Revisiting the evaporative Stirling engine: The mechanism and a case study via thermoacoustic theory.

In this work, a new explanation on an old idea to enhance the Stirling engine, i.e., the evaporative (two-phase two-component) Stirling engine, has been provided through thermoacoustic theory. In such a system, the periodical evaporation and condensation of an evaporative species is incorporated into the classical Stirling cycle to enhance the thermal-to-acoustic conversion. A theoretical model for an evaporative free piston Stirling engine has been developed, based on which the performance of the system has been investigated. Our calculated results show that the evaporative engine can operate with a temperature difference<50 K, much lower than that of the classical Stirling engine. In the optimal case, the proposed system can reach an exergy efficiency of 0.39 with the corresponding output power, efficiency and temperature difference to be 10 W, 0.032 and 33 K, respectively. The main reason for the improvement is the enhanced thermoacoustic conversion by the phase change of the evaporative species happening in the regenerator, embodied by the significantly increased acoustic source term. Moreover, the improved heat transfer in heat exchangers by phase change also helps. This demonstrates the promising potential of the evaporative Stirling engine for ultra-low-grade heat recovery. • Mechanism of evaporative Stirling engine has been explained via thermoacoustic theory. • A numerical model for an evaporative FPSE has been developed. • Phase change reduces the temperature difference and increases the specific power of the engine. • The improvement is due to wet thermoacoustic conversion, and the enhanced heat transfer. [ABSTRACT FROM AUTHOR]