ENERGY STORAGE AND RETRIEVAL SYSTEMS AND METHODS

Abstract

Energy storage and retrieval systems are disclosed, along with methods of storing and retrieving the energy. The systems include an energy storage system and a trilateral cycle. The energy storage system includes low- and high-temperature energy storage tanks storing one or more energy storage media that exchange heat with a working fluid in both a gradient heat exchanger and a substantially isothermal heat exchanger in the trilateral cycle. Pressure changing devices transport the energy storage medium/media between the storage tanks and through the heat exchangers. The working fluid rejects heat to the energy storage medium and drives a turbine when the system is charging, and the energy storage medium rejects heat to the working fluid when the system is discharging. In some embodiments, the energy storage medium drives a second turbine when the system is discharging.

Claims

1. An energy storage and retrieval system, comprising: a first low-temperature energy storage tank storing a first energy storage medium at a first temperature; a high-temperature energy storage tank storing the first energy storage medium at a second temperature higher than the first temperature and having a pressurized gas therein; one or more first pressure changing devices configured to transport the first energy storage medium between the first low-temperature energy storage tank and the high-temperature energy storage tank; a gradient heat exchanger through which the first energy storage medium passes, configured to exchange heat between the first energy storage medium and a working fluid over a temperature range; a substantially isothermal heat exchanger through which a second energy storage medium passes, configured to exchange heat at a substantially constant temperature between the working fluid and the second energy storage medium; a compressor/expander in fluid communication with the gradient heat exchanger and the substantially isothermal heat exchanger, configured to change a pressure of the working fluid between the gradient heat exchanger and the substantially isothermal heat exchanger; and one or more second pressure changing devices between and in fluid communication with the gradient heat exchanger and the substantially isothermal heat exchanger, configured to transport the working fluid from the substantially isothermal heat exchanger to the gradient heat exchanger when the energy storage and retrieval system is discharging and from the gradient heat exchanger to the substantially isothermal heat exchanger when the energy storage and retrieval system is charging.

2. The energy storage and retrieval system of claim 1, wherein the first low-temperature energy storage tank also has the pressurized gas therein, and the energy storage and retrieval system further comprises a first conduit fluidly connecting the pressurized gas in the first low-temperature energy storage tank and the pressurized gas in the high-temperature energy storage tank.

3. The energy storage and retrieval system of claim 2, wherein the first conduit passes through the gradient heat exchanger, and the pressurized gas in the first conduit exchanges heat with the first energy storage medium and/or the working fluid.

4. The energy storage and retrieval system of claim 1, wherein the first energy storage medium comprises water.

5. The energy storage and retrieval system of claim 1, wherein the one or more second pressure changing devices comprises: a first pump configured to transport the working fluid from the substantially isothermal heat exchanger to the gradient heat exchanger when the energy storage and retrieval system is discharging; and a first turbine between the gradient heat exchanger and the substantially isothermal heat exchanger, driven by the working fluid when the energy storage and retrieval system is charging.

6. The energy storage and retrieval system of claim 1, wherein the one or more first pressure changing devices comprise: a first circulation pump configured to transport the energy storage medium from the low-temperature energy storage tank to the high-temperature energy storage tank when the energy storage and retrieval system is charging; and a second circulation pump configured to transport the energy storage medium from the high-temperature energy storage tank to the low-temperature energy storage tank when the energy storage and retrieval system is discharging.

7. The energy storage and retrieval system of claim 2, wherein the one or more first pressure changing devices comprises a bidirectional pump.

8. The energy storage and retrieval system of claim 2, wherein the pressurized gas and the first conduit are configured to balance a first pressure in the high-temperature energy storage tank with a second pressure in the first low-temperature energy storage tank.

9. The energy storage and retrieval system of claim 1, further comprising: a second low-temperature energy storage tank, configured to store the second energy storage medium; and a third pump configured to move the second energy storage medium through the substantially isothermal heat exchanger when the energy storage and retrieval system is charging and when the energy storage and retrieval system is discharging.

10. The energy storage and retrieval system of claim 9, wherein the second energy storage medium comprises water.

11. The energy storage and retrieval system of claim 1, wherein the second energy storage medium comprises a man-made or natural body of water.

12. A method of storing energy, comprising: storing a first energy storage medium at a first temperature in a first low-temperature energy storage tank; passing the first energy storage medium through a gradient heat exchanger; compressing a working fluid; passing the compressed working fluid through the gradient heat exchanger; rejecting heat from the compressed working fluid to the first energy storage medium over a temperature range between the first temperature and a second temperature higher than the first temperature in the gradient heat exchanger, thereby heating the first energy storage medium and cooling the compressed working fluid; storing the heated energy storage medium in a high-temperature energy storage tank at the second temperature; and exchanging heat between the working fluid and a second energy storage medium at a substantially constant temperature, thereby changing a phase of the working fluid.

13. The method of claim 12, further comprising balancing a pressure in the high-temperature energy storage tank with a pressure in the first low-temperature energy storage tank using a pressurized gas in each of the low-temperature and high-temperature energy storage tanks and a conduit in fluid communication with each of the low-temperature and high-temperature energy storage tanks.

14. The method of claim 13, further comprising exchanging heat between the pressurized gas and at least one of the first energy storage medium and/or the working fluid, wherein the conduit passes through the gradient heat exchanger.

15. The method of claim 12, further comprising: driving a turbine with the cooled, compressed working fluid; and/or pumping the second energy storage medium through the isothermal heat exchanger from either a second low-temperature energy storage tank or a man-made or natural body of water.

16. A method of retrieving stored energy, comprising: storing a first energy storage medium at a first temperature in a high-temperature energy storage tank; passing the first energy storage medium through a gradient heat exchanger; passing a working fluid in a trilateral cycle through the gradient heat exchanger; rejecting heat from the first energy storage medium to the working fluid over a temperature range between the first temperature and a second temperature lower than the first temperature in the gradient heat exchanger, thereby heating the working fluid and cooling the first energy storage medium; storing the cooled first energy storage medium at the second temperature in a first low-temperature energy storage tank; expanding the heated working fluid; and exchanging heat between the expanded working fluid and a second energy storage medium at a substantially constant temperature, thereby changing a phase of the expanded working fluid.

17. The method of claim 16, further comprising balancing a pressure in the high-temperature energy storage tank with a pressure in the first low-temperature energy storage tank using a pressurized gas in each of the low-temperature and high-temperature energy storage tanks and a conduit in fluid communication with each of the high-temperature energy storage tank and the first low-temperature energy storage tank.

18. The method of claim 17, further comprising exchanging heat between the pressurized gas and at least one of the expanded working fluid and the first energy storage medium, wherein the conduit passes through the gradient heat exchanger.

19. The method of claim 16, further comprising pumping the second energy storage medium through the isothermal heat exchanger from either a second low-temperature energy storage tank or a man-made or natural body of water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIGS. 1A-B are temperature-entropy (TS) diagrams showing a conventional trilateral cycle (FIG. 1A) and specific trilateral cycles for dimethyl ether (DME; FIG. 1B).

[0027] FIG. 2 is a schematic diagram of an exemplary system in accordance with the present invention, including a trilateral expansion-heat exchange-compression-heat exchange cycle, a thermal energy storage subsystem including a low-temperature storage tank and a high-temperature storage tank, and a low-temperature/isothermal heat sink/heat source subsystem for the isothermal part of the trilateral cycle.

[0028] FIG. 3 shows an end of an exemplary gradient heat exchanger that includes straight heat conducting and heat isolating elements in accordance with one or more embodiments of the present invention.

[0029] FIG. 4 shows a cross-sectional view of a straight heat conducting and heat isolating element in the exemplary heat exchanger of FIG. 3.

[0030] FIGS. 5A-C show cross-sectional views of a section or level of alternative exemplary heat exchangers in accordance with embodiments of the present invention.

[0031] FIG. 6 shows a cross-sectional view of a section or level of another exemplary heat exchanger in accordance with one or more embodiments of the present invention.

[0032] FIGS. 7A-C show cross-sectional views of one or more exemplary heat exchangers in various stages of an exemplary method of making the exemplary heat exchanger(s) in accordance with one or more embodiments of the present invention.

[0033] FIG. 8 shows an exemplary two-tank energy storage system pressurized with an inert gas, working in conjunction with an exemplary trilateral cycle similar or identical to that shown in FIG. 2.

[0034] FIG. 9 shows an exemplary three-tank energy storage system including two tanks pressurized with an inert gas similar to the energy storage system shown in FIG. 8, and working in conjunction with an exemplary trilateral cycle similar or identical to that shown in FIG. 2.

[0035] FIG. 10 shows an alternative two-tank energy storage system, working in conjunction with an exemplary trilateral cycle similar to that shown in FIGS. 2, 8 and 9.

DETAILED DESCRIPTION

[0036] Reference will now be made in detail to one or more embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the following embodiments, it will be understood that the descriptions are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention. Furthermore, in the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be readily apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to unnecessarily obscure aspects of the present invention. Furthermore, it should be understood that the possible permutations and combinations described herein are not meant to limit the invention. Specifically, variations that are not inconsistent may be mixed and matched as desired.

[0037] The technical proposal(s) of embodiments of the present invention will be fully and clearly described in conjunction with the drawings in the following embodiments. It will be understood that the descriptions are not intended to limit the invention to these embodiments. Based on the described embodiments of the present invention, other embodiments can be obtained by one skilled in the art without creative contribution and are in the scope of legal protection given to the present invention.

[0038] Furthermore, all characteristics, measures or processes disclosed in this document, except characteristics and/or processes that are mutually exclusive, can be combined in any manner and in any combination possible. Any characteristic disclosed in the present specification, claims, Abstract and Figures can be replaced by other equivalent characteristics or characteristics with similar objectives, purposes and/or functions, unless specified otherwise.

[0039] For the sake of convenience and simplicity, part, portion, and region may be used interchangeably herein, but are generally given their art-recognized meanings. Wherever one such term is used, it also encompasses the other terms. Also, unless indicated otherwise from the context of its use herein, the terms known, fixed, given, certain and predetermined generally refer to a value, quantity, parameter, constraint, condition, state, process, procedure, method, practice, or combination thereof that is, in theory, variable, but is typically set in advance and not varied thereafter when in use.

[0040] The term ambient temperature, which may also be known as room temperature, refers to a temperature typically in the range of 15-30? C. (e.g., 18-25? C., or any temperature or range of temperatures therein).

[0041] In part, the present invention concerns a method and system for thermal energy storage and retrieval. FIG. 2 shows an exemplary system. The system includes a high-temperature energy storage tank 6. The tank 6 may store water as the energy storage medium at a relatively high temperature (e.g., 65-300? C., or any temperature or range of temperatures therein, such as 200? C.) and at a first pressure (e.g., 2-40 bar, or any pressure or range of pressures therein, such as 16 bar). The water may be stored in a low-temperature energy storage tank 5 at a relatively low temperature (e.g., 0-60? C., or any temperature or range of temperatures therein).

[0042] The expansion-heat exchange-compression-heat exchange cycle Aug. 18, 2017-22-8 (charging) and Aug. 9, 2021-17-18 (discharging) may use pentane(s) (i.e., n-pentane or a mixture of pentane isomers) as the working fluid, although other fluids, such as hexane(s), butane, propane, cyclopentane, acetone, cyclopentane, isopentane, undecane, HFO-1336mzz-Z, R123 (which is suitable, its potential phase-out in commercial use notwithstanding), R1233zd, or similar materials, may also be suitable. This cycle is implemented as a trilateral cycle, as shown in FIG. 1.

[0043] In the trilateral cycle, a gradient heat exchanger 7 (FIG. 2) exchanges heat between the working fluid and the energy storage medium. There may be a relatively large difference between the temperature of the working fluid entering the heat exchanger 7 and exiting the heat exchanger 7, corresponding to the slope 2-3 in the entropy diagram in FIG. 1. There may also be a relatively large difference (e.g., 100-250? C., or any temperature or range of temperatures therein, such as 150-200? C.) in the temperature of the energy storage medium after it passes through the heat exchanger 7.

[0044] When the system is charging, the energy storage medium is pumped by the pump 19 from the low-temperature energy storage tank 5, through the heat exchanger 7 from a first port 15 to a second port 16, to the high-temperature energy storage tank 6, and the working fluid (which may be at a relatively high pressure due to compression by the compressor 8) passes through the heat exchanger 7 from a third port 18 to a fourth port 17. When the system is discharging, the energy storage medium flows from the high-temperature energy storage tank 6, through the heat exchanger 7 from the second port 16 to the first port 15, through a turbine 20, to the low-temperature energy storage tank 5, and the working fluid is pumped by a pump 21 through the heat exchanger 7 from the fourth port 17 to the third port 18. The turbine 20 drives the generation of electricity or other form of usable energy (e.g., mechanical energy). Three-way valves 11 and 12 control the flow of the energy storage medium through the pump 19 and the turbine 20, and three-way valves 13 and 14 control the flow of the working fluid through the pump 21 and a second turbine 22 in the trilateral cycle.

[0045] A traditional heat exchanger 9 exchanges heat between the working fluid and the energy storage medium in the low-temperature energy storage tank 5. There may be a relatively small difference (e.g., about 0? C.) between the temperature of the working fluid entering the heat exchanger 9 and exiting the heat exchanger 9, corresponding to the horizontal (isothermal) part 4-1 of the trilateral cycle in the entropy diagram of FIG. 1. Optionally, there may also be a relatively small difference (e.g., 1-50? C., or any temperature or range of temperatures therein, such as 1-20? C.) in the temperature of the energy storage medium after it passes through the heat exchanger 9.

[0046] When the system is charging, part of the energy storage medium is pulled through the heat exchanger 9 by a two-way pump 10. The working fluid passes through the heat exchanger 9 from the second turbine 22 to the compressor 8. The second turbine 22 may also drive the generation of electricity or other form of usable energy (e.g., mechanical energy). Alternatively, the second turbine 22 may drive one or more of the pumps 10, 19 and 21. When the system is discharging, the two-way pump 10 moves some of the energy storage medium from the low-temperature energy storage tank 5 through the heat exchanger 9, while the working fluid passes through the heat exchanger 9 from the expander 8 (i.e., the reverse function/operation of the compressor 8) to the pump 21. Thus, the horizontal (isothermal) part 4-1 of the entropy diagram in FIG. 1 moves upwards during the discharge cycle and/or operation, a novel feature of the present system and method.

[0047] The gradient heat exchanger 7 may comprise a heat exchanger exemplified in FIG. 3. The heat exchanger 7 includes straight heat conducting (e.g., heat exchanging) and heat isolating elements 100. Tubes or pipes 101 and 102 transport the energy storage medium (e.g., 133) and a counter-flowing working fluid (e.g., 142), and pass through a plurality of straight heat transfer elements 100 made from two identical, mirrored and/or complementary sections (see, e.g., FIG. 4). Each straight heat transfer element 100 is surrounded (and thus thermally isolated) by a straight heat-isolating element 103 made from two identical (and preferably self-locking) sections. U-shaped tubes 111, 112 and 113 transport the energy storage medium and the working fluid from one heat exchanger section to the next heat exchanger section. The U-shaped tubes 111, 112 and 113 may be thermally insulated (and optionally thermally isolated from each other) to prevent heat exchange with the external environment.

[0048] FIG. 4 shows a cross-sectional view of a straight heat conducting and heat isolating element in the exemplary heat exchanger of FIG. 3. FIG. 4 shows tubes or pipes 101 and 102, a one-piece heat transfer unit or element 71 (corresponding to heat transfer elements 100 in FIG. 3), and (self-locking) sections 78 and 79 that make up the heat-isolating element 103 in FIG. 3. The heat transfer unit or element 71 (FIG. 4) may comprise a thermally conductive material 103 having an H-shaped cross-sectional profile (see FIG. 7A), with two or four relatively planar sheets of material extending from the middle, and two semi-circular or semi-cylindrical surfaces in the middle, facing in opposed directions. The tubes or pipes 101 and 102 have a diameter configured to fit in the semi-circular surfaces of the heat transfer unit or element 71. Although tubes or pipes 101 and 102 having a circular cross-section and a heat transfer unit or element 71 having two surfaces with a semi-circular cross-section are shown, the invention is not limited to such a configuration, and other cross-sectional profiles or shapes given herein are possible, as long as the heat transfer unit or element 71 has inner surfaces matching the external surfaces of the tubes or pipes 101 and 102 that face one another in the heat transfer elements 100. The sections 78 and 79 generally comprise a thermally non-conductive (insulative) material.

[0049] Alternative embodiments of the heat conducting and heat isolating element of FIG. 4 are shown in FIGS. 5A-C. For example, FIG. 5A shows a cross-sectional view of a section or level of an alternative heat exchanger, comprising the tubes or pipes 101/102 passing through a heat transfer element 72-73. The heat transfer element 72-73 may comprise or consist of a heat conducting material that transfers heat between the tubes or pipes 101 and 102. The tubes or pipes 101 and 102 and the heat transfer element 72-73 may comprise or consist of the same or different materials. The tubes or pipes 101 and 102 as shown have a circular cross-section, but the tubes or pipes 101 and 102 can have any cross-sectional shape that allows for heat exchange (e.g., oval, square, rectangular, half-circle, half-oval, etc.). The tubes or pipes 101 and 102 (or sections thereof) can comprise a single material (in which case each tube or pipe 101 or 102 can comprise the same material) or two or more different materials, depending on heat conduction requirements and/or a desired corrosion resistance. Such materials may include plastics (e.g., polyvinyl chloride [PVC], polyethylene [which may be cross-linked], polypropylene, polybutylene, acrylonitrile-butadiene-styrene [ABS], etc., any of which may be fiber-reinforced), metals (e.g., steel, aluminum, copper, iron, brass), glass, fiberglass, ceramics (e.g., concrete), etc. The tubes or pipes 101 and 102 may be inserted in the heat transferring element. The external surface of the tubes or pipes 101 and 102 and the internal surface of the heat transfer element are either in firm contact or a heat conducting material such as a heat conducting paste, glue or solder is between them, transferring heat between the surfaces. The firm contact is provided by the first securing mechanism.

[0050] The heat transfer element of FIG. 5A includes two heat transfer elements 72 and 73, bolted together with a bolt 74 and a securing element (e.g., a nut) 75. Each of the heat transfer elements 72 and 73 has a pair of semi-cylindrical troughs or depressions having a width substantially equal to the diameter of the tubes or pipes 101 and 102. The heat transfer elements 72 and 73 may comprise a material such as metal or a metal alloy that can be extruded or diecast, or a thermoplastic material that can be extruded or molded (e.g., by injection molding). The tubes or pipes 101 and 102 may have different diameters, in which case the heat transfer elements 72 and 73 are not identical. There is a wide variety of heat transfer materials and/or compounds available in the market having a variety of capabilities and/or rates of heat transfer (e.g., between the tubes 101 and 102, through the extruded or diecast profile of the heat transfer elements 72 and 73). In one example, the heat transfer elements 72 and 73 comprise a thermally conductive metal or metal alloy in the portion between the tubes or pipes 101 and 102. A compressible gasket or other fitting may line the troughs or depressions in the heat transfer elements 72 and 73, and the gasket or fitting may be made of a thermally conductive material. A thermally conductive paste may be applied between the tubes and the heat transfer elements to increase the heat transfer and to allow the different parts to move (e.g., due to thermal expansion). Additionally or alternatively, a washer or spring may be present between the tubes and the heat transfer elements to absorb any thermal expansion of the tubes and/or heat transfer elements.

[0051] FIG. 5B shows a cross-sectional view of a section or level of a further alternative heat exchanger. The alternative heat exchanger comprises two identical self-locking, snap-on heat transfer elements 171 and 172, secured to each other by complementary and/or mating, grooved and/or ridged locking mechanisms 179. In such an embodiment, the heat exchanger is straight or linear (e.g., it has an axis or plane of symmetry along its center). Otherwise, the heat exchanger of FIG. 2 is similar or substantially the same as the heat exchanger of FIG. 5A.

[0052] FIG. 5C shows a cross-sectional view of a section or level of a further alternative heat exchanger. The heat exchanger of FIG. 5C comprises a single heat transfer unit or element 21 having side walls that curve slightly inwards at the outermost edges or lips 29, configured to secure or self-lock the two tubes 101 and 102 therein. Thus, the diameter of the tubes or pipes 101 and 102 is slightly greater than the distance between the outermost edges or lips 29 of the side walls. The heat exchanger of FIG. 5C can be surrounded by a thermal insulator (e.g., similar to sections 78 and 79 in FIG. 4), in which case the thermal insulator functions as a thermal isolator between sections and/or layers of the heat exchanger (see, e.g., FIG. 3) and to the surroundings.

[0053] FIG. 6 shows a cross-sectional view of a section or level of another exemplary heat exchanger. The heat exchanger of FIG. 6 includes a 5-piece heat transfer unit or element through which four tubes or pipes 101 pass. The 5-piece heat transfer unit or element includes three heat-conducting elements 31, 32 and 33, and two isolator sections 41 and 42. The two isolator sections 41 and 42 may self-lock in the same manner as that shown in FIG. 4 or by another known self-locking mechanism, they may be bolted together as shown in FIG. 5A, or they may be held together or otherwise secured to each other by known means (e.g., a tightened band or strap around the outer periphery of the isolator sections 41 and 42, etc.). The heat conducting elements 31, 32 and 33 may be held in place with each other using known mechanisms (e.g., tongue-in-groove fittings, mating projection-and-hole, projection-and-opening or projection-and-depression fittings, etc., with or without an applied adhesive). Such mechanisms may also be used to secure the two isolator sections 41 and 42 to each other.

[0054] FIGS. 7A-C show cross-sectional views of a section or level of one or more exemplary heat exchangers during an exemplary method for making the heat exchanger(s). FIG. 7A shows two tubes or pipes 101 and 102, a one-piece heat transfer unit or element 71, and pressing tool sections or halves 78 and 79, substantially as depicted in FIG. 4. The heat transfer unit or element 71 has an H-profile, with two or four relatively planar sheets of material extending from the middle, and two semi-circular surfaces in the middle, facing in opposed directions. The tubes or pipes 101 and 102 have a diameter configured to fit in the semi-circular surfaces of the heat transfer unit or element 71. Although tubes or pipes 101 and 102 having a circular cross-section and a heat transfer unit or element 71 having two surfaces with a semi-circular cross-section are shown, the invention is not limited to such a configuration, and other cross-sectional profiles or shapes given herein are possible, as long as the heat transfer unit or element 71 has inner surfaces matching the surfaces of the tubes or pipes 101 and 102 that face one another (i.e., the inner surfaces of the tubes or pipes 101 and 102). The pressing tool sections 78 and 79 also have a cross-sectional profile matching that of the outer surfaces of the tubes or pipes 101 and 102, but larger by about the thickness of the sheets of material extending from the middle of the heat transfer unit or element 71.

[0055] In FIG. 7B, the pressing tool sections 78 and 79 are brought together, thereby bending the sheets of material extending from the middle of the heat transfer unit or element 71 around the outer surfaces of the tubes or pipes 101 and 102. As shown by the arrows in FIG. 7B, the pressing tool sections 78 and 79 are removed (see FIG. 7C), in which case the pressing tool sections 78 and 79 function as sections of a mold for forming the heat exchanger. The sheets of material extending from the middle of the heat transfer unit or element 71 may be held in place with a thermally-conductive adhesive applied to the inner surface of the material (e.g., after placing the tubes or pipes 101 and 102 against the inner surfaces of the heat transfer unit or element 71), and/or the ends of the sheets of material extending from the middle of the heat transfer unit or element 71 may be fastened or secured to each other by an interlocking mechanism as shown in FIG. 5B, or using a known mechanism (e.g., as described herein).

[0056] When the hot storage tank 6 in FIG. 2 empties, the pressure in the high-temperature energy storage tank 6 drops, whereby the water boils, and some of the water evaporates (e.g., turns into steam). The heat of vaporization taken from the water when it evaporates is significant, and the lost heat of vaporization lowers the system's capacity.

[0057] Referring now to FIG. 8, in order to increase the useful heat capacity in the system, the low-temperature energy storage tank 51 and the high-temperature energy storage tank 50 are pressurized with a gas 201 therein. The upper parts of the low- and high-temperature energy storage tanks 51 and 50 are connected with a pipe 202. When the pressure decreases in the high-temperature energy storage tank 50 (e.g., when water is pumped or transferred from the high-temperature energy storage tank 50), the pressurized gas 201 flows from the low-temperature energy storage tank 51 to the high-temperature energy storage tank 50. Thus, in its simplest form, the pressure in the low-temperature energy storage tank 51 and the high-temperature energy storage tank 50 is substantially the same. As a result, the power consumed by the pump (e.g., 52 and/or 53) that pumps the water between the low-temperature energy storage tank 51 and the high-temperature energy storage tank 50 is much less than without the pressurized gas 201. For example, the pumps 52 and 53 may comprise low-pressure circulation pumps or other, similar pumps that only need to overcome frictional losses in the pipes or tubes between the low-temperature energy storage tank 51 and the high-temperature energy storage tank 50. Pumps 52 and 53, and optionally, check valves 54 and 55 can be replaced with a single reversible propeller pump. In a more specific embodiment, the reversible propeller pump includes a relatively precise variable speed control, to optimize its operation.

[0058] In various modifications, there may also be a pressure sensor (not shown) in the pipe 202 to monitor and/or measure the pressure of the gas 210, a valve through which additional pressurized gas can be added to (or excess pressurized gas can be removed from) the pipe 202 or the low-temperature energy storage tank 51 (e.g., for safety reasons), and/or one or more pressure-activated valves (e.g., between a tank 51 or 50 and the pipe 202) to control when the pressurized gas 201 is transferred from one of the tanks 51 and 50 to the other (e.g., when pressure differential on opposite sides of the valve exceeds a predetermined value, such as in the range of 1-10 atm). On the other hand, when the water flows from the low-temperature energy storage tank 51 to the high-temperature energy storage tank 50, the pressure decreases in the low-temperature energy storage tank 51, and the pressurized gas 201 flows from the high-temperature energy storage tank 50 to the low-temperature energy storage tank 51.

[0059] The expansion-heat exchange-compression-heat exchange cycle 61-16-17-57-60-58-62-61 (charging) and 61-62-58-59-57-17-16-61 (discharging) is essentially the same as the expansion-heat exchange-compression-heat exchange cycle in FIG. 2, except for the embodiment(s) that include a section of the pipe or tube 202 in the gradient heat exchanger 56. The working fluid in the expansion-heat exchange-compression-heat exchange cycle in FIG. 8 may be the same as that in the expansion-heat exchange-compression-heat exchange cycle in FIG. 2.

[0060] In the trilateral cycle of FIG. 8, there may be a relatively large difference between the temperature of the working fluid entering the heat exchanger 56 and exiting the heat exchanger 56, corresponding to the slope 2-3 in the entropy diagram in FIG. 1. There may also be a relatively large difference in the temperature of the energy storage medium after it passes through the heat exchanger 56.

[0061] When the system is charging, the energy storage medium is pumped by the pump 52 from the low-temperature energy storage tank 51, through the heat exchanger 56, to the high-temperature energy storage tank 50, and the working fluid in the trilateral cycle passes through the heat exchanger 56 from port 16 to port 17, a turbine 60, and an isothermal heat exchanger 62 to a two-way compressor/expander 61. The turbine 60 drives the generation of electricity or other form of usable energy (e.g., mechanical energy). When the system is discharging, the energy storage medium is pumped by a second pump 53 from the high-temperature energy storage tank 50 to the low-temperature energy storage tank 51, and the working fluid in the trilateral cycle is pumped by a third pump 59 through the heat exchanger 56 from the port 17 to the port 16. In this case, because the pressure in the low-temperature energy storage tank 51, and the high-temperature energy storage tank 50 is essentially the same, the pump 53 moves the energy storage medium from the high-temperature energy storage tank 50 to the low-temperature energy storage tank 51, as there is no excess pressure in the high-temperature energy storage tank 50 to drive fluid flow through the heat exchanger 56 and a turbine. Check valves 54 and 55 control the flow of the energy storage medium through the pumps 52 and 53, and three-way valves 57 and 58 control the flow of the working fluid through the pump 59 and the turbine 60 in the trilateral cycle.

[0062] The isothermal heat exchanger 62 exchanges heat between the working fluid and the energy storage medium in the low-temperature energy storage tank 51. There may be a relatively small difference (e.g., about 0? C.) between the temperature of the working fluid entering the heat exchanger 62 and exiting the heat exchanger 62, corresponding to the horizontal (isothermal) part 4-1 of the trilateral cycle in the entropy diagram of FIG. 1A. Optionally, there may also be a relatively small difference in the temperature of the energy storage medium after it passes through the heat exchanger 62.

[0063] When the system is charging, the energy storage medium is pulled through the heat exchanger 62 by a two-way pump 63. The working fluid passes through the heat exchanger 62 from the turbine 60 to the compressor 61. Alternatively or additionally, the turbine 60 may drive one or both of the pumps 52 and 63. When the system is discharging, the two-way pump 63 moves some of the energy storage medium from the low-temperature energy storage tank 51 through the heat exchanger 62, while the working fluid passes through the heat exchanger 62 from the expander 61 (i.e., the reverse function/operation of the compressor 61) to the pump 59. Thus, the horizontal (isothermal) part 4-1 of the entropy diagram in FIG. 1A moves upwards during the discharge cycle and/or operation (see also the horizontal lines at 0? C. or greater in the temperature vs. entropy plots in FIG. 1B).

[0064] The pressurized gas 201 can be air when the high-temperature energy storage tank 50 is galvanized or coated with a protective layer. When the high-temperature energy storage tank 50 is made of steel, the pressurized gas 201 may be an inert gas, such as nitrogen or argon, to prevent corrosion of the steel.

[0065] In one advantageous embodiment shown in FIG. 8, the pressurized gas 201 can be heated or cooled as the gas 201 flows between the low-temperature energy storage tank 51 and the high-temperature energy storage tank 50. The pressurized gas 201 can be heated or cooled with the working medium (i.e., the medium in the cycle 61-62-58-59-57-56-61 or 61-56-57-60-58-62-61) by passing the pressurized gas 201 through the heat exchanger 31. The heat exchanger 31 can comprise the gradient heat exchanger 7 in FIG. 2 with a section of the pipe 202 therein, or a separate heat exchanger with an additional heat sink/source.

[0066] When the high-temperature energy storage tank 50 is not full for a sufficiently long time, the pressurized gas 201 in the tank 50 is saturated with water vapor. However, that heat (e.g., of vaporization) is not lost because it is returned to the hot water in the tank 50 via the heat exchanger 31 during charging. Alternatively, to prevent water vapor from evaporating and mixing with the pressurized gas 201 in the high-temperature energy storage tank 50, the water surface can be covered with a floating body or a layer of a liquid that does not mix with the water and that has a lower density than water (e.g., silicone oil or, at the operating temperatures in the high-temperature energy storage tank 50, paraffin wax).

[0067] FIG. 9 shows a three-tank energy storage system pressurized with preheated inert gas 211, driven by a trilateral cycle 40-44-45-34-35-37-40 (charging) or 40-37-35-43-42-45-44 (discharging), using a fluid other than water (e.g., pentane, DME) as the working fluid. The energy storage system is similar to that of FIG. 8, except that the low-temperature energy storage tank 51 in FIG. 8 is split into two low-temperature energy storage tanks 24 and 30. The low-temperature energy storage tank 24 stores the preheated inert gas 211 along with the high-temperature energy storage tank 25, and the low- and high-temperature energy storage tanks 24 and 25 operate together in substantially the same way as the low- and high-temperature energy storage tanks 50 and 51 in FIG. 8 and the low- and high-temperature energy storage tanks 5 and 6 in FIG. 2. However, a second low-temperature energy storage tank 30 stores water for use in isothermal heat exchanges in heat exchanger 37 with the trilateral cycle. In the heat exchanger 37, the water changes temperature, but the working fluid in the trilateral cycle changes phase (e.g., from gas to liquid, or from liquid to gas) without a substantial change in temperature (except perhaps at the entrance or exit of the heat exchanger 37).

[0068] The energy storage system and trilateral cycle of FIG. 9 operates substantially the same as that of FIG. 8, except that energy storage fluid in separate low-temperature energy storage tanks 24 and 30 exchange heat with the working fluid in the respective gradient and isothermal heat exchangers 44 and 37. This configuration allows for optimization of the quantities and, optionally, the temperatures of the water in the separate low-temperature energy storage tanks 24 and 30. The energy storage system and trilateral cycle of FIG. 9 also has the advantage that the storage tank 30 does not need to be pressurized (i.e., the energy storage medium therein can be stored at low pressure), which enables lower manufacturing costs (e.g., a relatively thin material for the storage tank 30) and use of a relatively low-pressure bidirectional pump 36 (e.g., a reversible propeller pump) to circulate the energy storage medium through the isothermal heat exchanger 37. On the other hand, the energy storage system and trilateral cycle of FIG. 8 has the advantage of having only two tanks for the entire system. The energy storage system and trilateral cycle of FIG. 2 can also use separate low-temperature energy storage tanks, similar to tanks 24 and 30 in FIG. 9.

[0069] FIG. 10 shows a two-tank, two-turbine system operating according to the principles of the systems of FIGS. 2, 8 and 9. The system of FIG. 10 is similar to the system of FIG. 2, except that the low-temperature energy storage tank 93 can be a large-volume water storage tank, a man-made or natural body of water, such as a pond, a lake, a river, a sea or an ocean, or even an external atmosphere (e.g., air outside a building). When the low-temperature energy storage tank 93 is a body of water or an external atmosphere, the heat exchanger 90 in the trilateral cycle 80-90-89-84 or 80-84-86-90 does not need to be close to (e.g., in thermal contact with) a similar heat exchanger in the energy storage system 70-84-88-93 or 93-85-84-70. When the low-temperature energy storage tank 93 is the external atmosphere, the water in the energy storage system simply passes through a pipe or tube (which may, for example, be thermally connected to a plurality of cooling fins) in the outside/outdoor air. The system of FIG. 10 operates similarly to the system of FIG. 2, in that the high-temperature energy storage tank 70 is self-pressurizing during charging. Thus, during discharging, the water from the high-temperature energy storage tank 70 can drive a turbine 88. Conversely, during charging, the trilateral cycle can drive a second turbine 86. When operating, each of the turbines 86 and 88 can drive a corresponding pump 85 or 89, respectively, in the other system.

[0070] The energy storage system in FIG. 10 comprises the high-temperature energy storage tank 70, a gradient heat exchanger 84, the turbine 88, a valve 82, a pump 85, a check valve 81, and the low-temperature energy storage tank 93. The trilateral cycle in FIG. 10 comprises a two-way expander/compressor 80, an isothermal (or substantially isothermal) heat exchanger 90 in or passing through the low-temperature energy storage tank 93, a pump 89, a check valve 87, the turbine 86, a valve 83, and the gradient heat exchanger 84. The valves 82 and 83 may comprise conventional manual or electronically-controlled valves. The trilateral cycle in FIG. 10 operates substantially identically to the trilateral cycles in FIGS. 2, 8 and 9.

[0071] The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.