Development of the free piston expander for work recovery in transcritical CO2 refrigeration cycle

Replacing the throttle valve with an expander is beneficial to the performance improvement on the transcritical CO2 refrigeration cycle. The expander is hereby developed for this purpose in this paper. With the required working conditions, performance potential, reliability, etc. considered, the free piston machine is selected as the most suitable concept after a comparison among some candidates. The single acting structure is adopted, in which an auxiliary compressor is arranged on a common shaft to utilize the work recovered by the expander. A slider-based scheme is applied to control the expander inlet/outlet, and an adjustable clearance volume is designed to modulate the operating ranges of the low/high pressures. A design model is established to determine the geometric parameters of the expander-compressor unit. A prototype expander is manufactured and preliminary test is carried out on a test rig with both air and CO2 as the working fluid. The experimental results show that the expander-compressor unit can work in some ranges of the low/high pressures, which proves that this type of expander concept as well as the inlet/outlet control method is feasible. A serious pressure pulsation is observed at the expander inlet, resulting in considerably large pressure drop during the filling process of the expander. The work to improve the expander is therefore being done, with the attenuation of the pressure pulsation focused on.