WO2015185880A1

WO2015185880A1 - Improved acaes system

Info

Publication number: WO2015185880A1
Application number: PCT/GB2014/052420
Authority: WO
Inventors: James Macnaghten
Original assignee: Isentropic Ltd
Priority date: 2014-06-06
Filing date: 2014-08-07
Publication date: 2015-12-10
Also published as: GB201413985D0; GB2526888A; GB201410083D0

Abstract

An adiabatic compressed air energy storage (ACAES) system comprising: a first compression stage (11), at least a first thermal energy storage TES system (40) for removing and returning thermal energy to compressed air passing through it upon charging and discharging the TES system, respectively, at least a first compressed air store (50), and, a first expansion stage (14), wherein the ACAES system is configured:- to store thermal energy in a charging mode in which air is compressed in the first compression stage (11) and passes through the first TES system (40) so as to heat the store; to retrieve thermal energy in a discharging mode in which air passes back through the first TES system (40) so as to cool the store for subsequent expansion in the first expansion stage (14) (e.g. to produce electrical power); wherein a pre-heater system (46) 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 (11) so as to increase the temperature of air entering the first TES system (40).

Description

Improved ACAES System

Field of the invention

The present invention relates to an adiabatic compressed air energy storage (ACAES) system and a method of operating such a system.

Background to the Invention

A number of different methods of compressed air energy storage (CAES) have been proposed including Diabatic CAES (DCAES), Adiabatic CAES (ACAES) and Isothermal CAES (ICAES). All of the systems have only medium round trip efficiencies in the region of 40-70%.

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.

It should be noted that while many systems for ACAES have been proposed, none have yet been built, mainly because it is difficult to deal with both high temperatures and high pressures. For high temperatures it is preferable to use sensible heat exchange to a solid as it is hard to find candidate liquids that can cover the temperature range of ambient to 500°C without requiring very high pressures to contain them. However, if thermal exchange is to be direct to the storage medium it is then necessary to contain the pressure. For a TES to be used with a gas cavern normally means that it will be a large system that must process large quantities of air. This implies that the TES needs to be quite large; however, there are significant structural and thermal issues with building large pressurised structures to contain hot materials. When transferring heat to the TES it is preferable that the flow rate of the compressed air is not too fast so as to allow efficient thermal exchange and avoid undesirable pressure drops.

Applicant's earlier application WO201 1/104556 describes a thermal store in which the size and type of media can be varied through the store to either reduce the irreversibilities that are created when a thermal front is generated or else to help reduce the pressure drop that develops across the store. This application also proposes a thermal storage system with a high pressure store for storing high temperature heat, wherein the high pressure store is selectively coupled and decoupled to a lower pressure store such that lower pressure gas may be circulated between the two stores so as to relocate the heat in a lower pressure (and hence lower cost) store.

Applicant's earlier application WO2012/127178 proposes 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.

EP 2581584 discloses an ACAES system and is focussed on increasing the work per unit mass of gas processed by increasing the pressure ratio while keeping the temperature after the compressor constant. It is proposed that this is achieved by cooling the inlet air to the compressor to allow for an increased pressure ratio, which results in a peak temperature that is constant, while the pressure (and pressure ratio) can be raised, but the TES apparatus (e.g. a direct transfer TES) must then be designed for these higher pressures.

The present invention is directed towards providing an improved ACAES system.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided an adiabatic compressed air energy storage (ACAES) system comprising:

a first compression stage,

at least a first thermal energy storage TES system for removing and returning thermal energy to compressed air passing through it upon charging and discharging the TES system, respectively,

at least a first compressed air store, and,

a first expansion stage,

wherein the ACAES system is configured:-

• to store thermal energy in a charging mode in which air is compressed in the first compression stage and passes through the first TES system so as to heat the store;

• to retrieve thermal energy in a discharging mode in which air passes back through the first TES system so as to cool the store for subsequent expansion in the first expansion stage (e.g. to produce electrical power);

wherein 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.

Use 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 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.

In this way, the energy density and efficiency of the system may be improved and the power required for any second stage machinery may also be reduced relative to the primary system. The heat addition may conveniently be by means of a heat exchanger.

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. As the mass flow rate through the first compressor will fall (due to less dense air) in the charging mode, it also allows for a reduction in the size of any second (or subsequent) power machinery (e.g. second compressor/expander).

The term ACAES is intended to cover any compressed air energy storage system in which at least part of the heat of compression is stored in a TES system.

The ACAES system may comprise further respective compression/expansion stages, such as a second compression stage and associated second expansion stage, and similarly, may comprise an optional further respective TES system associated 5 therewith, designed to store and return the heat of compression of such a further respective compression/expansion stage.

The ACAES 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 below 1000°C (for example less than 750°C, less than 700°C, less than 10 650°C or less than 450°C).

The ACAES 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 first TES system (e.g. within 10% or within 5%). In one embodiment, the expansion stage inlet 15 temperature may be equal to or lower than the temperature of the air exiting the first TES system.

In one embodiment, the ACAES system does not include any combustion mode of operation for raising the temperature of air exiting the first TES system.

The pre-heater system may be configured to supply thermal energy derived from 0 waste 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 preheated, 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 ACAES system (e.g. exhaust of 5 ACAES), associated systems (e.g. downstream), or other separate equipment co-located on-site.

In one embodiment, the pre-heater system preheats the air before it enters the first compressor in the charging mode. 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 30 not more than 100°C; however, advantages may be secured with only a small temperature rise, for example, one of at least 30°C.

The pre-heater system may comprise at least one heat exchanger provided upstream of the first compression stage, with respect to the charging mode, and configured in the charging mode to receive heat (in real time) from at least one further 35 heat exchanger that is located downstream of the first TES system, or a further downstream TES system (i.e. a TES system that is more downstream than the first TES system, for example, located after second stage power machinery), with respect to the charging mode. In one embodiment, in the charging mode, the at least one further 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 that heat exchanger in that mode. In one embodiment, during the previous discharge generation mode, the air inlet temperature to the first TES system, or further downstream TES system, is selectively raised by supplying at least some heat to the at least one further heat exchanger from an external source.

The upstream and downstream heat exchangers 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 (with the correct thermal gradient across them), it will be appreciated that gas circulating downstream of the first 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 gas inlet temperature on charging the store (from the hot end) and the last 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.)

In one embodiment, in the charging mode, the at least one further 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 that heat exchanger in that mode.

For example, during the previous discharge generation mode, the 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 at least one further heat exchanger from an external source e.g. the hot exhaust from the expander ( e.g. turbine).

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

The last gas inlet temperature when discharging the first TES or further downstream TES store (from the cold end) may selectively be raised, during the previous discharge mode, by choosing the degree, if any, at which to discard any of the waste heat generated by the power machinery. The simplest set-up is to configure the further heat exchanger located downstream of the TES in question so that they are bypassed or inoperative (ie bypassed to avoid any pressure drop through the heat exchanger or inoperative so that no HTF flows through them and hence the heat exchanger has no cooling effect after it is raised to approximately the air temperature in the circuit) during the discharge/generation mode, and hence, so that all the (low grade) waste heat becomes stored (at a higher "minimum store temperature") in the store. In the subsequent charging mode, the heat exchanger downstream of that store is then operative to transfer that heat (in effect, waste heat that was temporarily stored, for example, via a HTF circuit, to the upstream heat exchanger.

Heating the inlet air prior to compression is arguably counter-intuitive for a number of reasons. Usually, it is perceived that a rise or drift in air inlet temperature (e.g. in hot climates) in air compressors is undesirable. This is because warmer air is less dense so that the overall mass flow rate through the compressor falls while the work required to compress a certain mass of gas increases. However, in the present system, the increase in work per unit mass flow with increase in inlet temperature is exploited to advantage..

It will be appreciated that in the above aspect, the additional heat is stored during charging in the (hot end of the) first TES system downstream of the compressor, for subsequent discharge to the expander on discharge, but that the waste heat that may be used as a supply of that heat may be stored either in (the cold end of) the same TES system, or, a 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 upon charging.

The ACAES system may further comprise a yet further heat exchanger provided downstream of the first expansion stage, with respect to the discharging mode, and configured in the discharging mode to provide heat to the first TES system or further downstream TES system. In one embodiment, heat is provided from the yet further heat exchanger to the first TES system or further downstream TES system by supplying heat to the at least one further heat exchanger. In another embodiment, the at least one heat exchanger is also connectable so that it is disposed downstream of the first expansion stage, with respect to the discharging mode, and configured in the discharging mode to provide heat to the first TES system or further downstream TES system. 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 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 usually being reversed between the two modes.

In one embodiment, the CAES system does not contain any mode in which air can follow a gas flow path in which it is concurrently flowing through the one or more compression stages as well as through one or more expansion stages, that is, the CAES system is configured for operation either in a charging mode in which the compression stages are operative or in a discharging mode in which the expansion stages are operative.

In an alternative embodiment, the pre-heater system comprises a thermal store upstream of the first compression stage, with respect to the charging mode, and configured in the charging mode to preheat air entering the first compression stage.

The (at least one) thermal store may also be connectable so that it is disposed downstream of the first expansion stage, with respect to the discharging mode, and configured in the discharging mode to receive and store thermal energy from air exiting the first expansion stage. Flow connections associated with the thermal store may direct air flow as required through it in the respective charging and discharging modes, the direction of flow through the thermal store optionally being reversed between the two modes.

In the charging mode, thermal energy stored in the thermal store during a previous discharging mode may be transferred to air before subsequent compression of the air in the first compression stage.

The thermal store may comprise a gas permeable, solid thermal storage media disposed in the air flow path so as to allow an efficient direct transfer of heat between the air and storage media.

The use of a thermal store active in the charging mode upstream of the first compression stage to provide preheat to air passing through it, which pre-heat has been stored in the same (i.e. common) store in a (previous) discharging mode downstream of the first expansion stage in the manner of a regenerator provides high efficiency and minimises system costs. Where the thermal store comprises a gas permeable (preferably porous) solid thermal storage medium, the apparatus is preferably configured to pass air through the thermal store in one direction in the charging mode and in the opposed direction through the thermal store in the discharging mode.

The thermal store may correspond in structure to the first TES system but without the requirement for high pressures.

Any required flow valve/connector arrangements and mechanical coupling arrangements (e.g. clutches and/or gears) may be so configured as to provide the necessary flow and mechanistic connections to allow the ACAES to operate as required, and may be controlled by one or more controllers, which may be linked to sensors (e.g. temperature or pressure sensors) suitably located within the ACAES.

In accordance with a second aspect of the present invention, there is provided a method of operating an adiabatic compressed air energy storage (ACAES) system as specified above, the method comprising:

during a charging mode:

preheating air entering the first compression stage using the pre-heating system; compressing the preheated air using the first compression stage; and

passing the compressed air through the first TES system so as to heat the store; and

storing air cooled by the first TES system under pressure in the compressed air store;

during a discharging mode:

retrieving stored thermal energy by passing air stored under pressure back through the first TES system;

expanding the air heated by the first TES system using the first expansion stage (e.g. to generate electrical power).

In one embodiment, during the discharging mode the air entering the first expansion stage is at a temperature below 1000°C (for example less than 750°C, less than 700°C, less than 650°C or less than 450°C).

In one embodiment, during the discharging mode the air entering the first expansion stage is at a temperature within 20% of the temperature of the air exiting the first TES system (e.g. within 10% or 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. In one embodiment, the method does not include any combustion mode of operation in which the temperature of air exiting the first TES system is raised by combustion.

In one embodiment, during the charging mode the step of preheating the air using the pre-heater system comprises supplying thermal energy derived from waste heat to the air.

In one embodiment, the pre-heater system comprises at least one heat exchanger provided upstream of the first compression stage with respect to the charging mode; and during the charging mode the method comprises receiving at the at least one heat exchanger heat from at least one further heat exchanger that is located downstream of the first TES system, or a further downstream TES system with respect to the charging mode.

In one embodiment, the method further comprises: during the discharging mode storing heat in the first TES system, or the further downstream TES system, for use in preheating air entering the first compression stage during a subsequent charging mode.

In one embodiment, the ACAES system further comprises a yet further heat exchanger provided downstream of the first expansion stage with respect to the discharging mode; and during the discharging mode the method comprises receiving at the first TES system or further downstream TES system heat from the yet further heat exchanger.

In one embodiment, the step of receiving heat from the yet further heat exchanger at the first TES systems or further downstream TES system comprises receiving heat to the at least one further heat exchanger.

In another embodiment, the at least one heat exchanger is also connectable so that it is provided downstream of the first expansion stage, with respect to the discharging mode; and during the discharging mode the method comprises receiving heat at the first TES system or further downstream TES system from the at least one heat exchanger.

In an alternative embodiment, the pre-heater system comprises a thermal store upstream of the first compression stage, with respect to the charging mode; and during the charging mode the method comprises using thermal energy stored in the thermal store to preheat air entering the first compression stage.

The thermal store may correspond in structure to the first TES system, but without the requirement for high pressures.

In one embodiment, the thermal store is also connectable so that it is disposed downstream of the first expansion stage, with respect to the discharging mode; and during the discharging mode the method comprises receiving at the thermal store heat from air exiting the first expansion stage. In one embodiment, the method further comprises: during the charging mode transferring thermal energy stored in the thermal store during a previous discharging mode to air before subsequent compression of the air in the first compression stage.

In one embodiment, the method comprises passing air through the thermal store in one direction in the charging mode and in the opposed direction through the thermal store in the discharging mode.

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 a conventional adiabatic compressed air system (ACAES) of the prior art;

Figures 2a-c depict an ACAES system in accordance with a first embodiment of the present invention in which the ACAES system of Figure 1 has been modified to incorporate a pre-heater system, operating in charging and discharging modes respectively;

Figures 2d and 2e depict a further modification that may be made to the ACAES system of Figure 2a to form an ACAES system in accordance with a second embodiment of the present invention, operating in the different modes;

Figures 2f and 2g depict a yet further modification that may be made to the ACES system of Figure 2a to form an ACAES system in accordance with a third embodiment of the present invention, operating in charging and discharging modes respectively;

Figures 2h and 2i depict a modification that may be made to the ACES system of Figure 2f to form an ACAES system in accordance with a fourth embodiment of the present invention, operating in charging and discharging modes respectively;

Figures 3a-c depict a further modification that may be made to the ACAES system of Figure 2a to form an ACAES system in accordance with a fifth embodiment of the present invention, operating in different modes;

Figures 3d and 3e depict a modification that may be made to the ACAES system of Figure 3a to form an ACAES system in accordance with a sixth embodiment of the present invention, operating in different modes; Figures 4a and 4b show expected thermal profiles within a thermal store during charging and discharging modes respectively to illustrate how waste heat may be obtained from condensation of atmospheric air; and

Figures 5a-c 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 typical layout of a conventional prior art adiabatic compressed air energy storage system (ACAES) 10 used for peaking power generation, with an upstream first compressor (e.g. turbine compressor) 1 1 , a downstream expander (e.g. turbine expander) 14, selective connection to a motor/ generator 15 (e.g. connected to a transformer/grid), a first thermal energy storage TES system 40, and compressed gas storage 50. In a normal configuration either the compressor 1 1 or expander 14 is directly coupled to the motor generator on the same shaft by drive couplings (not shown). Filtered air enters the compressor at ambient conditions (e.g. 30°C, 1 bar) and is compressed up to a higher pressure and temperature (e.g. 650°C, 60 bar for a high pressure gas store or 420°C, 17bar for a low pressure gas store such as an underwater airbag).

First thermal energy storage TES system 40 comprises a first thermal store 41 comprising a thermally insulated vessel 42 and thermal storage media 43 which may be any suitable TES apparatus, a 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 must be designed so that the high pressure flow (usually at between 15 and 30 bar and between 450-600°C) can pass 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 WO2012/127178. As described above, the valved, layered store has functionality allowing it to store thermal energy in a controllable manner.

During a charging mode, the first compressor 1 1 compresses atmospheric air to a higher pressure and temperature. Power to drive compressor is supplied by motor/generator 15 that is normally drawing power from an electricity grid. Flow selector valve arrangement 31 diverts hot high pressure gas to the top of the thermal store 41 via pipe 32 and the gas passes through the thermal media 43 cooling as it progresses. The gas leaves the thermal store via pipe 33 where it may be cooled to near ambient temperature by heat exchanger 45. The cooled high pressure gas then enters compressed gas storage 50, which may be an underground cavern. Expander 14 is not in operation during the charging mode.

During a discharging mode, the gas exits compressed gas storage 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 entered the store during charging. The gas is then diverted by selector valve 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 1 1 is not in operation during the discharging mode.

If first compressor 1 1 raises the temperature of the air to high temperatures of between 450 and 600°C and to a pressure of around 18 bar, the thermal energy storage stores heat of this order and, after cooling of the compressed air has taken place, the compressed air storage stores gas at this order of pressure, such that advantageously, additional power stages or cooling stages are not required. For convenience, such storage may hereinafter be referred to as medium pressure storage (i.e. storage of the order of pressure of the compressor outlet pressure of first compressor 1 1) which, for example, will usually be about 10-30 bar, more likely, 15-25 bar, or even 18-23 bar. In principle, first compressor 1 1 may be selected to operate at higher pressures, e.g. using ratios of 30: 1. The upper limit for a compressor is the temperature that the last stage of the compressor can normally tolerate. This is currently around 600°C for continuous running of an uncooled blade although hotter temperatures can be achieved for short duration.

Figures 2a-c show a first embodiment of the present invention comprising an ACAES system with pre-heating prior to the first compressor 1 1.

An additional heat exchanger 46 is added to the air inlet flow that is coupled to heat exchanger 45 via a heat transfer fluid (or HTF). First thermal storage system is a simple TES store 41 based on direct thermal transfer as described in Figure 1 above.

As described further below, the system may operate in a charge only mode (Figure 2a) in which the expander 14 is declutched from the motor/generator 15, which then acts as a motor to drive the first compressor 1 1 , with all the compressor outlet flow entering the thermal store. This mode uses only electrical energy (e.g. from a local grid) for storage.

When charging the first thermal store 41 , valve 31 diverts hot high pressure gas to the top of the vessel 42 and the 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 second heat exchanger 46. In this way the heat is transferred via heat exchanger 46 to the incoming air prior to the first compressor 1 1.

The higher pressure gas leaves heat exchanger 45 and is diverted via pipe 33 to medium pressure gas storage 50.

There are a number of reasons why the temperature of the gas leaving the first thermal store during charging mode may be above ambient (or the original baseline store temperature).

The first 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 has been previously selectively stored in the thermal store as described for example in Figure 2c below, where heat is actively supplied in a previous mode.

Figure 2b 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 to receive its stored heat. There is no need for circulation of HTF between heat exchanger 45 and 46 during this process.

Figure 2c shows the system of 2a on discharging where heat exchanger 45 is used to selectively increase the air inlet temperature to the first TES system by supplying at least some heat to the heat exchanger located downstream 45 of the first TES system 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 2d and 2e show a modified version of the system of Figure 2a in which a yet further heat exchanger 48 is added after the expander 14 and is used during discharging (Figure 2d) to selectively increase the air inlet temperature to the first TES system by supplying at least some heat to the heat exchanger located downstream 45 of the first TES system. Due to machinery losses the temperature of gas leaving the expander 14 should be hotter than that of the gas post heat exchanger 46 (before it enters first compressor 1 1). As shown in Figure 2e, in the charging mode the inlet air is heated by heat exchanger 46 to a higher temperature using the higher grade waste heat that was stored during the previous discharge cycle prior to compression by compressor 1 1.

Figures 2f and 2g show another modified version of the system of Figure 2a in which first compressor 1 1 and expander 14 are configured such that heat exchanger 46 is connectable upstream of first compressor 1 1 during a charging mode (Figure 2f) and then subsequently connectable downstream of expander 14 during a discharging mode (Figure 2g). 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 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. Advantageously, this arrangement allows heat exiting expander 14 to be used to raise the air inlet temperature to the first TES system.

Figures 2h and 2i show a modified version of the system of Figure 2f in which heat exchanger 46 is replaced by a further thermal storage system 140 with a thermal store 141 comprising a vessel 142 housing a gas permeable thermal storage media 143.

During a discharging mode (Figure 2i), thermal store 141 is connected downstream of expander 14 whereby air exiting expander 14 is used to heat thermal store 141. During a subsequent charging mode (Figure 2h), thermal store 141 is connected upstream of first compressor 1 1 so that thermal energy stored in thermal store 141 during a previous discharging mode is transferred to air before compression of the air by first expander 1 1. The use of a thermal store active in the charging mode upstream of the first compression stage to provide preheat to air passing through it, which pre-heat has been stored in the same store in a previous discharging mode downstream of the first expansion stage in the manner of a regenerator provides high efficiency and minimises system costs. The apparatus is preferably configured to pass air through the thermal store in one direction in the charging mode and in the opposed direction through the thermal store in the discharging mode.

5 Figure 3a-c shows a modified version of the system of Figure 2a where the system includes a second compressor/expander stage such that air storage can occur at much higher pressures. In this embodiment, the first thermal storage system 40 is a simple (e.g. particulate bed) TES store 41 based on direct thermal transfer as described in Figure 2a above, followed by heat exchanger 45, and a second thermal storage system

10 580 also comprising a simple TES store 581 comprising thermally insulated vessel 582 and thermal storage media 583, which is based on direct thermal transfer, is provided downstream of the second stage power machinery 70, with an additional heat exchanger 47 downstream of second store 581 before the compressed gas storage 90. Again, an additional heat exchanger 46 is added to the air inlet flow. This may be coupled to heat

15 exchanger 45 (Fig 3a & b) or heat exchanger 47 (Fig. 3c) via a heat transfer fluid or HTF.

When charging (Figure 3a) the first thermal store 41 , valve 31 diverts hot high pressure gas to the top of the vessel 42 and the 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 0 exchanger 45 so that the temperature is close to ambient temperature and heat is transferred to a heat transfer fluid HTF which is coupled to upstream heat exchanger 46.

The higher pressure gas leaves heat exchanger 45 and is diverted via valve 71 so that on charging it passes through a second compressor 72 (as shown in Figure 3a) and on discharging through a second expander (e.g. second turbine) 73 (as shown in Figure 5 3b). Second compressor 72 and expander 73 are selectively coupled to second motor/generator 74. During charging, the temperature and pressure of the gas is raised by second compressor 72 so that the pressure is approximately equal (but slightly higher) than the pressure in the high pressure gas store 90.

Figure 3b shows the discharging process, which is the reverse. The hot high

30 pressure gas is then diverted by valves 71 so that it is expanded in expander 73 (with some power generation), before passing back through the first thermal store to receive its stored heat before it passes through expander 14 (with further power generation).

The last gas inlet temperature when discharging the store (from the cold end) may selectively be raised, during the previous discharge mode, by choosing the degree, if

35 any, at which to discard any of the waste heat generated by the second expander 73. It should be understood that due to certain irreversible losses during processing the temperature of the gas exiting expander 73 is usually going to be higher than that of the air entering compressor 72. The simplest set-up is to configure the heat exchanger 45 located downstream of the first TES so that it is bypassed or inoperative during the discharge/generation mode, and hence, so that all the (low grade) waste heat from the second expander 73 becomes stored (at a higher "minimum store temperature") in the first TES system. In the subsequent charging mode (Fig. 3a), the heat exchanger 45 downstream of the first TES system is then operative to transfer that heat (in effect, waste heat that was temporarily stored in the first TES), for example, via a HTF circuit, to the upstream heat exchanger 46.

Figure 3c shows a system that is identical to 2a, and again in charging mode, but where the cooled high pressure gas leaves the second thermal store 581 where it is still above ambient temperature. It is then further cooled in a heat exchanger 47 so that the temperature is close to ambient temperature and heat is transferred to a heat transfer fluid HTF which is coupled to the second heat exchanger 46.

Figures 3d and 3e show a modified version of the system of Figure 3a-c wherein a yet further heat exchanger 48 is added downstream after the expander 14 and is used to selectively increase the air inlet temperature to the second TES system (Figure 3d) by supplying at least some heat to the heat exchanger 47 located downstream of the second TES system. Due to machinery losses the temperature of gas leaving the expander 14 should be hotter than that of the gas post heat exchanger 46 (before it enters first compressor 1 1).

During a charging mode, as shown in Fig. 3e, the inlet air is heated to a higher temperature using the higher grade waste heat that was stored during the previous discharge cycle.

Use of such a pre-heater system 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.

If the ACAES does comprise a second stage of power machinery to allow storage of compressed air at higher pressures, then the higher pressure, compression/expansion stage may comprise positive displacement power machinery, preferably reciprocating linear machinery including piston based machinery, which is more suited than turbine machinery to higher operating pressures and will maintain a static pressure difference across it when the sub-system is actively storing, but not actively charging or discharging. Conveniently, the linear reciprocating (e.g. piston based) power machinery may be a single, reversible machine so as to act as both a compressor and an expander, as required during charging and discharging, respectively.

Where the at least one compressed air store is a variable pressure store, the second, higher pressure, compression/expansion stage preferably comprises variable pressure and/or variable mass flow rate power machinery, in particular, where the variable mass flow rate power machinery may be actively controlled. In particular, it is highly preferred to use positive displacement power machinery where variable pressure air storage is desired in excess of the first stage (e.g. fixed) operating pressures.

POWER CALCULATIONS

By way of example only, typical figures for a large ACAES plant are used to quantify the effect with and without pre-heating, in Table 1 below:-

Ambient Inlet preInlet pre¬

288K heat to 343K heat to 363K

Temperature after 1^st stage K 702 836 885

Mass Flow Air kg/s 430 430 430

1st stage pressure bar 16.6 16.6 16.6

2nd stage pressure bar 80 80 80

Charging first stage MW 179 213 226

Charging second stage MW 90 90 90

Total Charging MW 269 303 315

Discharging first stage MW 156 186 197

Discharging second stage MW 69 69 69

Total Discharging MW 225 255 266

Energy Density 100% 113% 118%

Size of second stage

machinery relative to first

stage 44% 37% 35%

Table 1

The table shows an ACAES system with a mass flow of 430 kg/s and a first stage pressure of 16.6 bar and a second stage pressure of 80bar would require 269MW to charge (note this includes thermodynamic losses, but excludes mechanical and electrical losses) at an ambient temperature of 288K. It would discharge 225MW of power. Using a pre-heater system (such as that shown in Figure 3a-e) that results in pre-heating of the compressor inlet to 343K, would increase the energy density of the system by 13% and would decrease the power of the second stage relative to the first from 44% of the size of the first stage to 37%. Pre-heating to 363K would increase this figure further to 18% increase in energy density and reduce the second stage machinery to just 35% of the size of the first stage. It can also be seen that preheating the inlet air by 75°C raises the temperature post compression by 183 °C (ie from 429 °C up to 612 °C).

The increase in energy density is related purely to the increased energy density of the first stage, which increases by approximately 26% in the 363K pre-heating case. However, the overall energy density (both thermal stores) of the system increases by 18% as shown.

The use of a pre-heater system is of more benefit where the ACAES system is able to use power machinery with lower pressure ratios, for example, where the air is compressed by no more than 40 times (its original pressure), or by no more than 30 times or by no more than 25 times (i.e. a compressor where gas inlet pressure:gas outlet pressure ratio is less than 1 :40 (e.g. 1 :39) or less than 1 :30, or less than 1 :25), since more heat can be added pre-compression without exceeding the maximum operating temperature of the compressor or downstream thermal store.

Figures 4a and 4b explain how waste heat may be obtained from condensation of atmospheric air. In the ACAES system described atmospheric air is drawn into the system 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, 1 kg of air can absorb 14.8 grams of moisture to become fully saturated. At 15 bar and 20°C the same 1 kg of air can only absorb 1 g of moisture. This means that if 1 kg 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 ACAES system where the thermal store is operating at a pressure of 15 bar, most of the condensation will occur in the 50- 70°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 4a. While the store starts with an original exit temperature of T^c1 (30°C), as condensation starts to occur (on the charge cycle) the exit temperature from the cold end of the store quickly rises to the temperature at which condensation starts to occur T eg 70°C. 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 preheater 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 4b shows the thermal profile within the thermal store on discharge. As the air is coming back from a high pressure cavern it is likely to be very dry. As a result the end discharge profile looks very similar to the original initial charge profile.

Figure 5a 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 T^c1. When charging has progressed, the temperature of the gas leaving the store starts to rise from the baseline T^c1 as the front of the thermal front starts to move out of the store.

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 say a selected temperature of 80°C so as to condition the store, if desired, to match (as discussed below) a subsequent cold 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 in reverse. At the outlet from the store gas leaves at close to T^h4 (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.

Figure 5b illustrates a discharge cycle where waste heat (as an example of low grade heat from e.g. an external or internal source) is added during the discharge mode such that the cold inlet temperature ^~r^c1 '²'^3,4 to the store is pre-heated to a temperature above T^c0. For example the temperature of the inlet air could be at 80°C. In this way, the lowest (or baseline) temperature of the thermal store is raised from (say 30°C) to a selected augmented (higher) temperature (say 80°C) such that once discharged, the media in the store is now sitting in that store at that higher temperature. That waste heat is thus stored until the next charge cycle.

Figure 5c shows the thermal store being charged after the Nth Charge Cycle. During charge, the gas exiting the store will be at a temperature that is near to T^c1 , say 80°C. That extra heat can be usefully captured by a heat exchanger and redirected to the pre-heat system such that air enters the compressed air store 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 T^c4, which could be 25°C hotter i.e. 105°C.

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

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.

Claims

Claims:

1. An adiabatic compressed air energy storage (ACAES) system comprising:

a first compression stage,

at least a first compressed air store, and,

a first expansion stage, wherein the ACAES system is configured:- · to store thermal energy in a charging mode in which air is compressed in the first compression stage and passes through the first TES system so as to heat the store;

• to retrieve thermal energy in a discharging mode in which air passes back through the first TES system so as to cool the store for subsequent expansion in the first expansion stage;

2. An ACAES system according to claim 1 , wherein the ACAES system is configured during the discharging mode to deliver air from the first TES system to the first expansion stage at an expansion stage inlet temperature below 1000°C.

3. An ACAES system according to claim 1 or claim 2, wherein the ACAES system is 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 first TES system.

4. An ACAES system according to any preceding claim, wherein the pre-heater system is configured to supply thermal energy derived from waste heat to the air.

5. An ACAES system according to any preceding claim, wherein the pre-heater system comprises at least one heat exchanger provided upstream of the first compression stage, with respect to the charging mode, and configured in the charging mode to receive heat from at least one further heat exchanger that is located downstream of the first TES system, or a further downstream TES system, with respect to the charging mode.

5 6. An ACAES system according to claim 5, wherein, in the charging mode, the at least one further 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 that heat exchanger in that mode.

10 7. An ACAES system according to claim 6, wherein, during the previous discharge generation mode, the air inlet temperature to the first TES system, or further downstream TES system, is selectively raised by supplying at least some heat to the at least one further heat exchanger from an external source.

15 8. An ACAES system according to claim 6, wherein, during the previous discharge generation mode, the air inlet temperature to the first TES system, or further downstream TES system, is selectively raised by selecting the degree to which the at least one further heat exchanger discards heat.

20 9. An ACAES system according to claim 5, wherein the ACAES system further comprises a yet further heat exchanger provided downstream of the first expansion stage, with respect to the discharging mode, and configured in the discharging mode to provide heat to the first TES system or further downstream TES system.

25 10. An ACAES system according to claim 9, wherein heat is provided from the yet further heat exchanger to the first TES system or further downstream TES system by supplying heat to the at least one further heat exchanger.

1 1. An ACAES system according to any of claims 5-8, wherein the at least one heat 30 exchanger is also connectable so that it is downstream of the first expansion stage, with respect to the discharging mode, and configured in the discharging mode to provide heat to the first TES system or further downstream TES system.

12. An ACAES system according to any of claims 1-4, wherein the pre-heater system 35 comprises a thermal store upstream of the first compression stage, with respect to the charging mode, and configured in the charging mode to preheat air entering the first compression stage.

13. An ACAES system according to claim 12, wherein the thermal store is also 5 connectable so that it is downstream of the first expansion stage, with respect to the discharging mode, and configured in the discharging mode to receive and store thermal energy from air exiting the first expansion stage.

14. An ACAES system according to claim 13, wherein in the charging mode thermal 10 energy stored in the thermal store during a previous discharging mode is transferred to air before subsequent compression of the air in the first compression stage.

15. An ACAES system according to any of claims 12 to 14, wherein the thermal store comprises a gas permeable, solid thermal storage media disposed in the air flow path.

15

16. A method of operating an adiabatic compressed air energy storage (ACAES) system according to claim 1 , the method comprising:

during a charging mode:

preheating air entering the first compression stage using the pre-heating system; 20 compressing the preheated air using the first compression stage; and

25 during a discharging mode:

expanding the air heated by the first TES system using the first expansion stage.

30 17. A method of operating an ACAES system according to claim 16, wherein during the discharging mode the air entering the first expansion stage is at a temperature below 1000°C.

18. A method of operating an ACAES system according to claim 16 or claim 17, 35 wherein during the discharging mode the air entering the first expansion stage is at a temperature within 20% of the temperature of the air exiting the first TES system.

19. A method of operating an ACAES system according to any of claims 16-18, wherein during the charging mode the step of preheating the air using the pre-heater system comprises supplying thermal energy derived from waste heat to the air.

20. A method of operating an ACAES system according to any of claims 16-19, wherein:

the pre-heater system comprises at least one heat exchanger provided upstream of the first compression stage with respect to the charging mode; and

during the charging mode the method comprises receiving at the at least one heat exchanger heat from at least one further heat exchanger that is located downstream of the first TES system, or a further downstream TES system with respect to the charging mode.

21. A method of operating an ACAES system as defined in claim 20, wherein the method further comprises:

during the discharging mode storing heat in the first TES system, or the further downstream TES system, for use in preheating air entering the first compression stage during a subsequent charging mode.

22. A method of operating an ACAES system according to claim 20 or claim 21 , wherein:

the ACAES system further comprises a yet further heat exchanger provided downstream of the first expansion stage with respect to the discharging mode; and

during the discharging mode the method comprises receiving at the first TES system or further downstream TES system heat from the yet further heat exchanger.

23. A method of operating an ACAES system according to claim 22, wherein the step of receiving heat from the yet further heat exchanger at the first TES systems or further downstream TES system comprises receiving heat to the at least one further heat exchanger.

24. An ACAES system according to any of claims 20 or 21 , wherein:

the at least one heat exchanger is also connectable so that it is provided downstream of the first expansion stage, with respect to the discharging mode; and during the discharging mode the method comprises receiving heat at the first TES system or further downstream TES system from the at least one heat exchanger.

25. A method of operating an ACAES system according to any of claims 16-19, wherein:

the pre-heater system comprises a thermal store upstream of the first compression stage, with respect to the charging mode; and

during the charging mode the method comprises using thermal energy stored in the thermal store to preheat air entering the first compression stage.

26. A method of operating an ACAES system according to claim 25, wherein:

the thermal store comprises a gas permeable, solid thermal storage media and the air is passed through that media for heat transfer therewith.

27. A method of operating an ACAES system according to claim 25 or 26, wherein: the thermal store is also connectable so it is disposed downstream of the first expansion stage, with respect to the discharging mode; and

during the discharging mode, the method comprises receiving at the thermal store heat from air exiting the first expansion stage.

28. A method of operating an ACAES system according to claim 26, wherein the method further comprises:

during the charging mode transferring thermal energy stored in the thermal store during a previous discharging mode to air before subsequent compression of the air in the first compression stage.

29. A method of operating an ACAES system according to claim 25 and 27, wherein the method comprises:

passing air through the thermal store in one direction in the charging mode and in the opposed direction through the thermal store in the discharging mode.

30. A method, use or apparatus substantially as hereinbefore described with reference to Figures 2a-5c of the accompanying drawings.