Thermal control elements for caloric energy conversion

Caloric energy conversion is an emerging field of cooling, heat-pumping and power-generation technologies. The potentially high energy efficiency and use of environmentally friendly and safe solid-state working substances in the form of refrigerants has stimulated increased research activity in the past two decades. Most of today's caloric devices apply so-called active regeneration, which involves the oscillation of the working fluid through the matrix of the caloric material - the caloric regenerator. However, the unavoidable, irreversible viscous and heat-transfer losses apply limits to the caloric device's performance as well as its size. The quest for better caloric-device performance has led to the development of thermal control elements, which control the heat flux on different size and time scales. In this paper we describe the working principles of these elements: thermal switches, thermal diodes and thermal regulators. This is followed by the first up-to-date critical review of the research activities and applications of thermal control elements in all types of caloric devices. We show that thermal control elements have the potential to improve the power density of caloric devices. Finally, we propose target features for these elements with respect to future research activities in the field of caloric technologies.