WO2013164563A1

WO2013164563A1 - Energy storage apparatus and method of operation of an energy storage system

Info

Publication number: WO2013164563A1
Application number: PCT/GB2013/050424
Authority: WO
Inventors: James Macnaghten
Original assignee: Isentropic Ltd
Priority date: 2012-04-30
Filing date: 2013-02-21
Publication date: 2013-11-07
Also published as: GB201207474D0; GB2501683A

Abstract

Apparatus for storing energy (100), comprising a first engine stage (130) acting as a compressor during a charging phase of a cycle and as an expander during a discharging phase of the cycle; a first heat store (120) for receiving and storing thermal energy from gas compressed by the first engine stage during the charging phase; a second engine stage (140) acting as an expander during the charging phase to expand gas received from the first heat store to below ambient temperature and as a compressor during the discharging phase; a second heat store(110) for transferring thermal energy to gas expanded by the expander during the charging phase; and thermal coupling means(170) for thermally coupling a part of the apparatus cooled below ambient temperature to a cooling chamber (190) for cooling an object or process within the cooling chamber, for example, to maintain a cryogenic temperature within it.

Description

ENERGY STORAGE APPARATUS AND METHOD OF OPERATION OF

AN ENERGY STORAGE SYSTEM

DESCRIPTION

The present invention relates to apparatus and methods for generating low temperatures, and particularly but not exclusively to apparatus and methods for generating cryogenic temperatures (e.g. temperatures below -150 °C).

There has been much interest in superconductors since the discovery of High Temperature Superconductors (HTS) in 1986. High Temperature superconductors are normally split into two groups - 1G based around BSCCO and 2G based around YBCO materials.

HTS power systems have been mooted as the next-generation of power systems technology, enabling the power grid to run with higher efficiency and lower cost. Superconductors are a subset of materials which show essentially zero resistance when cooled below their critical temperature (Tc), allowing, in principle, lossless power transmission. This is obviously of interest to electricity network operators, particularly in the case of underground/undersea DC cabling. Furthermore, since HTS cables can withstand much higher current density than conventional wires they can also be made to generate extremely large magnetic fields (> IT), a property which can be exploited to massively reduce the size and weight of inductive machinery (for example synchronous generators, transformers, wind turbines). Other uses include fault current limiters (FCLs), superconducting electromagnetic storage (SMES) and superconducting flywheels.

These so-called high-temperature superconductors are a class of cuprate alloys, for which Tc is around 30K or higher. The state-of-the art HTS material YBCO has a Tc of 92K. Crucially this is above the boiling point of Nitrogen (77K), meaning that YBCO will superconduct as long as it remains bathed in liquid N₂. Like any HTS, YBCO also has a critical current density Jc(T,B) above which it will quench. In fact the critical point is actually a surface in (B,T,J) space. Generally the lower T, the higher Jc, so depending upon application a T lower than 77K may be desirable.

There is an alternative LTS (Low Temperature Superconductor) material, Magnesium Diboride (MgB₂), which may be preferable under certain circumstances. MgB₂ has substantially colder Tc at 39K, which is much higher than other LTS materials and is many times cheaper to manufacture than HTS materials.

Most proposed superconducting applications require a cryocooler on site. As a general rule thermodynamic machinery efficiency always increases with size, which is unfortunate as most applications only require a relatively small amount of cooling to maintain the temperature of the HTS. For most current applications, only smaller cryocoolers are needed. However, whilst the efficiency of mechanical cryogenic refrigerators has been improving steadily over many years, for the devices of greatest interest to HTS applications their efficiency is typically <20% of Carnot efficiency, defined by: nc = Tc/(Th-Tc).

For an ambient temperature Th = 300 K and an operating temperature Tc = 77 K, nc = 0.345 (i.e. 34.5% efficiency so that 1J of input power will remove 0.345J of heat at 77K; so a typical efficiency is 20% of that, or 6.9% (i.e. 1J input will remove 0.069J of heat). Another way to represent this is through the Specific Power ( = l/η ), which is 14.5 in this case. This means that to remove one watt of heat from the 77 K region, it requires 14.5 watts of input energy. This requirement is known as the cryogenic penalty, because it is equivalent to a parasitic loss that diminishes the savings obtained by using HTS wire and eliminating the i2R loss of the particular electric device.

The present applicant has identified the need for an improved way of generating cryogenic temperatures that overcomes or at least alleviates problems associated with prior art devices.

In accordance with a first aspect of the present invention there is provided, apparatus for storing energy, comprising: a first engine stage configured to act as a compressor during a charging phase of a cycle and as an expander during a discharging phase of the cycle; a first heat store for receiving and storing thermal energy from gas compressed by the first engine stage during the charging phase; a second engine stage configured to act as an expander during the charging phase to expand gas received from the first heat store to below ambient temperature and as a compressor during the discharging phase; a second heat store for transferring thermal energy to gas expanded by the expander during the charging phase; and thermal coupling means (e.g. thermal coupling device) for thermally coupling a part of the apparatus cooled below ambient temperature to a cooling chamber for cooling an object or process within the cooling chamber.

In this way, apparatus is provided that both stores energy as heat (e.g. for subsequent conversion into electricity when there is a power demand) and cools or assists in cooling a cooling chamber (e.g. cryogenic cooling chamber) used to cooling an object or process. Since the energy storage apparatus is balanced, the removal of heat from the cooling chamber will increase the amount of heat that needs to be rejected from the apparatus and reduce the efficiency of energy storage cycle. However, whilst there is an incremental loss of efficiency in the energy storage cycle as a result of the additional cooling of the cooling chamber, the cooling is achieved at a much higher efficiency than would be achieved using a conventional small dedicated cryocooler.

In one embodiment, the apparatus comprises a circuit (e.g. gas circuit) configured to allow gas to pass cyclically between the first and second heat stores via the first and second engine stages during at least one of the charging and discharging phase.

In one embodiment the first and second engine stages each comprise separate compressor and expander devices. In another embodiment, the first and second engine stages each comprise a device configured to switch between operation as a compressor and operation as an expander. In one embodiment, the thermal coupling means is operable to transfer heat to a below ambient temperature part of the apparatus at a point between the second engine and the second heat store.

In one embodiment, the thermal coupling means is operable to transfer heat to a below ambient temperature part of the apparatus at a point inside the second heat store.

In one embodiment, the thermal coupling means is configured to allow gas cooled to below ambient temperature by the second engine during the charging phase or gas cooled to below ambient temperature by the second heat store to directly cool the cooling chamber. For example, the cooled gas may pass through the cooling chamber during charging or discharging.

In one embodiment, the thermal coupling means comprises a heat exchanger for transferring heat from a heat exchange fluid to the below ambient temperature part of the apparatus. In one embodiment, the heat exchanger is thermally coupled to the cooling chamber via an intermediate cooling process. The intermediate cooling process may be configured to cool the cooling chamber to a temperature which is lower (e.g. substantially lower) than the temperature of the below ambient temperature part of the apparatus.

In one embodiment, the thermal coupling means is selectively engageable. In one embodiment, the thermal coupling means is engaged during any stage (e.g. during charging, discharging or in a standby mode). In another embodiment, the thermal coupling means is engaged during the charging phase or discharging phase only. In a further embodiment, the thermal coupling means is engaged only during a charging phase.

In one embodiment, the cooling chamber is configured to cool the object or process to a temperature of at least -50°C, at least -100°C or at least -150°C.

In one embodiment, the temperature of the working gas of the apparatus may similarly be lowered to -50°C, at least -100°C or at least -150°C.

In one embodiment, the temperature of the working gas of the apparatus may be in the range -10 °C to -50 °C, -10 °C to -100 °C, or -10 °C to -150 °C. The cooling of the cooling chamber may use a small proportion of this range (e.g. raise the temperature of the working gas by no more than 10 °C ).

In one embodiment, the working gas of the apparatus is expanded (and hence further cooled) below a pressure level required for the second heat store prior to exposure to the thermal coupling means and the apparatus is configured to compress the working gas after exposure to the thermal coupling means to bring the pressure of the gas back up to the level required for the second heat store.

In one embodiment, the apparatus comprises further thermal coupling means for thermally coupling a part of the apparatus heated to above ambient temperature to an external process utilizing waste heat. In this way, any increase of heat generated by the apparatus by virtue of work done to cool the cooling chamber during a balanced process may be used rather than simply thrown away.

In one embodiment, the amount of energy used in providing cooling to the cooling chamber is less than 10% of the energy storage power input, less than 20% of the energy storage power input or less than 30% of the energy storage power input.

In one embodiment, the efficiency of the cooling of the cooling chamber is greater than 20% of Carnot cycle efficiency, greater than 30% of Carnot cycle efficiency, or greater than 40% of Carnot cycle efficiency.

In one embodiment, the cooling chamber is located within a cold storage vessel that is used in energy storage process performed by the apparatus.

In one embodiment, the cooling chamber is used to cool a process for separating gas (e.g. from air). In one embodiment, the process for separating gas is used to obtain a gas used by the apparatus (e.g. argon). The obtained gas may stored in liquid form until required.

In one embodiment, the cooling chamber cools a part of a superconducting device used by the apparatus. The superconducting device may be a superconducting motor and/or generator used by the apparatus during the charging or discharging phase. In another embodiment, the superconducting device may be a superconducting fault current limiter.

In accordance with a second aspect of the present invention, there is provided a method of modifying operation of an energy storage system in which a gas is cooled to below ambient temperature, comprising: thermally coupling a part of the energy storage system cooled below ambient temperature to a cooling chamber for cooling an object or process within the cooling chamber.

In one embodiment, the energy storage system comprises: a first engine stage configured to act as a compressor during a charging phase of a cycle and as an expander during a discharging phase of the cycle; a first heat store for receiving and storing thermal energy from gas compressed by the first engine stage during the charging phase; a second engine stage configured to act as an expander during the charging phase to expand gas received from the first heat store to below ambient temperature and as a compressor during the discharging phase; and a second heat store for transferring thermal energy to gas expanded by the expander during the charging phase.

In accordance with a third aspect of the present invention, there is provided a method of modifying operation of an energy storage system in which a gas is compressed, comprising: expanding at least a portion of the compressed gas to cool the gas below ambient temperature: and thermally coupling a part of the energy storage system cooled below ambient temperature to a cooling chamber for cooling an object or process within the cooling chamber.

In one embodiment of the second and third aspects of the present invention, the part of the energy storage system cooled below ambient temperature is located at a point between the second engine and the second heat store.

In one embodiment of the second and third aspects of the present invention, the part of the energy storage system cooled below ambient temperature is located at a point inside the second heat store.

In one embodiment of the second and third aspects of the present invention, the part of the energy storage system cooled below ambient temperature is used to directly cool the cooling chamber.

In one embodiment of the second and third aspects of the present invention, the step of thermally coupling the part of the energy storage system cooled below ambient temperature to the cooling chamber comprises transferring heat from a heat exchange fluid or a solid thermal conductor path to the below ambient temperature part of the energy storage system.

In one embodiment of the second and third aspects of the present invention, the heat exchange fluid is thermally coupled to the cooling chamber via an intermediate cooling process. The intermediate cooling process may be configured to cool the cooling chamber to a temperature which is lower (e.g. substantially lower) than the temperature of the below ambient temperature part of the energy storage system.

In one embodiment of the second and third aspects of the present invention, the step of thermally coupling the part of the energy storage system cooled below ambient temperature to the cooling chamber occurs at selected times. In one embodiment, the step of thermally coupling the part of the energy storage system cooled below ambient temperature to the cooling chamber occurs during any stage (e.g. during charging, discharging or in a standby mode). In one embodiment, the step of thermally coupling the part of the energy storage system cooled below ambient temperature to the cooling chamber 5 occurs during the charging phase or discharging phase. In a further embodiment, the step of thermally coupling the part of the energy storage system cooled below ambient temperature to the cooling chamber occurs only during a charging phase.

In one embodiment of the second and third aspects of the present invention, the cooling chamber is configured to cool the object or process to a temperature of at least - 10 50°C, at least -100°C or at least -150°C.

In one embodiment, the temperature of the working gas of the apparatus may be in the range -10 °C to -50 °C, -10 °C to -100 °C, or -10 °C to -150 °C. The cooling of the 15 cooling chamber may use a small proportion of this range (e.g. raise the temperature of the working gas by no more than 10 °C ).

In one embodiment of the second and third aspects of the present invention, the method further comprises thermally coupling a part of the energy storage system heated to above ambient temperature to an external process utilizing waste heat.

0 In one embodiment of the second and third aspects of the present invention, the amount of energy used in providing cooling to the cooling chamber is less than 10% of the energy storage power input, less than 20% of the energy storage power input or less than 30% of the energy storage power input.

In one embodiment of the second and third aspects of the present invention, the 5 efficiency of the cooling of the cooling chamber is greater than 20% of Carnot cycle efficiency, greater than 30% of Carnot cycle efficiency, or greater than 40% of Carnot cycle efficiency.

In one embodiment of the second and third aspects of the present invention, the cooling chamber is located within a cold storage vessel that is used in energy storage 30 process performed by the energy storage system.

In one embodiment of the second and third aspects of the present invention, the cooling chamber is used to cool a process for separating gas (e.g. from air). In one embodiment, the process for separating gas is used to obtain a gas used by the energy storage system (e.g. argon). The obtained gas may stored in liquid form until required.

In one embodiment of the second and third aspects of the present invention, the cooling chamber cools a part of a superconducting device used by the energy storage system. The superconducting device may be a superconducting motor and/or generator used by the energy storage system during the charging or discharging phase. In another embodiment, the superconducting device may be a superconducting fault current limiter.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figures 1A and IB show (during a charging phase and discharging phase respectively) a schematic illustration of an electricity storage system of the type disclosed in WO 2009/044139 adapted in accordance with a first embodiment of the present invention to provide cryogenic cooling;

Figures 1C and ID illustrate an expected P-V diagram for the system of Figure 1A during charging;

Figure 2 shows an electricity storage system according to a second embodiment of the present invention;

Figure 3 shows an electricity storage system according to a third embodiment of the present invention; and

Figures 4A and 4B show an electricity storage system according to a fourth embodiment of the present invention during a charging phase and a discharging phase respectively.

Figures 1A and IB show an electricity storage system 100 of the type disclosed in WO 2009/044139 adapted to cryogenically cool a cooling chamber 190 used for cooling an object or process within the cooling chamber.

System 100 comprises an insulated hot storage vessel 120 housing a first gas- permeable particulate heat storage structure 121, cold storage vessel 110 housing a second gas-permeable particulate heat storage structure 111, first and second multi-stage compressor/expanders 130, 140, first, second and third heat exchangers 150, 160 170 and interconnecting pipes 101, 102, 103 and 104 forming a gas circuit 105 for conveying working gas around the system. First and second heat exchangers 150, 160 operate to dissipate unwanted heat from the system in order to bring the datum temperature of the system down to as close to ambient temperature as possible to maximise efficiency of the apparatus. In contrast with the first and second heat exchangers, third heat exchanger 170 is in thermal communication with cooling chamber 190 to allow thermal energy to pass from cooling chamber 190 to gas circuit 105. In this way, third heat exchanger 170 acts to maintain or assist in maintaining a cryogenic temperature within cooling chamber 190.

In operation during a charging phase higher pressure gas at a temperature close to ambient temperature exits interconnecting pipe 103 (after passing through heat exchanger 160) and is expanded by compressor/expander 140 to a lower pressure. The gas is cooled during this expansion and passes via interconnecting pipe 104 to the cold storage vessel 110 receiving thermal energy from third heat exchanger 170 to cool cooling chamber 190. The gas then passes through particulate heat storage structure 111, where the gas is further heated. The now hotter gas leaves particulate heat storage structure 111 and passes into interconnecting pipe 101 where it is exposed to first heat exchanger 150 to bring the temperature of the gas down to a temperature closer to ambient temperature. The gas exits interconnecting pipe 101 and enter multi-stage compressor/expander 130, where the gas is compressed to the higher pressure. As the gas is compressed the temperature rises and the gas leaves the compressor at a higher temperature and passes into interconnecting pipe 102. The gas then enters hot storage vessel 120 and passes down through particulate heat storage structure 121, where the gas is cooled. The now cooler gas leaves particulate heat storage structure 121 and enters interconnecting pipe 103 where it is exposed to second heat exchanger 160 to bring the temperature of the gas down to a temperature closer to ambient temperature. The charging process can continue until the hot and cold stores are 'fully charged' or stop earlier if required.

This overall charging process absorbs energy that is normally supplied from other generating devices via the electric grid. The multi-stage compressor/expanders 130 and 140 are driven by a mechanical device, such as an electric motor (not shown).

In operation during a discharging phase high temperature gas at a higher pressure enters interconnecting pipe 102 and is expanded by multi-stage compressor/expander 130 to a lower pressure. The gas is cooled during this expansion and passes via interconnecting pipe 101 where it is exposed to first heat exchanger 150 to bring the temperature of the gas down to a temperature closer to ambient temperature. The gas then enters cold storage vessel 110 and passes down through particulate heat storage structure 111 where the gas is further cooled. The now colder gas leaves particulate heat storage structure 111 and enters interconnecting pipe 104 where it receives thermal energy from third heat exchanger 170 to cool cooling chamber 190. The gas then exits interconnecting pipe 104 and enters compressor/expander 140 where the gas is compressed to the higher pressure. As the gas is compressed the gas temperature rises and the gas leaves the compressor at a higher temperature and passes into interconnecting pipe 103 where it is exposed to second heat exchanger 160 to bring the temperature of the gas down to a temperature closer to ambient temperature. The gas then enters hot storage vessel 120 and passes up through particulate heat storage structure 121 where the gas is heated. The now high temperature gas leaves particulate heat storage structure 121 and passes into interconnecting pipe 102 and is expanded by multi-stage compressor/expander 130 with the energy of expansion being used to generate electricity for the electric grid. The discharging process can continue until the hot and cold stores are 'fully discharged' or stop earlier if required.

The overall discharging process generates energy that is normally supplied in an electrical form (e.g. back to the electric grid). In this mode the multi-stage compressor/expanders 130 and 140 drive a mechanical device, such as an electric generator (not shown).

Since the system is balanced, the removal of heat from cooling chamber 190 will increase the amount of heat that needs to be rejected from the apparatus and reduce the efficiency of energy storage system 100. However, whilst there is an incremental loss of efficiency in the energy storage cycle of system 100 as a result of the additional cooling of cooling chamber 190, the cooling is achieved at a much higher efficiency than would be achieved using a conventional small dedicated cryocooler. Furthermore, the additional waste heat rejected from the system may be in a form and at an elevated temperature that is useful, such as for the purpose of heating buildings or providing hot water.

Figures 1C and ID illustrate a theoretical P-V diagram for the operation of system 100 during charging. The area highlighted in Figure 1C is the component of the work done during the charging cycle that is attributable to the energy storage cycle and is comparable in shape with system 100 with third heat exchanger 170 removed. The area highlighted in Figure ID is the (much smaller) component of the work done during the charging cycle that is attributable to the cryogenic cooling and is comparable with the cycle for a Brayton cycle cryocooler. Of the two cycles, the work done is clearly much lower for the part of the cycle attributable to the cryogenic cooling (i.e. there is a large amount of mechanical work of compression and expansion compared to the actual work of the cycle). However, unlike with a Brayton cycle performed by a small dedicated cryocooler, the cryo-cooling achieved in system 100 benefits from being part of a much larger thermodynamic machine and is also much less susceptible to mechanical losses from compression and expansion, which is a practical problem with small Brayton cycle cryocoolers. The mechanical losses are already accounted for in the energy storage cycle, which is much less sensitive to them as it has a high work ratio. Work ratio is normally defined as the difference between compression and expansion work divided by the sum of compression and expansion work. The storage cycle might have a work ratio of 0.4 compared to a Brayton cycle cryocooler with a work ratio of .05. Accordingly, the cryogenic cooling cycle is effectively able to perform at much closer to ideal cycle performance than might be expected as it is 'piggybacked' (or "superimposed") on to the storage cycle.

Figure 2 shows an electricity storage system 200 based on system 100 and comprising an insulated hot storage vessel 220 housing a first gas-permeable particulate heat storage structure 221, cold storage vessel 210 housing a second gas-permeable particulate heat storage structure 211, first and second multi-stage compressor/expanders 230, 240, first, second and third heat exchangers 250, 260, 270 and interconnecting pipes 201, 202, 203 and 204 forming a gas circuit 205 for conveying working gas around the system. As in system 100 third heat exchanger 270 is in thermal communication with a cooling chamber 290 to allow thermal energy to pass from cooling chamber 290 to gas circuit 205. However, in system 200 cooling chamber 290 is thermally coupled to third heat exchanger via a closed heat pump circuit 280 (e.g. operating a Brayton cycle) comprising an expander 282 for expanding fluid cooled by third heat exchanger 270, a further heat exchanger 284 for transferring thermal energy from cooling chamber 290 to the working fluid of the heat pump circuit 280 and a compressor 286 for compressing the working fluid prior to exposure to third heat exchanger 270.

In this way, cooling chamber 290 may be cooled to a lower temperature than a minimum temperature of the working gas in gas circuit 205. Advantageously, the use of a secondary closed heat pump circuit 280 is that it allows the use of a working fluid that is more suited to very low temperatures (e.g. Neon, Helium or Hydrogen). A further advantage of using a closed heat pump circuit 280 is that the pressure ratio can be varied to match the temperature of the working gas entering cold storage vessel 210 (e.g. the pressure ratio can be adjusted upwards to compensate for changing (e.g. rising or falling) temperatures in the cold storage vessel 210).

Figure 3 shows an electricity storage system 300 based on system 100 and comprising an insulated hot storage vessel 320 housing a first gas-permeable particulate heat storage structure 321, cold storage vessel 310 housing a second gas-permeable particulate heat storage structure 311, first and second multi-stage compressor/expanders 330, 340, first, second and third heat exchangers 350, 360, 370 and interconnecting pipes 301, 302, 303 and 304 forming a gas circuit 305 for conveying working gas around the system. As in system 200 third heat exchanger 370 is in thermal communication with a cooling chamber 390 to allow thermal energy to pass from cooling chamber 390 to gas circuit 305. However, in system 300 a heat pump section 380 comprising: a first compressor/expander 382 for expanding the working gas prior to exposure to third heat exchanger 370 during a charging cycle and compressing the working gas after exposure to third heat exchanger 370 back to the starting pressure during a discharging cycle; and a second compressor/expander 386 for compressing the working gas back to the starting pressure after exposure to third heat exchanger 370 during the charging cycle and for expanding the working gas prior to exposure to third heat exchanger 370 during the discharging cycle. In this way, cooling chamber 390 may be cooled to a lower temperature than a minimum temperature of cold storage vessel 310 using the working gas in circuit 305 to provide the additional cooling.

Figures 4 A and 4B show an electricity storage system 400 based on system 100 and comprising an insulated hot storage vessel 420 housing a first gas-permeable particulate heat storage structure 421, cold storage vessel 410 housing a second gas-permeable particulate heat storage structure 411, first and second multi-stage compressor/expanders 430, 440, first, second and third heat exchangers 450, 460, 470 and interconnecting pipes 401, 402, 403 and 404 forming a gas circuit 405 for conveying working gas around the system. As in system 100 third heat exchanger 470 is in thermal communication with a cooling chamber 490 to allow thermal energy to pass from cooling chamber 490 to gas circuit 405.

During the discharge phase operation of system 400 is the same as operation of system 100. However, during the charging phase a controller 495 acts to direct working gas cooled by second heat exchanger 460 along a bypass pipe 406 connecting a Brayton cycle heat pump 480 between the second heat exchanger 460 and second compressor/expanders 440 for working directly on the working gas of the main system. Heat pump 480 comprises a compressor 482 for compressing working gas received form the second heat exchanger 5 460, a further heat exchanger 484 for receiving heat from working gas after compression by compressor 482 and dissipating heat from the system (e.g. to atmosphere), and an expander 486 for receiving working gas from further heat exchanger 484 and expanding the working gas to a temperature that is lower than the temperature of the gas exiting the second heat exchanger 460. In this way, cooling chamber 490 may be cooled to a lower temperature0 than would normally be achieved when expanding a gas from the temperature and pressure in interconnecting pipe 403 to the pressure in interconnecting pipe 404.

Claims

Claims:

1. Apparatus for storing energy, comprising:

a first engine stage configured to act as a compressor during a charging phase of a cycle and as an expander during a discharging phase of the cycle;

a first heat store for receiving and storing thermal energy from gas compressed by the first engine stage during the charging phase;

a second engine stage configured to act as an expander during the charging phase to expand gas received from the first heat store to below ambient temperature and as a compressor during the discharging phase;

a second heat store for transferring thermal energy to gas expanded by the expander during the charging phase; and

thermal coupling means for thermally coupling a part of the apparatus cooled below ambient temperature to a cooling chamber for cooling an object or process within the cooling chamber.

2. Apparatus according to claim 1, wherein the thermal coupling means is operable to transfer heat to a below ambient temperature part of the apparatus at a point between the second engine and the second heat store.

3. Apparatus according to claim 1, wherein the thermal coupling means is operable to transfer heat to a below ambient temperature part of the apparatus at a point inside the second heat store.

4. Apparatus according to any of the preceding claims, wherein the thermal coupling means is configured to allow gas cooled to below ambient temperature by the second engine during the charging phase or gas cooled to below ambient temperature by the second heat store to directly cool the cooling chamber.

5. Apparatus according to any of claims 1-3, wherein the thermal coupling means comprises a heat exchanger for transferring heat from a heat exchange fluid or a solid thermal conductor path to the below ambient temperature part of the apparatus.

6. Apparatus according to claim 5, wherein the heat exchanger is thermally coupled to the cooling chamber via an intermediate cooling process.

5 7. Apparatus according to claim 5 or claim 6, wherein the intermediate cooling process is configured to cool the cooling chamber to a temperature which is lower than the temperature of the below ambient temperature part of the apparatus.

8. Apparatus according to any of the preceding claims, wherein the thermal coupling 10 means is selectively engageable.

9. Apparatus according to claim 8, wherein the thermal coupling means is engaged during any stage.

15 10. Apparatus according to claim 9, wherein the thermal coupling means is engaged during the charging phase or discharging phase.

11. Apparatus according to claim 9, wherein the thermal coupling means is engaged only during a charging phase.

20

12. Apparatus according to any of the preceding claims, wherein the cooling chamber is configured to cool the object or process to a temperature of at least -50°C, at least -100°C or at least -150°C

25 13. Apparatus according to any of the preceding claims, wherein the apparatus comprises further thermal coupling means for thermally coupling a part of the apparatus heated to above ambient temperature to an external process utilizing waste heat.

14. Apparatus according to any of the preceding claims, wherein the amount of energy 30 used in providing cooling to the cooling chamber is less than 10% of the energy storage power input, less than 20% of the energy storage power input or less than 30% of the energy storage power input.

15. Apparatus according to any of the preceding claims, wherein the efficiency of the cooling of the cooling chamber is greater than 20% of Carnot cycle efficiency, greater than 30% of Carnot cycle efficiency, or greater than 40% of Carnot cycle efficiency.

5

16. Apparatus according to any of the preceding claims, wherein the cooling chamber is located within a cold storage vessel that is used in energy storage process performed by the apparatus.

10 17. Apparatus according to any of the preceding claims, wherein the cooling chamber is used to cool a process for separating gas.

18. Apparatus according to claim 17, wherein the process for separating gas is used to obtain a gas used by the apparatus.

15

19. Apparatus accordingly to claim 18, wherein the obtained gas is stored in liquid form until required.

20. Apparatus according to any of the preceding claims, wherein the cooling chamber 0 cools a part of a superconducting device used by the apparatus.

21. Apparatus according to claim 20, wherein the superconducting device is a superconducting motor and/or generator used by the apparatus during the charging or discharging phase.

5

22. Apparatus according to claim 21, wherein the superconducting device is a superconducting fault current limiter.

23. A method of modifying operation of an energy storage system in which a gas is 30 cooled to below ambient temperature, comprising:

thermally coupling a part of the energy storage system cooled below ambient temperature to a cooling chamber for cooling an object or process within the cooling chamber.

24. The method of claim 23, wherein the energy storage system comprises:

a second engine stage configured to act as an expander during the charging phase to expand gas received from the first heat store to below ambient temperature and as a compressor during the discharging phase; and

a second heat store for transferring thermal energy to gas expanded by the expander during the charging phase.

25. A method of modifying operation of an energy storage system in which a gas is compressed, comprising:

expanding at least a portion of the compressed gas to cool the gas below ambient temperature: and

26. A method according to claim 24, wherein the part of the energy storage system cooled below ambient temperature is located at a point between the second engine and the second heat store.

27. A method according to claim 24, wherein the part of the energy storage system cooled below ambient temperature is located at a point inside the second heat store.

28. A method according to any of claims 23-27, wherein the part of the energy storage system cooled below ambient temperature is used to directly cool the cooling chamber.

29. A method according to any of claims 23-27, wherein the step of thermally coupling the part of the energy storage system cooled below ambient temperature to the cooling chamber comprises transferring heat from a heat exchange fluid or a solid thermal conductor path to the below ambient temperature part of the energy storage system.

5 30. A method according to claim 29, wherein the heat exchange fluid is thermally coupled to the cooling chamber via an intermediate cooling process.

31. A method according to claim 29 or claim 30, wherein the intermediate cooling process is configured to cool the cooling chamber to a temperature which is lower than the

10 temperature of the below ambient temperature part of the energy storage system.

32. A method according any of claims 23-31, wherein the step of thermally coupling the part of the energy storage system cooled below ambient temperature to the cooling chamber occurs at selected times.

15

33. A method according to claim 32, wherein the step of thermally coupling the part of the energy storage system cooled below ambient temperature to the cooling chamber occurs during any stage.

20 34. A method according to claim 33, wherein the step of thermally coupling the part of the energy storage system cooled below ambient temperature to the cooling chamber occurs during the charging phase or discharging phase.

35. A method according to claim 34, wherein the step of thermally coupling the part of 25 the energy storage system cooled below ambient temperature to the cooling chamber occurs only during a charging phase.

36. A method according to any of claims 23-35, wherein the cooling chamber is configured to cool the object or process to a temperature of at least -50°C, at least -100°C or

30 at least -150°C.

37. A method according to claim of claims 23-36, wherein the method further comprises thermally coupling a part of the energy storage system heated to above ambient temperature to an external process utilizing waste heat.

38. A method according to any of claims 23-37, wherein the amount of energy used in 5 providing cooling to the cooling chamber is less than 10% of the energy storage power input, less than 20% of the energy storage power input or less than 30% of the energy storage power input.

39. A method according to any of claims 23-38, wherein the efficiency of the cooling of 10 the cooling chamber is greater than 20% of Carnot cycle efficiency, greater than 30% of

Carnot cycle efficiency, or greater than 40% of Carnot cycle efficiency.

40. A method according to any of claims 23-39, wherein the cooling chamber is located within a cold storage vessel that is used in energy storage process performed by the energy

15 storage system.

41. A method according to any of claims 23-40, wherein the cooling chamber is used to cool a process for separating gas.

20 42. A method according to claim 41, wherein the process for separating gas is used to obtain a gas used by the energy storage system.

43. A method according to claim 42, wherein the obtained gas is stored in liquid form until required.

25

44. A method according to any of claims 23-43, wherein the cooling chamber cools a part of a superconducting device used by the energy storage system.

45. A method according to claim 44, wherein the superconducting device is a 30 superconducting motor and/or generator used by the energy storage system during the charging or discharging phase.

46. A method according to claim 44, wherein the superconducting device is a superconducting fault current limiter.

47. An apparatus or method substantially as hereinbefore described with reference to the accompanying drawings.