Energy storage system

Abstract

An energy storage system (60) comprises a high temperature electrolyser (70), and a battery pack (65) with cells (10) that comprise a ceramic electrolyte, means (75) to supply steam at above 400°C to the high temperature electrolyser (70), and means to carry a gas stream (77) containing hydrogen away from the high temperature electrolyser (70). The system (60) includes means (78, 82) to maintain the battery pack at an operating temperature above 170°C by use of heat from the high temperature electrolyser (70). The system (60) may be used in conjunction with a renewable energy source (62) of variable power output.

Description

[0001] Energy Storage System

[0002] The invention relates to an energy storage system, particularly but not exclusively a system that produces a transportable fuel.

[0003] With current concern about greenhouse gas emissions, there is a desire to utilise renewable energy sources. Renewable energy sources such as wind power, tidal power and solar power can be used to generate electricity, but typically produce a variable power output, varying over timescales of hours or days. There is consequently a benefit in being able to store energy when more is generated than is required; the energy may be stored for example by charging an electric storage battery, or by performing electrolysis to form a fuel such as hydrogen. Subsequently the stored energy may be retrieved by discharging the storage battery, or supplying the hydrogen to a fuel cell, respectively. Hydrogen can also be used to produce other fuels such as ammonia or methanol, which may be easier to transport, or indeed hydrogen may be used as a feedstock for other products or processes.

[0004] Hydrogen can be produced by electrolysing water, and in particular can be produced by high temperature electrolysis of steam in a high temperature electrolyser. This typically is carried out at temperatures above 400°C, for example at between 500°C and 600°C, or somewhat higher, for example using a ceramic electrolyte. High temperature electrolysis is significantly more efficient than electrolysis at ambient temperatures or at up to 120°C, as is known with alkaline systems using KOH; however it does require a high temperature heat supply. The high temperature heat may be produced electrically, or by a solar concentrator, or may be obtained from an industrial process such as steel production.

[0005] According to the present invention there is provided an energy storage system comprising a high temperature electrolyser, and a battery pack with cells that comprise a ceramic electrolyte, means to supply steam at above 400°C to the high temperature electrolyser, means to carry a gas stream containing hydrogen away from the high temperature electrolyser, and means to maintain the battery pack at an operating temperature above 170°C by use of heat from the high temperature electrolyser.

[0006] The means to maintain the battery pack at the operating temperature may utilise the gas stream containing hydrogen that flows away from the electrolyser. The operating temperature may be above 200°C or above 250°C. Alternatively the battery pack may be arranged within thermal insulation for the electrolyser, so that heat flow through the insulation to the surroundings (at ambient temperature) maintains the battery pack at a desired temperature; this approach may be combined with use of a gas flow that carries heat from the electrolyser as mentioned above.

[0007] As mentioned above, the electrolyser may be supplied with high temperature steam, so it produces hydrogen. Alternatively the electrolyser may be provided with a high- temperature mixture of steam and carbon dioxide, and in this case the electrolyser may produce syngas, that is to say a mixture of hydrogen and carbon monoxide; if operated slightly differently it may produce methane.

[0008] The detailed operation of the electrolyser is affected by both the operating temperature and the voltage applied between the electrodes of the electrolyser. The steam electrolysis reaction is endothermic so running just above the reversible potential (equilibrium potential) for the reaction will mean that the electrolyser is actively cooling itself thermodynamically by the reactions taking place. This means that if the electrolyser is coupled to a source of high grade waste heat some of the heat is effectively driving the electrolysis (which makes the electrolyser appear very efficient, at least electrically), but the amount of hydrogen generated for the size of the electrolyser is not that large. The reversible potential for the reactions depends on the operating temperature, being lower at higher temperatures; at 800°C it is about 1 V per cell. As the voltage applied between the electrolyser electrodes is increased, the irreversible heat generated will, at a particular voltage (e.g. 1.286 V / cell at 800°C) balance the thermodynamic cooling of the cell; this is referred to as the thermoneutral point (this ignores heat losses to surroundings). Raising the cell voltage still higher above this thermoneutral point will then generate excess heat that can be used to balance heat loss, preheat the gas streams or provide heat to the battery pack. The electrolyser will also produce more gas.

[0009] Operating the electrolyser at high cell voltages can accelerate degradation, so if it is desirable to operate at a lower cell voltage external heat is needed for the electrolyser and also for the battery pack. This could come from electrical heating; or some of the produced gas may be combusted to provide this heat. Overall the energy still comes from the electrical supply but it just provides an alternative operating regime and heating method that could be advantageous for lifetime. The energy storage system is preferably operated in conjunction with a source of renewable energy whose output power varies with time on timescales of hours or days. Sometimes the electricity from the source may be used both to charge the battery pack and to operate the electrolyser; at times when less power is available from the source, the electricity from the source may be used only to operate the electrolyser; and at other times, if the power from the source is even less, power from the battery pack may be used to operate the electrolyser.

[0010] The battery pack may comprise planar cells, arranged in one or more stacks; the cells may be molten sodium / nickel chloride or molten sodium / iron chloride, with an electrolyte of a ceramic that conducts sodium ions such as beta alumina, or nasicon.

[0011] A number of different types of electrochemical cell are known that require an elevated temperature to operate. One such type of cell is a molten sodium / metal halide rechargeable battery, such as the sodium / nickel chloride cell which may be referred to as a ZEBRA cell (see for example J.L. Sudworth, "The Sodium / Nickel Chloride (ZEBRA) Battery (J. Power Sources 100 (2001) 149-163). A sodium / nickel chloride cell incorporates a liquid sodium negative electrode separated from a positive electrode by a solid electrolyte which conducts sodium ions. The solid electrolyte may for example consist of beta alumina. The positive electrode includes nickel, nickel chloride and sodium tetrachloroaluminate which is liquid during use and acts as a secondary electrolyte to allow transport of sodium ions from the nickel chloride to the solid electrolyte. The positive electrode also incorporates aluminium powder. Partial replacement of the nickel with other transition metals such as iron can result in additional discharge voltage levels. The cell operates at a temperature which is typically below 350°C, but must be above the melting point of the sodium tetrachloroaluminate, which is 157°C, and the operating temperature is typically between 270° and 300°C. During discharge the normal reactions are as follows:

[0012] Cathode (positive electrode): NiCb + 2 Na++ 2 e- Ni + 2 NaCI

[0013] Anode (negative electrode): Na -> Na++ e- the overall result being that anhydrous nickel chloride (in the cathode) reacts with metallic sodium (in the anode) to produce sodium chloride and nickel metal; and the cell voltage is 2.58 V at 300°C.

[0014] A modified type of a ZEBRA cell, that is to say a molten sodium-nickel chloride rechargeable cell, is described in WO 2019 / 073260. This uses an electrolyte element that comprises a perforated sheet of non-reactive metal, and a non-permeable layer of sodium- ion-conducting ceramic bonded to one face of the perforated sheet. In this electrolyte element the strength can therefore be provided by the metal sheet, and this enables the electrolyte thickness to be significantly reduced as compared to that required in a conventional ZEBRA cell. This results in a cell or a battery that can perform adequately at significantly lower temperatures, for example less than 200°C. Furthermore, a significantly thinner layer of ceramic also significantly reduces stresses induced by heating from ambient, so start-up times from ambient can be just a few minutes. These are both commercially advantageous benefits. The non-permeable layer is bonded to the perforated metal sheet, and this bonding may be by a porous ceramic sub-layer. Such a cell includes a metal case, which may have a peripheral flange.

[0015] A slightly different form of molten sodium / nickel chloride cell is described in WO 2022 / 123246 in which the electrolyte is again a planar sheet of ceramic that can conduct ions of the alkali metal, and a perforated planar sheet of an inert metal is immediately adjacent to the sheet of ceramic and in contact with the sheet of ceramic over substantially its entire area, to provide support to the sheet of ceramic. In this case the ceramic sheet is formed separately from the perforated planar sheet, rather than being formed by deposition onto it.

[0016] In another option the sheet of ceramic may be coated with graphite on the surface facing the anode, and provided with mechanical support by a metal sheet that is spring- loaded onto or towards that surface, optionally with a layer of carbon felt between them.

[0017] In each of these cells the electrolyte separates the anode chamber from the cathode chamber, and in use at the operating temperature there are liquid phases in each chamber. The electrochemical reactions at the electrodes are the same as those in the ZEBRA cell described above. The ceramic separator / electrolyte is planar, and the anode and cathode compartments are defined by metal plates that together form a case of generally planar form and with a projecting flange around the edge. The cells can therefore be arranged in one or more stacks, each stack being located in a frame, and the temperature of the cells can be controlled by causing a heat transfer fluid to flow between the flanges of adjacent cells. In the present invention the heat transfer fluid may be the gas stream containing hydrogen that flows away from the electrolyser, or may be a fluid stream that takes heat from that gas stream for example in a heat exchanger.

[0018] The invention will now be further and more particularly described with reference to, and as shown in the accompanying drawings, in which:

[0019] Figure 1 shows a cross-sectional view through an electrical cell that may be used in the invention;

[0020] Figure la shows a cross-sectional view of part of an electrolyte of a cell of figure 1;

[0021] Figure 2 shows a perspective view of a module that comprises a stack of cells as shown in figure 1, along with a frame;

[0022] Figure 2a shows a perspective view, partly broken away, of a modification to the module of figure 2;

[0023] Figure 3 shows a schematic diagram of an energy storage system of the invention, that includes a module as shown in figure 2 or figure 2a; and

[0024] Figure 4 shows a schematic diagram of an energy storage system of the invention, which is a modification to that of figure 3.

[0025] Referring to figure 1, a cell 10 which operates at an elevated temperature comprises metal electrode plates 11 and 12, between which is a sheet of electrolyte 13. The electrode plates 11 and 12, together with the electrolyte sheet 13, define an anode space 14 on one side of the electrolyte sheet 13 and a cathode space 15 on the other side of the electrolyte sheet 13, which contain chemicals that interact to generate electricity as a consequence of the passage of ions through the electrolyte sheet 13. Around their periphery the electrode plates 11 and 12 are sealed to the sheet of electrolyte 13 by a heat-resistant electrically- insulating sealant 17, and the edges of the electrode plates 11 and 12 and of the electrolyte sheet 13 form a projecting edge flange 20 around the periphery of the cell 10, the flange 20 being thinner than the remainder of the cell 10. The cell 10 is thus of generally planar form, with a thin projecting flange 20.

[0026] In this example the cell components are held together by crimping the edge of the electrode plate 11 around the edges of the electrolyte sheet 13 and the electrode plate 12, all of which are separated by the insulating sealant 17, but in some cases the sealant 17 alone may hold the components together, so no such crimping is needed, or the edges of the plates 11 and 12 and of the sheet of electrolyte 13 may be held together in other ways. In another modification there is no insulating sealant 17 between the edge of the electrolyte sheet 13 and the edge of one of the electrode plates 11 or 12, for example the plate 11 that forms the anode space 14; these edges may be hermetically welded together, and in this case the insulated sealant 17 only separates the edge of the plate 12 from both the plate 11 and the electrolyte sheet 13. The required operating temperature clearly depends upon the nature of the chemicals and the nature of the material of the electrolyte sheet 13. For example operation may require a temperature in the range 60°C and 300°C.

[0027] Preferably the cell 10 may be a sodium / metal halide cell. One such type of cell is a sodium / nickel chloride cell. The electrode plates 11 and 12 may be of stainless steel, and of dished form to define the anode space 14 and the cathode space 15, and each with a flat peripheral rim. The electrolyte sheet 13 may comprise a metal sheet 16 of a metal such as nickel, or aluminium-bearing ferritic steel (such as the type known as Fecralloy (trade mark)), or a steel that forms an electronically-conductive and adherent scale, for example a CrMn oxide scale, when heated in air. Most of the sheet 16 is perforated to produce a very large number of through holes 18, as shown schematically in figure la, the holes being of mean diameter 30 pm, potentially produced by a laser drilling process, or of mean diameter between 50 pm and 100 pm, and may for example be made by chemical etching. A margin 22 around the periphery of the metal sheet 16, typically of width 5 mm, is not perforated. The perforated portion of the sheet 16 is covered by a porous and permeable ceramic sublayer 26a which is itself covered by a non-permeable ceramic layer 26b, the ceramic layer 26b being of a sodium-ion-conducting ceramic. The non-permeable ceramic layer 26b may for example comprise beta alumina, but in addition it may contain a material that forms a glass during the sintering process. Thus although it is referred to as a ceramic layer, the term "ceramic" in this context includes combinations of ceramic and glass, as long as the layer is conductive to sodium ions during operation. The non-permeable ceramic layer must not be permeable, that is to say it would be impermeable to gases, and consequently impermeable to liquids during operation. The non-permeable layer 26b also covers the edges of the sublayer 26a.

[0028] The porous sub-layer 26a may be of the same sodium-ion-conducting ceramic as the non-permeable ceramic layer 26b, but would typically be formed from a slurry containing somewhat larger particles. The porous and permeable ceramic sub-layer 26a may be of thickness between 10 pm and 100 pm, while the non-permeable layer 26b may be of thickness in the range 5 pm to 50 pm, for example 20 pm, 30 pm or 40 pm.

[0029] In its charged state the cell 10 would contain sodium metal in the anode space 14 and nickel chloride in the cathode space 15. However, the cell would typically be assembled in a completely discharged state, with nickel powder mixed with sodium chloride in the cathode space 15. In practice the cathode space 15 would be initially filled with a powder mixture containing nickel powder, sodium chloride, and sodium aluminium chloride (sodium tetrachloroaluminate, NaAIC ) and preferably also a small proportion other ingredients such as iron sulphide and iron chloride, and aluminium powder, and there may also be an expanded mesh nickel sheet embedded within the powder mixture to ensure good electrical contact. The anode space 14 may initially contain carbon felt, and the surfaces of the anode space 14 may be coated with carbon black.

[0030] For the cell 10 to operate, it must first be heated to a temperature above 157°C, such as 200°C, at which the sodium aluminium chloride is molten, and at such a temperature the non-permeable ceramic layer 26b will conduct sodium ions sufficiently. The molten sodium aluminium chloride enables sodium ions to diffuse between the sodium chloride and the non-permeable ceramic layer 26b. The cell can therefore be charged by applying a voltage from an external power supply between the two electrode plates 11 and 12, so sodium ions pass through the electrolyte sheet 13 into contact with the carbon felt in the anode space 14, where sodium metal is formed, while within the cathode space 15 the remaining chloride ions react with the nickel to form nickel chloride. The cell 10 is readily reversible, so it can be charged and discharged multiple times.

[0031] Referring now to figure 2, twenty cells 10 (one is shown above the others) are assembled into a stack 30, all the cells 10 having the same orientation so that the cells 10 are electrically in series. At each end is an endplate 32 which defines two threaded bosses 34 for electrical connections to the stack 30. The stack 30, including the endplates 32, is held together by a heat-resistant fabric strip 33. Figure 2 also shows an insulating frame 35 of a porous ceramic material (such as a material produced by Rockwool) having a texture somewhat similar to cardboard, but being an electrical and thermal insulator, with a thermal conductivity of 0.035 W / m.K. The frame 35 is generally rectangular, with rounded external corners, and defines a rectangular duct 36 to accommodate the stack 35. Each side wall of the rectangular duct 36 defines a wide recess 38, and two circular apertures 40, one above the other, which communicate with that recess 38; the wide recess 38 extends to one end of the frame 35, but does not extend to the other end of the frame 35, terminating just beyond the location of the apertures 40.

[0032] The wall thickness of the frame 35 is greatest in the central portion into which the stack 30 can be placed; one end of the frame 35 defines a step 42 around the inside of the walls, so defining a projecting outer wall part 43, while the other end of the frame 35 defines a step 44 around the outside of the walls, so defining a projecting inner wall part 45. The dimensions are such that if two such frames 35 are aligned and pushed together, the projecting inner wall part 45 of one frame 35 fits closely within the projecting outer wall part 43 of the other frame 35. The frame 35 also defines a shallow recess 46 on the outside of each of the side walls, each such shallow recess extending the entire length of the frame 35, and communicating with the circular apertures 40.

[0033] When the stack 30 is located within the frame 35, the threaded bosses 34 reach the end of the recess 38 so the stack 30 can't be inserted further. In this position the apertures 40 are aligned with the threaded bosses 34 on the endplates 32. Electrical connections to each stack 30 are made through the circular apertures 40 to the threaded bosses 34. The combination of the stack 30 within the frame 35 may be referred to as a battery module 50. It will be appreciated that along the top and bottom surfaces of the stack 30 there are gaps 48 between the edge flanges 20 of adjacent cells 10, so in the battery module 50 there are multiple flow paths defined by the gaps 48 and the top and bottom walls of the frame 35.

[0034] Instead of providing a separate frame 35 for each stack 30, in an alternative module 50 a plurality of stacks 30 may be arranged side by side within a single frame that is correspondingly longer than the frame 35. The stacks 30 are side by side, so the flow paths defined by the gaps 48 between adjacent flanges 20 in each stack 30 are aligned with each other. In another modification the frame that encloses one or more stacks 30 may be of metal, as long as the frame is spaced from or electrically insulated from the cells 10 of each stack 30. By way of example figure 2a shows a partly broken away view of a metal frame 35a into which several stacks 30 are arranged side by side, showing two stacks 30 during their installation into the frame 35a to form a module 50. It will be appreciated that although a single cell 10 cannot provide large-scale energy storage, a battery or module 50 that consists of multiple cells, or multiple stacks of cells, can provide energy storage at a large scale.

[0035] Referring now to figure 3, an energy storage system 60 is shown schematically. It incorporates a renewable source of electricity 62, represented in the figure by a wind rotor, whose electrical output is supplied to a controller 63 which produces a DC output. The output from the controller 63 is supplied via a switch 64 to a battery 65 that comprises one or more modules 50 of cells 10 as described above, so the battery 65 can be charged or discharged by closing the switch 64.

[0036] The output from the controller 63 is also provided, via a switch 68, to a high temperature electrolysis unit 70, which is arranged in parallel with the battery 65, and also to external output contacts 66. The high temperature electrolysis unit 70 consists of two chambers separated by a ceramic electrolyte sheet 71 that can conduct oxygen ions, with an anode electrode 72 and a cathode electrode 73 at opposite faces of the electrolyte sheet 71. High temperature steam at a temperature above 400°C, for example at between 500°C and 600°C, or at a higher temperature such as 800°C, is produced by a heater 75, and fed into the chamber with the cathode 73. Electrolysis occurs, so hydrogen gas comes out of the chamber with the cathode 73 along with excess steam (shown as gas flow 77); oxygen ions migrate through the electrolyte sheet 71 to form oxygen gas which comes out of the chamber that contains the anode 72.

[0037] The outflowing hot mixture of superheated steam and hydrogen, gas flow 77, is passed through a heat exchanger 78, in which it exchanges heat with a flow of a heat transfer fluid; the resulting hot heat transfer fluid stream 80 can then be combined with a flow 81 of the heat transfer fluid at ambient temperature at a control valve 82, to provide a flow 83 of the heat transfer fluid at a temperature in the range from ambient up to 250°C. This heat transfer fluid flow 83 can then be provided to the battery 65, so each cell 10 is at a suitable operating temperature. The heat transfer fluid may be air.

[0038] In operation, the system 60 may be operated to produce hydrogen on a continuous basis, with the heater 75 continuously activated to provide the high temperature steam, and with the electrolyser 70 powered by the source 62; if the source 62 produces more power than that required by the electrolyser 70 then excess power can be used to charge the battery 65, while if the source 62 produces less power than that required by the electrolyser 70, the battery 65 can provide the power needed. Thus the battery 65 acts as a buffer, modulating the variable output from the source 62 to provide a steady power supply to the electrolyser 70. The hydrogen may be stored, or may be used as a feedstock for example to produce ammonia.

[0039] The external contacts 66 may therefore only be used if the power output by the source 62 is greater than that required by the electrolyser 70 and the battery is fully charged, in which case power may be output through the external contacts 66, for example to supply the grid; or alternatively if the battery 65 is discharged and the power from the source 62 is less than that required by the electrolyser 70, in which case power may be input through the external contacts 66, for example from the grid.

[0040] It will be appreciated that the system 60 may be modified in various ways while remaining within the scope of the invention. For example it will be appreciated that there are flows of both hydrogen and oxygen from the electrolyser 70, so heat to heat up the battery 65 might alternatively be taken from the outflowing oxygen.

[0041] Referring now to figure 4 there is shown schematically an energy storage system 80 which is a modification to the system 60; the electrical components are the same, and are referred to by the same reference numbers. Thus the system 80 incorporates a renewable source of electricity 62, represented in the figure by a wind rotor, whose electrical output is supplied to a controller 63 which produces a DC output. The output from the controller 63 is supplied to a battery 65 and also to a high temperature electrolysis unit 70, which is arranged in parallel with the battery 65.

[0042] The battery 65 and the electrolysis unit 70 are enclosed within thermal insulation 82 so as to form a chamber for the battery 65 at about 250°C and a chamber for the electrolysis unit 70 at about 600°C. The portion 82a of the thermal insulation 82 that separates the two chambers acts as a resistive thermal bridge, so some of the heat flow from the electrolysis unit 70 to the environment is utilised to maintain the temperature of the battery 65.

[0043] The electrolysis unit 70 generates an output stream 77 of hydrogen and steam and an output stream 85 of oxygen; a small proportion of the hydrogen-containing stream 77 is fed off as stream 83 to a catalytic combustor 84 to which air is supplied. The remainder of the stream 77 and the output stream 85 of oxygen are supplied to heat exchangers 87 and 88 respectively. The water supply is split so part flows through one heat exchanger 87 and the rest through the other heat exchanger 88; the preheated water / steam streams are recombined and fed through a heat exchanger 90 heated by the exhaust from the catalytic combustor 84. The heat exchanger 90 thus provides superheated steam at about 600°C to the electrolysis unit 70.

[0044] The exhaust gases from the catalytic combustor 84, after passing through the heat exchanger 90, may be supplied as flow 91 to a heat exchanger 92 to heat an air stream 94 used to control the temperature of the battery 65.

[0045] In this example the electrolysis unit 70 may be operated in an endothermic mode, by operating at a cell voltage below the thermoneutral point, and in addition the fluid flow paths are arranged to extract as much enthalpy as possible from both the H2 and O2 product streams 77 and 85, in heat exchangers 87 and 88, to preheat the water fed in. Splitting the water feed through the two heat exchangers 87 and 88 helps get the water / steam to a high temperature. The small flow 83 of H2 product through the catalytic burner 84 gives the final boost to the temperature of the steam to achieve the temperature needed by the electrolysis unit 70, so the steam provides the heating for the electrolysis unit 70. If any extra heat is required by the battery 65, it can be provided by the exhaust gas flow 91 from the catalytic combustor 84, after it has passed through the heat exchanger 90.

Claims

Claims1. An energy storage system comprising a high temperature electrolyser, and a battery pack with cells that comprise a ceramic electrolyte, means to supply steam at above 400°C to the high temperature electrolyser, means to carry a gas stream containing hydrogen away from the high temperature electrolyser, and means to maintain the battery pack at an operating temperature above 170°C by use of heat from the high temperature electrolyser.

2. An energy storage system as claimed in claim 1 wherein the means to maintain the battery pack at the operating temperature utilises the gas stream containing hydrogen that flows away from the electrolyser.

3. An energy storage system as claimed in claim 1 or claim 2 wherein the battery pack comprises planar cells, arranged in one or more stacks.

4. An energy storage system as claimed in any one of the preceding claims wherein the battery pack comprises molten sodium / metal chloride rechargeable cells, with a ceramic electrolyte separator.

5. An energy storage system as claimed in claim 4 wherein the ceramic electrolyte separator is a non-permeable layer of sodium-ion-conducting ceramic supported by a perforated sheet of non-reactive metal.

6. An energy storage system as claimed in claim 5 wherein the non-permeable layer is bonded to the perforated metal sheet by a porous ceramic sub-layer.

7. An energy storage system as claimed in any one of the preceding claims wherein the battery pack comprises planar cells, arranged in one or more stacks, each cell having a case of generally planar form and with a projecting flange around the edge, each stack being located in a frame, such that there are flow paths for a heat transfer fluid defined by gaps between the flanges of adjacent cells in the stack.

8. An energy storage system as claimed in any one of the preceding claims, in combination with a source of renewable energy whose output power varies with time.

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