TECHNICAL FIELD

The invention relates to a fuel cell system comprising a polymer electrolyte (proton exchange) membrane (PEM) fuel cell stack.

BACKGROUND

A PEM fuel cell stack generally requires humidification of its polymer electrolyte membranes during operation. In a fuel cell system comprising such a stack, water may be recovered from the cathode exhaust of the stack and used to humidify air and hydrogen fuel input to the cathode and anode sides of the stack respectively. The humidification system of such a fuel cell system accounts for roughly 20% to 25% of total air and fuel system weight and therefore has a substantial adverse impact on the performance of such fuel cell systems in mobile applications, especially aerospace applications. In addition, conventional water separators used to recover water from the cathode exhaust can be unreliable, having complex designs with moving parts, and remove water with substantially less than 100% efficiency. Increasing the efficiency with which water is recovered from the cathode exhaust of a fuel cell system would offer opportunities for reducing liquid cooling requirements of a PEM fuel cell stack.

BRIEF SUMMARY

According to a first aspect of the invention, a fuel cell system comprises a PEM fuel cell stack and an expansion valve, the cathode output of the PEM fuel cell stack being coupled to the expansion valve such that in operation of the fuel cell system gaseous output from the cathode side of the PEM fuel cell stack is provided to the expansion valve, the expansion valve being arranged to expand the gaseous output to condense water vapour within the gaseous output. Compared to conventional apparatus for recovering water from the cathode exhaust of a PEM fuel cell stack, an expansion valve is less complex, lighter and more efficient in terms of the fraction of water recovered from the cathode exhaust. Preferably the expansion valve is arranged to expand the cathode exhaust adiabatically or substantially adiabatically. The expansion valve may be a Joule-Thomson valve. The fuel cell system may be arranged such that in operation thereof at least a portion of condensed water produced by the expansion valve is provided to air input to the cathode side of the PEM fuel cell stack at the cathode input thereof to humidify the air. Alternatively, at least a portion of condensed water produced by the expansion valve may be injected directly into the PEM fuel cell stack to provide evaporative cooling and humidification thereof.

The fuel cell system may further comprise a turbocharger, the turbocharger charger comprising first and second compressors, a turbine and an electric motor, the first and second compressors being arranged to be driven by at least one of the turbine and the electric motor and wherein the fuel cell system is arranged such that in operation thereof air input to fuel cell system is provided to the cathode input of the PEM fuel cell stack via the first compressor and air output from the expansion valve is provided to the turbine via the second compressor.

A second aspect of the invention provides a propulsion system comprising a fuel cell system according to the first aspect of the invention and an electric propulsor, the electric propulsor being arranged to receive electrical power from the fuel cell system and to provide propulsive thrust using the electrical power.

A third aspect of the invention provides an aircraft comprising a propulsion system according to the second aspect of the invention.

A fourth aspect the of the invention provides a method of extracting water from the cathode exhaust of a PEM fuel cell or PEM fuel cell stack, the method comprising the step of expanding the cathode exhaust through an expansion valve to condense water vapour within the cathode exhaust. The step of expanding the cathode exhaust may be carried out adiabatically or substantially adiabatically. The expansion valve may be a Joule-Thomson valve.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from United Kingdom of Great Britain & Northern Ireland Patent Application No. GB 2200622.5, filed on 19 ^th January 2021, the entire contents of which are herein incorporated by reference. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described below by way of example only and with reference to the accompanying drawings in which:

Figure 1 shows a fuel cell system of the invention; and

Figure 2 shows a propulsion system comprising the fuel cell system of

Figure 1.

DETAILED DESCRIPTION

Referring to Figure 1, a PEM (polymer electrolyte membrane or proton exchange membrane) fuel cell system 100 of the invention comprises a PEM fuel cell stack 120 having cathode and anode sides 122, 126. The cathode side 122 has a cathode input 121 and a cathode output 123. The anode side 126 has an anode input 125 and an anode output 127. The fuel cell system 100 further comprises a turbocharger 105 which comprises first and second compressors 108, 109 arranged to be driven by a turbine 106 and/or an electric motor 110. The PEM fuel cell system 100 further comprises an ejector 130, a water trap 132, an air filter 104 and a humidifier 112. The humidifier 112 comprises an expansion valve 111 (for example a Joule-Thomson valve). The PEM fuel system 100 has air and fuel (gaseous hydrogen) inputs 102, 134, a fuel purge output 136 and a cathode exhaust output 114.

The PEM fuel cell stack 120 comprises a cooling circuit 140 in around which coolant fluid (in this case liquid water) is driven by a pump 146. The cooling circuit 140 further includes a splitter 142, a coolant fluid/air heat exchanger 144, a de-ioniser 148 and gas/coolant fluid heat exchangers 141, 143. The PEM fuel cell stack 120 has a coolant fluid input 128 and a coolant fluid output 129.

In operation of the PEM fuel cell system 100, air is drawn into the air input 102 and passes via the first compressor 108 of the turbocharger 105, the heat exchanger 141 and the humidifier 112 to the cathode input 121 of the PEM fuel cell stack 120. Gaseous hydrogen fuel enters the fuel input 134 and passes via the heat exchanger 143 and the ejector 130 to the anode input 125 of the PEM fuel cell stack 120.

Unused gaseous hydrogen fuel exits the anode side 126 and is re-circulated to the anode input 125 via the water trap 132 and the ejector 130. The anode side 126 may be intermittently purged by means of the fuel purge output 136. Heat resulting from compression of air at the first compressor 108 is transferred to hydrogen fuel input at the fuel input by operation of the heat exchangers 141 , 143 and the cooling circuit 140.

Heat generated by the PEM fuel cell stack 120 is lost to ambient air by the heat exchanger 144.

Cathode exhaust comprising air and water vapour exits the cathode side 122 of the PEM fuel cell stack 120 at the cathode output 123 and passes to the expansion valve 111 of the humidifier 112. Expansion of the cathode exhaust condenses water vapour within the cathode exhaust; the resulting liquid water is provided to compressed air exiting the heat exchanger 141 in order to humidify it prior to input to the cathode input 121. The remaining cathode exhaust passes to the turbine 106 of the turbocharger 105 via the second compressor 109. The expansion valve 111 may be a Joule- Thomson valve, the cathode exhaust being expanded adiabatically or substantially adiabatically.

In a variant of the PEM fuel cell system 100, liquid water produced by the expansion valve 111 is injected directly into the PEM fuel cell stack to achieve evaporative cooling and humidification of the PEM fuel cell stack. The requirement for liquid cooling of the PEM fuel cell stack may thereby be reduced or eliminated, for example the size and mass of the coolant fluid/air heat-exchanger may be reduced.

The PEM fuel cell system 100 has an electrical output 124. In operation of the PEM fuel cell system 100, electrical power is provided to the electric motor 110 of the turbocharger 105 via an inverter 156. High voltage output is provided at a high-voltage output 158 via a high-voltage bus 154. Electrical power is provided to the pump 146 via a low-voltage bus 152 which has a low-voltage output 151.

Figure 2 shows a propulsion system 200 of the invention comprising the PEM fuel cell system 100 of Figure 1 and an electric propulsor 210. The electric propulsor comprises an inverter 212 and an electric motor 214. The electric motor 214 is arranged to drive a propeller or fan 216. In operation of the propulsion system 200, electrical power output from the high-voltage output 158 of the PEM fuel cell system 100 is provided to the electric propulsor 210 which uses the electrical power to generate propulsive thrust. The propulsion system 200 may be comprised in an aircraft.