Patent Application
20130344406
1. Field
Disclosed herein is a recycling loop and methods that relate to fuel cell stacks, in particular, but not exclusively, to fuel cell stacks of the type having an electrolyte in the form of a polymer membrane (i.e. of PEFC (Polymer Electrolyte Fuel Cell) type).
2. Description of Related Art
It is known that fuel cell stacks enable electrical energy to be produced directly via an electrochemical redox reaction starting from hydrogen (the fuel) and oxygen (the oxidant), without passing via a mechanical energy conversion step. This technology seems promising, especially for motor vehicle applications. A fuel cell stack comprises in general the series combination of unitary components each consisting essentially of an anode and a cathode separated by a polymer membrane enabling ions to pass from the anode to the cathode.
In the case of “dead end” circuits, i.e. circuits that do not normally open into the ambient surroundings, which is generally the case for the anode circuit, and also the case for the cathode circuit for cells operating with pure oxygen, the recycling of the gases contained in the anode or cathode circuits of the fuel cell stack during normal operation is necessary so as to achieve a necessary oversupplying of the anode or cathode circuits without overconsumption of gas and also to wet the incoming fresh gas by virtue of the water contained in the recirculated gas.
Patent application WO 06/012953 and patent application EP 2 017 916 describe a fuel cell stack, in particular the gas supply channel thereof. In certain fuel cell stack uses, one is lead to increase the number of fluid pumps and/or compressors both at the anode circuit and at the cathode circuit, in order to be able to carry out quite sophisticated gas circuit controls, more particularly during the shutdown phases of a fuel cell stack. Reference may be made, for example, to patent application FR 2009/57644.
The objective of the embodiment of the present invention is to succeed in providing a sophisticated control of the gas recycling and purges or venting to the atmosphere that are necessary, whether operating in start-up or extinction phase or regime, without increasing the pumps, which are relatively bulky and expensive devices.
Disclosed herein is a recycling loop for a gas circuit of a fuel cell stack, the recycling loop forming a connecting line beginning at the outlet of one of the two anode or cathode circuits of said fuel cell stack and terminating in one of the two supply circuits, either in the fuel gas supply channel, or in the oxidant gas supply channel respectively, said recycling loop providing the recycling of the gas contained in the anode or cathode circuits of the fuel cell stack, said recycling loop comprising a recirculating pump that provides the recycling of the gas contained in the anode or cathode circuits of the fuel cell stack, characterized in that the recycling loop comprises a multi-way valve dividing said recycling loop into a first section and a second section, said multi-way valve having a first stable usage position, referred to as recycling position, providing the continuity between the first and second sections of said recycling loop and having a second stable usage position simultaneously providing the interruption of said continuity between the first and second sections of said recycling loop and a bringing of said recycling loop into contact with the atmosphere carried out by manoeuvring said multi-way valve.
In one preferred embodiment of the invention, the multi-way valve is a three-way valve. In the remainder of the description, examples using such a valve will be described. However, the present invention does not exclude the use of other types of valves, for example an arrangement of two two-way valves instead of one three-way valve, or any other arrangement of one or more multi-way valves.
In one particular embodiment of the invention, the pump installed in the recycling loop is capable of providing the recycling of the gas contained in the anode or cathode circuits of the fuel cell stack when the valve is in a first position, and is capable of providing the extraction or injection of gas when the valve is in a second position.
Embodiments disclosed herein make it possible to use a single pump for carrying out the recycling functions in normal operation of the fuel cell stack and the function of extracting fuel gas during particular operating phases such as a shutdown cycle of the fuel cell stack. This arrangement applies, on the anode circuit side, equally to cells supplied with atmospheric air as oxidant gas and to cells supplied with oxygen for the cathode side. The embodiments relate both to cells supplied with pure oxygen for the cathode side, but also to cells supplied with atmospheric air for this cathode side.
Embodiments disclosed herein make it possible to use a single pump for carrying out the mixing function for homogenization of the gas in the cathode circuit, and also the air injection function during particular operating phases such as a shutdown cycle of the fuel cell stack. This arrangement applies, on the cathode circuit side, equally to cells supplied with atmospheric air as oxidant gas and to cells supplied with pure oxygen. Moreover, in the case of cells supplied with pure oxygen, for the cathode circuit, the same pump provides, in addition, the recycling function in normal operation of the cell.
Also disclosed herein is a particular procedure for shutting down a fuel cell stack comprising the features described above, the shutdown procedure comprising the following actions:
(i) cutting off the supply of fuel gas and oxidant gas,
(ii) positioning the three-way valve of each of the two anode or cathode circuits in sequence in the following successive positions:
The remainder of the description serves to make all the aspects of the invention clearly understood by means of the appended drawings in which:
The installation illustrated in
The recycling loop 11R forms a connecting line beginning at the outlet of the anode circuit of the fuel cell stack 1a and terminating in the fuel gas supply channel 11A at the ejector 113. The ejector 113 provides the recycling of the fuel gas not consumed by the fuel cell stack and the mixing with fresh fuel gas originating from the pure hydrogen (H2) tank 11T. The recycling loop comprises a pump 115 providing a forced and controlled recycling of the gas not consumed by the fuel cell stack. The recycling loop comprises a three-way valve 119 dividing said recycling loop 11R into a first section 11R1 and a second section 11R2.
By positioning the three-way valve 119 at its first position (recycling position), the pump 115 is used for the function of recirculating the fraction of a fuel gas not consumed when crossing the anode circuit of the fuel cell stack.
During the shutdown of the fuel cell stack, one may be driven to have to extract the hydrogen forcibly from the anode circuit. In this case, by positioning the three-way valve 119 in its second position, the interruption of the communication of the recycling loop to the ejector 113 is provided. The first section 11R1 is isolated from the second section 11R2 of the recycling loop 11R. The first section 11R1 is then brought into contact with the atmosphere, via a first purge line 11D which terminates in an orifice 112 for venting to the atmosphere. In this case, the pump 115 is used for the function of extracting fuel gas during a shutdown phase of the fuel cell stack.
It should also be noted that the recycling loop 11R comprises a water separator 114, installed in the first section 11R1 of the recycling loop 11R. A second purge line 11C is installed beneath the water separator 114. A cut-off valve 118 is installed in this second purge line 11C. The latter terminates at the same orifice 112 for venting to the atmosphere. By controlling the cut-off valve 118, it is possible to provide the two-fold function of draining the water separator 114 and of purging the anode circuit when this is necessary.
An additional fuel gas accumulation chamber 116 is also visible, this being placed in the piping of the fuel gas supply circuit 11, between the cut-off valve 110 and a pressure regulating valve 117.
It should be noted that the additional fuel gas accumulation chamber 116 could be placed at any point in the fuel gas supply circuit, that is to say at any point between the cut-off valve 110 and the fuel cell stack 1, even in the recycling circuit 11R or in the circuit between the water separator 114 and the ejector 113. However, it is advantageous to place it at a point in the circuit where the pressure is higher, so as to reduce the volume thereof or, at an identical volume, so as to store a greater quantity of hydrogen. Moreover, the position upstream of the pressure regulating valve makes controlled discharge from said accumulation chamber possible.
It will now be described how it is possible to implement the invention at the cathode circuit of a fuel cell stack.
The installation illustrated in
The recycling loop 12Ra forms a connecting line beginning at the outlet of the cathode circuit of the fuel cell stack 1a and terminating in the oxygen supply channel 12A at the ejector 123. The ejector 123 provides the recycling of the oxygen not consumed and the mixing with fresh oxygen originating from the tank. The recycling loop 12Ra comprises a pump 125. An air supply channel 12D, beginning at an orifice 126 for venting to the atmosphere, is connected to the three-way valve 129.
By positioning the three-way valve 129 at its first position, it has been indicated that the continuity between the first section 12R1a and the second section 12R2a of said recycling loop 12Ra is provided. In this case, the pump 125 is used for the function of recirculating the gas contained in the cathode circuit of the fuel cell stack.
In certain operating phases of the cell, for example during a shutdown, it is possible to be driven to have to forcibly inject atmospheric air into the cathode circuit. In this case, by positioning the three-way valve 129 at its second position, the interruption of the communication from the recycling loop to the ejector 123 is provided. The first section 12R1a is isolated from the second section 12R2a of the recycling loop 12Ra. The second section 12R2a is then brought into contact with the atmosphere, via the pump 125 and the air supply line 12D. In this case, the pump 125 is used for the function of injecting air.
It should be emphasized that the invention, at the cathode circuit, may be applied both to fuel cells supplied with pure oxygen and to fuel cells supplied with atmospheric air as the oxidant gas. The implementation variants for fuel cells operating using atmospheric air as the oxidant gas will be examined below, based on
For cells using atmospheric air, in the cathode circuit 12b, it should be noted that there is no recycling to the cathode during the normal operation of the cell. Specifically, since the unconsumed gas is so poor in oxygen (depleted air), it is not advisable to recycle it. A recycling operation is used at the cathode only during the extinction of the fuel cell stack, not for mixing the unconsumed gas with fresh gas but solely for homogenizing, via mixing, the gas contained at the cathode so as to achieve a complete consumption of the oxygen without the risk of a locally higher oxygen concentration.
In normal operation of the fuel cell stack, the recycling circuit is not used, the pump 125 is shutdown, and no gas circulates in the recycling circuit 12Rb which becomes virtually non-existent. All of the gas not consumed by the cathode circuit is vented to the atmosphere through the pressure regulating valve 122b. If the pump 125 does not naturally provide the non-return function when it is stopped, it is necessary to provide a non-return valve in the recycling circuit 12Rb so as to guarantee the passage of all of the air supplied by the compressor to the cathode circuit of the fuel cell stack 1b.
The cut-off valve 128 makes it possible to isolate the cathode circuit from the atmospheric air when the cell is shutdown. This cut-off valve 128 may either be placed upstream or downstream of the compressor.
Represented in
By positioning the three-way valve 129 at its first position, just like in the first variant described above, the pump 125 is used for the function of recirculating the cathode gas of the fuel cell stack. When it is desired to forcibly inject atmospheric air into the cathode circuit, when carrying out an extinction procedure thereof, by positioning the three-way valve 129 at its second position, the interruption of the communication from the recycling loop to the connection 123b and the bringing of the second section 12R2c into contact with the atmosphere, via the pump 125 and the air supply line 12D, are simultaneously provided. In this case, the pump 125 is used for the function of injecting air.
The other elements that appear in
This variant is particularly useful if, as is generally done so, the compressor 125b is supplied with electrical energy directly by the fuel cell stack itself. Indeed, during the start-up and shutdown phases, the voltage over the fuel cell stack is not sufficient to supply the compressor 125b. Furthermore, the size of the pump 125 is much less than that of the compressor 125b. It is then advantageous to have another means of injecting air in order to initiate the start-up of the cell or in order to inject the air needed (in a small amount) for the generation of nitrogen during the extinction of the cell. The pump 125 is in general supplied by a low-voltage source that is always available even when the fuel cell stack is shutdown. For all of these reasons (available electrical voltage, amount of air to be injected) it is preferable to use the pump 125 for introducing air during the shutdown phase.
The procedure described below makes it possible to extinguish the fuel cell stack so as to guarantee storage with a hydrogen/nitrogen mixture therein, without requiring a nitrogen tank.
The shutdown procedure is essentially composed of the following phases:
1st phase: residual oxygen consumption phase, which occurs upon cutting off the fuel gas supply and oxidant gas supply, and by drawing a current IS at the terminals of the fuel cell stack. This current draw IS is maintained as long as an appropriate indicator indicates that the oxidant gas in the oxidant gas supply system has not been sufficiently consumed. An appropriate indicator is for example the voltage across the terminals of the fuel cell stack;
2nd phase: neutralization phase that occurs when filling the cathode circuit with nitrogen. In the embodiment described here, the nitrogen is that of the atmospheric air. Forced injection of atmospheric air then takes place, thereby again introducing a little oxygen, the consumption of which must be controlled by the current draw; and
3rd phase: forced extraction phase during which, after the electrochemical processes have been completely shut down, any excess fuel gas is forcibly removed (here, forced extraction of the excess hydrogen). It should be emphasized that, by virtue of the invention, this extraction takes place only after the fuel cell stack has been brought into a state in which the precautions for avoiding insufficient supply of hydrogen, the serious consequences of which are known, have been taken.
curve 1, the y-axis of which is labelled “Stack current [A]”, showing the current drawn from the fuel cell stack, expressed in amps;
curve 2, the y-axis of which is labelled “Average cell voltage [V]” showing the average electrical voltage across the terminals of the cells of the fuel cell stack, expressed in volts;
curve 3, the y-axis of which is labelled “Pressure out [bar]”, showing the pressure within the anode compartment (hydrogen: solid line) and in the cathode compartment (oxygen: dotted line), expressed in bara (as is usual in the field of fuel cell stacks, “mbara” means “millibar absolute”, the final letter “a” denoting “absolute”); and
curve 4, the y-axis of which is labelled “Anode H2 concentration [%]”, showing the hydrogen concentration in the anode compartment, expressed in vol %.
During the first phase of the extinction (0 to 11 s, marked “Oxygen depletion” in
Subsequently, the rest is consumed by drawing a current IS during a neutralization phase which will be explained below. The purge valve 122 remains closed during the rest of the extinction procedure and also during rest, so as to prevent air from penetrating the cathode.
As the first curve from
During the neutralization phase, the recycling and the air injection cannot be simultaneous. Depending on the position of the three-way valve, there is either recycling (first position), or injection (second position). This alternation in the controlling of the extinction clearly appears in the second part of the right-hand branch of
Furthermore, during the “nitrogen generation” phase, the pump 125 alternately provides the recycling function and the air injection function. These alternations of function result in pressure waves measured at the cathodes and in voltage waves measured in the cells. It should be noted that the cathode pressure waves and cell voltage waves are in phase opposition (see respectively the third curve and second curve from
The repeated air injections result in a voltage rise that is less and less high, to the extent that the presence of nitrogen in the cathode circuit becomes increasingly dominant. In the example illustrated here, with the aid of the curves from
The current draw IS is firstly established at a first constant level (around 60 amps) then it is reduced in proportion to the lowest of the voltages of the cells of the fuel cell stack. Conversely, it is seen in
The third curve of
From the start of the extinction procedure and up to the time 41 s, the pump 115 on the anode side is kept in operation and the three-way valve 119 in the recycling position so as to mix the anode gas and prevents any local shortage of hydrogen. Throughout the entire duration of the extinction, the shortage of hydrogen is avoided, as indicated by the hydrogen concentration represented in the fourth curve of
At the time 41 s, the hydrogen extraction phase is ordered by placing the three-way valve 119 in the extraction position (second position, see second to last block from
In this example, after six (6) alternations of air injections/recirculation, the cathode is essentially filled with nitrogen, and the voltage of the cells is virtually zero. This is only one example of a method for controlling the alternation of air injections/recirculation; other control methods resulting in an alternation of air injections/recirculation are possible.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1150749 | Feb 2011 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2012/051197 | 1/26/2012 | WO | 00 | 9/9/2013 |
