Development of Water Lubricated Compressor-Expander for Fuel Cell Application

Abstract

Fuel cells, currently being developed for automotive and stationary applications, require a supply of pressurized air at about 3 bars abs. This is expelled in the reaction products, mainly as water steam and nitrogen, at approximately 100 °C. The power input required to compress the air is typically 20–25% of the cell electrical output. Hence, for the cell efficiency to be acceptable and to recover water conveniently from the reaction products, power needs to be generated by expansion of the reaction products and used to reduce the electrical output from the cell that is consumed by the compressor. Then water may be condensed in a flue gas cooler and returned to process. Screw compressor is a good choice to supply pressurized air at about 3 bars for fuel cell application. 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. However, due to strict requirements for oil contamination, the rotors and bearings in such a machine have to be either absolutely sealed or left open and lubricated by water. A research performed at City University is described in the paper on the rotors and bearings lubricated exclusively by water which then can be used in the combined compressor-expander screw machines which are suitable for fuel cell application.