Fig. 4.

Demonstration of the harvest of low-grade heat by a prototype module. a Schematic of the structure and a photograph of the module. b Voltage and current curves of the module corresponding to the applied temperature difference. c Image of the module directly powering an LED array containing 29 red LED lamp beads. The module was sandwiched by two aluminum heat exchangers with water as the heat transfer fluid. The temperatures of the cold and hot water were controlled at approximately 278 and 333 K, respectively. Because of inevitable thermal conduction in the module and thermal contact resistance between the module and heat exchangers, the ΔT of 18 K built on the module at the steady state was smaller than that of the two heat exchangers. d Harvesting heat energy from the human body. The module was placed on a steel platform as a heat sink with a temperature of approximately 298 K and then covered by one hand. After several minutes, a stable temperature difference of 1.3 K was built, which induced an output of approximately 0.3 V by the module. The temperature and voltage were measured by a thermometer (left) and multimeter (right), respectively