Development and experimental investigation of an oil-free twin-screw air compressor for fuel cell systems

Compressor in air supply circuit influences flow rate of oxygen in cathode of polymer electrolyte membrane (PEM) fuel cell systems. In this paper, an air-cooled dry oil-free twin-screw compressor for fuel cell systems was developed and adopted successfully in a type of truck with fuel cell system. Extensive experimental investigations including the recording of p-V indicator diagram were carried out to study the performance of the prototype compressor. Rotation speed of the female rotor varied from 5000 rpm to 10,000 rpm, and discharge pressure from 0.14 MPa to 0.22 MPa. It was found that the volume flow rate decreases almost linearly with the reduction of rotation speed and this is beneficial to the fuel cell systems under part-load. Between speed of 7000 rpm and 10,000 rpm at a given discharge pressure, the variations of isentropic efficiency and specific power are very limited. This indicates the twin-screw compressor can operate efficiently in a wide range of power output from the fuel cell systems. Under the design conditions with discharge pressure of 0.2 MPa and female rotor rotation speed of 9000 rpm for a fuel cell output of 50 kW, the volumetric efficiency and isentropic efficiency of the prototype compressor developed are 70% and 55% respectively. And the mechanical efficiency and the polytropic process index regarding to discharge temperature are 80% and 1.58. These values can be used in the design of other air-cooled dry oil-free twin-screw compressors for similar applications.