GB2534914A

GB2534914A - Adiabatic liquid air energy storage system

Info

Publication number: GB2534914A
Application number: GB1501937.5A
Authority: GB
Inventors: Macnaghten James
Original assignee: Isentropic Ltd
Current assignee: Isentropic Ltd
Priority date: 2015-02-05
Filing date: 2015-02-05
Publication date: 2016-08-10
Also published as: GB201501937D0

Abstract

An adiabatic, liquid air, energy storage (ALAES) system, wherein air flows, in a charging mode, successively downstream through a first compression stage 11, a TES system 40 where thermal energy is removed and stored, and a Liquid Air (LAIR) sub-system 50 that liquefies the air and stores it on charging, and re-vaporises it on discharging, whereby discharged air flows successively back through the first TES system 40 where thermal energy is returned to it and a first expansion stage 14 where it is expanded to generate power. A pre-heater 46 provided upstream of the first compression stage 11 preheats its inlet air such that the first TES system receives and stores heat of a higher temperature, thereby increasing the energy density and efficiency of the system. The ALAES system may be integrated with a gas turbine power generating system. The preheater 46 may be a heat exchanger and may receive waste heat from a heat exchanger downstream of the TES system 40 during charging.

Description

Adiabatic Liquid Air Energy Storage System

Field of the invention

The present invention relates to an adiabatic liquid air energy storage (ALAES) system and method of operating the same. Such a system may include a hybrid, adiabatic 5 liquid air energy storage, combustion turbine power generation system.

Background to the Invention

A number of energy storage technologies have been developed including compressed air energy storage (CAES), pumped hydro energy storage (PHS), pumped heat energy storage (PHES) and cryogenic energy storage (CES) including liquid air energy storage (LAES). Unlike, CAES and PHS, PHES and CES/LAES have no geographical constraints.

CAES systems utilizing thermal energy storage (TES) apparatus to store heat have been known since the 1980's. In particular, ACAES systems store the heat of compression of the compressed air in thermal stores for subsequent return to the air as it leaves a compressed air store before undergoing expansion. The TES apparatus may contain a thermal storage medium through which the compressed air passes, releasing heat to the storage medium, thereby heating the store and cooling the air. The thermal storage medium may be in the form of a porous storage mass, which may be a packed bed of solid particles through which the air passes exchanging thermal energy directly, or, it may comprise a solid matrix or monolith provided with channels or interconnecting pores extending therethrough, or, the fluid may pass through a network of heat exchange pipes that separate it from the storage mass, such as a packed bed of particles (e.g. rocks). Alternatively, the compressed air may pass through a heat exchanger that is coupled to a separate thermal store, such that heat is transferred indirectly to the latter via a heat transfer fluid, in which case the thermal store need not be pressurised and could include a thermal storage medium such as a molten salt or high temperature oil.

LAES systems utilizing thermal energy storage (TES) apparatus to store heat have also been proposed. Similarly to an ACAES, an adiabatic liquid air energy storage ALAES system would store the heat of compression of the initially compressed inlet air in thermal stores for subsequent return to pressurised air that is returning from a liquid air energy air store, prior to expansion. Usually, such a liquid air energy store will form part of a liquefaction/evaporation system that includes means of recycling the cold of evaporation (generated upon discharge) to assist in liquefaction (upon charge) and this may involve a cold regenerator, or successive power machinery and heat exchanger/storage stages for progressive cooling of the air. US6,920,759 discloses various alternative LAES systems including ones with stores where heat is stored directly (e.g. regenerators).

It will be appreciated that where the storage of sensible heat in the TES apparatus is optimised, then the overall energy storage capacity of an ALAES will also be enhanced.

Thermal energy stores based on direct thermal transfer have much higher efficiencies than ones that store heat indirectly (e.g. usually involving heat exchangers coupled to remote stores via heat transfer fluid loops). Applicant's earlier application W02012/127178 proposes direct thermal transfer TES apparatus wherein the storage media is divided up into separate respective downstream sections or layers. The flow path of the heat transfer fluid through the layers can be selectively altered using valving in the layers so as to access only certain layers at selected times, so as to avoid pressure losses through inactive sections upstream or downstream of the sections where the thermal front is located and to maximise store utilisation. TES apparatus incorporating layered storage controlled by valves (more particularly, direct transfer, sensible heat stores incorporating a solid thermal storage medium disposed in respective, downstream, individually access controlled layers) can provide very efficient storage of heat up to temperatures of 600°C or even hotter. It should be noted that the flow velocity through such a bed may be as low as 0.5 m/s or even lower, promoting efficient thermal exchange.

The use of hybrid, energy storage power generation systems in which a combustion turbine (CT) is combined with an energy storage system have also been proposed. In these systems, the combustion turbine may operate in a normal power generation mode (in which a significant proportion of the power from the turbine is used to power the compressor), or the compressor and turbine may be decoupled for use at selected times. Thus, the compressor may operate alone in an energy storage mode to supply compressed air to the energy storage system, for example, to store compressed air or liquid air using low cost electrical energy (outside peak demand times), while the combustion turbine may be used to generate power in an energy recovery mode from the (release of the) compressed air or liquid air. The time shifting of the compressor operation allows higher power production which may be useful, for example, to meet peak demand.

Such hybrid systems may incorporate heat stores/regenerators and/or heat exchangers linked to stores to store thermal energy during the pressurising of the air in the energy storage mode, such that the compressed air or liquid air released in the energy recovery mode may be reheated and be returned to the combustion turbine at a suitable temperature (e.g. similar to the outlet temperature of the compressor during normal CT operation).

US6920759 proposes such a hybrid, liquid air energy storage, gas turbine electric power system and discloses the three operating modes referenced above, or a combination of those modes, as well as proposing combinations of plants such as a plurality of gas turbines combined with one liquefaction facility and one liquid air storage tank. The use of a heat recovery steam generator downstream of the gas turbine is also proposed to extract more energy from the combusted gas exhaust.

The present invention is directed towards providing an improved adiabatic liquid air energy storage system.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided an adiabatic, liquid air, energy storage (ALAES) system in which air from an inlet is directed, in a charging mode, successively downstream along a flow pathway comprising: a first compression stage where the air is compressed, a first thermal energy storage (TES) system where thermal energy is removed 15 from the air passing through it and stored; a LAIR sub-system that liquefies the air and stores it on charging, and re-vaporises it on discharging, respectively; wherein air from the LAIR sub-system is directed, in the discharging mode, successively downstream along a flow pathway comprising: the first TES system where thermal energy is returned to the air passing through it; and, a first expansion stage where the air is expanded to generate power; characterised in that a pre-heater system is provided upstream of the first compression stage with respect to the charging mode, and is configured in the charging 25 mode to preheat air entering the first compression stage so as to increase the temperature of air entering the first TES system.

Use of a pm-heater system to add heat at this upstream point during charging (e.g. by a substantially isobaric heat transfer), allows more heat to be stored in the first TES system, this being the first thermal store appearing downstream of that compressor (there may be subsequent downstream stores) without a commensurate rise in pressure (the pressure ratio and hence peak pressure can remain unchanged), which would add to TES system cost.

Heat addition to storage systems is counter-intuitive in that such systems normally require expensive heat exchangers in order to avoid a build-up of unwanted waste heat. 35 However, since heat added and stored in this way in the first TES system is discharged through the gas turbine during the discharge generation mode, it does not create a problem of waste heat build-up. Moreover, heating inlet air prior to a compressor per se is counter-intuitive, since it is well-known that the power output of a gas turbine GT falls as the air inlet temperature rises, so that the usual practice e.g. in hot countries is to cool intake air prior to compression.

In this way, the energy density and efficiency of the system may be improved. The reason for the improvement in efficiency is that the amount of work carried out per unit mass of gas (J/g) processed by the compressor increases (as it requires more work to compress a hotter gas over a certain pressure ratio), which means that the losses associated with processing a certain mass of gas actually fall. Furthermore, the amount of heat in the first TES system (and the amount of thermal storage media contained in the first TES system) is related to the mass of gas processed and the increased work translates to a higher energy density in the thermal stores and thus the potential for a smaller store.

The term adiabatic liquid air energy storage system or ALAES system is intended to cover any liquid air energy storage system in which at least part of the heat of compression (used to produce the liquid air) is stored in a thermal energy storage or TES system, for subsequent return to the air before it is expanded to generate power.

Other components performing minor functions may also be provided along the flow 20 pathway, such as for example, filtration devices, and devices for removing moisture/droplets or CO2. Additional TES systems may also be provided between the first TES system and the LAIR sub-system, or indeed form part of the latter.

The LAIR (liquid air) sub-system will have air liquefaction, storage, and evaporation functionality such as is known in the art. Any suitable downstream arrangement that allows liquid air to be stored in a liquid air store may be used, since the invention is directed towards an improvement to the upstream end of the overall system (i.e. concerning the first TES system downstream of the first compression stage). For example, the LAIR (liquid air) sub-system may comprise a combined facility with a liquid air store, or a liquefaction and evaporation facility leading to a separate, downstream liquid air store. Usually, an expansion valve or throttle valve will be used to expand the air into a liquid air store, and a liquid pump will be used to draw the liquid air back out of the liquid air store and re-compress it to high pressure prior to re-vaporisation. The liquid air store will usually store the air at a selected low e.g. near ambient pressure. There may be other TES systems or heating/cooling stages with associated heat transfer/storage located between the first TES system and the liquid air store.

The ALAES system may be configured during the discharging mode to deliver air from the first TES system to the first expansion stage at an expansion stage inlet temperature in Kelvin within 20% of the temperature of the air exiting the (hot end of the) first TES system (e.g. within 10% or within 5%). In one embodiment, the expansion stage inlet temperature may be equal to or lower than the temperature of the air exiting the first TES system. Thus, use of the pre-heater system may obviate the need for an ancillary heating stage to raise the temperature of air exiting the first TES system before it enters the first expansion stage.

The pre-heater system may be configured to supply thermal energy derived from waste heat (e.g. low grade heat) to the air. For example, if an industrial compressor with a pressure ratio of 17-18 is used then the inlet temperature to the TES is likely to be around 420°C. If pre-heated, for example, to say 100°C before compression then the inlet temperature to the TES will rise to 640°C. This waste heat may be waste heat available in real time or that has been stored and may originate either from the ALAES system (e.g. expander exhaust), or associated systems (e.g. downstream), or other separate equipment co-located on-site.

The pre-heater system operates to preheat the air before it enters the first compression stage and such pre-heating should preferably raise the air temperature by not more than 250°C, more preferably, by not more than 200°C or even not more than 100°C; however, advantages may be secured with only a small temperature rise, for example, of at least 20°C, or at least 40°C.

The heat addition may conveniently be by means of a heat exchanger, usually a counter-current heat exchanger, so that the pre-heater system may comprise at least one heat exchanger, hereinafter referred to as the "upstream heat exchanger", that is provided 25 upstream of the first compression stage with respect to the charging mode.

The upstream heat exchanger may be coupled, in the charging mode, so as to receive heat (in real time) from at least one further heat exchanger that is located downstream of the first TES system, hereinafter referred to as the "downstream heat exchanger', or that is located downstream of a further downstream TES system (that is, one that is more downstream than the first TES system, for example, located after second stage power machinery), with respect to the charging mode.

The upstream heat exchanger and downstream heat exchanger may transfer heat directly between them if configured so as to form a counter-current heat exchanger.

To achieve preheating with heat exchangers so linked across the gas flow pathway 35 (with the correct thermal gradient across them), it will be appreciated that gas circulating downstream of the first TES system, or a further downstream TES system, must be sufficiently hotter than that circulating upstream of the first compressor. A TES will usually be operated such that a thermal front is retained within, and moves backwards and forwards within the store with storage medium on the hot and cold sides of the thermal front respectively held at approximately the last hot end gas inlet temperature on charging the store (from the hot end) and the last cold end gas inlet temperature upon discharging the store (from the cold end). The latter temperature will therefore be the temperature exhibited by the gas exiting the first TES system during charging i.e. the last "minimum store temperature" of the first TES, which may or may not correspond to the very initial uncharged (e.g. ambient) temperature) and will normally be higher than ambient. Usually, once up and running, the store will operate between a maximum store temperature and minimum store temperature, with the thermal front confined to run between the two store ends, with preferably only a small part of the thermal front leaving the store during charge and discharge.

It will be appreciated that any TES system in the ALAES system may be coupled with a heat exchanger immediately downstream of it (in the charging direction), if that location allows that "downstream heat exchanger" access to sufficiently high quality heat that it can redirect back to the pre-heater system in the charging mode. Hence, in the context of the functionality of the downstream heat exchanger, this may be downstream of the first TES system (the most likely scenario) or a further downstream TES system.

Where the upstream and downstream heat exchangers are so coupled, then, in the charging mode, the downstream heat exchanger may be configured to receive heat that has been selectively stored in the first TES system, or further downstream TES system, during the previous discharge (generation) mode by selective operation of the downstream heat exchanger in that mode.

In that regard, during the previous discharge generation mode, the cold end air inlet temperature to the first TES system, or further downstream TES system, may be selectively raised by supplying at least some heat to the downstream heat exchanger from a heat source in the ALAES system or an external source.

Alternatively, during the previous discharge generation mode, the cold end air inlet temperature to the first TES system, or further downstream TES system, may be selectively raised by selecting the degree to which the downstream heat exchanger discards heat.

It will be appreciated that in the above embodiments, waste heat or low quality heat 35 that is (only) generated during a (previous) discharge mode is being stored in that mode in the first TES system, or further suitable downstream TES system (i.e. time shifted), so that it can be redirected in the next charge mode to the pre-heater system (via the heat exchanger couplings) and stored during that mode in the first TES system as high quality heat, so that advantageously on the next discharge mode the pressurised air with that additional high quality heat can pass through to the expander, or combustor of a gas turbine in the case of a hybrid system i.e.at a raised temperature. The high quality heat is being stored during charging in the (hot end of the) first TES system downstream of the compressor as an augmented maximum store temperature (due to the raised hot end inlet temperature), while the waste heat is being stored in (the cold end of) the same TES system as an augmented minimum store temperature (due to the raised cold end inlet temperature). As previously explained, the waste heat could alternatively be stored in another TES system further downstream, providing that the store in question is immediately upstream, upon charging, of the linked further heat exchanger which is collecting and redirecting that waste heat to the pre-heater system in the charge mode.

In one embodiment, the system comprises a heat exchanger, hereinafter referred to as the exhaust heat exchanger, that is provided downstream of the first expansion stage, with respect to the discharging mode, and that is configured in the discharging mode to retrieve waste heat from the exhaust air of the expander.

In a preferred arrangement, the exhaust heat exchanger and the upstream heat exchanger are the same heat exchanger, such that it is located upstream of the first compression stage with respect to the charging mode and in that mode is operational to preheat the inlet air to the compressor, and it is also located downstream of the first expansion stage with respect to the discharging mode and in that mode is operational to cool the outlet air from the expander. This heat exchanger is therefore multi-functional, acting as both the exhaust heat exchanger and the upstream heat exchanger, thereby reducing overall system cost.

The multi-functional heat exchanger will usually be a counter-current flow heat exchanger. Valve arrangements may provide selective connection of flow passageways (e.g. two) in the multi-functional heat exchanger to the first compressor stage and first expansion stage in the respective modes, and usually, to a heat exchanger elsewhere in the system, usually the downstream heat exchanger so that the multi-functional heat exchanger can provide heat via a HTF during charging (upstream of the first compressor stage) and remove heat via the HTF during discharging (downstream of the first expansion stage). Thus, the inlet air and exhaust air may flow alternately through one passageway, in reverse directions, respectively, while the HTF fluid flows alternately in reversing directions through the other passageway.

The exhaust heat exchanger may be coupled (i.e. fluidly coupled or connected by flow passageways) to the downstream heat exchanger, so as to supply it with waste heat for selective storage in the first TES system in the discharge mode.

The first thermal energy storage (TES) system may comprise at least one direct thermal energy store through which the compressed air has a flow path for direct exchange of thermal energy to a thermal storage medium contained within the thermal energy store; this may be a porous thermal mass in the form of, for example, a packed bed or particulate, especially a layered particulate store.

The ALAES system may be integrated with a gas turbine power generating system to form a hybrid system such that the compressor and expander of the gas turbine form the first compression stage and the first expansion stage, respectively, and can be decoupled from each other to permit their separate operation.

The compressor and expander may be mechanically or electrically or by some other means detachably coupled together; usually they will be mechanically coupled to a common power shaft by respective clutches. The hybrid system may be operable in a normal gas turbine, power generation mode where the first compressor, combustor and gas turbine are fluidly connected downstream of one another such that inlet air passes successively downstream through them to produce power, and the compressor is driven by and coupled to the turbine.

The hybrid system may be operable in a power generation from storage mode in which air passes down a flow pathway from the LAIR storage sub-system to the at least first TES and to the combustor of the gas turbine to produce power, by the selective use of flow connectors and with the first compressor inoperative and decoupled from the turbine.

The hybrid system may be operable in an energy storage mode in which inlet air passes down a flow pathway from the first compressor, to the at least first TES and to the LAIR storage sub-system where it is stored, by the selective use of flow connectors and with the first expander inoperative and decoupled from the first compressor.

The gas turbine power generating system may comprise a combined cycle gas 30 turbine CCGT system, wherein further energy is extracted from the gas turbine exhaust gas in a downstream steam turbine system (i.e. operating a steam bottoming cycle). In accordance with a further aspect, the present invention provides a hybrid, liquid air, energy storage, gas-turbine, electric power generating system comprising: a first compressor for compressing air, a first thermal energy storage TES system for storing thermal energy from the compressed air in a charging mode, and returning thermal energy to pressurised air in a discharging mode, a LAIR storage sub-system (e.g. liquefier/store/evaporator functionality) for liquefying and storing the compressed air as liquid air on charging, and 5 returning/vaporising the air to produce pressurised air on discharging, a combustor for burning fuel with the pressurised air from the thermal energy store to produce a combustion gas; a gas turbine driven by utilizing the combustion gas; and, a generator driven by said gas turbine for generating electric power; wherein a pre-heater system is provided upstream of the first compressor to preheat air entering the compressor, so as to increase the temperature of the air entering the first TES system.

The hybrid system may have any features or combination of features described above.

Use in this hybrid system of a pre-heater system to add heat at this upstream point during charging (e.g. by a substantially isobaric heat transfer), allows more heat to be stored in the first TES system, this being the first thermal store appearing downstream of the compressor (there may be subsequent downstream stores) without a commensurate rise in pressure (the pressure ratio and hence peak pressure can remain unchanged), which would add to TES system cost. The maximum power produced during the discharge-generation mode will remain unchanged if the pressure and the peak combustion temperature do not change. In this way, the energy density and efficiency of the hybrid system may be improved and in the discharge generation mode, the system is then able to provide a higher temperature pressurised gas to the combustor such that less fuel needs to be supplied to the combustor (to achieve the same expansion turbine power output). The heat addition may conveniently be by means of a heat exchanger and, because the additional heat stored in the first TES system is discharged through the gas turbine during the discharge generation mode, it does not create a problem of waste heat build-up.

There is further provided a method of operating an adiabatic, liquid air, energy storage (ALAES) system as described above, the method comprising: during a charging mode: preheating air entering the first compression stage using the pre-heater system; compressing the preheated air using the first compression stage; passing the compressed air through the first TES system so as to transfer and store thermal energy from the air in the store; and liquefying and storing the air as liquid air in the LAIR sub-system; and, during a discharging mode: revaporising the liquid air in the LAIR sub-system; passing the air back through the first TES system to retrieve the stored thermal energy; and, expanding the air heated by the first TES system using the first expansion stage.

In such a method, the system may be as specified above, and may operate as so specified.

In particular, in such a method the pre-heater system may comprise at least one upstream heat exchanger provided upstream of the first compression stage, with respect to the charging mode, which is coupled to and receives heat in the charging mode from a downstream heat exchanger that is located downstream of the first TES system, or of a further downstream TES system, with respect to the charging mode, and wherein, in the previous discharging mode, the downstream heat exchanger is selectively operated so as to store heat in the first TES system, or of a further downstream TES system by selectively raising the cold end inlet temperature to that TES system.

Selective operation of the downstream heat exchanger may include inoperation (inactivity), partial operation, or bypassing of the downstream heat exchanger in that previous discharging mode, all of which may be employed to ensure that the waste or low grade heat (e.g. resulting from irreversibilities) that is conveyed in the air returning from the LAIR sub-system is carried into the TES system in question as a raised cold end inlet temperature; thus, if the downstream heat exchanger becomes operational again in the charging mode, that waste heat is usefully transferred to the preheater system via the fluid coupling with the upstream heat exchanger.

Alternatively, in the discharging mode, the cold end inlet temperature to that TES system may be raised by the downstream heat exchanger receiving heat generated during that mode from either an external heat source, or, a heat source located in a different part of the ALAES system, for example, via a further fluid coupling to a heat exchanger in that location.

Brief Description of the Figures

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of an adiabatic liquid air energy storage (ALAES) 35 system of the prior art; Figure 2 is a schematic diagram of a hybrid, adiabatic liquid air energy storage (ALAES), gas turbine power generation system of the prior art; Figure 3 is a T-S diagram showing the changes of state of liquid air during operation of the prior art hybrid ALAES system of Figure 2; Figure 4 is a schematic diagram of a hybrid, adiabatic liquid air energy storage (ALAES), gas turbine power generation system according to a first embodiment of the present invention; Figure 5 is a T-S diagram showing the changes of state of liquid air during operation of the hybrid ALAES system of Figure 4; Figures 6a-6c depict an ALAES system in accordance with a second embodiment of the present invention operating in different modes; Figures 6d and 6e depict an ALAES system in accordance with a third embodiment of the present invention, operating in discharging and charging modes, respectively; Figures 6f and 6g depict an ALAES system in accordance with a fourth 15 embodiment of the present invention, operating in charging and discharging modes, respectively; Figure 7 depicts an ALAES system in accordance with a fifth embodiment of the present invention operating in charging mode; Figures 8a and 8b show expected thermal profiles within a thermal store during 20 charging and discharging modes respectively to illustrate how waste heat may be obtained from condensation of atmospheric air; and, Figures 9a-9c show the temperature profile through a simple packed bed thermal store at different states of charge as waste heat is added during the discharging mode. Figure 1 shows a conventional prior art, adiabatic liquid air energy storage (ALAES) 25 system 10. During a storage mode, ambient inlet air passes into a compressor 11 where it is compressed (adiabatically) before being cooled isobarically through a thermal energy storage system 40 such as a solid regenerator, which stores the heat of compression. The cooled pressurised air is then expanded through, for example, a throttle valve 18 to form liquid air which is stored in a cryogenic (e.g. insulated) tank 20.

In a discharge mode, the liquid air is pumped from the tank 20 by a liquid pump 22 up to a higher pressure before the liquid air is passed through, for example, the same thermal energy storage system 40, where it is isobarically reheated and vaporised, before being expanded through a turbine 14 to produce work. The throttle valve 18, liquid pump 22 and liquid air store 20 form part of a LAIR (liquid air) sub-system 50, hereinafter referred to in the examples as a liquefaction/storage/evaporation (LSE) facility 50, and that facility may also include additional power machinery and thermal transfer stages in order to cool/reheat the air as required, which could include further regenerators or thermal stores. Figure 2 shows a prior art, hybrid system 110 in which an ALAES system is incorporated into a gas turbine power production plant (i.e. a GTI-ALAES). The ALAES 5 system is the same as that of Figure 1 except that the compressor 11 and turbine 14 form part of a gas turbine which also includes a combustor 24 fluidly connected inbetween the compressor 11 and turbine 14 by valving (not shown) for selective connection. The compressor 11 and turbine 14 are respectively detachably mechanically coupled, by virtue of clutches 28, to a shaft leading to a motor generator 15.

The gas turbine 11/24/14 may operate in a normal gas turbine power production mode in which compressed air from compressor 11 passes directly into combustor 24 where fuel is added and combustion occurs to raise the gas temperature usually to about 1400°C before the gas passes into the turbine and is expanded to generate electricity via the motor/generator 15. Alternatively, the hybrid system may operate in a storage mode where the turbine 14 is mechanically decoupled and valving (not shown) is switched to send compressed air from the compressor 11 towards the thermal energy storage system 40 for storage of at least some heat of compression and thence to LSE facility 50 including tank 20 for storage. In a discharge/recovery mode, the compressor 11 and turbine 14 are, respectively, mechanically decoupled and coupled to the shaft by the clutches, and valving (not shown) is switched to send pressurised air which has been re-vaporised in the LSE facility 50 towards the combustor 24, and thence to the turbine 14 for power generation. Ideally, the pressurised air should be supplied to the expander 14 from the storage pathway at a similar temperature and pressure as it would have arrived along the normal gas turbine pathway. An example of such a hybrid system is disclosed in US6,920,759, where other combinations of operating modes are also described.

Figure 3 is a T-S diagram showing, by way of example, the changes of state of air during the storage and recovery modes of the LAES process as described in US6,920,759 for the Figure 2 system.

The bold dashes delineate the saturation curve/dome, below which mixed liquid phase and vapour phases of air exist. The top of the dome is the critical point, with the right hand side of the dome being the saturated vapour line with a superheated vapour region to the right thereof, while the left hand side of the dome is the saturated liquid line with a compressed liquid region to the left thereof. On the T-S diagram, the thinner dotted lines passing left to right through the dome and ascending sharply to the right represent isobars of equal pressure.

During the storage (air liquefaction) mode, ambient inlet air at 1 bar at point a is compressed by compressor 11 nearly isentropically up to 10 bar at point b at about 300°C, and then cooled through a high (+medium) temperature heat store/exchanger facility isobarically along the 10 bar isobar down to point c. It is then isentropically compressed up 5 to 40 bar at point d, before passing through and cooling isobarically again, this time through a low temperature, i.e. sub-ambient, heat store/exchanger facility isobarically across the top of the saturation dome to the point e. Next, in the expansion process of the expansion valve the air changes along the line from point e, through point f to lower temperature point g, where it is now a mixture of gas and liquid air. The very cold, gas 10 phase air at point i and at atmospheric pressure then increases its temperature back up to ambient along line i to a, as it is recycled through the respective low and (medium +) high temperature heat exchange facilities, so as to provide a source of recycled cold. The liquid air at point h is stored in the liquid air storage tank 20.

During the power recovery (vaporising) mode, the liquid air at point h is drawn out of the tank 20 by liquid pump 22 which isothermally increases its pressure up to about 200 bar to point j. Then the highly pressurised liquid air passes through the low temperature, heat store/exchanger facility where it nearly isobarically increases its temperature and vaporises to gaseous air to reach point k. The vaporised air is then isentropically expanded in an expansion turbine, reducing its temperature and pressure from point k to point c at 10 bar, before being reheated through the high (+medium) temperature heat store/exchanger facility isobarically to point b again. The air is then supplied at this temperature and pressure to the combustor 14 of the combustion turbine.

The simplified systems described in Figures 1 to 3 are merely illustrative of adiabatic liquid air energy storage systems and the present invention relates to any LAES system that includes a TES system for storage of the heat of compression, including any suitable liquefaction/storage/evaporating facility that results in storage of the air as liquid air.

Figure 4 is a schematic diagram of an ALAES system 210 according to a first embodiment of the present invention. System 210 is again a liquid air energy storage system integrated into a gas turbine power generation plant, but differs from that of Figure 2 in that it comprises a pre-heater system 6 upstream of the compressor 11 that is arranged to provide heating to the inlet air to the compressor 11 during charging. The pre-heater sub-system 6 may comprise an electrical heater or a heat exchanger coupled to a heat source or any other suitable heater.

The pre-heater system 6 is used during charging to add heat at this upstream point and effectively allows more heat to be stored in the first TES system 40 without a commensurate rise in pressure, which would add to TES system cost and hence overall ALAES system cost. (In an ALAES, maximum store operating pressure is a key factor influencing costs). Thus, upon discharge, gas will leave the TES system and arrive at the combustor at a higher temperature than otherwise, which, in the case of a hybrid, combined ALAES/gas turbine plant, will result in less fuel needing to be added to the air prior to combustion. This method of increasing energy density is counter-intuitive, given that the customary practice is to cool the intake air to a gas turbine to augment power output.

Figure 5 is a T-S diagram for the system of Figure 4 and shows the effect of the additional of a pre-heater system. Thus, inlet air arrives at the compressor 11 at one bar but at a higher inlet temperature, such that compression of the gas with the same compression ratio heats the gas up to a higher maximum temperature b', leading to a larger area under the dotted lines corresponding to a higher amount of energy stored.

Figures 6a-c depict an ALAES system in accordance with a second embodiment of the present invention provided with pre-heating prior to the first compressor 11.

Figure 6a shows the simple ALAES system 310 suitable for peaking power generation, with an upstream first compressor (e.g. turbine compressor) 11, a downstream expander (e.g. turbine expander) 14, selective connection to a motor/generator 15 (e.g. connected to a transformer/grid), a gas flow selector valve arrangement 31 including multiple valves disposed between the power machinery 11/14 and a first thermal energy storage TES system 40, and a combined liquefaction/storage/evaporation LSE facility 50 located downstream of the TES system 40. The flow selector valve arrangement 31 directs air flow from the compressor 11 to the system 40 upon charging, or, directs air flow from the system 40 to the expander, upon discharging, In a normal configuration, either the compressor 11 during charging, or, the expander 14 during discharging is directly coupled to the motor/generator 15 on the same shaft by drive couplings (not shown) such that, for example, power to drive the compressor is normally supplied by the motor/generator drawing power from an electricity grid.

According to the invention, a heat exchanger 46 forming part of a pre-heater system is located upstream of the first compressor 11. Heat exchanger 46 is coupled by heat transfer fluid HTF pipework to an additional heat exchanger 45 located immediately downstream of the TES system 40, such that heat transfer fluid in the pipework may transfer heat from (downstream) heat exchanger 45 to (upstream) heat exchanger 46 during a charging mode.

As described further below, the system may operate in a charge only mode, as shown in Figure 6a, in which the expander 14 is declutched from the motor/generator 15, which then acts as a motor to drive the first compressor 11, with all the compressor outlet flow entering the thermal store before going to storage in LSE facility 50. This mode uses only electrical energy (e.g. from a local grid) for storage. It may also operate in a discharge only mode, as shown in Figures 6b and 6c, where the liquid air returns from facility 50, via store 41, before passing out through the expander 14 which is now coupled to the generator 15.

Air (which may be filtered) enters the heat exchanger 46 at ambient conditions (e.g. 10 15°C, 1 bar) and is heated up to 70°C (a normal range might be to add between 10 and 130°C of heat depending upon the required compression ratio). It is then compressed up to a higher pressure, say 17 bar and temperature of 530°C, before being directed by valving 31 to TES system 40. It is preferable that the pressure is at least 10 bar to ensure that there is a sufficient work input to the system and that the temperature is high enough to maximise the amount of energy stored in the thermal store.

The first thermal energy storage TES system 40 comprises a simple TES store 41. The store comprises a thermally insulated vessel 42 and thermal storage media 43 which may be any suitable TES apparatus with direct thermal transfer, as mentioned above. Thermal media 43 may comprise a packed bed of suitable thermal media such as high temperature concrete, ceramic components, refractory materials, natural minerals (crushed rock) or other suitable material. Thermally insulated vessel 42 should be designed so that the high pressure flow (usually at between 10 and 30 bar and between 400-600°C) can pass, preferably at a suitably slow flow rate for efficient transfer, through the vessel transferring heat directly to/from the thermal media 43. As the media 43 is in the form of a packed bed with direct heat exchange to compressed gas, the thermally insulated vessel 42 will need to be an insulated pressure vessel.

An example of a thermal store that may be especially suitable for removing/returning thermal energy directly at high temperatures of at least 500-600°C, and pressures up to 30 bar, is the solid fill thermal store described in detail in Applicant's published application W02012/127178. As described above, the valved, layered store has functionality allowing it to store thermal energy in a controllable manner.

As shown in Figure 6a, during a charging mode, the first compressor 11 compresses atmospheric air to a higher pressure and temperature. Flow selector valve arrangement 31 diverts the hot high pressure gas to the top of the thermal store 41 and the 35 gas passes through the thermal media 43 cooling as it progresses. The cooled high pressure gas leaves the first thermal store 41 where it is still above ambient temperature. It is then further cooled in a heat exchanger 45 downstream of the thermal store so that the temperature is close to ambient temperature and heat is transferred to a heat transfer fluid HTF which is coupled to the other heat exchanger 46. In this way the heat is transferred via heat exchanger 46 to the incoming air prior to the first compressor 11. Expander 14 is not in operation during the charging mode.

During a discharging mode, as shown in Figs 6b and 6c, air returns from the LSE facility 50 via pipe 33 and enters thermal store 41, where the high pressure gas passes through the storage media and the temperature rises to close to that of the gas temperature that originally entered the store during charging. The gas is then diverted by selector valve arrangement 31 to expander 14 which expands the hot high pressure gas back to atmospheric pressure. Depending upon the efficiency of the machinery and the thermal stores this gas may be hotter than ambient at this stage. The expander 14 will generate power in this mode via motor/generator 15. First compressor 11 is not in operation during the discharging mode.

There are a number of reasons why the temperature of the gas leaving the first thermal store 41 during charging mode may be above ambient (or the original baseline store temperature).This is discussed in more detail in relation to Figures 8a-b and 9a-c below.

The first reason is that thermal losses from the store will tend to manifest themselves by a hotter gas exiting the store from the cold end than the gas that went into the cold end (in the same way that the gas exiting the hot end will be slightly cooler than the gas that originally entered the hot end).

The second is that depending upon the pressure and temperature, moisture will start condensing out at about 80°C. The heat of condensation for water is very high relative to sensible heat values of air and this heat of condensation will tend to add a large quantity of low grade heat to the store. As with other low grade heat generated from thermal losses, this will normally be deliberately rejected from the system as it cannot be usefully recovered on discharge.

The third is that heat may have been previously selectively stored in the thermal store as described in the examples of Figures 6b and 6c below. For Figure 6b, the LSE process is likely to have certain irreversible losses which mean that the system may well return air at a higher temperature back to the thermal store than it was received from the thermal store. In Figure 6c, heat is actively supplied in a previous mode.

Referring to Figure 6b, this shows the discharging mode or process, which is the reverse of the charging process. The high pressure air returns via pipe 33 passing back through the first thermal store 41 to receive its stored heat. There is no need for circulation of HTF between heat exchanger 45 and 46 during this process. In Figure 6b, one set-up is to configure the heat exchanger 45 located downstream of the first TES so that it is 5 bypassed or inoperative ("selective operation" of this heat exchanger here encompasses both of those concepts) during the discharge/generation mode, and hence, so that all the (low grade) waste heat from the LSE 50 becomes stored (at a higher "minimum store temperature') in the first TES system. In the subsequent charging mode (Fig. 6a), the heat exchanger 45 downstream of the first TES system is then operative to transfer that heat (in 10 effect, waste heat that was temporarily stored in the first TES), for example, via a HTF circuit, to the upstream heat exchanger 46.

Figure 6c shows the system of Figure 6a on discharging where downstream heat exchanger 45 is used to selectively increase the "cold end" air inlet temperature to the (cold end of the) first TES system by supplying at least some heat to that heat exchanger from an external source; this may therefore allow injection of higher grade heat, e.g. higher grade waste heat from downstream or associated systems operating concurrently in the discharge generation mode.

Figures 6d and 6e depict an ALAES system in accordance with a third embodiment of the invention. This is a modified version of the system of Figure 6a in which a yet further heat exchanger 48 is added after the expander 14 and is used during discharging (Figure 6d) to selectively increase the cold end air inlet temperature to the first TES system (so that the store has an augmented "minimum store temperature") by supplying at least some heat to the heat exchanger 45 which is located between the first TES system 40 and LSE facility 50. Due to machinery losses the temperature of gas leaving the expander 14 should be hotter than that of the gas post heat exchanger 46 i.e. before it enters first compressor 11. As shown in Figure 6e, in the charging mode, heat transfer fluid in the pipework transfers heat from heat exchanger 45 to heat exchanger 46 during the charging mode so that the inlet air to compressor 11 is heated by heat exchanger 46 to a higher temperature using the higher grade waste heat that was selectively stored (as an augmented "minimum store temperature") during the previous discharge cycle.

It will be noted that in the embodiment of Figures 6d and 6e, heat exchangers 46 and 48 operate at different times. In the embodiment of Figures 6f and 6g, the functionality of those exchangers is combined in a single exchanger.

Figures 6f and 6g show a modified version of the system of Figure 6a in which first 35 compressor 11 and expander 14 are configured such that heat exchanger 46 is connectable upstream of first compressor 11 during a charging mode (Figure 69 and then subsequently connectable downstream of expander 14 during a discharging mode (Figure 6g). Thus, conveniently, the same heat exchanger may act as the at least one heat exchanger active in the charging mode upstream of the first compression stage and active 5 in the discharging mode downstream of the first expansion stage, so as to minimise systems costs. Flow connections associated with the at least one heat exchanger may direct air flow as required through it in the respective charging and discharging modes, the direction of flow through the exchanger 46 usually being reversed between the two modes. Again, this arrangement allows heat exiting expander 14 to be used to raise the cold end 10 air inlet temperature to the first TES system such that once discharged the store rests at an augmented "minimum store temperature".

Figure 7 depicts an ALAES system in accordance with a fifth embodiment of the invention. This embodiment merely shows that it is possible for the upstream heat exchanger 46 to be coupled to any other heat exchanger in the system (via a heat transfer fluid HTF loop) that is operational during charging and that can supply heat down a thermal gradient. In this case, the embodiment involves second stage power machinery e.g. reversible compressor 70 (ie one that can operate as an expander on discharge) followed immediately downstream by a second stage thermal energy store 580 and heat exchanger 47 downstream of that store. Again, TES system 580 may comprise a simple TES store 581 comprising a thermally insulated vessel 582 and thermal storage media 583.

POWER CALCULATIONS

By way of example only, typical figures for a large ALAES plant are used to quantify the effect with and without pre-heating, in Table 1 below:-Ambient 288K Inlet preheat to 343K Inlet preheat to 363K Temperature after 1st stage K 702 836 885 Mass Flow Air kg/s 430 430 430 1st stage pressure bar 16.6 16.6 16.6 Charging 1st stage MW 179 213 226 Discharging 1st stage MW 156 186 197 Thermal Energy Density 100% 133% 144%

Table 1

The table shows an ALAES system with a mass flow of 430 kg/s and a first stage pressure of 16.6 bar would require 179MW to charge the first TES store (note this includes thermodynamic losses, but excludes mechanical and electrical losses) at an ambient temperature of 288K. It would discharge 156MW of power. Using a pre-heater system that 5 results in pre-heating of the compressor inlet to 343K would increase the power required per kg by 19% and the thermal energy density of the system by 33%. Pre-heating to 363K would increase the power figure further to 26% per kg processed and increase the energy density by 44%. The reason that the increase in power is different to the increase in thermal energy stored is that the thermal energy stored is related to the total thermal 10 energy stored and while the work per unit air processed increases, so does the base inlet temperature, which is an extra addition of thermal energy to the air. It can also be seen that preheating the inlet air by 75°C raises the temperature post compression by 183°C (i.e. from 429°C up to 612°C).

The use of a pre-heater system is of more benefit where the ALAES system is able to use power machinery with lower pressure ratios, for example, where the air is compressed by no more than 30 times (its original pressure), or by no more than 25 times or by no more than 20 times (i.e. a compressor where gas inlet pressure: gas outlet pressure ratio is less than 1:30 (e.g. 1:29) or less than 1:25, or less than 1:20), since more heat can be added pre-compression without exceeding the maximum operating temperature of the compressor or downstream thermal store.

Figure 8a explains how waste heat may be obtained from condensation of atmospheric air. In the ALAES system described atmospheric air is drawn into the system, preheated, and compressed to a higher temperature and pressure. This air is then cooled in a thermal store that might be a direct thermal store with a structure similar to a packed bed. The compressed air will have a certain amount of moisture contained within it and much of this will condense out in the thermal store as the air cools. This condensation process generates a significant amount of heat and can have an effect on the shape of the thermal front.

At 20°C and 1 bar, 1kg of air can absorb 14.8 grams of moisture to become fully saturated. At 15 bar and 20°C the same 1kg of air can only absorb 1g of moisture. This means that theoretically if 1kg of saturated air at 20°C is compressed from 1 bar to 15bar and then cooled back to 20°C, then 13.8 grams of water will condense out during this process. It should be understood that while the amount of water involved is relatively small compared to the amount of air, the latent heat of condensation is very high and, consequently, it can have a noticeable effect. For an ALAES system where the thermal store is operating at a pressure of 15 bar, most of the condensation will occur in the 4070°C temperature range. The enthalpy of condensation of 1 kg of water at is approximately 2260 kJ/kg of water. This is sufficient heat to raise 2260kg of dry air by approximately 1°C. The effect of this condensation upon the thermal front is shown in Figure Ba. While the store starts with an original exit temperature of Tc1 (30°C), as condensation starts to occur (on the charge cycle) it will have the effect of raising the exit temperature from the store at first charge to around 40-50°C dependent upon moisture content of the air. This moisture effect will be significant in hot humid climates, but only have a minimal effect in cold dry climates. The air leaving the store is fully saturated and it will require further cooling to cool the air and condense additional water out of the air. Some of this cooling can be provided by virtue of a downstream heat exchanger linked in real time to the pre-heater system such that, during charging, that heat is transferred to the air inlet stream to the compressor, thus advantageously providing real time pre-heating to the inlet air to the compressor.

Figure 8b shows the thermal profile within the thermal store on discharge if no additional heat were to be added. As the air is coming back from a liquid air store, it is likely to be very dry. As a result the end discharge profile looks very similar to the original initial charge profile.

Figure 9a shows the temperature profile through a simple packed bed thermal store at different states of charge, showing how the profile of the 'thermal front' or thermocline region changes with charge for a First Charge Cycle. The packed bed media could be crushed rock or some other low cost material thermal store that is being charged Gas leaves the store at its original uncharged temperature r. When charging has progressed, the temperature of the gas leaving the store starts to rise from the baseline Tcl as the front of the thermal front starts to move out of the store. This figure ignores the effect of condensation that was explained in Figure 8a.

Usually, it is desirable to stop at a selected small temperature rise, so that most of the thermal front is kept within the store (for reasons of efficiency). However, the exit temperature towards the end of charge (line 4) may be allowed to rise further, to for example a selected temperature of 80°C so as to condition the store, if desired, to match (as discussed below) a subsequent cold end inlet temperature that is being raised by the addition of waste heat.

Upon normal discharging, the thermal front would progress back through the store with a similar profile to Figure 9a but in reverse. At the outlet from the store, gas leaves at 35 close to Th4 (500°C) until towards the end of the discharging when the temperature at the outlet will start to fall. As some point the store is considered discharged, even though parts of the packed bed are still 'hot'. The reason for this is that the creation of the 'thermal front' normally results in a large efficiency loss. Hence it is much more efficient to store the 'thermal front' for use in the next cycle. Most of the store however will have returned to its original uncharged "minimum store temperature" Tc1.

Figure 9b illustrates a discharge cycle where waste heat (as an example of low grade heat from e.g. an extemal or internal source) is added during the discharge mode such that the cold end inlet temperature -rum to the store is pre-heated to a temperature above Tc°. For example the temperature of the inlet air could be at 80°C. In this way, the "minimum store temperature" (or baseline) temperature of the thermal store is raised from (say 30°C) to a selected augmented "minimum store temperature" (say 80°C) such that once discharged, most of the media in the store is now sitting in that store at that higher baseline temperature. That waste heat is thus stored in the store until the next charge cycle.

Figure 9c shows the thermal store being charged after the Nth Charge Cycle.

During charge, the gas exiting the store will be at the raised baseline temperature that is near to say 80°C. That extra heat can be usefully captured by a heat exchanger downstream of the thermal store and redirected to the pre-heat system upstream of the first compressor; as a result, the air emerging from that heat exchanger now proceeds to the LSE facility at the lower (e.g. usual) temperature. As the leading edge of the thermal front (line 4) reaches the end of the store the temperature will start to rise to r4, which could be 25°C hotter i.e. 105°C.

In this way the store can be charged (at its "hot end") with high grade heat (i.e. at a higher temperature), while low grade heat is being discharged from the ("cold end" of the) 25 store (i.e. at a lower temperature). And when high grade heat is being discharged from the (hot end of the) store, it is possible to store low grade heat.

It will be appreciated that use of a pre-heater system in an ALAES according to the invention allows more heat to be stored in the first TES system, without a commensurate rise in pressure, which would add to that TES system cost. Hence, the energy density and efficiency of the system may be improved and in the discharge mode, the system is then able to provide a higher temperature pressurised gas to the expander. The heat addition may conveniently be by means of a heat exchanger and, because that additional heat stored in the first TES system is discharged through the expander during the discharge mode, it does not create a problem of waste heat build-up.

While the present invention has been described in detail with reference to certain preferred embodiments, other embodiments of the invention are possible. Therefore, the scope of the appended claims should not be limited to the description of the preferred embodiments contained herein. Further, the present invention comprises any novel feature or novel combination of features hereinbefore described.

Claims

CLAIMS1. An adiabatic, liquid air, energy storage (ALAES) system in which air from an inlet is directed, in a charging mode, successively downstream along a flow pathway comprising: a first compression stage where the air is compressed, a first thermal energy storage (TES) system where thermal energy is removed from the air passing through it and stored; a LAIR sub-system that liquefies the air and stores it on charging, and re-vaporises it on discharging, respectively; wherein air from the LAIR sub-system is directed, in the discharging mode, successively downstream along a flow pathway comprising: the first TES system where thermal energy is returned to the air passing through it; and, a first expansion stage where the air is expanded to generate power; characterised in that a pre-heater system is provided upstream of the first compression stage with respect to the charging mode, and is configured in the charging mode to preheat air entering the first compression stage so as to increase the temperature of air entering the first TES system.
2. An ALAES system according to claim 1, wherein the pre-heater system is configured to supply thermal energy derived from waste heat to the air.
3. An ALAES system according to claim 1 or claim 2, wherein the pre-heater system comprises at least one heat exchanger, the "upstream heat exchanger", that is provided upstream of the first compression stage with respect to the charging mode.
4. An ALAES system according to claim 3, wherein the upstream heat exchanger is coupled in the charging mode so as to receive heat from at least one heat exchanger, the "downstream heat exchanger", that is located downstream of the first TES system, or of a further downstream TES system, with respect to the charging mode.
5. An ALAES system according to claim 4, wherein, in the charging mode, the downstream heat exchanger is configured to receive heat that has been selectively stored in the first TES system, or further downstream TES system, during the previous discharge generation mode by selective operation of the downstream heat exchanger in that mode.
6. An ALAES system according to claim 5, wherein, during the previous discharge generation mode, the cold end air inlet temperature to the first TES system, or further downstream TES system, is selectively raised by supplying at least some heat to the 35 downstream heat exchanger from a heat source in the ALAES system or an external source.
7. An ALAES system according to claim 5, wherein, during the previous discharge generation mode, the cold end air inlet temperature to the first TES system, or further downstream TES system, is selectively raised by selecting the degree to which the 5 downstream heat exchanger discards heat.
8. An ALAES system according to any preceding claim, wherein the system comprises a heat exchanger, the "exhaust heat exchanger", that is provided downstream of the first expansion stage, with respect to the discharging mode, and that is configured in the discharging mode to retrieve waste heat from the exhaust air of the expander.
9. An ALAES system according to claim 8, wherein the exhaust heat exchanger and the upstream heat exchanger are the same heat exchanger, such that it is located upstream of the first compression stage with respect to the charging mode and in that mode is operational to preheat the inlet air to the compressor, and is also located downstream of the first expansion stage with respect to the discharging mode and in that mode is operational to cool the outlet air from the expander.
10. An ALAES system according to claim 8 or claim 9, wherein the exhaust heat exchanger is fluidly coupled to the downstream heat exchanger, so as to supply it with waste heat for selective storage in the first TES system, or a further downstream TES system, in the discharge mode.
11. An ALAES system according to any preceding claim, wherein the first thermal energy storage (TES) system comprises a store comprising a packed bed or particulate store.
12. An ALAES system according to any preceding claim, wherein the ALAES system is integrated with a gas turbine power generating system such that the compressor and 25 expander of the gas turbine form the first compression stage and the first expansion stage, respectively, and can be decoupled from each other to permit their separate operation.
13. An ALAES system according to claim 12, wherein the gas turbine power generating system comprises a combined cycle gas turbine system.
14. A hybrid, liquid air, energy storage, gas-turbine, electric power generating system comprising: a first compressor for compressing air, a first thermal energy storage TES system for storing thermal energy from the compressed air in a charging mode, and returning thermal energy to pressurised air in a discharging mode, a LAIR storage sub-system for liquefying and storing the compressed air as liquid air on charging, and returning/vaporising the air to produce pressurised air on discharging, a combustor for burning fuel with the pressurised air from the thermal energy store to produce a combustion gas; a gas turbine driven by utilizing the combustion gas; and, a generator driven by said gas turbine for generating electric power; wherein a pre-heater system is provided upstream of the first compressor to preheat air entering the compressor, so as to increase the temperature of the air entering the first TES system.
15. A method of operating an adiabatic, liquid air, energy storage (ALAES) system according to claim 1, the method comprising: during a charging mode: preheating air entering the first compression stage using the pre-heater system; compressing the preheated air using the first compression stage; passing the compressed air through the first TES system so as to transfer and store thermal energy from the air in the store; and liquefying and storing the air as liquid air in the LAIR sub-system; and, during a discharging mode: revaporising the liquid air in the LAIR sub-system; passing the air back through the first TES system to retrieve the stored thermal energy; and, expanding the air heated by the first TES system using the first expansion stage.
16. A method according to claim 15, wherein the system is as specified in any of claims 2 to 14, and operates as so specified.
17. A method according to claim 15, wherein the pre-heater system comprises at least one upstream heat exchanger provided upstream of the first compression stage, with respect to the charging mode, which is coupled to and receives heat in the charging mode from a downstream heat exchanger that is located downstream of the first TES system, or of a further downstream TES system, with respect to the charging mode, and wherein, in the previous discharging mode, the downstream heat exchanger is selectively operated so as to store heat in the first TES system, or of a further downstream TES system by selectively raising the cold end inlet temperature to that TES system.
18. A method according to claim 17, wherein, in the discharging mode, the cold end inlet temperature to that TES system is raised by the downstream heat exchanger receiving heat generated during that mode from either an external heat source or a heat 35 source located in a different part of the ALAES system.
19. A method, use or apparatus substantially as hereinbefore described with reference to Figures 4, 5, 6a-g, 7, 8a-b or 9a-c of the accompanying drawings.