FUEL CELL SYSTEM

Abstract

A fuel-cell stack system includes a stack of electrochemical cells, a fuel gas supply circuit and an oxidant gas supply circuit, a cooling circuit, a micropump, a temperature measurement device, and a controller. The cells are separated by bipolar plates, with each bipolar plate including an anode, a cathode, and an ion-exchange membrane. The cooling circuit, which is structured to enable a coolant fluid to circulate therein, includes a secondary circuit and a primary circuit that is smaller in size than the secondary circuit, with the primary and secondary circuits being isolated from each other by a thermostatic valve. The micropump is installed at an outlet of the stack and enables a volume of water inside the stack to be mixed. The temperature measurement device determines an internal temperature of a core of the stack. The primary circuit is activated when the internal temperature rises above a predetermined threshold.

Claims

1-6: (canceled)

7: A fuel-cell stack system, comprising: a stack of electrochemical cells separated by bipolar plates, each of the bipolar plates including an anode, a cathode, and an ion-exchange membrane; a fuel gas supply circuit and an oxidant gas supply circuit; a cooling circuit structured to enable a coolant fluid to circulate therein, the cooling circuit including a secondary circuit and a primary circuit that is smaller in size than the secondary circuit, the primary and secondary circuits being isolated from each other by a thermostatic valve; a micropump installed at an outlet of the stack, the micropump being structured to enable a volume of water inside the stack to be mixed; and a core temperature measurement device arranged to determine an internal temperature of a core of the stack, wherein the primary circuit is activated when the internal temperature rises above a predetermined threshold.

8. The system according to claim 7, further comprising a cooling-circuit temperature measurement device arranged to determine a temperature in the primary circuit.

9. The system according to claim 7, wherein, when the primary circuit is activated, a pump installed in the primary circuit is activated in at least one of: a continuous mode and a pulsed mode.

10. The system according to claim 9, wherein the pump installed in the primary circuit is a variable-speed cooling pump that is activated only in the continuous mode.

11. The system according to claim 10, further comprising a dryer, which dries out the ion-exchange membranes when shutting down the fuel-cell stack system.

12. The system according to claim 9, further comprising a moisture measurement device arranged to measure a moisture content of the ion-exchange membranes.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0037] Other objectives and advantages of the invention will appear clearly in the following description of a preferred, but non-limiting, embodiment, illustrated by the following figures in which:

[0038] FIG. 1 shows a system according to the invention;

[0039] FIG. 2 shows the voltages across the terminals of the cells of a fuel cell stack in the case that the cooling pump is activated in continuous mode in a cold start phase.

[0040] FIG. 3 shows the variation in multiple temperatures within the fuel cell stack in the case that the cooling pump is started up after a delay, and activated in pulsed mode in a cold start phase.

[0041] FIG. 4 shows the voltages across the terminals of the cells of a fuel cell stack in the case that the cooling pump is started up after a delay, and activated in pulsed mode in a cold start phase.

DESCRIPTION OF THE BEST EMBODIMENT OF THE INVENTION

[0042] FIG. 1 shows a system according to the invention, including a stack of electrochemical cells 10. The system comprises a liquid cooling circuit, divided into a primary circuit 8 and a secondary circuit 9. The volume of the primary circuit is minimized with respect to the systems of the prior art, and isolated from the secondary circuit by a thermostatic valve 2.

[0043] The primary circuit is provided with a cooling pump 1. As described above, this pump may advantageously be activated in continuous and/or pulsed mode in the context of starting up the stack at temperatures substantially below zero.

[0044] The system also comprises a deionization filter 4, an expansion tank 5 and a radiator 3.

[0045] A small pump, also referred to as a “micropump” throughout the remainder of the description, 6 is installed at the outlet of the stack 10. This small pump allows the volume of water inside the stack to be mixed, with a minimum of external circuit. Such a construction makes it possible to homogenize the temperature at the core of the stack, thereby facilitating start-up at temperatures below zero by preventing the freezing of the water arising from the electrochemical reaction that takes place in the stack and by preventing the occurrence of local hot points without substantially increasing the amount of water to be warmed. Advantageously, it is useful for the micropump to be positioned as close to the stack as possible. Specifically, a greater distance would involve an increase in the volume of water to be warmed and additional losses, which could compromise the success of the cold start. Specifically, the possibility to cold-start a stack depends on the capacity of the stack to raise its core temperature above freezing point before the water produced by the reaction starts to be discharged.

[0046] The circuit shown in FIG. 1 must also be provided with a non-return valve 10 so as to guarantee that all of the flow produced by the micropump 6 passes through the stack. FIG. 2 shows the variation in the voltages across the terminals of the cells of a fuel cell stack during a cold start at −15° C. managed according to the methods of the prior art, namely by operating the cooling pump in continuous mode.

[0047] A gradual decrease in the voltage across the terminals of the set of cells is observed, followed by a collapse, starting at 13 seconds, of the voltage across the terminals of the first cell (lowest curve on the graph), followed shortly after by the voltage across the terminals of the second cell.

[0048] This rapid drop in voltage reveals a blockage linked to the freezing of the water produced in the fuel cell stack. As a result, the operation of the stack is negatively affected.

[0049] FIGS. 3 and 4 show the variation in parameters in a fuel cell stack for which a control method according to the invention is implemented. Thus, these two graphs show the variation for a cold start during which the stack is first operated with only the micropump active, then the main cooling pump is operated in pulsed mode.

[0050] In FIG. 3, the curve C1 shows the estimated temperature of the fuel cell stack, the curve C2 shows the control setpoint of the cooling pump and the curve C3 shows the temperature at the inlet of the stack. After around 65 seconds, the temperature, shown by curve C1, reaches a value of 20° C. This value corresponds to a first predetermined threshold in one embodiment of the invention. The cooling pump, or water pump, is then controlled in pulsed mode, as shown on the curve C2.

[0051] After 135 seconds of operation, the temperature of the coolant liquid at the inlet of the stack, shown on curve C3, becomes higher than 5° C. This value corresponds to a second predetermined threshold in one embodiment of the invention. The cooling pump is then operated in continuous mode. From this moment on, the coolant liquid circulates continuously, resulting in quite a rapid decrease, then disappearance, of the difference in temperature of the coolant liquid between the inlet and the outlet of the fuel cell stack.

[0052] At the same time, FIG. 4 shows the corresponding variation in the individual voltages of the cells of the fuel cell stack when a method according to the invention is implemented. It is observed in this figure that, unlike in FIG. 1, the first cells of the fuel cell stack retain an acceptable voltage level, or have a voltage level that quickly bounces back, when the cooling pump is activated. The cooling pump is activated in pulsed mode. It is observed that each injection of cold water results in a drop in the set of voltages, shown in FIG. 3 by ripples. The frequency of the pulses of the cooling pump, and hence of the injection of coolant liquid, is determined so as to allow time for the voltage across the terminals of the cells to return to an acceptable level before another injection. In the present example, one injection takes place every six seconds.

[0053] Thus, the use of a system according to the invention makes it possible to warm up the liquid contained in the cooling circuit while holding an acceptable voltage across the terminals of the cells of the fuel cell stack throughout the start-up phase and while guaranteeing a good level of uniformity of the temperature within the stack despite the delayed activation of the cooling pump.