Development of a double acting free piston expander for power recovery in transcritical CO2 cycle

To replace the throttling valve with an expander is considered as an efficient method to improve the performance of the transcritical CO2 refrigeration cycle. This paper presents the design and experimental validation of a double acting free piston expander, in which a slider-based inlet/outlet control scheme is used to realize a full expansion process for the expander. The power extracted from the expansion process is utilized by an auxiliary compressor, which is arranged in parallel with the main compressor. A design model is developed to determine the geometric parameters of the expander together with the auxiliary compressor. An expander prototype is manufactured and validated experimentally in the air test system, mainly by means of analyzing the dynamic pressures in the expander chamber. The experimental results show that the expander can work stably in a wide range of pressure differences/ratios at the frequency approximately linear with the pressure difference through the expander. The p-t diagrams in the expander indicate that the slider-based inlet/outlet control scheme enables the expander to have the proper suction, expansion and discharge processes. However, the prototype at high frequency doesn't present isobaric suction process, which results in insufficient gas suction and therefore decrease in the expander efficiency. With the p-t diagrams at various frequencies compared, the optimal working frequency is found to range from 10 to 17Hz in the air system. The isentropic efficiency of 62% is obtained from the p-V diagram analysis. Further validation of the expander in the CO2 system will be conducted in the near future.