The effect of working fluid properties on the performance of a miniature free piston expander for waste heat harvesting

Highlights: • A miniature free piston expander (FPE) operating in an open cycle is analyzed for different working fluids through a physics-based lumped-parameter model. • The FPE behavior and performance is studied for low temperature waste-heat harvesting application. • We report two observations that help achieve high effciencies. • An electrical output power of 2 W and output voltage of 20 V can be generated from a centimeter-sized FPE. -- Abstract: Power generation from waste heat sources at the miniature length scales may be generated by using phase change working fluids (liquid-to-vapor) in a traditional boiler-expander-condenser-pump system. This paper builds on our prior work of boiler and expander design by investigating the effects of working fluid properties on an expander unit based on a Free Piston (FPE) architecture. Here, using first principles, a lumped-parameter model of the FPE is derived by idealizing the FPE as a linear spring-mass-damper system. Moreover, a linear-generator model is incorporated to study the effects on useful power output from the FPE directly. As a result, insight into the thermodynamic processes within the FPE are detailed and general recommendations for working fluid selection are established. They include: (1) to achieve a higher FPE efficiency, it is desirable for a working fluid to have high specific heat ratio, and (2) a peak output voltage of about 20 V AC and peak output power of around 2 W can be generated by coupling a centimeter-scale electromagnetic energy converter to the FPE. Overall, this effort shows the promise of reliable miniature thermal power generation from low temperature waste heat sources.