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

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.

Recent progress on performance and control of linear engine generator

“…Because of low efficiencies in the MEMS scale, an open-cycle macro-scale FP-LEG was developed. 62,63 The intake duration and pressure influenced the performance indicators, similar to the result by Jia et al 26 An indicated efficiency of 18% at 2.5 bar, 373 K, was achieved. 62 Increased intake temperature did not significantly affect engine performance for a fixed intake duration, and an optimal intake duration existed, which maximised energy conversion, similar to the results in Ngwaka et al 64…”

Pressure Pulsation and Cavitation Phenomena in a Micro-ORC System

“…Commonly, the presence of simplified prototypes and/or different working conditions could determine several differences between the actual machine performance and the measured ones. To help the experimental analysis, a lumped-parameter model is often used for extending the analysis to other operating conditions or other fluids [7,8], increasing the design capabilities and limiting the cost and time.…”

Force and energy analysis of single-piston free-piston expander—linear generator

“…Burugupally et al indirectly studied the influence of viscous friction on the performance of FPE during operation by studying the different performance of FPE with different viscosity working fluid. However, this paper only analyzed the performance influence, and did not make a detailed analysis on the power consumption and magnitude of viscous friction [22].…”