GB2501795A

GB2501795A - Thermal energy storage system

Info

Publication number: GB2501795A
Application number: GB201303288A
Authority: GB
Inventors: Rowland Geoffrey Hunt
Original assignee: Isentropic Ltd
Current assignee: Isentropic Ltd
Priority date: 2012-04-30
Filing date: 2013-02-25
Publication date: 2013-11-06
Also published as: GB201207489D0; GB201303288D0

Abstract

An apparatus 100' for storing energy using a gas-based thermodynamic cycle comprising a circuit including a hot half-engine stage 130' acting as a compressor during charging and an expander during discharging; a cold half-engine stage 140' which acts as an expander during charging and a compressor during discharging; a first heat store 120' for receiving and storing thermal energy from gas compressed by the hot half-engine stage in charging mode, and which transfers energy to the gas compressed by the cold half-engine in discharging mode; and a second heat store 110' for transferring thermal energy to gas expanded by the cold half-engine stage during charging, and receiving and storing thermal energy from gas expanded by the hot half-engine stage during discharging. A first valve 220 connects a higher pressure part of the circuit to a gas reservoir 205 at a pressure equal to or below the higher pressure when gas needs to be withdrawn from the circuit and a second valve 210 connecting a lower pressure part of the circuit to the gas reservoir 205 at a pressure greater than or equal to the lower pressure when gas needs to be added to the circuit.

Description

ENERGY STORAGE SYSTEM

DESCRIPTION

S 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 refencd to as "electricity storage" apparatus).

The applicant's earlier application WO 2009/044 139 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). Tn 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 centriftigal. 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 connected to the atmosphere 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 the present invention, there is provided apparatus for storing energy using a gas based thermodynamic cycle comprising a circuit comprising: a hot half-engine stage which acts as a compressor during charging and as an expander during discharging; a (higher pressure) first heat store (or "hot store") for receiving and storing thermal energy from gas compressed by the hot half-engine stage in charging mode, and which transfers thermal energy to the gas compressed by the cold half-engine stage in discharging mode; a cold half-engine stage which acts as an expander for receiving gas from the first heat store during charging and which acts as a compressor driving gas into the first heat S storc during discharging; and a (lower pressure) second heat store (or "cold store") for transferring thermal energy to gas expanded by the cold half-engine stage during charging, and receiving and storing thermal energy from gas expanded by the hot half-engine stage during discharging; a first (HP) valve configured to connect a higher pressure part of the circuit to a gas reservoir at a pressure equal to or below the higher pressure when gas needs to be withdrawn from the circuit; a second (LP) valve configured to connect 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 this way, the total gas mass in the system may be maintained within a desired range. Hence, pressures in the thermal stores, for example, may be kept within a predetermined range by adding gas to the lower pressure part or withdrawing gas from the higher pressure part of the apparatus in response to a decrease or increase in pressure respectively, thereby allowing the thermal stores and hot and cold half-engine stages to operate with increased efficiency and with a more consistent power output or input for a given system operating speed over the duration of a charge or discharge cycle.

For convenience, the first and second valves cvill hereinafter be referenced as HP and LP valves. Each half-engine stage may comprise a single reversible machine capable of acting as a compressor and expander, or may comprise respective compression and expansion machines suitably connected (e.g. in parallel) for the gas to be alternatively directed through for compression or expansion, respectively.

Usually, the apparatus will be configured so that liquefaction of the gas (i.e. working fluid) in the reservoir does not occur. Specifically, the apparatus should not comprise a reservoir located in a lower plenum chamber of the second heat store (i.e. cold store), from which the gas is releasable directly into the lower plenum chamber, and that is configured for use with a gas that will liquef,i at a certain pressure when cooled to the cold temperature found in such a lower plenum chamber.

The reservoir may be a common sealed gas reservoir configured to store gas at constant volume (e.g. where the gas is a gas other than atmospheric air, e.g. argon). The gas is stored at a pressurc within the reservoir that is intermediate between the two pressures in the higher pressure part and thc lower pressurc part, respectively, to which thc gas buffer circuit is S connected. In this way, gas reservoir pressure may alternately increase and decrease during cyclic operation of the first and second valves to alternately decrease and increase pressure in the circuit.

Usually the apparatus will bc configured so that the HP valve is only open when a pressure measured somewhere in the system is above a desired upper threshold pressure, in order to allow gas to be removed fmm the circuit. As soon as the pressure returns below that upper threshold pressure, the HP valve will close. Similarly, usually, the LP valve is only open when a pressure measured somewhere in the system is below a desired lower threshold pressure, in order to allow gas to be added to the circuit from the gas buffer. As soon as the pressure returns above that lower threshold pressure, the HP valve will close. In this way, it will 1 5 be appreciated that flow through the gas buffering circuit is unidirectional and that the two valves will never be open simultaneously. While this gas buffering method does result in irreversible losses through higher pressure gas being expanded through valves, it has the advantage that it involves simple, robust apparatus (valves) which can be implemented with a simple control system. Usually, flow through the valves will be throttled or otherwise flow limited to a slow measured flow rate, in order that a sudden influx of gas is avoided and too much gas is added (or withdrawn).

The lower pressure part of the circuit may be a part located between the second heat store and the hot half-engine stage. This limb between the second heat store and the hot half-engine stage may be an, at or near ambient, temperature limb, and hence, one or more heat exchangers (rejecting to ambient) may be located upstream or downstream of the lower pressure part (i.e. the junction leading to the reservoir via the LP valve).

The higher pressure part of the circuit may be a part located between the first heat store and the cold half-engine stage. This limb between the first heat store and the cold half-engine stage may be an, at or near ambient, temperature limb, and hence, one or more heat exchangers may be located upstream or downstream of the higher pressure part (i.e. the junction leading to the reservoir via the HP valve).

The two parts (or the gas in the two parts) will preferably be at temperatures within 60°C, preferably within 40°C or even 20°C of each other. Ideally, the two parts are at or near ambient temperature, if the cyek is one with a high pressure ambient Emb and a low pressure ambient limb.

Ideally, the higher pressure and lower pressure parts (junctions) arc both in the S respective limbs referenced above, and each limb is at or near ambient temperature, which would be clear to the skilled addressee (e.g. due to the limitations of heat exchangers, at or ncar ambient may be within 30°C or 20°C or 10°C of ambient temperature).

In terms of pressure, it may be desirable to locate the higher pressure part and the lower pressure part at or close to the respective highest pressure and lowest pressure parts of the circuit. The larger the pressure differential across the reservoir circuit, the more gas that can be accommodated in this reservoir, which will normally need to be a pressuriscd vessel. However, this may not be practical if the temperatures of the gas at those two parts differ too much, as thermal mixing of different temperature gases may be undesirable or affect the cost of the pressure buffer vessel. Hence, the highest and lowest pressure parts might be selected only where the temperatures in the two parts do not differ by more than say 100°C.

As indicated above, it may be desirable to configure the apparatus such that the pressure buffer vessel does not receive gas at more than 300°C, or 200°C or 150°C, or, at less than -20°C, or -10°C or -5°C.

The apparatus may further comprise a control system (e.g. electronic or mcchanic& controller) for controlling operation of the first and second valves in response to a measured change in a variable associated with the cycle, normally a pressure or pressure related variable (e.g. temperature).

The measured change may be a change in pressure at a selected part of the circuit.

The detected change may be a pressure change at a selected part of the circuit, or a temperature change, which may indirectly reflect a pressure change. The measured pressure change may be detected by sensing equipment such as a pressure gauge or sensor located at the selected part of the circuit. The control system may be configured to maintain the pressure within a range defined by a threshold upper pressure and a threshold lower pressure.

The pressure may be measured anywhere suitable in the circuit by any suitable pressure gauge and this may depend upon how the system is set up and whether it is important, for example, to maintain the hot store pressure within a desired pressure range or the cold store pressure within a desired range.

Pressure may be measured at a lower pressure part of the circuit. Pressure may be measured at the outlet of the cold store, which will differ between charging and S discharging. For example, it may be measured by pressure sensing apparatus located on either or both sides of the cold store.

In a system running with an ambient limb after each store (in charging direction), the pressure may be monitored at a lower pressure part of the circuit (e.g. to maintain cold store pressure within a desired narrow range). The pressure may be monitored at the outlet to the cold store (where the pressure is lowest after the pressure drop through the cold store), which outlet will be on the low pressure, ambient limb during charging, and the low pressure, cold limb during charging. Pressure at the hot store could, for example, be measured at the inlet in the high pressure, high temperature (but temperatures there will be very high), or at the outlet in the high pressure, ambient limb, after the gas has cooled back to near ambient.

Where pressure is being monitored at a lower pressure part of the circuit, pressure may be kept within a range defined by a threshold upper pressure and a threshold lower pressure by opening of the first HP valve in response to a detected increase in pressure to a level above the threshold upper pressure at the lower pressure part of the circuit or opening of the second valve LP in response to a detected drop of pressure to a level below the threshold lower pressure at the lower pressure part of the circuit.

The present invention is intended to manage the total gas mass in the system so as to avoid uncontrolled pressure changes by providing a buffer to accommodate gas which is not required as a working fluid as the system charges or discharges.

The apparatus may maintain a system pressure (e.g. a system peak pressure or a minimum system pressure) withm a selected range and that range may be defined by an upper threshold value and a lower threshold value so that a control system may open the HP valve when the upper threshold value is exceeded or open the LP valve when the lower threshold value is exceeded. The range may be larger where, for example, the aim is to keep the hot store system pressure within say 0.5 bar of 12 bar (e.g. 11.5-12.Sbar), or the pressure variation may be more critical and require closer regulation. For example, if the cold store is an unpressurised vessel where cold store pressure is held slightly above ambient (to avoid collapse of low pressure pipes, etc), then it may be necessary to keep the system pressure within quite a narrow range (e.g. +1-0.lbar, or even +1-0.O5bar).

The apparatus may therefore be operated in a constant store pressure mode where the aim is generally to maintain either the hot store, or the cold store, or both hot and cold S stores within reasonably fixed pressure ranges (e.g. within 20%, preferably within 10%,, ideally within 5% of a target store pressure, which itself may need to change during running due to other factors). As indicated above, this has important design advantages. Hence, the LP valve may be configured to open if net contraction of working fluid has caused system pressure to fall below its set point value, and ffirther mass needs to be added to the circuit, and, similarly, the HP valve may be configured to open if net expansion of working fluid has caused system pressure to rise above its set-point value such that gas needs to be extracted from the circuit.

This system control is intended for (slow) control of total mass of working fluid as the system charges and discharges. In addition to accommodating slow changes in the required net mass of working fluid as a result of cyclical charge or discharge, it may accommodate changes in datum temperature (e.g. between daytime and night time temperatures). Other control systems, including pressure controlling systems, may be provided to control pressure in other ways, for example, to create net movement of working fluid from the cold store to the hot store, or vice versa. Typically, the gas buffer should be part of the slowest control feedback loop in a system management control system.

There is further provided, an apparatus for storing energy, comprising: a first stage comprising: a hot half-engine stage which acts as a compressor during charging and as an expander during discharging; and a first heat store for receiving and storing thermal energy from gas compressed by the hot half-engine stage in charging mode, and which transfers thermal energy to the gas compressed by the cold half-engine stage in discharging mode; a second stage comprising: a cold half-engine stage which acts as an expander for receiving gas from the first heat store during charging and which acts as a compressor driving gas into the first heat store during discharging; and a second heat store for transferring thermal energy to gas expanded by the cold half-engine stage during charging, and receiving and storing thermal energy from gas expanded by the hot half-engine stage during discharging; a first valve configured to connect a higher pressure part of the first stage to a gas reservoir at a pressure equal to or below the higher pressure when gas pressure in the lower pressure part exceeds a threshold upper pressure; and a second valve configured to connect a lower pressure part of the second stage to a S gas reservoir at a pressure greater than or equal to the lower pressure when gas pressure in the lower pressure part drops below the threshold lower pressure.

In one embodiment the controller is configured to allow only one of the first and second valves to be open at the same time.

An embodiment 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 2009/044 139; Figures 1 i)-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 an embodiment of the present invention; and, Figure 3 shows a ftrther schematic illustration of an electricity storage system according to an 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 plenum chambers 122, 123, cold storage vessel 110 housing a gas-permeable particulate heat storage structure Ill, upper and lower plenum chambers 112, 113, half-engine stages acting as compressor/expanders 130, 140 and intereonneetingpipes 101,102,103 and 104.

In operation, when charging, ambient temperature gas at a higher pressure exits interconnecting pipe 103 and is expanded by cold half-engine stage 140 to a lower pressure.

The gas is cooled during this expansion and passes via interconnecting pipe 104 to the cold storage vessel 110. The gas enters the lower plenum chamber 113 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 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 enters hot half-engine stage 130, where the gas is compressed to the higher pressure. As the gas is compressed the temperature rises and the gas leaves the hot haff-engine stage at a higher temperature and passes into interconnecting pipe 102. The gas then enters hot storage vessel 120 via upper plenum chamber 122 and passes down through particulate heat storage S 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 filly charged' or stop earlier if required.

This overall charging process absorbs energy that is normally supplied from other generating devices via the electric grid. The half-engine stages 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 interconnecting pipe 103 that maintains the gas in this pipe at or near ambient temperature; one may also be provided in the other ambient limb 101.

In operation, when discharging, high temperature gas at a higher pressure enters interconnecting pipe 102 and is expanded by hot half-engine stage 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 111, where the gas is cooled. The now colder gas leaves particulate heat storage structure 111 and passes into lower plenum chamber 113, from where it enters interconnecting pipe 104. The gas exits interconnecting pipe 104 and enters cold half-engine stage 140 where the gas is compressed to the higher pressure. As the gas is compressed the gas temperature rises and the gas leaves the cold half-engine stage 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 structure 121 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 hot half-engine stage 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 S upwards.

The overall discharging process generates energy that is normally supplied in an electrical form (e.g. back to the clcctric grid). In this mode the half-engine stages acting as comprcssor!cxpandcrs 130 and 140 drive a mechanical dcvicc, such as an electric generator (not shown).

Figures ii), lii) and liii) are examples demonstrating how charging and discharging can alter system conditions and why it is desirable to be able to remove or return gas mass to the system.

Figure Ii) 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 Ti to T2, the pressure must change. By way of example: THI (hot store uncharged temperature) = 300Kclvin TH2 (hot store charged temperature) = 773Kclvin PHI (hot store pressure uncharged) = 12 bar PH2 (hot store pressure charged) = 31 bar I-lowcver, for the same system the temperature within the cold store is also dropping at the same time: TC1 (cold store uncharged temperature) = 300 Kelvin TC2 (cold store charged temperature) = 1 l3Kclvin 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 lii) 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: S Cold Store from I 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 thc charged and unchargcd 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 bctten 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 Hence, to run in a constant store pressure mode, it is necessary to have the ability to remove and return a significant amount of the gas mass in the system.

Alternatively, this may be analysed algebraically. In the energy storage system the stores arc typically of larger volume than the volumes of the interconnecting pipes, and a simple approximate analysis demonstrating the benefit of a gas buffer may be carried out as follows.

As a close approximation within the normally anticipated range of operating temperatures and pressures, the ideal gas law states that pV=inRT where in is the mass of gas in volume V at temperature T and pressure p, and R is a gas constant.

Let the volume of the first (hot) store be VI, and the volume of the second (cold) store be V2. Let the system's high (hot store) pressure bepH and the system's low (cold store) pressure bep. Let the system's high temperature be TN at the junction between the hot store and the hot half-engine stage when the system is charged, and the low temperature be T at the junction between the cold store and the cold half-engine stage when the system is charged. Let the system have a uniform ambient temperature TA when filly discharged. As an approximation (neglecting small quantities of gas in pipes) it is assumed that all the mass of gas is either in V1 or in V2. Based on thermodynamic analysis ofa losslcss energy storage system of this type, the temperatures are related by TH/TA=TA/Tc=(pHpL)1 where y is the ratio of principal specific heat capacities, y =C/C1.

S When the system is uncharged and all gas is at ambient temperature, paVi/T4 + PL V2/TA = ma5ChR where 7disch is the total mass of gas in the system. Similarly when the system is charged, gas within the hot store is at T and gas within the cold store is at T, and PH VJ/TH + Pt. V2/T = !flc*argeR If the mass in of gas is unchanged, (e.g. there is no gas reservoir to take gas out of the system or to add gas back into the system depending on the state of charge), it is possible to write: PH V1/774 + p V/T4 = Pit V1/T + Pt V,/T( If pit, Pt, T, T.4, and T( are viewed as approximately constant for optimum system operation then this equation can be solved for the unique value of the ratio of V1 to V2 which allows the mass to remain constant: V1/V7 PLTH /pnTA=O/pJ' This is a restrictive condition on the design of the system, because other engineering considerations may lead to store volumes which are not in this ratio. Worse, the ratio pit/pt) may depend on variable conditions (such as the ambient temperature) which means that there is no single ratio V1/V2 which allows engine operating conditions to remain constant as the system is charged.

The present invention provides a solution to this problem by providing flexibility to vary the mass of gas in the working circuit of the system such that operating temperatures and pressures may remain optimal, independent of the state of charge of the system and depending as required on external conditions.

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 electricity storage system 200 in accordance with an embodiment S of the present invention. The apparatus comprises a circuit (e.g. gas circuit) configured to allow gas to pass cyclically around during at least one of a charging phase and a discharging phase. The system 100 comprises insulated hot storage vcssel 120' housing a gas-permeabic particulate heat storage structure 121', upper and lower 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' forming a gas circuit 150 for gas to flow eyelieallybetween vessels 120' and 110' via compressor/expanders 130', 140'.

System 200 further comprises an intermediate pressure buffer 205 connected to the gas circuit 150 at a point along interconnecting pipe 101' (a lower pressure part of the gas circuit 150) by interconnecting pipe 201 and at a point along interconnecting pipe 103' (a higher pressure part of the gas circuit 150) by interconnecting pipe 202. Intermediate pressure buffer 200 comprises a fixed volume storage tank 205 containing gas at a pressure that varies between the higher pressure and the lower pressure. Valves 210, 220 act to control flow of gas along interconnecting pipes 201 and 202 respectively to control gas flow between the gas circuit 150 and intermediate pressure buffer 200.

Where the cold storage vessel 110' is at atmospheric pressure then a small volume (atmospheric) buffer (spring-loaded piston in cylinder) 230 may, for example, act as a pressure sensor to provide a signal to controller 240 (e.g. electronic or mechanical controller) which is operative to control opening and closing of valves 210, 220 as discussed below. If the cold storage vessel 110' is not at atmospheric pressure then a volume of gas at the correct pressure must be attached to the opposite side of the small volume buffer 230 to allow it to work correctly.

In response to movement of the piston in buffer 230 upward to a position indicative of a gas pressure increase in the lower pressure part of the circuit beyond the threshold upper pressure, controller 240 acts to open valve 220 (and close valve 210 if open) to allow relatively higher pressure gas to pass from the circuit via connecting passageway 202 to intermediate pressure buffer 200. Once the position of the piston in buffer 230 indicates that the pressure in the lower pressure part of the circuit has returned to a level at or below the threshold upper pressure, controller 240 acts to close valve 220.

In response to movement of the piston in buffer 230 downward to a position indicative of a pressure decrease in the lower pressure part of the circuit below the threshold lower pressure, controller 240 acts to open valve 210 (and close valve 220 if open) to allow relatively higher pressure gas to pass from intermediate pressure buffer 200 via connecting passageway 201 to the circuit. Once the position of the piston in buffer 230 indicates that the pressure in the lower pressure part of the circuit has returned to a level at or above the threshold lower pressure, controller 230 acts to close valve 210. In this way, an efficient way of controlling pressure in the circuit is pmvided that involves minimal moving components.

Referring to Figure 3, this shows a generic pressure gauge/measuring device 280 controlled by a controller 240 which is operatively linked to both valves again. In this drawing, the pressure is shown being monitored at the lowest pressure part of the circuit during charging, namely, at the outlet to the cold store (where the pressure is lowest after the pressure drop through the cold store), which outlet will be on the low pressure, ambient limb (e.g. to maintain cold store pressure within a desired narrow range). This location is identified as A'. However, the pressure may be monitored during discharging at the cold store outlet which is now on the low pressure, cold limb at location B'. As mentioned earlier, where the pressure is measured depends on what sort of control of the cycle is required. Pressure at the hot store could, for example, be monitored and, in that case, it could be measured at the outlet in the high pressure, ambient limb, after the gas has cooled back to near ambient, as indicated by location C'.

As explained above, the intermediate buffer tank comprises a constant volume, variable pressure buffer arrangement intended for (slow) control of total mass of working fluid as the system charges and discharges, or datum temperature flutuates.

Claims

Claims: 1. Apparatus for storing energy using a gas based thermodynamic cycle comprising a circuit comprising: a hot half-engine stage which acts as a compressor during charging and as an expander during discharging; a first heat store for receiving and storing thermal energy from gas compressed by the hot half-engine stage in charging mode, and which transfers thermal energy to the gas compressed by the cold half-engine stage in discharging mode; a cold half-engine stage which acts as an expander for receiving gas from the first heat store during charging and which acts as a compressor driving gas into the first heat store during discharging; and a second heat store for transferring thermal energy to gas expanded by the cold half-engine stage during charging, and receiving and storing thermal energy from gas expanded by the hot half-engine stage during discharging; a first (HP) valve configured to connect a higher pressure part of the circuit to a gas reservoir at a pressure equal to or below the higher pressure when gas needs to be withdrawn from the circuit; a second (LP) valve configured to connect 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.
2. Apparatus according to claim 1, wherein the reservoir is a common sealed gas reservoir configured to store gas at constant volume.
3. Apparatus according to claim 1 or claim 2, wherein the lower pressure part of the circuit is a part located between the second heat store and the hot half-engine stage.
4. Apparatus according to any preceding claim, wherein the higher pressure part of the circuit is a part located between the first heat store and the cold half-engine stage.
5. Apparatus according to any preceding claim, wherein the two parts will be at temperatures within 60°C of each other.
6. Apparatus according to claim 5, wherein the two parts are at or near ambient temperature.
7. Apparatus according to any of the preceding claims, further comprising a control system for controlling operation of the first and second valves in response to a measured change in a variable associated with the cycle.
8. Apparatus according to claim 7, wherein the measured change is a change in pressure at a selected part of the circuit.
9. Apparatus according to claim 8, wherein the control system is configured to S maintain the pressure within a range defined by a threshold upper pressure and a threshold lower pressure.
10. Apparatus according to claim 8 or 9, wherein pressure is measured at a lower pressure part of thc circuit.
11. Apparatus according to claim 10, wherein pressure is measured by pressure sensing apparatus located on either or both sides of the cold store.
12. Apparatus according to any preceding claim, wherein the system is operated in a constant store pressure mode.
13. A method of operating apparatus according to claim I to keep the pressure in the heat stores within a predetermined range while charging or discharging the apparatus, wherein gas is added to the lower pressure part of the circuit from the reservoir, and, is withdrawn to the reservoir from the higher pressure part of the circuit, in response to a decrease or increase in pressure in the circuit, respectively.
14. Apparatus substantially as hercinbefore described with reference to the accompanying drawings.