GB2442252A

GB2442252A - Low Temperature operation of open fuel cell stacks using air circulation

Info

Publication number: GB2442252A
Application number: GB0618986A
Authority: GB
Inventors: Anthony Newbold; Paul Alan Benson; Tobias Reisch
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2006-09-27
Filing date: 2006-09-27
Publication date: 2008-04-02
Anticipated expiration: 2026-09-27
Also published as: WO2008038032A2; GB0618986D0; TW200836391A; GB2442252B; WO2008038032A3; AR063031A1

Abstract

A method and apparatus for operating an open cathode fuel cell stack. The fuel cell stack has a cathode air inlet and a cathode exhaust. A variable feedback path is provided by baffle 23 and air guides 27 for returning at least a portion of the cathode exhaust air to the cathode air inlet. Dependent on the ambient temperature conditions, a proportion of the cathode exhaust air is recycled to the air inlet which enables steady state operation even in sub-zero operating ambient conditions.

Description

LOW TEMPERATURE OPERATION OF OPEN CATHODE FUEL CELL

STACKS USING AIR RECIRCULATION

The present invention relates to proton exchange membrane (PEM) fuel cells, and in particular to open cathode REM fuel cells.

In the operation of a PEM fuel cell stack, hydrogen (supplied to all fuel cell anodes) and oxygen (supplied to all fuel cell cathodes) combine in an electrochemical reaction to generate electrical power, producing water as a physical product on cathode catalyst sites. The phenomenon of liquid water transport across the fuel cell membrane results in a water balance being established between anode and cathode.

A fuel cell stack using pure hydrogen as a fuel source delivered to the anodes will tend to be designed to operate either (i) with an anode recirculation loop in which hydrogen gas flow through anode delivery conduits is recirculated to consume unused hydrogen, or (ii) with a dead ended' anode configuration in which the anode delivery conduits are closed-ended, the fuel flow to the anode being limited solely by the rate of the fuel cell reaction. Either of these two methods are used to increase operational efficiency of the fuel cell stack by reducing unused hydrogen fuel (i.e. lowering the hydrogen stoichiometry).

However, in using these configurations, water in the stack must be removed entirely by the cathode reactant gas flow (except for intermittent anode purges -although these cannot occur too frequently or a poor operational efficiency is obtained). If this water is not removed from the cathode at a sufficient rate, flooding of anode and/or cathode electrodes will occur and the fuel cell stack will cease to produce power effectively as the transport of reactant gases to the electrode catalyst sites is gradually restricted. The flooding of the cathode side of a fuel cell has a much greater negative effect on fuel cell performance due to the relatively slow and complicated oxygen reduction reaction.

With pressurised' fuel cell stacks, a compressor typically supplies air at pressures between 0.5 bar g and 3.0 bar.g with the flow rate limited to 2-3 times the stoichiometric amount required (due to the high energy requirements of air compression). Here the problem of electrode flooding may be exacerbated due to the fact that pressurised stacks tend to use humidification sub-systems on at least one of the gas feeds to maintain a high water content in the polymer membrane structure (the water content of the polymer membrane structure is directly proportional to the limiting rate of proton transfer from anode to cathode). The transport of product and humidification water away from the stack is primarily undertaken by the high-pressure airflow which is, in turn, influenced by the design of flow conduits in faces of the cathode flow-field plates. Here the product water, in liquid phase, can be transported as a convective flow mechanism away from the fuel cells. This is in addition to the removal of water in vapour form which has been absorbed by the heated cathode exit gas.

One of the disadvantages of pressurised fuel cell stacks is that, due to the energy required to compress the air, up to 25% of the generated stack power may be consumed as parasitic loses. Open cathode' or sometimes called air-breathing' fuel cell stacks have much lower parasitic losses as air pressurization is not required. In a typical open cathode stack configuration, a low pressure fan is used as a combined cooling and air supply mechanism resulting in a much simpler stack and system design -no additional cooling or humidification subsystems are required. The cooling air flow passes through the active conduits of the stack cathodes, maintaining a set stack temperature, and the water required to keep the fuel cell membranes humidified is generated entirely from the fuel cell reaction. The open cathode configuration fuel cell stack operates with the air flow at or very close to atmospheric pressure -the oxygen stoichiometry can typically be in excess of 50 using such an arrangement.

In an open cathode fuel CC11 arrangement the cathode flow conduits are usually straight and short parallel channels across the width of the stack. The cathode plate design is usually one of two categories as shown in figure 7 The first of these is a corrugated design flow plate 70 (figure 7a) where a flat piece of material 71 (probably a metal alloy strip) has been pressed or stamped to form the plate 70, the flow channels 73 being formed between adjacent downward corrugations 72. The second is a fiat plate design of flow plate as shown in figure 7b which comprises a flat plate that has had the channels 76 formed in the planar surface by machining, etching or even moulding Note that in the case of the corrugated plate 70 there may also be non-active cooling channels which provide additional cooling compared to the flat plate 75.

The high stoichiometric flow rate of air through these cathode flow channels removes all of the reaction product water by the process of evaporation In each fuel cell cathode, the liquid product water is continually transported to outer sections of the diffuser layer adjacent to the flow channels where it is absorbed into the airflow The limiting rate of water removal is directly related to the ambient air temperature and humidity in addition to stack temperature as explained below Liquid water can be absorbed by air until a relative humidity of l00%RH is obtained, referred to as full saturation, which occurs when the partial pressure of water vapour in the air (PH2O) is equivalent to the saturation pressure of water vapour, Psat The saturation pressure of water vapour has an exponential relationship with temperature as shown in figure 8, therefore drier and warmer air results in a greater capacity for water absorption Conversely, cooler and damper air will have a lower capacity.

These concepts can be applied to an open cathode fuel cell stack by considering the water balance over a single cathode flow channel. Here, water in the liquid slate is produced by the fuel cell reaction and moves to the bottom surface of the flow channel (i.e. top of the cathode diffuser layer). Air is forced down the flow channel from entry to exit by the fan and gains heat from the four surfaces that form the channel cross sectional area. Water is absorbed into the air flow stream from the diffuser surface as it moves from channel entry to exit with the air leaving the stack unJikcly to be fully saturated, with the rate of absorption being relatively slow and the air speed being relatively high Therefore, depending on the ambient conditions of the air entering the flow channel and the rate of liquid product water generation, there will be either a balance. a net loss or a net gain of water over the channel. There are other factors affecting the water balance across the flow channel such as the rate of water absorption being influenced by the configuration of the cathode diffuser material -this may encourage large water droplet formations, slowing the rate of evaporation.

Therefore, under cold and humid ambient conditions (and also depending on the current produced by the open cathode stack), the air flow may not he sufficient to remove the product water at a rate equivalent to the rate of generation by the fuel cell reaction. When this situation arises, cathode flooding will occur and the electrochemical performance of the stack will eventually be compromised. In an open cathode fuel cell, cathode flooding is particularly undesirable as large water drops in the flow channels cannot be forcibly removed' by using a low pressure air fan The stack or system may have to be taken offline so the stack cathode channels can be unblocked. As a result of this known problem, the use of fuel cell systems equipped with open cathode stacks has previously been limited to operation at higher minimum ambient temperatures than pressurised stack systems.

It is an object of the present invention to provide an open cathode fuel cell stack capable of improved low ambient temperature performance.

The present invention recognises that at least a portion of the heat generated by the fuel cell under normal operation can be used, when operating in low ambient temperatures, to ensure that a suitable stack temperature is maintained sufficient to obtain an appropriate level of water evaporation from the cathodes. This is achieved by recirculating at least a portion of cathode exhaust air back to the cathode air inlet.

Although cathode air recirculation systems have been proposed in the art, none of these has been directed to maintaining temperature in an open cathode system.

For example, WO 03/041200 proposes recirculation in both the anode and cathode feed lines of a closed or pressurised cathode system. At least a part of the cathode exhaust is recirculated at a first recirculaton ratio and at least a part of the anode exhaust is recirculated at a second recireulation ratio so as to maintain or increase the total flow rate through the anode and cathode chambers for a given reactant stoichiometry. This gives a higher pressure drop across the fuel cell stack which in turn improves the uniformity of distribution of reactants and improves water management when the stack is operated at less than full power. By contrast, open p cathode fuel cell stacks cannot maintain a significant pressure drop since the cathode is operated at substantially atmospheric pressure.

US 4362789 also proposes a pressurised cathode system with a recirculation loop in which the pressurised (approximately 50 psi) oxidant make up source' and controlled vent of the pressurised cathode ioop are controlled "to discharge cathode reaction products and provide fresh oxidant such that the ratio of reaction product recirculation to fresh oxidant supply is approximately thirty to one ". The recirculation loop has to include a cooler to reduce the recirculation loop temperature.

JP 58164157 describes a fuel cell system with a cooling circuit including a recirculation loop for maintaining an optimum temperature of an air chamber in a fuel cell.

US 2006/0134495 describes a recirculation valve for a pressurised cathode recirculation flow path, which valve responds to increasing pressure of a cathode inlet stream by reducing the proportion of the cathode outlet stream that is recirculated to the cathode inlet.

According to one aspect the present invention provides an open cathode fuel cell stack having a cathode air inlet, a cathode exhaust, and a variable feedback path for returning at least a portion of the cathode exhaust air to the cathode air inlet.

According to another aspect, the present invention provides a method of operating an open cathode fuel cell stack comprising the steps of: driving air into an open cathode air inlet of the fuel cell stack, receiving air from a cathode exhaust of the fuel cell stack, and controlling a variable feedback path that returns at least a portion of the cathode exhaust air to the cathode air inlet to maintain a predetermined temperature of the fuel cell stack.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows a schematic side view of an open cathode fuel cell stack rn a chamber with a variable feedback path for cathode exhaust air; Figure 2 shows a schematic side view of an alternative arrangement of an open cathode fuel cell stack in a chamber with a variable feedback path for cathode exhaust air; Figure 3 shows a graph of cell potential and stack temperature as a function of time for an open cathode fuel ccli operating in an ambient temperature of approximately minus six degrees centigrade; Figure 4 shows a graph of cell potential and stack temperature as a function of time for an open cathode fuel cell operating in an ambient temperature of approximately minus six degrees centigrade but with cathode air recirculation; Figure 5 shows a schematic side view of an alternative arrangement of open cathode fuel cell stack with a variable feedback path for cathode exhaust air in a first configuration; Figure 6 shows a schematic side view of the arrangement of figure 5 with the variable feedback path in a second configuration; Figure 7 shows schematic cross-sectional views of two types of prior art flow plates; and Figure 8 shows a graph illustrating the saturation pressure of water as a function of temperature applicable to an open cathode fuel cell stack.

The present invention describes a method and apparatus for operating a fuel cell system, utilising an open cathode stack, under low and even sub-zero ambient temperatures and high humidities.

Basic energy balance calculations illustrate that a single fuel cell operating at around 0.6 to 0.7 volts produces a proportion of waste heat approximately equivalent to the amount of generated electrical power. In the case of open cathode fuel cells this is removed by the cathode exhaust air flow. For an open cathode fuel cell system, this heat is usually regarded as lost' as it cannot be recovered efficiently enough through any post-stack' heat transfer process.

S

The procedure described here involves recycling a proportion of the warm exhaust air back to the stack resulting in a higher operating temperature of the stack. As a direct result of this, a greater amount of product water can be removed from the cathode thus lowering the minimum ambient temperature at which the fuel cell stack can be S continuously operated. Other advantages of operating the fuel cell stack with recirculated cathode exhaust air are (i) an increase in electrochemical performance (corresponding to a higher stack operating temperature) and (ii) the potential for a faster low temperature (e.g. sub-zero) start-up of an open cathode fuel cell system.

With reference to figure 1, there is shown a first arrangement for an open cathode fuel cell stack deploying the principles of cathode air recirculation.

A fuel cell system 10 comprises an open cathode fuel cell stack 12 comprising a plurality of fuel cells arranged in series configuration in a stack in known manner.

The fuel cell stack has an open cathode, i.e the cathode fluid flow channels operate at substantially atmospheric (ambient) pressure with air flow through the cathode flow channels being assisted only by a fan 13. Preferably, the cathode plates are corrugated in design as shown in figure 7a. The stack 12 is housed in a chamber 11 substantially enclosing the fuel cell stack. Fuel feed lines (not shown) deliver hydrogen fuel to the anodes of the fuel cell stack 12 in a conventional manner and will not be discussed further here.

The fan is preferably positioned adjaceni to a downstream face of the stack 12 and pulls air through the stack from an air intake 14 which preferably includes a filter unit 15 for removal of chemicals and particulates that could be damaging to the cathode flow channels, the PEMs and catalyst sites in the cathodes. An air guide 16 is provided to guide the cathode ventilation air 17 around the chamber 11 to an upstream face l 8 of the fuel cell stack 12 where it can enter the open cathode flow channels.

An opposite, or downstream, face 19 of the fuel cell stack 12 provides the cathode exhaust. The air guide 16 may also serve to guide cathode exhaust air 20 through the fan 13 to a chamber air vent 21 and/or to a recirculation path 22 Preferably, the fan is of the centrifugal type as shown schematically.

II

in a first configuration as shown in figure 1, all of the cathode exhaust air 20 is directed straight to the chamber air vent 21 by exhaust air guides 25, where it is expelled from the chamber 11 by the positioning of a variable position baffle or flap 23 in a first, fully open, position. In the fully open position, 0 % of the cathode exhaust air is recirculated.

In a second configuration as shown in figure 2, all of the cathode exhaust air 20 is directed to the recirculation path 22 by the positioning of the variable position baffle or flap 23 in a second, fully closed position, occluding the chamber air vent 21. In this fully closed position, 100% of the cathode exhaust air is recirculated as shown by arrow 26. Air guides 27 may he provided in the recirculation path to assist in appropriate air flow from the baffle 23 to the upstream face 18 of the fuel cell stack 12. A balance is set up between air intake and expulsion through the filter unit 15 preventing a build-up of pressure within the chamber 11.

In general, the fan may be configured for forced ventilation of the stack by drawing air from a downstream face of the stack and towards variable feedback path. The variable feedback path may comprise air guides downstream of the fan confining the air flow to the recirculation path and / or to the chamber air vent according to the configuration of the variable position baffle A control circuit (not shown) controls the positioning of the baffle 23, e.g. by way of a small motor or similar device. The position of the baffle 23 may preferably be controlled to any position between the fuily open position of figure 1 corresponding to 0 % recirculation and the fully closed position of figure 2 corresponding to 100 % recirculation. The proportion of air recirculation required is preferably determined by the control circuit as a function of temperature. Preferably, the temperature measured to control the proportion of air recirculated is one or more of the stack temperature, the air inlet temperature and the stack exhaust temperature. The temperature may be measured by one or more suitably located thermocouples (not shown). Preferably, a control algorithm implemented in software or firmware in the control circuit determines a percentage of recirculation air required as a function of the measured temperature. )

In the preferred example discussed above, the control circuit for the recirculatiori baffle may set the proportion of recirculated exhaust air between 0 % at temperatures above a first predetermined temperature TI and 100 % at temperatures below a second predetermined temperature 12. Exemplary temperature values could be TI = degrees C and 12 = 0 degrees C. Preferably, at temperatures between TI and 12 the control circuit positions the baffle 23 at various angles or positions using an analogue control signal to vary the proportion of recirculation air between 0 and 100 % in order to maintain an optimum stack temperature. The setting of the baffle 23 could be determined by a pre-programmed scale or the system could be programmed to determine the optimum setting for any prevailing conditions, including one or more of stack power, temperature and humidity etc. The proportion of recirculated air may also be determined on a temporal basis, by having the recirculation baffle either fully open or fully closed for discrete periods of time and controlling the ratio of open to closed times by a control circuit. This configuration may have the advantage of not requiring a servo motor.

In another arrangement, the control system need not be an electronic control circuit.

Movement of the baffle 23 may be controlled by any sort of temperature sensitive device in, for example, the air exit path, which would control the angle of baffle 23.

Such a temperature sensitive device could be a bimetallic strip coil where two metals with dissimilar thermal coefficients are bonded such that a change in temperature results in a displacement change. This method is simpler than a control circuit but might give less flexibility in a control algorithm.

In a general sense, the exhaust air guides 25, the baffle 23 and recirculation path 22 provide an example of a variable feedback path in which the proportion of cathode exhaust air that is recirculated to the cathode air inlet 18 is controllable, preferably fromzerotol00% Other control functions may be included in the fuel cell system 10. For example, one potential issue in recycling air in a fuel cell system chamber II may be the build up of hydrogen due to any small leaks in the stack or pipe work. This can be resolved easily by periodically moving the baffle to the fully open position (figure 1) and purging the chamber II of recycled stack air This purge cycle can be implemented for a sufficiently short period such that no significant amount of heat is lost from the system. Such purge cycles could be implemented automatically on a timed basis, or could be triggered by detection of a predetermined level (e.g. a very low level) of hydrogen in the chamber 11 using a suitable sensor. A safely measure could be to provide no capabilit for a manual override of a hydrogen purge.

The embodiments described above provide a very high air stoichiometric flow using only a single fan for forced ventilation of the open cathode fuel cell stack. Providing that there is an adequate proportion of fresh air being drawn into the system there will be no water accumulation or oxygen depletion inside the fuel cell system.

The fuel cell system need not be housed in a chamber entirely enclosing the fuel cell stack. For example, as shown in figures 5 and 6. ducting is used to provide the feedback path. Figure 5 shows a close coupled' arrangement in which a fuel cell stack 52 is in close proximity to a filter unit 55 through which cathode ventilation air 17 is drawn by way of a centrifugal fan unit 53. The outlet of centrifugal fan 53 is coupled to a variable feedback path comprising a side vent 61, variable position baffle or flap 63, recirculation duct 67 with outlet 68. An air manifold block 56 enables the fan 53 to draw air through the stack 52 from downstream face 59 and expel the air via side vent 61 (as shown in figure 5) or, when the flap 63 is in the fully closed position (as shown in figure 6) to direct the air into the recirculation duct 67 arid back to the upstream face 58 of the stack 52.

As in the arrangement of figures 1 and 2, the flap 63 may be deployed in any position between the flrst, fully open position of figure 5 in which 0 % of cathode exhaust air is recirculated and the second, fully closed position of figure 6 in which 100 % of the cathode exhaust air is recirculated as air stream 66. Depending on the precise configuration of the system (e.g. degree of close coupling), a proportion of fresh air through filter unit 55 may continue to be delivered even with the flap 63 fully closed.

Thus, in a general sense the variable feedback path is provided by the recirculation duct 67 extending between a downstream face 59 of the stack 52 and an upstream face 58 of the stack; a vent 61 by which air flow can be expelled away from the stack 52; and the variable position baffle 63 that determines the proportion of exhaust air entering the recirculation duct 67 compared to the proportion of air passing through the vent 61.

Although the recirculated air is shown being delivered to an outlet 68 downstream of the filter unit 55, in another embodiment the system may be configured to deliver the recirculated air upstream of the filter unit.

Experimental results The results of tests with and without the air recirculation mechanism are shown here.

An open cathode fuel cell system was placed into an environmental chamber at a fixed temperature of minus 6 degrees Centigrade with the power loading on the stack being approximately 150 Watts. The air recirculatiori was implemented by a fixture which redirected a proportion of air from the exit air vent 21 and towards the air intake filter 14. The system was configured to switch off when any cell in the stack dropped to a potential of 400mV in order to avoid damage.

Figure 3 shows a graph of the system performance using no air recirculation (i.e. regular operation). Plot A (dashed) shows the mean cell potential. Plot B (dot-dash) shows the temperature of the stack as measured by a stack thermocouple. Plot C shows the environmental chamber (ambient) temperature for the system.

As shown in figure 3, the fuel cell system is about to switch off after approximately minutes operation, the effect of cathode flooding being clear from the potential plot A, in which the performance degrades initially steadily and then much more rapidly. The temperature of the stack (plot B) only reaches 10 degrees Centigrade.

Figure 4 shows a graph of the system performance with air recirculation activated. In figure 4, the same test has been repeated but with the use of the air recirculation fixture Plot E (dashed) shows the mean cell potential. Plot D (dot-dash) shows the temperature of the stack as measured by a stack thermocouple. Plot F shows the environmental chamber (ambient) temperature for the system.

The system commenced delivenng power and experienced no interruptions for six hours. Following this, the test was halted, it being clear that the fuel cell stack was operating under sustainable conditions (the stack potential was steady for the last five and a half hours). It can be observed how the temperature of the stack initially rose slowly, gradually equilibrated with the system internal temperature at a final temperature of approximately 33 degrees C with an external ambient temperature of rninus6degreesC.

The advantage of utilising air recirculation on system perfonnance in cold ambient conditions is clear from these data. An additional observation made from the tests was that where air recirculation was used, the system internal electronic components were completely dry and free from condensation This is a major operational benefit arid will increase the reliability and lifetime of electrical components in a system Other embodiments are intentionally within the scope of the accompanying claims.

Claims

An open cathode fuel cell stack having a cathode air inlet, a cathode exhaust, and a variable feedback path for returmng at least a portion of the cathode exhaust air to the cathode air inlet. )
2. The fuel cell stack of claim 1 in which the variable feedback path is adapted to vary a proportion of cathode exhaust air recirculated to the air inlet as a function of temperature
3. The fuel cell stack of claim 2 in which the variable feedback path is controlled as a function of stack temperature.
4. The fuel cell stack of claim 2 in which the variable feedback path is controlled as a function of the temperature of air entering the air intake.
5. The fuel cell stack of claim 2 in which the variable feedback path is controlled as a function of the temperature of the cathode exhaust air.
6. The fuel cell stack of claim I formed within a chamber, the chamber including: an air intake for ingress of ambient air external to the chamber; a fan for forced ventilation of the stack and cathode air inlet with air from the air intake; an air vent for egress of cathode exhaust air from the chamber; a recirculation path between the cathode exhaust and the air inlet; and a variable position baffle for determining a proportion of exhaust air entering the recirculation path and the air vent.
7. The fuel cell stack of claim I in which the variable feedback path is adapted to recirculate a variable portion between 0 and 100 % of the cathode exhaust air as a function of temperature and I or time.

I
8. The fuel cell stack of claim 1 in which the variable feedback path is adapted to periodically switch between recirculating 0 % and 100 % of the cathode exhaust air according to temperature and / or time.
9. The fuel cell stack of claim 6 in which the fan is configured for forced ventilation of the stack by drawing air from a downstream face of the stack and towards the variable feedback path, the variable feedback path comprising air guides downstream of the fan confining the air flow to the recirculation path and I or to the chamber air vent according to the configuration of the variable position baffle.
10. The fuel cell stack of claim 9 in which the recirculation path returns air from the downstream face of the stack to the upstream face of the stack.
Ii. The fuel cell stack of claim I in which the variable feedback path comprises: a duct extending between a downstream face of the stack and an upstream face of the stack; a vent by which air flow can be expelled away from the stack; and a variable position baffle for determining a proportion of exhaust air entering the duct and a proportion of air passing through the vent.
12. The fuel cell stack of claim 11 further including a fan having an inlet coupled to the downstream face of the stack by way of a manifold and an outlet directed towards said vanable position baffle.
13. A method of operating an open cathode fuel cell stack comprising the steps of: driving air into an open cathode air inlet of the fuel cell stack, receiving air from a cathode exhaust of the fuel cell stack, and controlling a variable feedback path that returns at least a portion of the cathode exhaust air to the cathode air inlet to maintain a predetermined temperature of the fuel cell stack.
14. A fuel cell stack substantially as described herein with reference to the accompanying drawings.