System comprising a fuel-cell stack, and associated control method

Abstract

A fuel cell system comprises a stack of electrochemical cells forming a fuel cell with an ion-exchange polymer membrane and a fuel gas supply circuit connecting a fuel gas reservoir to the anode of the fuel cell, the system being characterized in that it comprises: a hydrogen purge valve (305) installed on the anode outlet of the stack, a receiver (310) of the purged hydrogen, and means for redirecting the purged hydrogen to the anode inlet of the fuel cell. There is also an associated control method.

Claims

1. A fuel cell system comprising a stack of electrochemical cells forming a fuel cell with an ion-exchange polymer membrane and a fuel gas supply circuit connecting a fuel gas reservoir to an anode of the fuel cell, the system further comprising: a hydrogen purge valve installed on an anode outlet of the stack; a receiver of purged hydrogen, wherein residual hydrogen is purged upon opening the hydrogen purge valve and directed to the receiver; and means for depressurizing the receiver by redirecting the purged hydrogen to the anode inlet of the fuel cell, wherein the means comprises an ejector of a Venturi type, wherein a cross-section of a recirculation orifice of the ejector is calibrated for hydrogen.

2. The fuel cell system according to claim 1 further comprising a pressure measuring sensor in the hydrogen receiver.

3. The fuel cell system according to claim 1 further comprising purging means of the hydrogen receiver for when the pressure in the receiver is above atmospheric pressure.

4. A method of controlling the fuel cell system according to claim 1 comprising the following steps: supplying hydrogen to the anode of the fuel cell; purging the residual hydrogen at the anode outlet of the fuel cell toward the receiver; and depressurizing the receiver by selectively redirecting the purged hydrogen from the receiver to the anode inlet of the fuel cell by utilizing the ejector of the Venturi type, wherein the steps of purging the residual hydrogen and of depressurizing the receiver are alternated.

5. The method according to claim 4, wherein the step of purging the residual hydrogen corresponds to a step of opening the hydrogen purge valve installed on the anode outlet of the fuel cell and is started when pressure in the receiver is below atmospheric pressure.

6. The method according to claim 4, further comprising a step of purging water and gases from the receiver to atmosphere, wherein the gases include nitrogen.

7. The method according to claim 6, wherein the step of purging the water and the gases from the receiver is carried out when the pressure in the receiver is above atmospheric pressure.

8. The method according to claim 4, wherein the step of purging the residual hydrogen is stopped by closing the hydrogen purge valve when pressure in the receiver is above atmospheric pressure.

9. The method according to claim 4, further comprising a step of purging nitrogen from the receiver to atmosphere.

10. The method according to claim 9, wherein the step of purging the nitrogen from the receiver is carried out when the pressure in the receiver is no longer below a predetermined level.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other aims and advantages of the invention will become clear from the following description of a preferred but non-limiting embodiment, illustrated by the following figures in which:

(2) FIGS. 1 and 2, already described, show systems of the prior art,

(3) FIG. 3 shows a system according to the invention, and

(4) FIG. 4 is a graph showing the variation of pressure in the receiver of a system according to the invention.

DETAILED DESCRIPTION

(5) Thus, FIG. 3 shows a fuel cell system in which pressurized hydrogen is present at the level of the hydrogen supply tube 301. A pressure-regulating solenoid valve 302 associated with a pressure sensor 303 makes it possible to control the hydrogen pressure at the fuel cell inlet 304. A purge valve 305 is placed at the anode outlet. The hydrogen purged on opening the valve 305 is directed to a receiver 310. A Venturi ejector 307 associated with a non-return valve 308 is installed in such a way that it is able to aspirate the hydrogen contained in the receiver 310 and redirect it to the inlet of the anode chamber of the fuel cell.

(6) The receiver 310 also acts as a water separator. A receiver purge valve 311, installed at the outlet of the receiver 310, allows the elements that are not redirected to the fuel cell inlet to be purged to the exterior, namely: water separated in the receiver 310, which came from the anode channels, and nitrogen or other impure gases present in the receiver 310 and which preferably are not recirculated by the Venturi ejector 307.

(7) This system therefore advantageously allows effective removal of water from the anode channels without generating hydrogen losses, and without any risk of contaminating the fuel cell by reinjection of nitrogen or other impure gas.

(8) We shall now describe the control of a system of this kind, additionally on the basis of FIG. 4, which shows the pressure variation inside the receiver 310, measured by the sensor 312.

(9) In a first phase 1, the pressure is observed to increase. This phase corresponds to opening of the purge valve 305, which leads to purging of the residual hydrogen from the anode outlet of the cell to the receiver 310. The duration of this first phase, which corresponds to the valve opening time, must be sufficient to ensure sufficient over-stoichiometry within the fuel cell for example for between 0.5 and 10 seconds. Furthermore, prolonging the opening of the purge valve 305 would not lead to loss of hydrogen since it would be recirculated by the ejector. However, keeping it open for too long will be avoided, to allow sufficient periodicity of the purges, for example between 2 and 10 times per minute.

(10) Once valve 305 is closed, we then enter phase 2, during which the Venturi ejector evacuates the receiver 310 by aspiration of the hydrogen, redirecting it to the inlet of the cell. The duration of the second phase must be sufficient to reach a sufficient negative pressure within the receiver 310 but must not be too long, so as to allow a sufficient periodicity of the purges. For example, the duration of phase 2 may be set at between 2 and 10 seconds. The receiver 310 also acts as a water separator and it is necessary to evacuate this water to the surroundings 306 by means of a receiver purge valve 311. This valve may be operated periodically. However, as mentioned above, valve 311 is only opened when the pressure in the receiver indicated by the pressure sensor 312 is above atmospheric pressure.

(11) In another embodiment example, not illustrated in the figures, valve 311 may be replaced with a calibrated orifice for generating a controlled exhaust, combined with a non-return valve to prevent reintroduction of ambient air into the receiver 310.

(12) Furthermore, as noted above, the Venturi ejector is naturally selective, i.e. the cross-section of the recirculation orifice is calibrated for hydrogen. The presence of a gas with higher density mixed with the hydrogen, such as nitrogen for example, will cause saturation of the Venturi effect and the receiver 310 will no longer be able to reach the same level of vacuum.

(13) Thus, if it is detected, during phase 2, that the pressure in the receiver 310 no longer goes below a certain predetermined level, this signifies that an excessive amount of nitrogen has accumulated in the receiver. In one example, opening of the valve 311 is then operated, to remove the nitrogen while minimizing losses of hydrogen. This makes it possible to minimize the loss of hydrogen to values below 1%.