A thermally-coupled cascade free-piston Stirling engine-based cogeneration system

Abstract

Resonant Stirling/thermoacoustic cycle engines show great potential in efficiently recovering waste heat in small- or micro-scale applications. However, there is a significant research gap regarding the development of resonant Stirling/thermoacoustic cycle engines capable of effectively harnessing variable-temperature heat sources through cascade utilization. In this paper, a cogeneration system was proposed based on a thermally-coupled cascade dual-opposed free-piston Stirling engine. Through a multi-stage arrangement, the prototype enhances overall exergy efficiency by scavenging different grade heat. According to test results, with an input heating power of 20 kW for each stage, the corresponding heating temperatures for the three stages were 418.7 °C, 348.2 °C, and 302.8 °C, respectively. The demonstration setup provided simultaneous thermal power of 44.72 kW and electric power of 10.18 kW, resulting in an overall thermal-to-electric efficiency of 16.48% and an overall combined heat and power efficiency of 88.87%. Theoretically, compared with a single-stage system, the exergy efficiency improved from 36.3% to 43.9%, representing a relative improvement of more than 20%. This study provides valuable insights into the operating characteristics of multi-stage free-piston Stirling engine-based cogeneration systems and contributes to the development of waste heat recovery systems.