Performance improvement of twin-screw air expander used in PEMFC systems by two-phase expansion

Reducing the power consumption of the air supply system has become one of the bottleneck problems for developing PEMFCs. This paper proposes the two-phase expansion of air and liquid water in a twin-screw expander to recover the thermal energy of water simultaneously and achieve quasi-isothermal expansion, aiming at increasing the recovery power of the expander and thus reducing the power consumption of the air supply subsystem in PEMFCs. A mathematical model of the working process for a twin-screw expander is constructed to evaluate the performance improvement of the expander caused by the two-phase expansion at variational rotating speed, inlet pressure, and air stoichiometry ratio. The influence mechanisms of sealing and heat exchange of water are revealed. Results show that the sealing effect of water couldn't bring out obvious performance improvement of the expander mainly due to the limited water generated in the fuel cell stack and flowing into the expander. However, sufficient heat exchange between air and water can effectively improve the expander performance and would have a more significant effect under the condition of less gas leakage. The maximum increment of specific power could reach 10.8% compared to the dry-running condition.