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

Highlights: • A test rig of single-piston FPE-LG prototype is established and tested. • Intake timing control strategy based on piston displacement is presented. • Effects of intake duration and stroke length on FPE-LG performance are investigated. • The work-electric conversion efficiency is defined and analyzed. -- 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 (X₁ or X₂). 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.