WO2013026992A1

WO2013026992A1 - A system for heat storage

Info

Publication number: WO2013026992A1
Application number: PCT/GB2011/051597
Authority: WO
Inventors: Jonathan Sebastian Howes; James Macnaghten
Original assignee: Isentropic Ltd
Priority date: 2011-08-24
Filing date: 2011-08-24
Publication date: 2013-02-28

Abstract

Apparatus (200) for storing energy, comprising: a first stage comprising: a compressor (130a); and a first heat store (120a) for receiving and storing thermal energy from gas compressed by the compressor (130a); and a second stage comprising: an expander (140a) for receiving gas from the first heat store (120a); and a second heat store (110a) for transferring thermal energy to gas expanded by the expander (140a); wherein the apparatus further comprises pressure maintaining means (230) configured to substantially maintain gas pressure in at least a part of the apparatus within a predetermined pressure range.

Description

TITLE: A SYSTEM FOR HEAT STORAGE

DESCRIPTION

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

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

The cycles used in the system of WO 2009/044139 may be run as closed cycle processes or as open cycle systems (e.g. where there is one stage that is at near ambient temperature, atmospheric pressure and the working fluid is air). When running as a closed cycle, the working gas may be a monatomic gas such as argon which has a high isentropic index (i.e. for a given pressure change a higher temperature rise is achieved than for a diatomic gas such as nitrogen). This results in a lower peak system pressure which in turn lowers the amount of material required to contain the pressure and hence the cost of the thermal storage vessels.

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

In accordance with a first aspect of the present invention, there is provided apparatus for storing energy, comprising: a first stage comprising: a compressor; and a first heat store (e.g. higher pressure heat store) for receiving and storing thermal energy from gas compressed by the compressor; and a second stage comprising: an expander for receiving gas from the first heat store; and a second heat store (e.g. lower pressure heat store) for transferring thermal energy to gas expanded by the expander; wherein the apparatus further comprises pressure maintaining means (e.g. pressure maintaining device) configured to substantially maintain gas pressure in at least a part of the apparatus within a predetermined pressure range.

In this way, apparatus for storing energy is provided in which pressure in the thermal stores (e.g. pressure in both thermal stores) may be maintained within a narrow range allowing the thermal stores to be optimised to operate at or near a constant pressure regardless of the state of charge of the system. This also results in a more consistent power output or input for a given system operating speed over the duration of the charge or discharge cycle.

In one embodiment, the predetermined pressure range is within 20% of a specific target pressure (e.g. within 15% of the target pressure or within 10% of the target pressure).

In one embodiment, the pressure maintaining means is configured to substantially maintain a predetermined constant pressure.

In one embodiment, the pressure maintaining means is configured to maintain a substantially constant pressure ratio between the first and second heat stores.

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

In one embodiment, the part of the apparatus is a lower pressure part (e.g. between the second heat store and the compressor).

In one embodiment, the part of the apparatus is a higher pressure part (e.g. between the first heat store and the expander). In one embodiment, the part of the apparatus is an intermediate pressure part (e.g. between first and second expander stages of the expander).

In one embodiment, the part of the apparatus is a part nearest ambient temperature.

In one embodiment, the part of the apparatus is a cold part of the apparatus (e.g. lower pressure part of the second stage).

In one embodiment, the pressure maintaining means comprises a buffer for adding gas to or receiving gas from the at least one part of the apparatus (e.g. for adding gas to or receiving gas from the circuit). The buffer may comprises a sealed gas source (e.g. where the gas is a gas other than atmospheric air, e.g. argon).

In one embodiment, the sealed gas source is a substantially constant pressure gas source.

In one embodiment, the substantially constant pressure gas source has a pressure within the predetermined pressure range (e.g. substantially equal to the predetermined constant pressure).

In one embodiment, the substantially constant pressure gas source is in continuous fluid communication with the circuit during charging or discharging of the apparatus.

In one embodiment, the substantially constant pressure gas source comprises a sealed variable volume gas storage chamber biased by a balancing force and configured to increase or decrease in volume in order to maintain gas received in the chamber at substantially constant pressure (e.g. the chamber is configurable between an expanded configuration defining a first volume for receiving gas and a contracted configuration defining a second volume for receiving gas smaller than the first volume, and is biased by the balancing force to maintain the contracted configuration).

In one embodiment, the chamber comprises first and second chamber parts, with relative movement between the first and second parts resulting in a change in volume within the chamber. The chamber may further comprise a seal for preventing passage of gas between the first and second chamber parts. In one embodiment, the first and second chamber parts are sealed by a liquid seal.

In one embodiment, the chamber comprises an inflatable bladder. For example, the chamber may comprise an inflatable resilient bladder (e.g. formed from an elastomeric material) with the resilience of the bladder providing part of the balancing force. The inflatable resilient bladder may be configured to maintain gas pressure within the bladder above (e.g. slightly above) atmospheric pressure.

In one embodiment, the constant pressure gas source is configured to maintain gas contained in the chamber at substantially atmospheric pressure.

In one embodiment, the balancing force is provided at least in part by atmospheric air pressure.

In one embodiment, the balancing force is provided by a high pressure fluid (e.g. fluid having a pressure greater than atmospheric pressure). The high pressure fluid may comprise a head of fluid bearing upon the chamber.

In one embodiment, the high pressure fluid is a liquid.

In one embodiment, the liquid pressure is maintained by a balance pump.

In one embodiment, the high pressure fluid is a liquid-vapour mixture (e.g. volatile liquid- vapour mixture). The fluid pressure may be maintained by controlling the temperature of the liquid (e.g. volatile liquid).

In one embodiment, the buffer comprises: a high pressure gas source (e.g. non- constant pressure sealed gas source) having a pressure exceeding the predetermined pressure range (e.g. a pressure greater than the predetermined pressure); a selector for selectively connecting the high pressure gas source to the circuit; and a compressor for pumping gas from the circuit to the high pressure gas source when a reduction in gas pressure is required in the circuit.

In one embodiment, the buffer comprises: a low pressure gas source (e.g. non-constant pressure sealed gas source) having a pressure under the predetermined pressure range (e.g. a pressure lower than the predetermined pressure); a selector for selectively connecting the low pressure gas source to the circuit; and a compressor for pumping gas from the low pressure gas source to the circuit when an increase in gas pressure is required in the circuit.

In one embodiment, the low pressure gas source is located within the second heat store

(e.g. in a coldest part of the second heat store), hi this way, the volume of the low pressure gas source may advantageously be reduced relative to a gas source at ambient temperature.

In one embodiment, the low pressure gas source houses a liquid-vapour mixture. In one embodiment, the apparatus further comprises a heater for heating the liquid-vapour mixture.

In one embodiment, the buffer comprises a dust trap for collecting dust for subsequent removal. The apparatus may further comprises one or more isolation valves for isolating a lower pressure part of the apparatus from a higher pressure part of the apparatus. In one embodiment, the one or more isolation valves are configured to isolate the first heat store from the other of the first and second stage parts and/or isolate the second heat store from the other of the first and second stage parts. In this way, a system may be provided in which pressure in the higher and lower pressure parts (e.g. first and second heat stores) do not equalise when the apparatus is not running, whereby the first and second heat stores need only be designed to operate under their normal operating pressure range (e.g. near constant operating pressure in one embodiment of the invention).

In one embodiment, the first heat store comprises a first gas-permeable heat storage structure.

In one embodiment, the second heat store comprises a second gas-permeable heat storage structure.

In one embodiment, the second heat storage structure has a void fraction which is greater than a void fraction of the first heat storage structure.

In one embodiment, the second heat storage structure has a void fraction of greater than 40% (e.g. greater than 50%; greater than 60%; or greater than 70%). In this way, the volume of the chamber can be reduced together with the pressure drop through the second heat store.

In one embodiment, the second heat storage structure has a mean heat capacity per unit mass which is less than a mean heat capacity per unit mass of the first heat storage structure.

In one embodiment, the second heat storage structure has a volume which is greater than a volume of the first heat storage structure. In this way the a greater gross void space may be provided within the second heat storage structure thereby reducing the volume of the chamber required to buffer gas pressure within the second heat store or even negating the need for a buffer associated with the second heat store.

In accordance with a second aspect of the present invention there is provided, apparatus for storing energy, comprising: a first stage comprising: a compressor; and a first heat store for receiving and storing thermal energy from gas compressed by the compressor; and a second stage comprising: an expander for receiving gas from the first heat store; and a second heat store for transferring thermal energy to gas expanded by the expander; wherein the apparatus further comprises one or more isolation valves for isolating a lower pressure part of the apparatus from a high pressure part of the apparatus.

In one embodiment, the one or more isolation valves are configured to isolate the first heat store from the other of the first and second stage parts.

In one embodiment, the one or more isolation valves are configured to isolate the second heat store from the other of the first and second stage parts.

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

Figure 1 shows a schematic illustration of an electricity storage system of the type disclosed in WO 2009/044139;

Figures li)-iii) shows conditions in different parts of the system of Figure 1 during different operating conditions ;

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

Figure 3 shows a schematic illustration of an electricity storage system according to a second embodiment of the present invention;

Figure 4 shows a schematic illustration of an electricity storage system according to a third embodiment of the present invention;

Figure 5 shows a schematic illustration of an electricity storage system according to a fourth embodiment of the present invention;

Figure 6 shows a schematic illustration of an electricity storage system according to a fifth embodiment of the present invention;

Figure 7 shows a schematic illustration of an electricity storage system according to a sixth embodiment of the present invention;

Figure 8 shows a schematic illustration of an electricity storage system according to a seventh embodiment of the present invention;

Figure 9 shows a schematic illustration of an electricity storage system according to an eight embodiment of the present invention;

Figure 10 shows a schematic illustration of an electricity storage system according to a ninth embodiment of the present invention;

Figure 11 shows a schematic illustration of an electricity storage system according to a tenth embodiment of the present invention; and Figure 12 shows a schematic illustration of an electricity storage system according to an eleventh embodiment of the present invention.

Figure 1 shows an electricity storage system 100 comprising insulated hot storage vessel 120 housing a gas-permeable particulate heat storage structure 121, upper and lower 5 plenum chambers 122, 123, cold storage vessel 110 housing a gas-permeable particulate heat storage structure 111, upper and lower plenum chambers 112, 113, compressor/expanders 130, 140 and interconnecting pipes 101,102,103 and 104.

In operation, when charging, ambient temperature gas at a higher pressure exits interconnecting pipe 103 and is expanded by compressor/expander 140 to a lower pressure.

10 The gas is cooled during this expansion and passes via interconnecting pipe 104 to the cold storage vessel 110. The gas enters the lower plenum chamber 1 13 and then passes up through particulate heat storage structure 111, where the gas is heated. The now hotter gas leaves particulate heat storage structure 111 and passes into upper plenum chamber 112, from where it enters interconnecting pipe 101. The temperature of the gas at this point may

15 be around ambient or a temperature that is different to ambient. For example in one arrangement it could be at 500 degrees centigrade. The gas exits interconnecting pipe 101 and enter compressor/expander 130, where the gas is compressed to the higher pressure. As the gas is compressed the temperature rises and the gas leaves the compressor at a higher temperature and passes into interconnecting pipe 102. The gas then enters hot storage vessel

20 120via upper plenum chamber 122 and passes down through particulate heat storage structure 121 , where the gas is cooled. The now cooler gas leaves particulate heat storage structure 121 and passes into lower plenum chamber 123, from where it enters interconnecting pipe 103. The process can continue until the hot and cold stores are 'fully charged' or stop earlier if required.

25 This overall charging process absorbs energy that is normally supplied from other generating devices via the electric grid. The compressor/expanders 130 and 140 are driven by a mechanical device, such as an electric motor (not shown). In addition to these components there may also be heat exchangers contained within one or more of the interconnecting pipes - these are not shown. In one arrangement there is a heat exchanger in

30 interconnecting pipe 103 that maintains the gas in this pipe at or near ambient temperature.

In operation, when discharging, high temperature gas at a higher pressure enters interconnecting pipe 102 and is expanded by compressor/expander 130 to a lower pressure. The gas is cooled during this expansion and passes via interconnecting pipe 101 to the cold storage vessel 110. The gas enters upper plenum chamber 112 and then passes down through particulate heat storage structure 1 1 1 , where the gas is cooled. The now colder gas leaves particulate heat storage structure 1 1 1 and passes into lower plenum chamber 113, from where it enters interconnecting pipe 104. The gas exits interconnecting pipe 104 and enter compressor/expander 140 where the gas is compressed to the higher pressure. As the gas is compressed the gas temperature rises and the gas leaves the compressor at a higher temperature and passes into interconnecting pipe 103. The gas then enters hot storage vessel 120.via lower plenum chamber 123 and passes up through particulate heat storage structurel 21 where the gas is heated. The now high temperature gas leaves particulate heat storage structure 121 and passes into upper plenum chamber 122, from where it enters interconnecting pipe 102 and is expanded by compressor/expander 130 with the energy of expansion being used to generate electricity for the electric grid. The process can continue until the hot and cold stores are 'fully discharged' or stop earlier if required.

The cold thermal store may be charged with the flow entering from the bottom and travelling upwards and discharged with the flow entering from the top and travelling downwards.

The hot thermal store may be charged with the flow entering from the top and travelling downwards and discharged with the flow entering from the bottom and travelling upwards.

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

Figure li) shows that there are a number of changes that can occur to the gas pressure in system 100 as it is charged or discharged.

As a close approximation within the normally anticipated range of operating temperatures, the Ideal Gas Law states: PV = constant

T

So if there is a set volume of space within a thermal store, then as the temperature rises from Tl to T2, the pressure must change. By way of example:

TH1 (hot store uncharged temperature) =300Kelvin TH2 (hot store charged temperature) =773 elvin

PHI (hot store pressure uncharged) =12 bar

PH2 (hot store pressure charged) = 31 bar However, for the same system the temperature within the cold store is also dropping at the same time:

TCI (cold store uncharged temperature) =300 Kelvin

TC2 (cold store charged temperature) =1 13 Kelvin

PCI (cold store pressure uncharged) =1 bar

PC2 (cold store pressure charged) =0.36bar

From this it can be seen that if these stores were isolated the pressure ratio between the two stores would have gone from 12:1 (uncharged) to almost 90:1 when charged. However, the reason for the pressure change is that the hot thermal store now effectively has too much gas in it and the cold store has too little.

Figure Hi) shows the effect on pressure of allowing some of the gas to transfer from the hot store to the cold store. This still leaves too much gas in the system so the pressure in both stores has risen as follows:

Cold Store from 1 bar to 1.8 bar

Hot Store from 12 bar to 21.6 bar

This change leaves the pressure ratio constant at 1 : 12 in both the charged and uncharged state. From this it can be seen that if a constant mass of gas is maintained between the two thermal stored then the hot store must be designed for a peak pressure of 21.6 bar and the cold store for a peak pressure of 1.8 bar.

Figure liii) shows what happens if in accordance with the present invention gas is both allowed to transfer between stores and is also removed from the circuit during charging to maintain a near constant pressure in each store:

Mass of gas in system when uncharged = 21.1kg Mass of gas in system when charged = 11.9kg

Mass of gas removed from system = 9.2kg i.e. approx 44% of the gas in system is removed

Where a constant pressure is maintained in the stores it is then possible that the stores need only be designed for this one operating pressure. This has a number of advantages in that the cost of the stores is lower if the thermal stores are only designed for one pressure, in addition there is substantially no fatigue load on the stores as they are not regularly cycled between a higher and lower pressure (or minimal fatigue load where pressure is maintained within a narrow range). This fatigue load adds to the cost of the storage vessels over and above the cost of the basic pressure vessel. Finally the machinery power input and output is much more constant for a given temperature range as the pressure is not also varying.

Figure 2 shows an improved energy storage system 200 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) and comprising a buffer 230 connected to low pressure interconnecting pipe 101a via open connection 205. Buffer 230 comprises a gas container 233, the edge of which is sitting in liquid seal 231 which is contained by liquid vessel 232. Liquid vessel 232 is open on the upper side and is of sufficient depth to allow gas container 233 to rise and fall to vary the internal volume of the gas container 233. In this way a volume of gas 234 is enclosed by gas container 233 in a flexible sealed space that can vary in volume. In operation gas container 233 is able to move in response to the amount of gas 234 being increased or decreased by the addition or subtraction of gas from low pressure interconnecting pipe 101a via open connection 205. In this embodiment the gas buffer operates in much the same manner as a natural gas holder and maintains the gas in the interconnecting pipe 101a at or near atmospheric pressure. If an additional mass (not shown) or other biasing force is added to the top of gas container 233 it is possible to keep the pressure in the low pressure part of the system always just above atmospheric pressure.

Figure 3 shows an improved energy storage system 300 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) and comprising a buffer 330 connected to low pressure interconnecting pipe 101b via open connection 305. Buffer 330 comprises a gas container 331 which is attached to a gas lid 333 via seal 332. In this way a volume of gas 334 is enclosed by gas container 331 and gas lid 332 in a flexible sealed space that can vary in volume, where seal 332 prevents any external gas from contaminating the system. In operation gas lid 333 is able to move in response to the amount of gas 334 being increased or decreased by the addition or subtraction of gas from low pressure interconnecting pipe 101b via open connection 305. In this embodiment buffer 330 maintains the gas in the interconnecting pipe 101b at or near atmospheric pressure.

Instead of a seal or membrane it is possible to use an inflatable gas tight bladder or similar arrangement. If an additional mass (not shown) or other biasing force is added to the top of gas lid 333 it is possible to keep the pressure in the low pressure part of the system always just above atmospheric pressure.

Figure 4 shows an improved energy storage system 400 energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) and comprising a buffer 430 connected to high pressure interconnecting pipe 103 c via an open connection 405. Buffer 430 comprises pressurised container 431, which contains gas 434 from the high pressure interconnecting pipe 103c and a high pressure liquid 433, such as water, that is kept at substantially the same pressure as the gas 434, but which is separated from the gas 434 by a flexible membrane 432, which can accommodate changes in the volume of the high pressure liquid 433.

The volume of high pressure liquid 433 is adjusted via pressure balance pump 436 which is connected to both high pressure container 431 and low pressure container 438. Pressure balance pump 436 is connected to high pressure container 431 via pipe 435 and to low pressure container 438 via pipe 437. In this way pressure balance pump adds or removes liquid from high pressure container 431 in order to maintain the gas pressure 434 at or near a constant level and hence buffer 430 can maintain the pressure in high pressure interconnecting pipe 103c at or near a constant level.

Low pressure container 438 also contains low pressure liquid 439, which is the same as high pressure liquid 433, but which is held at a different pressure. In this example low pressure container 438 is open to the atmosphere and hence the liquid 439 is at the same pressure as the atmosphere 441. The low pressure liquid may be separated from the atmosphere by membrane 440 to avoid contamination from the atmosphere.

Both low pressure container 438 and high pressure container 431 are sized to allow for the correct amount of gas expansion or contraction and hence the correct amount of liquid 433 and 439. Instead of a seal or membrane it is possible to use an inflatable bladder or similar arrangement within the high pressure container 431 and the low pressure container 438.

Because liquid such as water is only very slightly compressible it requires small amounts of energy to add pressurised water to the container. This in turn maintains the gas in the container, and hence the high pressure part of the storage circuit, at a near constant pressure for only a small amount of energy. If this pressure balancing was done with a gas then it would require more significant amounts of energy and also the addition/removal of heat.

Figure 5 shows an improved energy storage system 500 again based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) and comprising a buffer 530 connected to high pressure interconnecting pipe 103d via an open connection 505. Buffer 530 comprises pressurised container 531, which contains gas 534 from the high pressure interconnecting pipe 103d and a high pressure vapour 533 and high pressure liquid 535, that is kept at substantially the same pressure as the gas 534, but which is separated from the gas 534 by a flexible membrane 532, which can accommodate changes in the volume of high pressure vapour 533 contained within it. A heat exchanger 536 within high pressure liquid 535 keeps the liquid within certain predefined temperature range.

The high pressure liquid is a volatile liquid that has a certain vapour pressure at a certain temperature. In this way if the temperature within the liquid is at a certain level then the vapour pressure can be kept at a near constant value. The advantage of this system is that there is no need for any additional tanks and there are minimal moving components. Heat or cold may be supplied by external heat exchangers, by a heater/chiller, from a large thermal mass or from different parts of the energy storage system.

Figure 6 shows an improved energy storage system 600 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) and further comprising a buffer 630 connected to low pressure interconnecting pipe lOle via connection 605. Buffer 630 comprises a compressor/expander 631 (controlled by a selector - not shown) connected to high pressure vessel 634 via pipe 632. High pressure vessel 634 contains high pressure gas 633 that has been compressed from the low pressure interconnecting pipe lOle by compressor/expander 631. In this way gas can be added or removed from the main storage system via connection 605 by the compressor/expander 631. High pressure gas 633 can be at a higher pressure than the gas in low pressure interconnecting pipe 101 e. The high pressure gas 633 may also be at a higher pressure than the gas in high pressure interconnecting pipe 103e. The pressure within high pressure vessel 634 may vary as additional gas is added and removed. The pressure within low pressure interconnecting pipe 101 e is kept at a near constant value by the addition and removal of gas from the circuit.

Figure 7 shows an improved energy storage system 700 based on the energy storage system that is shown in Figure 4 (with corresponding features labelled accordingly), but where there is a two stage compressor/expander 750, 751 on the low temperature part of the circuit. High pressure compressor expander 751 is connected to low pressure compressor/expander 750 by intermediate pressure pipe 752. Buffer 430' connected to intermediate pressure interconnecting pipe 752 via connection 405'. Buffer 430'comprises pressurised container 431 ' which contains gas 434' from the intermediate pressure interconnecting pipe 752 and a intermediate pressure liquid 433', such as water, that is kept at substantially the same pressure as the gas 434', but which is separated from the gas 434' by a flexible membrane 432', which can accommodate changes in the volume of intermediate pressure liquid 433'.

The volume of intermediate pressure liquid 433' is adjusted via pressure balance pump 436' which is connected to both pressurised container 43 Γ and low pressure container 438'. Pressure balance pump 436' is connected to pressurised container 431 ' via pipe 435' and to low pressure container 438' via pipe 437'. In this way pressure balance pump adds or removes liquid from pressurised container 431 ' in order to maintain the gas pressure 434' at or near a constant level and hence buffer 430' can maintain the pressure in intermediate pressure interconnecting pipe 752 at or near a constant level.

Low pressure container 438' also contains low pressure liquid 439', which is the same as intermediate pressure liquid 433', but which is held at a different pressure. In this example low pressure container 438' is open to the atmosphere and hence the liquid 439' is at the same pressure as the atmosphere 441 '. The low pressure liquid may be separated from the atmosphere by membrane 440' to avoid contamination from the atmosphere.

Both low pressure container 438' and pressurised container 431 ' are sized to allow for the correct amount of gas expansion or contraction and hence the correct amount of liquid 433' and 439'. Instead of a seal or membrane it is possible to use an inflatable bladder or similar arrangement within the pressurised container 431⁵ and the low pressure container 438'.

Because liquid such as water is only very slightly compressible it requires small amounts of energy to add pressurised water to the container. This in turn maintains the gas in the container, and hence the intermediate pressure part of the storage circuit, at a near constant pressure for only a small amount of energy. If this pressure balancing was done with a gas it would require more significant amounts of energy and also the addition/removal of heat.

Figure 8 shows an improved energy storage system 800 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly), but where two shut-off valves 810 and 811 are added to the high pressure interconnecting pipes 102g and 103g. These shut-off valves are designed to be used when the system is neither charging nor discharging. They are designed to allow only an insignificant leakage, such that the high pressure gas within the high pressure part of the system does not leak to the low pressure part of the system or the compressor/expanders 130g and 140g between charging and discharging operations.

Figure 9 shows an improved energy storage system 900 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly), but where two shut-off valves 920 and 921 are added to the high pressure interconnecting pipes lOlh and 104h. These shut-off valves are designed to be used when the system is neither charging nor discharging. They are designed to allow only an insignificant leakage, such that the high pressure gas within the high pressure part of the system does not leak to the low pressure store between charging and discharging operations.

Figures 10 shows an improved energy storage system 950 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) comprising a buffer 955 configured to receive gas from the high pressure part of the circuit and store the gas in a pressurised vessel 951 located within lower plenum chamber 1131. Pressure vessel 951 is not necessarily at the same pressure as the high pressure part of the system (i.e. not necessarily the same as the pressure in high pressure interconnecting pipe 103i). The pressure in the system is maintained by compressor/expander 953 (controller by a selector - not shown), which is in open communication with high pressure interconnecting pipe 103i via pipe 954 and maintains the pressure in the interconnecting pipe at or near a constant level. In this way the gas is transferred to and from pressure vessel 951 via interconnecting pipe 952. It is stored at low temperatures (e.g. near -160 deg C) in pressure vessel 951 and consequently the volume of the pressure vessel required to store this quantity of gas is greatly diminished. For example if gas is cooled to -160 deg C it only requires approximately 1/3 of the volume that the same mass of gas would require at 40 deg C.

Figures 1 1 shows an improved energy storage system 960 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) comprising buffer 965 configured to receive gas from the low pressure part of the circuit and store the gas in a pressurised vessel 961 located within lower plenum chamber 113j. Pressure vessel 961 is not necessarily at the same pressure as the high pressure part of the system (i.e. not necessarily the same as the pressure in high pressure interconnecting pipes 102j and 103j). The pressure in the system is maintained by compressor/expander 963 (controlled by a selector - not shown), which is in open communication with low pressure interconnecting pipe lOlj via pipe 964 and maintains the pressure in the Interconnecting pipe at or near a constant level. In this way the gas is transferred to and from pressure vessel 961 via interconnecting pipe 962. It is stored at low temperatures (e.g. near -160 deg C) and consequently the volume of the pressure vessel required to store this quantity of gas is greatly diminished.

However, certain gases will liquefy if they are cooled at high pressure, which will result in the pressure dropping until the gas no longer liquefies i.e. an equilibrium is reached.

Figure 12 shows an improved energy storage system 970 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) comprising a buffer 975 comprising a pressurised vessel 971 located within lower plenum chamber 113k and configured for use with a gas that will liquefy at a certain pressure when cooled to the cold temperature within the system that is found in lower plenum chamber 113k. In Figure 12 the gas is taken from the high pressure part of the circuit and stored in pressurised vessel 971. Pressure vessel 971 is not necessarily at the same pressure as the high pressure part of the system (i.e. not necessarily the same as the pressure in high pressure interconnecting pipe 103k). The pressure in the system is maintained by compressor/expander 973 (controlled by a selector - not shown), which is in open communication with high pressure interconnecting pipe 103k via pipe 974 and maintains the pressure in the interconnecting pipe at or near a constant level. In this way the gas stored in pressure vessel 971 is cooled down to low temperatures (e.g. near -160 deg C), so that part of it liquefies. As more gas is added the pressure in vessel 971 rises and more gas liquefies out. Consequently the volume of the pressure vessel required to store this quantity of gas is very greatly diminished, however some heat is added to the cold gas in lower plenum chamber 113k as the gas in pressure vessel 971 is liquefied. When the gas is to be recovered it is possible to remove gas from the pressure vessel 971 by using compressor/expander 973. As gas is removed from the system this will cause the pressure to drop and more gas will boil off. As the gas boils off it will absorb heat from the lower plenum chamber 113k. In one embodiment gas pressure is controlled without the need for additional heating (e.g. with the drop in vapour pressure releasing further vapour from the liquid-vapour mixture), but it is an option to provide additional heat if this process needs to occur quickly and this can be done by way of an electrical heater 978, which is connected via cables 977 to electrical supply 976. If an electrical supply is not to be used then it may be possible to use heat from other parts of the system or even background ambient heat to provide this heat.

In one embodiment the compressor/expander 973 is replaced with a throttle valve that lets gas into pressure vessel 971 via interconnecting pipe 972 when the pressure in interconnecting pipe 974 and hence in high pressure interconnecting pipe 103k starts to rise above a preset level. When the pressure in high pressure interconnecting pipe 103k starts to fall below a set level gas can be released directly into lower plenum chamber 113k via a second throttle valve (not shown) in pressure vessel 971.

Claims

Claims:

1. Apparatus for storing energy, comprising:

a first stage comprising: a compressor; and

a first heat store for receiving and storing thermal energy from gas compressed by the compressor; and

a second stage comprising: an expander for receiving gas from the first heat store; and a second heat store for transferring thermal energy to gas expanded by the expander;

wherein the apparatus further comprises pressure maintaining means configured to substantially maintain gas pressure in at least a part of the apparatus within a predetermined pressure range.

2. Apparatus according to claim 1, wherein the pressure maintaining means is configured to substantially maintain a predetermined constant pressure.

3. Apparatus according to claim 1 or claim 2, wherein the pressure maintaining means is configured to maintain a substantially constant pressure ratio between the first and second heat stores.

4. Apparatus according to any of the preceding claims, wherein the apparatus comprises a circuit configured to allow gas to pass cyclically between the first and second stages during at least one of a charging phase and a discharging phase.

5. Apparatus according to any of the preceding claims, wherein the pressure maintaining means comprises a buffer for adding gas to or receiving gas from the at least one part of the apparatus.

6. Apparatus according to claim 5, wherein the buffer comprises a sealed gas source.

7. Apparatus according to claim 6, wherein the sealed gas source is a substantially constant pressure gas source.

8. Apparatus according to claim 7, wherein the substantially constant pressure gas source has a pressure within the predetermined pressure range.

9. Apparatus according to claim 7 or claim 8, wherein the substantially constant pressure gas source is in continuous fluid communication with the circuit during charging or

5 discharging of the apparatus.

10. Apparatus according to any of claims 7-9, wherein the substantially constant pressure gas source comprises a sealed variable volume gas storage chamber biased by a balancing force and configured to increase or decrease in volume in order to maintain gas received in the

10 chamber at substantially constant pressure.

11. Apparatus according to any of claims 7-10, wherein the substantially constant pressure gas source is configured to maintain gas contained in the chamber at substantially atmospheric pressure.

15

12. Apparatus according to any of claims 10-11, wherein the balancing force is provided at least in part by atmospheric air pressure.

13. Apparatus according to any of claims 10-11, wherein the balancing force is provided 20 by a high pressure fluid.

14. Apparatus according to claim 13, wherein the high pressure fluid is a liquid.

15. Apparatus according to claim 14, wherein the liquid pressure is maintained by a 25 balance pump.

16. Apparatus according to any of claims 13-15, wherein the high pressure fluid is a liquid- vapour mixture.

30 17. Apparatus according to claim 16, wherein the fluid pressure is maintained by controlling the temperature of the liquid.

18. Apparatus according to claim 6 or claim 7, wherein the buffer comprises:

a high pressure gas source having a pressure exceeding the predetermined pressure range;

a selector for selectively connecting the high pressure gas source to the circuit; and a 5 compressor for pumping gas from the circuit to the high pressure gas source when a reduction in gas pressure is required in the circuit.

19. Apparatus according to claim 6 or claim 7, wherein the buffer comprises:

a low pressure gas source having a pressure under the predetermined pressure range; 10 a selector for selectively connecting the low pressure gas source to the circuit; and a compressor for pumping gas from the low pressure gas source to the circuit when an increase in gas pressure is required in the circuit.

20. Apparatus according to claim 19, wherein the low pressure gas source is located 15 within the second heat store.

21. Apparatus according to claim 20, wherein the low pressure gas source houses a liquid- vapour mixture.

20 22. Apparatus according to any of claims 5-21, wherein the buffer comprises a dust trap for collecting dust for subsequent removal.

23. Apparatus according to any of the preceding claims, further comprising one or more isolation valves for isolating a lower pressure part of the apparatus from a higher pressure part

25 of the apparatus.

24. Apparatus according to claim 23, wherein the one or more isolation valves are configured to isolate the first heat store from the other of the first and second stage parts.

30 25. Apparatus according to claim 23, wherein the one or more isolation valves are configured to isolate the second heat store from the other of the first and second stage parts.

26. An apparatus substantially as hereinbefore described with reference to accompanying drawings.