order to achieve an outlet temperature of 525 C. This means that if for instance the mass flow is changed due to load variations, the steam outlet temperature changes, which appears from Figure

19 Figure 14: Steam outlet temperature at changed mass flow conditions Here it is clear that steam outlet temperature increases beyond the desired set point. It requires additional measures to keep the temperature at or below set point Conclusion The above simulations show that it is possible to use an aluminium based metal PMC storage for steam production. They indicate that a steady steam outlet temperature throughout the discharge period makes the aluminium storage ideal for steam superheating. A disadvantage of the proposed storage is the rigidness of the conditions under which it operates. In order to get the desired outlet temperature, mass flow and inlet temperature must be fixed. Different design and configurations are possible in order to reach higher flexibility without adding much complexity Example of construction This report has not looked into the matter of charging the storage, as this is not within the ACSP scope. Figure 15 gives a design idea of a potential storage construction. Figure 15: Illustration of the tube bundle inside the storage 19

20 The physical dimensions of the storage depends on a number of factors such as capacity (discharge time), power (number of tubes) and due to the very high heat conductivity of aluminium it is expected that a more detailed design analysis will show that a much more simple tube configuration and arrangement is possible. 3.3 WP5: Business potential, fields of application and system cost analysis In order to assess the business case of a metal based PCM storage, one must take into account the entire system in which the storage facility is placed as it can take on many different forms, which more or less could be suitable for the purpose of the storage system. For instance, the storage unit could be installed in parallel to a boiler in an existing power plant. Another option is to have heat delivered into the storage unit by collecting and concentrating solar energy or to connect the storage unit to a wind farm, which at times would supply heat to the storage unit by means of electricity. In the following examples it is presumed that an energy storage unit is the only way to produce electricity when the main source is not able to do so. In other words, the energy system is considered to be isolated from a common grid, which would otherwise be activated and deliver electricity from other suppliers Preconditions This study is based on a number of key figures which are used in the economic calculations of the individual cases Aluminium Properties of aluminium: Latent heat of phase change 300 kwh/m 3 Density 2800 kg/m 3 Latent energy content kwh/kg Cost of aluminium 10 DKK/kg 1 Cost of an entire aluminium storage unit: assumed 6 x price of aluminium = 60 DKK/kg 1 Investments for an aluminium based energy storage unit is then 60/ DKK/kWh heat Assuming an efficiency of 35 % of converting thermal energy to electricity, the investments for electricity is 560/0.35 = 1600 DKK/kWh electricity A great advantage of the metal based PCM storage is that the metal itself will not be destroyed or polluted and thereby have a significant scrap value. This is not taken into consideration in the following analysis Battery storage (electrical energy) Cost of battery storage system DKK/kWh [17] 1 The cost of aluminium has dropped during the last couple of years, but the total cost of the aluminium phase change energy storage unit is thought to remain more or less the same since the deciding factor is the plant equipment rather than the aluminium itself. Thus, if the aluminium price is further reduced, the total plant costs would correspond equivalently to a higher factor of the aluminium cost price. An estimation of the total energy storage unit cost is presented in Appendix A on page

21 Diesel generator Cost of diesel generator DKK/kW electricity [18] Cost of fuel 4.75 DKK/liter Heating value of fuel 10 kwh Cost of fuel at 40 % efficiency of engine/generator 1.5 DKK/kWh Wind farm Assumed price of wind turbines 7200 DKK/kW [19] Assumed capacity factor of wind turbines of 50 % (like the Danish wind farm Horns Rev ) Steam turbine and generator Cost of steam turbine and generator 3600 DKK/kW [20] The analyzed configurations Two cases are considered in this study; one of the cases is calculated in two versions with different assumptions: 1. Aluminium heat storage unit connected to a concentrated solar power plant o Assumption: District heat is of no value 2. Aluminium heat storage unit supplied with excessive electricity from wind farms o Assumption: District heat is of no value o Assumption: District heat is valuable All cases compare investment costs (no operation or maintenance costs are included) of the three competing energy storage technologies: Aluminium latent heat storage, batteries and diesel generators. The background for selecting these technologies is the fact that they are all available for large-scale operation in Denmark. The geographical limitation naturally excludes technology like pumped hydro which is otherwise a well-known storage solution. The competing technologies are approached as equally as possible, but a few additional facts are to be kept in mind. For instance, the service life of the different technologies is not a part of this study, but in reality the aluminium storage would maintain its thermal capacity while the capacity of batteries would degrade over time. In this context the Compressed Air Energy Storage (CAES) technology is disregarded as a comparable storage technology, as heat is removed from the compression process, which means that a heat source, e.g. by burning natural gas, is needed when discharging the storage. However, the developers of CAES are working on a solution for storing the compression heat when compressing air. This would enable an isothermal compression of air which increases the efficiency of the storage process. LightSail Technologies is one of currently two developers of CAES with isothermal compression, but a scientific study has not yet been submitted proving the claimed system efficiency. Because of this, the isothermal CAES technology is not included in the present study, but if indeed the technology was to be proven and published scientifically, it should be part of economic comparative studies like the one in question. 21

22 The feasibility of three cases has been analyzed and compared to competing solutions: 1. Storage in parallel to the boiler in a conventional steam turbine based power plant 2. Storage for a high temperature concentrated solar power plant 3. Storage of excess wind power Case 1 covers the plant configuration where the storage is installed parallel to a conventional steam boiler. This way the storage can be operated as a full substitution of the operation of the boiler. Case 2 investigates a solar power plant which delivers heat to a steam cycle producing electricity. A portion of the heat is used to melt aluminium in the heat storage unit, allowing the energy to be used at some point in time where the production of electricity is feasible, but cannot be supplied directly through the collection of solar energy, e.g. at night or when the weather is cloudy. Case 3 concerns a system where the aluminium heat storage unit is supplied with excess electricity from wind turbines/farms. Unlike case 1, there is an additional cost of an extra steam turbine and generator, since the storage unit is not connected to a power plant turbine Conclusions on business potential Calculations carried out for Case 2 show that under these conditions, a heat storage unit based on aluminium is by far the most economically feasible choice. This was also the result for Case 3, both in terms of assuming district heating as being valuable and not valuable. Please, see Table 4 for the results of additional cost prices for including a storage technology. Additional cost per kwh [DKK/kWh] Aluminium latent heat storage Case 2 Storage connected to a solar power plant District heating is of no value Case 3 Storage supplied with excess electricity from wind farms District heating is of no value District heating is valuable Batteries > Diesel generators > 1.5 > 1.5 > 1.5 Table 4: Additional costs of storage technologies per kwh of produced electricity Figure 16 illustrates how the latent heat storage compares to other technologies, and given the calculations and assumptions made here, the aluminium heat storage unit is very cost effective and should be able to compete on the energy storage market. 22

23 Figure 16: Comparison of various energy storage technologies. The aluminium PCM heat storage unit (Al HSU) has a capital cost of approximately 240 USD/kWh. Adapted and modified from [21] Case 1: Aluminium heat storage installed parallel to a power plant boiler In the following case it is presumed that the aluminium heat storage unit will act as an extra boiler in an existing steam power plant using wood chips, straw or natural gas as fuel. In this case, it is important to emphasise that the heat storage unit will not provide the system with any additional power capacity, since the installed turbine and generator will already be operating at full capacity when extra power is needed. The maximum power production of the plant might be supplied by burning fuel in the boiler as well as by exploiting the heat storage unit. In other words, the heat storage unit can be thought of as an alternative fuel storage unit. However, since steam power plants normally use cheap fuels, it is almost impossible for the aluminium based heat storage unit to be competitive purely on economic terms. In Denmark, extra taxes would be imposed on such a system, which would naturally worsen the economic picture. In addition, today the owner of the storage system would still have to pay a gridtariff for the cheap electricity used to melt the aluminium when charging the storage unit. One could argue that the aluminium heat storage unit would provide the plant with a more rapid regulation of the steam power than a normal boiler and thereby offer a mean of regulation at various loads. However, the effect of this requires a more thorough investigation in order to be fully uncovered Case 2: Aluminium heat storage connected to a solar power plant In this context, it is presumed that a number of solar collectors supply a steam cycle with heat, which in the end powers a generator and produces electricity. Some of the heat is used to melt aluminium in the storage unit, and this ability to store heat can be used to drive the steam cycle at night or when varying solar radiation occurs, e.g. in cloudy weather. When evaluating the economy of this kind of system, the following questions must be considered: What is the actual benefit? What is the cost of the benefit? 23

24 In the present case, the benefit is production of electricity, when the sun cannot supply the required input of energy, and the costs are the payment and interests of having a heat storage unit installed. A steam turbine and a generator already exist in the solar power plant. Therefore, no extra investments are needed for this purpose. Moreover, district heating is assumed to be without any value in this case, since it is normally not an attractive commodity in countries where concentrated solar power utilised. An argument could be that a larger consumption of electricity at night would require an enlarged solar collection area which adds to the overall costs of payments and interests. However, this would also be the case for any other energy storage technology, e.g. batteries. Thus, this effect can be disregarded as long as the study exclusively concerns storage technologies Example: 1 kwh electricity to be stored A simple calculation can be carried out by assuming the purchase of an aluminium energy storage unit with the capacity of 1 kwh electricity, which is to be used every night. The cost of this has been determined in section at 1600 DKK/kWh, and the unit would therefore deliver 365 kwh/year. If the charges for this solution are assumed to be 8 % of the cost of the storage (3 % interests and 5 % payment), the annual costs come to: 0.08 * 1600 = 128 DKK and in terms of additional costs for having the option of storing electricity: 128/365 = 0.35 DKK/kWh. In comparison, a battery storage system costs DKK/kWh and would in this case require a larger turbine and generator to reach the same output because the batteries are charged during the day alongside the regular electricity production. This complicates a simple price calculation, but for the sake of completion of this study, the added cost of storage would be at least (0.08 * 4500)/365 = 0.98 DKK/kWh. Finally, a diesel generator set costs around DKK/kW electricity. The price of diesel oil on the world market without taxes is roughly 3 USD/gallon (4.75 DKK/liter), and one liter of diesel oil contains a heating value of around 10 kwh. With an assumed efficiency of the engine and the generator of 40 %, the fuel costs alone would be 1.5 DKK/kWh. Interests and payments for the unit with six hours of nightly operation would be 2000 * 0.08/(365 * 6) = 0.07 DKK/kWh. Furthermore, maintenance costs should be added in order to provide an accurate picture. In this case, the energy storage based on the latent heat of aluminium is by far the most economically feasible choice Case 3: Aluminium heat storage supplied with electricity from wind turbines In this case, a number of grid-connected wind turbines are assumed available for an aluminium heat storage unit. When a surplus of electricity from the wind farm is produced, it is used in the storage unit to melt aluminium. When there is an increased demand for electricity, the heat storage unit will drive a generator by a steam cycle. However, constructing a business case for this example is a bit more complicated, since the needed amount of storage capacity must be considered; whether or not district heating is a valued commodity and, if this is the case, whether the need for district heating complies with the present production or not. 24

25 Assumption: District heating is without any value As an example, considering that district heating is not of any value to the surrounding community, the storage unit should have 12 hours of electricity production, which is being delivered 200 times every year. In other words, we are considering an energy storage unit with 12 kwh of capacity, the power of 1 kw, and an annual conversion of 2400 kwh. Going through the competing storage technologies again with these assumptions and starting with a battery storage, the investments would be 12 * 4500 = DKK. Presuming a round trip efficiency of 80 %, 600 kwh is missing according to the target which corresponds to a 0.14 kw wind turbine or about 1000 DKK. This leads to an additional cost per kwh of produced electricity of (55000 * 0.08)/2400 = 1.83 DKK/kWh. In terms of the aluminium storage unit, the price calculations require a more complicated approach. Firstly, the storage unit cannot stand alone, but it needs additional investments in a turbine and a generator, since an existing power plant is not available in this example. Furthermore, more wind turbines would need to make it up for the aluminium steam plant s relatively low efficiency of the electricity production compared to the other storage technologies. For the aluminium energy storage unit, the investments are 12 * 1600 = DKK. The turbine and generator are about 3600 DKK/kW. Assuming a round trip efficiency in terms of electricity of 40 %, 3600 kwh electricity equivalent to a wind turbine of 0.82 kw or 6000 DKK is missing. The additional cost per kwh of produced electricity amounts to * 0.08/2400 = 0.96 DKK/kWh. A diesel generator is, as mentioned above, very expensive compared to the other technologies with more than 1.5 DKK/kWh in fuel expenses alone, and it is therefore not dealt with further in this study. In this case, the aluminium-based latent heat storage is, from an economic point of view, the best solution. 25

26 Assumption: District heating is valuable In the case where district heating has an actual value, the calculations should take the relationship between the demand for electricity and the demand for district heating into account. For the sake of this example, and to keep the calculations as simple as possible, it is assumed that the demand for district heating is in exact compliance with the actual amount of district heating produced by the steam power plant. If the demand is increased, it is covered by something else for all storage technologies. In other words, it is assumed that the installed wind power is sufficient for covering both electricity and district heating demands. The steam power plant is assumed to have an electricity production efficiency of 35 %, because district heating is used for the condensation process. This yields 4460 kwh heat for a 1 kw plant. The aluminium energy storage: 12 * 1600 = DKK. The turbine and the generator are about 3600 DKK/kW. In total, DKK or an additional cost per kwh of produced electricity of (22800 * 0.08)/2400 = 0.76 DKK/kWh. For batteries, the investment would be 12 * 4500 = DKK. However, it is rather unlikely that any waste heat from charge and discharge processes could be used for district heating. Assuming a round trip efficiency of 80 %, 600 kwh corresponding to a wind turbine of 0.14 kw or about 1000 DKK is missing. The added cost is then (55000 * 0.08)/2400 = 1.83 DKK/kWh. In the case of district heating being of some value, the aluminium latent heat storage is the most feasible choice. 26

27 4. Utilization of project results The results of this project form a solid foundation for a further project which will concentrate on experimental research concerning a latent heat storage using aluminium as PCM. Based on the business cases presented here, and based on the new experimental findings, a new business and application analysis will also be carried out in order to reassess the potential of this technology. If this technology turns out to be profitable, the associated companies (project partners) will benefit directly from these preliminary studies and also from the future experimental studies which will be carried out at the end of Spin-off technologies are also very likely to emerge which will also favour the participating private companies. Spin-off technologies can be of technical character. However, the participating partners can also benefit from the conducted market analysis, business cases and comparison of different energy storage systems in order to better understand and profit from the future energy market. 27

28 5. Project conclusion and perspective This project clearly indicates the great potential of latent heat storage systems relying on metal based PCMs. The project investigates the technical and economic feasibility of storing energy as the latent phase change energy of melting aluminium, i.e. as thermal energy at a high temperature. When a surplus of energy is available, the energy can be delivered into the storage system and thereby melt a quantity of aluminium at a constant temperature of approximately 660 C. The opposite process will take place in case of discharging, i.e. the energy is removed from the system and will therefore cause the aluminium to solidify. In the latter case, energy is used to convert thermal energy into electricity and possibly district heating. The investigation has not shown fundamental problems with respect to realization of a high temperature metal based phase change material (PCM) storage system. However, engineering challenges need to be addressed like mechanical stress due to expansion or thermal shock, insulation, material selection and corrosion, heat exchanger design, control, etc. A storage system utilized for superheating steam has been designed and analyzed in details. The feasibility of three cases have been analyzed and compared to competing solutions: 1. Storage parallel to the boiler in a conventional steam turbine based power plant 2. Storage for a high temperature concentrated solar power plant 3. Storage of excess wind power Based on the assumptions set up for the analysis only in Case 2, the technology gives the lowest power price. But the results are very dependent on constraints like tax on and cost of fuels and power transmission. This project has focused on the use of a latent heat storage charged by excess electricity from e.g. wind turbines. However, several other applications involving high temperature heat are also relevant like for instance fuel cells (SOFC) and electrolyser cells (SOEC) or industry processes where high temperature heat are dissipated. 28