US20130224611A1

US20130224611A1 - System for purging non-fuel material from fuel cell anodes

Info

Publication number: US20130224611A1
Application number: US13/852,707
Authority: US
Inventors: Tibor Fabian; Tobin J. Fisher; Daniel Braithwaite
Original assignee: Ardica Technologies Inc
Current assignee: Ardica Technologies Inc
Priority date: 2008-01-29
Filing date: 2013-03-28
Publication date: 2013-08-29
Also published as: CA2713022A1; US20090269634A1; EP2248213A1; US20090305112A1; US8192890B2; WO2009097149A1; CN101971402A; WO2009097146A1; JP2011511416A

Abstract

A fuel cell gas purge system is provided that includes at least one fuel cell, such as a fuel cell stack or a fuel cell array, a fuel supply, and an adjustable fuel cell current load. The system further includes at least one passive purge valve disposed to purge accumulated non-fuel matter in the fuel cell, and operates according to a pressure differential across the valve. The valve can be a passive bi-directional valve, such as a dome valve, or a passive unidirectional valve. Further included is a purge management module that has a purge request module to determine when to increase the pressure of the hydrogen fuel to initiate the purge, and a purge complete module to determine when to adjust the pressure of the hydrogen fuel to complete the purge. The non-fuel matter can include non-fuel gases or condensed water.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent application Ser. No. 12/322,337, filed 29 Jan. 2009, which claims the benefit from U.S. Provisional Application 61/062,961 filed 29 Jan. 2008, both of which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates generally fuel cells. More particularly, the invention relates to a system for purging hydrogen fuel cells and determining when the purge is necessary and when it is complete.

BACKGROUND

Fuel cell systems where oxygen is supplied from ambient air accumulate the non-reactive components of air (primarily nitrogen and some water vapor or condensation) in the fuel stream due to finite diffusion rates of gases through the fuel cell electrolyte. The inert gas accumulation eventually lowers the fuel cell output voltage due to drop of fuel concentration. As a consequence, continuous operation requires periodic purging of the fuel compartment. Additionally, fuel cell systems often employ safety valves that allow gas to escape if the internal pressure or vacuum builds to unsafe levels, preventing damage to the device and/or hazards to users. Two types of methods for addressing these issues include active and passive purge valves. In active purge systems, an electrically or mechanically controlled valve is employed at the outlet of the fuel gas flow path to allow the fuel and accumulated nitrogen to escape when necessary. In smaller micro-fuel cell systems, miniature valves are often used when minimum size and weight is desired, such as the X-Valve available from Parker Hannefin. These active valves suffer from a number of problems including high cost and high power consumption. Additionally, they are unreliable as a safety purge valve, as they require proper external control in order to function properly. Passive purge valve systems allow gas pressure or vacuum to be released at a specified pressure. Accumulated non-reactive gases can be purged by increasing the operating pressure of the system above the purge pressure of the valve, allowing gas to escape. These valves tend to be less expensive than active valves and do not require external control, making them more reliable. These passive valves include poppet valves, like those available from Smart Products and duck bill valves, like those available from Vernay. Nevertheless a purge system that is based on passive valves requires a good control of the pressure upstream of the purge valve to avoid fuel loss as well as excessive purging. In many hydrogen fuel cell systems, for example, hydrogen is generated on demand such as using binary chemical reactions. The response time of such systems is often characterized by latency and long time constants that are due to finite thermal mass and mass transfer limitations of the chemical hydrogen reactor systems. These limitations make frequent rapid pressure changes impossible and thus purging based on passive purge valves impractical.
Additionally, the current purge methods do not allow for detecting when all of the accumulated non-fuel gases have been purged from the system. To compensate for this ambiguity, systems with passive purge valves either need to purge excess fuel, creating a safety hazard and/or wasting fuel, or risk unsuccessful purges, resulting in reduced system power output and/or erratic performance.
Accordingly, there is a need to develop a simple, low-cost and effective purge system for fuel cells that minimizes system complexity while it maintains high fuel utilization.

SUMMARY OF THE INVENTION

The present invention provides a fuel cell gas purge system. The gas purge system includes at least one fuel cell, a fuel supply, and an adjustable fuel cell current load. The gas purge system further includes at least one passive purge valve disposed to purge accumulated non-fuel matter in the fuel cell, where the passive purge valve operates according to a pressure differential across the passive purge valve and a purge management module, where the purge management module includes a purge request module and a purge complete module. The purge request module determines when to increase the pressure of the fuel to initiate the purge and the purge complete module determines when to decrease the pressure of the hydrogen fuel to complete the purge.
In one aspect of the invention, the fuel cell can include a hydrogen fuel cell, a propane fuel cell, a butane fuel cell or a methane fuel cell.
In one aspect of the invention, the at least one fuel cell can be a single fuel cell, a fuel cell stack, a fuel cell array or any combination thereof.
In a further aspect of the invention, the non-fuel matter can include non-fuel gases or condensed water.
According to another aspect of the invention, when not purging, the adjustable fuel cell load is adjusted to keep the pressure upstream of the passive purge valve below a cracking pressure of the passive purge valve, while during the purge, the adjustable fuel cell load is adjusted to increase the pressure upstream of the passive purge valve above the cracking pressure.
In another aspect, the adjustable fuel cell load includes a battery charger circuit attached to a battery, where a charging current of the battery can be adjusted.
In yet another aspect, the passive purge valve can include a passive bi-directional valve or a passive unidirectional valve. Here, the bi-directional valve can include a dome valve.
According to one aspect, a cracking pressure of the passive purge valve is as low as 1 PSI.
In a further aspect of the invention, the passive purge valve is disposed at a distal end of at least two fuel cells having the fuel connected in series, where a source of the fuel is disposed at a proximal end of the array.
According to one aspect, the purge request module determines when the non-fuel matter needs to be purged by sensing when a voltage of any of the fuel cells drop below a predetermined threshold. Here, the purge request module can determine when the non-fuel matter needs to be purged by sensing when a voltage in the fuel cell that is most proximal to the passive purge valve drops below a predetermined threshold.
In another aspect of the invention, the purge complete module determines when the non-fuel material has been purged from at least one fuel cell by sensing when the purge gas is primarily the fuel gas.
In a further aspect of the invention, the purged non-fuel matter from the passive purge valve is directed across a cathode of one the fuel cells in an array of fuel cells, where the purge complete module determines when the non-fuel matter has been purged by sensing when a voltage of the one fuel cell drops below a threshold voltage.
In yet another aspect, the purged non-fuel matter from the passive purge valve is directed to a catalyst bed in the presence of ambient air, where the purge complete module determines when the non-fuel matter has been purged by sensing when a temperature of the catalyst bed exceeds a threshold level. Here the catalyst can include Platinum, Palladium, Ruthenium, Manganese oxide, Silver oxide and Cobalt oxide.
According to another aspect, the purge complete module determines when the non-fuel material has been purged by using a timer. Here, a duration of the timer is determined according to a current load in one of the fuel cells before the purge was initiated.
In a further aspect, the purged non-fuel matter from the passive purge valve is directed to the anode of an auxiliary fuel cell, where the purge complete module determines when the non-fuel matter has been purged by sensing when the output of the fuel cell exceeds a threshold level, where the output can be either voltage or current.

BRIEF DESCRIPTION OF THE FIGURES

The objectives and advantages of the present invention will be understood by reading the following detailed description in conjunction with the drawing, in which:

FIG. 1 shows a fuel cell gas purge system having detection for a completed purge based on a timer, according to the current invention.

FIG. 2 shows a fuel cell gas purge system having completed purge detection according to the present invention.

FIG. 3 shows a multiple discrete cell system that has the purge exhaust routed to one or more different cells in the system according to the present invention.

FIG. 4 shows a flow diagram of a software algorithm to monitor the system and use the voltage data from the purge cell to determine when a purge has been effectively completed, according to the current invention.

FIG. 5 shows a fuel cell gas purge system that has the purge exhaust routed to an anode of an auxiliary fuel cell, according to the current invention.

FIG. 6 shows a fuel cell gas purge system that has the purge exhaust routed to a catalyst bed in the presence of ambient air, according to the current invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the following detailed description contains many specifics for the purposes of illustration, anyone of ordinary skill in the art will readily appreciate that many variations and alterations to the following exemplary details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.
Referring to the figures, FIG. 1 shows a timer-based fuel cell purge system 100 according one embodiment of the current invention. A hydrogen fuel cell 102 is shown that uses an air source 104 for oxygen input to the cathodes (not shown) and exhausted through an air output 106 in the form of humidified air, where the air source can be driven by a fan, for example. Here an exemplary hydrogen fuel cell 102 is shown, but it is understood throughout this description that the hydrogen fuel cell 102 can be any one of the many types of gas fuel cells, for example a propane fuel cell, a butane fuel cell or a methane fuel cell. It is further understood that the fuel cell can be a single fuel cell a fuel cell stack, an array of fuel cells or any combination thereof. The present embodiment is a hydrogen fuel cell gas purge system 100 that includes at least one hydrogen fuel cell 102. The system 100 further includes a hydrogen fuel supply 108, and an adjustable hydrogen fuel cell current load 116. The gas purge system 100 further includes at least one passive purge valve 112 disposed to purge accumulated non-fuel matter 110 in the fuel compartment of the hydrogen fuel cell 102, where the passive purge valve 112 operates according to a pressure differential across the passive purge valve 112. It is understood throughout this document that the non-fuel matter 110 can include non-fuel gases or condensed water. It is further understood in this document that the passive purge valve 112 can be a passive bi-directional valve, such as a dome valve, or a passive unidirectional valve. Further included is a purge management module 118 that includes a purge request module 120 and a purge complete module 122. The purge request module 120 determines when to increase the pressure of the hydrogen fuel to initiate the purge, and the purge complete module 122 determines when to decrease the pressure of the hydrogen fuel to complete the purge. Here, the non-fuel matter no can include non-fuel gases or condensed water. According to the current invention many methods exist to determine when purge gases comprise primarily the fuel. In the current embodiment, the purge complete module 122 can determine when the non-fuel material 110 has been purged, by using a timer 124. Here, the duration of the timer 124 is determined according to a current load in one hydrogen fuel cell before the purge was initiated. The current load before purge is indicative of the hydrogen flow-rate before purge occurred. Based on the hydrogen flow-rate and known volume of fuel cell anode, the necessary duration of the purge can be determined.
FIG. 2 shows a hydrogen fuel cell gas purge system having detection for a completed purge 200. The end of purge detection fuel cell gas purge system 200 includes at least one hydrogen fuel cell 102, a hydrogen fuel supply 108, and an adjustable hydrogen fuel cell current load 116. The system 200 further includes at least one passive purge valve 112 disposed to purge accumulated non-fuel matter 110 in the hydrogen fuel cell 102. Further included is a purge management module 118 that includes a purge request module 120 and a purge complete module 122. The purge request module 120 determines when to increase the pressure of the hydrogen fuel to initiate the purge, and the purge complete module 122 determines when to decrease the pressure of the hydrogen fuel to complete the purge. According to the current embodiment, the purge request module 120 determines when the non-fuel matter 110 needs to be purged by sensing when a voltage in the fuel cell 102, for example the voltage of one fuel cell of a fuel cell stack, drops below a predetermined threshold.
While not purging, the adjustable hydrogen fuel cell load 116 is adjusted to keep the pressure that is upstream of the passive purge valve 112 below its cracking pressure, effectively matching the fuel flow-rate consumed by the fuel cell 102 to the fuel flow-rate of the fuel generator 108, while during the purge, the adjustable hydrogen fuel cell load 116 is adjusted to increase the pressure upstream of the passive purge valve 112 above the cracking pressure. This can be done by decreasing the fuel consumption by the fuel cell 102 (reducing the fuel cell load current) while keeping the generated fuel flow-rate constant, which leads to fuel pressure buildup. The adjustments of the current load can be done rapidly, and thus rapid variations of the fuel pressure are possible, which means that quick, controlled purges are possible.
According to the embodiment shown in FIG. 2, the passive purge valve 112 is disposed at the distal end of the fuel cell stack 102 having the hydrogen fuel 108 connected in parallel, where hydrogen fuel source 108 is disposed at a proximal end of the fuel stack 102. Here, the purge request module 120 determines when the non-fuel material 110 needs to be purged by sensing when a voltage of any of the fuel cells in the stack 102 drop below a predetermined threshold when under a load. For example, the purge request module 120 can determine when the non-fuel matter 110 needs to be purged by sensing when a voltage in the fuel cell 102 that is most proximal to the passive purge valve 112 drops below a predetermined threshold.
FIG. 2 shows a hydrogen fuel cell gas purge system 200 having purge detection, where the purge valve outlet 202 is routed over the cathode (not shown) of one or more of the cells 102 in the fuel cell system 200, having an adjustable load 116. Initially, when inert gas and other non-fuel matter 110 such as water vapor/condensation, is being purged over the cell cathode the cell voltage is minimally affected, in particular at low or no loads. Once all of the non-fuel matter 110 has been purged and instead pure hydrogen is being purged, it catalytically reacts with the air-present oxygen at the cathode catalyst layer; effectively starving the cell of oxygen. This creates a detectable decrease in cell voltage particularly in passive natural convection driven cathode flow systems (see FIG. 3), which can be used to confirm that a successful purge occurred, as hydrogen will only be released after the accumulated non-fuel matter 110 is purged, where nitrogen, for example, tends to collect at the end of the flow path. Purging hydrogen over the cell cathode also has beneficial effects on the operation of the cell 102, since hydrogen reduces catalyst contaminants such as oxides.
According to one embodiment, the purge exhaust 202 can be directed over the open cathode of the fuel cell 102 by a variety of means included but not limited to tubing routing the gas from the purge valve 112 to the surface of the cell 102 or positioning the purge valve 112 such that the exhaust 202 directed over the cell 102 is used for detecting purges.
The output of the purge valve 202 placed over the cathode of the fuel cell 102 can be placed in many positions over the cathode including on the center of the cell and closer to the edges. When the exhaust is placed closer to the edge of a cell 102, it is less sensitive to detecting purges, as the purged fuel gas can escape more readily. This can be advantageous in cases when there is limited control of the pressure of the fuel gas, potentially leading to excessive amounts of gas to be purged over the cell 102, and thus limiting the power output of that cell 102.
FIG. 3 shows a multiple discrete cell system 300, according to the current invention, that has the purge exhaust 202 routed to one or more different cells 304 in the system 300. In many fuel cell systems, such as with a fuel cell array 102, multiple cells are connected with serial fuel-gas flow 302, with a pressure sensor 310, and have a natural convection driven cathode flow systems. When the purged non-fuel matter no from the passive purge valve 112 is directed across one fuel cell 304 in the array 102 that is upstream from the passive purge valve 112, the purge complete module 118 determines when the non-fuel matter no has been purged by sensing when a voltage of the upstream fuel cell 304 drops below a threshold voltage. As shown, the passive purge valve 112 is disposed at a distal end of at least two hydrogen fuel cells 102 having the hydrogen fuel connected in series, where a source 108 of the hydrogen fuel is disposed at a proximal end of the array. In the types of systems shown in FIG. 3, nitrogen gas will accumulate in the last cell 306 over time, eventually causing its voltage and power output to fall. As a result, routing the purge exhaust 202 over the last cell 306 in the array 102 can be problematic, as the purge detection module cannot distinguish between a voltage drop due to needing a purge (due to accumulated nitrogen on the anode side) or having successfully completed a purge (due to catalytic oxygen starvation a the air side). According to one embodiment, it is preferable to use one of the first cells 304 in the array 102 (in order of receiving gas flow).
According to one embodiment, the adjustable hydrogen fuel cell load 116 can include a battery charger circuit attached to a battery 308, where a charging current of the battery 308 can be adjusted. One aspect here is that the battery 308 is not charged based on what it should be charged, for example with constant current etc., rather based on how much hydrogen is generated. Here, the battery 308 serves as a readily available energy storage needed to keep the pressure upstream of the purge valve 112 below cracking pressure as well as a hybridizing device that can support continuous (no power output interrupts during purges) as well as peak power output from the fuel cell system to an external user load.
According to the current invention, a number of methods exist for detecting the presence of hydrogen gas over the cathode of a fuel cell, a fuel cell stack or a fuel cell array. One method involves measuring the voltage of one cell and comparing it to surrounding cells. When the voltage of the cell receiving the purge output is substantially lower than its neighboring cells and the system pressure is within the range in which a purge is expected, it can be reliably concluded that the purge was successful.
One scheme for using the output of the purge detection method disclosed is to use a software algorithm to monitor the system and use the voltage data from the purge cell to determine when a purge has been effectively completed. One possible control scheme, without limitation, is the flow diagram 400 shown in FIG. 4. In this scheme the system CPU would first detect the need for a purge 402, generally looking for a voltage decrease in the last cell. The system CPU would then increase the system pressure 404. The means for doing this vary with the type of system. In one representative hydrogen fueled system, in which the hydrogen is produced on demand using a binary chemical reaction between a liquid and a solid, the system pressure can be increased by increasing the rate with which the hydrogen is produced, which is in turn done by increasing the rate that the fluid is pumped into the chamber containing the reactive solid. Alternatively, the system pressure can be increased by keeping the rate of hydrogen production constant, and decreasing the rate with which the hydrogen is consumed by reducing the load on the fuel cell system, which is accomplished with the adjustable load 116 shown in FIG. 2 and FIG. 3. In systems where there is a significant degree of latency in the hydrogen production rate, reducing the load to increase system pressure is the preferred embodiment. Once the pressure has increased above a minimum threshold purge pressure, the system CPU waits to see a voltage drop 406 in the purge detection cell, indicating a successful purge. The system can then either reduce the pumping rate or increase the load on the fuel cells to bring the system pressure back 408 below the purge pressure in order to prevent the purging of excess hydrogen.
The present invention uses at least one passive valve that allows flow in two directions at predetermined pressures. In one possible embodiment, a dome type valve is used as a passive purge valve in the fuel cell system. Dome valves allow flow in both directions once predetermined pressure thresholds are reached, enabling a single valve to be used for pressure relief, purging, and vacuum relief. A preferred cracking pressure for purging can be as low as 1 PSI.
The purge valve assembly can be a standalone part or integrated into another assembly. In one embodiment, the dome valve could be a silicone quadricuspid dome valve. These valves offer the additional benefits of being low cost and sealing reliably at very low pressures. Dome valves offer an additional benefit of some hysteresis in closing. This enables more rapid purges, which can be beneficial in fuel cell systems with parallel flow field structures.
FIG. 5 shows an auxiliary fuel cell embodiment 500 that includes a purge exhaust 202 routed to an anode of an auxiliary fuel cell 502, while the cathode of the auxiliary fuel cell 502 is supplied with oxygen by air, either from an active air-move 506 such as a fan or preferably passively by diffusion. The purge exhaust 202 from the passive purge valve 112 is directed to a hydrogen sensor such as an anode of an auxiliary fuel cell 502, where the purge complete module 122 determines when the non-fuel matter 110 has been purged by sensing when the output 504 of the auxiliary fuel cell 502 exceeds a threshold level, where the output 504 can be either voltage or current. Initially when inert gas and other non-fuel matter 110 such as water vapor/condensation, is being purged into the anode the open cell voltage of the auxiliary fuel cell 502 is low and cell current when loaded is minimal. Once all of the inert gas 110 has been purged and instead pure hydrogen is being purged the open cell voltage of the auxiliary cell 502 increases and the cell current under load increases appreciably. Comparing the load current of the auxiliary cell 502 to a threshold value can indicate hydrogen purity in the purge stream.
According to another embodiment FIG. 6 shows a catalyst bed embodiment 600 that includes a purge exhaust 202 routed to a catalyst bed 602 in the presence of ambient air 604. Here the purge complete module 122 determines when the non-fuel matter no has been purged by sensing when the temperature 606 of the catalyst bed 602 exceeds a threshold level. The structure of the catalyst bed allows mixing of the purge exhaust 202 with ambient air 604 e.g. by diffusion, or venturi entraining. Initially, when inert gas and other non-fuel matter no such as water vapor/condensation, is being purged into the catalyst bed the gases pass through the catalyst bed without any reaction. Once all of the inert gas no has been purged and instead pure hydrogen is being purged, the hydrogen mixed with oxygen from ambient air catalytically combust at the catalyst bed, releasing heat and water vapor. The measured temperature increase 606 of the catalyst bed 602 is a good indication of hydrogen purity in the purge stream. The catalysts suitable for this method are selected from the Platinum group, oxides of silver, cobalt, manganese or any other catalyst with suitable catalytic reactivity at room temperature.
The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. All such variations are considered to be within the scope and spirit of the present invention as defined by the following claims and their legal equivalents.

Claims

What is claimed:

1. A method of purging a fuel cell system, the fuel cell system including a fuel supply, a fuel cell that receives fuel from the fuel supply at a fuel flow rate, a purge valve, and an adjustable load that applies a load on the fuel cell system, the method comprising:

detecting a purge event; and

in response to detecting the purge event, reducing the load on the fuel cell system to increase the system pressure and purge the system.

2. The method of claim 1, wherein purging the system further comprises keeping the fuel flow rate constant before and during the purge.

3. The method of claim 1, wherein reducing the load on the fuel cell system comprises adjusting an amount of power drawn from the fuel cell system by the adjustable load.

4. The method of claim 3, wherein adjusting an amount of power drawn from the fuel cell system by the adjustable load comprises adjusting a charging current of a battery with a battery charger circuit electrically connected to the battery.

5. The method of claim 1, wherein reducing the load on the fuel cell system comprises reducing the load on the fuel cell system to increase the system pressure beyond a cracking pressure of the purge valve.

6. The method of claim 1, wherein a cracking pressure of the purge valve is approximately 1 PSI.

7. The method of claim 1, wherein detecting a purge event comprises sensing a drop in a voltage of the fuel cell below a predetermined voltage threshold.

8. The method of claim 7, wherein the fuel cell is a fuel cell most proximal to the purge valve within a fuel cell stack.

9. The method of claim 1, further comprising:

detecting a purge completion event; and

in response to detecting the purge completion event, increasing the load on the fuel cell system to decrease the system pressure and cease the purge.

10. The method of claim 9, wherein detecting the purge completion event comprises sensing a purge stream composition comprising fuel above a predetermined fuel threshold.

11. The method of claim 10, wherein detecting the purge completion event comprises directing the purge stream across a cathode of a second fuel cell in an array of said fuel cells, wherein sensing a purge stream composition comprising fuel above a predetermined fuel threshold comprises sensing a voltage of the second fuel cell dropping below a threshold voltage.

12. The method of claim 10, wherein detecting the purge completion event comprises directing the purge stream over a catalyst bed in the presence of ambient air, wherein sensing a purge stream composition comprising fuel above a predetermined fuel threshold comprises sensing when a temperature of the catalyst bed exceeds a threshold level.

13. The method of claim 12, wherein directing the purge stream over a catalyst bed comprises directing the purge stream over a bed comprising platinum.

14. The method of claim 10, wherein detecting the purge completion event comprises directing the purge stream over an anode of an auxiliary fuel cell, wherein sensing a purge stream composition comprising fuel above a predetermined fuel threshold comprises sensing an output of the auxiliary fuel cell exceeding a threshold level, wherein said output comprises a current or a voltage.

15. The method of claim 9, wherein detecting the purge completion event comprises detecting that a threshold time duration has been reached with a timer.

16. The method of claim 15, wherein the threshold time duration is determined based on a current load in the fuel cell before said purge was initiated.

17. A method of purging a fuel cell system, the fuel cell system including a fuel supply, a fuel cell that receives fuel from the fuel supply at a fuel flow rate, a purge valve, and an adjustable load that applies a load on the fuel cell system, the method comprising:

detecting a purge event;

in response to detecting the purge event, reducing the load on the fuel cell system to increase the system pressure and purge the system;

detecting a purge completion event; and

18. The method of claim 17, wherein reducing the load on the fuel cell system comprises reducing an amount of power drawn from the fuel cell system by the adjustable load, and increasing the load on the fuel cell system comprises increasing the amount of power drawn from the fuel cell system by the adjustable load.

19. The method of claim 17, wherein reducing the load on the fuel cell system comprises reducing the load on the fuel cell system to increase the system pressure beyond a cracking pressure of the purge valve.

20. The method of claim 17, wherein detecting the purge event comprises sensing a drop in a voltage of the fuel cell below a predetermined voltage threshold, and detecting a purge completion event comprises detecting that a threshold time duration has been reached with a timer.