SIZE LIMITS FOR REGENERATIVE HEAT ENGINES

Abstract

This article explores the lower size limit placed on regenerative heat engines by thermodynamics and heat transfer. Information derived in this work has direct relevance to the development of mesoscopic heat engines that are based on standard gas cycles employing regeneration. A model is developed for the Stirling cycle that incorporates a regenerator effectiveness term and an axial conduction term, both of which are dependent on the length scale of the device. The thermal efficiency for the engine is determined in terms of the cycle temperature ratio, the expansion ratio, regenerator effectiveness, and a nondimensional term called the conduction parameter. Results from this study show that a small-scale heat engine fabricated from a low-thermal-conductivity material can be made with a length scale approaching 1 mm. Such a device would undoubtedly be composed of numerous microscale components. Below the 1-mm limit, efficiency suffers to such a degree that solid-state thermoelectric devices would become a better choice for a particular application.