GB2516453A

GB2516453A - Thermal storage apparatus for rapid cycling applications

Info

Publication number: GB2516453A
Application number: GB1313051.3A
Authority: GB
Inventors: Jonathan Sebastian Howes
Original assignee: Isentropic Ltd
Current assignee: Isentropic Ltd
Priority date: 2013-07-22
Filing date: 2013-07-22
Publication date: 2015-01-28
Also published as: WO2015011438A1; GB201313051D0

Abstract

A thermal storage apparatus 81 for storing and returning thermal energy to a heat transfer fluid passing through the apparatus comprises two or more thermal stores 82, 84 connected in parallel with each other. Each thermal store comprises a fluid-permeable thermal storage media disposed in a chamber for transferring thermal energy to or from the heat transfer fluid. Flow controllers 86, 88 selectively alter the flow path of the heat transfer fluid through the apparatus, wherein the apparatus is configured to be operable in different respective flow modes. In at least one flow mode the apparatus is operable in alternative phases of charging and discharging, and at least one store is selectively configured to perform the charging while at least one other store is selectively configured to perform the discharging. The apparatus may form part of an energy storage system powered by electricity during charging and returns electricity during discharging. The energy storage system may be powered by solar or wind, may be a compressed air energy storage system or a pumped heat energy storage system.

Description

Thermal Storage Araratus for Raøid Cycling Ardications

Field of the invention

The present invention relates to thermal storage apparatus and methods for operating such apparatus, as well as systems incorporating such apparatus, especially energy storage systems.

Background to the Invention

Thermal energy storage (TES) apparatus (or thermal stores) is used to store thermal energy, which may be heat or cold (including cryogenic temperature energy) until subsequently required, and has found application within many industrial processes, such as chemical processing and metal refinement. More recently, IFS apparatus has found application in energy storage systems, including adiabatic compressed air energy storage (ACAES) plants or solar energy or wind based energy plants, especially where back-up capacity is required. TES apparatus is characterised as containing a thermal storage medium i.e. a mass of thermal energy absorbing or rejecting material through which a thermal exchange fluid or heat transfer fluid (HTF) (e.g. a gas or liquid) passes, either releasing heat to the storage medium, thereby heating the store and cooling the fluid, or absorbing heat from it, thereby cooling the store and heating the fluid. The thermal storage medium may be in the form of a porous storage mass, which may be a packed bed of solid particles through which the fluid passes exchanging thermal energy directly, or, it may comprise a solid matrix or monolith provided with HTF channels or interconnecting pores extending therethrough, or, the fluid may pass through a network of heat exchange pipes that separate it from the storage mass, such as a packed bed of particles (e.g. rocks).

In a sensible heat storage situation, a thermal front' is created in the store, i.e. a rise or a fall in temperature in the thermal storage medium with distance moved downstream, which occurs in the region of the store where thermal transfer is most active. Figure 1 illustrates the formation of a thermal front in a thermal store and shows how the process of charging a thermal store sets up a thermal front within a region of the store that progresses downstream and that is usually initially quite steep but which becomes progressively shallower as charging continues. Thus, the front may start with length Li, but as it moves down the vessel it extends in length to length L2 and then L3.

As the front will usually be asymptotic, the length of the front can be discussed in terms of the length of the front between TH2 and TA2, these being for example within 3% of the peak temperature and start temperature.

A similar front occurs in the heat transfer fluid with distance down the store, with the front of the storage medium inevitably lagging the front in the fluid. Often, a heat store will operate with a hotter end and a colder end so as to minimise exergy loss, whereby hotter heat transfer fluid arrives at the hotter end and passes down the length of the store losing heat until it exits the store at the cooler end in one of a charging or discharging phase, and, in the other phase, cooler heat transfer fluid is passed in the reverse direction through the stole before exiting as reheated fluid at the hotter end. Thus, in one phase the fluid has a hot thermal front moving down the store, and in the other phase, a cooler thermal front will make the return travel. Where it is important that apparatus either side or the store only sees a desired higher temperature or desired lower temperature, the practice will be to keep the thermal front held within the store by ceasing to charge or discharge just before the front leaves the store such that the HTF emerges at the same temperature as the nose or tail of the front.

For a certain store geometry a longer front will give lower thermal losses, but the length of the front will also reduce the useable amount of the store i.e. it will reduce the store utilization. If a store is 5m in diameter and lOm long and the thermal tront is 5m of this length, then the store utilization is reduced to approximately 50%-If the same sized store was used and the particle size was reduced, then the same level of thermal losses could be achieved with a much shorter front. So a smaller particle size in a packed bed or pore size in a porous media will tend to give better heat transfer, lower thermal losses and better store utilization. However, there is a pressure drop associated with the fluid flow through the bed and this pressure drop increases significantly as the particle or pore size reduces, so that the losses from the pressure drop will eventually outweigh the benefits of the smaller particle size.

Applicant's earlier application W02011/104556 describes a thermal store in which the size and type of media can be varied through the store to either reduce the irreversibilities that are created when a thermal front is generated or else to help reduce the pressure drop.

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 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.

TES apparatus typically operates in a charging phase, storage phase (e.g. part of fully charging) and discharging phase and, as mentioned above, usually operates such that HTF flow reverses between charging and discharging phases. However, in some systems (e.g. energy storage systems providing back-up capacity to the grid) the TES may be subject to "rapid cycling" i.e. short cycle (or low amplitude) flow reversals (e.g. such that a single direction flow pulse will only move a thermal front over a distance in a store that is very much less than the available front travel within that store). Sensible heat stores (as opposed to latent heat stores) within any rapid cycling system are prone to a loss of effectiveness over successive cycles. The thermal front behaviour associated with this class of heat store will reduce in gradient (dT/dx where x is distance along the store) with each successive flow reversal until a stable condition is achieved, in which each end of the thermal front reaches the store end on every flow reversal. When this occurs, if the flow reversals are very frequent, the thermal front may occupy almost the entire length of the store (as a linear gradient across the store) and the utilisation of the store will be severely compromised.

Applicant's earlier application WO 2009/044139 discloses an energy storage system using thermal stores, namely, a Pumped Heat Energy Storage system (PHES) which, in addition to being cycled between long phases of charging and discharging, may need to be cycled through a small amplitude (e.g. short period of charge followed by a short period of discharge). In the most basic configuration a hot store and a cold store are connected to each other by a compressor and expander acting on a working gas circulating in a circuit. In a charging (or heat pump) mode of the system, heat is effectively pumped from one store to the other by the working gas so as to heat the "hot store" (i.e. charging it with heat) and cool the "cold store" (i.e. charging it with cold) and in a discharge (or heat engine) mode of the system, the process is reversed with the "cold store" being used to cool gas prior to compression (i.e. discharging its cold) before the gas is heated in the "hot store" (i.e. discharging its heat) and expanded to generate power. The system can use a variety of different types of compressors and expanders, some examples are reciprocating, rotary screw, sliding vane, axial or centrifugal.

The present Applicant has identified the need for an improved thermal storage apparatus that is better suited to applications where there may be rapid cycling.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided thermal storage apparatus for storing and returning thermal energy to a heat transfer fluid passing through the apparatus, comprising: two or more thermal stores connected in parallel with each other, each thermal store comprising fluid-permeable thermal storage media disposed in a chamber for transferring thermal energy to or from the heat transfer fluid, and, flow controllers for selectively altering the flow path of the heat transfer fluid through the apparatus, wherein the apparatus is configured to be operable in different respective flow modes, wherein in at least one flow mode the apparatus is operable in alternative phases of charging and discharging as the nature of the incoming heat transfer fluid changes, and at least one store is selectively configured to perform the charging while at least one other store is selectively configured to perform the discharging, thereby minimising or avoiding switching of those individual stores between charging and discharging in that flow mode and the thermal front reversals associated therewith.

"Operable in alternative phases of charging and discharging as the nature of the incoming heat transfer fluid changes" is intended to mean that the apparatus as a whole in that flow mode (or flow set-up) is operable in, and able to switch between, a charging function or discharging function depending on the changing nature of the incoming heat transfer fluid requirement (e.g. temperature/direction); however, in addition to those active phases, the apparatus may also switch into other phases e.g. an inactive storing phase.

Charge can mean to charge with heat (or store heat) in which case the discharging is discharging or returning of the heat, or, it can mean to charge with cold (return heat), in which case the discharging is discharging of that cold (or storing heat). In either instance, the storage medium heats in one phase and cools in the other phase, but the charging and discharging nomenclature is of assistance when discussing thermal storage that forms part of energy storage systems.

In the at least one flow mode the at least one store may be selectively configured only to perform charging while the at least one other store may be selectively configured only to perform the discharging, so as to avoid any switching of those individual stores between charging and discharging and the thermal front reversals associated therewith.

The present invention is concerned with the problem of protecting the integrity of thermal fronts within a thermal storage apparatus required to switch between charging and discharging, i.e. as alternative active operational phases, and where the thermal fronts in individual stores would normally reverse their direction whenever the function of the individual store changes between charging and discharging, such that especially low amplitude, rapid cycling switching of the apparatus per se would cause degradation of the steepness of the thermal front (i.e. undesirable front lengthening) in those individual stores.

According to the present invention, stores are not caused to switch their individual charging or discharging functions (causing front reversal) every time the apparatus perse is required to switch its charging/discharging function (e.g. due to the heat transfer fluid reversing its direction through the thermal storage apparatus). Rather, the apparatus has at least one flow mode that protects individual stores from rapid, low amplitude cycling (i.e. where the apparatus is prone to frequent switching between charging and discharging) by virtue of the fact that, within that one flow mode, the selected at least one store and at least one other store have their dedicated respective charging and discharging roles.

In use, while the apparatus per se may switch rapidly between bursts of charging or discharging activity, within each of the respective selected stores a thermal front may progress steadily in one direction (e.g. in the discharging store in increments corresponding to the respective bursts of required discharging activity) along the store, usually from near or at one end of the store towards the other, without reversing.

Preferably, both the at least one store and the at least one other store are each configured such that a thermal front is only allowed to progress in a single direction along each respective store, without reversal, during the at least one flow mode.

Preferably, the apparatus is configured to switch from the at least one flow mode to a different flow mode when a selected end condition is reached in the at least one store or at least one other store. While the at least one flow mode may be interrupted before either store has become fully charged or discharged, ideally, the flow mode is maintained for as long as possible until one (or simultaneously both) of the selected stores is about to exceed a preselected maximum capacity (i.e. selected end condition) such that the apparatus cannot continue in that mode. Ideally, the apparatus is configured to switch from the at least one flow mode to a different mode just as a thermal front approaches the end of a thermal store, so as to retain and preserve that front in that store, for when the store is next required to perform the alternative charging/discharging function.

Preferably, the apparatus is configured such that, in the at least one flow mode, it can switch between charging and discharging without altering the setting of flow controllers that control the flow to the respective at least one store and at least one other store. Conveniently, the apparatus may be able to switch between charging and discharging merely by reversing the direction of the heat transfer fluid through the thermal storage apparatus.

In one embodiment, the apparatus comprises two thermal stores, wherein the apparatus is also operable in a second flow mode in which the at least one store is selectively configured to perform the discharging while the at least one other store is selectively configured to perform the charging. Thus, the two stores reverse their functions between the two flow modes. However, other stores may also be present and may be allocated those functions so that different respective pairs of stores are used in different respective flow modes. Further, two or three or more stores may be allocated the respective charging and discharging functions in each mode, and they may be selected on the basis of differing physical characteristics between different stores.

The heat transfer fluid may be a liquid or gas/vapour; the latter may apply where the fluid acts as a working fluid in a gas based thermodynamic cycle.

Thermal transfer will usually occur under isobaric conditions. The at least one thermal store and at least one other thermal store may be configured to receive a heat transfer fluid at substantially similar pressures.

The fluid-permeable thermal storage medium may comprise any materials suitable for direct thermal exchange with the heat transfer fluid at the temperatures and pressures contemplated. The media may comprise a monolithic porous structure provided with through passageways and/or a network of interconnecting pores, or, may comprise a particulate material.

In one embodiment, the gas-permeable thermal storage medium comprises particulate material. The particulate material may comprises solid particles and/or porous media and/or fibres and or foamed material (e.g. metallic, mineral or ceramic particles and/or fibres and/or foam) packed to form a gas-permeable thermal storage structure.

The invention is applicable to thermal stores in which the media is arranged as a continuous mass (e.g. single monolithic porous mass or particulate media), or where it is separated into respective downstream layers, as described above in relation to Applicant's layered store application.

The apparatus is operable both to charge and discharge as alternative active modes (i.e. modes where flow occurs), so that charging or discharging is conducted sequentially; there will, however, be other inactive modes e.g. where the apparatus is left in a fully charged, part charged or fully discharged state.) It is applicable to systems or equipment in which the working fluid flows through the thermal storage apparatus in one direction upon charging and the opposite direction upon discharging (e.g. where the thermal storage apparatus has an inlet/outlet at one end and an outlet/inlet at the opposed end), and to systems or equipment in which the heat transfer fluid acts as a working fluid (i.e. undergoing compression and/or expansion) doing work within a thermodynamic cycle. The thermal storage apparatus may be located within and may charge and discharge the heat transfer fluid to a single primary (closed) circuit (e.g. a pumped heat energy storage system) or limb (open circuit), which may form part of an energy storage system, or, may charge the heat transfer fluid to a primary circuit and discharge the heat transfer fluid to a secondary circuit of the system of which it forms a part.

The thermal storage apparatus may form part of an energy storage system. This may be powered by electricity in a charging phase and/or return electricity (e.g. to the electricity grid) in a discharging phase.

The energy storage system may be powered by solar power or wind power.

Thermal storage apparatus as configured above is particularly suited for use in an energy system powered by wind, solar or other power sources prone to fluctuating behaviour.

The energy storage system may comprise a compressed air energy storage CAES system, especially an adiabatic CAES.

The energy storage system may comprise a pumped heat energy storage system for storing and returning energy comprising: a first engine stage configured to act as a compressor during a charging phase of a cycle and as an expander during a discharging phase of the cycle; a first thermal storage arrangement (or hot storage arrangement") for receiving and storing thermal energy from gas compressed by the first engine stage during the charging phase; a second engine stage configured to act as an expander during the charging phase to expand gas received from the first thermal store and as a compressor during the discharging phase; a second thermal storage arrangement (or "cold storage arrangement") for transferring thermal energy to gas expanded by the expander during the charging phase; wherein one or both of the first and second thermal storage arrangements comprises thermal storage apparatus as defined above.

In a further aspect, there is provided thermal storage apparatus for storing and returning thermal energy to a heat transfer fluid passing through the apparatus, comprising: two or more thermal stores connected in parallel with each other, each thermal store comprising fluid-permeable thermal storage media disposed in a chamber for transferring thermal energy to or from the heat transfer fluid, and, flow controllers for selectively altering the flow path of the heat transfer fluid through the apparatus, wherein the apparatus is configured such that heat transfer fluid will pass through each store in a first direction if that store is storing thermal energy and in the reverse direction if that store is returning thermal energy, and wherein the apparatus is configured to be operable in at least one flow mode wheiein the apparatus is operable both to store thermal energy and return thermal energy, but not simultaneously, and at least one store is selectively configured only to store the thermal energy while at least one other store is selectively configured only to return the thermal energy, thereby minimising or avoiding switching of those individual stores between the storing and returning functions and the thermal front reversals associated therewith.

In accordance with a further aspect of the present invention, there is provided a method of operating thermal storage apparatus to store and return thermal energy to a heat transfer fluid passing through the apparatus, wherein the apparatus comprises: two or more thermal stores connected in parallel with each other, each thermal store comprising fluid-permeable thermal storage media disposed in a chamber for transferring thermal energy to or from the heat transfer fluid, and, flow controllers that selectively alter the flow path of the heat transfer fluid through the apparatus, wherein the apparatus operates in different respective flow modes, wherein in at least one flow mode the apparatus switches between operating in alternative charging and discharging phases, and at least one store is selectively configured to perform the charging while at least one other store is selectively configured to perform the discharging, thereby minimising or avoiding switching of those individual stores between charging and discharging and the thermal front reversals associated therewith.

The thermal storage apparatus may cycle between phases of charging and discharging activity (i.e. that activity is not simultaneous) in the at least one flow mode while in each store a thermal front advances progressively in only a single direction relative to each respective store in steps corresponding to the respective phases of activity.

The apparatus may switch to a different flow mode in response to a thermal front reaching the other end of one of the respective stores.

In a yet further aspect, there is provided a method of operating thermal storage apparatus so as to minimise thermal front reversals therein, wherein the apparatus comprises two or more thermal stores connected in parallel with each other, each thermal store comprising fluid-permeable thermal storage media disposed in a chamber for transfer of thermal energy to or from a heat transfer fluid passing through the media, and flow controllers for selectively altering the flow path of the heat transfer fluid through the apparatus, wherein the apparatus is configured such that heat transfer fluid will pass through each store in a first direction if that store is storing thermal energy and in the reverse direction if that store is returning thermal energy, and wherein the apparatus is operable in different respective flow modes, the method comprising operating the apparatus in at least one flow mode in which the apparatus switches alternately between storing thermal energy and returning thermal energy and at least one store is assigned solely for storing thermal energy and at least one other store is assigned solely for returning thermal energy such that, during that flow mode, the heat transfer fluid does not reverse direction within either store when the apparatus switches.

In accordance with a further aspect of the present invention, there is provided apparatus for storing and returning energy, comprising: a first engine stage configured to act as a compressor during a charging phase of a cycle and as an expander during a discharging phase of the cycle; a first thermal storage arrangement (or "hot storage arrangement") for receiving and storing thermal energy from gas compressed by the first engine stage during the charging phase; a second engine stage configured to act as an expander during the charging phase to expand gas received from the first thermal store and as a compressor during the discharging phase; a second thermal storage arrangement (or "cold storage arrangement") for transferring thermal energy to gas expanded by the expander during the charging phase; wherein at least one of the first and second thermal storage arrangements comprise: a first thermal store; and a second thermal store connected in parallel to the first thermal store; wherein: in a first mode of operation the apparatus is operative to pass gas through the first thermal store during the charging phase and to pass gas through the second thermal store during the discharging phase; and in a second mode of operation the apparatus is operative to pass gas through the first thermal store during the discharging phase and to pass gas through the second thermal store, or through a further thermal store connected in parallel to the first and second thermal stores, during the charging phase.

In this way, apparatus for storing energy is provided in which the at least one of the first and second thermal storage arrangements may be switched between a charging phase and discharging phase without the need to reverse gas flow through a partially discharged/charged thermal store.

In one embodiment, the apparatus is operative to: charge the first thermal store from a substantially uncharged configuration to a substantially fully charged configuration without any intermediate discharging of the first thermal store; or, to discharge the second thermal store from a substantially fully charged configuration to a substantially uncharged configuration without any intermediate charging the second thermal store.

In one embodiment the first and second engine stages comprise separate apparatus. In each stage, the apparatus may comprise separate apparatus for respectively compressing and expanding gas, or may comprise a device operable in a first mode to compressor gas and operable in a second mode to expand gas.

In another embodiment, the first and second engine stages may involve the use of the same apparatus for compressing and expanding gas (e.g. with compression and expansion occurring sequentially by alternative connections).

In one embodiment, the apparatus comprises: a valve arrangement operative to configure the apparatus between the first and second modes of operation; and a controller (e.g. mechanical or electronic controller) operative to control operation of the valve arrangement to switch between the first and second modes of operation.

In one embodiment, the apparatus is configured to generate gas flow in a first direction during charging and in a second direction, opposed to the first direction, during discharging, and the valve arrangement is operable: in the first mode to allow gas flow in the first direction to pass through the first thermal store and to prevent gas flow in the first direction passing through the second thermal store; and in the second mode to allow gas flow in the second direction to pass through the second thermal store and to prevent gas flow in the second direction passing through the first thermal store.

In one embodiment, the valve arrangement comprises first and second valves (e.g. first and second interruptible non-return valves) for controlling gas flow through one of the first and second thermal stores, the first and second valves being connected in series and being configurable between: a first valve mode in which flow is permitted through the first valve in the first direction only and through the second valve in both the first and second directions; and a second valve mode in which flow is permitted through the second valve in the second direction only and through the first valve mode in both the first and second directions.

In one embodiment the one of the first and second thermal stores is positioned in series between the first and second valves.

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

In one embodiment, the first and second thermal stores are configured to receive gas at substantially similar pressures.

In one embodiment, at least one of the first and second thermal stores comprise: a chamber for receiving gas; and a gas-permeable thermal storage medium housed in the chamber (i.e. to allow direct heat transfer between the gas flow and the thermal storage medium).

In one embodiment, the second thermal store comprises first and second thermal sub-stores connected in parallel and the apparatus is configured to pass gas through either the first thermal sub-store or through second thermal sub-store.

In one embodiment, the apparatus is operative to pass gas through the first thermal sub-store when a temporary change between the charging and discharging phases is determined to last for a time period below a predetermined threshold and operative to pass gas through the second thermal sub-store when a temporary change between the charging and discharging phase is determined to last for a time period above the predetermined threshold.

In one embodiment, the second thermal sub-store is configured to provide a thermal charge/discharge efficiency which is higher (e.g. substantially higher) than that of the first thermal sub-store.

In accordance with another aspect of the present invention, there is provided a method of operating an energy storage system comprising: during a charging phase of a cycle: compressing a gas; transferring heat from the compressed gas to a first thermal storage arrangement for receiving and storing thermal energy from the compressed gas; expanding the gas after heat from the gas has been transferred to the first thermal storage arrangement; and transferring heat from a second thermal storage arrangement to the expanded gas compressed during the charging phase; and during a discharging phase of the cycle: cooling a gas by transferring heat from the gas to the second thermal storage arrangement; compressing the gas cooled by the second thermal storage arrangement; heating the compressed gas by transferring heat from the first thermal storage arrangement to the gas; and expanding the gas heated by the first thermal storage arrangement to generate a power output; wherein at least one of the first and second thermal storage arrangements comprise: a first thermal store; and a second thermal store connected in parallel to the first thermal store; and during a first mode of operation: passing gas through the first thermal store during the charging phase; and passing gas through the second thermal store during the discharging phase; and during a second mode operation: passing gas through the first thermal store during the discharging phase; and passing gas through the second thermal store, or through a further thermal store connected in parallel to the first and second thermal stores, during the charging phase.

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 graph illustrating thermal front progression within a storage vessel; Figures 2a and 2b show thermal storage apparatus according to a first embodiment of the present invention operating in two respective flow modes; Figures 3a to 3h are schematic illustrations of the thermal storage apparatus of Figure 2 in a plurality of charging configurations during use in two flow modes; Figure 4 shows a schematic illustration of an energy storage system according to a second embodiment of the present invention; Figures 5a to 5e illustrate the energy storage system of Figure 4 in a plurality of charging configurations during use; Figures 6a to 6c illustrate in more detail the operation of a hot storage arrangement of Figure 4; Figures 7a and 7b illustrate the operation of the hot storage arrangement of Figure 4 in a first flow mode; Figures Ba and 8b illustrate the operation of the hot storage arrangement of Figure 4 in a second flow mode; and, Figuie 9 is a schematic illustration of a hot stolage airangement according to a further embodiment of the invention.

Detailed Description of Specific Embodiments

Figuies 2a and 2b show thermal stolage appalatus 81 accoiding to a first embodiment of the piesent invention for storing and returning thermal eneigy, especially in applications wheie the apparatus may be subjected to successive pad charge/part discharge cycles. Such appalatus may, for example, form pait of a chemical processing plant oi an energy storage system.

Figuies 2a and 2b respectively depict two respective flow modes (oi flow set-ups) in which the apparatus 81 may operate, with the apparatus being able to both charge and discharge in each respective flow mode.

Referring to Figure 2a, the apparatus 81 comprises two pressure vessels 82, 84 acting as thermal stores arranged in parallel. In order to minimise convection effects, the apparatus and stores are arranged vertically between a downward Hot Feed HF, such that hot working fluid approaches the apparatus from above and moves downwardly through the apparatus/stores, and an upward Cold Feed CF, whereby, alternatively, relatively colder working fluid approaches the apparatus from below and moves upwardly through the apparatus/stores. Thus, a hot front (bringing relatively hotter fluid) will always move down each store and a cold front (bringing relatively colder fluid) will always move up a store, such that in any part-charge situation heat is already located at the top of a store.

In this embodiment, each store 82, 84 has an upper port branching into two conduits each containing a non-return valve NRV 88, which valves are in opposed arrangements so that they permit fluid flow in opposed directions in the respective conduits. The conduits rejoin into a single upward conduit 83 at a junction provided with a selector valve 86 which, as depicted, can be alternatively adjusted so that one or other of the conduits is in open communication with conduit 83. Conduits 83 from each store 82, 84 meet and join in a single upper conduit 85 through which the Hot Feed would be supplied. At the bottom of the apparatus, a conduit 87 supplying the Cold Feed branches into respective conduits leading to the lower port of each store without any valving present to inhibit flow. In this embodiment, the two thermal stores 82, 84 are provided in parallel with this valving with the aim of maintaining a common flow direction (thermal front direction) within each individual store despite periodic reversing of the flow direction within the apparatus per se, which would otherwise cause undesired lengthening of the thermal front due to the successive flow reversals in a single store.

Looking at a generalised case first, a system including a heat storage function may exist in two HTF flow conditions defined by reversal of a flow of heat transfer fluid across the heat storage element, where the flow in one direction will be arriving as a hotter flow and the flow in the other direction will be arriving as a relatively cooler flow.

These may be defined as "HTF flow condition one" and "HTF flow condition two" and relate to the heat transfer fluid. The heat storage element may comprise two heat stores connected to the system fluid flow circuit in parallel (but further stores in parallel could also be used). Each heat store may be either heated by the fluid flow, thus cooling the flow, or the store cooled by the flow, hence heating the flow and these states are referred to as HS "heating store" or CS "cooling store" respectively Naming the stores as store A and store B, in HIF flow condition one, store A is a cooling store and flow is prevented (eg by a valve) from passing through store B. In HTF flow condition two, store B is a heating store and flow is prevented from passing through store A. Therefore, if the system flow is repeatedly changed or cycled between HTF flow conditions one and two, store A only acts as a cooling store and store B only acts as a heating store with the thermal fronts in each store progressing intermittently in a single direction on each cycle of switching between flow conditions one and two, as the direction (and nature) of the heat transfer fluid changes.

Eventually the front in one store will reach a point of travel where it is desirable to stop the process i.e, a "selected end condition" (discussed below) where the front has reached the end of one store and further travel will, for example, lead to an unacceptable exit flow temperature variation with respect to the needs of the system. When this selected end condition is reached, the operation of stores A and B can be reversed.

Up until now, the system can be regarded as being in a first flow mode of operation, where flow controllers have assigned the store A and B the above respective functions. If the system alters to a different flow mode of operation (upon reaching the selected end condition) where the flow controllers now assign the opposite functionality to the stores, then store A becomes the heating store under HTF flow condition two, while store B becomes a cooling store under HTF flow condition one (i.e. as heat transfer fluid direction changes).

Thus, referring back to Figures 2a and 2b, the downward arrow next to HF "Hotter Fluid denotes the downward direction that a hotter heat transfer fluid will approach the apparatus, while CF Cooler Fluid" next to the upward arrow denotes the upward direction that a cooler heat transfer fluid will approach the apparatus, in the alternative flow condition.

Figure 2a shows a first flow mode of operation where selector valves 86 have assigned store 82 and 84 the respective functionalities of CS Cooling Store and HS Heating Store, regardless under which HTF flow condition the heat transfer fluid is operating (i.e. whether HF or CF is arriving).

Referring to Figure 2a, it will be seen that fluid flow CF from bottom to top of the diagram is prevented from passing through the right hand store 84 (acting as the "heating store" HS) by the right hand selector valve 86 blocking feed from the relevant non-return valve. The flow CF will pass through the left hand cooling store 82 CS as the left hand selector valve 86 has opened the flow path to flow in this direction via the appropriate non-return valve Similarly, if the flow condition of the transfer fluid changes (e.g. a switch from charging to discharging is required in an energy storage system containing this apparatus) and there is HF flow from top to bottom, then the valve combination on the left prevents flow from passing through the cooling store 82 CS but allows it to pass through the heating store 84 HS.

Upon reaching the selected end condition in the first flow mode of operation, the system needs to alter to a different flow mode of operation and this is accomplished in this embodiment merely by adjustment of the selector valves 86, without adjustment of the parallel NRV's.

Thus, Figure 2b shows a second flow mode of operation where selector valves 86 have assigned store 82 and 84 the now respective reverse functionalities of HS Heating Store and CS Cooling Store, regardless under which flow condition the heat transfer fluid is operating (i.e. whether HF or CF is arriving). Hence, in each individual store, the thermal front directions change and the left hand store becomes the "heating store" while the right hand store becomes the "cooling store". (The arrows in the stores in both figures are intended merely to indicate the respective directions of the thermal fronts in each store; there is no simultaneous reverse flow in both stores.) In a respective flow mode, a selected end condition should be chosen for each store since either may reach that condition first. The selected end condition may be chosen to be such that the thermal front is close to but has not yet exited the store (it being desirable to keep a front within a store, rather than needing to create a new one), and it may be detected, for example, by monitoring for a certain rise or fall in exit gas temperature from the system. A selected end condition may only correspond for example to -50-75% of the full theoretical capacity of the storage medium (if 100% corresponds to the whole stole rising to say close to the entry temperature of a hotter transfer fluid), with steeper fronts leading to higher capacities.

The switching of the selector valves 86 may be made automatic by sensing of the store downstream temperatures. When these show a variation beyond a limit acceptable to the system, then both selector valves are switched and the fronts in each store change direction while the stores swap roles from "cooling" to "heating" and vice versa.

The above example references "heating" and "cooling". Altematively, for convenience, the more generic terms "charging" and "discharging" may be employed, as in the remainder of the embodiments, this sometimes being more helpful in the context of thermal storage apparatus forming part of an overall system, particular an energy storage system References to charging and discharging of an individual store will involve both heating and cooling of the store, and in the context of the present specification, references to charging should be understood to mean either charging with heat, in which case discharging will be discharging of that heat, or, charging with cold, in which case the discharging will be discharging of that cold. (It will be appreciated that discharging a store that is intended to provide a sub-ambient temperature reservoir will result in the heating of that store, while charging a super-ambient temperature store will also require the heating of the store, such that charging and discharging are ambiguous as to whether thermal energy is increasing or decreasing in the store.) In summary, by the use of thermal storage apparatus comprising at least two parallel thermal stores that are assigned respective charge and discharge roles within at least one flow mode, together with appropriate associated flow controllers, it is possible to maintain thermal front quality in situations where the apparatus is subjected to rapid cycling. The "heating" and "cooling" stores may then be configured to switch function by the flow controllers switching the apparatus to a different flow mode, as and when either store is incapable of preserving its original function due to its charge state; in the different flow mode, the same pair of stores may be assigned the reverse functionality, or one or more different new stores may be assigned that functionality. Referring to Figures 3a to 3h, these are schematic illustrations of the thermal storage apparatus of Figure 2 illustrating how it progresses through a plurality of charging states during use in two flow modes Ml and M2. In flow mode Ml (corresponding to Figure 2a flow mode), Cooler Fluid flow can only pass up through the apparatus via left hand store 82. In flow mode M2 (corresponding to Figure 2b flow mode), Cooler Fluid flow can only pass up through the apparatus via right hand store 84. Hatching denotes charging with heat with thermal fronts 82a and 84a in stores 82 and 84 shown as horizontal borders for simplicity (although their profiles would more resemble those of Figure 1).

Figure 3a shows that in operative mode Ml store 82 is fully charged with heat and is assigned as Cooling Store CS, while store 84 is empty of heat (although preferably with thermal fronts parked near the respective ports) and is assigned as Heating Store HS. At this point, HF is arriving with heat for storage and transfer fluid is passing through the apparatus from top to bottom. By Figure 3b, store 84 has undergone some charging with heat and the thermal front has progressed downwards somewhat. At this point, the apparatus is subject to a reversal of heat transfer fluid, with cooler fluid arriving at the bottom of the apparatus. In response, store 82 assigned as Cooling Store CS starts to discharge its heat (i.e. cool), with a cold front 82a starting to move up from the bottom of that store, as shown in Fig 3c. The hot thermal front in store 84 remains stationary during this temporary period of reversal. A further reversal of heat transfer fluid back to HF arriving leads to the cold front 82a in store 3d remaining static, whereas the hot front 84a in store 84 assigned as HS progresses down in a further increment.

After further reversals of heat transfer fluid, Figure 3e shows the two stores have reached a point where the flow mode needs to switch to flow mode M2 because the selected end condition for the Heating Store HS 84 has been reached with the hot front just adjacent the end of that store so that any further charging of that store would generate excessive outlet temperatures. Flow mode M2 is therefore selected by the selector valves so that stores 82 and 84 swap such that they are assigned as HS and CS respectively, as shown in Figure 2b. It will be noted that the front in Figure 3e is therefore forced to change direction before it reaches the end of that store.

In flow mode M2, between Fig. 3f and 3h, the valving has changed the functionality. Figure 3g shows a hot front moving down store 82 as it acts as HS. If the pair of stores is not thereafter subjected to a requirement to discharge heat, it is possible for the hot front in store 82 to reach the end such that both stores become fully charged with heat. Thereafter no further charging is possible.

There are now various options. The reaching of a selected end condition in flow mode M2, can trigger the apparatus swapping back to flow mode Ml so store 82 acts as Cooling Store CS. Alternatively, the apparatus could remain in flow mode M2, such that store 84 continues in its function of Cooling Store CS. Thus, when one store reaches its selected end condition acting in a particular function such that further functionality of that type is required, flow mode switching may be initiated, or, in the event that no further (e.g. third parallel store) functionality of that type is available, the apparatus may remain in the same mode, or swap to a further flow mode in which, for example, functionality is not assigned to preserve front quality but where, for example, stores are used in parallel to achieve a high charge or discharge rate.

As an example of a suitable control logic, operation of the at least two parallel stores in the at least one flow mode may be expressed as:-If charging a store continue to use this store ONLY to charge until full OR if this store is the only remaining supply of stored energy switch this store to a different flow mode such that it is discharging.

If discharging a store continue to use this store ONLY to discharge until empty OR if this store is only remaining space to store energy then switch this store to a different flow mode such that it is charging.

It will be appreciated that in each of modes Ml and M2, thermal fronts have remained static in each store or progressed in increments in only one direction in that store. Thermal front reversal in each store is not therefore linked to a change of direction of the heat transfer fluid through the apparatus (i.e. associated with a change in requirement between the alternative functions ot charging and dischaiging of the apparatus), but rather is linked to a change of flow mode preferably initiated by the reaching of a selected end condition in one store, and achieved by a change of valving.

Figure 4 shows an electricity storage system 10 comprising a hot storage arrangement 20, a cold storage arrangement 30, first and second compressor/expanders 40, 50, interruptible non-return valves (NRVs) 60, and an arrangement of interconnecting pipes 70 for conveying working gas around the system to form a gas circuit 80. Hot storage arrangement 20 comprises first and second insulated hot storage vessels 22, 24 each housing a gas-permeable particulate thermal storage structure 22a, 24a respectively. Cold storage arrangement 30 comprises first and second insulated cold storage vessels 32, 34 each housing a gas-permeable particulate thermal storage structure 32a, 34a respectively.

In operation, when charging, gas that may be near ambient temperature or at a different temperature, but at a higher pressure exits hot storage arrangement 20 and is expanded by second compressor/expander 50 to a lower pressure. The gas is cooled during this expansion and passes to cold storage arrangement 30 where the cooled gas is heated (thereby cooling the cold storage arrangement 30). The now hotter gas leaves cold storage arrangement 30 at a temperature that may be around ambient or a temperature that is different to ambient. The gas then enters first compressor/expander 40 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 hot storage arrangement 20 where the gas is cooled (thereby heating the hot storage arrangement 20). The now cooler gas leaves hot storage arrangement 20. The process can continue until the hot and cold stoles are fully charged' (i.e. fully heated and cooled respectively) or stop earlier if required.

This overall charging process absorbs energy that is normally supplied from other generating devices via the electric grid. The compressor/expanders 40, 50 are driven by a mechanical device, such as an electric motor (not shown).

In operation, when discharging, high temperature gas at a higher pressure exits hot storage arrangement 20 and is expanded by first compressor/expander 40 to a lower pressure. The gas is cooled during this expansion and passes to cold storage arrangement 30 where the gas is cooled (thereby warming the cold storage arrangement 30). The now colder gas leaves the cold storage arrangement 30 and then enters second compressor/expander 50 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 hot storage arrangement 20 where the gas is heated (thereby cooling the hot storage arrangement 20). The now high temperature gas leaves hot storage arrangement 20 and is expanded by compressor/expander 40 with the energy of expansion being used to generate electricity for the electric grid. The process can continue until the hot and cold storage arrangements 20, 30 are fully discharged' (e.g. returned to ambient temperature) or stop earlier if required.

As illustrated, cold storage arrangement 30 is charged with cold with the flow entering from the bottom and travelling upwards and discharged of cold with the flow entering from the top and travelling downwards. Hot storage arrangement 20 is charged with heat with the flow entering from the top and travelling downwards and discharged of heat 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 40, drive a mechanical device, such as an electric generator (not shown).

Figures 5a)-e) show in more detail a preferred manner in which hot and cold storage arrangements 20, 30 of a PHES system are charged/discharged.

It should be noted that, in this example, fully charged configurations A and B refer to respective operatively linked fully charged pairs of stores (e.g. 22 and 34, or, 32 and 24); in fact, in this example, only half of the total thermal storage capacity of the four stores is ever used in order to ensure that any low amplitude cycling can be met without any thermal front reversals in individual stores. Further a fully discharged store in this example may have some thermal energy still remaining in that store due to deliberate retention of a region of thermal front: the creation of a new thermal front can create losses so that it may be more efficient to keep' it to use with the next charge cycle.

Figure 5a shows the electricity storage system 10 in a first charged configuration "A" in which first insulated hot storage vessel 22 of hot storage arrangement 20 and second insulated cold storage vessel 34 of cold storage arrangement 30 are each fully charged and second insulated hot storage vessel 24 of hot storage arrangement 20 and first insulated cold storage vessel 32 of cold storage arrangement 30 are each fully discharged.

Figure 5b shows electricity storage system 10 during a subsequent charging phase in which charging of the second insulated hot storage vessel 24 and first insulated cold storage vessel 32 has begun.

Figure 5c shows electricity storage system 10 during a subsequent short discharging phase triggered (e.g. by a sudden brief demand for electricity to be generated by the system 10) before charging of the second insulated hot storage vessel 24 and first insulated cold storage vessel 32 is complete. As illustrated, the flow of gas is not reversed through second insulated hot storage vessel 24 and first insulated cold storage vessel 32 but instead is reversed through first insulated hot storage vessel 22 and second insulated cold storage vessel 34 that are each primed for discharge.

As shown in Figure 5d, once the short discharging phase is over the system returns to the charging phase and charging of the second insulated hot storage vessel 24 and first insulated cold storage vessel 32 continues. If the charging phase needs to be interrupted again before second insulated hot storage vessel 24 and first insulated cold storage vessel 32 are fully charged, the system again operates to direct the reversed flow of gas through first insulated hot storage vessel 22 and second insulated cold storage vessel 34 in the manner illustrated in Figure 5c to continue the discharge of these stores.

Figure 5e shows electricity storage system 10 in a second fully charged configuration "B" in which second insulated hot storage vessel 24 and first insulated cold storage vessel 32 are each fully charged and first insulated hot storage vessel 22 and second insulated cold storage vessel 34 are each fully discharged.

The activity shown in Figures 5a to 5e have taken place in a first operative flow mode with the stores assigned the functionality shown above. The process may then be reversed by swapping to a second operative flow mode, where the functionality and hence the flows in each of the stores are reversed. This may return the system back to charged configuration A' with occasional low amplitude cycling reversing the overall flow although, due to the use of delegated stole functions, in each store a front merely remains static or progresses in only a single direction towards the other end of the store.

It should be noted that, after Figure 5e, even if the valves and dedicated roles reverse, it may be possible if the stores are only required to charge, for all four stores to become fully charged (i.e. two hot stores fully storing heat and two cold stores fully storing cold).

Figures 6a to 6c, 7a, 7b, 8a and 8b show in more detail the operation of the electricity storage system 10, and in particular, the operation of NRVs 60 to control gas flow through hot storage arrangement 20 (operation of the apparatus on the cold storage side is equivalent).

As illustrated in Figure 4, an NRV 60 is positioned at each end of each of insulated hot storage vessels 22, 24 and each of insulated cold storage vessels 32, 34 (with each NRV 60 being operative when acting to restrict the direction of gas flow to allow gas flow into its respective storage vessel). As shown in Figures 6b and 6c, NRVs have two modes of operation: a first mode in which they are configured to act as conventional non-return valves and permit gas flow in one direction only; and a second mode in which the operation of the valves is "interrupted" to permit gas flow in opposed directions. As illustrated in Figure 6b, each NRV 60 may be a simple mechanical flap valve 62 that pivots around a pivot point "P" with a lever 64 also attached at the pivot point and capable of being acted upon by an actuating force as shown in Figure 6c so as to hold the valve 62 open ("the interrupted" state).

As shown in Figure 6a, during a first flow mode intended to be primarily charging (with occasional discharge cycling) first insulated hot storage vessel 22 is defined as being in a "charge" state, as shown, whilst second insulated hot storage vessel 24 is assigned the opposed functionality i.e. is in a "discharge" state. In this operative mode, as shown in Figures 7a and Yb, the valve states remain the same regardless of whether system 10 is charging (Figure Yb) or discharging (Figure 7a). For the "charge" store 22 the NRV 60 that prevents flow in a hot to cold direction is interrupted (i.e. held open) and the NRV 60 that prevents flow in a cold to hot direction will allow flow in a hot to cold direction (i.e. charge direction). For the "discharge" store 24, the NRV 60 that prevents flow in a hot to cold direction will allow flow in a cold to hot direction (i.e. discharge direction) and the NRV 60 that prevents flow in a cold to hot direction is interrupted (i.e. held open). In this way, working gas entering the manifold at the hot end of the stores will only pass through the "charge" store 22, as shown. If, on the other hand, working gas enters the manifold at the cold end of the stores, it will only pass through the "discharge" store 24 (not shown). When the gas flows are intermittent and reversed frequently, the thermal fronts within "charge" and "discharge" stores 22, 24 will nevertheless progress steadily along their respective stores over the course of multiple cycles without any reversal in the directions of the respective fronts in the stores, until the selected store capacity (or selected end condition) is reached in one store.

At this point, the stores are swapped over to a different operative flow mode by changing the flow controllers, namely, the interrupted NRVs 60 are released to normal operation (e.g. by an electronic or mechanical controller) and the previously active NRVs 60 are then interrupted (e.g. by the same controller). Accordingly, the previous "charge" store 22 becomes the new "discharge" store and the previous "discharge" store 24 becomes the new "charge" store and the direction of progression of the front within each store is then reversed as shown in Figures 8a and 8b. Accordingly, the action of the two stores 22, 24 becomes identical with that of a conventional store undergoing either a simple charge or discharge cycle and the front management techniques disclosed by the prior art are then valid for this low amplitude charge/discharge mode of operation.

In Figs 8a and 8b, the valve states again remain the same regardless of whether system 10 is charging (Figure 8b) or discharging (Figure 8a), since those valve states only needed to alter when a requirement for further capacity (e.g. a front in one of the stores approaching a store end) caused them to reverse the two stores 22, 24 respective functions.

The above-described arrangement and operation only using interruptible NRVs 60 is advantageous because the valve states do not need to be adjusted in order to alternate between the charging mode and discharging mode; all that is required is for the gas flow direction to reverse, which may occur very rapidly. However, as will be appreciated by the skilled reader, a variety of other valve types and valve arrangements (e.g. two-way valves, or NRV's combined with selector valves) may also be used, requiring other operating regimes, in order to implement the present invention.

Figure 9 shows a modified hot storage arrangement 20' including an alternative valve arrangement in which pairs of NRVs 60' are located at one end of the stores 22, 24.

As illustrated, the NRV5 60' of each pair are of opposed directionality when operative to restrict flow to one direction with each NRV 60' of the pair facing the other NRV 60' of the pair. Operation of this valve arrangement is equivalent to valve arrangement discussed in relation to Figure 6a and this alternative valve arrangement is equally suitable for use with cold storage arrangement 30.

While the present invention has been described in detail with reference to certain preferred embodiments, other embodiments of the invention are possible. Therefore, the scope of the appended claims should not be limited to the description of the preferred embodiments contained herein. For example, while two stores in parallel are preferred, the charging and discharging functions may be shared among more stores e.g. three or four stores; further, the at least one store and at least one other store may each comprise sub-stores connected in series or in parallel.

Claims

Claims 1. Thermal storage apparatus for storing and returning thermal energy to a heat transfer fluid passing through the apparatus, comprising: two or more thermal stores connected in parallel with each other, each thermal store comprising fluid-permeable thermal storage media disposed in a chamber for transferring thermal energy to or from the heat transfer fluid, and, flow controllers foi selectively altering the flow path of the heat transfer fluid through the apparatus, wherein the apparatus is configured to be opeiable in different respective flow modes, wheiein in at least one flow mode the apparatus is operable in alternative phases of charging and discharging as the nature of the incoming heat transfer fluid changes, and at least one store is selectively configured to perform the charging while at least one other store is selectively configured to perform the discharging, thereby minimising 01 avoiding switching of those individual stores between charging and discharging in that flow mode and the thermal front reversals associated therewith.
2. Thermal storage apparatus according to claim 1, wherein both the at least one store and the at least one other store are each configured such that a thermal front is only allowed to progress in a single direction along each respective store, without reversal, during the at least one flow mode.
3. Thermal storage apparatus according to claim 1 or claim 2, wherein the apparatus is configured to switch from the at least one flow mode to a different flow mode when a selected end condition is reached in the at least one store, or, in the at least one other store.
4. Thermal stolage apparatus accoiding to claim 3, wheiein the apparatus is configured to switch from the at least one flow mode to a different mode just as a thermal front approaches the end of a thermal store, so as to retain the front within the stole.
5. Thermal storage apparatus accoiding to any pieceding claim, wheiein the apparatus is configured such that, in the at least one flow mode, it can switch between charging and discharging phases without altering the setting of flow controllers that control the flow to the respective at least one store and at least one other store.
6. Thermal storage apparatus according to any preceding claim and comprising two thermal stores, wherein the apparatus is also operable in a second flow mode in which the at least one store is selectively configured to perform the discharging while the at least one other store is selectively configured to perform the charging.
7. Thermal storage apparatus according to any preceding claim, wherein the at least one thermal store and at least one other thermal store are configured to receive a heat transfer fluid at substantially similar pressures.
8. Thermal storage apparatus according to any preceding claim, wherein the thermal storage apparatus forms part of an energy storage system.
9. Thermal storage apparatus according to claim 8, wherein the energy storage system is powered by electricity in a charging phase and/or returns electricity in a discharging phase.
10. Thermal storage apparatus according to claim 8, wherein the energy storage system is powered by solar power or wind power.
11. Thermal storage apparatus according to claim 8, wherein the energy storage system comprises a compressed air energy storage CAES system.
12. Thermal storage apparatus according to claim 8, wherein the energy storage system comprises a pumped heat energy storage system for storing and returning energy comprising: a first engine stage configured to act as a compressor during a charging phase of a cycle and as an expander during a discharging phase of the cycle; a first thermal storage arrangement (or "hot storage arrangement") for receiving and storing thermal energy from gas compressed by the first engine stage during the charging phase; a second engine stage configured to act as an expander during the charging phase to expand gas received from the first thermal store and as a compressor during the discharging phase; a second thermal storage arrangement (or "cold storage arrangement") for transferring thermal energy to gas expanded by the expander during the charging phase; wherein one or both of the first and second thermal storage arrangements comprises thermal storage apparatus as defined above.
13. A method of operating thermal storage apparatus to store and return thermal energy to a heat transfer fluid passing through the apparatus, wherein the apparatus comprises: two or more thermal stores connected in parallel with each other, each thermal store comprising fluid-permeable thermal storage media disposed in a chamber for transferring thermal energy to or from the heat transfer fluid, and, flow controllers that selectively alter the flow path of the heat transfer fluid through the apparatus, and, wherein the apparatus operates in different respective flow modes, wherein in at least one flow mode the apparatus switches between alternative charging and discharging phases, and at least one store is selectively configured to perform the charging while at least one other store is selectively configured to perform the discharging, thereby minimising or avoiding switching of those individual stores between charging and discharging and the thermal front reversals associated therewith.
14. A method according to claim 13, wherein the thermal storage apparatus cycles between phases of charging and discharging activity in the at least one flow mode while in each stole a thermal front advances progressively in only a single direction relative to each respective store in steps corresponding to the respective phases of activity.
15. A method according to claim 14, wherein the apparatus switches to a different flow mode in response to the thermal front reaching the other end of one of the respective stores.
16. A method or apparatus substantially as hereinbefore described with reference to the accompanying drawings.