A fully passive thermal circuit for generating constant temperature difference from an oscillating heat source.

• A fully passive thermal circuit was proposed for energy harvesting. • The superior thermoelectric conversion performance was experimentally validated. • Working principle of energy harvesting thermal circuit was investigated. • Feasibility of the designed thermal circuit for practical electricity supply was proved. The low-grade thermal energy with oscillating temperature is abundant and pervasive due to periodic solar radiation, intermittent working of the devices and so on; while it cannot be readily harvested because temperature difference is generally required for thermal energy collection. A novel thermal circuit composed of passive thermal diode and thermal mass is designed to effectively collect low-grade thermal energy, and the mathematical model of the thermal circuit is built to comprehensively reveal the working mechanism. The results show that the design criteria of thermal circuit require a dimensionless time period less than 0.1 and a thermal rectification ratio larger than 10, indicating the effective working region of thermal circuit. Based on the analytical results, a fully passive thermal circuit is constructed and the capability of outputting a stable direct voltage is experimentally demonstrated. Meanwhile, the thermoelectric conversion performance of the thermal circuit is over 1.99 mW K-2 m-2, significantly superior to the reported results in the literature. In addition, the previous thermal circuit is only applicable for the oscillating heat source with the period of a few hours. For this, low-melting point metallic phase change material with both large specific heat capacity and high thermal conductivity is introduced into the thermal circuit. It is found that the improved thermal circuit can output the stable direct voltage in the range of 0.695 V to 0.793 V, showing a great feasibility for the scenarios with real-world diurnally oscillating temperature. This study paves a feasible way for electricity supply of electronic equipment in remote region. [ABSTRACT FROM AUTHOR]