Fuel cell system and method for controlling this system
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- SYMBIO FRANCE
- Filing Date
- 2024-08-22
- Publication Date
- 2026-07-01
Smart Images
Figure EP2024073543_27022025_PF_FP_ABST
Abstract
Description
[0001] TITLE: Fuel cell system and method for controlling this system
[0002] The present invention relates to a fuel cell system. It also relates to a method for controlling this fuel cell system.
[0003] A fuel cell is a device for generating electricity by electrochemical reaction between a fuel, in particular dihydrogen, otherwise known more simply as hydrogen, and an oxidant, in particular dioxygen, otherwise known more simply as oxygen, typically that of air. We are primarily interested here in fuel cells of the proton exchange membrane type, with solid electrolyte, commonly designated by the English acronym PEMFC, which usually comprise a stack, otherwise known as a "stack" in English, of unit cells each constituting an electrochemical generator.
[0004] Schematically, each unit cell comprises two separators, also called polar plates, between which is interposed a solid electrolyte in the form of a proton exchange membrane. The membrane is made for example of a sulfonated perfluorinated polymer material. Within each cell, each separator delimits with the corresponding membrane a reactive compartment. One of the two reactive compartments is a cathode compartment, in which oxygen circulates, while the other reactive compartment is an anode compartment, in which hydrogen circulates. Within the stack, the cells are stacked so as to alternate the cathode and anode compartments. The invention is particularly relevant to fuel cells in which, for two neighboring cells, a separator of one of the two cells is back-to-back with a separator of the other cell.These two separators together form a bipolar separator, also called a bipolar plate. A cooling compartment, in which a cooling fluid such as glycolated water circulates, is generally arranged between the two separators of the bipolar separator.
[0005] Hydrogen, air and coolant are delivered to the fuel cell by respective dedicated circuits. In particular, the air circuit comprises a supply line and an exhaust line, respectively connected to a cathode inlet and a cathode outlet of the fuel cell. A compressor is arranged in the supply line to drive pressurized air into the air circuit and into the cathode compartments of the fuel cell. In addition, a humidifier is often arranged partly in the supply line and partly in the exhaust line, to humidify the air flowing in the supply line while capturing the moisture from the air flowing in the exhaust line.
[0006] When the fuel cell is to be shut down, one or both of the reactants must be removed from the fuel cell, so as not to leave the cell energized. Purging the fuel cell of its hydrogen carries the risk of inducing reactions that could degrade the metal parts of the fuel cell. Purging the fuel cell of its air is therefore generally preferred, but faces the difficulty that, when the air supply is cut off, only the oxygen in the air present in the cathode compartments of the fuel cell is consumed, inerting the membrane of the fuel cell cells with a "nitrogen skin".After a certain time, for example a few hours, oxygen present in the air that has remained captive in the air circuit gradually migrates into the cathode compartments of the fuel cell: this oxygen can reactivate the cell, by raising its voltage above zero volts, which is potentially dangerous. Furthermore, this oxygen risks infiltrating the anode compartments, which can cause an overvoltage when the fuel cell is restarted.
[0007] To circumvent this difficulty, WO 2007 / 044971 A proposed depleting the above-mentioned captive air of oxygen by forcing this air to circulate in the fuel cell under the driving effect of the compressor of the air circuit supply line. However, this approach is cumbersome and expensive to implement because it requires the compressor to be able to deliver air over a wide range of flow rates, so that this compressor can operate the fuel cell both in steady state and for the limited needs of circulating the above-mentioned captive air when the fuel cell is shut down. In addition, this approach is restrictive in the sense that it complicates, or even prevents, the use of the compressor for purposes other than circulating the captive air when the fuel cell is shut down.
[0008] The aim of the present invention is to propose a fuel cell system whose shutdown is efficient, simple, economical and flexible.
[0009] To this end, the invention relates to a fuel cell system, comprising:
[0010] - a fuel cell, which is provided with an anode inlet, an anode outlet, a cathode inlet and a cathode outlet, and
[0011] - an air circuit, which is adapted to circulate air outside the fuel cell, which air circuit comprises:
[0012] - an air supply line, which: o connects an air intake to the cathode inlet, o is provided with a compressor, and o is also provided with a first flow control valve, arranged downstream of the compressor,
[0013] - an evacuation line, which: o connects the cathode outlet to an air evacuation, and o is provided with a second flow control valve,
[0014] - a humidifier which: o is partially arranged in the air supply line so as to humidify the air flowing in the air supply line, the humidifier being arranged downstream of the flow control valve along the air supply line, and o is partially arranged in the discharge line so as to capture moisture from the air flowing in the discharge line, the humidifier being arranged upstream of the second flow control valve along the discharge line, and
[0015] - a recirculation branch, which: o connects the exhaust line to the air supply line, o is connected to the exhaust line both downstream of the humidifier and upstream of the second flow control valve, o is connected to the air supply line both downstream of the first flow control valve and upstream of the humidifier, and o is provided with a pump adapted to drive air in the recirculation branch from the exhaust line to the air supply line, such that, when the first and second flow control valves are closed and the pump is activated, the pump circulates air in a closed loop passing through the fuel cell and the humidifier.
[0016] One of the ideas underlying the invention is to allow, when the fuel cell system is shut down, to circulate, without using the compressor of the air supply line, the captive air in a closed loop including the fuel cell in order to gradually deplete this captive air in oxygen, by means of the electrochemical reaction of the latter within the fuel cell which therefore continues to produce electricity in a corresponding manner, until it is inerted once all the oxygen in the captive air has been consumed. To do this, the invention provides that the closed loop includes a dedicated recirculation branch, which belongs to the air circuit and which is provided with a pump dedicated to driving the air in this closed loop.Thus, this pump, which is separate from the compressor, is advantageously sized based solely on the need to drive the captive air in the closed loop and can therefore be much more compact and less expensive than the compressor of the air supply line. In addition, since the compressor of the air supply line is not included in the closed loop, it can be activated independently of the aforementioned pump and can therefore in particular be activated if necessary, if necessary punctually, during the shutdown of the fuel cell system according to the invention, this activation of the compressor being able to have various purposes linked in particular to the optimal operation and durability of the fuel cell.Furthermore, the invention provides that the humidifier is included in the closed loop: in this way, the captive air which circulates in the closed loop when the fuel cell system according to the invention is stopped includes the air present in the humidifier, so that the oxygen depletion of the captive air in the closed loop is also effective within the humidifier, the proper functioning of which is thus preserved, in particular when the fuel cell system is restarted.
[0017] The invention also relates to a fuel cell system, comprising:
[0018] - a fuel cell, which is provided with an anode inlet, an anode outlet, a cathode inlet and a cathode outlet, and
[0019] - an air circuit, which is adapted to circulate air outside the fuel cell, which air circuit comprises:
[0020] - an air supply line, which: o connects an air intake to the cathode inlet, o is provided with a compressor, and o is also provided with a first flow control valve, arranged downstream of the compressor,
[0021] - an evacuation line, which: o connects the cathode outlet to an air evacuation, and o is provided with a second flow control valve,
[0022] - a humidifier which: o is partially arranged in the air supply line so as to humidify the air flowing in the air supply line, the humidifier being arranged downstream of the flow control valve along the air supply line, and o is partially arranged in the discharge line so as to capture moisture from the air flowing in the discharge line, the humidifier being arranged upstream of the second flow control valve along the discharge line, and
[0023] - a recirculation branch, which: o connects the exhaust line to the air supply line, o is connected to the exhaust line both downstream of the humidifier and upstream of the second flow control valve, o is connected to the air supply line both downstream of the first flow control valve and upstream of the humidifier, and o is provided with a pump adapted to drive air in the recirculation branch from the exhaust line to the air supply line, such that, when the first and second flow control valves are closed and the pump is activated, the pump circulates air in a closed loop passing through the fuel cell and the humidifier, wherein the recirculation branch is:
[0024] - connected to the air supply line at the outlet of the first flow control valve, and
[0025] - connected to the discharge line at the inlet of the second flow control valve.
[0026] According to additional advantageous characteristics of the fuel cell system according to the invention, taken in isolation or in all technically possible combinations:
[0027] - the pump is sized so that its maximum flow rate is at most 5% of that of the compressor;
[0028] - the recirculation branch is also provided with a non-return valve which prevents the flow of air in the recirculation branch from the air supply line to the exhaust line;
[0029] - the air circuit also comprises a first bypass line, which (i) is connected in parallel to the discharge line, being connected to the discharge line so as to extend from upstream of the humidifier to downstream of the second flow control valve, and (ii) is provided with a third flow control valve, the pump being adapted to circulate air in the closed loop when the first, second and third flow control valves are closed;
[0030] - the first bypass line is connected to the discharge line at the inlet of the third flow control valve;
[0031] - the air circuit further comprises a bypass branch, which (i) connects the air supply line to the discharge line, (ii) is connected to the air supply line upstream of the first flow control valve, (iii) is connected to the discharge line downstream of the second flow control valve, and (iv) is provided with a fourth flow control valve which is adapted to be opened when the first and second flow control valves are closed so that air leaving the compressor flows, via the bypass branch, from the air supply line to the discharge line, independently of the circulation of air in the closed loop;- the exhaust line is provided with an exhaust device which is arranged downstream of the connection of the bypass branch to the exhaust line, and the fuel cell system further comprises a hydrogen circuit, which is adapted to flow hydrogen outside the fuel cell and which comprises:;
[0032] - a hydrogen supply line which: o connects a hydrogen tank to the anode inlet, and o is provided with a mixer adapted to mix hydrogen from the hydrogen tank and hydrogen recirculated from the anode outlet and to send the corresponding hydrogen mixture to the anode inlet, and
[0033] - a recirculation line, which: o connects the anode outlet to the mixer, and o is provided with a separator adapted to separate a flow from the anode outlet into recirculated hydrogen, sent to the mixer via the recirculation line, and into effluents, sent to the exhaust device via a purge line;
[0034] - the air circuit further comprises a second bypass line, which (i) is connected in parallel to the air supply line, being connected to the air supply line from the compressor outlet to the cathode inlet, and (ii) is provided with a fifth flow control valve;
[0035] - the second bypass line is connected to the air supply line at the outlet of the fifth flow control valve.
[0036] The invention also relates to a method for controlling the fuel cell system defined above, in which:
[0037] - during a steady state phase, the pump is deactivated and the compressor is activated so as to supply the cathode inlet with air leaving the compressor, while supplying hydrogen to the anode inlet, and
[0038] - during a shutdown phase, the first and second flow control valves are closed, as well as the third flow control valve when the fuel cell system is provided with one, and the pump is activated to circulate air in the closed loop, while continuing to supply hydrogen to the anode inlet.
[0039] The method according to the invention makes it possible to inert the fuel cell in an efficient, simple, economical and flexible manner when the fuel cell system is shut down.
[0040] According to additional advantageous characteristics of the method according to the invention, taken in isolation or in all technically possible combinations:
[0041] - during the shutdown phase and / or during a restart phase in which the first and second flow control valves are opened while the pump is deactivated, the compressor is activated and the fourth flow control valve is opened to cause air to flow from the compressor to the exhaust device via the bypass branch;
[0042] - during a start-up phase in ambient air at negative temperature:
[0043] - first, the compressor is activated, the fifth flow control valve is opened and the first flow control valve is closed, so as to supply the cathode inlet only with air flowing in the second bypass line, then
[0044] - in a second step, the first flow control valve is opened while simultaneously keeping the fifth flow control valve open and the compressor activated, so as to supply the cathode inlet with both air flowing in the second bypass line and air flowing in the air supply line via the humidifier;
[0045] - we move from the first time to the second time when the temperature of a cooling fluid at a cooling inlet of the fuel cell exceeds a predetermined value.
[0046] The invention will be better understood on reading the following description, given solely by way of example and with reference to the drawings in which [Fig. 1] Figure 1 is a diagram of a fuel cell system according to the invention.
[0047] Figure 1 shows a fuel cell system 1 which is for example intended to be integrated into an electric motor vehicle, so that the fuel cell system 1 produces electrical energy to operate the aforementioned electric motor.
[0048] The fuel cell system 1 comprises a fuel cell 10 in which fluids circulate for the purposes of the operation of the fuel cell 10. Thus, during the operation of the fuel cell 10, the latter is supplied, at the same time, with a combustible gas, typically pure dihydrogen, more commonly called "hydrogen" for the sake of simplification, with an oxidizing gas, typically dioxygen from the air, more commonly called "oxygen" for the sake of simplification, and with a cooling fluid, for example glycolated water.
[0049] In practice, the fuel cell 10 is provided for this purpose:
[0050] - an anode inlet 11 through which the fuel cell 10 is intended to be supplied with hydrogen intended to react inside the fuel cell,
[0051] - an anode outlet 12 through which the fuel cell 10 is designed to evacuate hydrogen that has not been consumed inside the fuel cell, - a cathode inlet 13 through which the fuel cell 10 is designed to be supplied with oxygen, typically oxygen from the air, intended to react inside the fuel cell,
[0052] - a cathode outlet 14 through which the fuel cell 10 is designed to evacuate oxygen which has not been consumed inside the fuel cell, typically mixed with the other components of the air supplying the fuel cell,
[0053] - a cooling inlet 15 through which the fuel cell 10 is provided to admit cooling fluid, and
[0054] - a cooling outlet 16 through which the fuel cell 10 is designed to discharge cooling fluid.
[0055] As shown schematically in Figure 1, the fuel cell 10 generally comprises a stack 17 of electrochemical cells which each have an anode compartment and a cathode compartment, separated from each other by a proton exchange membrane. Depending on the direction in which the electrochemical cells of the stack 17 are stacked, a cooling compartment is interposed between the electrochemical cells of each pair of electrochemical cells adjacent to each other. In practice, the fuel cell 10 comprises an integer N of electrochemical cells, N preferably being between one and several hundred.
[0056] When the fuel cell 10 is operating in steady state, hydrogen feeds, via the anode inlet 11, the anode compartments while, at the same time, oxygen, typically oxygen from the air, feeds, via the cathode inlet 13, the cathode compartments. The hydrogen that has not been consumed in the anode compartments is evacuated, via the anode outlet 12, from the anode compartments, typically being mixed with nitrogen, while, at the same time, the oxygen that has not been consumed in the cathode compartments is evacuated, via the cathode outlet 14, from the cathode compartments, typically being mixed with the other components of the air that fed these cathode compartments.It is understood that the respective anode compartments of the stack 17 jointly form an anode of the fuel cell 10 while the respective cathode compartments of this stack jointly form a cathode of the fuel cell.
[0057] Also when the fuel cell 10 is operating in steady state, cooling fluid is supplied, via the cooling inlet 15, to the cooling compartments, from which the cooling fluid is discharged, via the cooling outlet 16, the cooling fluid having, at the cooling outlet 16, a temperature higher than that which the cooling fluid has at the cooling inlet 15.
[0058] Regardless of the embodiment of the fuel cell 10, the fuel cell system 1 comprises a hydrogen circuit 20, an air circuit 30 and a cooling circuit 40, which are adapted to flow outside the fuel cell 10, respectively, hydrogen, air and cooling fluid.
[0059] As shown schematically in Figure 1, the hydrogen circuit 20 comprises a hydrogen supply line 21 which connects a hydrogen tank 2 to the anode inlet 11 so as to be able to supply the fuel cell 10 with hydrogen from the hydrogen tank 2. When the fuel cell is operating in steady state, hydrogen from the hydrogen tank 2 flows in the supply line 21 from the hydrogen tank 2 to the anode inlet 11. The supply line 21 thus has an upstream end, which opens into the hydrogen tank 2, and a downstream end, which opens into the anode inlet 11.
[0060] In practice, the hydrogen tank 2 is a pressurized tank which is, for example, on board the vehicle mentioned above.
[0061] In the embodiment considered here, the supply line 21 is provided with a mixer 21.1 which makes it possible to mix two distinct hydrogen flows, namely a hydrogen flow, which comes from the hydrogen tank 2, and a hydrogen flow, which is recirculated from the anode outlet 12. In addition to being adapted to mix the two aforementioned hydrogen flows, the mixer 21.1 is adapted to send the mixture of these two hydrogen flows to the anode inlet 11. When the fuel cell 10 operates in steady state, the hydrogen flow from the hydrogen tank 2 flows in the supply line 21 from the hydrogen tank 2 to the mixer 21.1, while the aforementioned mixture flows in the supply line 21 from the mixer 21.1 to the anode inlet 11. In addition, the hydrogen circuit 20 comprises a line recirculation 22 which connects the anode outlet 11 to the mixer 21.1 so as to be able to evacuate from the fuel cell 10 hydrogen that has not been consumed inside the latter and to cause this hydrogen to flow from the anode outlet 11 to the mixer 21.1. The recirculation line 22 thus has an upstream end, which opens into the anode outlet 12, and a downstream end, which opens into the mixer 21.1. When the fuel cell 10 is operating in steady state, hydrogen from the anode outlet 12 flows into the recirculation line 22 from the anode outlet 12 to the mixer 21.1, forming, in a downstream end portion of the recirculation line 22, the flow of hydrogen recirculated from the anode outlet 12, mentioned above.The recirculation line 22 makes it possible to avoid discharging, outside the fuel cell system 1, hydrogen which has not been consumed by the fuel cell 10, by recirculating, towards the anode inlet 11, hydrogen discharged by the fuel cell 10 at its anode outlet.
[0062] 12. This hydrogen recirculation is advantageous because it makes it possible to improve the performance of the fuel cell 10 without increasing the hydrogen consumption. In particular, this recirculation makes it possible to ensure a sufficient hydrogen flow rate within the anode compartments of the fuel cell 10 to avoid any accumulation of liquid water in the anode compartments and thus avoid local hydrogen shortages, which consequently makes it possible to ensure optimal efficiency and durability of the fuel cell 10.
[0063] In practice, the recirculation line 22 is provided with a separator 22.1 which makes it possible to separate a discharge stream, which leaves the fuel cell 10 at the anode outlet 12, into two separate streams, namely recirculated hydrogen, which is sent to the mixer 21.2 from the anode outlet 12, and effluents, which are evacuated from the recirculation line 22 by a purge line 50. The separator 22.1 thus makes it possible to separate a portion of the gaseous hydrogen contained in the discharge stream coming from the anode outlet.
[0064] 12, with respect to the rest of this discharge flow, that is to say with respect to the aforementioned effluents which contain liquid water and nitrogen, mixed with hydrogen. In practice, the separator 22.1 is based on technology known per se, which will not be detailed further here.
[0065] According to a preferred embodiment, the mixer 21.1 is a Venturi ejector which makes it possible to draw the hydrogen recirculated from the anode outlet 12 into the recirculation line 22, by the Venturi effect. Thus, the recirculation of hydrogen via the recirculation line 22 is controlled by the Venturi effect produced by this Venturi ejector.
[0066] For its part, the air circuit 30 comprises an air supply line 31 which connects an air intake 3 to the cathode inlet 13 so as to be able to supply the fuel cell 10 with air coming from the air intake 3, the oxygen contained in the air thus supplying the fuel cell 10 being intended to react inside the latter. When the fuel cell is operating in steady state, air admitted via the air intake 3 flows into the supply line 31 from the air intake 3 to the cathode inlet
[0067] 13. The supply line 31 thus has an upstream end, which opens into the air intake 3, and a downstream end, which opens into the cathode inlet 13.
[0068] In practice, the air intake 3 is an air handling device which is provided with one or more orifices used for introducing ambient air into the supply line 31, this air handling device being for example on board the vehicle mentioned above.
[0069] As clearly visible in Figure 1, the supply line 31 is provided in series, from its upstream end to its downstream end: - with a compressor 31.1 which is adapted to compress the air admitted at its inlet and deliver the resulting compressed air at its outlet,
[0070] - an intercooler 31.2 which is adapted to cool the air leaving the compressor 31.1, typically by heat exchange with a fluid supplying the intercooler 31.2 according to arrangements not shown in FIG. 1, and
[0071] - a flow control valve 31.3, which makes it possible to control and adjust the flow rate of the air flowing in the supply line 31, this flow control valve 31.3 typically being a proportional valve, in particular designed to deliver a flow rate proportional to its opening.
[0072] The air circuit 30 also comprises an evacuation line 32 which connects the cathode outlet 14 to an air evacuation 4 so as to be able to evacuate from the fuel cell 10 air containing oxygen which has not been consumed inside the fuel cell. When the fuel cell 10 is operating in steady state, air leaving the fuel cell 10 flows in the evacuation line 32 from the cathode outlet 14 to the air evacuation 4. The evacuation line 32 thus has an upstream end, which opens into the cathode outlet 14, and a downstream end, which opens into the air evacuation 4.
[0073] In practice, the air evacuation 4 is an airflow device which ensures the discharge of air outside the fuel cell system 1, typically into the ambient air.
[0074] As clearly visible in Figure 1, the evacuation line 32 is provided in series, from its upstream end to its downstream end:
[0075] - a flow control valve 32.1, which makes it possible to control and adjust the flow rate of the air flow flowing in the discharge line 32, this flow control valve 32.1 typically being a proportional valve, and
[0076] - an exhaust device 32.2 which is adapted to relax, mix and calm, typically in an ad hoc chamber, the fluid(s) that this exhaust device receives at the inlet, before evacuating them in the form of an exhaust flow at its outlet.
[0077] In the embodiment considered here, the purge line 50 is, at its downstream end, connected to the inlet of the exhaust device 32.2. In addition, this purge line 50 is here provided with a flow control valve 50.1 which makes it possible to control and adjust the flow rate of the effluents flowing into the purge line 50 from the separator 22.1. In this way, when the flow control valve 50.1 is open, effluents from the separator 22.1 are sent, via the purge line 50, to the exhaust device 32.2 where they are mixed with the air flowing in the discharge line 32, before joining the air discharge 4 mixed with this air. The exhaust device 32.2, which is based on technology known per se, thus makes it possible to dilute the effluents from the separator 22.1 in the air leaving the fuel cell 10, in particular for safety reasons or reasons related to environmental standards.
[0078] The air circuit 30 also comprises a humidifier 33 which is arranged both on the supply line 31 and on the discharge line 32. This humidifier 33, which is a piece of equipment based on technology known per se, makes it possible to control the humidity of the air entering the fuel cell 10 in order to optimize the operation of the latter, it being noted that the air leaving the fuel cell is necessarily more humid than the air entering the fuel cell, due to the electrochemical reactions occurring inside the fuel cell. The humidifier 33 is thus able to capture part of the humidity of the air leaving the fuel cell and to “transfer” this humidity into the air sent to the inlet of the fuel cell.Whatever its embodiment, the humidifier 33 thus includes both a first part, which is arranged in the supply line 31 so as to be able to humidify the air flowing in the supply line 31 when this air passes through the humidifier 33, and a second part, which is arranged in the discharge line 32 so as to capture the humidity of the air flowing in the discharge line when this air passes through the humidifier 33. The aforementioned first part of the humidifier 33 is arranged on the supply line 31 downstream of the flow control valve 31.3. The aforementioned second part of the humidifier 33 is arranged on the discharge line 32 upstream of the flow control valve 32.1.
[0079] In the embodiment considered here, the air circuit 30 also comprises a bypass line 34 associated with the humidifier 33. More precisely, the bypass line 34 is connected in parallel to the discharge line 32, being connected to the latter on either side of the humidifier 33. In other words, the bypass line 34 is closed on the discharge line 32, extending from upstream of the humidifier 33 to downstream of the flow control valve 32.1. Here, the connection of the bypass line 34 to the discharge line 32, downstream of the flow control valve 32.1, is located upstream of the exhaust device 32.2. In addition, the bypass line 34 is provided with a flow control valve 34.1 which makes it possible to control and adjust the flow rate of the air flow in the bypass line 34, this flow control valve 34.1 typically being a proportional valve.It is understood that the air discharged by the fuel cell 10 at its cathode outlet 14 flows:.
[0080] - completely through the humidifier 33, without bypassing the latter via the bypass line 34, when the flow control valve 32.1 is open while the flow control valve 34.1 is closed, - completely in the bypass line 34, bypassing the humidifier 33, when the flow control valve 32.1 is closed while the flow control valve
[0081] 34.1 is open, or
[0082] - partially in the humidifier 33 and partially in the bypass line 34 when the flow control valves 32.1 and 34.1 are both open.
[0083] It is also understood that by adjusting the degree of opening of the flow control valves 32.1 and 34.1, the respective flow rates of the air flowing into the part of the humidifier 33, arranged in the discharge line 32, and of the air flowing into the bypass line 34 are each adjustable.
[0084] Also in the embodiment considered here, the air circuit 30 further comprises a bypass branch 35 which, as clearly visible in FIG. 1, connects the supply line 31 to the discharge line 32. The bypass branch 35 is connected to the supply line 31 upstream of the flow control valve 31.3 and downstream of the compressor 31.1, here downstream of the intercooler 31.2. The bypass branch 35 is connected to the discharge line 32 downstream of the flow control valve 32.1, here upstream of the exhaust device 32.2. In addition, the bypass branch
[0085] 35 is provided with a flow control valve 35.1 which makes it possible to control and adjust the flow rate of the air flowing in the bypass branch 35, this flow control valve 35.1 being typically designed to deliver a flow rate proportional to its opening. It is understood that, when the flow control valve 35.1 is closed, it isolates the supply line 31 and the discharge line 32 from each other at the bypass branch 35. Conversely, when the flow control valve 35.1 is open, and as soon as the compressor 31.1 is activated, air leaving this compressor
[0086] 31.1 flows from the supply line 31 to the discharge line 32 via the bypass branch 35, with a flow rate controlled by the flow control valve 35.1: the corresponding air flow is referenced A1 in FIG. 1, being drawn in dotted lines incorporating arrows which indicate the direction of flow of this air flow A1 from the outlet of the compressor 31.1 into the discharge line 32, here to the outlet of the exhaust device 32.2. The bypass branch 35 thus makes it possible to bring into the discharge line 32 an additional air flow to that leaving the fuel cell 10, the compressor 31.1 being controlled accordingly, typically by increasing its output flow rate when the flow control valve 35.1 is open.
[0087] In all cases, the air circuit 30 also includes a recirculation branch
[0088] 36 which connects the discharge line 32 to the supply line 31. As clearly visible in FIG. 1, the recirculation branch 36 is connected to the discharge line 32 both downstream of the humidifier 33 and upstream of the flow control valve 32.1. The recirculation branch 36 is connected to the supply line 31 both downstream of the flow control valve 31.3 and upstream of the humidifier 33. In addition, the recirculation branch 36 is provided with a pump 36.1 for driving air into the recirculation branch 36 from the discharge line 32 to the supply line 31, so that, when the pump 36.1 is activated, air flows, via the recirculation branch, from the discharge line 32 to the supply line 31. Thus, the recirculation branch 36 has an upstream end, which is connected to the discharge line 32 between the humidifier 33 and the flow control valve 32.1, and a downstream end, which is connected to the supply line 31 between the flow control valve 31.3 and the humidifier 33.
[0089] In the embodiment considered here, the recirculation branch 36 is advantageously provided with a non-return valve 36.2 which prevents the flow of air in the recirculation branch 36 from the supply line 31 to the discharge line 32. In this way, whatever the activation state of the pump 36.1, the recirculation branch 36 does not allow air to pass from the supply line 31 to the discharge line 32. In practice, the embodiment of the non-return valve 36.2 is not limiting as long as the operation of the non-return valve is based on a mechanism capable of preventing the flow of air in the recirculation branch 36 from the supply line 31 to the discharge line 32. The non-return valve 36.2 thus has the advantage of being compact, reliable and inexpensive, operating automatically, particularly compared to a controlled valve.
[0090] For its part, the pump 36.1 also has a non-limiting embodiment since, when the flow control valves 31.3 and 32.1, as well as, in the embodiment considered here, the flow control valve 34.1, are closed, the activation of the pump 36.1 circulates air in a closed loop passing through the fuel cell 10 and the humidifier 33, this closed loop being referenced A2 in FIG. 1, being drawn in long dotted lines incorporating arrows which indicate the direction of air flow in the closed loop A2. As clearly visible in FIG. 1, the closed loop A2 thus includes:
[0091] - recirculation branch 36,
[0092] - a downstream segment of the supply line 31, extending from the outlet of the flow control valve 31.3 to the cathode inlet 13 of the fuel cell 10, this downstream segment therefore including the part of the humidifier 33 arranged on the supply line 31, - the cathode compartments of the fuel cell 10, that is to say, more generally, the part of the fuel cell, in which the air flows from the cathode inlet 13 to the cathode outlet 14, and
[0093] - an upstream segment of the discharge line 32, extending from the cathode outlet 14 to the inlet of the flow control valve 32.1, this upstream segment therefore including the part of the humidifier 33 arranged on the discharge line 32.
[0094] Thus, the closed loop A2 does not pass through the compressor 31.1, being at least separated from it by the flow control valve 31.3, as clearly visible in figure 1.
[0095] It is understood that, when the flow control valves 31.3, 32.1 and 34.1 are closed, that is to say completely closed so as to prohibit any flow of air through them, the closed loop A2 is filled with captive air, which is trapped inside the closed loop A2 and which the pump 36.1 circulates throughout this closed loop A2 as soon as the pump 36.1 is activated. Thus, this captive air flows, exclusively under the effect of the pump 36.1, from the latter until it returns to this pump 36.1, by flowing successively through a downstream segment of the recirculation branch 36, connecting the pump 36.1 at the downstream end of the recirculation branch 36, by the part of the humidifier 33 arranged on the discharge line 32, by the cathode inlet 13 of the fuel cell 10, by the cathode compartments of the fuel cell 10, by the cathode outlet 14, by the part of the humidifier 33 arranged on the discharge line 32, and by an upstream segment of the recirculation branch 36, extending from the upstream end of this recirculation branch to the pump 36.1. The circulation of the air in the closed loop A2 is independent of the activation state of the compressor 31.1.
[0096] The interest of the closed loop A2 is notably linked to the implementation of a shutdown phase of the fuel cell system 1. Such a shutdown phase is distinguished notably from a steady state phase, in which the pump 36.1 is deactivated and the compressor 31.1 is activated so that the compressor 31.1 supplies air to the cathode inlet 13 of the fuel cell 10 while, at the same time, the anode inlet 11 is supplied with hydrogen, so that the fuel cell 10 generates electricity by electrochemical reaction. In the shutdown phase, the flow control valves 31.3 and 32.1 are closed, as well as the flow control valve 34.1 for the embodiment considered here, and the pump 36.1 is activated to circulate air in the closed loop A2, as detailed just above, while continuing to supply hydrogen to the anode inlet 11 of the fuel cell 10.In this way, during the shutdown phase, the captive air, which circulates in the closed loop A2 under the effect of the pump 36.1, gradually becomes depleted in oxygen due to the consumption of the latter by electrochemical reaction inside the fuel cell 10 whose hydrogen supply is maintained. The fuel cell 10 continues to generate electricity, as long as oxygen is present in the captive air of the closed loop A2. As this captive air becomes depleted in oxygen, the fuel cell 10 gradually depolarizes, until the voltage at the terminals of the fuel cell 10 reaches zero volts. It is understood that it is thus possible to finely control this depolarization of the fuel cell 10, in particular thanks to a direct current transformer connected to the terminal of the fuel cell 10.
[0097] According to a preferred arrangement aimed at limiting the volume of captive air in the closed loop A2, the recirculation branch 36 is, at the same time, connected to the supply line 31 at the outlet of the flow control valve 31.3 and connected to the discharge line 32 at the inlet of the flow control valve 32.1. Similarly, here, the bypass line 34 is advantageously connected to the discharge line 32 at the inlet of the flow control valve 34.1. In other words, the flow control valves 31.3, 32.2 and 34.1 are arranged as close as possible to the closed loop A2, limiting the dead volume of connection between these flow control valves and the closed loop A2. In this way, the volume of air to be depleted of oxygen during the shutdown phase and therefore the duration of the latter are minimized.
[0098] During the shutdown phase, it is advantageously possible to activate the compressor 31.1 and simultaneously open the flow control valve 35.1, in order to cause air to flow from the compressor 31.1 to the exhaust device 32.2 via the bypass branch 35, in other words in order to generate the air flow A1 described above. This generation of the air flow A1 is thus independent of the circulation of air in the closed loop A2. In practice, the generation of the air flow A1 can thus be punctual and, if necessary, repeated during the shutdown phase. Thus, during the shutdown phase, the effluents from the separator 22.1 can be managed in the same way as during the steady-state phase, in particular by being able to be sent to the exhaust device 32.2 in order to be mixed there with the air flow A1 from the compressor 31.1.
[0099] Taking into account the explanations given so far, it is understood that the delivery capacity of the pump 36.1 does not have to be sized with the same order of magnitude as the delivery capacity of the compressor 31.1. Indeed, the activation of the pump 36.1 aims to drive a volume of captive air which is restricted, by circulating the latter in the closed loop A2 which is limited in length. Therefore, in particular so that the fuel cell system 1 is particularly economical and compact, the pump 36.1 is advantageously sized so that its maximum flow rate is at most 5% of that of the compressor 31.1. In all cases, as the shutdown phase progresses, the pressure of the captive air in the closed loop A2 decreases due to its oxygen depletion, until this pressure reaches a value close to atmospheric pressure.At the end of the shutdown phase, the hydrogen pressure in the anode compartments of the fuel cell 10 advantageously remains at a static overpressure value which does not affect the durability of the fuel cell 10, in particular of its proton exchange membranes.
[0100] Once the fuel cell 10 is depolarized to zero volts, the shutdown phase is completed: the compressor 31.1 is deactivated if it was still activated, the hydrogen circuit 20 is closed, and the pump 36.1 is deactivated. The fuel cell system 1 is switched off.
[0101] In the case where a prolonged shutdown of the fuel cell system 1 follows the shutdown phase, the anode overpressure of hydrogen in the fuel cell 10 can slowly equilibrate with the cathode pressure, due to the migration of hydrogen into the cathode compartments through the membranes of the cells of the fuel cell. However, the presence of hydrogen in the cathode compartments is then harmless to the fuel cell 10 because this hydrogen is found, in the cathode compartments, not in a mixture with oxygen, but in a mixture only with nitrogen which is captive in the cathode compartments. During a restart phase of the fuel cell system 10, it is advantageous to keep the pump 36.1 deactivated, to activate the compressor 31.1 and to open the flow control valves 31.3, 32.1 and 35.1, in order to both remove from the fuel cell 10 the mixture of nitrogen and hydrogen, present in the cathode compartments of the fuel cell, and to dilute this mixture in the exhaust device 32.2 with the air flow A1.
[0102] According to an optional arrangement which is advantageously implemented by the fuel cell system 1, the air circuit 30 further comprises a bypass line 37 which is distinct from the bypass line 34. This bypass line 37 is connected in parallel to the supply line 31, being connected to the latter from the outlet of the compressor 31.1 to the cathode inlet 13 of the fuel cell 10. In practice, here, the bypass line 37 is connected, at its upstream end, to the supply line 31 between the compressor 31.1 and the intercooler 31.2, and is connected, at its downstream end, to the supply line 31 between the humidifier 33 and the cathode inlet 13.
[0103] In addition, the bypass line 37 is provided with a flow control valve 37.1 which makes it possible to control and adjust the flow rate of the air flow in the bypass line 37, this flow control valve 37.1 typically being a proportional valve. For reasons similar to those mentioned above, the bypass line 37 is preferably connected to the supply line 31 at the outlet of the flow control valve 37.1.
[0104] When the flow control valve 37.1 is open, air leaving the compressor 31.1 flows into the bypass line 37, from the upstream end to the downstream end of the latter, to the cathode inlet 13, thus forming an air flow which bypasses both the intercooler 31.2 and the humidifier 33, this air flow being referenced A3 in FIG. 1, being drawn in short dotted lines incorporating arrows which indicate the direction of flow of this air flow A3.
[0105] The advantage of the air flow A3 is particularly linked to the possible start-up of the fuel cell system 10 in ambient air at a negative temperature, for example in ambient air whose temperature is around -30°C. Indeed, in such cold ambient air, starting the fuel cell 10 risks irreversibly degrading it due to the formation of ice on the membranes of its cells. During a start-up phase in ambient air at a negative temperature, it is advantageously provided here that:
[0106] - firstly, the compressor 31.1 is activated, the flow control valve 37.1 is opened and the flow control valve 31.3 is closed, so as to supply the cathode inlet 13 only with air flowing in the bypass line 37,
[0107] - then, in a second step, the flow control valve 31.3 is opened while simultaneously keeping the flow control valve 37.1 open and the compressor 31.1 activated, so as to supply the cathode inlet 13 with both air flowing in the bypass line 37 and air flowing in the supply line 31, via the humidifier 33.
[0108] In this way, during the first aforementioned time, all the heat of the air leaving the compressor 31.1, resulting in particular from its heating by the compressor to be compressed, is directly brought by the air flow A3, via the bypass line 37, to the cathode inlet 13, and this by bypassing the intercooler 31.2 and the humidifier 33 where this heat would have been partially dissipated and therefore lost before reaching the cathode inlet 13. The air thus admitted into the fuel cell 10 is particularly hot and dry, which makes it possible to avoid the formation of ice inside the fuel cell during its start-up.
[0109] During the second aforementioned step, a thermal transition is carried out for the air entering the fuel cell 10, providing that this air results from the mixing of the air flow A3 and air which, while coming from the compressor 31.1, has passed through the intercooler 31.2 and the humidifier 33 before reaching the cathode inlet 13. This mixture is therefore a mixture between the hot and dry air of the air flow A3, and the less hot and more humid air flowing in the supply line 31 at the outlet of the humidifier 33. It is thus possible to smoothly change in this mixture the respective proportions of the hot and dry air and the less hot and more humid air, in order to optimize the conditions in which the fuel cell 10 starts.
[0110] The transition from the first time to the second time is preferably controlled by the temperature of the cooling fluid in the cooling circuit 40: it is thus advantageously provided that one passes from the first time to the second time when the temperature of the cooling fluid at the cooling inlet 15 exceeds a predetermined value. It is understood that, at the initiation of the start-up of the fuel cell system 1 in ambient air at negative temperature, for example in ambient air whose temperature is of the order of -30°C, the cooling fluid in the cooling circuit 40 has a temperature substantially equal to that of the ambient air; then, during the aforementioned first time, the temperature of the cooling fluid in the cooling circuit 40 rises progressively, under the effect of the heat generated by the fuel cell 10 during start-up.For example, the temperature of the cooling fluid thus gradually changes from approximately -30°C to positive values, so that when the temperature of the cooling fluid at the cooling inlet 15 becomes greater than -15°C or +5°C or, more generally, the aforementioned predetermined value, the start-up phase of the method for controlling the fuel cell system 1 is changed from the aforementioned first time to the aforementioned second time. Thus, this predetermined value corresponds to a transition threshold between this first and this second time, which is advantageous.
[0111] Finally, various arrangements and variants of the fuel cell system 1, as well as its control method, described so far are conceivable.
Claims
CLAIMS 1. Fuel cell system (1), comprising: - a fuel cell (10), which is provided with an anode inlet (11), an anode outlet (12), a cathode inlet (13) and a cathode outlet (14), and - an air circuit (30), which is adapted to circulate air outside the fuel cell (10), which air circuit (30) comprises: - an air supply line (31), which: o connects an air intake (3) to the cathode inlet (13), o is provided with a compressor (31.1), and o is also provided with a first flow control valve (31.3), arranged downstream of the compressor (31.1), - an evacuation line (32), which: o connects the cathode outlet (14) to an air evacuation (4), and o is provided with a second flow control valve (32.1), - a humidifier (33) which: o is partially arranged in the air supply line (31) so as to humidify the air flowing in the air supply line, the humidifier (33) being arranged downstream of the flow control valve (31.3) along the air supply line, and o is partially arranged in the discharge line (32) so as to capture moisture from the air flowing in the discharge line, the humidifier (33) being arranged upstream of the second flow control valve (32.1) along the discharge line, and - a recirculation branch (36), which: o connects the exhaust line (32) to the air supply line (31), o is connected to the exhaust line (32) both downstream of the humidifier (33) and upstream of the second flow control valve (32.1), o is connected to the air supply line (31) both downstream of the first flow control valve (31.3) and upstream of the humidifier (33), and o is provided with a pump (36.1) adapted to drive air in the recirculation branch (36) from the exhaust line to the air supply line, such that, when the first and second flow control valves (31.3, 32.1) are closed and the pump is activated, the pump circulates air in a closed loop (A2) passing through the fuel cell (10) and the humidifier (33), in which the recirculation branch (36) is: - connected to the air supply line (31) at the outlet of the first flow control valve (31.3), and - connected to the discharge line (32) at the inlet of the second flow control valve (32.1).
2. Fuel cell system according to claim 1, wherein the pump (36.1) is dimensioned so that its maximum flow rate is at most 5% of that of the compressor (31.1).
3. Fuel cell system according to one of claims 1 or 2, wherein the recirculation branch (36) is also provided with a non-return valve (36.2) which prohibits the flow of air in the recirculation branch (36) from the air supply line (31) to the exhaust line (32).
4. Fuel cell system according to any one of the preceding claims, wherein the air circuit (30) also comprises a first bypass line (34), which: - is connected in parallel to the discharge line (32), being connected to the discharge line so as to extend from upstream of the humidifier (33) to downstream of the second flow control valve (32.1), and - is provided with a third flow control valve (34.1), the pump (36.1) being adapted to circulate air in the closed loop (A2) when the first, second and third flow control valves (31.3, 32.1, 34.1) are closed.
5. Fuel cell system according to claim 4, wherein the first bypass line (37) is connected to the discharge line (32) at the inlet of the third flow control valve (34.1).
6. Fuel cell system according to any one of the preceding claims, wherein the air circuit (30) further comprises a bypass branch (35), which: - connects the air supply line (31) to the exhaust line (32), - is connected to the air supply line (31) upstream of the first flow control valve (31.3), - is connected to the discharge line (32) downstream of the second flow control valve (32.1), and - is provided with a fourth flow control valve (35.1) which is adapted to be opened when the first and second flow control valves are closed so that air leaving the compressor (31.1) flows, via the branch bypass (35), from the air supply line (31) to the exhaust line (32), independently of the air circulation in the closed loop (A2).
7. Fuel cell system according to claim 6, wherein the exhaust line (32) is provided with an exhaust device (32.2) which is arranged downstream of the connection of the bypass branch (35) to the exhaust line, and wherein the fuel cell system (1) further comprises a hydrogen circuit (20), which is adapted to flow hydrogen outside the fuel cell (10) and which comprises: - a hydrogen supply line (21) which: o connects a hydrogen tank (2) to the anode inlet (11), and o is provided with a mixer (21.1) adapted to mix hydrogen from the hydrogen tank (2) and hydrogen recirculated from the anode outlet (12) and to send the corresponding hydrogen mixture to the anode inlet (11), and - a recirculation line (22), which: o connects the anode outlet (12) to the mixer (21.1), and o is provided with a separator (22.1) adapted to separate a flow from the anode outlet (12) into recirculated hydrogen, sent to the mixer (21.1) via the recirculation line (22), and into effluents, sent to the exhaust device (32.2) via a purge line (50).
8. Fuel cell system according to any one of the preceding claims, wherein the air circuit (30) further comprises a second bypass line (37), which: - is connected in parallel to the air supply line (31), being connected to the air supply line from the compressor outlet (31.1) to the cathode inlet (13), and - is provided with a fifth flow control valve (37.1).
9. Fuel cell system according to claim 8, wherein the second bypass line (37) is connected to the air supply line (31) at the outlet of the fifth flow control valve (37.1).
10. Method for controlling the fuel cell system (1) according to any one of the preceding claims, wherein: - during a steady state phase, the pump (36.1) is deactivated and the compressor (31.1) is activated so as to supply the cathode inlet (13) with air leaving the compressor (31.1), while supplying hydrogen to the anode inlet (11), and - during a shutdown phase, the first and second flow control valves (31.3, 32.1) are closed, as well as the third flow control valve (34.1) when the fuel cell system is in accordance with claim 5, and the pump (36.1) is actuated to circulate air in the closed loop (A2), while continuing to supply hydrogen to the anode inlet (12).
11. Control method according to claim 10, wherein the fuel cell system (1) is according to claim 7, and wherein during the shutdown phase and / or during a restart phase in which the first and second flow control valves (31.3, 32.1) are opened while the pump (36.1) is deactivated, the compressor (31.1) is activated and the fourth flow control valve (35.1) is opened to cause air to flow from the compressor to the exhaust device (32.2) via the bypass branch (35).
12. Control method according to one of claims 10 or 11, in which the fuel cell system is in accordance with claim 8, and in which during a start-up phase in ambient air at negative temperature: - firstly, the compressor (31.1) is activated, the fifth flow control valve (37.1) is opened and the first flow control valve (31.3) is closed, so as to supply the cathode inlet (13) only with air flowing in the second bypass line (37), then - in a second step, the first flow control valve (31.3) is opened while simultaneously keeping the fifth flow control valve (37.1) open and the compressor (31.1) activated, so as to supply the cathode inlet (13) with both air flowing in the second bypass line (37) and air flowing in the air supply line (31) via the humidifier (33).
13. Control method according to claim 12, in which the first time is switched to the second time when the temperature of a cooling fluid at a cooling inlet (15) of the fuel cell (10) exceeds a predetermined value.