Compressed gas energy storage system using a geological thermal store

Abstract

A compressed gas energy storage system which can also store thermal energy. The storage system can have a surface subsystem, a geological heat storage subsystem, and a compressed gas storage subsystem. The surface subsystem can comprise gas compression, gas expansion, and heat exchange functions. The geological heat storage subsystem can comprise a cold and hot zone in fluid communication with the surface subsystem. The compressed gas storage subsystem can also be in fluid communication with the surface subsystem. The systems and methods can be implemented in porous rock formations. This allows for the use of extremely large, natural formations rather than man-made salt caverns, as is the current state of the art. Existing wells drilled by oil and gas producers can be utilized, thereby drastically reducing cost and time for implementation. Further, use of sub-surface thermal management provides a complete solution for long term energy storage at high efficiency.

Description

APPLICATION FOR PATENTTITLE:COMPRESSED GAS ENERGY STORAGE SYSTEM USING A GEOLOGICALTHERMAL STORESPECIFICATIONCROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to and the benefit of co-pending US Provisional Patent Application Serial No. 63 / 523,157 filed on June 26, 2023, titled “Compressed Gas Energy Storage System Using A Geological Thermal Store.” This reference is incorporated in its entirety.FIELD

[0002] The present disclosure relates generally to compressed gas energy storage, and more particularly to compressed gas energy storage systems which include a geological thermal store.BACKGROUND

[0003] Renewable power sources are now economically competitive with or cheaper than traditional forms of power generation in several geographic markets. However, these renewable power sources are often variable and intermittent, and their supply may not always match peak demand. Energy storage is hence critical to the renewable energy grid system transition as it can smooth out the delivery of variable or intermittent resources, such as wind energy and / or solar energy, by storing excess energy when the wind is blowing and the sun is shining and delivering it when they are not available.

[0004] Energy storage refers to various technologies capable of storing electricity generated at one time for later use. Long duration energy storage of 10+ hours to time scales of weeks and months for seasonal storage is critical to achieve the goal of a net zero carbongrid. In the current state of the art, lithium-ion batteries can become prohibitively expensive beyond 4 to 6 hours of storage at grid scale. Other alternatives, such as pumped storage hydropower, are geographically limited, particularly for long duration energy storage at scale.

[0005] Compressed air energy storage is a viable candidate for long duration energy storage. In the charging cycle, ambient air can be compressed and injected into underground caverns or porous reservoirs for storage. In the discharging cycle, when electricity is needed, the stored high-pressure air can be produced to flow through turbines for power generation and discharged to the atmosphere.

[0006] Storage in porous reservoirs is particularly suited for long duration because of low absolute costs as well as low marginal costs for increasing storage capacity. The storage area in a porous reservoir is primed by first creating a large, compressed air cushion that can then provide pressure support for the subsequent injection and production cycles of compressed air. Typical volumes of the air cushion can be 10 to 100 times the air injection and production cycle volumes.

[0007] Exemplary porous reservoirs can be either depleted hydrocarbon reservoirs or aquifers. The latter is the more attractive of the two, as it minimizes any potential contamination of the air by hydrocarbons. This not only eliminates any potential flammability hazard but also allows atmospheric discharge of air from the turbines without costly treatments to remove environmental contaminants. Aquifer candidates selected for compressed air energy storage are typically those that are confined and isolated from underground drinking water sources.

[0008] In a diabatic compressed air energy storage process, the heat generated during compression is wasted by rejection to the environment. The heat needed for the subsequent expansion cycle is provided by burning natural gas. The advanced adiabatic compressed air energy storage process overcomes the limitations of the diabatic compressed air energy storage process by storing the compression heat in thermal stores for later reuse during expansion, thus resulting in higher efficiencies and fossilfuel -free operation.

[0009] Several thermal store options have been proposed, such as pressurized hot water, molten salts, concrete blocks, packed alumina beds, etc. However, the time duration over which such thermal stores can store the heat is typically lower than 24 hours. Furthermore, the cost of such thermal storage systems, that need to operate at high pressure and temperature, increases steeply with the power output capacity and the energy storage duration hours of the compressed air energy storage plant.

[0010] Hence, there is a need for a low-cost thermal storage option that allows heat storage for durations beyond 10 hours to time scales of weeks to months to enable long duration energy storage.

[0011] The present invention meets these needs.SUMMARY OF THE INVENTION

[0012] The present disclosure relates generally to compressed gas energy storage, and more particularly to compressed gas energy storage systems which include a geological thermal store.

[0013] The systems and methods described in this invention provide for more cost-effective and longer duration thermal storage by using geological thermal stores for compressed gas energy storage systems.

[0014] In one embodiment of the present invention, a system for a geological thermal store is provided by geological heat storage, whereby the heat released during the charging compression cycle of a compressed gas energy storage process is transferred to water produced from a first zone of an aquifer and then geologically stored by injecting the heated water into a second zone of the aquifer. The first and second zones are separated by a distance such that there is zero to minimal heat flow between the two zones toensure the heat storage capacity of the second zone is maximized. The circulation of water is reversed during the discharge cycle, where the geologically stored hot water in the second zone is produced from the aquifer to provide heat for gas expansion, and the cooled water is reinjected back into the first zone of the aquifer.

[0015] In another embodiment of the present invention, a system for a geological thermal store comprises a geothermal heat source, such as a hydrothermal source or an unconventional geothermal source, to provide the heat for the expansion process in the discharge cycle by circulating water between the geothermal heat source and the surface heat exchangers using two or more wells.

[0016] The present disclosure is unique in that the systems and methods can be implemented in porous rock formations. This allows for the use of extremely large, natural formations rather than man-made salt caverns, as is the current state of the art. Existing wells drilled by oil and gas producers can be utilized, thereby drastically reducing cost and time for implementation. Further, the present disclosure makes use of sub-surface thermal management to provide a complete solution for long term energy storage at high efficiency.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The detailed description will be better understood in conjunction with the accompanying drawings as follows:

[0018] Figures 1A and IB show a subsurface energy storage system.

[0019] Figures 2A and 2B show a subsurface energy storage system wherein the source of heated water is a geological hydrothermal source.

[0020] Figures 3A and 3B show a subsurface energy storage system with a constant heatsource using an unconventional or enhanced geothermal heat system.

[0021] The embodiments of the present disclosure are detailed below with reference to the listed Figures.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] Before explaining the present disclosure in detail, it is to be understood that the disclosure is not limited to the specifics of particular embodiments as described and that it can be practiced, constructed, or carried out in various ways.

[0023] While embodiments of the disclosure have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the disclosure. The embodiments described herein are exemplary only, and are not intended to be limiting.

[0024] Specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis of the claims and as a representative basis for teaching persons having ordinary skill in the art to variously employ the present embodiments. Many variations and modifications of embodiments disclosed herein are possible and are within the scope of the present disclosure.

[0025] Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations.

[0026] The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and / or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.”

[0027] The word “about” means plus or minus 5% of the stated number.

[0028] The use of the term "optionally" with respect to any element of a claim is intended to mean that the subject element is required, or alternatively, is not required. Both alternatives are intended to be within the scope of the claim. Use of broader terms such as comprises, includes, having, etc. should be understood to provide support for narrower terms such as consisting of, consisting essentially of, comprised substantially of, and the like.

[0029] When methods are disclosed or discussed, the order of the steps is not intended to be limiting, but merely exemplary unless otherwise stated.

[0030] Accordingly, the scope of protection is not limited by the description herein, but is only limited by the claims which follow, encompassing all equivalents of the subject matter of the claims. Each and every claim is hereby incorporated into the specification as an embodiment of the present disclosure. Thus, the claims are a further description and are an addition to the embodiments of the present disclosure.

[0031] The inclusion or discussion of a reference is not an admission that it is prior art to the present disclosure, especially any reference that may have a publication date after the priority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference, to the extent they provide background knowledge; or exemplary, procedural or other details supplementary to those set forth herein.

[0032] The embodiments of the present disclosure generally relate to compressed gas energy storage, and more particularly to compressed gas energy storage systems which include a geological thermal store.

[0033] The present disclosure provides embodiments of systems and methods for a geological thermal store comprising geological storage of compression heat from a charging cycle for subsequent use in a discharge cycle of a compressed gas energy storage process. In another embodiment, the disclosure provides systems and methods for a geological thermal store comprising the use of a geothermal heat source in a discharge cycle of acompressed gas energy storage process. While the systems and methods are described for a compressed air energy storage system that uses an aquifer for storing the compressed air, they are also applicable to compressed air energy systems that store air in underground caverns or depleted gas and oil reservoirs. The systems and methods described are also applicable to compressed gas energy storage systems that include other gases such as carbon dioxide, nitrogen, hydrogen, and natural gas.

[0034] Geological thermal store comprising a geological heat storage system:

[0035] System Description: FIG. 1A and FIG. IB illustrate an embodiment of the invention comprising a surface subsystem comprising a compression subsystem 110 with one or more compressors, a heat exchange subsystem 120 with one or more heat exchangers and an expansion subsystem 130 with one or more air expanders. It further comprises a geological thermal store which is a geological heat storage subsystem comprising a confined aquifer with one or more wells for transfer of cold water between a first zone 130 of the aquifer and the heat exchanger subsystem and one or more wells for transfer of hot water between the heat exchanger subsystem and a second zone 140 of the aquifer. The embodiment further comprises a compressed air storage subsystem 150 comprising one or more wells for transfer of compressed air between one or more of the surface subsystems and the air cushion in the aquifer.

[0036] During the charging cycle, shown in FIG. 1A, when renewable energy is available, ambient air is adiabatically compressed in the compression subsystem 110 to a storage pressure in one or more compression stages with interstage cooling. A higher compression ratio per stage (outlet to inlet pressure for that stage) is more cost effective as it allows for the compression to be achieved with fewer stages. A higher compression pressure ratio also results in a higher discharge temperature for that stage allowing storage of heat in water at higher temperatures and hence requiring lower total water circulation volumes. However, the maximum compressor air discharge temperature is limited by factors such as the temperature rating of the compressor material and seals used and hence limits the maximum compression ratio per stage.

[0037] The heat exchange subsystem 120 comprises one or more heat exchangers. Any heat exchanger known to persons having ordinary skill in the art can be used, but preferably an indirect heat exchanger such as a shell and tube-type configuration. In the charging cycle, water from the first zone 130 of the aquifer, which is initially at native aquifer water temperature and is the cold thermal store, is used to cool the hot air discharged from each of the compression stages in the compression subsystem 110, with air from the last stage being cooled to its storage temperature. The hot water coming out of the heat exchanger subsystem is injected back into the aquifer into the second zone 140 to create the hot thermal store. The cooled high-pressure air from the heat exchanger subsystem is injected into the air cushion in compressed air storage subsystem 150.

[0038] In the discharge cycle shown in FIG. IB, the stored air in compressed air storage subsystem 150 is produced and adiabatically expanded. Adiabatic expansion causes the air temperature to drop as a function of the expansion pressure ratio (inlet to outlet pressure). To avoid potential problems such as moisture freezing, the expansion is carried out in multiple stages with interstage heating. To enable this interstage heating of air, hot water from the second zone 140 is produced and circulated through the heat exchanger subsystem 120 to heat the high-pressure air, and the cooled water exiting the heat exchanger is reinjected into the first zone 130 of the aquifer.

[0039] The heat recovery factor (defined as the cumulative quantity of heat recovered divided by the cumulative quantity of heat added) in the initial charge / discharge cycles will be poor as there will be heat loss to the geological formation, both radially in the aquifer layer and vertically to the confining layers above and below the aquifer. A “poor” heat recovery factor refers to one in which the system is not economically feasible to operate.

[0040] There is also conductive heat loss from a wellbore to the geological formation surrounding it. Conductive heat loss from the wellbore to the surrounding formation can be mitigated by using insulated tubing in the wellbore. Multiple cycles of heat injection and withdrawal will cause the aquifer and formation temperatures to slowlyincrease, resulting in a reduction of the heat loss to the formation as the difference between the injection temperature and formation temperature decreases.

[0041] A quasi-steady state (a state where the temperatures are changing slowly enough that it can be considered to be constant) is finally reached, where there are still some finite heat losses, but the heat recovery factor can approach values of 90% or higher. An initial period of only charging heat and not withdrawing it prior to starting cycles of heat injection and withdrawal can accelerate the evolution to this quasi-steady state. Properties that contribute to the time to reach a quasi-steady state include aquifer hydraulic properties (such as permeability and porosity) and aquifer thermal properties (such as thermal conductivity and volumetric heat capacity of the aquifer rock and water).

[0042] Hydraulic and thermal properties of the adj acent confining layers as well as the thermal properties of the wellbore tubing, casing, casing cement, and the formation surrounding the wellbore also impact the magnitude of heat losses. Aquifer layer thickness, initial aquifer temperature, injection and production water flow rate and hot water injection temperature also impact the temperature profiles in the aquifer and their evolution time to a quasi-steady state condition. For a given water flow rate, a thicker aquifer layer means lower radial penetration of the hot water from the wellbore into the aquifer and hence lower surface area for heat loss through conduction into the aquifer.

[0043] A higher temperature of the injected hot water will cause more initial heat loss due to greater temperature differentials. The charging and discharging cycle durations also impact the depth of radial penetration of heat and fluid flux into the aquifer and the time evolution of temperature profiles. In addition to the conduction heat loss, convective heat transfer through water flow as well as convection induced by the temperature gradient can also impact the temperature profiles in the aquifer. Preferred aquifer candidates for geological storage of heat are those that are confined within sealing rock layers and with little to no pressure driven flow of water in the aquifer layer. Any flow of water in the aquifer layer would facilitate heat loss between the hotand cold thermal store zones and reduce the heat storage capacity of the hot thermal store zone.

[0044] System Design Considerations: System design optimization requires design of the different subsystems to be coupled and aligned as the operation and design parameters of one influence the other. Persons having ordinary skill in the art can determine optimal parameters based upon the specific application.

[0045] The water flow rate needed for the heat transfer depends on the compressed air energy storage system’s rated power capacity and the design parameters of the compression and expansion subsystems, such as the flow rate of air, number of compression stages, compressor stage inlet and discharge air temperature, number of expansion stages, expander stage inlet and discharge temperature, and the differential temperature approach between air and water in the indirect heat exchanger.

[0046] The pressure and temperature of the compressed air storage system determine the air pressure and temperature of the last compression stage’s outlet stream and the first expansion stage’s inlet stream. For compressed air storage in aquifers, storage pressure can be between the aquifer native pressure (typically hydrostatic) and the fracture pressure, which is usually determined from the minimum principal stress in the aquifer formation. The air injection temperature can be at the aquifer temperature but can also be higher as a way of storing some thermal heat in the air cushion.

[0047] The number of wells required for water injection into and production from the geological thermal store zones in the aquifer depends on the total water flow rate required for the heat transfer as explained above and also on the injectivity and productivity of the aquifer wells transferring the water between the geological thermal store zones to the surface heat exchangers.

[0048] The injectivity of the well is defined as the injection rate divided by the difference between the wellbore pressure and the aquifer pressure and is a function of the product of permeability and thickness of the aquifer layer. The injectivity of the well limits themaximum injection flow rate per well to ensure that the injection pressure is lower than the fracture pressure, The productivity of the well is defined as the production rate divided by the difference between the aquifer pressure and the wellbore pressure and is also a function of aquifer layer permeability and thickness. Very high pressure drops near the wellbore could cause near wellbore formation damage and reduce permeability. The well productivity determines the maximum production flow rate from each well for which the pressure drop is acceptable.

[0049] Thermal separation between the aquifer cold and hot thermal store zones should be such that there is zero to minimal heat flow between the two zones to ensure the heat storage capacity of the hot thermal store zone is maximized. Aquifer thermal and hydraulic properties as well as the aquifer geometry (flat or dipping layers, fold or pinch-out structure, anticline or dome) are the main factors impacting the inter-well distances. Significant heat exchange between the two zones will lower the temperature of the hot store zone and increase the temperature of the cold store zone and compromise the heat transfer process in the heat exchangers.

[0050] Similarly, when the compressed air storage subsystem is located in the same aquifer system, it needs to be at a distance from the aquifer cold and hot thermal store zones such that there is zero to minimal interference between their temperature and pressure fields and the energy storage capacity of the compressed air storage subsystem is maximized.

[0051] The surface subsystems can be designed by modeling the thermodynamic processes, unit operations, and heat and energy balances. Thermo-hydro-mechanical simulation of simultaneous fluid flow and heat transport in the subsurface can be used to design the subsurface system of wells and their operational control settings (flow rates and pressures). It is important to look at the integrated system design and optimization by coupling all subsystems to ensure alignment of each of their individual operation settings.

[0052] While the embodiment described above uses different zones within the same aquifersystem for the geological thermal stores and the storage of the compressed air, the system and methods also apply to other embodiments where the compressed air storage zone and the geological thermal store are in different aquifer layers of the same reservoir.

[0053] Geological thermal store comprising a geothermal heat source:

[0054] FIG. 2A and FIG. 2B show an embodiment of the system wherein the source of heated water is a geological hydrothermal source. FIG. 2A shows the charging cycle and FIG. 2B shows the discharge cycle.

[0055] In this embodiment, the compression heat from the compression subsystem 110 in the charging cycle is rejected to the atmosphere or used as waste heat elsewhere, and the heat needed for the expansion is provided by a hydrothermal source 210, which is a constant source of heat that is considered as a renewable resource. Cooled water discharged from the heat exchanger subsystem 120 can be re-injected back into hydrothermal source 210 to sustain water balance in the hydrothermal source rock.

[0056] A cooled water injection zone and a hot water production zone can be situated within the hydrothermal heat source 210. The separation distance between the cooled water injection zone and the hot water production zone within the hydrothermal heat source 210 should be such that any heat loss from the hot water production zone is much lower than its heat gain from the hydrothermal heat source 210 and the drop in the temperature of the hot water being produced in the discharge cycle is minimized.

[0057] FIG. 3A and FIG. 3B show an embodiment with a constant heat source using an unconventional or enhanced geothermal heat system 310. Unconventional or enhanced geothermal system 310 can be man-made geothermal reservoirs. For a subsurface region that is hot but contains little permeability and / or fluid, pumping pressurized water into these rocks can stimulate and create a geothermal reservoir by creating shear or tension fractures in the rock.

[0058] In this embodiment, in the first discharge cycle of the compressed air storage subsystem 150, water from an external source 320 is pumped into the hot rock matrix to extract heat, and the produced as hot water for heating air in the expansion subsystem. The cooled water out of the heat exchanger subsystem 120 is then reinjected back into the hot rock matrix to be reheated and reused for subsequent discharge cycles. The external source of water for the first discharge cycle can be an aquifer or a surface water body.

[0059] A cooled water injection zone and a hot water production zone can be situated within unconventional or enhanced geothermal system 310. The separation distance between the cooled water injection zone and the hot water production zone within the unconventional or enhanced geothermal system 310 should be such that any heat transfer between the two zones is much lower than the heat gain from the unconventional geothermal source and the drop in the temperature of the hot water being produced in the discharge cycle is minimized.

[0060] In these embodiments, there is a constant or renewable source of geological heat that controls the evolution of the temperature distribution in the rock and enables very long duration energy storage systems of the order of weeks and months. Heat loss along the wellbore to the surrounding formation still occurs and will evolve to a quasi-steady state over time as a function of the thermal properties of the tubing, casing, cement, and surrounding rock formation. This heat loss can be mitigated by using insulated tubing in the wellbore. Design considerations for the system are similar to what was described earlier for the geological heat storage system. Additional factors to consider for the number of wells and the water circulation pressure include the temperature of the geothermal source and any operating constraints related to single- or two-phase circulation flow of water as well as the pressure needed to keep fractures open in the case of enhanced geothermal heat source systems to enable flow.

[0061] Other embodiments of compressed gas energy storage system with geological thermal store

[0062] While the system and methods of all the embodiments in this disclosure have beendescribed using a compressed air storage system in an aquifer, they are equally applicable to compressed air storage in a cavern or depleted gas and oil reservoir.

[0063] While all the embodiments described in this disclosure use a geological thermal store as the sole heat supply source to heat the air for expansion, persons having ordinary skill in the art can also use the geological thermal store as a supplemental heat supply system to a surface thermal heat store. It can also be used as a supplemental heat supply system to a compressed air storage system that simultaneously stores heat. Such a compressed air thermal store may be created by injecting high-temperature air discharged from the last compression stage with little or no cooling into the air storage system.

[0064] While the system and methods of all the embodiments in this disclosure have been described using air, they are equally applicable to other gases as chosen by persons having ordinary skill in the art, such as carbon dioxide, nitrogen, hydrogen, and natural gas.

[0065] While the present disclosure emphasizes the embodiments, it should be understood that within the scope of the appended claims, the invention might be practiced other than as specifically described herein.

Claims

CLAIMSWhat is claimed is:

1. A compressed gas energy storage system comprising: a) a surface subsystem comprising: i. a gas compression subsystem with one or more compressors; ii. a gas expansion subsystem with one or more expanders; iii. a heat exchange subsystem comprising one or more heat exchangers; b) a geological heat storage subsystem comprising: i. a first zone in fluid communication with the heat exchange subsystem; ii. a second zone in fluid communication with the heat exchange subsystem; c) a compressed gas storage subsystem in fluid communication with the surface subsystem; wherein, during a charging cycle, water from the first zone cools compressed gas from the heat exchange subsystem creating heated water and cooled compressed gas, the heated water is injected into the second zone and the cooled compressed gas is injected into the compressed gas storage subsystem and, wherein, during a discharging cycle, water from the second zone heats compressed gas in the heat exchange subsystem for expansion in the gas expansion subsystem, thereby creating cooled water, and the cooled water is injected into the first zone.

2. The compressed gas energy storage system of Claim 1, wherein the separation distance between the first zone and the second zone of the geological heat storage subsystem is configured to minimize heat loss from second zone to the first zone.

3. The compressed gas energy storage system of Claim 1, wherein the compressed gas storage subsystem is in an aquifer of the geological heat storage subsystem and a separation distance of the compressed gas storage subsystem from the geological heat storage subsystem is configured to minimize interference of the compressed gas storage subsystem with the temperature and pressure fields of the geological heat storage subsystem.

4. The compressed gas energy storage system of Claim 1, wherein the compressed gas storage subsystem is in a different aquifer from an aquifer containing the geological heat storage subsystem.

5. The compressed gas energy storage system of Claim 1, wherein the compressed gas storage subsystem is in a depleted gas or oil reservoir.

6. The compressed gas energy storage system of Claim 1, wherein the compressed gas storage subsystem is in a cavern.

7. The compressed gas energy storage system of Claim 1, wherein thermally insulated tubing is used to minimize heat loss.

8. The compressed gas energy storage system of Claim 1, further comprising a surface based thermal storage subsystem.

9. The compressed gas energy storage system of Claim 1, wherein the compressed gas storage subsystem also functions as a thermal storage subsystem when compressed gas discharged from the gas compression subsystem is injected into the compressed gas storage subsystem.

10. A compressed gas energy storage system comprising: a) a surface subsystem comprising: i. a gas compression subsystem with one or more compressors; ii. a gas expansion subsystem with one or more expanders; iii. a heat exchange subsystem comprising one or more heat exchangers; b) a geothermal heat source subsystem comprising: i. a first zone in fluid communication with the heat exchange subsystem; ii. A second zone in fluid communication with the heat exchange subsystem; c) a compressed gas storage subsystem in fluid communication with the surface subsystem; wherein, during a discharge cycle, water from the second zone heats compressed gas in the heat exchange subsystem for expansion in the gas expansion subsystem, thereby creating cooled water, and the cooled water is injected into the first zone.11 . The compressed gas energy storage system of Claim 10, wherein the separation distance between the first zone and the second zone in the geothermal heat source subsystem is configured to minimize any drop in the temperature.

12. The compressed gas energy storage system of Claim 10, wherein a separation distance of the compressed gas storage subsystem from the geothermal heat source subsystem is configured to minimize interference of the compressed gas storage subsystem with the temperature and pressure fields of the geothermal heat source subsystem.

13. The compressed gas energy storage system of Claim 10, wherein the compressed gas storage subsystem is in an aquifer.

14. The compressed gas energy storage system of Claim 10, wherein the compressed gas storage subsystem is in a depleted gas or oil reservoir.

15. A compressed gas energy storage system of Claim 10, wherein the compressed gas storage subsystem is a cavern.

16. The compressed gas energy storage system of Claim 10, wherein thermally insulated tubing is used to minimize heat loss.

17. The compressed gas energy storage system of Claim 10, wherein the heat source in the geothermal heat source subsystem is a hydrothermal heat source.

18. The compressed gas energy storage system of Claim 10, wherein the heat source in the geothermal heat source subsystem is an unconventional geothermal heat source subsystem comprising: a) a hot rock matrix; b) injection of an external source of water through the second set of one or more wells to fracture the hot rock matrix, absorb the heat and flow to the first zone in a first discharge cycle; c) production of hot water from the first set of one or more wells to the heat exchange subsystem to heat compressed gas for expansion; and d) injection of cooled water from the heat exchange subsystem into the second zone to extract heat for subsequent discharge cycles.

19. The compressed gas energy storage system of Claim 1, wherein the gas is one among a group of air, carbon dioxide, nitrogen, hydrogen, or natural gas.

20. The compressed gas energy storage system of Claim 10, wherein the gas is one among a group of air, carbon dioxide, nitrogen, hydrogen, or natural gas.

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