DESCRIPTION

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

The applicant's earlier application WO 2009/044139 discloses a thermodynamic electricity storage system using thermal stores. In the most basic configuration a hot store

10 and a cold store are connected to each other by a compressor and expander (the latter is often referred to as a turbine in the context of fluid dynamic machinery). In a charging mode heat is pumped from one store to the other (i.e. heating the hot store and cooling the cold store) and in a discharge mode the system process is reversed (i.e. with the cold store being used to cool gas prior to compression and heating in the hot store). The system can use a

15 variety of different types of compressors and expanders, some examples are reciprocating, rotary screw, sliding vane, axial or centrifugal. The system 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 ran as closed cycle processes (e.g. with nitrogen or a noble gas such as argon being re-circulated around the

20 apparatus) or as open cycle systems (e.g. where there is one stage that is at near ambient temperature, atmospheric pressure and the working fluid is air). In both cases, cycles may be sensitive to pressure losses created as the gas flows through different parts of the system (e.g. pressure, loss associated with the gas flow passing through a bed of porous thermal storage media forming part of the hot or cold stores). For example in the system of WO

25 2009/044139 a 2% pressure drop across a thermal store (relative to the absolute pressure of the system at the point where the pressure drop occurs) may lead to approximately a 1.6% drop in the round-trip efficiency of the system, i.e. the amount of electricity that is recovered from the system versus the amount that was used to charge it.

The present applicant has identified the need for an improved heat storage system

30 which overcomes or at least alleviates problems associated with the identified prior art.

In accordance with a first aspect of the present invention, there is provided apparatus for storing energy, comprising: a heat store for receiving a gas flow, the heat store comprising a chamber housing a gas-permeable heat storage structure; characterised in that the apparatus further comprises a filter for removing contaminants from the gas flow.

In this way, contaminants may be removed from the gas flow to protect components of the apparatus and improve operational efficiency. The present applicant has identified that thermal cycling of the porous thermal storage media can lead to fracturing when in use, which in turn may create dust or similar contamination in the gas flow. In addition, certain porous thermal storage media may also by their nature or as a result of the manufacture process be coated with dust or similar small particles.

In one embodiment, the pressure drop across the filter is less than 1% of the absolute pressure in the system at the location of the filter. In another embodiment, the pressure drop across the filter is less than 0.5% of the absolute pressure of the system at the location of the filter. In a yet further embodiment, the pressure drop across the filter is less than 0.1% of the absolute pressure of the system at the location of the filter.

In one embodiment, the heat storage structure comprises particulate material housed in the chamber. The particulate material may comprise at least one of: solid particles; porous media; fibres; and foamed material packed to form a gas-permeable structure.

In one embodiment, the apparatus further comprises gas processing apparatus connected to the heat store by at least one passageway. The gas processing apparatus may comprise one or more of a compressor and an expander.

In one embodiment, the apparatus is configured to transfer thermal energy between gas processed by the gas processing apparatus and the gas-permeable heat storage structure by passing the processed gas through the gas-permeable heat storage structure (i.e. direct without use of a heat exchanger).

In one embodiment, the at least one passageway forms a circuit for circulating gas between the gas processing apparatus and the heat store (e.g. for cyclically charging or discharging the heat store).

Removal of contaminants may be particularly advantageous where the particulate material comprises crushed material (e.g. low cost crashed minerals used to minimize the overall system cost). Such crushed material will usually include a proportion of dust (e.g. small particles of the same or similar material covering the crashed material) and may also have the potential to generate further dust (small particles that can be carried in the gas flow). These small dust particles may cause damage to other parts of the apparatus as they pass through the apparatus. For example, seals, compressor/expander blades or valves might all be damaged.

In one embodiment, the apparatus comprises a higher pressure part and a lower pressure part. (e.g. a first part which receives gas at a higher pressure than a second part receives gas).

In one embodiment, the filter is located in the higher pressure part. In this way, the filter may generate a fractional pressure drop which is relatively low relative to the fractional pressure drop the same filter would generate in the lower pressure part of the apparatus.

In one embodiment, the filter is located in the low pressure part of the system (e.g. a part of the system at or near atmospheric pressure (e.g. within +/-20% or +/- 10% of atmospheric pressure) at a point that is close to ambient temperature (e.g. within +/- 50 deg C, +/-30 deg C of ambient temperature), h this way, a filter with a large cross-section which would be unsuited to use in a high pressure, high temperature gas flow may be used to provide filtration (i.e. with a reduced impact on the overall system manufacturing cost).

In one embodiment, the filter is configured to remove contaminants from the gas flow at a location along the at least one passageway. For example, the filter may be located within the at least one passageway.

In one embodiment, the at least one passageway comprises an expansion chamber (e.g. chamber having a cross-sectional area which is greater than a mean cross-sectional area of the at least one passageway) and the filter is configured to remove contaminants from the gas flow at a location in the expansion chamber. For example, the filter may be located within the expansion chamber.

In one embodiment, the at least one passageway includes a section comprising a primary passageway and a (discrete) bypass passageway for conveying a portion of the gas flow in parallel to the primary passageway between end points of the section and the filter is configured to remove contaminants from the gas flow at a location in the bypass passageway. For example, the filter may be located within the bypass passageway. In one embodiment, the apparatus further comprises a pump for pumping gas flow through the bypass passageway. In one embodiment, the gas flow through the bypass passageway is less than l/3 ^rd of the gas flow through the primary passageway. In one embodiment, the gas flow through the bypass passageway is less than 1/5* of the gas flow through the primary passageway. In one embodiment, the gas flow through the bypass passageway is less than 1/10 ^th of the gas flow through the primary passageway. In one embodiment, the gas flow through the bypass passageway is less than l/50 ^,h of the gas flow through the primary passageway.

In one further embodiment, the flow of gas through the bypass passageway may be varied relative to the gas flow through the primary passageway (e.g. by controlling the pump associated with the bypass passageway). In this way, the bypass flow may be varied depending upon the level of contamination in the gas flow through the primary passageway and/or the direction of the gas flow through the primary passageway (e.g. in dependence upon whether the apparatus is charging or discharging).

Advantageously the provision of the filter in an expansion chamber or bypass passageway allows filtration to occur with a reduced pressure drop since the filter in each case will be exposed to a reduced gas flow. The dimensions of the expansion chamber and/or bypass passageway may be configured to obtain a desired pressure drop within the ranges indicated above.

hi one embodiment, the filter comprises an insulated housing. In this way, heat transfer between the filter and gas flow may be minimised when the filter is located within a hottest part of the system (e.g. a part of the system having a gas temperature of greater than 100 deg C, greater than 200 deg C or greater than 300 deg C) or a coldest part of the system (e.g. a part of the system having a gas temperature below -30 deg C, below -50 deg C or below -70 deg C).

In one embodiment, the filter comprises a gas-permeable screen configured to prevent passage of particulate matter therethrough. The gas-permeable screen may be configured to substantially (e.g. entirely) span a cross-sectional area of the at least one passageway at the location where the filter is located (i.e. to ensure that all or substantially all the gas flow passes through the gas-permeable screen). The gas-permeable screen will have an effective pore size dictated by the size of the contaminants (e.g. small particles) the filter is to remove from the gas flow.

In one embodiment, the gas-permeable screen comprises at least one of: a woven fabric; metallic foamed material; porous ceramic; or any other material forming a porous structure. The choice of gas-permeable screen material may depend upon the temperature range and flow rate of the gas flow in which the filter will be located, together with the particle size of the contaminants to be filtered. The filter may have a cross-sectional area which is substantially equal to or greater than a cross-sectional area of the heat storage structure (e.g. mean cross-sectional area of the heat storage structure). In this way, the pressure drop generated by the gas-permeable screen material may be kept low (e.g. relative to the pressure drop generated by the gas flow through the gas-permeable heat storage structure) and within the ranges indicated above.

In one embodiment, the filter comprises an inertial separator. In the case of apparatus comprising a bypass passageway, the inertial separator may be located in the bypass passageway. In one embodiment, the inertial separator is a cyclonic separator. The inertial separator and/or bypass passageway may be configured to obtain a desired pressure drop within the ranges indicated above.

In one embodiment, the filter comprises sorption media for absorbing or adsorbing contaminants (e.g. unwanted gases or contaminants) from the gas stream.

In a fully closed cycle system there are a number of benefits to controlling the quality of the working gas, such as longer machine life. In one embodiment, Argon may be used as the working fluid, primarily because it is a monatomic gas, but also because of certain benefits that can be derived from the fact that it is an inert gas, such as the potential to use carbon compressor blades or graphite seals that can both break down at high temperatures in the presence of oxygen. Materials such as carbon and graphite can only be used for long periods if very high gas purities are achieved (e.g. there is minimal oxygen in the system). However, even industrially pure gases have a small percentage of contaminants and there is likely to be some contamination from outgassing of storage media with entrapped oxygen or when the system is initially charged. An oxygen content of 1% is enough to significantly reduce the life of the carbon blades.

The sorption media may be configured to remove at least one of: oxygen; water; and carbon dioxide from the gas flow, but not limited to this list. The sorption media may be configured to maintain the purity of the gas flow to: greater than or substantially equal to 99%; greater than or substantially equal to 99.3%; greater than or substantially equal to 99.5%; or greater than or substantially equal to 99.7%.

In one embodiment, the sorption media comprises at least one of: activated carbon; hyperbranched aluminosilica; silica; or any other suitable sorption media.

In one embodiment, the heat store, at least one passageway and gas processing apparatus are all kept at a pressure that is above ambient pressure to stop any external gas contamination from entering the apparatus. In one embodiment, the heat storage structure (e.g. particulate material) housed in the chamber comprises a magnetic material (e.g. permanently magnetic material or magnetisable material). The magnetic material may be a ferromagnetic, ferrimagnetic or paramagnetic material. The magnetic material may comprise a (e.g. crushed) magnetic ore such as magnetite.

In one embodiment, the filter comprises a magnetic filtration device for separating magnetised contaminants from the gas flow (e.g. originating from the magnetic material forming the heat storage structure). In this way, highly effective filtering of contaminants may be achieved with substantially no pressure drop being generated by the filter. The magnetic filtration device may comprise a permanent magnet, an electromagnet or both.

In one embodiment, the magnetic filtration device is configured to remove magnetised contaminants from the gas flow at a location in the chamber.

In one embodiment, the magnetic filtration device comprises a plurality of magnetic elements (e.g. permanent magnets, electromagnets or a combination of both) located within the heat store. For example, the plurality of magnetic elements may be located within the chamber (e.g. confined within the chamber). In one embodiment, the plurality of magnetic elements are configured to maintain a predetermined minimum separation. For example, the plurality of magnetic elements may be fixed in a grid-like configuration inside the chamber (e.g. extending through the heat storage structure).

In one embodiment, the heat store comprises at least one magnetisable part (e.g. non- permanently magnetised part). For example, the heat store may comprise at least one magnetisable casing part or the heat storage structure itself may comprise at least one magnetisable part (e.g. in the case of a heat storage structure comprising particulate material, the particulate material may comprise magnetisable particulate material). In this way, the magnetic filtration device may induce magnetism in the magnetisable part, thereby potentially further increasing the effectiveness of the magnetic filtration device.

In one embodiment, the apparatus comprises a higher temperature part and a lower temperature part (e.g. a first part receiving gas at a higher temperature than a second part receives gas) and the magnetic filtration device is configured to remove magnetic contaminants from the gas flow at a location in the lower temperature part of the apparatus (e.g. to ensure that magnetism of the magnetic contaminants is maintained at the point where the magnetic filtration device operates efficiently). In one embodiment, the apparatus comprises at least one valve for allowing transfer of material collected in the filter from the at least one passageway to a collector. In this way, collected contaminants may be removed from the filter with little impact on the working gas in the system as a whole.

In one embodiment, the filter (e.g. gas-permeable screen filter) is configured to remove contaminants from the gas flow at a location between an inlet into the chamber and the gas-permeable heat storage structure or at a location between an outlet from the chamber and the gas-permeable heat storage structure.

In one embodiment, the filter substantially covers an end surface of the heat storage structure, whereby substantially all the gas flow passing through the heat storage structure passes through the filter, h this way, gas flow through the filter may be minimised to reduce pressure drop across the filter and obtain a desired pressure drop within the ranges indicated above.

In one embodiment, the filter comprises a gas-permeable layer (e.g. packed bed) of particulate material housed in the chamber, the layer of particulate material defining a mean gap size (e.g. mean effective pore size) substantially smaller than a corresponding mean gap size of the heat storage structure (e.g. 2 times, 5 times or even 10 times smaller).

In one embodiment the particulate material forming the filter has a mean particle size (e.g. effective diameter) of less than 2 mm (e.g. less than 1 mm, less than 0.5 mm or less than 0.1 mm).

In one embodiment, the particulate material defines a mean gap size (e.g. mean effective pore size) configured to trap gas-borne contaminants.

In one embodiment, particulate material defines a mean gap size (e.g. mean effective pore size) configured to trap particles that have a particle size (e.g. effective diameter) of less than 0.5 mm (e.g. less than 0.3 mm, less than 0.15 mm or less than 0.075 mm).

Since the filter will cause a much greater pressure drop than material used in the heat storage structure the filter will usually be a thin layer. In one embodiment, the filter has a thickness (e.g. in a direction of the gas flow) of less than 10% of a depth (e.g. length) of the heat storage structure (e.g. 5% of the depth of the heat storage structure. In one embodiment, the filter has a thickness which is less than 50 mm (e.g. less than 30mm or less than 10mm).

In one embodiment, the layer of particulate material comprises at least one of: solid particles; porous media; fibres; and foamed material packed to form a gas-permeable structure. In one embodiment, the layer of particular material comprises at least one of a ceramic and a metal.

In one embodiment, the layer of particulate material comprises a different material to the heat storage structure.

The apparatus may comprise a further heat store as previously defined.

In one embodiment, the first-defined heat store is a higher pressure heat store and the at least one further heat store is a lower pressure heat store (e.g. the first-defined heat store receives gas (or is configured to receive gas) at a higher pressure than the at least one further heat store receives gas).

In one embodiment, the first-defined heat store (e.g. higher pressure heat store) is configured to store thermal energy from gas compressed by a compressor and the second- defined heat store (e.g. lower pressure heat store) is configured to transfer thermal energy to gas expanded by an expander after passing through the first-defined heat store (e.g. during a charging phase of operation).

In another embodiment, the apparatus further comprises a connector for connecting the higher pressure heat store to a gas store for storing pressurised gas after exposure to the higher pressure heat store or to a gas processing stage for receiving pressurised gas after exposure to the higher pressure heat store; and gas transfer apparatus for transferring gas at lower pressure between the higher pressure heat store and the lower pressure heat store, whereby stored thermal energy is transferred between the higher pressure heat store and the lower pressure heat store by passing lower pressure gas between the higher pressure heat store and the lower pressure heat store (e.g. with heat transfer between the gas and heat stores occurring direct without the use of a heat exchanger).

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

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

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

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

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

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

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

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

Figure 1 shows an electricity storage system 100 comprising an insulated hot storage vessel 120 housing a gas-permeable particulate heat storage structure 121, upper and lower plenum chambers 122, 123, insulated cold storage vessel 110 housing a gas-permeable particulate heat storage structure 1 11, upper and lower plenum chambers 1 12, 113, compressor/expanders 130, 140 and interconnecting pipes 101,102,103 and 104.

In operation, when charging, ambient temperature gas at a higher pressure exits interconnecting pipe 103 and is expanded by compressor/expander 140 to a lower pressure. The gas is cooled during this expansion and passes via interconnecting pipe 104 to 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 heated gas leaves the particulate heat storage structure 111 and passes into upper plenum chamber 1 12, 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 compressor/expander 130, where the gas is compressed to the higher pressure. As the gas is compressed the gas temperature rises and the gas leaves the compressor at a higher temperature and passes into interconnecting pipe 102. The gas then enters hot storage vessel 120 via upper plenum chamber 122 and passes down through particulate heat storage structure 121, where the gas is cooled. The cooled gas leaves particulate heat storage structure 121 and passes into lower plenum chamber 123, from where it enters interconnecting pipe 103. The process can continue until the hot and cold stores are 'fully charged' or stop earlier if required. The gas in the hot storage vessel 120 is at a higher pressure than the gas in the cold storage vessel 110.

This overall charging process absorbs energy that is normally supplied from other generating devices (e.g. via the electric grid). The compressor/expanders 130 and 140 are driven by a mechanical device, such as an electric motor (not shown). In addition to these components there may also be heat exchangers contained within one or more of the interconnecting pipes - these are not shown. In one arrangement there is a heat exchanger in interconnecting pipe 103 that maintains the gas in this pipe at or near ambient temperature. 5 In operation, when discharging, high temperature gas at a higher pressure enters interconnecting pipe 102 and is expanded by compressor/expander 130 to a lower pressure. The gas is cooled during this expansion and passes via interconnecting pipe 101 to cold storage vessel 110. The gas enters the upper plenum chamber 112 and then passes down through particulate heat storage structure 111, where the gas is cooled. The cooled gas leaves

10 particulate heat storage structure 1 1 1 and passes into lower plenum chamber 113, from where it enters interconnecting pipe 104. The gas exits interconnecting pipe 104 and enters compressor/expander 140 where the gas is compressed to the higher pressure. As the gas is compressed the gas temperature rises and the gas leaves the compressor at a higher temperature and passes into interconnecting pipe 103. The gas then enters hot storage vessel

15 120. The gas then enters lower plenum chamber 123 and passes up through particulate heat storage structure 121 where the gas is heated. The now high temperature gas leaves the particulate heat storage structurel21 and passes into upper plenum chamber 122, from where it enters interconnecting pipe 102 and is expanded by compressor/expander 130 with the energy of expansion being used to generate electricity for the electric grid. The process can

20 continue until the hot and cold stores are 'fully discharged' or stop earlier if required.

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

Figure 2 shows an improved energy storage system 200 based on the energy storage 5 system that is shown in Figure 1 (with corresponding features labelled accordingly) and further comprising filters 220, 221 at the top and bottom of the particulate heat storage structure 1 11 ' in cold store 110' and filters 222, 223 at the top and bottom of particulate heat storage structure 12Γ in hot store 120'. Filters 220, 221, 222 and 223 are configured to remove contaminants/particles above a predetermined size from the gas flow and are

30 designed for minimal pressure loss. Each filter 220, 221 , 222 and 223 may comprise a ceramic, metal or other material in the form of a fibres, particles or foamed material. In one embodiment, filters 220, 221, 222 and 223 may be part of the particulate heat storage structure (e.g. form upper and lower layers of a packed bed of particulate material having a smaller particle size and/or a different material to the particulate heat storage structure). Each filter 220, 221 , 222 and 223 may be substantially span the whole cross-sectional area of the particulate heat storage structure. The filters may be designed to operate over the 5 relevant temperature range for their location within the cycle and may be designed to be easily accessible so that they can be cleaned and/or changed as necessary.

In operation, when charging the hot thermal store, hot gas enters upper plenum chamber 122' via high pressure interconnecting pipe 102' and passes through filter 222. Filter 222 will absorb heat from the gas until it is at or very close to the gas temperature.

10 The gas passes into particulate heat storage structure 121 ' and transfers its remaining heat direct to the particulate heat storage structure 121 ' . The gas leaves the particulate heat storage structure 121 ' and passes through filter 223. Again filter 223 will absorb heat from the gas until it is at or very close to the gas temperature. The now cooled gas passes into lower plenum chamber 123' and leaves via high pressure interconnecting pipe 103 '.

15 In operation, when charging the cold thermal store, cold gas enters lower plenum chamber 1 13' via low pressure interconnecting pipe 104' and passes through filter 221. Filter 221 will transmit heat to the gas until it is at or very close to the gas temperature. The gas passes into particulate heat storage structure 1 1 1 ' and receives heat direct from the particulate heat storage structure 1 1 1 '. The gas leaves particulate heat storage structure 1 1 1 '

20 and passes through filter 220. Again filter 220 will transmit/receive heat to/from the gas until it is at or very close to the gas temperature. The now warmed gas passes into upper plenum chamber 1 12' and leaves via low pressure interconnecting pipe 101 '.

In operation, when discharging the hot thermal store, cooler gas enters lower plenum chamber 123' via high pressure interconnecting pipe 103 ' and passes through filter 223.

25 Filter 223 will transfer heat to the gas until it is at or very close to the gas temperature. The gas passes into particulate heat storage structure 121 ' and receives heat direct from the particulate heat storage structure 121 '. The gas leaves the particulate heat storage structure 121 ' and passes through filter 222. Again filter 222 will transfer heat to the gas until it is at or very close to the gas temperature. The now heated gas passes into upper plenum chamber

30 122' and leaves via high pressure interconnecting pipe 102'.

In operation, when discharging the cold thermal store, warm gas enters upper plenum chamber 1 12' via low pressure interconnecting pipe 101 ' and passes through filter 220. Filter 220 will receive heat from the gas until it is at or very close to the gas temperature. The gas passes into particulate heat storage structure 11 1 ' and transfers heat direct to the particulate heat storage structure 111 '. The gas leaves the particulate heat storage structure 1 1 1 ' and passes through filter 221. Again filter 221 will receive heat from the gas 5 until it is at or very close to the gas temperature. The now cooled gas passes into lower plenum chamber 113' and leaves via low pressure interconnecting pipe 104'.

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

10 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.

Figure 3 shows an improved energy storage system 300 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly)

15 comprising cold filter 331, hot filters 332 and ambient filters 330 and 333 in the interconnecting pipes, each filter comprising a gas-permeable screen configured to prevent passage of particulate matter therethrough. Cold filter 331 operates in the coldest part of the system. Hot filter 332 operates in the hottest part of the system, ambient filters operate either near ambient temperature, an intermediate temperature between the hot and cold

20 temperatures or a combination of these. For example in the system proposed in WO 2009/044139 the cold temperature might be -160 deg C, the hot temperature might be 500 deg C and the two ambient filters near ambient temperature.

Hot filter 332 and ambient filter 333 are located within the higher pressure part of the system. Hot filter 332 is located within hot filter housing 337 which in turn is within

25 high pressure interconnecting pipe 102". Ambient filter 333 within ambient filter housing 338 which in turn is within high pressure interconnecting pipe 103".

Cold filter 331 and ambient filter 330 are located within the lower pressure part of the system. Cold filter 331 is located within cold filter housing 336 which in turn is within low pressure interconnecting pipe 104". Ambient filter 330 within ambient filter housing

30 335 which in turn is within low pressure interconnecting pipe 101 ".

In operation, when charging the hot thermal store, hot gas enters the hot filter housing 337 via high pressure interconnecting pipe 102" and passes through hot filter 332. Hot Filter 332 will absorb heat from the gas until it is at or very close to the gas temperature. Accordingly, hot filter housing 337 may be insulated for improved efficiency. The gas passes via the high pressure interconnecting pipe 102" into upper plenum chamber 122" and then through particulate heat storage structure 121 ". As the gas passes through particulate heat storage structure 121 " it transfers heat direct to particulate heat storage structure 121 ". The cooled gas leaves particulate heat storage structure 121 " and passes via lower plenum chamber 123" and high pressure interconnecting pipe 103" into ambient filter housing 338. The gas passes through ambient filter 333 and if there is a temperature difference between the filter and gas then filter 333 will absorb heat from the gas until it is at or very close to the gas temperature. The gas leaves via high pressure interconnecting pipe 103". In many applications it is beneficial to reject heat from the system at or around the point of the ambient filter 333. Consequently it is an option to not insulate filter housing 337 to allow it to assist in heat rejection.

In operation, when charging the cold thermal store, cold gas enters the cold filter housing 336 via low pressure interconnecting pipe 104" and passes through cold filter 331. Cold filter 331 will transfer heat to the gas until it is at or very close to the gas temperature. Cold filter housing 336 may be insulated for improved efficiency. The gas passes via the low pressure interconnecting pipe 104" into lower plenum chamber 113" and then through particulate heat storage structure 111 ". As the gas passes through particulate heat storage structure 111 " it receives heat direct from particulate heat storage structurel 1 1 ". The warmed gas leaves the particulate heat storage structure 111 " and passes via upper plenum chamber 112" and low pressure interconnecting pipe 101 " into ambient filter housing 335. The gas passes through ambient filter 330 and if there is a temperature difference between filter and gas then filter 330 will absorb heat from the gas until it is at or very close to the gas temperature. The gas leaves via low pressure interconnecting pipe. In many applications it is beneficial to reject heat from the system at or around the point of the ambient filter 330. Consequently it is an option to not insulate filter housing 335 to allow it to assist in heat rejection.

In operation, when discharging the hot thermal store, cooler gas enters the ambient filter housing 338 via high pressure interconnecting pipe 103". The gas passes through ambient filter 333. Ambient filter 333 will transfer heat to the gas until it is at or very close to the gas temperature. Via high pressure interconnecting pipe 103", the gas passes into lower plenum chamber 123" and then into particulate heat storage structure 121 " and transfers its heat direct to the particulate heat storage structurel21 ". The gas leaves particulate heat storage structure 121 " and passes through upper plenum chamber 122" into high pressure interconnecting pipe 102". From here the hot gas enters hot filter housing 5 337 and passes through hot filter 332 and then out via high pressure interconnecting pipe 102".

In operation, when discharging the cold thermal store, warmer gas enters the ambient filter housing 335 via low pressure interconnecting pipe 101 ". The gas passes through ambient filter 330. Ambient filter 330 will receive heat from the gas until it is at or0 very close to the gas temperature. Via low temperature interconnecting pipe 101 ", the gas passes into upper plenum chamber 1 12" and then into particulate heat storage structure 1 1 1 " and transfers heat direct to the particulate heat storage structure 1 1 1 ". The gas leaves particulate heat storage structure 1 1 1 " and passes through plenum 113 " into low pressure interconnecting pipe 104". From here the cold gas enters the cold filter housing 336 and5 passes through cold filter 331 and then out via low pressure interconnecting pipe 104".

Figure 4 shows an improved energy storage system 400 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) wherein particulate heat storage structure 1 1 1 '" of cold store 1 10"' comprises a magnetic material such as magnetite and particulate heat storage structure 12Γ" of hot store 120"' also0 comprises a magnetic material. A filter 422 is installed in the top of the hot store 120" ' as previously described and magnetic filters 440, 441 and 442 are added to the storage system in interconnecting pipes 101 '", 103 '" and 104"'. Using a magnetic filter minimizes the impact of the dust collection system upon the gas flow path. Depending upon the store material and temperatures of the system, the magnetic filter may be located in the low5 temperature areas of the system as normal materials normally start to lose their magnetism as they are heated to high temperatures.

Sorption media 450 and 452 are inserted within interconnecting pipes 10Γ " and 103" ' to remove, oxygen, water, carbon dioxide and any other unwanted trace gases. Sorption media 450 and 452 are located in the ambient temperature areas of the system for0 convenience. There may be some advantages in locating the sorption media 450 and 452 in the higher temperature areas of the system as certain materials may be used at the higher temperatures that would be ineffective or unreactive at ambient temperatures. Magnetic filters 440, 441 and 442 may be designed to be emptied while the system remains charged with an operating gas that is not air and where there is minimal loss of the operating gas or contamination with air (e.g. by means of a valve adjacent to the magnetic filters).

The sorption media may be designed to be replaced while the system remains charged with an operating gas that is not air and where there is minimal loss of the operating gas or contamination with air (e.g. by means of a valve adjacent to the sorption media).

Figure 5 shows an improved energy storage system 500 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) comprising a bypass filter 550 at lower pressure part of the system comprising an input pipe 551, a variable flow gas pump 552, transfer pipe 553, inlet plenum chamber 554, gas- permeable fabric screen filter 555, outlet plenum chamber 556, outlet pipe 557 and dust trap 558.

In operation a 'dirty' bypass flow is drawn from the main gas flow in low pressure interconnecting pipe 101 "" via inlet pipe 551 into gas pump 552. The gas is pumped via transfer pipe 553 into inlet plenum chamber 554, where the flow slows down and passes through filter 555. Filter 555 is designed to remove dust and contamination. The now 'clean' bypass flow passes into outlet plenum chamber 556 and then outlet pipe 557 where it j oins the main gas flow again in low pressure interconnecting pipe 101 "".

As filter 555 removes contaminants from the bypass flow the filter tends to build up a thick layer of dust. This layer of dust may be periodically removed from the filter by applying a back pulse of high pressure air to the filter. The dislodged dust is collected in a dust trap 558 located below filter 555. Dust trap 558 may be designed such that it can be emptied or cleaned without contaminating the rest of the gas in the system. The filter 555 may be designed such that it can be emptied or cleaned without contaminating the rest of the gas in the system.

Figure 6 shows an improved energy storage system 600 based on the energy storage system that is shown in Figure 1 (with corresponding features labelled accordingly) comprising a bypass dust separation system 660 at a lower pressure part of the system and a bypass dust separation system 670 at a higher pressure part of the system. Bypass dust separation system 660 comprises an input pipe 661, cyclonic separator 662, outlet pipe 663 and dust trap 664. Bypass dust separation system 670 comprising an input pipe 671, cyclonic separator 672, outlet pipe 673 and dust trap 674.

In operation bypass dust separation system 660 works as follows: a 'dirty' bypass flow is drawn from the main gas flow in low pressure interconnecting pipe 101 ""' via inlet pipe 661 into cyclonic separator 662 which removes dust by spinning the gas at high speed. The relatively heavy dust particles are forced to the outside of the separator where they can be slowed down and eventually collected in a dust trap. Dust is collected in dust trap 664 and the now 'clean' bypass flow passes into outlet pipe 663 where it joins the main gas flow again in low pressure interconnecting pipe 101 "'". Cyclonic separators 662, 672 each include an integral pumping facility to draw gas through the machine during operation.

In operation bypass dust separation system 670 works as follows: a 'dirty' bypass flow is drawn from the main gas flow in low pressure interconnecting pipe 103""' via inlet pipe 671 into cyclonic separator 672. Dust is collected in dust trap 674 and the now 'clean' bypass flow passes into outlet pipe 673 where it joins the main gas flow again in low pressure interconnecting pipe 103""'.

Dust traps 664 and 674 may be designed such that they can be emptied or cleaned without contaminating the rest of the gas in the system. Cyclonic separators 662 and 672 may be variable flow devices.

In different embodiments the one or more bypass dust separation systems may be located in different parts of the system, which may be hotter or colder, and it may be preferable to insulate them if they are operating at gas temperatures that are not at or near ambient temperature.

Figure' 7 shows an electricity storage system 701 of the type disclosed in the applicant's earlier application PCT/GB2011/050370 comprising a compressor/expander (e.g. compressor/expander turbine) 702 powered by electricity supply 703 and connected to high pressure thermal store 710 and gas store 720. High pressure thermal store 710 is in turn connected to low pressure thermal stores 711 and 712. Air enters and leaves the system through pipe 730 and is transferred via pipes 731 , 732, 733, 734, 735, 736, 737 and 738. Valves 740, 741 , 742, 743, 744 and 46 can be used to selectively close/open different pipes. Air Pump 750 is connected to pipe 736 and can pump air in either direction. Heat exchanger 760 is used to keep the temperature of the gas passing through pipe 736 at a substantially ambient or fixed base temperature. The high pressure thermal store 710 comprises an insulated high pressure vessel 713 housing a gas-permeable particulate heat storage structure 714 that the compressed gas can pass through and transfer its heat when charging and receive its heat from when discharging.

The low pressure thermal stores 711 and 712 comprise an insulated low pressure vessel 715 and 717 housing a gas-permeable particulate heat storage structures 716 and 718 respectively that the gas can pass through and transfer its heat when charging and receive its heat from when discharging.

The gas store 720 may be a pressurised underground cavern, such as a salt dome, an aquifer or other suitable underground space or pressure vessel. Alternatively gas store 720 may comprise a store including a gas processing stage.

The compressor/expander 702 acts as a compressor driven by an electric motor (not shown) when charging and as an expander (i.e. turbine if a rotary machine) driving a generator (not shown) when discharging. The compressor and expander may be the same equipment as shown or they may be separate units optimised for each process.

As illustrated, the high pressure vessel 713 has a storage volume which is substantially less than the storage volume of each of the low pressure vessels 715, 717.

In operation, when storing electricity during a charge phase, atmospheric air is drawn in through pipe 730 and compressed in compressor/expander 702 before entering pipe 731. Valves 740 and 741 are both open. Valves 742 and 743 are both closed. The air in pipe 731 is both higher pressure and higher temperature than when it entered the compressor/expander 702. Particulate heat storage structures 714, 716 and 718 are initially at substantially ambient temperature.

The air enters high pressure thermal store 710 through valve 740 and passes inside high pressure vessel 713 and through particulate heat storage structure 714. As the high pressure air enters particulate heat storage structure 714 it transfers its heat of compression to particulate heat storage structure 714. The now cooled high pressure air leaves particulate heat storage structure 714 and passes out of the high pressure vessel 713 via valve 741 and into pipe 732. Pipe 732 may have an additional heat exchanger fitted to further cool any air prior to entering gas store 720. The air then enters gas store 720, which volumetrically is much larger than high pressure vessel 713.

When particulate heat storage structure714 has stored a sufficient quantity of the heat of compression the compressor/expander 702 is stopped. The valves 740 and 741 are both closed and the pressure within the high pressure vessel 713 is lowered to the pressure within the low pressure vessels 715 and 717 using a balance pump (not shown).

When the pressures are substantially equal, valves 742, 743 and 744 are set to an open position and valve 746 is closed. Pump 750 is activated and pumps air from pipe 736 via heat exchanger 760 through valve 743 and into the high pressure vessel 713. The air passes through particulate heat storage structure 714 where it receives heat from the particulate heat storage structure. The air passes out of the high pressure vessel and enters pipe 733 via valve 742. The air passes into pipe 734 via valve 744 and enters the low pressure vessel 715. The air passes through particulate heat storage structure 716 and transfers heat to the particulate heat storage structure. The air leaves particulate heat storage structure at near to ambient temperature and exits the low pressure vessel 715 via pipe 737 and enters pipe 736. The air returns to the pump 750 and the process of transferring heat from the high pressure thermal store to the low pressure thermal store continues. When a suitable proportion of the heat has been transferred pump 750 is stopped and valves 742 and 743 are closed.

Air is added to the high pressure thermal store (e.g. using a balance pump (not shown) comprising a compressor for receiving and raising the pressure of atmospheric air) until the pressure within the store is substantially equal to that within pipes 731 and 732. Valves 740 and 741 are opened and the compressor/expander 702 starts to compress air again.

The above process repeats until low pressure thermal store 71 1 is 'fully charged' with heat. At this stage valve 744 is closed and valve 746 is opened and low pressure thermal store 712 can now be charged in a similar manner.

When all stores are charged the system is 'full', however it is possible to recover the electricity stored at any stage, even when stores are part charged. The charge/discharge efficiency of the system will always be less than 100% as there are a number of losses in the different processes.

To 'recover' the electricity in a discharge phase, pressurised air is drawn in through pipe 732 and enters high pressure vessel 713 via valve 741. If fully charged each particulate heat storage structure 714, 716 and 718 should be in a 'hot' state. Valves 740 and 741 are both open. Valves 742 and 743 are both closed. The high pressure air passes through particulate heat storage structure 714 and receives heat from the particulate heat storage structure. The now heated air leaves the high pressure vessel 713 via valve 740 and enters pipe 731. The air enters the compressor/expander 702 and is expanded generating work in the process that drives a generator to produce electricity that is transmitted into electricity supply 703.

This process continues until particulate heat storage structure 714 has transferred a suitable quantity of heat i.e. it is fully discharged. In cyclic operation it may be beneficial to leave part of the thermal front in the store for reuse in a subsequent stage. The compressor/expander 702 is stopped. The valves 740 and 741 are both closed and the pressure within the high pressure vessel 713 is lowered to the pressure within the low pressure vessels 715 and 717.

When the pressures are substantially equal valves 742, 743 and 744 are set to an open position and valve 746 is closed. Pump 750 is activated and pumps air from pipe 736 into pipe 737 and enters low pressure vessel 715. The air passes through particulate heat storage structure? 16 and receives heat from the particulate heat storage structure. The air passes out of the low pressure vessel 715 into pipe 734 and via valve 744 into pipe 733. The air enters the high pressure vessel 713 via valve 742. The air passes through particulate heat storage structure? 14 and transmits heat to the particulate heat storage structure. The air leaves the particulate heat storage structure at a temperature that is near ambient or the base temperature and passes into pipe 736 via valve 743. The air passes through heat exchange 760 where it is cooled further if necessary and leaves the heat exchanger at near ambient or base temperature.

The air returns to the pump 750 and the process of transferring heat from the low pressure thermal store to the high pressure thermal store continues. When a suitable proportion of the heat has been transferred pump 750 is stopped and valve 742 and 743 are closed.

Air is added to the high pressure thermal store until the pressure within the store is substantially equal to that within pipes 731 and 732. Valves 740 and 741 are opened and the compressor/expander 702 starts to expand air again.

This process repeats until the low pressure thermal store 711 is 'fully discharged'.

At this stage valve 744 is closed and valve 746 is opened and low pressure thermal store 712 can now be discharged in a similar manner. In accordance with the present invention, electricity storage system 701 further comprises low pressure gas filter 770, high pressure gas filter 771 in an insulated housing, upper and lower filters 772 located in each of the different thermal stores 710-712, magnetic filter 773 for filtering one or more magnetic store materials and bypass vortex separator 774.