WO2024020322A1 - Regenerative brayton based thermal energy storage system - Google Patents

Abstract

A regenerative Brayton based thermal energy storage system includes two rock bed tanks, at least one compressor and at least one expander (e.g., turbine) that provide a heat pump. The heat pump is operated in a charging mode to use electricity to store thermal energy. The heat pump is operated in a reverse or discharging mode to generate electricity with the stored thermal energy.

Description

REGENERATIVE BRAYTON BASED THERMAL ENERGY STORAGE SYSTEM

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

[0001] Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57. This application claims priority to U.S. Provisional Patent Application No. 63/368,839 filed July 19, 2022, the entirety of which is incorporated herein by references and is considered part of the specification.

BACKGROUND

Field

[0002] The present invention is directed to a thermal energy storage system and more particularly to a thermal energy storage system utilizing two volumes of rocks for thermal energy storage and as heat exchangers.

Description of the Related Art

[0003] There is an increased focus on reducing the use of fossil fuels to reduce the greenhouse gas emissions to the atmosphere. Power generation from renewable energy sources (e.g., solar power such as concentrated solar power, wind power, hydroelectric power, biomass, etc.) continues to grow. However, many of these renewable energy sources (e.g., solar power, wind power) are intermittent and unpredictable, limiting the amount of electricity that can be delivered to the grid from intermittent renewable energy sources.

SUMMARY

[0004] There is a need for thermal energy storage systems that can be used to store energy (e.g., generated using renewable energy sources), for example during the day, and that can be used to generate electricity, for example at night (e.g., when renewable energy sources are not in operation).

[0005] In accordance with one aspect of the disclosure, a regenerative Brayton based thermal energy storage system is provided. The system includes two rock bed tanks, at least one compressor and at least one expander (e.g., turbine) that provide a heat pump. The heat pump is operated in a charging mode to use electricity to store thermal energy. The heat pump is operated in a reverse or discharging mode to generate electricity with the stored thermal energy.

[0006] In some aspects, the techniques described herein relate to a regenerative Brayton based thermal energy storage system. The thermal energy storage system includes a first tank filled with a first heat storing material and a second tank filled with a second heat storing material. The thermal energy storage system also includes and at least one compressor in fluid communication with the first tank and the second tank and at least one expander in fluid communication with the first tank and the second tank. The at least one compressor and the at least one expander provide a heat pump. The heat pump is operable in a charging mode to store thermal energy in the first heat storing material of the first tank. The heat pump is further operable in a discharging mode to generate electricity with the stored thermal energy in the first tank.

[0007] In some aspects, the techniques described herein relate to a method for operating a regenerative Brayton based thermal energy storage system. The method includes operating one or more compressors and one or more expanders in fluid communication with a first tank and a second tank to store thermal energy in a first heat storing material in the first tank in a charging mode and to generate electricity with stored thermal energy in the first tank in a discharging mode. Operating in a charging mode include delivering a flow of air to the second tank, flowing the air through a second heat storing material in the second tank, and exhausting the flow of air from the second tank at a higher temperature. Operating in a charging mode also includes flowing the exhausted flow of air from the second tank to one or more compressors and compressing said air. Operating in a charging mode also includes delivering the flow of compressed air to the first tank, flowing the compressed air through the first heat storing material in the first tank, and exhausting the flow of air from the first tan. Additionally, operating in a charging mode includes flowing the exhausted flow of air from the first tank to one or more expanders and expanding said air.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a schematic view of a regenerative Brayton based thermal energy storage system. [0009] Figure 2 is a schematic view of a regenerative Brayton based thermal energy storage system.

[0010] Figure 3 is a schematic view of a regenerative Brayton based thermal energy storage system.

[0011] Figure 4 A is a schematic view of a tank illustrating a change in a thermocline in the material in the tank during a charging process.

[0012] Figure 4B is a schematic view of a tank illustrating a change in a thermocline in the material in the tank during a discharging process.

DETAILED DESCRIPTION

[0013] Disclosed herein are Brayton based thermal energy storage systems operable to store electricity as thermal energy when the system is operated in a charging mode and operable to generate electricity from thermal energy when the system is operated in a discharging mode. The systems disclosed herein utilize only two volumes of thermal storage medium (e.g., rocks, gravel), which advantageously also operate as heat exchangers that effect heat transfer with the medium (e.g., air) that flows through the volumes of thermal storage medium. In one implementation, as described further below, the system is an open loop system. The system provides a heat pump that operates in a charging mode to store energy, and operates in a discharging mode to generate electricity.

[0014] Figure 1 shows a schematic view of a Brayton based thermal energy storage system 1000 (the “system”) that includes a first tank 100 filled with rocks (e.g., gravel) and a second tank 200 filled with rocks (e.g., gravel). In one implementation, the second tank 200 can be buried in the ground. The rocks (e.g., gravel, pebbles) can in one example be rounded pieces and approximately 30-40 mm in size (e.g., diameter, width). Such rocks can be of the type used to make concrete. The first tank 100 can be a hot thermal storage tank that operates under pressure (e.g., at 3 bar) and the second tank 200 can be a warm thermal storage tank (e.g., that operates at a lower temperature than the first tank 100), and operate at atmospheric pressure (e.g., at 1 bar). In one implementation, the volume of rocks in the first tank 100 is about the same as the volume of rocks in the second tank 200. In one example, the first tank 100 and the second tank 200 can each have an outer diameter of about 30 m and have a height of about 20 m. The first tank 100 can include an outer steel jacket, an insulating material disposed adjacent an inner surface of the outer steel jacket, and a concrete pressure vessel disposed inward of the insulating material so that the insulating material is interposed between the concrete pressure vessel and the steel jacket. The concrete pressure vessel can define a chamber that receives the rocks (e.g., gravel, pebbles). Advantageously, use of the insulating material between the concrete pressure vessel and the steel jacket allowed the steel jacket to be made of carbon steel (e.g., because the steel jacket is thermally insulated by the insulating material from the rocks in the concrete pressure vessel).

[0015] With configured reference to Figure 1, the system 1000 includes a compressor 300 and an expander or turbine 400 (e.g., interchangeable expander or turbine) in fluid communication with the first and second tanks 100, 200. The system 1000 can operate in a charging mode 120 (e.g., use electricity to store thermal energy in the first tank 100) and in a discharging mode 170 (e.g., to generate electricity from thermal energy in the first tank 100). The charging mode 120 is shown in FIG. 1 by the dash-dot-dash arrows and the discharging mode 170 in shown in FIG. 1 by the dashed arrows. Optionally, an air filter 150 can be positioned upstream of the compressor 300 and/or upstream of the expander or turbine 400.

[0016] In the charging mode 120 (e.g., to store thermal energy), air flow enters 122 (e.g., delivered to) the second tank 200 (e.g., via the bottom end 202 of the second tank 200) and exits 124 (e.g., exhausts from) the second tank 200 (e.g., via the top end 204 of the second tank 200). In one example, the air enters 122 the second tank 200 at a temperature (e.g., ambient temperature) of about 15°C and a pressure of about 1 bar, and at a flow rate of 315 kg/s and exits 124 the second tank 200 at a temperature of about 393 °C. The heated air then enters 126 the compressor 300, where the air is compressed, and exits 128 the compressor 300 at a pressure of about 3 bar and a temperature of about 650°C . The air flow then enters 130 the first tank 100 (e.g., via the top end 104 of the first tank 100) and flows through the packed rock bed in the tank 100 to transfer heat to the rocks. The air flow exits 132 the first tank 100 (e.g., via the bottom end 102 of the first tank 100) at a pressure of about 3 bar and a temperature of about 156°C. The air flow then enters 134 the turbine 400, where the air is expanded (e.g., the lower temperature air is expanded), and exits 136 the turbine 400 at a pressure of about 1 bar and a temperature of about 57°C. In the charging mode 120, the compressor 300 can operate with about 81 MW of power and the turbine 400 can generate about 31 MW of power, with the difference of about 50 MW of power being provided to the system 1000 by an electric motor operated, for example, by the electric grid 165 or via renewable energy (e.g., wind, solar such as concentrated solar power, etc.).

[0017] In the discharging mode 170 (e.g., to use thermal energy to generate electricity and, for example transfer it to the electric grid 165), air flows enters 172 the compressor 300 at a temperature (e.g., ambient temperature) of about 15°C and a pressure of about 1 bar, where the air is compressed, and exits 174 the compressor 300 at a pressure of about 3 bar and a temperature of about 126 °C . The air flow then enters 176 (e.g., delivered to) the first tank 100 (e.g., via the bottom end 102 of the first tank 100) and flows through the packed rock bed in the tank 100, where the air is heated by the rocks. The air flow exits 178 (e.g., exhausts from) the first tank 100 (e.g., via the top end 104 of the first tank 100) at a pressure of about 3 bar and a temperature of about 620°C. The air flow then enters 180 the turbine 400, where the air is expanded, and exits 182 the turbine 400 at a pressure of about 1 bar and a temperature of about 415°C. The airflow is then directed toward the second tank 200 where it enters 184 the second tank 200 (e.g., via the top end 204 of the second tank 200) and flows through the second tank 200 to transfer heat to the rocks in the second tank 200. The air flow exits 186 the second tank 200 (e.g., via the bottom end 202 of the second tank 200 at a pressure of about 1 bar and a temperature of about 37°C. In the discharging mode 170, the compressor 300 can operate with about 35 MW of power and the turbine 400 can generate about 65 MW of power, with the difference of about 29.8 MW of power being provided to a generator 160 of the system 1000 (e.g., which can transfer the power to the electric grid 165).

[0018] In the implementation described above, the efficiency of the compressor 300 is 87%, the efficiency of the turbine 400 is 92%, and the compression ratio in the charging mode 120 and discharging modes 170 is 2.7 (e.g., minimum compression ratio). In another implementation, the compression ratio of the charging mode 120 and the discharging mode 170 can be different. The temperature difference in the first tank 100 between the air entering 130 the first tank 100 during the charging mode 120 and the air exiting the first tank 100 during the discharging mode 170 due to rock storage is 30°C (e.g., difference between 650°C in charging mode and 620°C in discharging mode). The system 1000 has an efficiency of approximately 59.4% (e.g., ratio of power generated, 29.8 MW, in the discharging mode to power input, 50.1 MW, in the charging mode). Over a period of 12 hours of operation, the system 1000 can deliver about 357 MW-hr of power (e.g., 29.8MW x 12 hours), for example to the electric grid 165, at a cost of $68/KW-hr.

[0019] Figure 2 shows a schematic view of a Brayton based thermal energy storage system 1000’ (the “system”). Some of the features of the system 1000’ are similar to features of the system 1000 in FIG. 1. Thus, reference numerals used to designate the various components of the system 1000’ are identical to those used for identifying the corresponding components of the system 1000 in FIG. 1, except that a “ ’ ” has been added to the numerical identifier. Therefore, the structure and description for the various features of the system 1000 in FIG. 1 and how it’s operated are understood to also apply to the corresponding features of the system 1000’ in FIG. 2, except as described below.

[0020] The system 1000’ differs from the system 1000 in that the compressor 300’ and turbine 400’ are mounted on one shaft 250’ and connected to a motor-generator 500’. In the charging mode, power is provided by the motor-generator 500’ (e.g., via a renewable energy source) to power the system 1000’. hr one implementation, the system 1000’ can have one or more gear boxes 600’ (e.g., two gear boxes, clutches ) to change the rotational speed (e.g., of the turbine 400’, of the compressor 300’) between the charging mode 120 and discharging mode 170. In one example, the one or more gear boxes 600 can have a ration of 3:1. In another implementation, instead of a gear box 600’, electronics can be used to change the rotational speed of the compressor 300’ and the turbine 400’. In another implementation, the motor and generator can be separate components. When charging, the compressor 300’ can rotate more quickly and the turbine 400’ can rotate more slowly, and when discharging, the compressor 300’ can rotate more slowly and the turbine 400’ can rotate more quickly. This is because though there is the same amount of mass flow in the charging modes 120’ and discharging modes 170’, the density is different between the different modes. For example, to move the same mass flow, when the density is lower (e.g., higher temperature air), the compressor 300’ needs to rotate more quickly.

[0021] In the discharging mode 170’, power is provided by the system 1000’ to the motor-generator 500’, which can be transferred, for example, to the electric grid 165’. The system 1000’ also includes valves V (e.g., three-way valves) that connect the different flow paths, where the valves V can he operated (e.g., selectively operated by a controller) to effect a particular flow path when in the charging mode 120 ‘(^c-g-> ^scc solid arrows) and a different flow path when in the discharging mode 170’ (e.g., see dashed arrows).

[0022] Figure 3 shows a schematic view of a Brayton based thermal energy storage system 1000” (the “system”). Some of the features of the system 1000” are similar to features of the system 1000 in FIG. 1. Thus, reference numerals used to designate the various components of the system 1000” are identical to those used for identifying the corresponding components of the system 1000 in FIG. 1, except that a “ ” ” has been added to the numerical identifier. Therefore, the structure and description for the various features of the system 1000 in FIG. 1 and how it’s operated are understood to also apply to the corresponding features of the system 1000” in FIG. 3, except as described below.

[0023] The system 1000” differs from the system 1000 in that it has two separate compressors 300A, 300B and two separate expanders or turbines 400A, 400B. Not shown are the generator and electric motor, which can operate as described above for the system 1000. In the charging mode 120”, the compressor 300A and expander or turbine 400A operate, and in the discharging mode 170”, the compressor 300B and expander or turbine 400B operate. The different equipment (e.g., compressors, turbines) can be sized symmetrically or asymmetrically depending on how the system 1000” is to operate (e.g., to charge in 10 hours but discharge in 20 hours). Though the system 1000” shows the use of two compressors 300A, 300B and two turbines 400A, 400B, in another implementation, the system can have two compressors but one turbine (e.g., that operates in both charging and discharging modes). In another implementation, the system can have two expanders or turbines but one compressor (e.g., that operates in both charging modes 120” and discharging modes 170”).

[0024] Figures 4A-4B schematically show thermoclines T in the first tank 100 during operation in a charging mode (FIG. 4A) and discharging mode (FIG. 4B), where the hottest portion of the tank is toward the top. During the charging mode, shown in FIG. 4A, the heated air enters 130 the first tank 100 from the top end 104 and moves the thermocline T down as it passes therethrough (e.g., transferring heat to the rocks and increasing the temperature of more of the rocks in the tank), and exits 132 the bottom end 102 of first tank 100 at a lower temperature (e.g., in a warm state). In the discharging mode 170, shown in FTG. 4B, the warm air 176 enters the first tank 100 from the bottom end 102 and moves the thermocline T up as it passes therethrough (c.g., absorbing heat from the rocks and decreasing the temperature of more of the rocks in the tank), and exits 178 the top end 104 of the first tank 100 at a higher temperature (e.g., in a hot state). In one implementation, about 1/3 to Vi of the rocks in the first tank 100 will be used in the charging modes 120 and discharging modes 170.

[0025] While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the systems and methods described herein may be made without departing from the spirit of the disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure. Accordingly, the scope of the present inventions is defined only by reference to the appended claims.

[0026] Features, materials, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example are to be understood to be applicable to any other aspect, embodiment or example described in this section or elsewhere in this specification unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The protection is not restricted to the details of any foregoing embodiments. The protection extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0027] Furthermore, certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations, one or more features from a claimed combination can, in some cases, be excised from the combination, and the combination may be claimed as a subcombination or variation of a subcombination.

[0028] Moreover, while operations may be depicted in the drawings or described in the specification in a particular order, such operations need not be performed in the particular order shown or in sequential order, or that all operations be performed, to achieve desirable results. Other operations that are not depicted or described can be incorporated in the example methods and processes. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the described operations. Further, the operations may be rearranged or reordered in other implementations. Those skilled in the art will appreciate that in some embodiments, the actual steps taken in the processes illustrated and/or disclosed may differ from those shown in the figures. Depending on the embodiment, certain of the steps described above may be removed, others may be added. Furthermore, the features and attributes of the specific embodiments disclosed above may be combined in different ways to form additional embodiments, all of which fall within the scope of the present disclosure. Also, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products.

[0029] For purposes of this disclosure, certain aspects, advantages, and novel features are described herein. Not necessarily all such advantages may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the disclosure may be embodied or carried out in a manner that achieves one advantage or a group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.

[0030] Conditional language, such as “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements, and/or steps. Thus, such conditional language is not generally intended to imply that features, elements, and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements, and/or steps arc included or arc to be performed in any particular embodiment.

[0031] Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z. Thus, such conjunctive language is not generally intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z.

[0032] Language of degree used herein, such as the terms “approximately,” “about,” “generally,” and “substantially” as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately”, “about”, “generally,” and “substantially” may refer to an amount that is within less than 10% of, within less than 5% of, within less than 1% of, within less than 0.1% of, and within less than 0.01% of the stated amount. As another example, in certain embodiments, the terms “generally parallel” and “substantially parallel” refer to a value, amount, or characteristic that departs from exactly parallel by less than or equal to 15 degrees, 10 degrees, 5 degrees, 3 degrees, 1 degree, or 0.1 degree.

[0033] The scope of the present disclosure is not intended to be limited by the specific disclosures of preferred embodiments in this section or elsewhere in this specification, and may be defined by claims as presented in this section or elsewhere in this specification or as presented in the future. The language of the claims is to be interpreted broadly based on the language employed in the claims and not limited to the examples described in the present specification or during the prosecution of the application, which examples are to be construed as non-exclusive.

[0034] Of course, the foregoing description is that of certain features, aspects and advantages of the present invention, to which various changes and modifications can be made without departing from the spirit and scope of the present invention. Moreover, the devices described herein need not feature all of the objects, advantages, features and aspects discussed above. Thus, for example, those of skill in the art will recognize that the invention can be embodied or carried out in a manner that achieves or optimizes one advantage or a group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. Tn addition, while a number of variations of the invention have been shown and described in detail, other modifications and methods of use, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is contemplated that various combinations or subcombinations of these specific features and aspects of embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the discussed devices.

Claims

WHAT IS CLAIMED IS:

1. A regenerative Brayton based thermal energy storage system, comprising: a first tank filled with a first heat storing material; a second tank filled with a second heat storing material; and at least one compressor in fluid communication with the first tank and the second tank; at least one expander in fluid communication with the first tank and the second tank, the at least one compressor and the at least one expander providing a heat pump, wherein the heat pump is operable in a charging mode to store thermal energy in the first heat storing material of the first tank, and wherein the heat pump is operable in a discharging mode to generate electricity with the stored thermal energy in the first tank.

2. The thermal energy storage system of claim 1, wherein the second tank is buried underneath a ground surface.

3. The thermal energy storage system of claims 1-2, wherein the at least one compressor is two compressors.

4. The thermal energy storage system of claims 1-3, wherein the at least one expander is at least one turbine.

5. The thermal energy storage system of claim 1, wherein the first heat storing material and the second heat storing material are a same material.

6. The thermal energy storage system of claims 1-5, wherein the second tank operates at a lower temperature than the first tank.

7. The thermal energy storage system of claims 1-6, wherein the first heat storing material and the second heat storing material are a plurality of rocks.

8. The thermal energy storage system of claim 7, wherein the plurality of rocks are rounded.

9. The thermal energy storage system of claims 7-8, wherein the plurality of rocks are approximately 30 mm in size.

10. The thermal energy storage system of claims 1-9, wherein the first tank includes an outer steel jacket and an insulating material disposed adjacent to an inner surface of the outer steel jacket.

1 1. The thermal energy storage system of claim 10, wherein a concrete pressure vessel is disposed inward of the insulating material.

12. The thermal energy storage system of claims 10-11, wherein the outer steel jacket is made of carbon steel.

13. The thermal energy storage system of claims 1-12 wherein a volume of the first heat storing material in the first tank is approximately equal to a volume of the second heat storing material in the second tank.

14. The thermal energy storage system of claims 1-13, wherein the at least one expander is configured to generate power deliverable to an electric grid.

15. The thermal energy storage system of claims 1-14, wherein at least one air filter is in fluid communication with the at least one compressor.

16. The thermal energy storage system of claims 1-15, further comprising a motorgenerator connected to a shaft, wherein the at least one compressor and the at least one expander are coupled to the shaft.

17. The thermal energy storage system of claims 1-16, further comprising one or more gear boxes, wherein the one or more gear boxes are configured to change a rotational speed of the at least one expander.

18. The thermal energy storage system of claims 1-17, further comprising one or more valves coupled to one or more flow paths between the first tank or the second tank and the at least one compressor and the at least one expander.

19. A method for operating a regenerative Brayton based thermal energy storage system, comprising: operating one or more compressors and one or more expanders in fluid communication with a first tank and a second tank to store thermal energy in a first heat storing material in the first tank in a charging mode and to generate electricity with stored thermal energy in the first tank in a discharging mode; wherein operating in a charging mode includes: delivering a flow of air to the second tank, flowing the air through a second heat storing material in the second tank, and exhausting the flow of air from the second tank at a higher temperature; flowing the exhausted flow of air from the second tank to one or more compressors and compressing said air; delivering the flow of compressed air to the first tank, flowing the compressed air through the first heat storing material in the first tank, and exhausting the flow of air from the first tank; and flowing the exhausted flow of air from the first tank to one or more expanders and expanding said air.

20. The method of claim 19, wherein operating in a discharging mode includes delivering a flow of air to the one or more compressors and compressing said air, delivering the flow of compressed air to the first tank, flowing the compressed air through the first heat storing material in the first tank and exhausting the flow of air from the first tank, flowing the exhausted flow of air from the first tank to one or more expanders and expanding said air, delivering the flow of expanded air to the second tank, flowing the air through the second heat storing material in the second tank and exhausting the flow of air.

21. The method of claims 19-20, wherein rotating the one or more expanders generates electricity.

22. The method of claims 19-21, wherein the one or more expanders are configured to generate power deliverable to an electric grid.

23. The method of claims 19-22, wherein the first heat storing material and the second heat storing material is a plurality of rocks.

24. The method of claims 19-23, wherein the second tank operates at atmospheric pressure.

25. The method of claims 19-24, wherein the first tank operates at a pressure greater than atmospheric pressure.

26. The method of claims 19-25, wherein the first tank operates at a pressure of approximately 3 bar.

27. The method of claims 19-26, wherein the second heat storing material is at a lower temperature than the first heat storing material.