GB2501685A

GB2501685A - Apparatus for storing energy

Info

Publication number: GB2501685A
Application number: GB201207486A
Authority: GB
Inventors: Jonathan Sebastian Howes; James Macnaghten
Original assignee: Isentropic Ltd
Current assignee: Isentropic Ltd
Priority date: 2012-04-30
Filing date: 2012-04-30
Publication date: 2013-11-06
Also published as: GB201207486D0; WO2013164562A1

Abstract

Apparatus 300 for storing energy including a first engine stage 330 acting as a compressor during a charging phase of a cycle and as an expander during a discharging phase; a first heat store 320 for receiving and storing thermal energy from gas compressed by the first engine stage during the charging phase; a second engine stage 340 acting as an expander during the charging phase to expand gas received from the first heat store 320 and as a compressor during the discharging phase; a second heat store 310 for transferring thermal energy to gas expanded by the expander during the charging phase; a heat rejection device 385, 380 for dissipating unwanted heat from a gas flow and control means 370 for varying the degree of thermal coupling between the gas flow and the heat rejection device, or, its level of heat rejection performance, between a lower level during a first part of the cycle and a higher level during a second part of the cycle.

Description

TITLE: APPARATUS FOR STORING ENERGY

DESCRIPTION

The present invention relates to apparatus for storing energy, and particularly but not exclusively to apparatus for receiving and returning energy in the form of electricity (hereinafter referred to as "electricity storage" apparatus).

The applicant's earlier application WO 2009/044139 discloses a thermodynamic electricity storage system using thermal stores. In the most basic configuration a hot store and a cold store are connected to each other by a compressor and expander (the latter is often referred to as a turbine in axial flow machinery). In a charging mode heat is pumped from one store to the other (i.e. heating the hot store and cooling the cold store) and in a discharge mode the system the process is reversed (i.e. with the cold store being used to cool gas prior to compression and heating in the hot store). The systems can use a variety of different types of compressors and expanders, some examples are reciprocating, rotary screw, sliding vane, axial or centrifugal. The systems can use a thermal storage media, such as a refractory 111cc alumina, or a natural mineral like quartz.

The cycles used in the system of WO 2009/044139 may be run as closed cycle processes or as open cycle systems (e.g. where there is one stage that is at near ambient temperature, atmospheric pressure and the working tluid is air). When running as a closed cycle, the working gas may advantageously be a monatomic gas such as argon which has a high isentropic index (i.e. for a given pressure change a higher temperature rise is achieved than for a diatomic gas such as nitrogen). This results in a lower peak system pressure which in turn lowers the amount of matenal required to contain the pressure and hence the cost of the thermal storage vessels. However, in order to maximise the efficiency of such as closed cycle system it is generally important to reject waste heat from the system to allow working gas exiting the hot and cold stores during charging (and entenng the cold and hot stores during discharging) to be cooled to a temperature Tdaum that is as close to ambient temperature Tambieut as possible. Since the amount of energy stored in the system during charging will depend upon the temperature difference between the maximum temperature of the hot store and Tjaiam, both Tanihielit and the temperature difference between TdCft and Tanihieii[ will have an effect on energy storage density.

The present applicant has identified the need for an improved heat storage system which allows potential for improved operation over the identified prior art.

hi accordance with a first aspect of the present invention, there is provided apparatus for stonng 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 and as a compressor during the discharging phase; a second heat store for transfelTing thermal energy to gas expanded by the expander during the charging phase; a heat rejection device for dissipating unwanted heat from a gas flow; and control means (e.g. a controller) for varying the degree of thermal coupling between the gas flow and the heat rejection device or varying the level of heat rejection performance provided by the heat rejection device between a lower level during a first part of the cycle and a higher level during a second part of the cycle, with the change from the lower level to the higher level being configured to provide an increase in heat rejection dunng the second part of the cycle for boosting energy density over the cycle.

lii this way, energy storage apparatus is provided in which the system datum temperature can be artificially lowered below a natural level achievable through continually connected heat exchangers to improve energy capacity and power output during periods when additional storage or output are required. Whilst this increase in storage or output will reduce efficiency of operation. for situations where the second phase of the cyde is alTanged to coincide substantially with a period when electricity is very low cost (e.g. free) or in situations in which the apparatus is used in warm climates where storage and output may be limited by high ambient temperatures during periods of the day, this energy penalty may be worth paying for the added value of extra capacity and power output at peak times.

Furthermore, where there is use for the heat generated by the decreased efficiency some or all of the increased heat output from the apparatus may be used in a further process.

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 heat rejection device is configured to receive heat from at least a portion of the gas flow that is directed to the heat rejection device and the control means is configured to vary the degree of thermal coupling by varying the proportion of the gas flow that is directed to the heat rejection device relative the gas flow that bypasses the heat rejection device.

In one embodiment, the level of heat rejection performance provided by the heat rejection device is varied by increasing or decreasing a flow of coolant through the heat rejection device or varying a lower temperature of the coolant or a combination of both of these techniques.

In one embodiment, at the lower level heat rejection from the gas flow via the heat rejection device is substantially zero (e.g. substantially all of the gas flow bypasses the heat rejection device (e.g. thermal coupling is substantially zero) or the heat rejection performance provided by the heat rejection device is reduced to a lower limit by stopping a heat rejection process performed by the heat rejection device).

In one embodiment, the second part of the cycle occurs substantially during the charging phase of the cycle.

In one embodiment, the second part of the cycle substantially corresponds to the charging phase of the cycle.

In one embodiment, the heat rejection device comprises heat pump means (e.g. heat pump) operable to remove heat from the gas. In one embodiment, the heat pump means compnses means for compressing the at least a portion of gas flow (e.g. a further compressor) and a heat exchanger for transferring heat from the at least a portion of gas flow prior to expansion of the at least a portion of gas flow (e.g. by the expander or by a further expander). In one embodiment, the heat pump means is a Brayton cycle heat pump.

In one embodiment, the heat pump means is thermally coupled to the apparatus by a heat exchanger.

In one embodiment, the heat pump means is operable to remove heat from the apparatus at a point between the first heat store and the second engine stage.

In one embodiment, the heat pump means is operable to remove heat from the apparatus at a point between the second heat store and the first engine stage.

In one embodiment, during the charging phase the first engine stage is configured to act as a multi-stage compressor comprising first and second (e.g. final) compressor stages and the heat rejection device comprises a heat exchanger connectable between the first and second compressor stages to remove heat from gas compressed by the first compressor stage prior to compression of the gas by the second compressor stage.

In one embodiment, the apparatus further comprises at least one fixed heat exchanger with a constant degree of thermal coupling to the gas flow for dissipating unwanted heat from the gas flow (e.g. to ambient).

In accordance with a second aspect of the present invention, there is provided a method of operating an energy storage system comprising: during a charging phase of a cycle: compressing a gas using a first engine stage; transferring heat from the compressed gas to first heat store for receiving and storing thermal energy from the compressed gas; expanding the gas using a second engine stage after heat from the gas has been transferred to the first heat store; and transferring heat from a second heat store to the expanded gas during the charging phase; and during a discharging phase of the cycle: cooling a gas by transferring heat from the gas to the second heat store; compressing the gas cooled by the second heat store; heating the compressed gas by transferring heat from the first heat store to the gas; and expanding the gas heated by the first heat store to generate a power output; characterised by the step of varying the degree of thermal coupling between a gas flow and a heat rejection device configured to dissipate unwanted heat from the system or varying the level of heat rejection performance provided by the heat rejection device between a thwer level during a first part of the cycle and a higher level during a second part of the cycle, with the change from the lower level to the higher level being configured to provide an increase in heat rejection during the second part of the cycle for boosting energy density over the cycle.

hi one embodiment, the heat rejection device is configured to receive heat from at least a portion of the gas flow that is directed to the heat rejection device and the step of varying the degree of thermal coupling comprises varying the proportion of the gas flow that is directed to the heat rejection device relative to the gas flow that bypasses the heat rejection device.

hi one embodiment, the level of heat rejection performance provided by the heat rejection device is varied by increasing or decreasing a flow of coohmt through the heat rejection device or varying a lower temperature of the coolant or a combination of both of these techniques.

hi one embodiment, at the lower level heat rejection from the gas flow via the heat rejection device is substantially zero (e.g. substantially all of the gas flow is made to bypass the heat rejection device or the heat rejection performance provided by the heat rejection device is reduced to a lower limit by stopping a heat rejection process performed by the heat rejection device).

hi one embodiment, the second part of the cycle occurs substantially during the charging phase of the cycle.

hi one embodiment, the second part of the cycle substantially colTesponds to the charging phase of the cycle.

hi one embodiment, the heat rejection device comprises heat pump means (e.g. heat pump) operable to remove heat from the gas.

hi one embodiment, the heat pump means comprises means for heating the at least a portion of gas flow (e.g. a further compressor) and a heat exchanger for transferring heat from the at least a portion of gas flow prior to expansion of the at least a portion of gas flow (e.g. by the expander or by a further expander).

hi one embodiment, the heat pump means is thermally coupled to the apparatus by a heat exchanger.

In one embodiment the heat pump means is operable to remove heat from the apparatus at a point between the first heat store and the second engine stage.

hi one embodiment, the step of compressing the gas during the charging phase comprises compressing the gas using first and second compressor stages of a multi-stage compressor and the heat rejection device compnses a heat exchanger connectable between the first and second (e.g. final) compressor stages to remove heat from gas compressed by the first compressor stage prior to compression of the gas by the second compressor stage.

hi accordance with a third 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 theimal energy from gas compressed by the first engine stage during the charging phase; a second engine stage configured to act as an expander dunng the charging phase to expand gas received from the first heat store and as a compressor during the discharging phase; a second heat store for transferring thermal energy to gas expanded by the expander dunng the charging phase; and a heat rejection device for dissipating unwanted heat from a gas flow at a point between the first heat store and the second engine stage; wherein the second engine stage is operable during the discharging phase to compress gas cooled by the second heat store to an elevated pressure exceeding mean peak gas pressure in the first heat store during the charging phase.

hi this way, energy storage apparatus in provided in which additional heat is dissipated during the discharging phase by effectively pumping heat from the second heat store to the heat rejection device) providing the potential for an increase in energy density during a subsequent charging phase since at least a part of the second heat store will be at a lower temperature than if the additional heat had not been dissipated whilst the increase in temperature of at least a part of the first heat store is less then the reduction in temperature of the at least a part of the second heat store as a result of the operation of the heat rejection device. Advantageously, this technique for increasing energy density is achieved by using the second engine stage without the need for any further compressor/expander stage.

hi one embodiment, the elevated pressure is at least 10% higher than the mean peak gas pressure in the first heat store during the charging phase. For example, the elevated pressure may be at least 20% higher than the mean peak gas pressure in the first heat store during the charging phase.

lii accordance with a fourth aspect of the present invention, there is provided a method of operating an energy storage system comprising: during a charging phase of a cycle: compressing a gas using a first engine stage; transferring heat from the compressed gas to first heat store for receiving and storing thermal energy from the compressed gas; expanding the gas using a second engine stage after heat from the gas has been transferred to the first heat store; and transferring heat from a second heat store to the expanded gas; and during a discharging phase of the cycle: cooling a gas by transferring heat from the gas to the second heat store; compressing the gas cooled by the second heat store and subsequently dissipating unwanted heat from the gas flow; heating the compressed gas by transfening heat from the first heat store to the gas; and expanding the gas heated by the first heat store to generate a power output; characterised in that the step of compressing the gas cooled by the second heat store comprises compressing the gas to an elevated pressure exceeding mean peak gas pressure in the first heat store during the charging phase.

In one embodiment, the elevated pressure is at least 10% higher than the mean peak gas pressure in the first heat store during the charging phase. For example, the elevated pressure may be at least 20% higher than the mean peak gas pressure in the first heat store during the charging phase.

lii accordance with a fifth 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 dunng 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 and as a compressor dunng the discharging phase; a second heat store for transferring thermal energy to gas expanded by the expander dunng the charging phase; heat pump means (e.g. heat pump) operable to remove unwanted heat from a gas flow.

lii one embodiment, the heat pump means comprises means for heating the at least a portion of gas flow (e.g. a further compressor) and a heat exchanger for tnmsferring heat from the at least a portion of gas flow prior to expansion of the at least a portion of gas flow (e.g. by the expander or by a further expander).

hi one embodiment, the heat pump means is operable to remove heat from the apparatus at a point between the second heat store and the first engine stage.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figures 1A and lB show a schematic illustration of an electricity storage system of the type disclosed in WO 2009/044 139 during a charging phase and discharging phase respectively; Figures 2A and 2B show an electricity storage system according to a first embodiment of the present invention during a charging phase and a discharging phase respectively; Figure 3 shows an electricity storage system according to a second embodiment of the present invention during a charging phase; Figures 4A and 4B show an electncity storage system according to a third embodiment of the present invention during a charging phase and a discharging phase respectively; and Figures 5A and SB 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 lB show an electricity storage system 100 of the type disclosed in WO 2009/044139 comprising 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 and second heat exchangers 150, 160 and interconnecting pipes 101, 102, 103 and 104 forming a gas circuit 105 for conveying lii operation during a charging phase higher pressure gas at a temperature close to ambient temperature exits interconnecting pipe 103 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. The gas passes through particulate heat storage structure 111, where the gas is heated. The now hotter gas leaves particulate heat storage structure ill 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 nses 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 frilly charged' or stop earlier if required.

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

In this example. material properties limit the system to a maximum temperature Tmax of 500 °C. First and second heat exchangers 150. 160 are designed to bring Tdaa,m down to as close to ambient air temperature as possible. The larger the pumped temperature difference between Tjaam and Tmax the greater the energy stored during charging/electrical output during discharging. Tdaum is generally 10-20°C hotter than external ambient depending upon the size of the heat exchangers 150,160. The higher Tdaum the lower the compression ratio that will result in a peak temperature of 500 °C. Accordingly, the energy density stored and output power generated by system 100 may be significantly lower in a wanner climate country than in a cooler climate country. Furthermore, in certain warmer climate countries peak electricity demand is driven by air conditioning, i.e. discharge of the system is required during periods when the ambient temperature is at its highest. Countries with large differences in temperature could see large swings in storage capacity.

Figures 2A and 2B show 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 and second heat exchangers 250, 260 and interconnecting pipes 201, 202, 203 and 204 forming a gas circuit 205 for conveying working gas around the system.

Dunng the discharge phase operation of system 200 is the same as operation of system 100. However, during the charging phase a controller 270 acts to direct working gas cooled by second heat exchanger 260 along a bypass pipe 206 connecting a Brayton cycle heat pump 280 between the second heat exchanger 260 and second compressor/expanders 240 for working directly on the working gas of the main system. Heat pump 280 comprises a compressor 282 for compressing working gas received form the second heat exchanger, a further heat exchanger 284 for receiving heat from working gas after compression by compressor 282 and dissipating heat from the system (e.g. to atmosphere), and an expander 286 for receiving working gas from further heat exchanger 284 and expanding the working gas to a temperature that is lower than the temperature of the gas exiting the second heat exchanger 260.

When heat pump 280 is connected to gas circuit 205 during the charging phase there is an increase in heat rejection relative to the level of heat rejection if heat pump 280 was not connected. This increase in heat rejection during the discharging phase acts to lower the temperature of cold storage vessel 210 for boosting energy density and power output over the complete charge/discharge cycle. Advantageously, system 200 allows the energy storage capacity to be increased during periods in which increased capacity is required or the efficiency penalty justified (e.g. to meet demand during a high electricity price period or to take advantage of low off-peak prices when efficiency is less important than capacity).

By placing heat pump 280 on the high pressure side of the system between hot storage vessel 220 and second multi-stage compressor/expander 240, heat pump 280 (which processes the same amount of gas as each compressor/expander 230, 240) may be smaller and work more efficiently than if positioned on a lower pressure side of the system.

Furthermore, location on the high pressure side reduces the effect of pressure drop through connecting valves, mechanical friction and pressure loss from further heat exchanger 284 relative to a heat pump working directly on the working gas at a low pressure part of the system. Whilst heat pump 280 will create a high pressure region for heat exchange, this high pressure is restricted to the heat pump part of the system and does not impose these additional gas pressures on the rest of the system.

Figure 3 shows an electricity storage system 300 based on system 200 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 and second heat exchangers 350, 360 and interconnecting pipes 301, 302, 303 and 304 forming a gas circuit 305 for conveying working gas around the system.

However, instead of using a direct heat pump to cool the working gas during the charging phase system 300 comprises an external heat pump 380 thermally coupled to gas circuit 305 at a point between first heat exchanger 350 and first multi-stage compressor/expanders 330 via a further heat exchanger 385. A controller 370 operates to switch extemal heat pump 380 on during the charging phase to generate a flow of low temperature co&ant through further heat exchanger 385 for receiving heat from the working gas of the system. In this way, the working gas is cooled below a temperature that can be achieved by just cooling gas via first heat exchanger 350. Tn the discharging phase controller 370 operates to switch external heat pump 380 off so that flow of coolant is stopped and coolant is allowed to rise to substantially ambient temperature with heat transfer from the working gas to the coolant being minimal in this mode of operation.

Like with system 200, when heat pump 380 is switched on during the charging phase there is an increase in heat rejection relative to the level of heat rejection if heat pump 380 was not switched on. This increase in heat rejection during the charging phase allows an increased pressure ratio between hot storage vessel 320 and cold storage vessel 310 and acts to lower the temperature of gas entering cold storage vessel 310 from compressor/expander 340 during charging, for boosting energy density over the complete charge/discharge cycle.

Figures 4A and 4B show an electncity 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 and second heat exchangers 450, 460 and interconnecting pipes 401, 402, 403 and 404 forming a gas circuit 405 for conveying working gas around the system.

During the discharge phase operation of system 400 is the same as operation of system 100. However, during the charging phase a controller 470 acts to direct working gas along a bypass pipe 406 connecting a further heat exchanger 480 between first and last stages of first multi-stage compressor/expander 430.

When further heat exchanger 480 is connected to gas circuit 405 dunng the charging phase there is an increase in heat rejection relative to the level of heat rejection if further heat exchanger 480 was not connected. This increase in heat rejection during the charging phase acts to lower the temperature of cold storage vessel 410 for boosting energy density over the complete charge/discharge cycle.

Figures 5A and SB show an electricity storage system 500 based on system 100 and comprising an insulated hot storage vessel 520 housing a first gas-permeable particulate heat storage structure 521, cold storage vessel 510 housing a second gas-permeable particulate heat storage structure 511, first and second multi-stage compressor/expanders 530, 540, heat exchanger 560 and interconnecting pipes 501, 502, 503 and 504 forming a gas circuit 505 for conveying working gas around the system.

During the charging phase operation of system 500 is broadly the same as operation of system 100. However, during the discharging phase second multi-stage compressor/expander 540 is configured to compress gas cooled by cold storage vessel 510 S to an elevated pressure exceeding mean peak gas pressure in the hot storage vessel 520 during the charging phase. This change in the high pressure ratio between charging and discharging effectively acts to pump heat from an upper part of cold storage vessel 510 out though heat exchanger 560 to reduce the temperature of the upper part of cold storage vessel 510 by more than the temperature in a lower part of hot storage vessel 520 is increased. hi this example, during the discharging phase second multi-stage compressor/expander 540 compresses gas to a pressure of approximately i5.i bar (i.e. 3 bar higher than the approximately 12.1 bar mean peak gas pressure in the hot storage vessel 520 during the charging phase representing an approximately 25% increase in gas pressure.

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 sccond cnginc stagc configured to act as an cxpandcr during the charging phasc to cxpand gas rcccivcd from thc first hcat store and as a comprcssor during thc discharging phase; a second heat store for transferring thermal energy to gas expanded by the expander during the charging phase; a heat rejection device for dissipating unwanted heat from a gas flow; and control means for varying the degree of thermal coupling between the gas flow and the heat rejection device or varying the level of heat rejection performance provided by the heat rejection device between a lower level during a first part of the cycle and a higher level during a second part of the cycle, with the change from the lower level to the higher level being configured to provide an increase in heat rejection during the second part of the cycle for boosting energy density over the cycle.
2. Apparatus according to claim 1, wherein the heat rejection device is configured to receive heat from at least a portion of the gas flow that is directed to the heat rejection device and the control means is configured to vary the degree of thermal coupling by varying the proportion of the gas flow that is directed to the heat rejection device relative the gas flow that bypasses the heat rejection device.
3. Apparatus according to claim 1 or claim 2, wherein at the lower level heat rejection from the gas flow via the heat rejection device is substantially zero.
4. Apparatus according to any of the preceding claims, wherein the second part of the cycle occurs substantially during the charging phase of the cycle.
5. Apparatus according to claim 4, wherein the second part of the cycle substantially corresponds to the charging phase of the cycle.
6. Apparatus according to any of the preceding claims, wherein the heat rejection device comprises heat pump means operable to remove heat from the gas.
7. Apparatus according to claim 6, wherein the heat pump means comprises means for compressing the at least a portion of gas flow and a heat exchanger for transferring heat from the at least a portion of gas flow prior to expansion of the at least a portion of gas flow.
8. Apparatus according to claim 6, wherein the heat pump means is thermally coupled to the apparatus by a heat exchanger.
9. Apparatus according to any of claims 6-8, wherein the heat pump means is operable to remove heat from the apparatus at a point between the first heat store and the second engine stage.
10. Apparatus according to any of claims 6-8. wherein the heat pump means is operable to remove heat from the apparatus at a point between the second heat store and the first engine stage.
11. Apparatus according to any of claims 1-5, wherein during the charging phase the first engine stage is configured to act as a multi-stage compressor comprising first and second compressor stages and the heat rejection device comprises a heat exchanger connectable between the first and second compressor stages to remove heat from gas compressed by the first compressor stage prior to compression of the gas by the second compressor stage.
12. Apparatus according to any of the preceding claims, wherein the apparatus further comprises at least one fixed heat exchanger with a constant degree of thermal coupling to the gas flow for dissipating unwanted heat from the gas flow.
13. A method of operating an energy storage system comprising: during a charging phase of a cycle: compressing a gas using a first engine stage; transfening heat from the compressed gas to first heat store for receiving and storing thermal energy from the compressed gas; S expanding the gas using a second engine stage after heat from the gas has been transfened to the first heat store; and transfcning hcat from a sccond heat store to the expanded gas; and during a discharging phase of the cycle: cooling a gas by transferring heat from the gas to the second heat store; compressing the gas cooled by the second heat store; heating the compressed gas by transferring heat from the first heat store to the gas; and expanding the gas heated by the first heat store to generate a power output; eharaeterised by the step of varying the degree of thermal coupling between a gas flow and a heat rejection device configured to dissipate unwanted heat from the system or varying the level of heat rejection performance provided by the heat rejection device between a lower level during a first part of the cycle and a higher level during a second part of the cycle, with the change from the lower level to the higher level being configured to provide an increase in heat rejection during the second part of the cycle for boosting energy density over the cycle.
14. A method according to claim 13, wherein the heat rejection device is configured to receive heat from at least a portion of the gas flow that is directed to the heat rejection device and the step of varying the degree of thermal coupling comprises varying the proportion of the gas flow that is directed to the heat rejection device relative to the gas flow that bypasses the heat rejection device.
15. A method according to claim 13 or claim 14, wherein at the lower level heat rejection from the gas flow via the heat rejection device is substantially zero.
16. A method according to any of claims 13-15, wherein the second part of the cycle occurs substantially during the charging phase of the cycle.
17. A method according to claim 16, wherein the second part of the cycle substantially corresponds to the charging phase of the cycle.
18. A method according to any of claims 13-17, wherein the heat rejection device comprises heat pump means operable to remove heat from the gas.
19. A method according to claim 18, wherein the heat pump means comprises means for compressing the at least a portion of gas flow and a heat exchanger for transferring heat from the at least a portion of gas flow prior to expansion of the at least a portion of gas flow.
20. A method according to claim 18, wherein the heat pump means is thermally coupled to the apparatus by a heat exchanger.
21. A method according to any of claims 18-20, wherein the heat pump means is operable to remove heat from the apparatus at a point between the first heat store and the second engine stage.
22. A method according to any of claims 18-20, wherein the heat pump means is operable to remove heat from the apparatus at a point between the second heat store and the first engine stage.
23. A method according to any of claims 13-17, wherein the step of compressing the gas during the charging phase comprises compressing the gas using fir st and second compressor stages of a multi-stage compressor and the heat rejection device comprises a heat exchanger connectable between the fir st and second compressor stages to remove heat from gas compressed by the first compressor stage prior to compression of the gas by the second compressor stage.
24. 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 S expand gas received from the first heat store and as a compressor during the discharging phase; a sccond hcat store for transferring thermal energy to gas expanded by the expander during the charging phase; and a heat rejection device for dissipating unwanted heat from a gas flow at a point between the first heat store and the second engine stage; wherein the second engine stage is operable during the discharging phase to compress gas coo'ed by the second heat store to an devated pressure exceeding mean peak gas pressure in the first heat store during the charging phase.
25. Apparatus according to claim 24, wherein the elevated pressure is at least 10% higher than the mean peak gas pressure in the first heat store during the charging phase.
26. A method according to claim 25, wherein the elevated pressure is at least 20% higher than the mean peak gas pressure in the first heat store during the charging phase.
27. A method of operating an energy storage system comprising: during a charging phase of a cycle: compressing a gas using a first engrne stage; transferring heat from the compressed gas to fir st heat store for receiving and storing thermal energy from the compressed gas; expanding the gas using a second engine stage after heat from the gas has been transferred to the first heat store; and transferring heat from a second heat store to the expanded gas; and during a discharging phase of the cycle: cooling a gas by transferring heat from the gas to the second heat store; compressing the gas cooled by the second heat store and subsequently dissipating unwanted heat from the gas flow; heating the compressed gas by transferring heat from the first heat store to the gas; and expanding the gas heated by the first heat store to generate a power output; characterised in that the step of compressing the gas cooled by the second heat store S comprises compressing the gas to an elevated pressure exceeding mean peak gas pressure in the first heat store during the charging phase.
28. A mcthod according to claim 27, wherein thc elevatcd prcssurc is at least 10% higher than the mean peak gas pressure in the first heat store during the charging phase.
29. A method according to claim 28, wherein the elevated pressure is at least 20% higher than the mean peak gas pressure in the first heat store during the charging phase.
30. 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 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; heat pump means operable to remove unwanted heat from a gas flow.
31. Apparatus according to claim 30, wherein the heat pump means comprises means for heating the at least a portion of gas flow and a heat exchanger for transferring heat from the at least a portion of gas flow prior to expansion of the at least a portion of gas flow.
32. Apparatus according to claim 30, wherein the heat pump means is thermally coupled to the apparatus by a heat exchanger.
33. Apparatus according to any of claims 30-32, wherein the heat pump means is operable to remove heat from the apparatus at a point between the first heat store and the second engine stage.
34. Apparatus according to any of claims 30-32, wherein the heat pump means is operable to remove heat from the apparatus at a point between the second heat store and the first cnginc stagc.
35. An apparatus or mcthod substantially as hcrcinbcforc dcscribcd with rcfcrcncc to thc accompanying drawings.