GB2537126A

GB2537126A - Hybrid energy storage system

Info

Publication number: GB2537126A
Application number: GB1505892.8A
Authority: GB
Inventors: Sebastian Howes Jonathan; Macnaghten James
Original assignee: Isentropic Ltd
Current assignee: Isentropic Ltd
Priority date: 2015-04-07
Filing date: 2015-04-07
Publication date: 2016-10-12
Also published as: GB201505892D0

Abstract

A hybrid energy storage system 410 comprises a liquid air energy storage (LAES) system integrated with a pumped heat energy storage (PHES) system 200. The LAES system and PHES system are thermally coupled via a heat exchanger 90. The LAES comprises first LAES power machinery for compression and expansion of working fluid, thermal energy storage (TES) system 36, the heat exchanger 90, and a liquefaction apparatus including a cryogenic tank 120. The PHES system comprises first PHES power machinery 212, a PHES thermal store 236, further PHES power machinery 214 and the heat exchanger 90. The PHES operates in a charging mode when the LAES system operates in a charging mode and in a discharge mode when the LAES system is in a discharge mode. The heat exchanger 90 transfers thermal energy from air in the LAES system to working fluid in the PHES system during LAES charging, and transfers thermal energy to the air in the LAES system from the working fluid during LAES discharging. The LAES system may be an adiabatic(ALAES) system with a TES and may form part of a gas turbine power generation system having a variable pressure, liquid air tank 120.

Description

Hybrid Energy Storage System

Field of the invention

The present invention relates to a liquid air energy storage system and method of operating the same, including, in particular, a liquid air energy storage, combustion turbine power generation system. It also relates to a hybrid energy storage system in which one energy storage system, in particular, a cryogenic energy storage system such as, for example, a liquid air energy storage system, is synergistically integrated with another energy storage 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.

The use of hybrid, energy storage power generation systems in which a combustion turbine (GT) is combined with an energy storage system, such as a CAES or LAES system, has also been proposed. In these systems, the combustion turbine may operate in a normal power generation mode where a significant proportion of the power from the turbine is used to power the compressor, or for example, the compressor and turbine may be decoupled for selective individual use; in an energy storage mode, the compressor may supply compressed air to the energy storage system to store compressed air or liquid air using low cost electrical energy (e.g. 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 be adiabatic CAES (ACAES) or adiabatic LAES (ALAES) systems that incorporate heat stores/regenerators and/or heat exchangers linked to stores to store the heat of compression 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 returned to the combustion turbine at a suitable temperature, for example, similar to the outlet temperature of the compressor during normal CT operation.

It will be appreciated that where the storage of sensible heat in the thermal energy storage (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.

US6,920,759 proposes a hybrid, liquid air energy storage, gas turbine electric power system with various hybrid operating modes, combined with an optional steam bottoming cycle to extract more energy from the combusted gas exhaust. US6,920,759 proposes a CES cycle in which liquid air stored at constant pressure is pumped from its storage tank by a liquid pump to a higher pressure, then evaporated and the cold energy stored in a cold thermal store or regenerator before the pressurised air flows through the turbine to produce power; during the storage mode, ambient air is compressed, and then cooled through the cold thermal store to form liquid air which is stored in the tank. Instead of a regenerator, US6,920,759 also proposes a multi-stage heat exchanging facility storing heat at progressively lower temperatures in each stage (e.g. the use of propane with a temperature range of -188°C to -42°C is mentioned), to cool incoming air, before the pressurised cooled gas is expanded through an expansion valve separating it into gas and liquid; the liquid air is stored in the tank while the cold gaseous air is recycled as a source of cold in reverse order to the heat exchangers. Other prior art CES systems propose alternative ways of recycling the cold of evaporation to assist in the liquefaction process.

Such systems are based on conventional liquefaction techniques that aim to produce cryogenic liquid as product, but that are not designed with reversibility in mind.

Hence, conventional LAES systems have a relatively low round trip efficiency of about 50%. Disregarding CAPEX, the price of electricity during the discharging phase needs to be at least double the price during charging to break even. Moreover, storage of cold required to improve the efficiency of this cycle is problematic: it is difficult to find suitable liquid thermal storage mediums that operate over sub-ambient ranges and that are safe to use. On the other hand, if a solid sensible storage medium is used, the store may be large and expensive in view of the fact that the heat capacity of solids decreases with temperature.

Applicant's published PCT Application No. W02014/162129 describes an alternative energy storage and recovery system based upon a closed system in which a 35 working fluid is transferred, during charging and discharging, between a first pressure vessel and a second pressure vessel, via power machinery, and where the working fluid is stored in each tank as a saturated liquid/vapour air mixture under equilibrium pressure and temperature conditions which march in the sense that condensation of the air in a tank causes a progressive increase in the equilibrium vapour pressure and temperature of the saturated mixture, and evaporation of the air in a tank causes a progressive decrease in the equilibrium vapour pressure and temperature of the saturated mixture.

The present invention is directed towards providing an improved hybrid energy storage system including an improved liquid air energy storage (LAES) system.

Summary of the Invention

1st Aspect In accordance with a first aspect of the present invention, there is provided a liquid air energy storage (LAES) system in which liquid air is stored in a cryogenic tank, the system comprising a flow pathway leading from an air inlet successively to: first stage power machinery to compress the air upon charging and to expand the air to produce work upon discharging; an optional first thermal energy storage (TES) system to store and return the heat of compression upon charging and discharging, respectively; (iii) second stage power machinery to expand the air into the tank upon charging, and to withdraw the air from the tank and compress it upon discharging; and, (iv) the cryogenic tank for storing the liquid air; wherein the system is configured such that the air enters and leaves the cryogenic tank in gaseous form, and is stored in the cryogenic tank as a saturated liquid/vapour air mixture under equilibrium pressure and temperature conditions, whereby condensation of the air in the tank during charging causes a progressive increase in the equilibrium vapour pressure and temperature of the saturated mixture, and evaporation of the air in the tank during discharging causes a progressive decrease in the equilibrium vapour pressure and temperature of the saturated mixture.

In contrast to a conventional prior art LAES system, the air both enters and leaves the cryogenic tank as a gas and is stored in it as a saturated liquid/vapour mixture under marching equilibrium conditions. Under such conditions, the pressure and temperature of the liquid air marches upwards during charging due to warming from latent heat as the air condenses in the tank, and marches downwards during discharging due to cooling from latent heat absorption as the air evaporates and leaves the tank. It will be appreciated that while the cryogenic tank stores the liquid air, the liquid air itself may act as a thermal storage medium to store its own latent heat of condensation. This process can be made highly reversible leading to a higher round-trip efficiency than has previously been possible for a LAES system.

It should be appreciated that, in the present system, air is converted into liquid air from the right hand side of the liquid-vapour saturation dome such that the air in the form of superheated vapour is expanded down to intersect the saturated-vapour line of the saturation dome. The present system forces all the air to be condensed and stored in the tank (except for the vapour in equilibrium), such that it can all be returned. Furthermore, the superheated vapour should not experience a large change in heat capacity and hence any heat transfer processes that cool or reheat the air can be made more efficient since heat can be transferred over smaller temperature differences.

By contrast, conventional LAES systems are based upon expansion of supercritical air and throttling through a J-T valve on the left hand side of the saturation dome; on discharge, the liquid air is pumped out of the (e.g. unpressurised) tank as a liquid and heated back to a supercritical fluid. Such systems are based on conventional liquefaction techniques that aim to produce cryogenic liquid as product, but that are not designed with reversibility in mind. For example, cooling air isobarically from ambient at a pressure above the supercritical pressure will lead to a significant variation in its heat capacity making it difficult to achieve efficient thermal transfer, in contrast to cooling air on the right hand side of the dome. Conventional LAES systems thus suffer from a low round-trip efficiency of no more than about 50%.

The phrase "first stage power machinery" is used to refer to upstream machinery performing compression during charging (and expansion during discharging), and "second stage power machinery' is used to refer to downstream machinery performing expansion during charging (and compression during discharging). There may be further first stage machinery, or further second stage machinery, or indeed other power machinery, present in addition.

The first stage machinery will process air from an air inlet, usually at roughly constant temperature and pressure (e.g. ambient pressure and temperature), except for example for diurnal variations. The inlet air will normally always be less than 40°C and less than 2 bar, whereas the pressure and temperature at which the liquid air is stored at the other end of the flow pathway will usually be gradually rising during charging and falling during discharging.

The first stage and second stage power machinery may each comprise separate machines that perform the respective expander and compressor functions.

In one embodiment, the second stage power machinery comprises reversible machinery able to function as an expander and compressor.

In order to expand the air into the tank and to withdraw it from the tank and compress it, the second stage power machinery will be in fluid communication with the cryogenic tank. Thus, the flow pathway downstream of the second stage power machinery, ie between its outlet and the tank, will be at (or nearly at) the current equilibrium pressure of the saturated liquid/vapour air mixture in the cryogenic tank.

In one embodiment, at least one of the first and second stage power machinery, or, 5 further power machinery, comprises variable pressure ratio machinery so that along the flow pathway from the air inlet at least one pressure altering device is able to compensate for the varying pressure of the air emerging from and entering the cryogenic (marching LAIR) tank. The variable pressure ratio power machinery may be selected from, for example, turbo machinery or positive displacement machinery, including for example, 10 reciprocating piston based machinery.

Usually, the second stage power machinery will comprise variable pressure ratio machinery since that machinery is already in fluid communication with the tank, and usually, the machinery will automatically expand the gas down to the equilibrium pressure in the tank. For example, the power machinery may be axial machinery or piston based machinery that is configured to open automatically upon pressure equalisation with the pressure downstream of the second power machinery outlet. However, the second stage power machinery could operate over a fixed pressure ratio if a control system arranged for machinery elsewhere (e.g. first stage power machinery) to operate over a selectively varying pressure ratio.

The first (stage) machinery is likely to be large machinery operating over a fixed pressure ratio to provide significant compression/expansion work upon charge/discharge, respectively, and operatively coupled to a generator to generate electricity. This power machinery may form part of a gas turbine (GT) or gas turbine derivative; this may form part of a simple cycle gas turbine (OCGT) plant or a combined cycle gas turbine plant (CCGT).

A number of arrangements are possible for the flow pathway downstream of the first stage power machinery. The second stage power machinery may be in direct fluid communication with the tank such that it is directly coupled by the flow pathway to the tank and air is fed directly from it to the tank. Alternatively, the second stage power machinery may be in indirect fluid communication with the tank such that it is indirectly coupled to the tank by the flow pathway via intervening apparatus, the air being fed indirectly from the second power machinery via the intervening apparatus ultimately to the tank. The intervening apparatus may be heat transfer apparatus configured to provide isobaric cooling and heating.

Possible arrangements downstream of the first stage power machinery may 35 include:- (i) An arrangement with the second stage power machinery in indirect fluid communication with the cryogenic tank: (a) the first TES system ("hot store") for cooling and reheating the air, by storing/returning the heat of compression during charge/discharge, respectively; (b) optional heat exchanger for cooling to ambient and optional moisture 5 remover; (c) second stage power machinery with outlet in (indirect) fluid communication with the tank providing expansion into, or compression out of, the tank at a marching pressure ratio upon charge/discharge; (d)(i) cold store/regenerator for cooling and reheating the air, by storing/returning 10 sub-ambient heat at a marching inlet temperature during charge/discharge, respectively; or, (d)(ii) heat exchanger for cooling and reheating the air, by removing (optionally to temporary storage)/returning (optionally from temporary storage) sub-ambient heat at a marching inlet temperature during charge/discharge, respectively; (e) cryogenic tank.

(ii) An arrangement with the second stage power machinery in direct fluid communication with the cryogenic tank: (a) the first TES system ("hot store") for cooling and reheating the air, by storing/returning the heat of compression during charge/discharge, respectively; (b) optional heat exchanger for cooling to ambient (e.g on charging and/or discharging) and optional moisture remover; (c) further power machinery providing further compression/expansion work upon charge/discharge, respectively, at a selectively varying pressure ratio; (d)(i) hot store/regenerator ("hot store") for cooling and reheating the air, by storing/returning the heat of compression during charge/discharge; this will be subjected to a varying (above ambient) inlet temperature; (d)(ii) heat exchanger for cooling and reheating the air, by removing (optionally to temporary storage)/returning (optionally from temporary storage) the heat of compression during charge/discharge; this will be subjected to a varying (above ambient) inlet temperature; (e) optional heat exchanger for cooling to ambient and optional moisture remover; second stage power machinery with outlet in (direct) fluid communication with the tank providing expansion into, or compression out of, tank at a marching pressure 35 upon charge/discharge; (g) cryogenic tank.

(iii) An arrangement with the second stage power machinery in direct fluid communication with the cryogenic tank: (a) the first TES system ("hot store") for cooling and reheating the air, by storing/returning the heat of compression during charge/discharge, respectively; (b) optional heat exchanger for cooling to ambient and optional moisture remover; (c)(i) cold store/regenerator ("cold store") for cooling and reheating the air, by storing/returning sub-ambient heat (cold) during charge/discharge; (c)(ii) heat exchanger for cooling and reheating the air, by removing (optionally to 10 temporary storage)/returning (optionally from temporary storage) during charge/discharge; (d) second stage power machinery with outlet in (direct) fluid communication with the tank providing expansion into, or compression out of, tank at a marching pressure upon charge/discharge; (e) cryogenic tank.

In the above arrangements, an optional heat exchanger for cooling to ambient may be used on charging and/or on discharging, while the optional moisture remover may be used on charging.

In one embodiment, the second stage power machinery is in direct fluid communication with the cryogenic tank.

In one embodiment, the second stage power machinery is located directly upstream of the cryogenic tank and is configured to expand the air isentropically (or near isentropically) into the tank upon charging and/or withdraw and compress the air out of the cryogenic tank isentropically (or near isentropically) during discharging. Both the first and second stage power machinery will usually expand and/or compress the air as isentropically or nearly isentropically as possible, to provide a reversible change in pressure and temperature that can be used to absorb or generate power.

If it is intended to cool the air with a heat exchanger, it is simpler for the heat exchanger to be located upstream of the second stage power machinery so that it is not exposed to the marching pressures and temperatures that will exist between the second stage power machinery and the tank; in this way, such a heat exchanger may operate with fixed inlet and outlet temperatures.

In one embodiment, the second stage power machinery is in indirect fluid communication with the cryogenic tank.

Heat transfer apparatus providing isobaric (or near isobaric) heat transfer, for example, a thermal store or heat exchanger may be interposed between the second stage power machinery and the tank so that the air may be cooled isobarically or near isobarically into the right hand (high entropy) side of the saturation dome until it reaches the equilibrium temperature for that equilibrium pressure of the saturated liquid/vapour air mixture in the tank. By "near isobarically" it is meant that an individual packet of gas will be cooled along an isobar although, over time, successive packets of gas will of course enter the tank at progressively higher pressures due to the rising equilibrium vapour pressure in the tank.

During charging, the second stage power machinery will operate over a decreasing pressure ratio as the tank pressure marches upwards. So, over time, less cooling will be carried out by the (isentropic) expansion and more cooling will be required by the heat transfer apparatus. The heat transfer apparatus may, for example, be a thermal store storing at least some sub-ambient heat, i.e. a "cold store", and this will advantageously be a solid regenerator; a regenerator can accommodate the changing temperature range over which cooling is required with time. In the case of a heat exchanger, the counter-current flow will ideally need to vary in its inlet temperature to the heat exchanger during charging (as the temperature range over which the exchanger works will need to change during a charge cycle), and may optionally be actively controlled. Heat transferred on charging may be stored for subsequent return on discharging; for example, the heat exchanger may be coupled to one or more liquids stores, or a stratified liquid store where thermal energy is stored at respective successive temperature levels.

In one embodiment, a thermal store is interposed between the second stage power machinery and the cryogenic tank. The thermal store may comprise a regenerator comprising a porous (matrix or bed of) solid thermal storage medium. Cooling through a solid regenerator is highly preferred because the passing of successive packets of gas at marching temperatures ensures that thermal transfer processes occur over a small temperature difference. The tank end of the regenerator will automatically be maintained close to the current temperature of the liquid air in the tank, so that advantageously the saturated vapour will enter the tank at (if some condensation has occurred in the store) or very close to the temperature therein. Furthermore, the entire length of the (e.g. cold) store will inevitably be active in heat transfer due to the continually rising sub-ambient inlet temperature upon charging (associated with the continually rising vapour pressure of the liquid in the cryogenic tank) meaning that all the media along the store continuously rises in temperature.

In an alternative embodiment, a heat exchanger is interposed between the second stage power machinery and the cryogenic tank. Such a heat exchanger may, during charging, transfer the (e.g. sub-ambient) heat to a heat transfer fluid that requires the sub-ambient heat in real time to rewarm it, and vice versa upon discharge. Alternatively, the heat exchanger may be coupled to an (indirect) thermal store for temporary storage and return of the stored sub-ambient heat during charging and discharging.

In one embodiment, the cryogenic tank is an insulated, pressurised vessel. The cryogenic tank will usually be an insulated, closed (except for fluid communication with the flow pathway), pressurised vessel. The maximum operating pressure of the cryogenic tank during operation will be below the critical pressure of the air. During operation, the tank will usually be subjected to (and will usually only be built to withstand) a maximum operating pressure of less than 25 bar, or even less than 20 bar or 10 bar. For cost reasons, it may be preferable to configure the system so that the cryogenic tank is subjected to a maximum operating pressure of less than 5 bar. While the tank could be an unpressurised vessel operating only within a range of sub-atmospheric pressures, this has the disadvantage that the efficiency will be more significantly impacted by small pressure losses.

In one embodiment, the system is configured such that a selected minimum 15 amount of liquid air is always kept in the cryogenic tank as ballast air.

The presence of additional ballast air to absorb the heat of condensation will advantageously reduce the overall rate of marching of the equilibrium vapour pressure and temperature) in the tank, which is helpful for reducing the ranges over which the power machinery and heat transfer processes are required to operate. The selected minimum amount is likely to be at least two or three times, more likely four or even five times the normal amount transferred (e.g. the maximum amount transferred between the system being in one of its fully charged or discharged states, and the other of those states). It will thus be appreciated that such a tank is likely to be quite large, as well as being pressurised, in strict contrast to conventional LAIR tanks.

Other mechanisms may be used to reduce (but not eliminate) the rate of marching.

However, sub-systems that remove or return heat to the tank will increase complexity, while the presence of other ballast materials, for example, solid materials, are likely to have lower heat capacities than that of liquid air itself, leading to large storage volumes.

The LAES system may comprise a sub-system to replenish the ballast air by addition of liquid air at the same temperature and pressure as that in the cryogenic tank. The LAES system will usually comprise an adiabatic LAES (ALAES) system comprising said first thermal energy storage (TES) system. The first TES may be a direct or indirect TES. Conveniently, the first stage power machinery may operate over a fixed pressure ratio and hence, the first TES system may operate with a fixed inlet temperature 35 upon charging.

After the first TES system, optionally a first heat exchanger is provided to discard heat to ambient (usually on charge and discharge) and an optional moisture removal device may also be provided (for use on charge) to ensure that subsequent expansion downstream does not cause icing up of power machinery.

Optionally, the first stage power machinery forms part of a (conventional) gas turbine or gas turbine derivative, so that the hot high pressure air from the compressor is mixed with fuel (e.g. natural gas) and combusted in a combustor chamber heating the gas to a much higher temperature (e.g. 1400°C, 23 bar) before it is expanded back to atmospheric pressure in the turbine. The gas turbine derivative may be one in which the compressor and turbine are decouplable for separate operation and where gas flow paths are controlled by valving so as to permit alternative modes of operation.

The first stage power machinery is likely to operate over the normal working conditions for the gas turbine, for example, a fixed pressure ratio of not more than 30:1. The gas turbine or gas turbine derivative may form part of an OCGT or CCGT power plant. Whereas some prior art LAES processes rely upon recycling waste heat from the gas turbine exhaust to assist with reheating the cold air during discharge, in the present aspect a steam bottoming cycle/plant can be added after the gas turbine exhaust.

In the first aspect, there is further provided a method of storing liquid air in a liquid air energy storage system comprising:-storing liquid air in a cryogenic tank as a saturated liquid/vapour air mixture under 20 equilibrium pressure and temperature conditions; and charging the cryogenic tank by:-compressing air, cooling the air and storing the heat of compression, expanding the compressed air into the tank such that it enters the cryogenic tank as a gas (that is, through the superheated vapour region on the right hand side of the saturation dome), and condenses into the mixture causing a progressive increase in the pressure and sub-ambient temperature (i.e. warming) of the saturated liquid/vapour air mixture and in its liquid/vapour equilibrium phase change pressure and temperature; and discharging the cryogenic tank by:-drawing gaseous air out of the tank such that its evaporation causes a progressive decrease in the pressure and sub-ambient temperature (i.e. cooling) of the saturated liquid/vapour air mixture and in its liquid/vapour equilibrium phase change pressure and temperature, heating the air by returning the stored heat of compression, and expanding the air to produce work.

In the method, during charging, the air may be cooled near isobarically in the flow pathway upstream of the tank, for example, immediately before entering the cryogenic tank; this may be by means of a heat exchanger in the flow pathway upstream of the tank.

Alternatively, the air may be cooled near isobarically through a thermal store (e.g. cold regenerator) just before entering the cryogenic tank. As mentioned above, by "near isobarically", it is meant that an individual packet of gas will be exposed to nearly constant pressure, although, over time, successive packets of gas will enter the tank at progressively higher pressures during charging. Usually, the air will be cooled in heat transfer apparatus capable of adjusting the rate of heat removal over time.

The temperature range over which the air is isobarically cooled during charging, or 10 reheated during discharging, will usually change over time due to the marching conditions in the tank.

In the method, the air may be expanded near isentropically just before entering the cryogenic tank. The air may be expanded near isentropically, or rather, in a nearly reversible adiabatic manner into the right hand side of the saturation dome, usually through power machinery located directly upstream of the tank, preferably, to produce useful work. The gas should be expanded until it reaches the equilibrium temperature (and pressure) of the saturated liquid/vapour air mixture in the tank i.e. intersects the saturated-vapour line of the saturation dome; dry rather than wet expansion will be achieved, which is preferable to avoid the irreversiblilities associated with droplet formation and/or damage to the equipment (e.g. turbine blades).

There is further provided in the first aspect a liquid air energy storage (LAES) system comprising a flow pathway leading from an air inlet successively to: first stage power machinery, a first thermal energy storage (TES) system, second stage power machinery, and, a cryogenic tank for storing liquid air, wherein the system is configured such that the air enters and leaves the cryogenic tank in gaseous form and is stored in the cryogenic tank as a saturated liquid/vapour air mixture, whereby condensation of the liquid air during charging causes a progressive increase in the equilibrium vapour pressure and temperature of the saturated mixture, and evaporation of the liquid air during discharging causes a progressive decrease in the equilibrium vapour pressure and temperature of the saturated mixture.

2nd Aspect In accordance with the second aspect of the invention, there is provided a hybrid 35 energy storage system comprising a liquid air energy storage (LAES) system integrated with a pumped heat energy storage (PHES) based system; wherein the LAES system comprises a first flow pathway leading from an air inlet successively to: i) first stage power machinery configured to compress the air from the air inlet upon charging and, upon discharging, to expand the air to produce work; (before 5 discharging the air e.g. to an air outlet, or to other apparatus); ii) an optional first thermal energy storage (TES) system configured to store and return thermal energy to air passing through it upon charging and discharging, respectively; iii) a heat exchanger configured to cool and reheat air passing through it upon 10 charging and discharging, respectively, wherein thermal energy is removed by, and returned from a heat transfer fluid flowing through a second flow pathway in the heat exchanger; and, iv) liquefaction apparatus for liquefying, storing and re-vaporising the air, including a cryogenic tank for storing the liquid air, wherein the LAES system and PHES based system are coupled by means of the (common) heat exchanger, the second flow pathway forming part of an open or closed circuit of the PHES based system in which the heat transfer fluid is the circulating working fluid, wherein the PHES circuit comprises, in turn, first PHES power machinery, a first 20 PHES thermal store, second PHES power machinery, and the heat exchanger; wherein the PHES based system is configured to operate a gas-based thermodynamic cycle in which the working fluid circulates in a charging mode when the LAES system is charging, and in a discharging mode when the LAES system is discharging; and wherein the PHES based system is configured such that:-the first PHES power machinery compresses the working fluid during charging, and expands the working fluid during discharging; the first PHES thermal store receives and stores thermal energy from the compressed working fluid during charging, and returns thermal energy to the expanded 30 working fluid during discharging; the second PHES power machinery expands the working fluid leaving the first PHES thermal store during charging, and compresses the working fluid leaving the heat exchanger during discharging; and, the heat exchanger transfers thermal energy from the air in the LAES system to the 35 working fluid leaving the second PHES power machinery during charging, and transfers thermal energy to the air in the LAES system from the working fluid during discharging.

The provision of a cold thermal store for sub-ambient thermal energy storage in a prior art LAES or PHES system can be problematic. It is difficult to provide liquid cold stores for LAES systems that operate over this range as suitable liquids often have environmental or safety implications. If a solid sensible heat storage medium is used in 5 either system, the store may be large and expensive. For example, in a PHES system, the cold store may need to be twice the size of the hot store because the heat capacity of solids decreases with temperature. By integrating a LAES system and PHES system in accordance with the second aspect of the invention, it is possible to obviate the need for a cold store in either system. Accordingly, it is possible to very significantly increase the 10 energy density in each system.

The absence in the PHES based system of a cold store may allow it to operate as an over-pressurised system where, for example, the lowest pressure in the system is more than 2 bar, or even more than 3 or 4 bar.

A control system including synchronising apparatus may control the hybrid energy 15 storage system to synchronise simultaneous charging, simultaneous storage, and simultaneous discharging in the respective LAES and PHES based systems.

Either the mass flow rates through the PHES system or through the LAES system may be varied to ensure that there is optimum heat transfer between the two circuits. For example, on a hot day the mass flow rate through the LAES system may be reduced as the air density drops. In this case it is preferable to adjust the mass flow through the PHES circuit to match this. As there are likely to be different gases in each circuit the mass flow rates may be different, but balanced having regard to their respective heat capacities, so that they give and receive (thermal) energy at the same rate (equal rate of energy transfer). For example if Argon is used in the PHES circuit then there will be a significantly higher mass flow of Argon passing through the PHES circuit than air passing through the LAES circuit. The reason for this is that the heat capacity of Argon is almost half that of nitrogen or oxygen.

The LAES system and PHES based system may be configured such that they are both able to operate for at least a selected minimum charging time and minimum discharging time. Any thermal stores disposed in their respective first and second flow pathways may be configured to have a selected minimum storage capacity, as will the cryogenic tank in the first flow pathway, such that all stores/tanks are individually able to operate for at least the selected minimum charging time and selected minimum discharging time.

By "gas-based thermodynamic cycle" it is meant that the working fluid does not undergo any phase changes. The working fluid in the PHES based circuit may be atmospheric air, especially if the circuit is an open circuit. It may also be nitrogen or a monatomic gas, for example, one of the inert noble gases argon or helium.

The LAES system will operate conventionally such that the air passes from the air inlet successively through each of the apparatus listed in i) to iv) above, where such apparatus is present, during charging to be stored in the cryogenic tank (also hereinafter referred to as liquid air or LAIR tank), and whereby, upon discharging, the air usually follows a reverse path along the first flow pathway except for discharging to an air outlet, or to other apparatus.

Advantageously, the LAES system comprises an adiabatic LAES (ALAES) system 10 comprising said first thermal energy storage (TES) system (according to ii) aboveyln this way, a significant proportion of the energy stored in the LAES system will be stored as thermal energy.

Usually, a heat exchanger for cooling to ambient is provided downstream of the first TES system (with respect to the charging direction), to discard heat to ambient (usually on charge and on discharge), and an optional moisture removal device may also be provided (for use on charge) to ensure that subsequent expansion downstream does not cause icing up of power machinery.

As explained above, this second aspect provides an arrangement that may act as an alternative to a cold store in both systems. To this end, the heat exchanger will usually operate at least partially or fully over a sub-ambient temperature range, to remove and return sub-ambient heat, and may operate partially or fully over a sub-zero temperature range. Thus, in one embodiment, during charging, the temperature of the air leaving the downstream outlet of the heat exchanger may be sub-ambient, or even sub-zero (degrees centigrade). That temperature, during charging, may be even lower than -50°C, or -100°C, or -120°C, or even lower than -140°C.

The cryogenic tank may be a conventional LAIR tank configured to store the liquid air in an insulated tank at substantially constant pressure, for example, at or close to ambient pressure, an unpressurised tank being preferred. The LAIR tank may form part of traditional liquefaction apparatus in which cold, high pressure, supercritical air is expanded back to the pressure in the LAIR tank, for example, through a throttle (J-T) valve, so as to form mostly liquid air that is stored in the tank. A small fraction (-10% depending on the cold air temperature) will not be liquefied during charging. Upon discharging, the liquid air will be pumped up to a higher pressure liquid before being heated to form cold supercritical air again. The unliquefied air, effectively produced as a by-product of liquefaction, contains useful cold energy and may therefore be directed back though the apparatus in reverse flow to provide cooling to incoming air during the charging process.

Thus, in one conventional LAIR tank arrangement, during charging, unliquefied air (produced as a by-product of liquefaction) is directed (e.g. from the liquefaction apparatus) as a counter-current flow to cool air flowing in the first flow pathway towards the cryogenic tank. The unliquefied air may be directed through a counter-current flow passageway of a further heat exchanger that is disposed in the first flow pathway, or, may be directed through an additional counter-current (third) flow pathway of said heat exchanger as identified in iii) above.

In an alternative conventional LAIR tank arrangement, during charging, unliquefied air (produced as a by-product of liquefaction) is re-compressed and re-combined with air flowing in the first flow pathway towards the cryogenic tank. In this way, the mass of liquid air that is stored in the LAIR tank is conserved/optimised, such that it is available when discharging the system; this is advantageous in the case of adiabatic LAES, where total mass flow through any thermal stores in the first flow pathway should be as similar as possible during charge and discharge, in order fully to deplete the thermal stores of thermal energy.

Preferably, the air is re-compressed to the same or similar high pressure to that with which it is being combined. In view of the reduced mass flow rate, only a small compressor may be required for this function. If the air is hotter than ambient, and a heat exchanger rejecting heat to ambient is present in the first flow pathway, then that air is preferably recombined upstream of that heat exchanger.

Instead of a conventional LAIR tank, the system may comprise:-second stage power machinery to expand the air before entry to the cryogenic tank upon charging, and to withdraw the air from the tank and compress it upon discharging; and, wherein the system is configured such that the air enters and leaves the cryogenic tank in gaseous form, and is stored in the cryogenic tank as a saturated liquid/vapour air mixture under equilibrium pressure and temperature conditions, whereby condensation of the air in the tank during charging causes a progressive increase in the equilibrium vapour pressure and temperature of the saturated mixture, and evaporation of the air in the tank during discharging causes a progressive decrease in the equilibrium vapour pressure and temperature of the saturated mixture.

Thus, in one embodiment, a marching LAIR tank, as described above in relation to the first aspect of the invention, is incorporated in the hybrid system according to the second aspect, with the same attendant advantages.

In a marching LAIR embodiment, the second stage power machinery may comprise variable pressure ratio machinery.

In a marching LAIR embodiment, the second stage power machinery may be in direct fluid communication with the cryogenic tank. In this way, the heat exchanger is not exposed to the marching pressure and temperature in the LAIR tank as it is upstream of the second stage power machinery. However, in this embodiment, it is preferably selectively to adjust the outlet temperature of the air leaving the heat exchanger outlet upon charging, in order to ensure that it is at a temperature at which, upon (near) isentropic expansion in the second stage power machinery, the air will be expanded to a fully or close to fully saturated state (i.e intersecting the saturation dome and avoiding a wet expansion). Thus, the air outlet temperature should decrease during charging.

Other features described above in relation to the first aspect may also be incorporated in this marching LAIR embodiment such as, for example:-i. a solid regenerator may be interposed between the second stage power machinery and the cryogenic tank.

ii. the cryogenic tank may be an insulated, pressurised vessel; iii. the system may be configured such that a selected minimum amount of liquid air is always kept in the cryogenic tank as ballast air; and, iv. the LAES system may comprise a sub-system to replenish the ballast air by addition of liquid air at the same temperature and pressure as that in the cryogenic tank.

In one embodiment, either, or both, of the first and the second PHES power machinery comprises a reversible machine capable of acting both as a compressor and an expander.

Each of the PHES power machinery may be a single reversible machine capable of acting both as a compressor and an expander, or each may comprise separate respective compressor and expander to carry out the respective functions, for example, disposed in parallel flow pathways. The reversible machinery may comprise positive displacement reciprocating machine.

In one embodiment, the PHES circuit is a closed circuit and a gas buffer is provided in the PHES circuit selectively to remove gas from, or add gas to, the PHES circuit (e.g. so 30 as to maintain the pressures within a pre-designated range).

In a closed PH ES circuit, it may be desirable selectively to remove gas from, or add gas to, the PHES circuit, for example, to maintain gas pressure within at least a part of the PHES circuit within a predetermined range; in particular, it may be desirable to minimise pressure fluctuations causing temperature changes across the heat exchanger.

The gas buffer may be a constant pressure gas buffer, which may store the gas in a variable volume reservoir at the constant pressure for removal from or addition to the PH ES circuit.

Alternatively, the gas buffer may be a constant volume gas buffer that stores the gas at a varying pressure for removal from or addition to the PHES circuit, for example, as described in Applicant's published patent application GB2501795.

In one embodiment, the first thermal energy storage (TES) system comprises a direct TES.

In one embodiment, the first stage power machinery forms part of a gas turbine or gas turbine derivative (including a combustor to combust the returning air up to very high temperatures prior to expansion). The gas turbine or gas turbine derivative may form part 10 of an OCGT or CCGT power plant.

There is further provided a method of operating a hybrid energy storage system as specified above, comprising the steps of: i) operating the PHES based system in a charging mode when the LAES system is operating in a charging mode; and, ii) operating the PHES based system in a storage mode when the LAES system is operating in a storage mode; and, iii) operating the PHES based system in a discharging mode when the LAES system is operating in a discharging mode.

In one embodiment, the mass flow rate through the first and the second PHES 20 power machinery in the PHES circuit is selectively adjusted, during charging and/or discharging, in order to modify heat transfer to or from the LAES system in the heat exchanger.

The system may be configured, usually by means of a control system (e.g. including sensors), selectively to adjust the mass flow rate through (e.g. simultaneously through both) the first and the second PHES power machinery (e.g. to adjust the PHES circuit mass flow rate). The flow rate may be adjusted having regard to the mass flow rate in the LAES first flow pathway, for example, to match the mass flow rate in the LAES first flow pathway or to match a combination (the product) of mass flow and heat capacity if different gases are in each circuit.

The mass flow rate may be adjusted as between charging and discharging in the PHES circuit, for example, to select a first mass flow rate during charging and to select a second mass flow rate during discharging. This may be required if the mass flow rate in the LAES system differs between charging and discharging.

The mass flow rate may be adjusted during charging, if required, and similarly, the 35 mass flow rate may be adjusted during discharging. Again, this may be having regard to, or in order to match the mass flow rate in the LAES first flow pathway.

In one embodiment, the mass flow rate through the first and the second PHES power machinery in the PHES circuit is selectively adjusted, during charging and/or discharging, in order to modify heat transfer to or from the LAES system in the heat exchanger.

The hybrid energy storage system may be configured selectively to adjust the pressure ratio across the first and the second PHES power machinery.

The system may be configured, usually by means of a control system (e.g. including sensors), selectively to adjust the pressure ratio across the first and the second PHES power machinery (one half of the PHES circuit will be at a higher pressure (the other half at the lower pressure). In this way, the temperature of the PHES working fluid may be selectively adjusted (e.g. at a desired location in the PHES circuit, for example, at the end of the heat exchanger closer to the cryogenic tank). The pressure ratio may be adjusted having regard to the air outlet temperature of the air leaving the downstream outlet of the heat exchanger during charging, or having regard to the air inlet temperature of the air approaching the inlet of the heat exchanger during discharging.

The pressure ratio may be adjusted as between charging and discharging in the PHES circuit, for example, to select a first pressure ratio during charging and to select a second pressure ratio during discharging. This may be required if the air temperature in the LAES system differs between charging and discharging.

The pressure ratio may be adjusted during charging, if required, and similarly, the pressure ratio may be adjusted during discharging. Again, this may be having regard to changes in the air temperature in the LAES first flow pathway.

Ina preferred method, the pressure ratio across the first and the second PHES power machinery in the PHES circuit is selectively adjusted, during charging and/or 25 discharging, in order to modify heat transfer to or from the LAES system in the heat exchanger.

In a preferred method, temperature control apparatus is provided at a location within the PHES circuit and is used, during charging and/or discharging, selectively to raise or lower the temperature of the PHES working fluid at that location, in order to modify heat 30 transfer to or from the LAES system in the heat exchanger.

Temperature control apparatus may be disposed within the PHES circuit to selectively raise or lower the temperature of the PHES working fluid at the apparatus outlet. This may, for example, allow the inlet temperature of the working fluid entering the heat exchanger in the PH ES circuit to be selectively adjusted, during charging, which will correspondingly indirectly allow the temperature at which the cooled air exits the heat exchanger outlet to be adjusted before it enters the cryogenic tank.

The apparatus may be arranged in parallel with a bypass pathway so that it can be selectively bypassed if required, during charging or discharging. The apparatus may include a heat exchanger arrangement to provide selective isobaric heating or cooling; alternatively, the apparatus may include a heat pump arrangement, for example, with heating or cooling to ambient via a heat exchanger disposed between a (e.g. matched pressure ratio) compressor/expander pair (or expander/compressor pair) to provide selective heating or cooling.

In a further aspect, there is provided an hybrid energy storage system in which a first energy storage system is integrated with a pumped heat energy storage (PH ES) 10 based system, wherein the first energy storage system comprises a first flow pathway through which a first working fluid circulates in one direction upon charging and in the reverse direction upon discharging, wherein a heat exchanger is disposed in the first flow pathway and is configured to cool the first working fluid circulating in the one direction upon charging and to reheat the first working fluid circulating in the reverse direction upon discharging such that the thermal energy is transferred to and from a second working fluid flowing in counter-flow through a second flow pathway in the heat exchanger; and, wherein the first energy storage system and PHES based system are coupled by means of the (common) heat exchanger, the second flow pathway forming part of an open or closed circuit of the PHES system in which the second working fluid is circulating, wherein the circuit respectively comprises first PHES power machinery, a first PHES thermal store, second PHES power machinery, and the heat exchanger; wherein the PHES based system is configured to operate a gas-based thermodynamic cycle in which the working fluid circulates in a charging mode when the first energy storage system is charging, and in a discharging mode when the first energy storage system is discharging; and wherein the PHES based system is configured such that:-the first PHES power machinery compresses the working fluid during charging, and 30 expands the working fluid during discharging; the first PHES thermal store receives and stores thermal energy from the compressed working fluid during charging, and returns thermal energy to the expanded working fluid during discharging; the second PHES power machinery expands the working fluid leaving the first 35 PHES thermal store during charging, and compresses the working fluid leaving the heat exchanger during discharging; and, the heat exchanger transfers thermal energy from the first working fluid in the first energy storage system to the second working fluid leaving the second PHES power machinery during charging, and transfers thermal energy to the first working fluid in the first energy storage system from the second working fluid during discharging.

The hybrid system according to the further aspect may have any of the features detailed above in respect of the second aspect.

In particular, the first working fluid may comprise air and the first flow pathway lead from an air inlet, via at least the heat exchanger, ultimately to a liquid air cryogenic tank that forms part of liquefaction apparatus for liquefying, storing and re-vaporising the air. In 10 this variant, the first flow pathway may lead from the air inlet successively to: i) first stage power machinery to compress the air upon charging and to expand the air to produce work upon discharging; ii) a first thermal energy storage (TES) system to store and return thermal energy to air passing through it upon charging and discharging, respectively; iii) the heat exchanger; and, iv) the liquid air cryogenic tank.

The hybrid energy storage system may include synchronising apparatus to synchronise simultaneous charging, simultaneous storage, and simultaneous discharging in the respective first energy storage system and PHES based system.

As explained above in relation to the second aspect, the integration of two energy storage systems in this way, via a heat exchanger, provides an arrangement that obviates the need for a store in that location in each respective system, and may for example, eliminate the need for a cold store in that location. To this end, the heat exchanger will usually operate at least partially or fully over a sub-ambient temperature range, to remove and return sub-ambient heat, and may operate partially or fully over a sub-zero temperature range. Thus, in one embodiment, during charging, the temperature of the first working fluid leaving the downstream outlet of the heat exchanger may be sub-ambient, or even sub-zero (degrees centigrade). That temperature, during charging, may be even lower than -50°C, or -100°C, or -120°C, or even lower than -140°C.

There is further provided, according to this further aspect, a method of operating such a hybrid energy storage system comprising the steps of: i) operating the PHES based system in a charging mode when the first energy storage system is operating in a charging mode; and, ii) operating the PHES based system in a storage mode when the first energy 35 storage system is operating in a storage mode; and, iii) operating the PHES based system in a discharging mode when the first energy storage system is operating in a discharging mode.

In one embodiment, the mass flow rate through the first and the second PHES power machinery in the PHES circuit is selectively adjusted, during charging and/or discharging, in order to modify heat transfer to or from the first energy storage system in 5 the heat exchanger.

In one embodiment, the pressure ratio across the first and the second PHES power machinery in the PHES circuit is selectively adjusted, during charging and/or discharging, in order to modify heat transfer to or from the first energy storage system in the heat exchanger.

The present invention further provides any novel feature or combination of novel features hereinbefore described.

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 prior art, liquid air energy storage system; Figure 2 is a schematic diagram of a prior art, hybrid, liquid air energy storage, gas turbine power generation plant; Figure 3 is a T-S diagram showing the changes of state of air during the storage and recovery modes of the prior art hybrid system of Figure 2; Figure 4 illustrates an adiabatic liquid air energy storage (ALAES) system in accordance with a first aspect of the present invention; Figure 5a is a T-S diagram showing the changes of state of air during the storage and recovery modes in the system of Figure 4, and Figure 5b is an enlargement of the sub-ambient part of the T-S diagram of Figure 5a; Figure 6 illustrates a hybrid, adiabatic liquid air energy storage, gas turbine power generation system in accordance with a first aspect of the present invention; Figure 7 is a T-S diagram showing the changes of state during the storage and recovery modes in the system of Figure 6; Figures 8a, 8b and 8c are schematic diagrams showing the temperature profiles 30 with distance along the hot store and cold store of Figure 4 and the hot store of Figure 6, respectively, during charging; Figure 9 is a schematic diagram of a sub-system for replenishing the liquid air ballast in the liquid air storage tank of Figures 4 and 6; Figure 10 is a schematic diagram of a prior art, pumped heat energy storage 35 (PHES) system; Figures 11a,11bi and 11c illustrate a hybrid, adiabatic liquid air energy storage, gas turbine power generation system in accordance with a second aspect of the present invention, operating respectively, in power generation, charging and discharging modes, while Figure 11bii depicts a slightly modified apparatus operating in the charging mode; Figure 12 is a T-S diagram showing the changes of state during the charging mode in the system of Figure 11bi; Figures 13a, 13b and 13c are schematic diagrams of alternative pumped heat energy storage (PH ES) based sub-systems for use in accordance with the second aspect of the present invention; Figures 14a, 14b and 14c illustrate a further hybrid, adiabatic liquid air energy 10 storage, gas turbine power generation system in accordance with a second aspect of the present invention, operating respectively, in power generation, charging and discharging modes; Figure 15 is a T-S diagram showing the changes of state during the storage and recovery modes in the system of Figure 14; and, Figures 16a and 16b illustrate an adiabatic liquid air energy storage (ALAES) system, in accordance with a second aspect of the present invention, operating respectively, in its charging and discharging modes.

Figure 1 shows a conventional prior art, liquid air energy storage (LAES) system 10. During a storage mode, ambient inlet air passes into a compressor 12 where it is compressed before being cooled isobarically through a heat exchange and/or storage facility 16. The cooled pressurised air is then expanded through, for example, a throttle valve 18 to form liquid air which is stored in a cryogenic 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 heat 25 exchange and/or storage facility 16, where it is isobarically reheated and vaporised, before being expanded through a turbine 14 to produce work.

Figure 2 shows a prior art, hybrid system in which a LAES system 11 is incorporated into a gas turbine power production plant 30 (i.e. a GTI-LAES). The LAES system is the same as that of Figure 1 except that the compressor 12 and turbine 14 form part of a gas turbine which also includes a combustor 24 fluidly connected inbetween the compressor 12 and turbine 14 by valving (not shown) for selective connection. The compressor 12 and turbine 14 are respectively detachably mechanically coupled, by virtue of clutches 28, to a shaft leading to a motor generator 26.

The gas turbine 12/24/14 may operate in a normal gas turbine power production 35 mode, or in a storage mode where the turbine 14 is mechanically decoupled and valving (not shown) is switched to send compressed air from the compressor 12 towards the heat exchange and/or storage facility 16 and tank 20 for storage. In a discharge/recovery mode, the compressor 12 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 reheated in the heat exchange and/or storage facility 16 (after emerging as liquid air from the storage tank 20) towards the combustor 24, and thence to the turbine 14 for power generation, ideally, the pressurised air being supplied from the storage pathway at a similar temperature and pressure as it would have arrived directly from the gas turbine pathway. An example of such a hybrid system is disclosed in US6,920,759 mentioned previously, where other combinations of operating modes are also described.

Figure 3 is a T-S diagram showing the changes of state of air described in US6,920,759 during the storage and recovery modes conducted in the prior art, hybrid, liquid air energy storage, gas turbine power generating system of Figure 2.

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 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 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 on the left side of the dome. 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 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 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, whereupon it is combusted in a the gas to a much higher temperature (e.g. 1400°C, 23 bar) before it is expanded back to atmospheric pressure in the turbine.

The above descriptions of prior art LAES and GTI-LAES are simplified and the precise arrangement of the heat transfer or heat storage systems may vary and there may be various compressor or expander stages. For example, US6,920,759 either proposes a liquefaction/vaporising facility comprising high temperature, medium temperature and lower temperature heat transfer stages leading to an expansion valve and gas-liquid separator, or the use of cold solid regenerators which store and return cold thermal energy directly and in which gas liquefaction and vaporisation occur. Other prior art CES systems propose various modifications but a common aim is to recycle the cold of evaporation to assist in the liquefaction process, with a variety of methods/systems being proposed to accomplish this. In such systems, ambient air is usually stored in a liquid air storage tank at a fixed pressure, usually close to ambient pressure, and the liquid air is withdrawn from the tank by a liquid pump at the start of the power recovery mode and compressed (on the left hand side of the saturation dome) up to a highly pressurised liquid prior to vaporisation.

Figure 4 Figure 4 is an adiabatic LAES (ALAES) system according to the first aspect of the present invention. In contrast to prior art LAES systems, in this aspect, the liquid air is stored in a pressurised tank under varying conditions of pressure and temperature (denoted by double-headed arrow). The air both enters and leaves the tank as a gas that is forced to condense into the tank and where the latent heat of condensation is stored highly reversibly for subsequent return. The air is delivered to and held in the tank as a saturated vapour/liquid mixture under equilibrium conditions by virtue of single or combined power machinery upstream of the tank that is able to supply the gas at the progressively increasing pressure inside the tank (e.g. on charging). For high round-trip efficiency, a thermal store also upstream of the tank is arranged to store thermal energy efficiently at the progressively changing inlet temperatures associated with the varying pressure in the downstream flow pathway.

Figures 4 and 6 show, by way of example, two alternative combinations of power 35 machinery and thermal storage that may achieve the above result, with power cycles as shown in Figures 5 and 7 respectively, but other combinations operating other power cycles are also possible.

In both arrangements, in order to expand the air into the tank and to withdraw it from the tank and compress it, the second stage power machinery is in fluid communication with the cryogenic tank (e.g. the flow pathway leads to the open tank and, for example, no valves or pressure altering devices are present in the flow pathway to disrupt flow). Thus, the flow pathway downstream of the second stage power machinery, ie between its outlet and the tank, will be at (or nearly at) the current equilibrium pressure of the saturated liquid/vapour air mixture in the cryogenic tank.

In Figure 4, the system 110 comprises a compressor 12 and turbine 14 each detachably coupled to a shaft linked to a motor/generator 26. A three-way valve 32 located between the compressor 12 and turbine 14 allows selective fluid communication between each of them and a fluid pathway 34, which leads ultimately to storage via, respectively, a first heat store 36 comprising a thermal storage medium 38, a heat exchanger 40 and optional CO2 and moisture removal system 42, second stage power machinery 55, a second heat store 56 comprising a thermal storage medium 58, and an insulated cryogenic tank 120 for storing liquid air (LAIR tank).

In this embodiment, first and second heat stores 36 and 56 respectively store hot and cold. A heat exchanger 40 to ambient, or, an alternative heat exchange/cooling device such as a counter-current exchanger cooled by seawater, is preferably located in the fluid pathway between the stores to enable waste heat (generated by irreversibilities inherent in heat transfer processes) to be discarded, thereby minimising heat build-up in the cryogenic tank 120. In view of sub-ambient conditions downstream, it is also desirable to have a device 42 to remove CO2 and moisture at this stage using known methods, to avoid damage to downstream machinery.

In Figure 4, second stage power machinery 55 acting as an expander during charging is located downstream of the heat exchanger. This expands and cools the air (i.e. reversible adiabatic expansion) down to the pressure in the tank 120, with which it is in (indirect) fluid communication via the flow pathway, before the air passes through cold heat store or regenerator 56 for additional isobaric cooling to the saturated condition and passes into the tank. On discharge, the second stage machinery, which will usually be reversible machinery, acts as a compressor to draw the air back out and compress it. The second stage power machinery may be variable pressure ratio machinery. The flow pathway between the second stage power machinery and the tank 120 may contain intervening apparatus providing it does not hinder air flow so that the machinery outlet and (open) tank remain in fluid communication at the common pressure (of the tank) via the flow pathway (e.g. no valves).

During charging, the power machinery forces air to enter the tank 120 and condense as liquid air (e.g. by bubbling through the liquid) to produce a progressive increase in the pressure and sub-ambient temperature (i.e. warming due to introduction of heat of condensation) of the saturated liquid/vapour air mixture and in its liquid/vapour equilibrium phase change pressure and temperature. During discharging, gaseous air is drawn by the power machinery acting as a compressor out of the tank such that its evaporation causes a progressive decrease in the pressure and sub-ambient temperature (i.e. cooling due to withdrawal of heat for evaporation) of the saturated liquid/vapour mixture and in its liquid/vapour equilibrium phase change pressure and temperature, before being heated and expanded to produce work.

Thus, referring to Figure 4, during a charging/storage mode, ambient inlet air passes into the compressor 12 where it is compressed before being cooled isobarically through hot store 36, before passing through the heat exchanger 40, where it is cooled close to ambient temperature to discard waste heat in the system, and moisture remover device 42. The cooled pressurised air is then expanded through power machinery 55 acting as an expander to the current pressure existing in the (openly connected) tank 120, and passed through cold store or regenerator 66 where the air is further cooled isobarically, or near isobarically, stored, before passing into the cryogenic tank 120, where it is forced to condense under equilibrium conditions and is stored (mostly as liquid air) as 20 a saturated vapour/liquid mixture under equilibrium conditions.

During a recovery/discharge mode, gaseous air is drawn by power machinery 55 acting as a compressor out of the tank such that its evaporation (requiring latent heat) progressively cools the equilibrium mixture causing a decrease in the its liquid/vapour equilibrium phase change pressure and temperature of the saturated liquid/vapour mixture.

The gaseous air is drawn through the (relatively warmer) "cold" regenerator 66 and rewarmed before being compressed by the power machinery 55 up to a higher pressure (the usually constant pressure of the flowpath between the first and second machinery) and temperature, one usually close to ambient, with optional heat exchange to ambient, before being further isobarically heated by the hot store 36, and passed to the turbine 14 at a suitable inlet temperature for expansion therein to produce power.

Figures 5a & 5b Referring to the T-S diagram of Figure 5a, and the enlarged ambient section thereof in Figure 5b, this provides an example of a LAES cycle that may be run in the Figure 4 system in terms of the changes in state of the air working fluid. The cycle would be suitable for gas turbine integration. The hot compression is from standard atmosphere (15°C, 1.01325 bar) to 20 bar and is appropriate to the compression stage of a gas turbine; the hot store is thus at a constant 20 bar. The pressures within the cold liquid tank march gradually from 1.01325 bar (standard atmosphere) through 5, 10, 15 and 20 bar.

At the start of the charge mode, ambient air at point A is compressed nearly isentropically (i.e. nearly reversibly adiabatically) in compressor 12 up to point B at around 670k and about 20 bar, and is then cooled through hot store 36 isobarically to point C around ambient temperature, the last section including some cooling to ambient in the heat exchanger 40.

The pressurised, ambient air at point C is then expanded (as isentropically/ adiabatically as possible) in the expander 55 and cooled to about 122K, the vertical line from C to point D. The expander is in fluid communication with the LAIR tank and hence expands the gaseous air down to the current pressure in the LAIR tank (subject to initial conditioning steps -see below). In the first charge cycle, this expansion has the largest expansion ratio expanding from about 20 bar to about 1 bar at D (20:1) and provides the coolest and lowest pressure air that will enter the LAIR tank 120. Successive expansions expand the air less because the pressure in the tank rises with charging until the tank is fully charged at about 20 bar.

The cooled sub-ambient air at about 1 bar then passes through the cold regenerator 56, undergoing further isobaric cooling along the 1 bar line until, at point E, it reaches the saturated vapour condition (as it intersects the saturation dome on the saturated-vapour line). The saturated vapour is then passed through the thermal ballast liquid air (eg, bubbled through it) such that high-quality latent heat exchange may take place between the liquid and the saturated vapour air as it condenses.

Devices in the LAIR tank may assist the condensation process. An inlet at the bottom of the tank may be provided such that the liquid is caused to enter and bubble through the tank from the bottom, preferably through a bubble screen or similar device; if the vapour fails to condense, the pressure will inevitably rise causing a use in the condensation temperature at which condensation occurs such that this is now initiated and hence, this is self-limiting.

As the air condenses into the tank, the liberated heat of condensation will cause a slight overall increase in the equilibrium pressure and temperature of the saturated liquid/vapour mixture (depending on the quantity of liquid air present as "ballast" that may act as the "liquid thermal storage material"), such that the next packet of air to arrive will encounter a slightly higher pressure such that the expander does not need to expand the air packet as much as the previous packet. Hence, in successive charges, as depicted by way of example along the 5 bar and 10 bar isobars, the expander expands the air from point C down to point D' on the 5 bar isobar, and C to point D" on the 10 bar isobar.

Charging is discontinued when either the tank is full or has reached a desired maximum operating pressure beneath the top of the saturation dome (e.g. below the critical pressure), usually less than 25 or even less than 20 bar.

On discharging, the process operates in reverse (or in close approximation thereof) on the T-S diagram, such that the air vapour at say point E" is, in effect, boiled off the mixture by being drawn out of the tank (e,g from a top outlet) by the reversible machinery 55 acting as a compressor to lower the pressure between it and the tank with which it is in fluid communication along the fluid pathway 34. Thus, vapour at E" passes through the cold regenerator 56 and is subjected to near isentropic compression in power machinery 55, ascending at near constant entropy from D" to point C where it reaches roughly ambient temperature and 20 bar. After passing through the heat exchanger 40, where any waste heat may be discarded (i.e. there may be some irreversible warming to say -20 degrees above ambient in the power machinery), the gaseous air at 20 bar is reheated as it passes in reverse through the hot store 36 reaching close to the initial temperature of about 670K at point B. The high pressure, high temperature air at B is then expanded nearly isentropically through the turbine 14 to produce power which may be sent to the grid via generator 26.

The conditions depicted for the stores in the Figure 4 and Figure 6 systems according to the invention are summarised in Table 1 below.

Store Figures Gas Inlet Temp. Sensible Heat Transfer Latent Heat Transfer 36 8a -Constant Yes -hot store e.g. No (subject to layered particle bed fluctuations in ambient) 56 8b Marching Yes -cold store e.g. Yes, a little phase porous regenerator change (condensation) 66 8c Marching Yes -hot store e.g. No stratified liquid tank or porous regenerator N/A Marching No (or minimal) Yes, phase change

Table 1

In the Figure 4 system, heat is stored in as reversible a manner as possible in both stores 36 and 56 to provide high round-trip efficiency, while latent heat is stored alongside the liquid air in the LAIR tank in a highly reversibly manner, as described below, with additional liquid air being initially provided as thermal ballast (higher in heat capacity than solid ballast). The choice of a direct or indirect store will depend on various factors.

A "direct heat transfer" store may be in the form of a porous storage mass, which 5 may be a packed bed of solid particles through which the air passes exchanging thermal energy directly, or may be a solid matrix or monolith provided with HTF channels or interconnecting pores extending therethrough. Alternatively, 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); this may be a better design option where for example the 10 working fluid is at very high pressures, as then the latter may be confined to the pipework, without needing to build a containment vessel to contain the storage mass at a high pressure.

An "indirect heat transfer" store may comprise 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 again the thermal store need not be pressurised and could include a thermal storage medium such as a molten salt or high temperature oil. Such a store may include a stratified liquid or solid store for storing heat at different temperatures in distinct regions. Generally, solid stores are better suited to high temperatures and larger temperature ranges. Solid cold stores are likely to be large due to their lowered heat capacity, so that indirect liquid stores may be preferred, but these are dependent upon being able to select a suitable heat transfer liquid for the temperature range in question.

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 W02012/127178 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.

For a store with a fixed gas inlet temperature, a direct transfer layered store is likely to produce the most reversible heat transfer. Where a marching inlet temperature is involved, a simple packed bed or porous mass store without layers is simpler and may suffice, as more of such a store is thermally active. An indirect heat store where a heat exchanger is linked to a stratified, liquid store or other form of stratified store may also be used to store temperature in respective temperature regions or bands, such that the heat from those regions can be returned in reverse order.

Hot store 36 stores relatively high temperatures and is configured to store sensible heat and to operate with fixed inlet temperatures on charge and discharge. As such, it is preferably a store based on direct heat transfer, preferably with layered storage as described above, to allow selective gas flow paths dependent upon the progress of thermal transfer.

Figures 8a-8c Figure 8a shows how a thermal front progresses through a store such as hot store 36 that is storing only sensible heat at a fixed inlet temperature. The profiles represent the temperature profile in the porous mass at different states of advancing charge with this profile taking the form of an advancing front, or thermocline, moving through the store in the direction of the flow. The porous mass downstream (ahead) of the thermocline remains substantially at the original store temperature, whereas the porous mass upstream (behind) the thermocline is substantially at the gas inlet temperature. Both ahead of, and behind the thermocline, the store is thermodynamically inactive, ie, no heat transfer is taking place although gas is still passing through this material and pressure losses, and hence entropy generation will occur, which will reduce the overall reversibility of the process. The evident lengthening of the thermocline region as the charge progresses may result in a reduced proportion of the porous mass achieving the inlet gas temperature.

Such pressure losses and reduced usage may be mitigated by the use of a layered store structure.

The second heat store 56 of Figure 4 is a cold store that stores sub-ambient temperatures and stores sensible heat (as well as a small amount of latent heat due to condensation starting there) at inlet temperatures that increase upon charging and decrease upon discharging. In the present embodiment, cold store 56 is preferably a porous, solid regenerator.

Figure 8b shows how the temperature profile along the store changes with charging and shows the progression of the temperature profile of the vapour and regenerator with time during the storage mode (charge). The initial negative gradient reflects sensible heat transfer, while the flat downstream section reflects latent heat transfer. Both vapour and solid temperatures are shown and the marching of the L-V phase change temperature is evident on the right hand side. Initially, the store sees the coldest inlet temperature of about 122K at the largest expansion ratio at the start of charging; during charging, this rises to 288K as that ratio drops as the LAIR store 120 pressure rises. It will be noted that only a small temperature difference exists between the solid and vapour and also that the whole of the store is thermally active i.e. rising in temperature (unlike store 36 with a fixed inlet temperature where only one the region around the thermal front is thermally active) giving high density storage.

The marching in the regenerator happens if the regenerator/heat store is in direct fluid communication with the otherwise sealed LAIR tank 120 into which the vapour is condensing. This is typically achieved by passing the vapour though a body of the liquid condensate such that further condensation occurs at the bubble boundaries. As this is a closed vessel, as energy in the form of latent heat is added to this body of liquid, the temperature of the liquid rises. This also raises the equilibrium pressure between the vapour and the liquid, ie, the pressure in the liquid air tank increases as further condensation takes place. The vapour transiting the heat store, to continue to flow through the store, must therefore be supplied at progressively higher pressure as the pressure in the liquid air tank is increased. Since the air vapour entering the store is both dry and superheated, this means that the expander that is feeding the store, if it takes input vapour at a constant pressure, must operate over progressively decreasing pressure ratio. This pressure ratio is entirely dependent upon the vapour-liquid equilibrium pressure in the liquid air tank.

The function of the cold store is to cool the superheated air vapour isobaically (i.e. down along the isobar) until it is very close in temperature to the air mixture in the tank i.e. so that the store outlet temperature closely matches that temperature. With careful design, this can happen automatically in a direct store/regenerator. It will be desirable to provide a condensate collection apparatus, optionally using gravity, which arranges for any air that does condense to liquid in the cold store to drain into the LAIR tank 120.

The cold store could, however, be an indirect heat store and/or heat exchanger, but in that case the inlet temperature of the other heat transfer fluid entering the heat exchanger in counter-flow (which will result in a closely similar outlet temperature for the air leaving that exchanger) needs to be actively controlled (possibly with feedback sensors) so as to match the rising temperature in the LAIR tank as closely as possible; this ensures the equilibrium vapour pressure of the mixture rises as gradually as possible, and hence reversibly as possible. (Air arriving at a temperature that is significantly above that in the tank will cause an undesirably large increase in the equilibrium vapour temperature and pressure with an associated undesirably large temperature difference (delta T) between the air and the liquid in the tank.) For example, the indirect store may comprise a heat exchanger disposed in the working fluid pathway carrying a heat transfer fluid that moves, for example, between two liquid tanks, or a single stratified liquid tank, where hot thermal transfer liquid is added at the top on charging and removed from the bottom upon discharging, preferably with horizontal baffles disposed across the tank at vertical intervals to discourage convection in such a tank and allow storage of liquid at different respective heights and temperatures in an ordered manner, to allow recovery of heat in reverse order. However, such apparatus inevitably involves multiple heat transfer processes and hence, the use of a solid regenerator such as a simple particle bed or porous high surface area mass is preferred. Furthermore, it is difficult to find thermal transfer liquids that can operate within the sub-ambient temperature range and that are safe to use (e.g. proximity of stored hydrocarbon to a stored liquid air tank).

In any heat transfer process, it is desirable for the heat exchange temperature difference (which always leads to a non-reversible increase in entropy) to be small. Unlike a normal heat transfer process between a phase changing material and a non-phase changing material, where the former will hold itself at a constant temperature, while the latter heats up so that the temperature difference between them varies, in the present invention, liquid air can be stored under conditions in which the temperature difference upon phase change to liquid is a nearly constant increment. This is accomplished by the use of the marching equilibrium conditions in the LAIR tank preferably combined with the use of a solid (e.g. porous) regenerator for careful management of the incoming vapour temperature. Due to the constant changing of the temperature and pressure at which the phase change occurs, and the use of an intermediate regenerator which cools the vapour close to the point it condenses, saturated vapour may arrive in the tank only slightly warmer than the liquid to which it evolves its latent heat, ensuring as reversible a latent heat transfer as possible.

The Figure 4 system requires conditioning prior to first use so that the tank and regenerator achieve the correct (lowest) temperature and pressure for the fully discharged state. Prior to start up, it is necessary to fill the cryogenic liquid tank 120 with liquid air (or liquid nitrogen) to provide thermal 'ballast. This is likely to be about 5-6x the quantity of the actual liquid being stored each cycle i.e. total amount entering the tank on charge. Sufficient liquid air should be added to the tank to equal a fully charged system. The temperature of the system should then be allowed to rise as heat leaks into the tank. When the system is at peak pressure, start a discharge cycle for the second stage machinery only, i.e. evaporate air from the tank and pass through the regenerator. Air will enter the 3-3 regenerator at progressively lower temperatures, as the equilibrium saturated vapour pressure and temperature fall. Compress this air up to the pressure of the first store and then reject to atmosphere through a valve. When the pressure in the tank has fallen to its minimum normal operating pressure, a quantity of air equal to a normal discharge cycle should have been evaporated from the tank. The liquid air mixture in the tank is now at its coldest temperature and pressure and the regenerator has been cooled to a similar temperature. The cold store and LAIR tank are now conditioned for use and a charge cycle can now take place.

Figure 4 is one example of an arrangement with the second stage power machinery in indirect fluid communication with the cryogenic tank.

Possible arrangements downstream of the first power machinery may be: (a) the first TES system ("hot store") for cooling and reheating the air, by storing/returning the heat of compression during charge/discharge, respectively; (b) optional heat exchanger for cooling to ambient and optional moisture 15 remover; (c) second stage power machinery with outlet in (indirect) fluid communication with the tank providing expansion into, or compression out of, the tank at a marching pressure ratio upon charge/discharge; (d)(i) cold store/regenerator for cooling and reheating the air, by storing/returning 20 sub-ambient heat at a marching inlet temperature during charge/discharge, respectively; or, (d)(ii) heat exchanger for cooling and reheating the air, by removing (optionally to temporary storage)/returning (optionally from temporary storage) sub-ambient heat at a marching inlet temperature during charge/discharge, respectively; (e) cryogenic tank.

Referring to (d)(ii) above, the cold store may therefore be replaced by a heat exchanger.

Figure 6 Figure 6 shows a hybrid, LAES system incorporated into a gas turbine power plant according to a second embodiment. This system is more complex and expensive, but illustrates how alternative combinations of power machinery and stores may be combined in an LAES system including a LAIR tank maintained under marching equilibrium conditions.

The system 210 is a split gas turbine, where the compressor and turbine are 35 detachably coupled to one another for separate respective operation (for example, the compressor may operate in a charging mode without the turbine, or the turbine may operate without the compressor in a generation from storage/discharge mode), or there may be combined operation in a normal GT generation mode. An example of such a system is described in US5,778,675 to Nakhamkin.

Thus, the system 210 comprises a gas turbine with compressor 12, turbine 14 and combustor 24, where the compressor and turbine are each detachably coupled to a double-ended motor/generator 26. A three-way valve arrangement 32 located between the compressor 12 and turbine 14 allows selective fluid communication between each of them and a fluid pathway 34 downstream ultimately leading to the marching LAIR tank 120.

Downstream (during charging) of the compressor 12 there is disposed successively in a fluid pathway 34, a first hot store 36 comprising a thermal storage medium 38, a heat exchanger 40, an optional chiller/droplet separator 42 (for separation of moisture and CO2), reversible, second stage power machinery 75, a second hot store 66 comprising a thermal storage medium 58, a further optional heat exchanger 50 to ambient, third stage power machinery 65, and an insulated cryogenic tank 120 for storing liquid air (LAIR tank).

Store 36 is a hot store that stores only sensible heat at a fixed inlet temperature similarly to the hot store 36 of Figure 4.

Second stage power machinery 75 acts as a compressor during charging to deliver compressed gas to second heat store 66 at a varying selected store inlet pressure and temperature. That pressure needs selectively to rise with charging and be appropriate having regard to the extent of charging in the LAIR tank, so as to avoid a wet expansion for example (as discussed with reference to Figure 7); hence, a control system is needed, and optional sensors to provide feedback regarding the pressure or temperature in the LAIR tank, to set the (rising) pressure ratio in the compressor so as to match the marching pressure further downstream in the tank.

Store 66 is a further "hot" store that stores only sensible heat, but at a rising inlet temperature due to the varying pressure ratio in the compressor. Figure 8c shows how a thermal front progresses through the store. In the early stages of the charge, the thermocline comprises a short region of heated store with a negative temperature gradient in the direction of flow, followed by a region in which this gradient blends out to the original, un-charged store temperature. As the charge progresses, this temperature profile advances along the store with an unvarying form. The inlet temperature rises from about 300K to 480 K. In this embodiment, the store is sized and used such that the blend region is not allowed to exit the store so as to avoid the outlet temperature rising. As in the marching store (56) of Figure 4, every part of the store 66 is now thermally active and undergoing heat exchange because each new "packet" of gas encountered has a higher temperature than the last one. In this case, sensible heat is transferred to the mass, thereby cooling the gas, but it is still supersaturated and will not condense in the store.

In this embodiment, the store 66 need not be conditioned but is at ambient temperature before charging starts. Cooling of the re-compressed air through the store back to ambient occurs and stops once the store is full in the sense that further gas would lead to a significant rise in the outlet temperature.

A heat exchanger 50 discarding waste heat to ambient may be used at this point. The gas then passes to a third stage power machinery 65 where it is expanded down to the equilibrium pressure of the equilibrium mixture in the LAIR tank, with which the power machinery is in fluid communication. Power machinery 65, like 55 of the previous embodiment, has its expansion ratio governed by the equilibrium pressure of the LAIR mixture (akin to feedback) because it is is in fluid communication with the cryogenic tank.

In this embodiment, the gas is cooled and expanded rapidly under nearly isentropic conditions as it passes into the LAIR tank. This is different to the Figure 4 embodiment where a more gradual, isobaric cooling into the tank occurs but has the advantage that it does not require a cold store.

Figure 7 Referring to the T-S diagram of Figure 7, the changes in state of the air working fluid in the Figure 6 system will now be described. In the Figure 6 embodiment, prior to start up it is again necessary to fill the cryogenic liquid tank 120 with liquid air or liquid nitrogen to provide thermal 'ballast'. This is likely to be about 5-6x the quantity of the actual liquid being transferred/stored each cycle.

Note that all compression and expansion processes are shown as ideal, however this will not be achieved in practice and depending upon the type of machinery there will be an increase in entropy during each process. For example the first compression process could achieve the same pressure, but is likely to have a higher temperature than the ideal process.

A charge cycle can now take place in the Figure 6 system as follows: At the start of the charge mode, the constant entropy line extending vertically from point (a) represents compression of ambient air by the compressor 12 of the gas turbine power plant over a pressure ratio appropriate to a gas turbine, typically from around 15 to 30:1. This results in the air after the turbine having an elevated temperature. The heat from this air is then transferred to the first sensible heat store 36, preferably as described for the hot store of Figure 4 above.

On exit from this first sensible heat store 36, the air is still at approximately the 35 same high pressure but is close to ambient temperature. In order to provide a means of the removal of irreversible heat build-up, the air is then passed through heat exchanger 40 at point (b) that is cooled by ambient air or some other source of environmental cooling, eg, a water reservoir or stream. The air may then be passed through an optional chiller /optional droplet separator 42.

From point (b), at the start of charging for a short initial period the air is passed through compressor 75 without any compression (ratio 1:1), and hence any heat exchange in heat store 66 and heat exchanger 50 before passing to the expander 65, where it is expanded adiabatically until it reaches a saturated vapour condition, shown on the figure as a vertical (constant or near constant entropy) line descending from point (b) to intersect the saturation dome on the T-S diagram.

The air tank 120 is maintained to always contain at least a minimum quantity of liquefied air as thermal ballast. Saturated vapour is then passed through this thermal ballast (eg, bubbled through it) such that high-quality heat exchange takes place between the liquid and the saturated vapour. If the vapour passes through the ballast without condensing then the pressure in the tank/store will, inevitably rise. This results in a raising of the temperature at which condensation will occur and so the saturated vapour will then start to condense within the thermal ballast liquid. As condensation of the vapour releases latent heat, this raises the temperature of the entire liquid mass within the store and so, as described earlier, the phase change temperature and pressure march upwards as vapour is supplied to the tank and the temperature difference between the vapour and liquid during the phase change process becomes constant and minimum and hence highly reversible.

The air tank 120 now has a rising pressure and so successive feed of saturated vapour must occur at a higher pressure. This is achieved via the variable compression ratio power machinery 75 acting as a compressor after the heat exchanger 40 at point (b).

Thus, in successive charging cycles, the air follows normal fluid pathway 34 to machinery (compressor) 75, where it is compressed nearly isentropically, as indicated by compression from point (b) up to point (c), whereupon the compressed gas is again cooled within a second sensible heat "hot' store 66 at a higher pressure. After passing through an heat exchanger 50 to return the air to a near ambient temperature, the air is then expanded adiabatically by variable pressure ratio, second stage power machinery 65 from the pressure preselected by the machinery (compressor) 75 down to the current pressure in the tank. In order to avoid a "wet expansion" (droplet formation being undesirable), the machinery (compressor) 75 should compress the air to a pressure which, after isobaric cooling to ambient, preferably leaves the air at the correct entropy for an isentropic expansion to cause the expansion line to terminate at the current pressure and temperature in the tank which coincides with/ intersects the saturation dome (rather than further wet expansion below the dome to reach the current tank pressure). The simplest way to change pressure within the volume that is located between compressor 75 and power machinery 65 is to adjust the mass flow rates ie if the mass entering the space is greater than that leaving then the pressure will rise. The vapour is then passed through the liquid within the tank 120 further raising its temperature and pressure while itself condensing to liquid and adding to the mass of liquid air within the tank 120.

As both compression stages are work-in processes and require more work than is released by the expansion to the saturated vapour condition, the overall process is an energy storage cycle.

Recovery is the inverse of the charge process with the expansion to saturation being replaced by compression in reversible power machinery 65, before optionally passing through the heat exchanger 50 to ensure that it is at or close to ambient temperature prior to passage through, and heating by second sensible heat store. The reason that this heat exchanger may be useful is that with power machinery that is non-ideal and operating of large compression ratios, the air is likely to be substantially above ambient temperature. This heat is not useful and needs to be rejected from the system in some manner, hence the heat exchanger. Heat is now added to the gas by passage through this second heat store 66, prior to expansion in reversible power machinery 75 back to point (b) at 30bar (the first stage pressure), where the gas is passed through the heat exchanger 40 to ensure that it is at or close to ambient temperature prior to passage through, and heating by the first sensible heat store 36. This is a net work-release process. On exit from the first heat store 36, the gas is now in a similar condition to that found on exit from the turbine compressor and it may be passed to the turbine combustion chamber 24 prior to passing through the gas turbine expander 14. Optionally, the exhausted air may then pass through an HRSG for further power production in a steam cycle.

Figure 9 Successive use is likely to result in depletion of the ballast air in tank 120 so means for maintaining a minimum quantity of liquefied air (i.e. ballast) within the liquid tank may be provided. Figure 9 shows one example of an apparatus 313 that may be used to replenish the liquid air in tank 120. Ideally, liquid air should be added at or very near to (e.g. within 10K of) the temperature and pressure already in the tank, in order not significantly to disrupt the equilibrium conditions, especially if top-up is occurring during the LAES charge or discharge cycle. This may be from another tank where the liquid to be added is again at saturated equilibrium conditions. Conveniently, addition of liquid will occur when the tank is fully charged or fully discharged.

The apparatus 313 comprises a delivery tank 314 connected via a fluid connection/pipe including a liquid pump 302 and a shut-off valve 304 to a conditioning tank 312. This contains a heater 316 and a stirrer 318 and is connected by a different fluid connection/pipe via a shut-off valve 306 and one-way valve 308 to the liquid air tank 120. The delivery tank 314 could be an on-site liquefaction facility.

The filing process is as follows. Shut off valve 304 is opened and shut off valve 306 is shut. The pump 302 is turned on and a selected amount of liquid air is pumped from the delivery tank 314 into the conditioning tank 312. Usually the delivery tank will be at ambient pressure. Then the pump is turned off and shut off valve 304 is closed. Shut off valve 306 is opened. The heater 316 is switched on so that the liquid air in the conditioning tank 312 rises slowly in temperature, the rate being slow enough to maintain equilibrium conditions with assistance from a continuously operating stirrer 318 to mix the contents. The liquid air temperature and hence its equilibrium vapour pressure rises slowly. Eventually, those conditions in the conditioning tank 312 equilibrate with those of the store 120 (vapour pressures match etc.) so that air will transfer to the LAIR tank 120 via the one-way valve 308 with minimal disruption to the temperature and pressure conditions within the LAIR tank 120.

Once that tank has been suitably replenished, shut off valve 306 is closed, and the heater and stirrer in conditioning tank are turned off.

It will be appreciated that, in the first aspect, the present invention employs a highly reversible liquid-vapour phase change to be used to store energy in the form of liquefied air in a highly reversible manner, using power machinery similar to that which may be used in CAES or hybrid, gas turbine CAES power generation systems, but with the advantages of being much more compact and allowing the economic use of an insulated, pressurised storage tank rather than a structure such as a salt cavern, making a system so defined independent of specific geological characteristics.

Second Aspect The following embodiments relate to the second aspect of the present invention which is concerned with a hybrid energy storage system whereby two energy storage systems are integrated with one another so as to eliminate the need for a thermal store, usually a "cold store" (i.e. storing at least some sub-ambient heat), in either system. Some embodiments relate to both the first and second aspects.

The integrated energy storage system is derived from a pumped heat energy storage PHES system. Figure 10 shows an example of such a prior art pumped heat energy storage (PH ES) system operating according to a closed Brayton gas cycle. Such a system 35 is described in Applicant's published PCT application W02009/044139.

The system comprises first and second TES 136 and 116 and, in this example, reversible piston machinery 112 and 114 disposed within a working gas circuit. The TES each respectively comprise a pressure vessel containing a thermal storage medium through which the working fluid passes for direct transfer of thermal energy to, or from, the storage medium, where it is stored as sensible heat.

In a charge mode, using electrical power (e.g. supplied from the grid via a motor/generator), the working gas circulates in a clockwise direction. The gas leaves the top of TES 116 and flows through a heat exchanger rejecting any waste heat to atmosphere, before being compressed to a higher temperature and pressure by the machinery 112 acting as a compressor. The gas flows through TES 136 where it is cooled to near ambient and in the process transfers heat (the heat of compression) to the thermal media. The gas is further cooled to near ambient temperature by another heat exchanger, again to reject waste heat. Gas is expanded from near ambient to a lower pressure and sub-ambient temperature in expander 114. The now cold gas enters TES 116 at the bottom and is warmed up as it passes through the thermal media, before gas exits the TES 160 to pass around the system again. The work of the compressor 112 is greater than the work of the expander (turbine) 114 and hence this system requires work to be input to drive the process. This normally occurs via an electric motor and is how electrical energy is input to the system.

Charging therefore leads to heating of first TES 136 and cooling of second TES 116 relative to their respective starting temperatures such that, in effect, heat is "pumped" up a thermal gradient from the second TES to the first TES; these are often referred to as the cold store and hot store, respectively, since they respectively become colder and hotter during charging; however, whether or not the second store of a particular system operates at sub-ambient or sub-zero temperatures will depend on the working gas and pressure ratios selected. Charging may continue until the two stores are part or fully charged.

In a discharge mode, the working gas reverses the direction of flow through the TES circulating in an anti-clockwise direction. Gas leaves the top of TES 136 at the higher pressure and temperature (stored in the medium) and is expanded to a lower temperature and pressure by machinery 112 acting as an expander. The gas is further cooled by heat exchanger before it flows through TES 116 where it is cooled to sub-ambient temperatures and in the process transfers heat to the thermal media. The gas exits TES 116 and is compressed by machinery 114 acting as a compressor to a higher temperature and pressure. It is then cooled to near ambient temperature by the heat exchanger. The gas then enters TES 136 where it is now heated up to high temperatures. The now hot gas exits the TES 136 and can pass around the system again. The work of the compressor 114 is less than the work of the expander 112 and hence this system generates power that can be converted to electricity. This normally occurs via an electric generator and is how electrical energy is output from the system. The amount of energy that can be stored is limited by the store sizes and cost.

Figures 11a, llbi, llbii, 11c These show a hybrid GTI-ALAES power generation system 310 with a second stage compression and associated thermal storage, but thereafter incorporating a PHES based energy storage sub-system, instead of a cold store, prior to LAIR tank 220.

The upstream gas turbine may be a split gas turbine as described earlier in relation to the embodiment of Figure 6.

Alternatively, the gas turbine may be a standard GT that can allow air injection (from the LAIR storage) or air withdrawal (to LAIR storage) post the compressor, so that in addition to a normal GT generation mode, an augmented generation mode may operate where the GT is still operating (at less than full capacity for example because of altitude or raised ambient temperatures) but air from LAIR storage is injected at or upstream of the combustor to augment the normal GT air flow from the compressor to the combustor, and a storage mode may operate where some of the air from the GT compressor is diverted to LAIR storage. US5,934,063 to Nakhamkin describes such a modified GT. It requires suitable fluid connections (e.g. ports/inlets/outlets) provided in the gas turbine for air injection and withdrawal and suitable valve structure and flow pathways to and from air storage. In this case, all modes of operation require the GT to be operating.

Also, an additional (usually smaller as it is only processing diverted air flow) compressor may be provided in parallel with the compressor of the gas turbine, so that charging to storage can occur when the GT is not running. This is also described in US5,934,063.

In Figure 11a, the system is acting in a normal gas turbine (either OCGT or CCGT) generation mode, where gas is burnt in combustor 24 to generate power and the flowpath way to storage and associated components are inactive.

In Figure 11bi, the system 310 is acting in a charge mode. Compressor 12 is compressing air to a higher pressure, normally that associated with the Gas Turbine peak 30 pressure ie 15-20 bar or more broadly 10 -30 bar depending upon the type of GT. Thermal store 36 is a direct thermal store that is storing the heat of compression. Heat exchanger 40 cools the exit air to near ambient. In doing so there will be a certain amount of condensation formed. After heat exchanger 40, any remaining moisture is removed in moisture removal device 42, together with any CO2. One skilled in the art will 35 understand there are a number of different technologies that can achieve this that are used regularly in the air separation industry.

The now dry air is compressed to a higher pressure in compressor/expander 75 which may be either a single reversible machine or two (or more) machines in parallel, where at least one can work as a compressor and one as an expander.

The hot compressed high pressure air is passed through (hot) thermal store 166 (also direct) where the heat of compression is stored. It will be understood that thermal store 166 could also be configured as an indirect thermal store ie with a heat exchanger to transfer heat to/from the compressed air stream and into a thermal storage media (ie a mineral oil) that will store the heat in a separate lower pressure vessel.

After "hot' thermal store 166, the air is further cooled to close to ambient in heat exchanger 50. The now high pressure dry air (which is at a pressure that is above the critical pressure in the supercritical region of 40 to 100 bar (even higher pressures may be used eg 200 bar) is cooled as it passes through heat exchanger 90. This is in counter-flow with the gas in the PHES sub-system 200 and also the cold return air stream 121 (discussed below) from the low pressure liquid tank 220 (at or close to ambient pressure).

However, the majority of the cooling is provided by the PHES sub-system 200.

PHES based sub-system 200 is similar to the prior art system of Figure 10 except that there is no cold thermal (low pressure) store. Instead the gas in the PHES circuit during charging is heated back up by heat exchanger 90 while in counter-flow with the dry high pressure air.

The PHES based sub-system charges as described for the prior art system. The working gas is compressed from near ambient (note the dry high pressure air is at near ambient having passed through heat exchanger 50) to a higher pressure by compressor/expander 212. Many different gases may be used in this circuit, however a monatomic gas, such as argon, allows for a lower pressure ratio for a given temperature rise. If for example the working gas is Argon and the compression ratio in the region of 10:1, then the temperature of the gas post compression will be around 500 °C.

The hot compressed gas passes through thermal store 236, which is a direct thermal store and the heat is stored within thermal media 238. In this way cooled high pressure gas exits the thermal store and is further cooled in heat exchanger 240 so that it is close to ambient. This cooled high pressure gas is now expanded to a lower pressure and temperature. The work of expansion generated by expander/compressor 214 is used to offset some of the work of the compressor/expander 212. The cold gas passes through heat exchanger 90 where it is heated back up to ambient in counter-flow with the high pressure dry air.

The system is similar to the prior art system during a charging (energy storage) mode of operation. However, the energy is stored only within a hot thermal store during charging. The remaining power input to the system to drive compressor/expander 212 comes from an electric motor (not shown), which is how the system converts electrical work to heat.

The compressor/expander 212 and expander/compressor 214 may each be a reversible positive displacement reciprocating machine. Alternatively they may consist of two or more different machines with suitable ducting and valves, which may be made up of different types of compressor or expanders, such as turbo-compressors, axial flow compressors, centrifugal compressors, sliding vane or screw compressors.

The now cold high pressure dry air passes through throttle (JT) valve 123, where the pressure is dropped back to that of low pressure liquid air tank 220. Normally this will be at or around ambient pressure for a traditional LAIR tank where the air is held at constant pressure. Depending upon the temperature of the high pressure air, the majority of it will be converted to liquid air as it passes through the throttle valve. This liquid air is added to the liquid air 124 already in the tank 220. The remaining unliquefied cold, but gaseous, air (i.e. by product of liquefaction) will be fed back through return pipe 121 into heat exchanger 90, where it will be heated back up to ambient temperature while in counter-flow with the high pressure dry air. As has been previously explained, it is preferable if the majority (>90%) of the cooling should be provided by the PHES subsystem.

Figure 11bii shows a slightly modified system where an alternative approach is used, and where the unliquefied cold gas is redirected and recompressed in a small additional compressor 125 (note the mass flows through this device will be much lower than through compressor 12) up to the same high pressure as the air post thermal store 166. This high pressure air is then added to the main high pressure air flow either before or after heat exchanger 50. The air should be injected before heat exchanger 50 if the temperature of the injected air is likely to be above ambient temperature. The advantage of this approach is that the mass of liquid air that is available when discharging this system is broadly equal to the amount that is required to fully deplete thermal stores 36 and 166. If, for example, the system of Figure 11bi was used and only 90% of the air was stored as liquid air then only 90% of the heat in the thermal stores can be used upon discharge. The disadvantage of this 11 bii system is that it requires additional machinery in the form of high pressure compressor 125. The size of this machinery can be kept low if the high pressure air entering the throttle (JT) valve 123 is sufficiently cold. For example if the air is at -170° C then at least 90% of the high pressure air should turn to liquid upon expansion through the valve.

In this way, upon charging the system, energy is stored in hot thermal stores 36, 166 and 236, while liquefying a quantity of air. The advantage of this approach is that it eliminates the need for either the PHES system or LAES system to have any cold thermal stores. Hence the system will have a much higher energy density than either a separate LAES system or PHES system. For example in a standalone PHES system the mass of the cold storage media is normally almost twice that of the hot storage media. Consequently, this PHES based sub-system 200 stores the same quantity of energy as a standalone PHES system, but only requires one third of the thermal media ie the energy density is now three times higher in this system. Likewise for a LAES system, if a cold sensible heat store (with solid media) is used in place of heat exchanger 90 then the thermal storage mass of this cold store will normally be greater than the combined thermal mass of both hot stores. Hence, the energy density of the LAES part of the cycle is also much higher.

Figure 11c shows the system in a discharging and generation mode. Liquid air 124 is pumped as a liquid up to a high pressure in pump 122 and then passed, as a supercritical fluid, through heat exchanger 90 where the supercritical air is warmed while cooling down a counter-current flow of gas in the PHES sub-system 200.

The PHES based sub-system 200 in discharge mode works as follows. A working fluid is cooled as it passes through heat exchanger 90. It is then compressed to a higher pressure in expander/compressor 214 before passing through heat exchanger 240. There are certain irreversible losses associated with compression/expansion and heat exchange between the gas and the air streams. If the same pressure ratio is used on charge and discharge for the PHES sub-system, then it is very likely that the temperature of the gas post compression by expander/compressor 214 will be well above ambient temperature. This warm gas will pass though heat exchanger 240 where it will be cooled to close to ambient temperature before passing through hot thermal media 238 where the gas is heated to a high temperature. It will be understood by one skilled in the art that heat exchange does not need to happen on both the charge and discharge cycles, but it is often beneficial for efficiency reasons. The hot high pressure gas leaves the thermal store 236 and is expanded by compressor/expander 212 back to the starting pressure, where it can again be cooled as it passes through heat exchanger 90. The compressor/expander 212 acting as an expander can drive both the expander/compressor 214 (acting as a compressor) and an electric generator (not shown).

It should also be understood by one skilled in the art that the pressure ratio of the PHES sub-system may be varied between charge and discharge. This will have the effect of changing the temperatures of the gases before/after compression/expansion from that which a fixed pressure ratio system would have. For example, if the pressure ratio on discharge is increased, then the temperature of the gas after expansion in compressor/expander 212 may be lower. Likewise the temperature post compression in expander/compressor 214 may be higher.

In the LAES flow pathway, the heated high pressure dry air passes through heat exchanger 50, where it may exchange heat with ambient if it is above ambient temperature (note this depends upon the temperature of the gas entering the heat exchanger 90 from the PHES sub-system part of the system). This hot high pressure air is further heated as it passes through thermal store 166. The hot dry high pressure air is then expanded in compressor/expander 75 where it generates power by driving a generator (not shown). It is cooled (if necessary) as it passes through heat exchanger 40 and then it is further heated as it passes through thermal media 38. The hot high pressure gas then enters combustion chamber 24 where gas is added and combusted to raise the temperature of the mixture. This is then expanded through a gas turbine. As is understood the hot exhaust can then be passed through a HRSG to extract useful work from the remaining heat by driving a steam 15 turbine.

Figure 12 Figure 12 shows a Temperature-Entropy plot of the LAES part of the system in Figure 11. It is assumed in this LAES system that the charge and discharge pressures are kept broadly the same between charge and discharge. However, it will be understood by someone skilled in the art, that it is possible to vary the charge and discharge pressures and they may differ if the liquid is compressed to a higher pressure upon leaving the tank. For example the charge pressure might be only 80 bar and the discharge pressure 100 bar. Changing the pressure ratio means that the compressor/expander 75 will have to work over a different peak pressure and pressure ratio. It is assumed that the pressure range of the GT will be left broadly unchanged if the GT is based upon a standard industrial GT design. However, this is not an essential feature and there may be reasons why varying the pressure ratio of the first or both stages is preferable.

i) Upon charge, Figure 12 shows:-a-b First stage compression through compressor 12 b-c Cooling of the air in the first "hot" thermal store 36 and heat exchanger 40 c-dl Second stage compression in compressor/expander 75 dl-d2 Cooling of the air in the second "hot' thermal store 166 d2-d3 Further cooling of the air in heat exchanger 50 d4 This is the temperature of the cold air from the liquid tank 220 that is re-compressed to add it back into the main airflow. It can be seen on the T-s diagram that this is likely to be colder than the air post heat exchange (unless the compressor is very inefficient). As this return flow should be both quite small and cooler than the main flow at this point it is likely to be mixed in with main airflow at point d3, where it will warn up, but in the process slightly cool the main airflow.

d3-e Further cooling through heat exchanger 90 e-f-g Gas is expanded through throttle (JT) valve 123 and converts a proportion to liquid air. Some of the air becomes gaseous and it is this air which is recompressed and returned to the main flow (see Figure 11 bii).

In the version of the system shown in Figure 11bi, the exiting air passes up the line i-a where it is heated in counter-flow with high pressure air passing from d3-e.

ii) Upon discharge, Figure 12 shows:-j-e liquid is compressed to high pressure (which raises the temperature slightly) e-d3 High pressure liquid air is further warmed up in heat exchanger 90 and is converted to supercritical air d3-d1 high pressure supercritical air is further heated in second "hot thermal store 166 dl-c high pressure supercritical air is expanded in compressor/expander 75 c-b gas is further heated up in first "hot" thermal store 36 At this stage in the cycle the gas at point b enters the combustor and is heated to significantly higher temperatures. Note the cycle may complete the b-a cycle in reverse if it 20 is an Adiabatic Liquid Air Energy Storage system ie where there is no gas combustion and no integration with a gas power plant.

Figures 13a to 13c These figures depict three alternative configurations for a PHES based subsystem/cycle, although many other configurations are also possible.

Figure 13a shows an embodiment as previously described with reference to Figure 11a, etc. Figure 13b shows an embodiment where there are two thermal stores and two stages of compression and heat exchanger 250 to ambient. For a given peak temperature this will give a lower cold temperature post expansion upon charging, allowing the air in counter-flow to be cooled to a lower temperature. For example the first thermal store could have a peak temperature of 500°C and be a direct thermal store. The second thermal store could be an indirect thermal store with a peak temperature of 300°C; it may be a liquid store where the liquid is also the heat transfer medium. Some examples of potential liquids that may be used (depending upon the temperature range required) are water, mineral oil, synthetic oil or molten salts. Using Argon as the working fluid, the pressure in the cold limb of the circuit could be 2 bar, the first thermal store could be at 18 bar (ie a pressure ratio of 9:1), the second stage pressure in the heat exchanger could be 42 bar (ie pressure ratio 2.33:1) and the temperature post expansion could be 99K and 2 bar pressure.

In certain circumstances it may be beneficial to increase the amount of heat rejection that occurs after the ambient heat exchanger. For example, by a further heat 5 pumping process that increases the amount of heat being rejected from this system. It allows a colder temperature to be achieved post expansion than would otherwise be possible. Temperature control apparatus may be provided in the (flow path of the) PHES based circuit to achieve this and it may be provided in parallel with a bypass circuit, in order that it can be selectively used.

Figure 13c shows an embodiment that is designed to increase heat rejection in the PHES based sub-system during either the charge phase, the discharge phase or both. There are certain irreversible losses within the system that lead to an increase in entropy. This entropy increase must be rejected from the system to allow steady state operation. This is achieved by heat rejection to ambient, normally via heat exchanger 240 or 250.

Figures 14a-14c Figures 14a -14c show a hybrid GTI-ALAES power generation system 410 incorporating a PHES based sub-system according to the second aspect, and where the LAES system incorporates a marching LAIR tank according to the first aspect.

The system comprises the first stage GT power machinery and associated further thermal store 36. The flow pathway 34 downstream thereof leads again to heat exchanger 40 and moisture removal device 42, and then passes through a common heat exchanger 90 that is integrated with the PHES sub-system before a second stage power machinery 76 acting as an expander upon charging, which expands the air into a marching LAIR tank 120.

Expander/compressor 76 may be a single reversible positive displacement reciprocating machine. Alternatively it may comprise two or more different machines with suitable ducting and valves, which may be made up of different types of compressor or expanders, such as turbo-compressors, axial flow compressors, centrifugal compressors, sliding vane or screw compressors.

Using a marching LAIR tank 120 has the advantage of being a more reversible system and is likely to lead to a higher round-trip efficiency. In contrast to the Figure 11 embodiment, there is no loss of mass from the system: all the liquid air arriving upon charging is held in the tank (nearly all condensing as liquid) and is subsequently returned through the stores upon discharge. On the other hand, the LAIR tank 120 needs to be designed to be both pressurised and also significantly larger than the unpressurised LAIR tank of the Figure 11 system. The reason for the increase in size is the desirability for (liquid) thermal ballast in the form of additional liquid air in the tank to reduce the rate of the marching pressure and temperature. This ballast may be in the region of 4-5 times the working mass of the tank.

In Figure 14a, the system is working as a normal gas turbine generation system as 5 previously explained for Figure 11.

In Figurel4b, the system is working in a charging mode. Compressor 12 is compressing air to a higher pressure, normally that associated with the Gas Turbine peak pressure ie 15-20 bar or more broadly 10 -30 bar depending upon the type of GT. Thermal store 36 is a direct thermal store that is storing the heat of compression. Heat exchanger 40 cools the exit air to near ambient. In doing so there will be a certain amount of condensation formed. After heat exchanger 40 any remaining moisture is removed in device 42 along with any 002. One skilled in the art will understand there are a number of different technologies that can achieve this that are used regularly in the air separation industry. The dry pressurised air is cooled as it passes through heat exchanger 90. This is in counter-flow with the gas in PHES sub-system 200.

PHES based sub-system 200 is similar to the PHES sub-system described in Figure 11, and charges and discharges in the same way, again, co-terminously with the charging and discharging of the LAES system respectively.

There is a difference between the operation of this PHES based sub-system 200 and that of Figure 11. In this system, it is preferable that the dry compressed air stream is cooled to a lower temperature (i.e. the outlet temperature from heat exchanger 90 in the LAES flow pathway 34 is progressively lowered over time) as the system is charged. This is explained further in Figure 15, but it improves efficiency if the cold air expanded in expander/compressor 76 is expanded to a fully saturated state.

This reduction in exchanger outlet temperature as the charge cycle progresses can be achieved in a number of ways, for example:- (i) by increasing the pressure ratio across the PHES based system as it is charged; (this is described below using the Fig. 13a arrangement) by changing the amount of heat rejected in a second stage (this is 31 described below using the Fig. 13c heat pump/bypass arrangement).

The cold compressed air is then expanded in expander/compressor 76 to the pressure of the pressurised cold liquid tank 120. If the (e.g. preselected) temperature of the cold compressed air pre-expansion is correct, then the air can be expanded to a fully or close to fully saturated state. The saturated gas may be bubbled through the pressurised liquid 224 in order to condense it out. Any liquid that does not condense will increase the pressure in the tank and thereby provide feedback to increase the rate of condensation. The result is that the pressure and temperature within the tank march upwards as more and more liquid air is condensed out. To reduce the rate at which the pressure and temperature marches upwards it is desirable to have a certain amount of ballast, preferably, liquid air 'ballast'. This consists of liquid air that remains in the tank even when fully discharged. It may equal 4-5 times the amount of working liquid added to the tank or removed in each charge cycle, and should be selected so as to ensure the fully charged vapour pressure in the tank when a selected amount of energy/air has been stored does not exceed the maximum design pressure of the tank or other equipment within the process.

Figure 14c shows the system in a discharging mode. Air is boiled off from the liquid air 224 that is kept within pressurised tank 120. This air is then compressed by expander/compressor 76 up to the same pressure as the GT needs to operate. The compressed air is further heated up to near or just above ambient in heat exchanger 90, heat being supplied by PHES sub-system 200. The compressed air is further heated in thermal store 36 before entering combustion chamber 24 where gas is added and combusted to raise the temperature of the mixture. This is then expanded through a gas turbine. As is understood the hot exhaust can then be passed through a HRSG to extract useful work from the remaining heat by driving a steam turbine.

As the liquid air in the tank boils off the temperature and pressure will fall. As has 20 previously been discussed, it is preferable to adjust the PHES sub-system circuit so that the two cycles are closely matched in terms of mass flow and heat capacity.

One of the reasons for the improved efficiency offered by having a varying pressure liquid air tank 120 is that the heat capacities of the (superheated) air and the gas in the PHES sub-system circuit are likely to be fairly constant over the temperature range involved in heat exchanger 90. Consequently it is relatively simple to ensure that there is a balanced heat flow between the two circuits. This is not the case when air is at a supercritical pressure, as in the case of the constant pressure LAIR tank embodiment of Figure 11, where there can be significant variations in heat capacity with temperature.

The overall aim is to maximise the amount of energy recovered and this is normally achieved by ensuring that where heat rejection does occur it is close to ambient temperature.

Figure 15 Figure 15 shows a Temperature-Entropy plot of the LAES part of the system in Figure 14 showing the changes of state in the air. Note an actual system may deviate from this due to various irreversible losses. What is shown, by way of example, is an ideal system that does not include these losses. The liquid air tank pressure will be very close to the saturation pressure of the liquid in the tank. (Point B is merely indicated by an upwards arrow because it is much higher than the scale of the plot allows.) i) Upon charge in the LAES system: a-b First stage compression through compressor 12 b-c Cooling of the air in the first thermal store 36 and heat exchanger 40 c-dl Cooling of the air in heat exchanger 90 at start of charge c-d2 Cooling of the air in heat exchanger 90 at mid-charge c-d3 Cooling of the air in heat exchanger 90 at end of charge d1-el Expansion of cold compressed air in expander/compressor 76 at start of charge d2-e2 Expansion of cold compressed air in expander/compressor 76 at mid charge d3-e3 Expansion of cold compressed air in expander/compressor 76 at end of charge el-fl Condensation of liquid air at start of charge e2-f2 Condensation of liquid air at mid charge e3-f3 Condensation of liquid air at end of charge ii) Upon discharge in LAES system: f3-e3 Evaporation of liquid air at start of discharge f2-e2 Evaporation of liquid air at mid discharge fl-el Evaporation of liquid air at end of discharge e3-d3 Compression of cold compressed air in expander/compressor 76 at start of discharge e2-d2 Compression of cold compressed air in expander/compressor 76 at mid discharge el-dl Compression of cold compressed air in expander/compressor 76 at end of discharge d3-c Heating of the air in heat exchanger 90 at start of discharge d2-c Heating of the air in heat exchanger 90 at mid-discharge dl-c Heating of the air in heat exchanger 90 at end of discharge c-b gas is further heated up in first thermal store 36 At this stage in the cycle the gas at point b enters the combustor and is heated to significantly higher temperatures. Note the cycle may complete the b-a cycle in reverse if it is merely an Adiabatic Liquid Air Energy Storage system ie where there is no gas combustion and no integration with a gas power plant.

As mentioned above, the reduction in exchanger outlet temperature as the charge cycle progresses in the embodiment of Figure 14 with a marching LAIR tank can be achieved: by increasing the pressure ratio across the PHES based system, as it is charged. One example using the arrangement of Figure 13a would be:-During charging: air in the LAES pathway enters heat exchanger 90 in counterflow at 298K. Argon exits the heat exchanger 90 at 293K (5°C temp difference) 1 bar and is compressed to 11 bar 800K in compressor 212. Argon is cooled in hot thermal store 236 and then passes through heat exchanger 240, where it is cooled further to 298K. Argon is then expanded in expander 214 back to 1 bar 118K and passes through heat exchanger 90 where argon is heated back up to 293K and the cycle repeats. Air passing through heat exchanger 90 is cooled to 123K (-150°C).

As the cycle progresses, the pressure in the hot store is gradually increased to 13 bar while maintaining the cold side pressure at 1 bar. The hot store inlet temperature increases to 850K and the argon now enters the cold heat exchanger at 108K (-165°C). Air passing through the heat exchanger 90 is cooled to 113K (-160°C). So, during charging, the air is cooled progressively through heat exchanger 90 to a lower outlet temperature.

On discharge: this process is reversed, and the pressure ratio is gradually decreased back to the original ratio. However the temperature difference in heat exchanger 90 means that the Argon is only cooled to -155°C at the start of the discharge process and -145°C at the end of the discharge process. Consequently the temperature post compression in expander/compressor 214 and pre-heat exchanger 240 will be 70°C tie almost 50°C hotter). These figures assume highly efficient reciprocating machinery is used. Using turbo-machinery is likely to require an expansion ratio of at least 15:1 for state of the art machinery to achieve the 108K and possibly as high as 19:1 for machinery that only has an 85% polytropic efficiency. The mass flow of Argon passing through the PHES cycle should be 1.95 times the mass flow of air passing through the LAIR side.

The above-mentioned reduction in exchanger outlet temperature as the charge cycle progresses can also be achieved, for example:- (ii) by changing the amount of heat rejected in a second stage using the heat pump/bypass arrangement of Fig. 13c with a marching LAIR as per Figure 14/15 as follows:-Argon working fluid and heat exchanger 240 can cool argon to 298K. During charging air entering heat exchanger in counterflow also at 298K. Argon exits heat exchanger at 293K (5°C temp difference) 1 bar and is compressed to 9 bar 731K. Argon is cooled in hot thermal store and is then further compressed to 11 bar 324K before passing through heat exchanger 250' where it is cooled further to 298K. Argon is then expanded back in 2 stages to 1 bar 118K and passes through heat exchanger 90 where argon is heated up to 293K and cycle repeats. Air passing through heat exchanger 90 is cooled to 123K (-150°C). As the cycle progresses the pressure in the seconds stage machinery is slowly increased to 13 bar (347K) while maintaining the cold side pressure at 1 bar. The hot store inlet temperature increases remains constant at 731K and the argon now enters the cold heat exchanger at 108K (-165°C). Air passing through the heat exchanger 90 is cooled to 113K (-160°C). On discharge this process is reversed, however the temperature difference in heat exchanger 90 means that the Argon is only cooled to -155°C at the start of the discharge process and -145°C at the end of the discharge process. Consequently the temperature post compression in expander/compressor 214 and pre-heat exchanger 240 will be 70°C (ie almost 50°C hotter). On discharge it is not necessary to use the second stage machinery and heat rejection can be achieved via heat exchanger 240. These figures assume highly efficient reciprocating machinery is used. Using turbo-machinery is likely to require an expansion ratio of at least 15:1 for state of the art machinery to achieve the 108K and possibly as high as 19:1 for machinery that only has an 85% polytropic efficiency. The mass flow of Argon passing through the PHES cycle should be 1.95 times the mass flow of air passing through the LAIR side.

Figures 16a and b These show the charge and discharge cycles for an adiabatic LAES system 510 combined with PHES based sub-system. There is no fundamental difference between this cycle and that shown in Figures 14a-c except for the lack of combustion and integration within a generation cycle. However, an advantage of this system is that the first stage compression and pressure ratio is not determined by the gas turbine operating conditions. Consequently the first stage pressure ratio may be lower than that required for a normal industrial gas turbine i.e. below 15 bar or even below 10 bar. This may be beneficial for reducing the peak pressure of the pressurised cold liquid air tank 120 store and, hence, for reducing the cost of the store.

In the case where the PHES circuit is a closed circuit, a gas buffer may be provided in the PHES circuit selectively to remove gas from, or add gas to, the PHES circuit, for example, to maintain gas pressure within at least a part of the PHES circuit within a predetermined range; in particular, it may be desirable to minimise pressure fluctuations causing temperature changes across the heat exchanger. (The free volume available in the PHES based circuit will be markedly less than that available in a conventional PHES system with a hot and cold store.) In Figure 16, by way of example, a gas reservoir/buffer 404 and associated fluid connections (dotted lines) is shown positioned across the PHES circuit, so that it extends from a high pressure part of the circuit (bottom right corner) to a lower pressure part of the circuit (top left corner). In use, a first high pressure (HP) valve connects the higher pressure part of the circuit to the (e.g. sealed) gas reservoir at a pressure equal to or below the higher pressure when gas needs to be withdrawn from the circuit, and, a second low pressure (LP) valve connects a lower pressure part of the circuit to the gas reservoir at a pressure greater than or equal to the lower pressure when gas needs to be added to the circuit.

In all the embodiments described above in relation to the second aspect, the first energy storage system, i.e. the LAES system and PHES based system will be configured such that they are both able to operate at the same time for at least a selected minimum charging time and discharging time. Any thermal stores disposed in their respective first and second flow pathways will thus need to have a selected minimum storage capacity. Similarly, the LAIR tank in the LAES system will also need to be matched in their storage capacity to the respective storage capacities of the respective thermal stores.

While the invention has been described by reference to specific embodiments, it should be understand that the invention is not limited to the described embodiments and numerous modifications may be made within the scope of the present invention.

Claims

CLAIMS1. A hybrid energy storage system comprising a liquid air energy storage (LAES) system integrated with a pumped heat energy storage (PH ES) based system, wherein the LAES system comprises a first flow pathway leading from an air inlet 5 successively to: i) first stage power machinery configured to compress the air from the air inlet upon charging and, upon discharging, to expand the air to produce work; ii) an optional first thermal energy storage (TES) system configured to store and return thermal energy to air passing through it upon charging and discharging, 10 respectively; iii) a heat exchanger configured to cool and reheat air passing through it upon charging and discharging, respectively, wherein thermal energy is removed by, and returned from a heat transfer fluid flowing through a second flow pathway in the heat exchanger; and, iv) liquefaction apparatus for liquefying, storing and re-vaporising the air, including a cryogenic tank for storing the liquid air, wherein the LAES system and PHES based system are coupled by means of the heat exchanger, the second flow pathway forming part of an open or closed circuit of the PHES based system in which the heat transfer fluid is the circulating working fluid, wherein the PHES circuit comprises, in turn, first PHES power machinery, a first PHES thermal store, second PHES power machinery, and the heat exchanger; wherein the PHES based system is configured to operate a gas-based thermodynamic cycle in which the working fluid circulates in a charging mode when the LAES system is charging, and in a discharging mode when the LAES system is discharging; and wherein the PHES based system is configured such that:-the first PHES power machinery compresses the working fluid during charging, and expands the working fluid during discharging; the first PHES thermal store receives and stores thermal energy from the compressed working fluid during charging, and returns thermal energy to the expanded working fluid during discharging; the second PHES power machinery expands the working fluid leaving the first PHES thermal store during charging, and compresses the working fluid leaving the heat exchanger during discharging; and, the heat exchanger transfers thermal energy from the air in the LAES system to the working fluid leaving the second PHES power machinery during charging, and transfers thermal energy to the air in the LAES system from the working fluid during discharging.
2. A hybrid energy storage system according to claim 1, wherein the LAES system comprises an adiabatic LAES (ALAES) system comprising said first thermal energy storage (TES) system.
3. A hybrid energy storage system according to claim 2, wherein a heat exchanger for cooling to ambient is provided downstream of the first TES system.
4. A hybrid energy storage system according to any preceding claim, wherein the outlet temperature of the air leaving the downstream outlet of the heat exchanger during charging is lower than -50°C.
5. A hybrid energy storage system according to claim 4, wherein the outlet temperature of the air leaving the downstream outlet of the heat exchanger during charging is lower than -100°C.
6. A hybrid energy storage system according to any preceding claim, wherein the cryogenic tank is configured to store the liquid air in an insulated tank at substantially 15 constant pressure.
7. A hybrid energy storage system according to claim 6, wherein, during charging, unliquefied air is directed, as a counter-current flow, to cool air flowing in the first flow pathway towards the cryogenic tank.
8. A hybrid energy storage system according to claim 6, wherein, during charging, unliquefied air is re-compressed and re-combined with air flowing in the first flow pathway towards the cryogenic tank.
9. A hybrid energy storage system according to any preceding claim, wherein the system further comprises:-second stage power machinery to expand the air before entry to the cryogenic tank 25 upon charging, and to withdraw the air from the tank and compress it upon discharging; and, wherein the system is configured such that the air enters and leaves the cryogenic tank in gaseous form, and is stored in the cryogenic tank as a saturated liquid/vapour air mixture under equilibrium pressure and temperature conditions, whereby condensation of the air in the tank during charging causes a progressive increase in the equilibrium vapour pressure and temperature of the saturated mixture, and evaporation of the air in the tank during discharging causes a progressive decrease in the equilibrium vapour pressure and temperature of the saturated mixture.
10. A hybrid energy storage system according to claim 9, wherein the second stage power machinery is in direct fluid communication with the cryogenic tank.
11. A hybrid energy storage system according to claim 9 or claim 10, wherein the second stage power machinery comprises variable pressure ratio machinery.
12. A hybrid energy storage system according to any preceding claim, wherein either, or both, of the first and the second PHES power machinery comprises a reversible 5 machine capable of acting both as a compressor and an expander.
13. A hybrid energy storage system according to any preceding claim, wherein the PHES circuit is a closed circuit and a gas buffer is provided in the PHES circuit selectively to remove gas from, or add gas to, the PHES circuit.
14. A hybrid energy storage system according to claim 2, wherein the first thermal energy storage (TES) system comprises a direct TES.
15. A hybrid energy storage system according to any preceding claim, wherein the first stage power machinery forms part of a gas turbine or gas turbine derivative, which optionally forms part of an OCGT or CCGT power plant.
16. A method of operating a hybrid energy storage system according to any preceding claim, comprising the steps of: i) operating the PHES based system in a charging mode when the LAES system is operating in a charging mode; and, ii) operating the PHES based system in a storage mode when the LAES system is operating in a storage mode; and, iii) operating the PHES based system in a discharging mode when the LAES system is operating in a discharging mode.
17. A method of operating a hybrid energy storage system according to claim 16, wherein the mass flow rate through the first and the second PHES power machinery in the PHES circuit is selectively adjusted, during charging and/or discharging, in order to 25 modify heat transfer to or from the LAES system in the heat exchanger.
18. A method of operating a hybrid energy storage system according to claim 16, wherein the pressure ratio across the first and the second PHES power machinery in the PHES circuit is selectively adjusted, during charging and/or discharging, in order to modify heat transfer to or from the LAES system in the heat exchanger.
19. A method of operating a hybrid energy storage system according to claim 16, wherein temperature control apparatus is provided at a location within the PHES circuit and is used, during charging and/or discharging, selectively to raise or lower the temperature of the PHES working fluid at that location, in order to modify heat transfer to or from the LAES system in the heat exchanger.
20. A hybrid energy storage system, or method of operating, substantially as hereinbefore described with reference to Figures 11a, 11 bi, 11 bii, 11c, 12, 13, 14a-b, 15 and 16a-b of the accompanying drawings.