GB2534549A

GB2534549A - Fuel cell track system

Info

Publication number: GB2534549A
Application number: GB1500697.6A
Authority: GB
Inventors: Akhtar Naveed
Original assignee: AFC Energy PLC
Current assignee: AFC Energy PLC
Priority date: 2015-01-16
Filing date: 2015-01-16
Publication date: 2016-08-03
Also published as: GB201500697D0

Abstract

A system for supplying air to multiple fuel cell stacks 11, whereby the air is arranged to pass through a plurality of fuel cells before emerging through an outlet 15c. At least one of the fuel cell stacks is provided with a flow-restrictor 52 downstream of the stack to restrict flow through the outlet. The flow-restrictors may be provided to multiple stacks which restrict air flow to different extents, such that the flow rate through each stack is uniform. The flow-restrictor may consist of a plate defining an orifice (53, fig 4). The extent of flow restriction may be obtained by providing orifices of different sizes, or alternatively by providing a plate with multiple orifices.

Description

Fuel Cell Stack System The present invention relates to a system that includes several fuel cell stacks, enabling fluids to flow through all the cell stacks; it relates to the supply of 5 gases, and of air in particular.

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 suitable for operation in an industrial environment. Acid fuel cells and fuel cells employing other aqueous 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. Typical electrodes for alkaline fuel cells comprise a conductive metal mesh, typically nickel, that provides mechanical strength to the electrode. Onto the metal mesh is deposited a catalyst which may for example contain activated carbon and a catalyst metal such as platinum. A single fuel cell does not produce a large voltage, and it is usually desirable to assemble a number of fuel cells into a stack to provide a larger electrical power output.

For some purposes it may be necessary to assemble a number of such stacks into a system that can provide still greater electrical power output; and in this context there is a requirement to provide the electrolyte to each of the stacks, and to supply gases to each cell stack, and to withdraw these fluids from each of the stacks. For example there may be a requirement to feed electrolyte to and from each stack; to supply air to and from each stack; and to supply fuel gas to and from each stack. It may be beneficial to ensure that the flow rates of the fluids are the same to all the stacks.

As discussed in US 6 893 756 (General Motors), a fuel cell system is normally operated with excess fuel and oxidant feed stream flow rates, so the quantities of reactants supplied are greater than those expected from the stoichiometry deduced from the equation of the reaction. For example, the anode portion of a fuel cell may require approximately 130% of the required fuel to generate a given electrical output while the cathode portion may require approximately 200% of the oxygen required to complete the reaction. The excess oxidant and fuel feed stream flows are indicated by a parameter lambda, A, which is defined as the ratio of the molar rate at which a reactant is supplied to the fuel cell, divided by the molar rate at which it is consumed by the fuel cell. The corresponding value of this parameter for hydrogen (at the anode) may be termed AA and the corresponding value for oxygen (at the cathode) may be termed Ac. In the previous example, the value of lambda for hydrogen AA would be 1.3, while the value of lambda for the oxygen Ac would be 2.0. This means that 1.0 part hydrogen is converted to electrical energy for every 1.3 parts provided to the anode, with the remaining 0.3 parts hydrogen exiting the fuel cell stack in anode effluent.

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

problems of the prior art.

In accordance with the present invention there is provided a system for supplying air to a plurality of fuel cell stacks, wherein within each fuel cell stack the air is arranged to pass through a plurality of fuel cells before emerging through an air outlet, wherein at least one of the fuel cell stacks is provided with a flow restrictor downstream of the fuel cell stack to restrict flow through the air outlet, the flow restrictor reducing the flow rate through that fuel cell stack.

Within each fuel cell stack the air typically passes in parallel through the plurality of fuel cells, although in some cases it may pass in series through the plurality of fuel cells. Where there are multiple fuel cell stacks it may be desirable to provide several fuel cell stacks with such flow restrictors, selected so as to ensure substantially consistent and uniform flow rates through each of the fuel cell stacks.

The flow restrictor may comprise an orifice plate, and the different degrees of flow restriction may be provided by orifices of different sizes, and/or different numbers of orifices.

In the case of fuel cell stacks to which air is provided as the oxidant, and a 35 fuel gas such as hydrogen or methane is provided as the fuel, it has been found that there is a particular issue with air flow, as the rate of flow of air has an impact on the power output of the cells, and may be a limiting parameter. The flow rate of hydrogen is a critical parameter when considering efficiency and fuel utilisation, and the reaction rate at the anode is comparatively fast; in contrast the flow rate of the air may significantly affect the power output, because the reaction rate at the cathode is comparatively slow. Where multiple fuel cell stacks are provided with air through a common header, the stacks being fed successively from the header, the pressure in the header and the air flow rate will decrease along the length of the header. Providing flow restrictors that impose different degrees of restriction to the fuel cell stacks may make it possible to achieve the same air flow rate through each of the stacks. The flow restrictors may be adjustable, to help achieve such uniformity in air 10 flow rates.

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 perspective view of a system of the invention incorporating sixteen fuel cell stack assemblies arranged at four different levels; Figure 2 shows a perspective view of a fuel cell stack assembly on a support, showing a connection means, which is part of the system of figure 1; Figure 3 shows a diagrammatic flow diagram for the air supplied to the system of 20 figure 1; and Figure 4 shows a cross-sectional view through an air outlet pipe for use in the system of figure 1.

A fuel cell consists of two electrodes, an anode and a cathode, separated by an electrolyte, and each electrode is in contact with a respective gas stream. Chemical reactions that take place at the electrodes cause ions to migrate through the electrolyte, and generate an electric current in an external circuit. It is customary to arrange fuel cells in stacks, to obtain a larger voltage or power output than is available from a single fuel cell. Each such fuel cell stack must be supplied with appropriate fluids. For example the electrolyte may be an aqueous solution of potassium hydroxide (KOH), and the gas streams may be hydrogen, and air or oxygen. In the system described below a KOH electrolyte and an air stream are passed in parallel through the fuel cell stacks, and similarly a hydrogen stream is passed in parallel through the cell stacks.

Referring now to figure 1, a system 10 of the invention is shown in which sixteen fuel cell stack assemblies 11 are arranged at four different heights, with four fuel cell stack assemblies 11 at each different height. Ducts 12 are provided to supply electrolyte to an inlet port 15 of each cell stack assembly 11 via a shut-off valve 14 that is normally open. Each fuel cell stack assembly 11 also has an outlet port 15a near its base for electrolyte. Each assembly 11 also has an inlet port 15b for air and an outlet port 15c for air; and an inlet port 15d for hydrogen and an outlet port 15e for hydrogen. The outlet duct for electrolyte, and the inlet and outlet ducts for carrying hydrogen and air are not shown in figure 1, for clarity.

Between the shut-off valve 14 and the electrolyte inlet of the assembly 11 is a T-junction, arranged so that a drain pipe 16 branches off from the duct 12, and the drain pipe 16 incorporates a drain valve 17 that is normally closed. The drain pipe 16 extends down to an electrolyte storage tank 20. In operation, if the shut-off valve 14 is closed for any reason, so preventing electrolyte flowing into the assembly 11, then the drain valve 17 would normally be opened to allow electrolyte that is within the module 11 to drain down into the electrolyte storage tank 20. The electrolyte is circulated from the storage tank 20 by a pump 22 through a pipe 24 to a succession of constant head tanks 25 which ensure that the electrolyte pressure is substantially identical for all the stack assemblies 11 at the same height in the system 10, as excess electrolyte overflows through overflow pipes 26 from each constant head tank 25 to the next one down, or in last case into the storage tank 20.

Figure 1 primarily shows the connections to supply electrolyte to each of the fuel cell stack assemblies 11. Obtaining consistent and uniform electrolyte flow through each of the fuel cell stack assemblies 11 requires care, because a difference of height within a liquid electrolyte, for example in a header, will lead to a difference of pressure. The issues with obtaining consistent and uniform gas flows are somewhat different, as differences in height are of less significance. On the other hand, the gas flows are directly related to the supply of reactants to the electrodes, so the gas flows have a more direct impact on the power output of the fuel cell stack assemblies 11. Where air is provided as the oxidant, the air flow rate may be a limiting factor.

As discussed above, the gas flow rates may be characterised by the parameter lambda, A, indicating the ratio of the supply rate of a reactant to the rate of consumption of that reactant. In the case of the anode, the hydrogen may be supplied at a value AA of 1.25 (i.e. a 25% excess). In contrast, if oxygen is the oxidant, it is usually supplied at a value of Ac of more than AA, because the oxygen reduction reaction takes place at a slower rate, and may be the rate limiting factor. The value of Ac may for example be 2.0. Since the oxygen is provided in air supplied to the cathode, and air is about 80% nitrogen, which does not react in the fuel cell, this implies that the flow rate of air is a significant consideration. It is therefore advantageous to ensure uniform flow rates of air to all the fuel cell stack assemblies 11.

Referring now to figure 2, part of the system 10 is shown. A single fuel cell stack assembly 11 is shown supported on a frame 30, and connected to feed ducts 12 and outlet or return ducts 12 for air, hydrogen and electrolyte. In this case each feed duct 12 is provided with a respective pneumatically-controlled valve 32. As regards the feed ducts for air and hydrogen, the pneumatically-controlled valve 32 is a shut-off valve, whereas for the electrolyte feed duct 12 the pneumatically-controlled valve 32 is a two-way valve, which serves the function of the valves 14 and 17 described above. Each fuel cell stack assembly 11 includes a stack of fuel cells within a casing 34. The casing 34 includes an end plate 36 on which the electrolyte, air and hydrogen inlet and outlet ports 15 and 15a to 15e (shown in Figure 1) are defined.

Referring now to figure 3, this shows the air supply of the fuel cell system 10; the flow control valves 32 are not shown, and the ducts 12 for hydrogen and electrolyte are also not shown. Air is supplied by a blower 40 and any carbon dioxide is removed by passing the air through a scrubber 42 and a filter 44. The air then flows through a duct 46 which leads in succession through a heat exchanger 47 and a humidification chamber 48. The cleaned, warmed and humidified air is then supplied to a feed duct 12 which supplies the air to all the air inlet ports 15b of the fuel cell stack assemblies 11 (only three are shown). The spent air that emerges from each air outlet port 15c is fed to an outlet duct 49 in which is a condenser 50 to separate water from air.

In this arrangement the feed duct 12 is supplying air to the fuel cell stack assemblies 11 in succession, the feed duct 12 acting as a header, and the flow rate along the feed duct 12 becomes less the further along the feed duct 12. With the features as described in the previous paragraph, although all the fuel cell stack assemblies 11 are supplied with air flows that are in parallel, the air flows are not equal. It has been observed that the flow rates of air through the fuel cell stack assemblies 11 that are connected further along the feed duct 12 are less than those through the fuel cell stack assemblies 11 that are connected nearer the start of the feed duct 12. Considering just two fuel cell stack assemblies 11, each connected in parallel to the feed duct 12, it was observed that the flow rate through the first fuel cell stack assembly 11 (i.e. the assembly 11 connected nearer to the start of the feed duct 12) was 0.19 m3/s, whereas the flow rate through the second assembly 11 (i.e. the assembly 11 connected further from the start of the feed duct 12) was only 0.14 m3/s.

The present invention therefore also provides a flow restrictor which in this example is provided by an orifice plate 52 to restrict flow through the outlet port 15c of each fuel cell stack assembly 11 (or at least those assemblies 11 apart from the fuel cell stack assembly 11 furthest from the start of the feed duct 12). The orifice plates 52 provide different restrictions to the air flow: a greater restriction is applied for those casings 34 connected nearer the start of the feed duct 12, and a lesser restriction is applied for those casings 34 connected further from the start of the feed duct 12. Two orifice plates 52 are indicated schematically in figure 1, and the orifice plates 52 are represented schematically in figure 3.

Considering the example discussed above, where there are just two fuel cell stack assemblies 11 an orifice plate 52 that provides a restriction at the outlet port 15c is required only in the first casing 34. In this arrangement, with an appropriate flow-restricting orifice plate 52 at the outlet port 15c of the first casing 34 but with no flow-restricting orifice plate at the outlet port 15c of the second casing 34, the corresponding flow rates were both 0.16 m3/s, without changing any of the other parameters.

In practice, rather than attaching an orifice plate 52 to the outside of the casing 34, as shown schematically in figure 1, it may be convenient to incorporate the orifice plate 52 within the outlet duct 12 connected to the air outlet port 15c.

Referring now to figure 4, this shows a pipe connector 60 which may be incorporated within this outlet duct 12. The pipe connector 60 comprises a threaded collar 62 which defines an internal flange 63 at one end, and first and second internally-threaded sleeves 64 and 65. The first internally-threaded sleeve 64 defines an external flange 66 to locate against the internal flange 63, while the second internally-threaded sleeve 64 defines an externally-threaded portion 67 to engage the thread of the collar 62. Such a pipe connector 60 is known.

The first internally-threaded sleeve 64 can therefore be connected by the internal screw thread onto a pipe 68 which communicates with the air outlet port 15c, while the second internally-threaded sleeve 65 is connected by the internal screw thread onto the outlet duct 12. A circular orifice plate 52 which defines a circular orifice 53 is clamped between the opposed ends of the first and second sleeves 64 and 65, so that the circular orifice 53 forms part of the air flow path for the air emerging from the air outlet port 15c.

Figure 4 shows the orifice plate 52 installed within a threaded pipe connector 60 of a conventional type. It will be appreciated that the orifice plate 52 may instead be installed within other types of connector, for example within a push-fit pipe connector for plastic pipes. A range of known and conventional pipe connectors are readily available, within which the orifice plate 52 may be installed so that the orifice 53 forms a part of the air flow path for air emerging from the air outlet port 15c.

The air outlet ports 15c from different fuel cell stack assemblies 11 will generally require orifice plates 52 that impose different restrictions on the air flow rate, for example by defining orifices 53 of different sizes. The appropriate size of the orifice 53 may be found by trial and error, while monitoring the air flow rate through each fuel cell stack assembly 11, to find the appropriate size of orifice 53 for which the air flow rates are substantially the same through each fuel cell stack assembly 11. As explained above in relation to figure 3, the orifice plates 52 are arranged to provide a greater flow restriction for those fuel cell stack assemblies 11 connected nearer the start of the feed duct 12, which may be achieved by providing a smaller orifice 53; and are chosen to provide a lesser restriction for the fuel cell stack assemblies 11 connected further from the start of the feed duct 12, which may be provided by a larger orifice 53. Instead of changing the size of the orifice 53 to change the flow restriction, in some cases an orifice plate 52 may define more than one orifice 53.

Claims

Claims 1. A system for supplying air to a plurality of fuel cell stacks, wherein within each fuel cell stack the air is arranged to pass through a plurality of fuel cells before emerging through an air outlet, wherein at least one of the fuel cell stacks is provided with a flow-restrictor downstream of the fuel cell stack to restrict flow through the air outlet, the flow-restrictor reducing the flow rate through that fuel cell stack.
2. A system as claimed in claim 1 in which there are multiple fuel cell stacks, 10 and in which several fuel cell stacks are provided with such flow-restrictors that restrict the air flow to different extents, selected so as to ensure substantially uniform flow rates through each of the fuel cell stacks.
3. A system as claimed in claim 1 or claim 2 wherein each flow-restrictor 15 consists of a plate defining at least one orifice.
4. A system as claimed in claim 3 when dependent on claim 2 wherein the different extent of flow restriction is obtained by providing orifices different sizes.
5. A system for supplying air to a plurality of fuel cell stacks substantially as hereinbefore described, with reference to and as shown in the accompanying drawings.