Performance analysis of a single-piston free piston expander-linear generator with intake timing control strategy based on piston displacement

Abstract A single-piston free piston expander-linear generator (FPE-LG) prototype for a small-scale Organic Rankine Cycle system is presented. The steady-state operation of the prototype is successfully achieved, which indicates the feasibility of single-piston FPE-LG with intake timing control strategy based on piston displacement. The effects of intake duration, intake pressure, theoretical stroke, and external load resistance ( R ) on operational performance and energy conversion efficiency are investigated. Results show that the piston is running at a relatively high velocity when it approaches the predetermined special displacement point (X 1 or X 2 ). The peak voltage, current, and power output increase linearly as the intake pressure and duration increase. The theoretical stroke exerts a minimal effect on peak velocity and peak voltage output but considerably affects the operating frequency. The peak power output and work-electric conversion efficiency ( η w-e ) increase first and then decrease with increasing external load resistance. The peak output power and η w-e reach the maximum value of 32.5 W and 30.8%, respectively, with an optimal R =  80 Ω. As the intake duration increases, the indicated efficiency increases gradually, whereas η w-e shows an increasing and then decreasing trend. The optimal intake duration for reaching the maximum η w-e is variable under different operation conditions.