WO2015015184A2

WO2015015184A2 - Compressed air energy storage system or other system with cold storage

Info

Publication number: WO2015015184A2
Application number: PCT/GB2014/052317
Authority: WO
Inventors: Mark Aidan HOWITT
Original assignee: Storelectric Limited
Priority date: 2013-07-29
Filing date: 2014-07-29
Publication date: 2015-02-05
Also published as: WO2015015184A3

Abstract

Compressed Air Energy Storage System or Other System with Cold Storage An energy storage apparatus comprises a compressor arrangement configured to compress air or other process gas and to supply the compressed air or other process gas to a compressed gas store; an expander arrangement configured to expand the air or other process gas from the compressed gas store; a first heat transfer device associated with the expander arrangement; a thermal store including a thermal storage medium; and a second heat transfer device associated with the compressor arrangement. The first heat transfer device is configured to transfer heat to the air or other process gas which has passed through the expander arrangement from the thermal storage medium, the transfer producing a reduction in the thermal energy of the thermal storage medium in order to store cold in the thermal store. The second heat transfer device is configured to transfer heat between the thermal storage medium and the air or other process gas before the air or other process gas is stored in the compressed gas store, the transfer producing an increase of the thermal energy of the thermal storage medium. The thermal store is configured to store cold such that the temperature of the thermal storage medium in use is below ambient temperature. The thermal transfer fluid and/or the thermal storage medium may be a phase change material.

Description

Compressed Air Energy Storage System or Other System with Cold Storage

BACKGROUND [01 ] The present invention relates to an energy storage apparatus and a method of storing energy. In particular, the present invention relates to a Compressed Air Energy Storage apparatus and method of storing energy using a Compressed Air Energy Storage device. [02] Compressed Air Energy Storage (CAES) is well known, having been first implemented on a large scale at Huntorf in Germany in 1978. In such a system, electricity is converted into compressed air energy using a compressor arrangement. An electric motor associated with the compressor arrangement powers the compressor arrangement to generate compressed air. The compressed air is then stored, in Huntorf's case in salt caverns beneath the plant. As such, the stored compressed air constitutes a store of energy. When electricity is wanted from the plant, the stored energy is extracted. In particular, the compressed air is extracted from the caverns and expanded to generate (or to help generate) electricity, that is, for the electricity regeneration process. In Huntorf's case, the regeneration process is achieved by mixing the compressed air with natural gas and then feeding this mixture into a gas turbine and associated electric generator to generate electricity: the addition of the compressed air increases the efficiency of the gas turbine. The gas turbine is one form of expander - the mixture of natural gas and compressed air is combusted and then passed through the turbine and expanded to atmospheric pressure. Thus this configuration feeds compressed air into an open-cycle gas turbine power station: it is essentially a gas-fired power station rendered more efficient by the compressed air. There has been a similar implementation since 1992 in Macintosh, Alabama, USA. Many similar schemes have been proposed but not implemented since.

[03] During compression, significant amounts of heat are generated. Likewise, during expansion the air needs to absorb significant amounts of heat if it is not to be very cold. This is inescapable due to the first law of thermodynamics, and reduces the intrinsic efficiency of CAES as measured by the ratio of electrical energy out (generated by the generator associated with the expander) to electrical energy in (used to power the compressor arrangement via the motor). Therefore, in some arrangements of CAES, in order to increase the intrinsic efficiency of CAES, some or all of the thermal energy generated during compression is stored, and re-used later in another part of the process (for example to heat the expanded air and/or expander). These arrangements in which heat is stored and later re-used may be referred to as adiabatic CAES arrangements. [04] The Adele Project, proposed in the year 2000 by GE of America and RWE of Germany, envisages a system whereby this heat is stored in a solid heat store, such as one comprising bricks, stones or ceramic materials. The hot compressed air is passed through the solid heat store and transfers much of its heat to it. During the energy recovery process the cooled compressed air is passed back through the heat store, heating it up prior to electricity regeneration.

Although the known adiabatic CAES arrangements produce an improvement in the efficiency of the CAES apparatus, there are still inefficiencies within the system which limit the possible efficiency of such systems. It is an aim of the present invention to provide an energy storage apparatus and method of storing energy which is better than known CAES systems in terms of efficiency, capital cost, size, minimisation of environmental impact, and/or other aspects that may be considered important. It is a further aim of the present invention to obviate or mitigate any disadvantages of known energy storage apparatuses and methods, whether discussed above or otherwise. It is a further aim of the present invention to provide an alternative energy storage apparatus and method.

SUMMARY

[06] According to a first aspect of the invention there is provided a process or system that creates heat; a means of transferring heat from the fluid to the air or process gas before, while or after expanding the air or process gas; a means of storing the fluid that will accommodate its reduced temperature; a means of recovering the cooled fluid; and a means of transferring heat from the air or process gas to a process or system that uses heat.

[07] In other words, there is provided a process or system that uses heat; a means of transferring heat from a fluid (which may be a thermal transfer fluid) to the air or process gas (also referred to as transferring cold from the air or process gas to the fluid) before, while or after expanding the air or process gas; a means of storing the fluid that will accommodate its reduced temperature (also referred to as storing the cold) ; and a means of transferring heat from the process or system that uses heat to the fluid (also referred to as transferring cold from the fluid to the process or system that uses heat).

The process or system that creates heat may be a means of compressing air or another process gas.

The process or system that uses heat may be a means of expanding air or another process gas.

Instead of transferring heat from the process or system that creates heat, the heat may be transferred from the air or process gas itself to the fluid. For example, if the process or system that creates heat is a compressor arrangement, then the heat may be transferred from the air or process gas itself, at a location before/upstream of the compressor arrangement, to the fluid.

Instead of transferring heat to the process or system that uses heat, the heat may be transferred to the air or process gas itself from the fluid. For example, if the process or system that uses heat is an expander arrangement, then the heat may be transferred from the fluid to the air or process gas itself at a location after/downstream of the compressor arrangement.

The air or process gas compression (system that creates heat) and expansion (system that uses heat) system(s) may form a part of a Compressed Air Energy Storage (CAES) system. The compressed gas may be stored in one or more geological structures, whether man-made, natural but enhanced by human intervention, or entirely natural, and/or in one or more vessels, and/or in one or more bladders.

The fluid may be stored in one or more geological structures, whether man- made, natural but enhanced by human intervention, or entirely natural, and/or in one or more vessels, and/or in one or more bladders. The fluid may be stored in a vessel. The fluid may be stored in a bladder.

The fluid (which may be referred to as a thermal transfer fluid) may be a gas at the elevated temperature and a liquid at the lower temperature.

[16] There may be a plurality of thermal transfer fluids and thermal transfer subsystems.

[17] The system may be fitted to a mobile device.

The system may be configured such that energy of any kind (such as, but not restricted to, electric, mechanical, chemical and fluid energy) is converted into compressed air energy.

[19] The compressed air and heat energy may be converted from energy of any kind ("input energy") (such as, but not restricted to, electric, mechanical, chemical, potential and fluid energy).

[20] The compressed air and heat energy may be converted into energy of any kind (such as, but not restricted to, electric, mechanical, chemical, potential and fluid energy). The compressed air and heat energy may be converted into energy which may or may not be the same kind as the input energy.

[21 ] The process and/or system of storing and recovering cold may be applied to a process and/or system other than compressed air energy storage.

[22] The cold may be stored in a solid. The cold may be stored in a thermal storage medium. [23] The cold may be stored in a material (the "thermal storage material") that solidifies upon cooling. Alternatively the cold may be stored in a material that liquefies or crystallises or changes state in any other way upon cooling.

[24] One or more thermal transfer fluids may be the same as one or more thermal storage materials.

The cold may be transferred to the cold storage means by conduction through solid means from the heat source to the cold storage means.

[26] The cold may be transferred from the cold storage means by conduction through a solid means from the cold storage means to the process or system that uses heat.

[27] The cold may be transferred to the cold storage means (cold thermal store) by a thermal transfer fluid (alternatively known as a thermal management fluid) from the heat source to the cold storage means.

The cold may be transferred from the cold storage means (cold thermal store) by a thermal transfer fluid (alternatively known as a thermal management fluid) from the cold storage means to the process or system that uses heat.

The cold storage means (cold thermal store) may comprise a thermal storage medium which is configured to store cold. The thermal storage material may be a fluid. The thermal transfer fluid and the thermal storage material may be a single fluid.

[30] In an alternative arrangement of this first aspect of the invention, the thermal transfer fluid and the thermal storage medium may not be the same. The thermal storage medium may be stored in one or more vessels, bladders, geological features (which may or may not have been affected by human intervention), or other storage means. [31 ] A first thermal transfer fluid may transfer the cold from the expansion means (system that uses heat) to the thermal storage means prior to, during or after expansion of the air or process gas.

[32] A second thermal transfer fluid may transfer the cold from the thermal storage means to the compression means (system that creates heat) prior to, during or after compression of the air or process gas.

[33] The first and second thermal transfer fluids may be the same fluid.

[34] The first and second thermal transfer means may be linked.

[35] The temperature of the fluid in the hot storage means may be substantially higher than ambient temperature.

[36] Said ambient temperature may be the temperature at one of: an inlet to the system, an inlet to the compressor, an inlet to the expander, an outlet to the system, the environment outside the system or apparatus, or inside the compressed gas store.

According to a second aspect of the invention there is provided an energy storage apparatus comprising a compressor arrangement configured to compress air or other process gas and to supply the compressed air or other process gas to a compressed gas store; an expander arrangement configured to expand the air or other process gas from the compressed gas store; a first heat transfer device associated with the expander (or with the air or process gas being expanded); a thermal store including a thermal storage medium; a second heat transfer device associated with the compressor arrangement (or with the air or process gas being compressed); the first heat transfer device being configured to transfer heat to the air or other process gas which has passed through the expander arrangement from the thermal storage medium, the transfer producing a reduction in the thermal energy of the thermal storage medium in order to store cold in the thermal store (such that the temperature of the thermal storage medium in use in this part of the process is substantially below ambient temperature); the second heat transfer device being configured to transfer heat between the thermal storage medium and the air or other process gas before the air or other process gas is stored in the compressed gas store, the transfer producing an increase of the thermal energy of the thermal storage medium ; wherein the thermal store is configured to store heat such that the temperature of the thermal storage medium in use is below ambient temperature.

The ambient temperature may be the temperature at one of: an inlet to the compressor arrangement, an inlet to the expander arrangement or inside the compressed gas store. The ambient temperature may be the temperature at one of: an inlet to the apparatus, an outlet to the apparatus, the environment outside apparatus. If at least part of the apparatus is located in or near soil, rocks or a body of water, the ambient temperature may be the temperature of the soil, rocks or body of water. The ambient temperature may be the temperature of any other object that may provide a suitable reference temperature.

The temperature of the thermal storage medium in use may be below at least one of: about 20 °C, about M3 °C, about 0 °C, about -10Ό, about -20 "C, about - 30 °C, about -40 "C, about -50 "C, about -70 "C, about -80 "C, about -90 °C and about -100 Ό. The temperature of the thermal storage medium in use may be at substantially lower temperatures. For example, the temperature of the thermal storage medium in use may be substantially colder than 100 °C. [40] The process gas may be air and the energy storage apparatus may be a Compressed Air Energy Storage (CAES) apparatus.

The apparatus may be open loop such that the compressor compresses air from the atmosphere and such that the air that has passed through the expander arrangement is released into the atmosphere.

[42] The expander arrangement may be associated with a transducer. The transducer may be configured to convert energy resulting from the expander expanding the compressed air into a different form of energy. Said different form of energy may be electrical energy. Said transducer may be a generator arrangement.

The compressor arrangement may be associated with a compressor transducer, the compressor transducer being configured to convert energy into potential energy of a compressed gas from a different form of energy. The different form of energy may be electrical energy.

The second heat transfer device may be configured to transfer heat between the thermal storage medium and the air or other process gas before the air or other process gas is compressed by the compressor arrangement.

The second heat transfer device may be configured to transfer heat between the thermal storage medium and the air or other process gas after the air or other process gas is compressed by the compressor arrangement.

[46] The second heat transfer device may be configured to transfer heat between the thermal storage medium and the air or other process gas whilst the air or other process gas is being compressed by the compressor arrangement.

[47] The energy storage apparatus may comprise the compressed gas store.

The compressed gas store may be a vessel or bladder. The compressed gas store may comprise a subterranean cavern or other suitable geological feature. Any such subterranean cavern or suitable geological feature may have been modified by human intervention to make it suitable for compressed gas storage.

The thermal storage medium may be a solid. That is to say the thermal storage medium may be formed from a material which is a solid material at all of the operating temperatures of the thermal storage medium.

The thermal storage medium may be a liquid. That is to say the thermal storage medium may be formed from a material which is a liquid material all of the operating temperatures of the thermal storage medium. [51 ] The thermal storage medium may be a phase change material. That is to say the thermal storage medium may be formed from a material which is in one phase while it is at a higher temperature, and in another phase while it is at a lower temperature.

[52] The first heat transfer device may be configured such that said heat transferred from the thermal storage medium to the air or other process gas which has passed through the expander arrangement to is transferred by conduction through a solid to the air or other process gas from the thermal storage medium.

[53] The first heat transfer device may be configured such that said heat transferred from the thermal storage medium to the air or other process gas which has passed through the expander arrangement is transferred by a fluid to the air or other process gas from the thermal storage medium.

[54] The second heat transfer device may be configured such that said heat transferred between the thermal storage medium and the air or other process gas before the air or other process gas is stored in the compressed gas store is transferred by conduction through a solid between the air or other process gas and the thermal storage medium.

[55] The second heat transfer device may be configured such that said heat transferred between the thermal storage medium and the air or other process gas before the air or other process gas is stored in the compressed gas store is transferred by a fluid between the air or other process gas and the thermal storage medium.

[56] Any circulation of the thermal transfer fluid(s) may in one or more closed loops. [57] There may be a single thermal store, or a single set of thermal stores performing similar functions.

[58] There may be a plurality of thermal stores performing different functions, for example a cold store and a hot store.

[59] One or more of the thermal transfer fluids may be phase change material(s). [60] One or more thermal transfer fluids may also be used as one or more thermal storage material(s). [61 ] One or more thermal transfer fluids may differ(s) from one or more thermal storage material(s).

[62] One or more thermal storage material(s) may be phase change material(s). [63] According to a third aspect of the present invention there is provided a method of storing energy using an energy storage apparatus, the energy storage apparatus comprising a compressor arrangement, a compressed gas store, an expander arrangement, a first heat transfer device associated with the expander arrangement (and/or with the air or process gas being expanded), a thermal store including a thermal storage medium, and a second heat transfer device associated with the compressor arrangement (and/or with the air or process gas being compressed); the method comprising: the compressor arrangement compressing air or another process gas; supplying the compressed air or other process gas to the compressed gas store; storing the compressed air or other process gas in the compressed gas store; the expander arrangement expanding the air or other process gas from the compressed gas store; the first heat transfer device transferring heat from the thermal storage medium to the air or other process gas which has passed through the , the transfer reducing the thermal energy of the thermal storage medium in order to store cold in the thermal store, such that the temperature of the thermal storage medium is below ambient temperature; and the second heat transfer device transferring heat between the thermal storage medium and the air or other process gas before the air or other process gas is stored in the compressed gas store, the transfer increasing of the thermal energy of the thermal storage medium.

[64] The ambient temperature may be the temperature at one of: an inlet to the compressor arrangement, an inlet to the expander arrangement or inside the compressed gas store. The ambient temperature may be the temperature at one of: an inlet to the apparatus, an outlet to the apparatus, the environment outside apparatus. If at least part of the apparatus is located in or near soil, rocks or a body of water, the ambient temperature may be the temperature of the soil, rocks or body of water. The ambient temperature may be the temperature of any other object that may provide a suitable reference temperature.

[65] The temperature of the thermal storage medium in use may be below at least one of: about 20 °C, about M3 °C, about 0 °C, about -10Ό, about -20 "C, about - 30°C, about -40 "C, about -50 "C, about -70 "C, about -80 "C, about -90 °C and about -100 Ό. The temperature of the thermal storage medium in use may be at substantially lower temperatures. For example, the temperature of the thermal storage medium in use may be substantially colder than 100 °C.

The process gas may be air and the energy storage apparatus may be a Compressed Air Energy Storage (CAES) apparatus.

The apparatus may be open loop such that the compressor arrangement compresses air from the atmosphere and such that the air that has passed through the expander arrangement is released into the atmosphere. The method of storing energy using such an apparatus may be referred to as an open loop method.

The apparatus may further comprise one or more thermal transfer fluid(s) which may or may not be the same as the thermal storage medium. [69] The energy storage apparatus may further comprise a transducer associated with the expander arrangement. The method may further comprise the transducer converting energy resulting from the expander arrangement expanding the compressed air into a different form of energy. The different form of energy may be electrical energy. Said transducer may be a generator arrangement.

The second heat transfer device may transfer heat between the thermal storage medium and the air or other process gas before the air or other process gas is compressed by the compressor arrangement. [71 ] The second heat transfer device may transfer heat between the thermal storage medium and the air or other process gas after the air or other process gas is compressed by the compressor arrangement. [72] The second heat transfer device may transfer heat between the thermal storage medium and the air or other process gas whilst the air or other process gas is being compressed by the compressor arrangement.

[73] The compressed gas store may be a vessel or bladder. The compressed gas store may comprise a subterranean cavern or other suitable geological feature.

Any such subterranean cavern or suitable geological feature may have been modified by human intervention to make it suitable for compressed gas storage.

[74] The thermal storage medium may be a solid. That is to say the thermal storage medium may be formed from a material which is a solid material at all of the operating temperatures of the thermal storage medium.

[75] The thermal storage medium may be a liquid. That is to say the thermal storage medium may be formed from a material which is a liquid material all of the operating temperatures of the thermal storage medium.

[76] According to a fourth aspect of the invention there is provided an energy storage system and/or apparatus substantially as described in the description with reference to figure 1 or figure 2 or figure 3 or figure 4 or figure 5 or figure 6 or figure 7.

[77] According to a fifth aspect of the invention there is provided a method of storing energy substantially as described in the description with reference to figure 1 or figure 2 or figure 3 or figure 4 or figure 5 or figure 6 or figure 7.

[78] According to a sixth aspect of the invention there is provided a means of compressing air or another process gas; a means of transferring heat from the air or process gas to a phase change material; a means of storing the phase change material that will accommodate its elevated temperature; and a means of transferring heat from the phase change material to the air or process gas before, while or after expanding the air or process gas.

[79] The apparatus may additionally comprise a means of transferring heat from the air or process gas to a first fluid before, while or after the air or gas is compressed; a means of transferring heat from the first fluid to a phase change material; a means of transferring heat from the phase change material to a second fluid, wherein such second fluid may or may not be the same as the first fluid; and a means of transferring heat from the second fluid to the air or process gas before, while or after expanding the air or process gas.

[80] The first fluid may differ from the second fluid.

[81 ] The first fluid may be the same as the second fluid.

[82] The first and second fluids may be the same, and may be in the same fluid system that is in thermal contact (whether directly or indirectly) with all of the compression means, the expansion means and the phase change material. [83] The air or process gas compression and expansion system(s) may form a part of a Compressed Air Energy Storage (CAES) system.

[84] The phase change material may be solid in its cooled state and either liquid or gaseous in its heated state.

[85] The phase change material may be a liquid in its cooled state and gaseous in its heated state.

[86] The phase change material may be crystalline in its cooled state and amorphous in its heated state.

[87] Said change in phase may be a change between solid and either liquid or gas.

[88] Said change in phase may be a change between liquid and gas. [89] Said change in phase may be a change between crystalline and amorphous.

[90] Other phase changes are possible, such as but not restricted to deionisation and recombination, eutectic phase change and peritectic phase change.

[91 ] The phase change material may be stored in a subterranean cavern.

[92] The phase change material may stored in a bladder. [93] The phase change material may be stored in a vessel.

[94] The system may be fitted to a mobile device.

[95] Energy of any kind (such as but not restricted to electric, mechanical, chemical and fluid) may be converted into compressed air energy.

[96] The compressed air and heat energy may be converted from energy of any kind (such as but not restricted to electric, mechanical, chemical, potential and fluid). [97] The compressed air and heat energy may be converted into energy of any kind (such as but not restricted to electric, mechanical, chemical, potential and fluid).

[98] The temperature of the hot reservoir may be no higher than 500^SC.

[99] According to a seventh aspect of the invention there is provided a compressed air energy storage apparatus comprising: a compressor arrangement configured to compress air or another process gas; a heat transfer arrangement configured to transfer heat between the air or process gas and a thermal storage medium; a thermal store arrangement configured to store the thermal storage medium and accommodate substantially elevated or reduced temperature of the thermal storage medium; the heat transfer arrangement comprising: a first heat transfer device configured to transfer heat between the air or process gas and a first thermal transfer fluid before, while or after the air or process gas is compressed; a second heat transfer device configured to transfer heat between the thermal transfer fluid and the first thermal storage medium; a third heat transfer device configured to transfer heat between the thermal storage medium and a second thermal transfer fluid; a fourth heat transfer device configured to transfer heat between the second thermal transfer fluid and the air or process gas before, while or after the air or process gas is expanded by an expander arrangement; wherein the thermal storage medium comprises a phase change material, the phase change material being configured to store heat or cold in the form of latent heat by changing phase upon being heated or cooled. The seventh and eighth (see below) aspects of the invention are defined such that the first heat transfer device is configured to transfer heat between the air or process gas and a first thermal transfer fluid before, while or after the air or process gas is compressed, and such that a fourth heat transfer device configured to transfer heat between the second thermal transfer fluid and the air or process gas before, while or after the air or process gas is expanded by an expander arrangement. The phrase "before, while or after" may refer to time and/or location. For example, the first heat transfer device may be configured to transfer heat between the air or process gas and the first thermal transfer fluid at a time before, while or after the air or process gas is compressed by the compressor arrangement. Alternatively or in addition, the first heat transfer device may be configured to transfer heat between the air or process gas and the first thermal transfer fluid at a location upstream (with respect to the flow direction of process fluid through the compressor arrangement) of the compressor arrangement, at a location in the compressor arrangement or at a location downstream (with respect to the flow direction of process fluid through the compressor arrangement) of the compressor arrangement. Furthermore, the fourth heat transfer device may be configured to transfer heat between the air or process gas and the second thermal transfer fluid at a time before, while or after the air or process gas is expanded by the expander arrangement. Alternatively or in addition, the fourth heat transfer device may be configured to transfer heat between the air or process gas and the second thermal transfer fluid at a location upstream (with respect to the flow direction of process fluid through the expander arrangement) of the compressor arrangement, at a location in the expander arrangement or at a location downstream (with respect to the flow direction of process fluid through the expander arrangement) of the expander arrangement.

[101 ] The phase change material may be configured to store heat and the heat stored by the phase change material may be heat generated by the compressor arrangement.

[102] The phase change material may be configured to store cold and the cold stored by the phase change material may be cold generated by the expander arrangement.

[103] The first thermal transfer fluid may differ from the second thermal transfer fluid.

[104] The second and third heat transfer devices may constitute (or be formed as) a combined thermal store heat transfer device.

[105] Each thermal transfer fluid may be a liquid, a gas, a vapour or a phase change material.

[106] The phase change of such phase change material may be between the liquid and gas or vapour phase.

[107] The first thermal transfer fluid may differ from the second thermal transfer fluid.

The second thermal transfer fluid may be the same as the first thermal transfer fluid such that the first and second thermal transfer fluid are collectively a single thermal transfer fluid.

[108] The single thermal transfer fluid may be contained within a fluid system that is in thermal contact with each of the compressor arrangement, the expander arrangement and the thermal storage medium.

[109] The phase change material may be a solid in its cooled state (i.e. before it has been heated) and either liquid or gaseous in its heated state. [1 10] The thermal storage material may comprise one or more of rock, sand, ceramic or other solid material.

[1 1 1 ] The thermal storage material may wholly or partially comprise one or more of a liquid, a gel, a suspension, a gas, a vapour or another fluid.

[1 12] The thermal storage medium may be stored in a subterranean cavern.

[1 13] The first thermal transfer fluid and/or second thermal transfer fluid may be stored in a rock formation. The rock formation may be a porous geological feature.

[1 14] The first thermal transfer fluid and/or second thermal transfer fluid and/or thermal storage medium may be stored in a vessel. The vessel may be a tank, cylinder or pipe.

[1 15] The apparatus may comprise an arrangement configured to vary the size and/or shape of a cavity that contains the first and/or second thermal transfer fluid.

[1 16] Energy of any kind (such as but not restricted to electric, mechanical, chemical and fluid) may be converted into compressed air energy.

[1 17] The compressed air and heat energy is converted from energy of any kind (such as but not restricted to electric, mechanical, chemical, potential and fluid).

[1 18] The compressed air and heat energy may be converted into energy of any kind (such as but not restricted to electric, mechanical, chemical, potential and fluid).

[1 19] The compressor arrangement and expander arrangement may both be formed at least in part from a common portion of apparatus, the common portion of apparatus may have a first operating mode when the compressor arrangement is compressing the air or other process gas, and may have a second operating mode when the expander arrangement is expanding the air or other process gas. [120] The compressor arrangement may comprise a rotating compressor and/or the expander arrangement may comprise a rotating expander.

[121 ] The compressor arrangement may be coupled with a transducer converting an extrinsic power source into motive force for the compressor arrangement. The transducer may include an axle and/or shaft and/or gears and/or other rotating arrangement. The transducer may include an electrically driven motor.

[122] The expander arrangement may be coupled with a transducer configured to convert the motive force of the expander into an extrinsic power source. The transducer may include an axle and/or shaft and/or gears and/or other rotating arrangement. The transducer may include an electrical generator.

All or part of the generator arrangement and the motor arrangement may be substantially the same arrangement, operated in a different mode for each function of driving and generation.

[124] The expander and the generator may be substantially the same arrangement, as an expander generator.

[125] The apparatus outputs may include one or more other effects such as (but not restricted to) cooling; heating; distillation; vacuum; separation or splitting of chemicals, compounds, materials or products; purification; filtration; osmosis; desalination.

[126] The apparatus may be fitted to a mobile device.

[127] The temperature of the hot reservoir may be no higher than 500^SC. [128] According to an eighth aspect of the invention there is provided a method of storing energy using an energy storage apparatus, the energy storage apparatus comprising a compressor arrangement, an expander arrangement, a thermal store arrangement, and a heat transfer arrangement comprising first, second, third and fourth heat transfer devices; the method comprising: the compressor arrangement compressing air or another process gas; the thermal store arrangement storing a thermal storage medium and accommodating a substantially elevated or reduced temperature of the thermal storage medium ; the heat transfer arrangement transferring heat between the air or process gas and the thermal storage medium ; the first heat transfer device transferring heat between the air or process gas and a first thermal transfer fluid before, while or after the air or process gas is compressed; the second heat transfer device transferring heat between the first thermal transfer fluid and the thermal storage medium ; the third heat transfer device transferring heat between the thermal storage medium and a second thermal transfer fluid; the fourth heat transfer device transferring heat between the second thermal transfer fluid and the air or process gas before, while or after the air or process gas is expanded by an expander arrangement; wherein the thermal storage medium comprises a phase change material, the phase change material storing heat or cold in the form of latent heat by changing phase upon being heated or cooled.

[129] The phase change material may store heat and the heat stored by the phase change material may be heat generated by the compressor arrangement.

[130] The phase change material may store cold and the cold stored by the phase change material may be cold generated by the expander arrangement.

[131 ] The first thermal transfer fluid may differ from the second thermal transfer fluid.

[132] The second and third heat transfer devices may constitute (or be formed as) a combined thermal store heat transfer device.

[133] The second thermal transfer fluid may be the same as the first thermal transfer fluid such that the first and second thermal transfer fluid are collectively a single thermal transfer fluid.

[134] The single thermal transfer fluid may be contained within a fluid system that is in thermal contact with each of the compressor arrangement, the expander arrangement and the thermal storage medium. [135] The phase change material may be a solid in its cooled state and either liquid or gaseous in its heated state.

[136] The phase change material may be liquid in its cooled state and gaseous in its heated state.

[137] The phase change material may undergo different phase changes.

[138] The thermal storage material may comprise one or more of rock, sand, ceramic or other solid material.

[139] The thermal storage material may wholly or partially comprise one or more of a liquid, a gel, a suspension, a gas, a vapour or another fluid.

[140] The thermal storage medium may be stored in a subterranean cavern.

[141 ] The first thermal transfer fluid and/or second thermal transfer fluid may be stored in a rock formation.

[142] The first thermal transfer fluid and/or second thermal transfer fluid and/or thermal storage medium may be stored in a vessel.

[143] The apparatus may comprise an arrangement configured to vary the effective size and/or shape of a cavity that contains the first and/or second thermal transfer fluid.

[144] The compressor arrangement and expander arrangement may both be formed at least in part from a common portion of apparatus, the common portion of apparatus being operated in a first operating mode when the compressor arrangement is compressing the air or other process gas, and being operated in a second operating mode when the expander arrangement is expanding the air or other process gas.

[145] The compressor arrangement may include a rotating compressor and/or the expander arrangement may include a rotating expander. [146] The expander arrangement may be coupled or integrated with a transducer, the transducer converting the motive force of the expander arrangement into an extrinsic power source.

[147] It will be appreciated that any of the optional features discussed above in relation to the first, second, third, sixth, seventh or eighth aspects of the invention, may, where appropriate, be utilised with any of the other aspects of the present invention.

[148] Other aspects and preferred features of the present invention will be apparent from the following description and the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[149] The invention will now be further described by way of example and with reference to the accompanying drawings, in which:

[150] Figure 1 is a schematic diagram of an embodiment of the present invention, showing the direction of flow of both air and fluid;

[151 ] Figure 2 is a schematic diagram of a further embodiment of the present invention which includes a feedback loop of a thermal transfer fluid that incorporates cold storage and hot storage, showing the direction of flow of both air and fluid. The hot storage may optionally be at either approximately ambient or elevated temperature;

[152] Figure 3 is a schematic diagram of a further embodiment of the present invention which includes a single thermal store and two circulating subsystems for thermal transfer fluid. Such circulating subsystems may be fluidly unconnected with each other and/or with the thermal storage;

[153] Figure 4 is a schematic diagram of an embodiment of the invention which includes a thermal storage medium comprising a phase change material; [154] Figure 5 shows a schematic diagram of variant of the arrangement shown in Figure 4, depicting a closed loop thermal transfer subsystem;

[155] Figure 6 shows a schematic diagram of a variant of the arrangement shown in Figure 5 depicting a second thermal store; and

[156] Figure 7 is a schematic diagram that shows another embodiment of the invention depicting an arrangement whereby the process gas itself carries the thermal energy to and from the thermal store.

[157] In all of these figures, for ease of depiction and description, separate pipes, conduits etc. are shown for process gas entering and exiting the cavern. Optionally the gas may enter and exit through a single pipe, conduit or other arrangement.

[158] In all of these figures, for ease of depiction and description, separate pipes, conduits etc. are shown for the thermal transfer fluid travelling to and from the thermal store. Optionally the gas may pass each way through a single pipe, conduit or other arrangement.

[159] Within the figures equivalent features are numbered with the same reference numbers.

DETAILED DESCRIPTION OF THE DRAWINGS

[160] Figure 1 depicts a system in which energy 1 and air 2 is fed in, the air is compressed 3 using the energy, then piped or transported 4 into a storage means for the compressed air 5, from which it can then be piped or transported 6 into an expansion means 7 where it generates energy of another form 8 (that is, other than compressed air), following which the air is expelled 9 from the expansion means. The expansion means may incorporate or be coupled with a transducer (e.g. generator) generates energy of another form.

[161 ] Cold is transferred by a heat transfer means 10 such as (but not limited to) a heat exchanger into the fluid from the expansion means, then piped or transported 1 1 into a cold fluid storage means 12. Such coupling may include coupling with the compressed air in order to heat it prior to expansion, or coupling with the cold expanded air, or coupling with the air at any intermediate stage. Such heating of the fluid by the means of compression may be performed either directly or indirectly. The cold fluid is then piped or transported 13 to the means whereby cold is transferred 14 to the compression means. The expelled air may optionally be fed back into the expansion means in a closed loop or similar system. The compression means 3 could be any heat source, and the expansion means 7 could be any heat sink or equipment or process that is to be heated, or which requires the air or any input or output fluid to be heated.

[162] In other words, Figure 1 depicts an energy storage apparatus according to an embodiment of the present invention. The energy storage apparatus includes a compressor arrangement 3. The compressor arrangement 3 includes an inlet 2 and an outlet 4. An electric motor 3a is associated with the compressor arrangement 3. The electric motor 3a is configured to receive energy 1 (in this case in the form of electrical energy) and use this energy to power the compressor arrangement 3. In other embodiments, the motor may be replaced with any appropriate transducer which uses any appropriate form of energy to power the compressor arrangement. The compressor arrangement 3 receives air from the atmosphere via inlet 2 and outputs compressed air at elevated pressure (relative to the pressure of the air at the inlet 2) at the outlet 4. The compressed air is transported along a conduit 4 into a compressed gas store 5. The compressed gas store 5 is configured to store the compressed gas at a pressure which is elevated compared to that of the atmosphere.

[163] When required, the compressed gas is transported along a conduit 6 to an expander arrangement 7. The expander arrangement 7 is associated with a transducer 7a. The transducer 7a converts the kinetic energy of the expansion of the compressed gas in the expander arrangement 7 into another form of useful energy 8. For example, the transducer 7a may be an electrical generator arrangement which converts the kinetic energy of the expansion of the compressed gas into electrical energy. However, it will be appreciated that in other embodiments the transducer may be of any appropriate type which converts the kinetic energy of the expansion of the compressed gas into any appropriate type of useful energy. The expanded air which has passed through the expander arrangement is output to atmosphere via an expander outlet 9.

[164] The energy storage apparatus also includes a first heat transfer device 10 associated with the expander arrangement 7. The first heat transfer device is configured such that it can exchange heat with the expander arrangement 7 and/or the expanded air. The heat exchange may be direct (e.g. via conduction) and/or remote (e.g. via radiation). The first heat transfer device 10 is configured to transfer heat from a thermal storage medium of a thermal store 12 to the air or other process gas which has passed through the expander. In some embodiments the heat transfer device 10 may transfer heat to the expanded air or other process gas directly (for example, the first heat transfer device may include a heat exchanger which is located directly in the flow path of the expanded air or other process gas). In other embodiments the heat transfer device 10 may transfer heat to the expanded air indirectly (for example, the first heat transfer device may include a heat exchanger which is associated with an intermediate body (e.g. the expander arrangement), the intermediate body being located in the flow path of the expanded air. In such embodiments the expanded air cools the intermediate body and the heat transfer device exchanges heat with the intermediate body.

[165] In one example the first heat transfer device includes a heat exchanger which is coupled to the expander arrangement. A heat transfer fluid is piped or otherwise transported (by conduit 1 1 in Figure 1 ) between the heat exchanger and the thermal store 12 in order to transfer heat between the expander arrangement

(and hence the expanded air) and the thermal storage medium of the thermal store 12. As previously discussed, the first heat transfer device may, in some embodiments transfer heat between the thermal store and the air which has been expanded by the expander arrangement. In other embodiments the first heat transfer device may transfer heat between the thermal store and the expander arrangement. In still further embodiments the first heat transfer device may transfer heat between the thermal store and air as it is transported between the compressed gas store and the expander arrangement. [166] The energy storage apparatus further includes a second heat transfer device 14 associated with the compressor arrangement 3. In this embodiment the second heat transfer device is configured such that it can exchange heat with the compressor arrangement 3. In other embodiments, additionally, or in the alternative, the second heat transfer device may be configured such that it can exchange heat with the air or other process gas provided to the inlet 2 to the compressor 3 and/or with the air or other process gas provided to the compressed gas store 5 by the compressor arrangement 3. The heat exchange may be direct (e.g. via conduction) and/or remote (e.g. via radiation). The second heat transfer device 10 is configured to transfer heat to the thermal storage medium of the thermal store 12 from the air or other process gas before it is stored in the compressed gas store. In some embodiments the second heat transfer device 14 may directly transfer heat from the air before it is stored in the compressed air store (for example, the second heat transfer device may include a heat exchanger which is located directly in the flow path of the air before it is stored in the compressed air store). In other embodiments the second heat transfer device 14 may transfer heat from the air indirectly (for example, the second heat transfer device may include a heat exchanger which is associated with an intermediate body (e.g. the compressor arrangement), the intermediate body being located in the flow path of the air before it is stored in the compressed air store. In such embodiments the air heats the intermediate body and the second heat transfer device exchanges heat with the intermediate body.

[167] In one example the second heat transfer device includes a heat exchanger which is coupled to the compressor arrangement. The heat transfer fluid is piped or otherwise transported (for example in conduit 13, as shown in figure 1 ) between the heat exchanger and the thermal store 12 in order to transfer heat between the compressor arrangement and the thermal storage medium of the thermal store 12. The second heat transfer device may, in some embodiments transfer heat between the thermal store and the air before the air is compressed by the compressor arrangement (i.e. at a position upstream (with respect to the direction of flow of air through the compressor arrangement) of the compressor arrangement). In other embodiments the second heat transfer device may transfer heat between the thermal store and the air after the air is compressed by the compressor arrangement (i.e. at a position downstream (with respect to the direction of flow of air through the compressor arrangement) of the compressor arrangement). In still further embodiments the second heat transfer device may transfer heat between the thermal store and air whilst the air is compressed by the compressor arrangement.

[168] Known CAES (Compressed Air Energy Storage) systems may have a similar structural layout to that described in relation to Figure 1 above, however, they operate in a very different manner. [169] Known CAES systems may utilise a heat transfer device associated with the compressor in order to extract heat from the compressor and/or compressed air. This extracted heat is supplied to a heat store. The supply of the extracted heat to the heat store results in an increase in the thermal energy of the heat store (i.e. an increase in temperature of the heat store). The temperature of the heat store when the extracted heat has been supplied to the heat store will be significantly above the ambient temperature (e.g. the temperature of the environment to the exterior of the heat store). The ambient temperature may be in the range of about 0 °C to about 40 °C. The temperature of the heat store when the extracted heat has been supplied to the heat store may be in excess of 50 °C and, in many cases may be in excess of 100 °C.

[170] The previously described known CAES systems may utilise a further heat transfer device associated with the expander in order to supply heat from the heat store to the expander. The supply of the heat from the heat store to the expander will result in an increase in the thermal energy (i.e. temperature) of the expander and air passing through the expander. Conversely, the supply of the heat from the heat store to the expander will result in a decrease in the thermal energy of the heat store. [171 ] To the contrary of the previously described known CAES systems, the energy storage apparatus according to the present invention operates as follows. The first heat transfer device associated with the expander transfers cold from the expander arrangement and/or air which passes through the expander arrangement to the thermal storage medium of the thermal store. The transfer of cold to the thermal store results in a decrease (or reduction) in the thermal energy of the thermal store, and, in particular, the thermal storage medium of the thermal store. This is equivalent to a decrease (or reduction) in the temperature of the thermal store, and, in particular, the thermal storage medium of the thermal store, and may also (or instead) result in a change of phase of the thermal storage medium of the thermal store. The temperature and/or phase change of the thermal store (and, in particular, the thermal storage medium of the thermal store) when the cold has been transferred to the thermal store will be below the ambient temperature (e.g. the temperature of the environment to the exterior of the thermal store). The ambient temperature may be in the range of about 0 °C to about 40 °C. The temperature of the thermal storage medium of the thermal store when the heat has been transferred to the thermal store may be below at least one of: about 20 ^qC, about -\ 0 °C, about 0 °C, about -10 Ό, about -20 °C, about -30 °C, about -40 °C, about -50 °C, about -70 °C, about -80 °C, about -90 °C and about -Ι ΟΟ Ό, or even colder. A thermal store which stores heat such that the temperature of the thermal store (and, in particular, the thermal storage medium of the thermal store) is less than the ambient temperature may be referred to as a cold store.

[172] The second heat transfer device associated with the compressor transfers heat to the thermal store, in particular, the thermal storage medium of the thermal store from the compressor arrangement and/or air or other process gas before it is stored in the compressed gas store. The transfer of heat to the thermal store from the compressor arrangement and/or air or other process gas before it is stored in the compressed gas store will result in a decrease in the thermal energy (i.e. temperature) of the compressor arrangement and/or air or other process gas. Conversely, the transfer of the heat to the thermal store from the compressor arrangement and/or air or other process gas will result in an increase in the thermal energy (i.e. temperature) of the thermal store. [173] Within this specification the transfer of heat from a first entity to a second entity may refer to transmission of heat from the first entity to the second entity which results in the thermal energy of the first entity decreasing and the thermal energy of the second entity increasing. This may be also be referred to as hot transfer from a first entity to a second entity or the first entity heating the second entity. Alternatively, heat may be transmitted to the first entity from the second entity which results in the thermal energy of the first entity increasing and the thermal energy of the second entity decreasing. This type of heat transfer may be referred to as the first entity cooling the second entity or cold transfer (or transferring cold) from the first entity to the second entity. It will be appreciated that hot transfer from the first entity to the second entity is equivalent to cold transfer from the second entity to the first entity. If heat is transferred to a thermal storage medium such that the thermal energy of the thermal storage medium increases, then the thermal storage medium may be said to be storing heat. Conversely, if heat is transferred to a thermal storage medium such that the thermal energy of the thermal storage medium decreases, then the thermal storage medium may be said to be storing cold.

[174] Embodiments of the invention in which the second heat transfer device enables cold to be transferred from the thermal store to the compressor arrangement and/or air before or during compression of the air by the compressor arrangement may reduce the temperature of the air before it is compressed by the compressor arrangement. Pre-cooling the air in this manner before compression may make compression of the air by the compressor arrangement more efficient (as compared to the compressor arrangement compressing air of a higher temperature). This will in turn increase the efficiency of the energy storage apparatus.

[175] The inventor submits that it is neither obvious nor straightforward to utilise the thermal store of the present invention in which the temperature of the thermal storage medium of the thermal store in use is substantially below ambient temperature.

[176] Furthermore, it is submitted that it is not obvious (with reference to Figure 2, below) to have a "hot" store operating at or close to ambient temperature. Regarding some embodiments, nor is it obvious to transfer such cold to the air or other process gas prior to or during compression. Regarding some of these embodiments, nor is it obvious to place the heat exchangers before compression or after expansion. There is considerable technical prejudice within the field towards storing heat in a heat store such that the temperature of the thermal storage medium of the heat store is significantly above ambient temperature and such that the stored heat is used to pre-heat air before it is expanded by the expander in order to make the expansion more efficient. Because the air must be heated (i.e. have its temperature increased) in order to make the expansion more efficient, it is desirable that the temperature of the heat store is as high as possible, and in particular greater than ambient, so that it can be used to raise the temperature of the air to be expanded as much as possible therefore resulting in the greatest possible efficiency of the expander.

[178] It would not be possible to replace a heat store of a known CAES system which stores heat at a temperature which is greater than ambient with a thermal store according to the present invention which stores heat at a temperature below ambient, because by doing so the thermal store according to the present invention which stores heat at a temperature below ambient would not be capable of sufficiently raising the temperature of the air to be expanded to thereby increase the efficiency of the expander. [179] Another reason why a person may be motivated away from storing cold in a thermal store such that the temperature of the thermal storage medium of the thermal store is below ambient temperature it that, in embodiments in which heat is transferred to and from the thermal store by a liquid, the liquid will become more viscous at temperatures below the ambient temperature (as compared to at temperatures significantly above ambient temperatures). The increased viscosity of the liquid will reduce the flow rate of the heat transfer liquid to and from the thermal store. This may either significantly, adversely affect the rate at which heat can be transferred to and from the thermal store and/or may necessitate the use of costly pumps to pump the heat transfer liquid to/from the thermal store.

[180] There are several advantages of storing cold (i.e. storing heat such that the temperature of the thermal storage medium is less than the ambient temperature) as opposed to storing heat in the known manner (i.e. such that the temperature of the thermal storage medium is substantially greater than the ambient temperature). These are discussed below.

[181 ] One advantage of storing cold rather than hot storage is that cold fluids are, in general, more compact than hot fluids. As such, a thermal store which stores heat such that its temperature is below the ambient temperature may take up less space as compared to a heat store which stores heat such that its temperature is above the ambient temperature. This may be advantageous in applications in which there is limited space for a thermal store. Furthermore, because a cold store may be smaller in size than an equivalent hot store, the cold store may be cheaper and easier to manufacture.

[182] In addition, the relatively small size of the cold store compared to an equivalent hot store may mean that it is possible to locate the cold store closer to other components of the energy storage device. This not only makes assembly of the energy storage device less expensive and more straightforward, but also results in reduced heat losses (and therefore greater efficiency of the energy storage apparatus) between the thermal store and the components of the energy storage device which transfer heat to/from the thermal store. This is because heat loss is usually greater the greater the distance heat must travel, and the distance heat must travel between the thermal store and the components of the energy storage device which transfer heat to/from the thermal store is reduced when the cold store is located closer to other components of the energy storage device.

[183] If a phase change material is used as a thermal storage medium, the cold state normally requires a much smaller volume than the hot state for storage, piping, pumping etc. [184] It is also desirable to store cold rather than heat because it is easier to utilise the latent heat of phase change at such temperatures than at temperatures significantly above ambient.

[185] The use of a phase change from gas to liquid may employ the latent heat of vaporisation to store a greater amount of energy with reduced temperature differential and/or volume of cold storage required. A large amount of energy is stored while minimising temperature difference.

[186] An additional advantage of a cold store as compared to a hot store is that the technologies for transporting and storing cold fluids is well known, particularly in liquefied gas industries such as natural gas, nitrogen, oxygen and carbon dioxide.

[187] In some embodiments the temperature, in use, of the thermal storage medium of a thermal store according to the present invention is not only below ambient temperature, but is also much closer to ambient temperature than the temperature in use of the thermal storage medium of a known hot storage thermal store. The reduced difference between ambient temperature and the temperature of the thermal storage medium means that the rate of any heat transfer between the thermal storage medium and the ambient environment is reduced. This means that, for a given time the potential for heat to undesirably be transferred between the thermal store and the ambient environment is reduced using a cold store according to the present invention. Furthermore, the reduced difference between ambient temperature and the temperature of the thermal storage medium means that the thermal store may require less insulation to prevent heat from undesirably being transferred between the thermal store and the ambient environment. The reduced requirement for insulation is a further reason that a cold store according to the present invention takes up less space and is less costly than a previously known equivalent type of hot storage thermal store.

[188] This system is equally applicable to any system wherein there is a process or system that creates heat 3 (e.g. compressor) and a process or system that uses heat 7 (e.g. expander) are separated in time and/or location from each other.

[189] Figure 2 depicts a further embodiment of energy storage device in accordance with another embodiment of the invention. This embodiment is the same as the embodiment of the invention shown in figure 1 except that it additionally comprises a second thermal store 16 which includes a thermal storage medium. This may be a second thermal storage medium. Likewise, it may be the same as the thermal transfer fluid. A third conduit 17 links the first heat transfer device 10 with the second thermal store 16, and a fourth conduit 15 links the second heat transfer device 14 with the second thermal store 16.

[190] This embodiment functions in the same manner as the embodiment described in relation to Figure 1 except that, in addition to the second heat transfer device 14 transferring heat between the thermal storage medium of the thermal store 12 and the air or other process gas before the air or other process gas is stored in the compressed gas store (such that thermal energy of the thermal storage medium of the thermal store 12 is increased, thereby reducing the thermal energy of the air or other process gas), the second heat transfer device 14 also transfers heat between the air or other process gas and the second thermal store 16 via conduit 15 such that the transfer produces an increase in the thermal energy of the second thermal storage medium of the second thermal store 16.

[191 ] Likewise, in addition to the first heat transfer device 10 transferring heat between the air which has passed through the expander arrangement 7 and the thermal storage medium of the thermal store 12 (such that thermal energy of the thermal storage medium of the thermal store 12 is decreased, thereby increasing the thermal energy of the air), the first heat transfer device 10 also transfers heat between the air and the second thermal store 16 via conduit 1 1 such that the transfer produces an increase in the thermal energy of the second thermal storage medium of the second thermal store 16.

[192] In this way the second thermal store 16 operates as a hot store. The conduits 1 1 , 13, 15 and 17, heat transfer devices 10 and 14, and thermal stores 12 and 16 act as a heat cycle - conduits 1 1 and 13 and thermal store 12 provide a transfer of cold from the first heat transfer device 10 to the second heat transfer device 14, and conduits 15 and 17 and thermal store 16 provide hot transfer from the second heat transfer device 14 to the first heat transfer device 10. [193] Alternatively, this embodiment may be operated such that the second thermal store, its thermal storage material and the thermal transfer fluid(s) in conduits 15 and 17 operate at or close to ambient temperature. [194] As previously discussed the conduits 1 1 , 13, 15 and 17 carry a thermal transfer fluid which is used to transfer heat between the various parts of the system. Any appropriate thermal transfer fluid may be used. Also, one or more of the thermal transfer fluids may or may not be the same as one or more of the thermal storage materials. In some embodiments, a single fluid (optionally changing phase through the cycle) is used in both thermal stores and all conduits.

[195] Utilising a cycle (or closed loop) for the thermal transfer fluid in this manner means that the thermal transfer fluid can be reused. That is to say, the thermal transfer fluid is cooled by the first heat transfer device 10. The cooled thermal transfer fluid is then stored in the first thermal store 12 and transported to the second heat transfer device 14. The thermal transfer fluid is then heated by the second heat transfer device. The heated thermal transfer fluid is then stored in the second thermal store 16 and transported back to the first heat transfer device. The cycle then repeats.

[196] The second thermal store 16 may store heat such that the temperature of the second thermal storage medium in use is above ambient temperature.

[197] In other words, in this embodiment, hot (heat transfer) fluid is piped or transported 15 from the means whereby heat is transferred from the compression means (compressor arrangement). It is then stored in its hot state 16 (in the second thermal store) and then piped or transported 17 to the means whereby heat is transferred to the expansion means (expander). This forms a closed loop or similar system for the fluid.

[198] In the previously described embodiment the first heat transfer device receives hot thermal transfer fluid from the second thermal store and produces cold thermal transfer fluid for the first thermal store. In some embodiments the first heat transfer device may only produce cold thermal transfer fluid for the first thermal store - in such situations a third heat transfer device may receive hot thermal transfer fluid from the second thermal store.

[199] In the previously described embodiment the second heat transfer device produces hot thermal transfer fluid for the second thermal store and receives cold thermal transfer fluid from the first thermal store. In some embodiments the second heat transfer device may only receive cold thermal transfer fluid from the first thermal store - in such situations a fourth heat transfer device may produce hot thermal transfer fluid for the second thermal store.

[200] The heat transfer fluid used in either of the previously described embodiments is used to store the cold generated from expansion of air, such as (but not restricted to) the expanded air used to generate electrical energy. The heat transfer fluid used in embodiment described in relation to Figure 2 may also be used to store the heat generated from compression of air, such as (but not restricted to) the compressed air used to store energy in the compressed gas store. That is to say, in the previously described embodiments the thermal transfer fluid and thermal storage medium are one and the same - the thermal transfer fluid is stored by the thermal store(s) to store heat. In other embodiments the thermal transfer fluid and thermal storage medium may be different. For example, in some embodiments the thermal storage medium of the thermal store(s) may be a solid material. In such embodiments the thermal transfer fluid flows past the thermal storage medium and heat is transferred between the thermal transfer fluid and thermal storage medium. The thermal storage medium may be formed from any appropriate material in any appropriate state provided it is capable of storing heat.

[201 ] Figure 3 shows a schematic view of a further embodiment of the present invention. This embodiment is substantially the same as that shown in Figure 1 . As in the embodiment shown in Figure 1 , in this embodiment, there is a thermal store 12. Again, similar to the embodiment shown in Figure 1 , conduits 1 1 and 13 respectively link the first and second heat transfer devices 10 and 14 to the thermal store 12. As previously discussed the conduits 1 1 and 13 allow heat transfer between the thermal store 12 and the first and second heat transfer devices 10 and 14 respectively via the flow of heat transfer fluid through the conduits. The embodiment shown in Figure 3 differs from that shown in Figure 1 in that it includes additional conduits 15a and 17a. Conduit 15a runs between the second thermal transfer means 14 and the thermal store 12. Conduit 17a runs between the first thermal transfer means 14 and the thermal store 12.

[202] During expansion of the air or process gas by the expander arrangement 7, the thermal transfer fluid takes cold from the thermal transfer device 10 via conduit 1 1 to the thermal store 12. The thermal transfer fluid then flows along conduit 17a back to the thermal device 10. The thermal transfer fluid transfers its cold from the thermal transfer device 10 to the thermal storage medium of the thermal store. As such, the temperature of the thermal transfer fluid in conduit 1 1 is less than in conduit 17a. This may optionally form a closed loop subsystem for the thermal transfer fluid on the expander arrangement side of the system or apparatus.

[203] Likewise, during compression of the air or process gas by the compressor arrangement 3, the thermal transfer fluid takes cold from the thermal store 12 via conduit 13 to the thermal transfer device 14. The thermal transfer fluid then flows along conduit 15a back to the thermal store 1 2. The thermal transfer fluid transfers its cold from the thermal storage medium of the thermal store 12 to the thermal transfer device 14. As such, the temperature of the thermal transfer fluid in conduit 13 is less than in conduit 15a. This may optionally form a closed loop subsystem for the thermal transfer fluid on the compressor arrangement side of the system or apparatus.

[204] It is preferred within this embodiment that the thermal transfer fluid in each closed loop subsystem differs from the thermal storage medium. However, in some applications the thermal transfer fluid in each closed loop subsystem and the thermal storage medium may be the same. If they are the same, then there may be some transfer of fluid between the conduits and the thermal store.

[205] As previously discussed, some embodiments of the present invention may include a thermal transfer fluid and/or thermal storage medium which includes a phase change material. Discussed below are some further aspect of the invention relating to the use of thermal transfer fluid and/or thermal storage medium which include a phase change material.

[206] Figure 4 depicts a system in which energy 1 and air 2 is fed in, the air is compressed 3 using the energy (by a compression arrangement), then piped or transported 4 into a storage means for the compressed air 5, from which it can then be piped or transported 6 into an expansion means 7 (or expansion arrangement) where it incorporates or is coupled with a means that generates energy of another form 8 (that is, other than compressed air), following which the air is expelled 9 from the expansion means. Heat is transferred by a heat transfer means 3 such as (but not limited to) a heat exchanger into a first fluid from the compression means, then piped or transported 13 into a first heat transfer means 12a thermally coupled to a storage means 12 for a phase change material 12b.

[207] When heat is required for expansion of the air, heat is transferred using a second heat transfer means (also denoted 12a in the diagram) into a second fluid 1 1 . Such second heat transfer means may or may not be the same as the first heat transfer means, and/or such second fluid may or may not be the same as the first fluid. The hot fluid is then piped or transported 1 1 to the means whereby heat is transferred 10 to the expansion means 7. The expelled air 9 (i.e. air expelled by the expander arrangement) may optionally be fed back into the compression means in a closed loop or similar system. [208] Figure 5 depicts a similar system to Figure 4, in which the hot thermal management fluid 13 is transported to a heat exchange arrangement 12a which exchanges heat with a phase change material 12b that is contained within a vessel 12. In this arrangement, no cold fluid storage vessel is required, though one may be included within the system (see Figure 6). In other words, in this embodiment, the thermal management fluid (also referred to as the thermal transfer fluid), instead of passing into a hot reservoir, passes through a heat exchanger to exchange its heat with a phase change material 12b that is contained within a hot reservoir 12. In this embodiment, no cold reservoir is required (though one may be added) as the thermal management fluid can circulate freely. [209] Within the described embodiments the hot thermal management fluid is transported through pipes, conduits or other means coupled with the means of compression (compression arrangement) such that the means of compression heats the fluid prior to transferring its heat to the phase change material. Such coupling includes coupling with the hot compressed air, or coupling with the cold uncompressed air in order to cool it prior to compression, or coupling with the air at any intermediate stage. Such heating of the fluid by the means of compression may be performed either directly or indirectly.

[210] The hot thermal management fluid would then transfer its heat through a heat transfer means to the phase change material. The thermal management fluid cools as its heat is transferred (to the phase change material). The fluid may then optionally be transported back to the means of compression in a closed loop, to be made available for heating again by such means of compression.

[21 1 ] In some embodiments, the thermal storage material may be a solid or a fluid instead of a phase change material.

[212] A cold second thermal management fluid 1 1 (which may or may not be the same as the first thermal management fluid) would then receive heat transferred to it through a heat transfer means 12a from the phase change material 12b. The fluid 1 1 heats up as the heat is transferred. Such heated fluid 1 1 is transported through pipes or other means coupled with the means of expansion 7 such that the means of expansion 7 cools the fluid. Such coupling includes coupling with the compressed air in order to heat it prior to expansion, or coupling with the cold expanded air, or coupling with the air at any intermediate stage, such as when the air is being expanded by the expander arrangement 7. Such heating of the process fluid by the means of compression may be performed either directly or indirectly.

[213] The second thermal management fluid may then optionally be transported back to the means of compression in a closed loop, to be made available for heating again by such means of compression. [214] The hot and/or cold (thermal transfer) fluid(s) may be contained within a pressurised system in order to enable the fluid (such as but not limited to water, brine, ammonia or other appropriate liquid or gas, preferably with high specific heat capacity). In some embodiments the thermal transfer fluid is pressurised such that it may be heated to a temperature above its boiling point at atmospheric pressure. This may enable thermal transfer fluids which could otherwise not be used at atmospheric pressure to used. The use of pressurised thermal transfer fluids may also enable greater possible temperature changes between the fluid's (or fluids') hot and cold states.

[215] The phase change material may be solid in its cold state and liquid in its heated state, or liquid when cold and gaseous when hot, or undergo some other phase change(s) - for example - crystallisation or sublimation. The material is selected such that the temperature at which the phase change occurs is within the range of temperatures over which the thermal management system (or heat transfer arrangement) operates in order for a significant portion of the thermal energy to be stored in the form of latent heat (in addition to or as an alternative to heat stored as a function of the specific heat capacity of a substance). [216] In any embodiment, the selection of phase change material would be made subject to various considerations such as, but not restricted to, one or more of: the preferred containment means, the piping or transportation means, the ease of heat transfer to and from the fluid, the nature of the material itself, and the failsafe or containment systems in case of failure of any item of equipment.

[217] In any of these embodiments, factors that may be considered (when determining an appropriate phase change material and/or thermal transfer fluid) under the title of the nature of the material itself may include, but not be restricted to, one or more of: the heat capacity of the material at the operational temperatures, its melting and/or boiling point, its corrosiveness, its flammability or explosiveness, its usefulness for other purposes such as in a fuel cell, its cost, the cost and means of its disposal, and any aspect(s) of its environmental friendliness. [218] The phase change material vessel may optionally be a pressure vessel in order to hold the phase change material at elevated pressure. This is particularly useful if the phase change material is gaseous when hot, in order to reduce the necessary size of the vessel.

[219] In other embodiments, the same pipe (or transfer means, or conduit) may be used between the phase change heat storage and the compressed air storage means for both directions of travel of the compressed air. Likewise, if the compression means is the same as the expansion means, or is generally proximate to it, the same pipe (or transfer means) may be used for all or part of both directions of travel of the compressed air. This applies equally to all embodiments, where practicable.

[220] In other embodiments, the compressed air itself may pass through the heat transfer means, without the need for an intermediate thermal management fluid.

[221 ] Figure 6 depicts another version of the system depicted by Figure 5, in which there is an additional heat transfer arrangement 16a to exchange heat with a phase change material 16b in a cold vessel (thermal store for storing cold) 16. It is likely, but not necessary, that the phase change material in the cold vessel differs from that in the hot vessel, in order to ensure that the temperature at which the phase change occurs corresponds with the operational temperatures of that part of the thermal management system. [222] In other embodiments, the compressed air itself may pass through the heat transfer means, without the need for an intermediate thermal management fluid.

[223] Figure 7 is a schematic diagram that shows another embodiment, in which the compressed air 4a, 4b is fed through the phase change heat storage 12 (and/or a connected heat transfer device 12a) to the compressed air storage means 5, and back from such compressed air storage means through the heat storage (and/or a connected heat transfer device) 12a to the expansion means 7.

[224] Alternatively, the embodiments shown in Figures 4 to 7 may be used to store cold instead of heat. "Cold", when used to depict a physical property, is the inverse of heat, that is, the energy of thermal difference wherein such difference is a reduction in temperature below ambient, or below the temperature denoted by "hot".

In particular, in the embodiment shown in Figure 4, cold may be transferred by the heat transfer means 10 from the expander or the exhaust of said expander such as (but not limited to) a heat exchanger into a first fluid from the compression means, then piped or transported 1 1 into a first heat transfer means 12a thermally coupled to a storage means 12 for a phase change material 12b.

[226] When cold is required for compression of the air, cold is transferred using the second heat transfer means into the second fluid 13. Such second heat transfer means may or may not be the same as the first heat transfer means, and/or such second fluid may or may not be the same as the first fluid. The cold fluid is then piped or transported 13 to the means whereby cold is transferred 14 to the compression means 7. The expelled air 9 (i.e. air expelled by the expander arrangement) may optionally be fed back into the compression means in a closed loop or similar system.

[227] In the embodiment shown in Figure 5, the cold thermal management fluid 1 1 may be transported to the heat exchange arrangement 12a which exchanges heat with a phase change material 12b that is contained within a vessel 12. In this arrangement, no hot fluid storage vessel is required, though one may be included within the system (see Figure 6) as the thermal management fluid can circulate freely. In other words, in this embodiment, the thermal management fluid (also referred to as the thermal transfer fluid), instead of passing into a cold reservoir, passes through a heat exchanger to exchange its cold with a phase change material 12b that is contained within a cold reservoir 12.

[228] Within the described embodiments the cold thermal management fluid is transported through pipes, conduits or other means coupled with the means of expansion (expansion arrangement) such that the means of expansion cools the fluid prior to transferring its cold to the phase change material. Such coupling includes coupling with the hot compressed air, or coupling with the cold uncompressed air in order to cool it prior to compression, or coupling with the air at any intermediate stage such as but not restricted to the compression arrangement 3. Such cooling of the fluid by the means of expansion may be performed either directly or indirectly.

[229] The cold thermal management fluid would then transfer its cold through a heat transfer means to the phase change material. The thermal management fluid heats as its heat is transferred (to the phase change material). The fluid may then optionally be transported back to the means of expansion in a closed loop, to be made available for cooling again by such means of expansion.

[230] In some embodiments, the thermal storage material may be a solid or a fluid instead of a phase change material.

[231 ] A second thermal management fluid 13 (which may or may not be the same as the first thermal management fluid) would then receive cold transferred to it through a heat transfer means 12a from the phase change material 12b. The fluid 13 cools down as the heat is transferred. Such cooled fluid 1 1 is transported through pipes or other means coupled with the means of compression 7 such that the means of compression 7 heats the fluid. Such coupling includes coupling with the uncompressed air in order to cool it prior to compression, or coupling with the hot compressed air, or coupling with the air at any intermediate stage, such as but not restricted to the compression arrangement 3. Such cooling of the process fluid by the means of expansion may be performed either directly or indirectly.

The second thermal management fluid may then optionally be transported back to the means of expansion in a closed loop, to be made available for cooling again by such means of expansion.

[233] The hot and/or cold thermal transfer and/or thermal storage fluid(s) may be contained within a pressurised system in order to enable the fluid (such as but not limited to water, brine, ammonia or other appropriate liquid or gas, preferably with high specific heat capacity and/or good fluid properties for the temperatures encountered in the system). In some embodiments the thermal transfer fluid is pressurised such that it may be heated to a temperature above its boiling point at atmospheric pressure. This may enable thermal transfer fluids which could otherwise not be used at atmospheric pressure to used. The use of pressurised thermal transfer fluids may also enable greater possible temperature changes between the fluid's (or fluids') hot and cold states.

[234] The phase change material may be solid in its cold state and liquid in its heated state, or liquid when cold and gaseous when hot, or undergo some other phase change(s) - for example - crystallisation or sublimation. Other phase changes may be employed. The material is selected such that the temperature at which the phase change occurs is within the range of temperatures and pressures over which the thermal management system (or heat transfer arrangement) operates in order for a significant portion of the thermal energy to be stored in the form of latent heat (in addition to or as an alternative to heat stored as a function of the specific heat capacity of a substance).

[235] In any embodiment, the selection of phase change material would be made subject to various considerations such as, but not restricted to, one or more of: the preferred containment means, the piping or transportation means, the ease of heat transfer to and from the fluid, the fluid's fluid properties (such as but not restricted to viscosity, propensity for turbulence and heat transfer efficiency), the nature of the material itself, and the failsafe or containment systems in case of failure of any item of equipment.

[236] In any of these embodiments, factors that may be considered (when determining an appropriate phase change material and/or thermal transfer fluid) under the title of the nature of the material itself may include, but not be restricted to, one or more of: the heat capacity of the material at the operational temperatures, its melting and/or boiling point, its corrosiveness, its flammability or explosiveness, its usefulness for other purposes such as in a fuel cell, its cost, the cost and means of its disposal, and any aspect(s) of its environmental friendliness. [237] The phase change material vessel may optionally be contained in a pressure vessel in order to hold the phase change material at elevated pressure. This may be particularly useful if the phase change material is gaseous when hot, in order to reduce the necessary size of the vessel.

[238] In other embodiments, the same pipe (or transfer means, or conduit) may be used between the phase change heat storage and the compressed air storage means for both directions of travel of the compressed air. Likewise, if the compression means is the same as the expansion means, or is generally proximate to it, the same pipe (or transfer means) may be used for all or part of both directions of travel of the compressed air. This applies equally to all embodiments, where practicable.

[239] In the embodiment shown in Figure 6 the additional heat transfer arrangement 16a may exchange heat with a phase change material 16b in a hot vessel

(thermal store for storing heat) 16. It is likely, but not necessary, that the phase change material in the cold vessel differs from that in the hot vessel, in order to ensure that the temperature at which the phase change occurs corresponds with the operational temperatures of that part of the thermal management system.

[240] The embodiments of the invention shown in Figures 4 to 7 are now described in more detail. [241 ] Figure 4 shows a compressed air energy storage apparatus comprising a compressor arrangement 3 configured to compress air or another process gas. Air or other process gas is provided to the compressor arrangement 3 via inlet 2, compressed by the compressor arrangement and the compressed gas is output via a conduit 4 to a compressed gas store 5. When required the, compressed air or other process gas is provided via a conduit 6 to an expander arrangement 7. The expander arrangement 7 expands the compressed air or other process gas and it is output from the expander arrangement via an outlet 9. [242] A first transducer is connected to the compressor arrangement. The first transducer and compressor arrangement are configured to cooperate to convert an input energy 1 into potential energy in the form of compressed gas. A second transducer is connected to the expander arrangement. The second transducer arrangement are configured to convert the kinetic energy of the expanded air or process gas into another form of usable energy, such as electricity.

[243] The apparatus also includes a heat transfer arrangement configured to transfer heat from the air or process gas to a thermal storage medium 12b. The thermal storage medium 12b forms part of (or is stored by) a thermal store arrangement

12. The thermal store arrangement 12 is configured to accommodate a substantially elevated or reduced temperature of the thermal storage medium.

[244] The heat transfer arrangement comprises a first heat transfer device 14 configured to transfer heat between the air or process gas and a first thermal transfer fluid whilst the air or process gas is compressed by the compressor arrangement 3. The first thermal transfer fluid is contained within conduit 13 which runs between the first heat transfer device 14 and a second heat transfer device 12a. In other embodiments first heat transfer device 14 may be configured to transfer heat between the air or process gas and a first thermal transfer fluid before or after the air or process gas is compressed by the compressor arrangement 3.

[245] The heat transfer arrangement also comprises the second heat transfer device 12a. The second heat transfer device 12a is configured to transfer heat between the first thermal transfer fluid and the thermal storage medium 12b.

[246] The heat transfer arrangement also comprises a third heat transfer device 12a configured to transfer heat between the thermal storage medium 12b and a second thermal transfer fluid. The second thermal transfer fluid is contained in conduit 1 1 which runs between the third heat transfer device 12a and a fourth heat transfer device 10.

[247] In this embodiment the fourth heat transfer device 10 is associated with the expander arrangement 7 and is configured to transfer heat between the second thermal transfer fluid and the air or process gas while the air or process gas is expanded by an expander arrangement 7. In other embodiments the fourth heat transfer device 10 may be configured to transfer heat between the second thermal transfer fluid and the air or process gas before or after the air or process gas is expanded by an expander arrangement 7.

[248] The thermal storage medium 12b includes a phase change material. The phase change material is configured to store heat or cold in the form of latent heat by changing phase upon being heated or cooled.

[249] In one example the phase change material is configured to store heat and the heat stored by the phase change material is heat generated by the compressor arrangement 3. In particular, the compression of the air or process gas by the compressor arrangement 3 generates heat. This heat is transferred from the compressed process gas to the first heat transfer device 14 via the compressor arrangement 3. The first heat transfer device 14 transfers the heat to the first thermal transfer fluid. The first thermal transfer fluid is free to move within conduit 13. The second heat transfer device 12a transfers heat from the first thermal transfer fluid to the thermal storage medium 12b. The heat transferred to the thermal storage medium 12 causes the phase change material to undergo a change in phase which absorbs at least part of the transferred heat and stores the heat as latent heat. Subsequently, the phase change material undergoes a further change in phase which results in the phase change material emitting heat. The heat emitted by the phase change material is transferred by the third heat transfer device 12a to the second thermal transfer fluid. The second thermal transfer fluid is free to move within conduit 1 1 . The fourth heat transfer device 10 transfers heat from the second thermal transfer fluid via the expander arrangement 7 to the air or process gas in the expander arrangement 7. The expansion of the air or process gas by the expander arrangement 7 results in cooling which may absorb/use the heat transferred by the fourth heat transfer device 10.

[250] In another example the phase change material is configured to store cold and the cold stored by the phase change material is cold generated by the expander arrangement 7. In particular, the expansion of the air or process gas by the expander arrangement 7 generates cold. This cold is transferred from the expanded process gas to the fourth heat transfer device 10 via the expander arrangement 7. The fourth heat transfer device 10 transfers the cold to the second thermal transfer fluid. The second thermal transfer fluid is free to move within conduit 1 1 . The third heat transfer device 1 2a transfers cold from the second thermal transfer fluid to the thermal storage medium 12b. The cold transferred to the thermal storage medium 12 causes the phase change material to undergo a change in phase which absorbs at least part of the transferred cold and stores the cold as latent heat. Subsequently, the phase change material undergoes a further change in phase which results in the phase change material absorbing heat (i.e. emitting cold). The cold emitted by the phase change material is transferred by the second heat transfer device 12a to the first thermal transfer fluid. The first thermal transfer fluid is free to move within conduit 13. The first heat transfer device 14 transfers cold from the first thermal transfer fluid via the compressor arrangement 3 to the air or process gas in the compressor arrangement 3. The compression of the air or process gas by the compressor arrangement 3 results in heating which may absorb/use the cold transferred by the first heat transfer device 14. The arrows shown on the Figure depict the direction of flow of the thermal transfer fluid(s) in this example.

[251 ] In this embodiment the second and third heat transfer device are one and the same and form a combined thermal store heat transfer device - as such they have been given the same reference numeral. Furthermore the first and second thermal transfer fluids in this embodiment are also one and the same. In this way, in this embodiment, a single thermal transfer fluid is free to move within conduits between the first and fourth heat transfer devices. The thermal transfer fluid transfers heat with the combined thermal store heat transfer device which is in thermal communication with the thermal storage medium.

[252] In other embodiments the second and third heat transfer device are separate.

Furthermore in other embodiments the first and second thermal transfer fluids in are separate. In an alternative embodiment to that shown in figure 4, the energy storage apparatus may store and transfer cold. In such an embodiment the first heat transfer device 10 configured to transfer cold between the air or process gas and the first thermal transfer fluid whilst the air or process gas is expanded by the expander arrangement 7. The first thermal transfer fluid is contained within conduit 1 1 which runs between the first heat transfer device 10 and a second heat transfer device 12a. In other embodiments first heat transfer device 10 may be configured to transfer cold between the air or process gas and a first thermal transfer fluid before or after the air or process gas is expanded by the expander arrangement 7.

[254] The heat transfer arrangement also comprises the second heat transfer device 12a. The second heat transfer device 12a is configured to transfer heat between the first thermal transfer fluid and the thermal storage medium 12b.

[255] The heat transfer arrangement also comprises a third heat transfer device 12a configured to transfer cold between the thermal storage medium 12b and a second thermal transfer fluid. The second thermal transfer fluid is contained in conduit 13 which runs between the third heat transfer device 12a and a fourth heat transfer device 14. The second and third heat transfer devices may optionally be a single device.

[256] In this embodiment the fourth heat transfer device 14 is associated with the compressor arrangement 3 and is configured to transfer cold between the second thermal transfer fluid and the air or process gas while the air or process gas is compressed by a compressor arrangement 3. In other embodiments the fourth heat transfer device 14 may be configured to transfer cold between the second thermal transfer fluid and the air or process gas before or after the air or process gas is compressed by a compressor arrangement 3.

[257] The apparatus shown in Figure 5 differs from that shown in Figure 4 in that it comprises an additional conduit 15b that links the first heat transfer device 14 and second heat transfer device 10. This provides a fluid flow path between the first heat transfer device 14 and second heat transfer device 10 other than via the thermal storage arrangement 12. [258] In this embodiment when the phase change material is configured to store heat and the heat stored by the phase change material is heat generated by the compressor arrangement 3, the additional conduit 15b allows thermal transfer fluid which has been cooled by the expander arrangement 7 at the fourth heat transfer device 10 to flow back to the first heat transfer device 14 to be reheated by the compressor arrangement 3.

[259] In this embodiment when the phase change material is configured to store cold and the cold stored by the phase change material is cold generated by the expander arrangement 7, the additional conduit 15b allows thermal transfer fluid which has been heated by the compressor arrangement 3 at the first heat transfer device 14 to flow back to the fourth heat transfer device 10 to be re- cooled by the expander arrangement 7. The arrows shown on the Figure depict the direction of flow of the thermal transfer f luid(s) in this example.

[260] Cycling the thermal transfer fluid in the manner permitted by the additional conduit may lead to an increase in efficiency of the energy storage apparatus, a reduction in the amount of thermal transfer fluid used over multiple cycles, and the use of a much wider range of thermal transfer fluids because such fluid is contained within a closed loop thermal management subsystem.

[261 ] The apparatus in Figure 6 is similar to that shown in Figure 2 and differs from that shown in Figure 5 in that the additional conduit 15b is replaced with conduits 15 and 17 and a second thermal store arrangement 16. The second thermal store arrangement 16 includes a second thermal storage medium 16b which includes a phase change material. The second thermal store arrangement also includes at least one heat transfer device in thermal communication therewith. The second thermal store arrangement includes a thermal storage medium which includes a phase change material which exchanges heat/cold with the air or other process gas (and hence stores heat or cold) via the first and fourth heat transfer devices as previously discussed above in relation to the first thermal store arrangement 12. As such, needless repetition of the manner in which heat/cold is transferred/stored by the phase change material of the second thermal store arrangement 16 is avoided. [262] The cyclic nature of the thermal transfer conduits and thermal storage arrangements is such that whilst the phase change material of the first thermal store arrangement 12 is configured to store heat and the heat stored by the phase change material of the first thermal store arrangement is heat generated by the compressor arrangement 3, the phase change material of the second thermal store arrangement 16 is configured to store cold and the cold stored by the phase change material of the second thermal store arrangement is cold generated by the expander arrangement 7. Conversely, whilst the phase change material of the first thermal store arrangement 12 is configured to store cold and the cold stored by the phase change material of the first thermal store arrangement is cold generated by the expander arrangement 7, the phase change material of the second thermal store arrangement 16 is configured to store heat and the heat stored by the phase change material of the second thermal store arrangement is heat generated by the compressor arrangement 7. The arrows shown on the Figure depict the direction of flow of the thermal transfer fluid(s) in this latter example.

[263] Figure 7 shows a schematic diagram of another embodiment of energy storage apparatus. In this embodiment air or process gas which has been compressed by the compressor arrangement 3 is supplied via conduit 4a to a first heat transfer device 12a. The compressed gas then passes via conduit 4b to the compressed gas store 5. When desired, the compressed gas from the compressed gas store 5 passes along conduit 6b to the first heat transfer device 12a and then via conduit 4a to the expander arrangement 7.

[264] The first heat transfer device 12a is configured to transfer heat to/from a thermal storage medium 12b which includes a phase change material. The thermal storage medium 12b is accommodated by a thermal store arrangement.

[265] In use as the air or process gas passes through the first heat transfer device 12a, heat is transferred between the air or process gas and the thermal storage medium which may optionally be a phase change material. As previously discussed, heat or cold transferred to the thermal storage medium 12 causes the phase change material to undergo a change in phase which absorbs at least part of the transferred heat or cold and stores the heat or cold as latent heat. Subsequently, the phase change material may undergo a change in phase which causes at least part of the heat or cold stored as latent heat to be transferred to the air or process gas.

[266] The thermal transfer fluid and/or the thermal storage material may be any appropriate material or combination of materials. For example they may be water, ammonia, alcohol, carbon dioxide, or a hydrocarbon. The choice of phase change material will be determined by the heat transfer characteristics of a particular apparatus, or by the fluid's fluid properties (such as but not restricted to viscosity, propensity for turbulence and heat transfer efficiency), the nature of the material itself, and the failsafe or containment systems in case of failure of any item of equipment. Any phase change material will be chosen such that the temperature changes in the thermal storage medium due to heating by the compressor and/or cooling by the expander result in a desired phase change of the phase change material.

[267] In other embodiments, the same pipe (or transfer means) may be used between the phase change heat storage and the compressed air storage means for both directions of travel of the compressed air. Likewise, if the compression means is the same as the expansion means, or is generally proximate to it, the same pipe (or transfer means) may be used for all or part of both directions of travel of the compressed air. This applies equally to all embodiments, where practicable. [268] In further means, the heat transfer between the hot process fluid (which may be hot air) and the phase change material may be affected by passing the fluid through one or more pipes (or conduits) through or in thermal contact with the phase change material or any material in thermal contact with the phase change material.

[269] The phase change material is used to store the heat generated from compression of air, such as (but not restricted to) the compressed air used to store electrical energy. In such a case the compressed air could be stored in one or more caverns underground. In some embodiments the compressed air may be compressed using other types of energy, such as but not restricted to mechanical, fluid, chemical and potential.

[271 ] In some embodiments the compressed air may be used to generate other types of energy, such as but not restricted to mechanical, fluid, chemical and potential.

[272] In some embodiments the energy used to compress the air may differ from the energy generated by the compressed air.

[273] The hot first fluid would be transported through pipes or other means coupled with the means of compression such that the means of compression heats the fluid prior to transferring its heat to the phase change material. Such coupling includes coupling with the hot compressed air, or coupling with the cold uncompressed air in order to cool it prior to compression, or coupling with the air at any intermediate stage. Such heating of the fluid by the means of compression may be performed either directly or indirectly.

[274] The hot first fluid would then transfer its heat through a heat transfer means to the phase change material. The fluid cools as its heat is transferred. The fluid may then optionally be transported back to the means of compression in a closed loop, to be made available for heating again by such means of compression. [275] In some embodiments, the phase change material changes phase upon being heated, from solid to liquid; in others, from liquid to gas; in others, from solid to gas. Such phase change(s) enable substantial amounts of energy to be stored in the material in the form of latent heat, without requiring temperature and volume differences between its heated and its cooler states to be as great as in similar materials that do not undergo phase change at such temperatures.

[276] A cold second fluid (which may or may not be the same as the first fluid) would then receive heat transferred to it through a heat transfer means from the phase change material. The fluid heats up as the heat is transferred. Such heated fluid is transported through pipes or other means coupled with the means of expansion such that the means of expansion cools the fluid. Such coupling includes coupling with the compressed air in order to heat it prior to expansion, or coupling with the cold expanded air, or coupling with the air at any intermediate stage. Such heating of the fluid by the means of compression may be performed either directly or indirectly.

[277] The second fluid may then optionally be transported back to the means of compression in a closed loop, to be made available for heating again by such means of compression.

[278] Optionally, the phase change material would be stored in one or more insulated containment vessels. Such an embodiment could be fixed or mobile, large or small, and of variable (such as using a bladder) or fixed size. [279] In any aspect of this embodiment storing the phase change material in one or more insulated containment vessels, the entire system could be fitted onto a mobile device such as a platform, ship or vehicle.

[280] In an alternative embodiment, the phase change material would be stored in one or more underground caverns. In this case, one suitable material would be brine which would reduce ongoing cavern erosion. Other materials could be used though attention would then need to be given to the interaction of the material with the cavern. Consideration may also be given to the proximity of the phase change temperature(s) to the natural temperature of the cavern.

[281 ] Other embodiments are possible, for example but not limited to containment of the phase change material in a bladder or some other storage means.

In any of these embodiments, the temperature of the hot fluid may be maintained at up to δΟΟΌ in order to limit the costs of maintaining the fluid at the hot temperature.

[283] It is also understood that the heat may alternatively be stored in any material without such material undergoing a phase change under the operational conditions of the system, and that such heat storage material may be heated by heat transfer from the compressed gas after compression, and may in turn heat the compressed gas prior to expansion.

[284] Other embodiments store the thermal energy in a fluid that is subject to phase change during the thermal cycles involved in this system. The fluid is transported in its hot state to a means whereby it can transfer its heat into a phase change material. Any means of storage may be used for such phase change material, though better embodiments will normally minimise the heat lost by the fluid during storage and transfer.

[285] In some embodiments, the air itself may transport the heat to the phase change material. One example of such embodiments is to pass the hot compressed air through the phase change material in at least one pipe; another would be to have the hot compressed air as the heat source of a heat exchanger. Both of these example embodiments could be operated in reverse in order to transfer the heat back from the phase change material into the air prior to (or following) expansion.

[286] In an alternative embodiment, the thermal storage material would be stored in one or more underground caverns, such caverns optionally being lined to prevent thermal or other damage to the cavern surfaces. In this case, one suitable material would be brine which would reduce ongoing cavern erosion. Other materials could be used though attention would then need to be given to the interaction of the material with the cavern. Consideration may also be given to the proximity of the phase change temperature(s) to the natural temperature of the cavern.

[287] When the compressed air is expanded to generate some other form of energy (for example, electricity or mechanical energy), it requires heat input. The heat is then extracted from the phase change or other thermal storage material by passing cold fluid into a heat exchange means to heat up. This hot fluid is then drawn out and heat is transferred back into the air. This may be at any part of the process of expansion, generation and exhaust. [288] The main advantage of storing the heat from compression to re-use during expansion is that additional energy is not required during expansion to prevent freezing or localised changes to atmospheric conditions.

Heat storage and re-use also enables the use of a wider range of types of expander within the expander arrangement, as the expander will not have to operate under such cold conditions.

[290] One advantage of this system over a system that stores heat in materials that do not change phase at such temperatures is that a greater amount of thermal energy can be stored in proportion to the difference in temperature between the material's hot state and its cold state. In addition the use of a thermal storage medium including a phase change material means that, due to using the latent heat of the phase change material (in addition to its thermal capacity) to store heat or cold, a greater amount of heat or cold can be stored per unit mass of thermal storage medium. As such, the thermal store which includes the thermal storage medium can be of reduced size (and potentially cost) as compared to a thermal store including a thermal storage medium which does not include a phase change material.

[291 ] The compressed air in any of the previously described embodiments could be stored in one or more caverns underground. However, in other embodiments the compressed gas may be stored in any appropriate vessel or bladder. [292] In the previously described embodiments the air is compressed using an electrically powered compressor arrangement (i.e. using electrical energy). In other embodiments the compressed air may be compressed using other types of energy, such as but not restricted to mechanical, fluid, chemical and potential. [293] In the previously described embodiments the compressed air is expanded by the expander arrangement and the associated transducer converts the kinetic energy of the expanded gas into electrical energy. In other embodiments the compressed air may be used (with and appropriate transducer associated with the expander arrangement) to generate other types of energy, such as but not restricted to mechanical, fluid, chemical and potential. [294] In other embodiments the type of energy used to compress the air may differ from the type of energy generated by the compressed air. [295] The cold fluid (heat transfer fluid) may be stored in a storage means (thermal store) coupled with the means of expansion (expander arrangement) such that the means of expansion (expander arrangement) cools or refrigerates the fluid prior to storage. Such cooling or refrigeration of the fluid by the means of expansion may be performed either directly or indirectly. The fluid (heat transfer fluid) may also be coupled after storage (thermal store) to the means of compression (compressor arrangement) such that the means of compression (compressor arrangement) heats the fluid. Such heating of the fluid by the means of compression (compressor arrangement) may be performed either directly or indirectly. Such coupling may include coupling with the compressed air in order to heat it prior to expansion, or coupling with the cold expanded air, or coupling with the air at any intermediate stage.

[296] The cold fluid may be stored in a thermal store which comprises one or more insulated containment vessels.

[297] The thermal transfer fluid and/or thermal storage medium may be maintained at elevated pressure, for example in order to permit a different selection of material and/or fluid. [298] The energy storage apparatus may be fixed or mobile, large or small. In particular, the thermal store may be fixed or mobile, large or small, and of variable (such as using a bladder) or fixed size.

[299] In the previously described embodiments the heat transfer fluid is a liquid. In other embodiments, the fluid may be a gas. The selection of gas or liquid would be made subject to various considerations such as, but not restricted to, one or more of: the preferred containment means, the piping or transportation means, the ease of heat transfer to and from the fluid, the nature of the fluid itself, and the failsafe or containment systems in case of failure of any item of equipment. The fluid may also be selected to be liquid at the lower (sub-ambient) temperatures and gaseous at the higher (approximately ambient or elevated) temperature. These considerations would be readily apparent to a person skilled in the art. [300] In any embodiment, the thermal transfer fluid could be a gas at the elevated temperature and a liquid at the lower temperature in order to store a greater amount of energy per degree increase in temperature by virtue of the latent heat of vaporisation. [301 ] In any of these embodiments, factors that may be considered under the title of the nature of the fluid itself may include, but not be restricted to, one or more of: the heat capacity of the fluid at the operational temperatures, its boiling point, its corrosiveness, its flammability or explosiveness, its usefulness for other purposes such as in a fuel cell, its cost, the cost and means of its disposal, and any aspect(s) of its environmental friendliness.

[302] In some embodiments, the entire energy storage apparatus may be fitted (or mounted) onto (or into) a mobile device such as a platform, ship or vehicle.

[303] As previously discussed, in some embodiments the thermal store for the cold fluid (heat transfer fluid) may be a bladder.

[304] In some embodiments (such as, for example, that shown in Figure 2), after the heat transfer fluid has been used to transfer cold from the expander arrangement to the air before it is stored in the compressed air store, the fluid may be stored in a second heat storage means, at a higher temperature than that in the first heat storage means. Then this hot fluid may be used again for taking cold energy from (that is, putting heat energy into) the means of expansion (expander arrangement). This would form a closed cycle for the fluid in which the fluid may be used any number of times.

[305] In embodiments of the invention, the energy being converted into compressed air (and heat energy) could be any appropriate form of energy such as (but not restricted to): Electricity, at high power such as from an electricity grid or network, power station or other generating device;

Electricity, at low power such as from photovoltaic cells, regenerative braking or other such source;

Mechanical energy such as from the movement of a vehicle, turbine or other equipment;

Chemical energy such as from a battery, a chemical reaction or a fuel cell; Fluid energy such as from a dam or reservoir, or flowing water or gas or other fluid;

Potential energy such as from an elevated body whose descent is used to provide the energy used for compression.

[306] In embodiments of the invention, the energy being converted from compressed air (and heat energy) could be any form of energy such as (but not restricted to): Electricity, at high power such as to put into an electricity grid, network, heating system, or equipment;

Electricity, at low power such as to use in a vehicle, low powered equipment, or lighting;

Mechanical energy such as to use to enhance the movement of a vehicle, turbine, ship or other equipment;

Chemical energy such as to charge a battery, support a chemical reaction or charge a fuel cell;

Fluid energy such as to refill a dam or reservoir, or power a ship or other water- borne device or gas or other fluid;

Potential energy such as raising a body.

[307] In the embodiments described above the compressor arrangement and expander arrangement are each shown as a single item, for simplicity and clarity of depiction and description. However, it will be appreciated that energy storage apparatus according to other embodiments of the invention may include compressor arrangements and/or expander arrangements may be of any appropriate configuration. For example, the compressor and/or expander arrangements may consist of a plurality of components which may be in series, in parallel or any combination thereof. [308] The transducer, compressed air storage means, thermal storage means and/or conduits (in any part of the system or apparatus) may also be of any appropriate configuration.

In some embodiments the expansion arrangement (expander arrangement) may use some or all of the same equipment and/or arrangement as the compressor arrangement, in which case at least some of the arrangement is operated in the reverse direction for expansion as for compression. In other words, in some embodiments, at least part of the expander arrangement may be at least part of the compressor arrangement. In such an arrangement, the relevant shared parts may be operated in a first mode in which they function to compress the air, and a second mode in which they function to expand the air.

[310] The concept of storing cold in this way can be applied to any system wherein cold is created at one time and/or in one place, and cooling is required at another time and/or place.

[31 1 ] The concept of storing cold in this way can also be applied to any system wherein the cold storage means is solid, or solidifies during cooling (i.e. the use of a thermal storage medium which includes a phase change material).

[312] As previously discussed, in some embodiments the cold storage means (i.e. the thermal store) may comprise a thermal storage medium which is a solid. In such embodiments heat may be transferred between the thermal store and the first/second heat transfer devices by a piped heat transfer fluid which transfers the cold to the solid cold storage means (thermal store) using a heat transfer means- for example a heat exchanger in contact with the solid thermal storage medium through which the heat transfer fluid flows. [313] In embodiments wherein the cold storage means is a fluid that solidifies when cooled, the piped fluid transfers the cold to the solid cold storage means using a heat transfer means. Such an embodiment would employ the latent heat of melting to store a greater amount of energy with reduced temperature differential and/or volume of cold storage required. [314] Other embodiments of this invention may use conducted cold through a solid material (instead of or as well as using a piped heat transfer fluid) between the heat generation means (e.g. compressor arrangement) and the cold storage means (thermal store), or between the cold storage means (thermal store) and the process or system that uses heat (e.g. expander arrangement), or for both of these energy transfer stages.

[315] Other embodiments may store both cold in the cold storage means (first thermal store) and heat in the hot storage means (second thermal store), in order to maintain a larger heat differential between the hot and cold states of the fluid, and thereby increase the overall efficiency of the system. This would have the additional benefit of maximising heat differential between the two parts of the system while minimising divergence of temperatures from ambient. Minimising the temperature difference between the thermal storage medium of each of the first and second thermal stores and ambient reduces that rate of any potential heat loss between the first and second thermal stores and the ambient environment (this has been discussed in more detail previously). This may reduce the amount of insulation that is required by the first and second thermal stores.

[316] Although the previously described embodiments use air as the process gas (i.e. the gas which is compressed, stored and expanded), in other embodiments the process gas may be any appropriate gas. For example the process gas may be natural air or any other gas or mixture of gases that may be compressed.

[317] Likewise the heat transfer fluid may be any suitable liquid or gas, or mixture of liquids or mixture of gases.

[318] "Hot" is used to denote any temperature higher than the temperature denoted by "cold".

[319] Energy is transformed from any type (for example, electric, mechanical or chemical) into compressed air using a means of compression. This compression will generate heat, both in the compressor due to the work being done, and in the air due to its state of compression. Any means of compression may be used. [320] The transferring of cold refers to a cooling or refrigeration process. The storage of cold refers to the storing of the cold fluid in such a way as to reduce its rate of heating (i.e. to reduce its rate of increasing in thermal energy).

[321 ] Some or all such cold is transferred to a fluid by any heat exchange process.

Any means of heat exchange may be used. That is to say, the first and second (and, where appropriate, third and fourth) heat transfer devices may be any appropriate form of device capable of transferring heat.

[322] In some embodiments the thermal store is such that cold is stored by the thermal store storing heat transfer fluid in a cold state. In other embodiments appropriate form of thermal store may be used (for either the first thermal store, and, if applicable, the second thermal store). Any means of storage may be used, though preferred embodiments will normally minimise the heat absorbed/or lost by the thermal store during storage and minimise the heat absorbed/or lost during transfer of heat to/from any thermal store.

[323] When the compressed air is compressed by some other form of energy (for example, electricity or mechanical energy), it emits heat. The cold fluid is then drawn out of its storage and cold is transferred back into the air. This may be at any part of the process of compression, feed-in to compression, feed-out from compression, or related process. [324] The exhaust from the generation process (i.e. the gas which has passed through the expander arrangement) may pass into a vessel for later re-use, in which case the heat from the compression could be applied to the vessel containing the exhaust gas. [325] It will be appreciated that the detail of operation of the expander arrangement and compressor arrangement within the energy storage apparatus has been omitted because the operation of expander and compressor arrangements (in particular, expanders and compressors) will be well understood by a person skilled in the art. Any appropriate type of expander arrangement or compressor arrangement may be used. Furthermore, any or all of the conduits through which the air or other process gas flows, and/or the conduits through which the thermal transfer fluid flows may include any appropriate flow control device, the operation of which is well understood by the person skilled in the art. For example, the conduit between the compressed gas store and the expander arrangement may include a flow control device (e.g. a valve and/or a pump). The flow control device may be opened when it is desired for gas from the compressed gas store to be supplied to the expander arrangement (for example to generate electricity via an appropriate transducer) and closed when gas from the compressed gas store is not required by the expander arrangement. Configuration and operation of such devices are well understood by a person skilled in the art. Within the previously described embodiments ambient temperature has been defined as the temperature of the environment to the exterior of the thermal store. The ambient temperature may be the temperature of any appropriate location to the exterior of the thermal store. For example, the ambient temperature may be the temperature at one of: an inlet to the compressor, an inlet to the expander or inside the compressed gas store. If at least part of the apparatus is located in or near soil, rocks or a body of water, the ambient temperature may be the temperature of the soil, rocks or body of water. The ambient temperature may be the temperature of any other object that may provide a suitable reference temperature.

Claims

CLAIMS:

1 . A method of storing energy using an energy storage apparatus, the energy storage apparatus comprising a compressor arrangement, a compressed gas store, an expander arrangement, a first heat transfer device associated with the expander arrangement, a thermal store including a thermal storage medium and a second heat transfer device associated with the compressor arrangement; the method comprising: the compressor arrangement compressing air or another process gas;

supplying the compressed air or other process gas to the compressed gas store;

storing the compressed air or other process gas in the compressed gas store;

the expander arrangement expanding the air or other process gas from the compressed gas store;

the first heat transfer device transferring heat to the air or other process gas which has passed through the expander from the thermal storage medium, the transfer reducing the thermal energy of the thermal storage medium in order to store cold in the thermal store, such that the temperature of the thermal storage medium is below ambient temperature; and

the second heat transfer device transferring heat between the thermal storage medium and the air or other process gas before the air or other process gas is stored in the compressed gas store, the transfer increasing of the thermal energy of the thermal storage medium.

2. A method according to claim 1 , wherein said ambient temperature is the temperature at one of: an inlet to the compressor, an inlet to the expander, inside the compressed gas store, or external to the energy storage apparatus.

3. A method according to any preceding claim, wherein the temperature of the thermal storage medium, when the first heat transfer device transfers heat from the air or other process gas which has passed through the expander to the thermal storage medium, is below at least one of: about 20 °C, about -\ 0 °C, about 0 °C, about -10 Ό, about -20 °C, about -30°C, about -40 °C, about -50 °C, about -70 °C, about -80 °C, about - 90°C and about -100 Ό.

4. A method according to any preceding claim, wherein the process gas is air and wherein the energy storage apparatus is a Compressed Air Energy Storage (CAES) apparatus.

5. A method according to any preceding claim, wherein the apparatus is open loop such that the compressor arrangement compresses air from the atmosphere and such that the air that has passed through the expander arrangement is released into the atmosphere.

6. A method according to any preceding claim, wherein the energy storage apparatus further comprises a transducer associated with the expander arrangement, and wherein the method further comprises the transducer converting energy resulting from the expander arrangement expanding the compressed air or other process gas into a different form of energy.

7. A method according to any preceding claim, wherein the energy storage apparatus further comprises a transducer associated with the compressor arrangement, the transducer being configured to convert energy from a different form of energy into potential energy of a compressed gas.

8. A method according to claim 6 or 7, wherein said different form of energy is electrical energy.

9. A method according to any preceding claim, wherein the second heat transfer device transfers heat between the thermal storage medium and the air or other process gas before the air or other process gas is compressed by the compressor arrangement.

10. A method according to any of claims 1 to 8, wherein the second heat transfer device transfers heat between the thermal storage medium and the air or other process gas after the air or other process gas is compressed by the compressor arrangement.

1 1 . A method according to any of claims 1 to 8, wherein the second heat transfer device transfers heat between the thermal storage medium and the air or other process gas whilst the air or other process gas is being compressed by the compressor arrangement.

12. A method according to any preceding claim, wherein the energy storage apparatus comprises the compressed gas store.

13. A method according to any preceding claim, wherein said compressed gas store comprises a subterranean cavern or other suitable geological feature.

14. A method according to any preceding claim, wherein said compressed gas store comprises a vessel.

15. A method according to any preceding claim, wherein thermal storage medium comprises a phase change material(s).

16. An energy storage apparatus comprising:

a compressor arrangement configured to compress air or other process gas and to supply the compressed air or other process gas to a compressed gas store;

an expander arrangement configured to expand the air or other process gas from the compressed gas store;

a first heat transfer device associated with the expander arrangement;

a thermal store including a thermal storage medium;

a second heat transfer device associated with the compressor arrangement;

the first heat transfer device being configured to transfer heat to the air or other process gas which has passed through the expander arrangement from the thermal storage medium, the transfer producing a reduction in the thermal energy of the thermal storage medium in order to store cold in the thermal store;

the second heat transfer device being configured to transfer heat between the thermal storage medium and the air or other process gas before the air or other process gas is stored in the compressed gas store, the transfer producing an increase of the thermal energy of the thermal storage medium;

wherein the thermal store is configured to store cold such that the temperature of the thermal storage medium in use is below ambient temperature.

17. An apparatus according to claim 16, wherein said ambient temperature is the temperature at one of: an inlet to the compressor arrangement, an inlet to the expander arrangement or inside the compressed gas store.

18. An apparatus according to any of claims 16 to 17, wherein the temperature of the thermal storage medium in use is below at least one of: about 20 °C, about ~\ 0 °C, about 0 °C, about -10 Ό, about -20 "C, about -30 "C, about -40 "C, about -50 "C, about - 70 °C, about -80 °C, about -90 °C and about -100 Ό.

19. An apparatus according to any of claims 16 to 18, wherein the process gas is air and wherein the energy storage apparatus is a Compressed Air Energy Storage (CAES) apparatus.

20. An apparatus according to any of claims 16 to 19, wherein the apparatus is open loop such that the compressor arrangement compresses air from the atmosphere and such that the air that has passed through the expander arrangement is released into the atmosphere.

21 . An apparatus according to any of claims 16 to 20, wherein the second heat transfer device is configured to transfer heat between the thermal storage medium and the air or other process gas before the air or other process gas is compressed by the compressor arrangement.

22. An apparatus according to any of claims 16 to 20, wherein the second heat transfer device is configured to transfer heat between the thermal storage medium and the air or other process gas after the air or other process gas is compressed by the compressor arrangement.

23. An apparatus according to any of claims 16 to 20, wherein the second heat transfer device is configured to transfer heat between the thermal storage medium and the air or other process gas whilst the air or other process gas is being compressed by the compressor arrangement.

24. An apparatus according to any of claims 16 to 23, wherein the energy storage apparatus comprises one or more compressed gas stores.

25. An apparatus according to any of claims 16 to 24, wherein said compressed gas store(s) comprises a vessel.

26. A method of storing energy using an energy storage apparatus, the energy storage apparatus comprising a compressor arrangement, an expander arrangement, a thermal store arrangement, and a heat transfer arrangement comprising first, second, third and fourth heat transfer devices; the method comprising:

the compressor arrangement compressing air or another process gas;

the thermal store arrangement storing a thermal storage medium and accommodating a substantially elevated or reduced temperature of the thermal storage medium ;

the heat transfer arrangement transferring heat between the air or process gas and the thermal storage medium;

the first heat transfer device transferring heat between the air or process gas and a first thermal transfer fluid before, while or after the air or process gas is compressed;

the second heat transfer device transferring heat between the first thermal transfer fluid and the thermal storage medium;

the third heat transfer device transferring heat between the thermal storage medium and a second thermal transfer fluid;

the fourth heat transfer device transferring heat between the second thermal transfer fluid and the air or process gas before, while or after the air or process gas is expanded by an expander arrangement;

wherein the thermal storage medium comprises a phase change material, the phase change material storing heat or cold in the form of latent heat by changing phase upon being heated or cooled.

27. A method according to claim 26, wherein the phase change material stores heat and the heat stored by the phase change material is heat generated by the compressor arrangement.

28. A method according to claim 26, wherein the phase change material stores cold and the cold stored by the phase change material is cold generated by the expander arrangement.

29. A method according to any of claims 26 to 28, wherein the first thermal transfer fluid differs from the second thermal transfer fluid.

30. A method according to any of claims 26 to 29, wherein the second thermal transfer fluid is the same as the first thermal transfer fluid such that the first and second thermal transfer fluid are collectively a single thermal transfer fluid.

31 . A method according to claim 30, wherein the single thermal transfer fluid is contained within a fluid system that is in thermal contact with each of the compressor arrangement, the expander arrangement and the thermal storage medium.

32. A method according to any of claims 26 to 31 , wherein the second and third heat transfer devices constitute a combined thermal store heat transfer device.

33. A method according to any of claims 26 to 32, wherein said change in phase is a change between solid and either liquid or gas.

34. A method according to any of claims 26 to 32, wherein said change in phase is a change between liquid and gas.

35. A compressed air energy storage apparatus comprising:

a compressor arrangement configured to compress air or another process gas;

a heat transfer arrangement configured to transfer heat from the air or process gas to a thermal storage medium;

a thermal store arrangement configured to store the thermal storage medium and accommodate a substantially elevated or reduced temperature of the thermal storage medium ;

the heat transfer arrangement comprising:

a first heat transfer device configured to transfer heat between the air or process gas and a first thermal transfer fluid before, while or after the air or process gas is compressed;

a second heat transfer device configured to transfer heat between the first thermal transfer fluid and the thermal storage medium ;

a third heat transfer device configured to transfer heat between the thermal storage medium and a second thermal transfer fluid;

A fourth heat transfer device configured to transfer heat between the second thermal transfer fluid and the air or process gas before, while or after the air or process gas is expanded by an expander arrangement; wherein the thermal storage medium comprises a phase change material, the phase change material being configured to store heat or cold in the form of latent heat by changing phase upon being heated or cooled.

36. An apparatus according to claim 35, wherein the phase change material is configured to store heat and the heat stored by the phase change material is heat generated by the compressor arrangement.

37. An apparatus according to claim 35, wherein the phase change material is configured to store cold and the cold stored by the phase change material is cold generated by the expander arrangement.

38. An apparatus according to any of claims 35 to 37, wherein the first thermal transfer fluid differs from the second thermal transfer fluid.

39. An apparatus according to any of claims 35 to 37, wherein the second thermal transfer fluid is the same as the first thermal transfer fluid such that the first and second thermal transfer fluid are collectively a single thermal transfer fluid.

40. An apparatus according to claim 39, wherein the single thermal transfer fluid is contained within a fluid system that is in thermal contact with each of the compressor arrangement, the expander arrangement and the thermal storage medium.

41 . An apparatus according to any of claims 35 to 40 wherein the second and third heat transfer devices constitute a combined thermal store heat transfer device

42. An apparatus or method substantially as hereinbefore described with reference to figure 1 , figure 2, figure 3, figure 4, figure 5, figure 6 and figure 7.