Viability of low-grade heat conversion using liquid piston Stirling engines

Liquid piston Stirling engines (LPSEs) are low-cost, simple, maintenance-free, and flexible external combustion engines with the potential to operate from low-grade heat sources. This study reviews the major components, operations, and variants of LPSEs along with the mathematical models to assess their performance, which have been developed since the inception of LPSEs. Consequently, the technical feasibility of LPSEs in different applications is deduced from this complete systematic review, especially useful for both theoretical and applied researchers. The operation is based on a four-stage Stirling cycle with most engines having three main components as per the findings of this study: the displacer columns, the regenerator, and tuning columns. The two main variants categorized by this study are the dry operating fluidyne and the wet operating thermofluidic. The accurate analyses of these engines are mostly within the spheres of experimental and numerical techniques since analytical mathematical models are difficult to develop owing to the complexity of the operating thermo-physics. Pumping, cooling, and the direct generation of electricity are frequent applications of these engines reported in the literature. However, since their inception in 1969, these engines are yet to be commercialized owing to their low energy efficiency, power density, and reliability. The findings of this research conclude that LPSEs are essential for environment-friendly pumping applications using low-grade heat, especially in the irrigation sector of remote regions with limited access to electricity grids. Additionally, by replacing the electricity-consuming status quo pumps, LPSEs ensure sustainability with minimal greenhouse gas emissions.