Development and experimental validation of novel free-piston expander

A double acting free piston expander with a novel inlet/outlet control mechanism was developed to replace the throttling valve and recover the expansion work in transcritical CO₂ (carbon dioxide) refrigeration cycle. The expander together with the inlet/outlet control mechanism was experimentally validated in the air test rig, and the expander performance was investigated with the p-t and the p-V diagrams. The experimental results show that the inlet/outlet control mechanism can realize the suction, expansion and discharge processes properly, which is required by a typical expander. The expander can work stably in a wide range of pressure differences. The frequency of the expander exhibits approximately linear change with the pressure difference, and the optimal frequency is from 10 Hz to 17 Hz. The adiabatic efficiency of the expander at 10.2 Hz is about 62%. Further experiment investigations on the working processes of the expander show that too long suction passage increases the response time during the suction process, which results in insufficient gas suction and thus lower efficiency at higher frequency.