Combined Compressor-expander in Fuel Cell Application

To improve efficiency of fuel cells working on the proton exchange membrane principle, which are currently being developed for automotive applications, a supply of pressurized air at about 3 bars abs is required. This is expelled in the reaction products, mainly as water and nitrogen, at approximately 100C. The power input required to compress the air consumes approximately 20% of the cell electrical output. To recover a part of the compressor power and to extract water, the reaction products must be expanded. It is shown that a twin screw machine can perform both the compression and expansion functions using only one pair of rotors. Moreover, by proper location of the ports, mechanical friction losses within it will be less than if the two functions are performed in separate machines.