GB2516095A

GB2516095A - Fuel cell systems

Info

Publication number: GB2516095A
Application number: GB1312461.5A
Authority: GB
Inventors: Richard Michael Tracy
Original assignee: AFC Energy PLC
Current assignee: AFC Energy PLC
Priority date: 2013-07-11
Filing date: 2013-07-11
Publication date: 2015-01-14
Also published as: GB201312461D0

Abstract

A fuel cell system (10, figure 1) comprises at least one fuel cell stack (20) and a recirculation network for circulating a liquid electrolyte (12) through each fuel cell stack, the network including a decoupling wheel 44 comprising two opposed end plates 62 rotatable about a horizontal axis and linked by vanes 64 subdividing the space between the plates into a multiplicity of chambers 80 that are open around the periphery, the system further comprising an inlet duct 40 having at least one port 60 above the wheel to one side of the axis and a collecting chamber positioned below the wheel and having an outlet duct 46. The wheel preferably includes a sill 82 extending adjacent to the outer edge of each vane, wherein each sill most preferably comprises a plate joined to the outer edge of each vane and to the end plates. The collecting chamber may be provided with an overflow duct 48 at a higher position than the outlet duct and the system may comprise a device (50) to break up the electrolyte flow from the overflow duct.

Description

Fuel Cell System The present invention relates to liquid electrolyte fuel cells, and to a fuel cell system that incorporates such liquid electrolyte fuel cells.

Background to the invention

Fuel cells have been identified as a relatively clean and efficient source of electrical power. Alkaline fuel cells are of particular interest because they operate at relatively low temperatures, are efficient and mechanically and electrochemically durable. Acid fuel cells and fuel cells employing other liquid electrolytes are also of interest. Such fuel cells typically comprise an electrolyte chamber separated from a fuel gas chamber (containing a fuel gas, typically hydrogen) and a further gas chamber (containing an oxidant gas, usually air). The electrolyte chamber is separated from the gas chambers using electrodes. To provide adequate power and adequate voltage, fuel cells are commonly arranged as a stack, that is to say multiple fuel cells are operated together; by connecting the fuel cells electrically in series, a larger output voltage is obtained. The liquid electrolyte may be circulated through the cells in the stack, and through a recirculation network. However there can be shunt currents flowing through the electrolyte, which lower the efficiency of the fuel cell stack.

Discussion of the invention The fuel cell system of the present invention addresses or mitigates one or more

problems of the prior art.

According to the present invention there is provided a fuel cell system comprising at least one fuel cell stack, and a recirculation network for circulating a liquid electrolyte through each fuel cell stack, the system including a decoupling wheel in the recirculation network, the decoupling wheel comprising two opposed end plates linked by vanes, the end plates being mounted so as to be rotatable about a horizontal axis, the vanes subdividing the space between the end plates into a multiplicity of chambers that are open around the periphery; an inlet duct having at least one port above the decoupling wheel to one side of the horizontal axis; and a collecting chamber below the decoupling wheel, the collecting chamber having an outlet duct.

In operation, the electrolyte is fed into the inlet duct, and falls out of each port onto the decoupling wheel to one side of the horizontal axis, therefore falling into one of the chambers. The weight of the electrolyte in that chamber causes the decoupling wheel to rotate about the horizontal axis; and when the decoupling wheel has turned through a sufficient angle, the next vane will intercept the falling electrolyte, so the electrolyte will start to enter the next chamber. After a certain degree of rotation, the electrolyte from the first chamber will overflow. Electrolyte that falls out of the decoupling wheel falls into the collecting chamber, and may then flow out of the outlet duct.

Preferably the vanes define at least three chambers; and there may be more than ten chambers. It has been found that satisfactory operation can be achieved with between four and eight chambers, for example four, five or six chambers.

In one embodiment the decoupling wheel also includes a sill extending adjacent to the outer edge of each vane. In this case the electrolyte that falls into a chamber is caught by the sill and the adjacent vane.

The effect of the decoupling wheel is to break up the stream of electrolyte between the inlet duct and the outlet duct, and thereby to suppress electric current carried by ions in the electrolyte within that part of the recirculation network. The dimensions of the elements defining a chamber must therefore be selected in relation to the rate of rotation of the decoupling wheel and the flow rate of the electrolyte, such that each chamber does not fill with electrolyte, before the decoupling wheel has rotated such that electrolyte enters the next chamber.

Such a decoupling wheel may be provided in a duct which carries spent electrolyte from a fuel cell stack.

Such a decoupling wheel may be provided in a duct that carries electrolyte to a fuel cell stack. In this case the collecting chamber may also be provided with an overflow duct at a higher position than the outlet duct. In operation the flow of electrolyte into the inlet duct should be at least equal to, and preferably greater than, that flowing out of the outlet duct, so the excess electrolyte flows out of the overflow duct, the depth of electrolyte therefore reaching a steady state corresponding to the position of the overflow duct. The electrolyte is therefore supplied to the fuel cell stack at a substantially constant pressure.

In this situation there is the possibility of ionic current flowing in the electrolyte that flows through the overflow duct. The system may therefore also include a device to break up the electrolyte flow from the overflow duct, and this may be a secondary decoupling wheel into which the overflow duct feeds the excess electrolyte. This may have substantially the same features as the decoupling wheel described above, but it may be of a smaller size, as the flow rate of the excess electrolyte may be less than the flow of electrolyte through the inlet duct.

Hence a fuel cell system may comprise a plurality of fuel cell stacks, and a recirculation network for supplying electrolyte to each fuel cell stack, where there are at least as many decoupling wheels as fuel cell stacks, at least one decoupling wheel for each fuel cell stack. There may be decoupling wheels in both the duct that carries electrolyte to a fuel cell stack, and the duct which carries spent electrolyte from that fuel cell stack. Each decoupling wheel, if it is in a duct carrying electrolyte to a fuel cell stack, may also include a secondary decoupling wheel as described above.

Such a decoupling wheel has been found suitable for breaking up an electrolyte flow of as much as 60 litres/mm or more, for example up to 40 litres/mm, for example 10,20 or litres/mm. Typically the secondary decoupling wheel, where this is provided, would carry a flow rate no more than half that of the decoupling wheel, more typically no more than a fifth, for example a tenth or a twentieth or less.

It will be appreciated that the decoupling wheel can suppress ionic leakage currents in the electrolyte recirculation network, those currents being generated by one or more of the fuel cell stacks, but does not have any effect on any ionic leakage currents that may occur between cells of the fuel cell stack. Those ionic leakage currents may be suppressed in other ways, for example by feeding electrolyte into a single cell through a long and narrow flow channel; or by arranging electrolyte to flow into a single cell in discrete drops; or by other means.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a schematic flow diagram of a liquid electrolyte fuel cell system, including a recirculation network; Figure 2 shows a perspective view of part of the recirculation network; Figure 3 shows a fragmentary Gloss-sectional view on the line 3-3 of figure 2; Figure 4 shows a cross-sectional view on the line 4-4 of figure 2; Figure 5 shows a view corresponding to that of figure 4, showing a modification; and Figure 6 shows a view corresponding to that of figure 4, showing an alternative modification.

Referring now to figure 1, a fuel cell system 10 is shown which includes a number of fuel cell stacks 20 (three fuel cell stacks are shown, schematically). As indicated by broken lines, there may be a larger number of fuel cell stacks 20.

Each fuel cell stack 20 uses an aqueous electrolyte 12, for example an aqueous solution of potassium hydroxide, which might for example be at a concentration of 6 moles/litre. Each fuel cell stack 20 is supplied with a fuel, such as hydrogen gas; an oxidant such as air, and electrolyte 12, and operates at an electrolyte temperature of for example about 65° or 70°C. Inlet ducts 14 and 16 for hydrogen and for air are shown schematically for one of the fuel cell stacks 20.

Each fuel cell stack 20 is represented schematically, but it consists of a stack of several fuel cells, for example there might be between five and 200 fuel cells, for example 5, 10, 20, 40, 80, 100, 120 or 140 fuel cells or more, as required to provide a desired output voltage. In each fuel cell stack 20, each fuel cell comprises a liquid electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode. In each cell, air flows through a gas chamber adjacent to the cathode, to emerge as spent air. Similarly, in each cell, hydrogen flows through a gas chamber adjacent to the anode, and emerges as an exhaust gas stream.

The electrolyte 12 is stored in an electrolyte storage tank 25 which is provided with a vent 26; in a modification the vent 26 may not be provided as the pressure may be regulated in another fashion. The storage tank 25 is also provided with a heat exchanger 27 which forms part of a coolant circuit 28 that also includes a circulation pump 29 and an air-cooled heat exchanger 30 with an air fan 31. The coolant circuit 28 enables the temperature of the electrolyte 12 to be maintained at a desired value.

A pump 35 circulates electrolyte 12 from the storage tank 25 into a header duct 37 from which a plurality of feed ducts 40 provide the electrolyte 12 to each fuel cell stack 20, one feed duct 40 for each fuel cell stack 20. Each feed duct 40 communicates via a tank 42 containing a decoupling wheel 44, and then through an outlet duct 46 to the fuel cell stack 20. Each tank 42 is provided with a vent 47, and has an overflow pipe 48 so that electrolyte returns to the storage tank 25 via a secondary decoupling wheel 50 and an outflow pipe 52; in a modification the vent 47 may not be provided. The overflow pipe 48 communicates with the tank 42 above the level of the outlet duct 46, and so ensures that the level of electrolyte in the tank 42 is constant, as long as there is more electrolyte provided through the feed duct 40 than required by the fuel cell stack 20. Consequently the electrolyte 12 is supplied at constant pressure through the outlet duct 46 to the fuel cell stack 20; and spent electrolyte returns to the storage tank 25 through a return duct 54. The return ducts 54 from all the fuel cell stacks 20, and the outflow pipe 52 carrying the excess electrolyte from each tank 42, all feed into the storage tank 25 through a common return pipe 55.

Referring now to figure 2, this shows a perspective view of one of the tanks 42, showing the decoupling wheel 44 and also showing the secondary decoupling wheel 50.

The walls of the tank 42, and the walls of the secondary tank 56 which encloses the secondary decoupling wheel 50, are shown as transparent, so the features within the tanks 42 and 56 are visible; they would not necessarily be transparent in practice.

The tank 42 is of rectangular shape. The feed duct 40 extends between opposite ends of the tank 42, just below the top of the tank 42, and defines multiple flow apertures 60 (shown in figure 4) along its length, on its underside. The decoupling wheel 44 is also of rectangular shape, with square end plates 62 linked by vanes 64 (shown in more detail in figure 4), the end plates 62 being fixed to projecting cylindrical shafts 65 at the centre, and these shafts 65 are mounted on opposite end walls of the tank 42 so as to be free to rotate, so defining an axis of rotation A (shown in figure 3). The flow apertures 60 should not be directly above the axis of rotation A, and in this example the axis of rotation A is slightly to one side of the centre plane of the tank 42, whereas the feed duct 40 is displaced to the other side of the centre plane of the tank 42.

Referring also to figure 3, which shows a fragmentary sectional view of one of the cylindrical shafts 65, the cylindrical shafts 65, and indeed the end plates 62, may for example be of an engineering plastic such as ABS (acrylonitrile butadiene styrene) or polypropylene, and may be mounted in bushes 66 of a low friction material such as solid PTFE (polytetrafluoroethylene). The bushes 66, in this example, locate in grooves 68 down the inner surface of the end walls of the tank 42, so the bushes 66 and the decoupling wheel 44 can be slid into position, or removed.

The outlet duct 46 communicates with an end wall of the tank 42 just above the bottom, whereas the overflow duct 48 communicates with that end wall of the tank 42 above the bottom, but below the bottom of the decoupling wheel 44. The overflow duct 48 feeds into the tank 56 that contains the secondary decoupling wheel 50, the overflow duct 48 extending through a wall of the tank 56 to terminate at a spout 70. (In a modification, instead of a single spout 70, there may be multiple outlet ports analogous to the flow apertures 60 described above.) The secondary decoupling wheel 50 is also of rectangular shape, with square end plates 72 linked by vanes 74, the end plates 72 being joined to projecting cylindrical shafts 75 at the centre, and these shafts 75 locate in bushes 76 mounted on opposite end walls of the tank 56, so defining an axis of rotation. The axis of rotation of the secondary decoupling wheel 50 should not be directly below the spout 70, and in this example the axis of rotation is on the centre plane of the tank 56, whereas the spout 70 is to one side of the centre plane.

The cylindrical shafts 75 can be of the same materials, and mounted in the same way, as the corresponding components in the decoupling wheel 44. So for example the cylindrical shafts 75, and indeed the end plates 72, may be of an engineering plastic such as ABS or polypropylene, while the bushes 76 may be of a low friction material such as solid PTFE. The bushes 76 may locate in grooves 78 down the inner surfaces of the end walls of the tank 56, so the bushes 76 and the decoupling wheel 50 can be slid into position, or removed.

Referring now to figure 4, this shows a transverse sectional view through the tank 42. In this example, the decoupling wheel 44 includes four vanes 64 which extend from the axis of rotation A to the corners of the end plates 62, and which extend from one end plate 62 to the opposite end plate 62, so subdividing the space between the end plates 62 into four chambers 80 between successive vanes 64. Each chamber 80 is open along its periphery, between the sides of the opposing end plates 62, apart from rectangular sill plates 82 which extend between the opposing end plates 62 starting at a corner, each sill plate 82 being joined to the vane 64 that extends to that corner. The vanes 64 and the sill plates 82 should be of an electrically-insulating material, and use of an engineering plastic (such as ABS or polypropylene) is appropriate.

Thus, in opeiation, the electrolyte 12 emeiges through the flow apertuies 60 and falls onto the decoupling wheel 44 to the right-hand side (as shown) of the axis of rotation A. The electrolyte 12 therefore collects within the chamber 80 at the top right-hand side (as shown), happed between the vanes 64 on either side of that chamber 80, and by the sill plate 82. The weight of the electrolyte in that chambei 80 causes the decoupling wheel 44 to rotate clockwise, with the cylindrical shafts 65 turning freely in the bushes 66 (as shown in figure 3). As a consequence of this rotation, the next vane 64 will pass under the flow apertuies 60, so the electrolyte will start to collect in the next chambei 80; and then electrolyte will overflow over the sill plate 82 of the chamber 80 that had recently been at the top right-hand side, the electrolyte 12 therefore falling to the bottom of the tank 42. Thus the decoupling wheel 44 will continuously rotate as the electrolyte 12 flows through the apertuies 60, collecting electiolyte 12 in the chamber 80 at the top light-hand side, emptying electrolyte 12 from the chamber 80 at the bottom right-hand side, and completely emptying the chamber 80 at the bottom left hand side, so the chamber 80 at the top left-hand side is always empty. The inflowing electrolyte 12 from the feed duct 40 is electrically and ionically insulated fioni the electiolyte 12 at the bottom of the tank 42, so that ionic leakage curients aie substantially prevented.

The secondary decoupling wheel 50 is of substantially the same cross-sectional shape, with the vanes 74 defining four chambers 90, and the chambers also including a sill plate 92 (as shown in figure 2), so that the electrolyte 12 falling from the spout 70 makes the decoupling wheel 50 continuously rotate (anticlockwise as shown in figure 2). The electrolyte therefore falls from the spout 70 into a chamber 90, and then as the decoupling wheel 50 rotates the electrolyte 12 falls out of a chamber 90 into the bottom of the tank 56, and then flows out of the outflow pipe 52. This again leads to the consequence that the inflowing electrolyte 12 from the spout 70 is electrically and ionically insulated from the electrolyte 12 at the bottom of the tank 56.

It will be appreciated that the sizes of the decoupling wheel 44 and of the secondary decoupling wheel 50 must be appropriate for the electrolyte flow rates that they are to carry.

The size of the decoupling wheel 44 or 50 must therefore be selected in relation to the rate of rotation of the decoupling wheel 44 0150 and the flow rate of the electrolyte 12, such that electrolyte does not overflow from a chamber 80 or 90 before the decoupling wheel 44 or has rotated through such an angle that the falling electrolyte starts to collect in the next chamber 80 or 90. In the decoupling wheel 44 or 50 where the vanes 74 are in orthogonal orientations, this poses a restriction on the height of each sill plate 82 or 92: the volume of electrolyte that can be held behind the sill plate 82 or 92 without overflowing, when the decoupling wheel 44 or 50 is just about to start collecting electrolyte in the next chamber 80 or 90, must be at least equal to the amount of electrolyte that flows through the feed duct 40 or the spout 70 in the time for a quarter of a revolution of the decoupling wheel 44 or 50.

By way of example the decoupling wheel 44 may be of length 460 mm, and 128 mm square, with a sill plate 82 of height between 10 and 20 mm high, for example 12 mm or 13mm high; this has been found suitable for a flow rate of 20 litres/mm. The decoupling wheel 50, in this example, may carry a significantly smaller flow rate. In one example the flow rate is less than 2 litres/mm, and so the decoupling wheel 50 may be smaller, for example 80 mm square and 250 mm long, and with the sill plate 92 of height between 5 mm and 15mm.

It will be appreciated that the decoupling wheel 44 or 50 may differ from the one described above, while remaining within the scope of the invention. For example a decoupling wheel 44 or 50 may be of length up to 1000 mm or more, although it is usually more convenient to have a length between 50mm and 600 mm, for example between 150 mm and 500 mm. Similarly a decoupling wheel 44 or 50 may have a different cross-sectional size, for example the end plates 62 or 72 may have sides of length between 50 mm and 300 mm or more, more generally between 75 mm and 200 mm. The end plates might have a different shape, and might for example be circular or polygonal, while the number of vanes 64 or 74, and so the number of chambers 80 or 90 may be different, for example five or six. Where sill plates 82 or 92 are provided, they may for example project between 2 mm and 50 mm above the vane 64 or 74, more typically between 5 mm and 35 mm. The sill plates 82 or 92 may lie against the periphery of the end plates, as shown; if the end plates have a different shape, such as polygonal or circular, then the sill plates would consequently be at a different angle to the adjacent vanes 64 or 74 to that shown in figure 4.

Referring now to figure 5, this shows a fragmentary view of a modification to the decoupling wheel 44, differing in having circular end plates 100 to support the vanes 64, and in having arcuate sill plates 102.

In any event the sill plates may have a different orientation relative to the adjacent vanes, and may for example be orthogonal. Referring now to figure 6, this shows a fragmentary view of another modification to the decoupling wheel 44, differing in having circular end plates 100 to support the vanes 64, and in having a sill defined by a plate 104 which extends orthogonally to the vane 64 near its outer edge.

It will also be appreciated that the sill need not be of uniform thickness, so it may not be in the forrri of a plate; for example in figure 6, the sill rriight occupy the volume 106 shown radially outside the plate 104, instead of or in addition to the volume of the plate 104, so that the sill is thicker where it is attached to the vane 64, than at its end remote from the vane 64. Furthermore the sill, whether or not it is in the form of a plate, may be integral with the adjacent vane 64.

It will also be appreciated that if a fuel cell system incorporates both a decoupling wheel 44 and a secondary decoupling wheel 50, these may, as shown in figure 2, have the axes of rotation of the decoupling wheels 44 and 50 in orthogonal planes; or alternatively the axes of rotation may be in parallel planes, or at any other convenient relative orientation. It will also be appreciated that the secondary decoupling wheel 50 may be arranged directly underneath the tank 42 that contains the decoupling wheel 44, if this provides a more compact and convenient layout; in this case the outlet duct 46 and the overflow pipe 48 might emerge through the bottom of the tank 42, rather than through the end wall.

Claims

Claims 1. A fuel cell system comprising at least one fuel cell stack, and a recirculation network for circulating a liquid electrolyte through each fuel cell stack, the system including a decoupling wheel in the recirculation network, the decoupling wheel comprising two opposed end plates linked by vanes, the end plates being mounted so as to be rotatable about a horizontal axis, the vanes subdividing the space between the end plates into a multiplicity of chambers that are open around the periphery; an inlet duct having at least one port above the decoupling wheel to one side of the horizontal axis; and a collecting chamber below the decoupling wheel, the collecting chamber having an outlet duct.
2. A fuel cell system as claimed in claim 1 wherein the vanes define at least three chambers.
3. A fuel cell system as claimed in claim 2 wherein the number of chambers is between four and forty inclusive.
4. A fuel cell system as claimed in any one of the preceding claims wherein the decoupling also includes a sill extending adjacent to the outer edge of each vane.
5. A fuel cell as claimed in claim 4 wherein each sill comprises a plate joined to the outer edge of a vane, and to the end plates.
6. A fuel cell system as claimed in claim 5 wherein the sill is integral with the vane.
7. A fuel cell as claimed in any one of claims 4 to 6 wherein the height to which the sill projects above the vane is between 5 mm and 50 mm.
8. A fuel cell system as claimed in any one of the preceding claims wherein such a decoupling wheel is provided in a duct which carries spent electrolyte from a fuel cell stack.
9. A fuel cell system as claimed in any one of the preceding claims wherein such a decoupling wheel is provided in a duct that carries electrolyte to a fuel cell stack.
10. A fuel cell system as claimed in claim 9 wherein the collecting chamber is also provided with an overflow duct at a higher position than the outlet duct.
11. A fuel cell system as claimed in claim 10 also comprising a device to break up the electrolyte flow from the overflow duct.
12. A fuel cell system as claimed in claim 11 wherein the device to break up the electrolyte flow comprises a secondary decoupling wheel, the secondary decoupling wheel comprising two opposed end plates linked by vanes, the end plates being mounted so as to be rotatable about a horizontal axis, the vanes subdividing the space between the end plates into a multiplicity of chambers that are open around the periphery, and with a sill extending adjacent to the outer edge of each vane; and the overflow duct communicates with at least one port above the decoupling wheel to one side of the horizontal axis.
13. A fuel cell system as claimed in claim 12 wherein the vanes in the secondary decoupling wheel define at least three chambers.
14. A fuel cell system as claimed in claim 13 wherein the number of chambers in the secondary decoupling wheel is between four and eight inclusive.
15. A fuel cell system as claimed in any one of the preceding claims comprising a plurality of fuel cell stacks, a recirculation network for supplying electrolyte to each fuel cell stack, where there are at least as many decoupling wheels as fuel cell stacks, at least one decoupling wheel for each fuel cell stack.
16. A fuel cell system as claimed in claim 15 comprising decoupling wheels in both the duct that carries electrolyte to a fuel cell stack, and the duct which carries spent electrolyte from that fuel cell stack, for each fuel cell stack.
17. A fuel cell system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.