Exploring a Novel Route for Low-Grade Heat Harvesting: Electrochemical Brayton Cycle

Efficient technologies to convert huge amounts of low-grade heat to power are urgently desired. Although various electrochemical technologies have been proposed, the low thermal efficiency and power density still limit their practical applications. Starting from the ideal cycle of thermo-chemical engines and the decoupling of heat transfer and charge transport, this study proposes the electrochemical Brayton cycle (EBC) for power generation for the first time, which is realized by flow batteries and heat exchangers. By establishing energy and entropy generation models, the thermal efficiency and power density of ideal and actual EBC systems, as well as the isentropic efficiency and entropy generation are analyzed and discussed. The results indicate the ideal EBC without overpotential performs a maximum power density of 690 W m−2 and a relative efficiency to Carnot efficiency of 52.3% when operated between 25 and 75 °C. With 70% heat regeneration, the relative efficiency and power density of actual EBC could reach 45.3% and 1.6 W m−2 at an electrolyte flow velocity of 0.01 mm s−1, or 23.7% and 6.8 W m−2 at a velocity of 0.1 mm s−1, respectively. Further entropy generation analysis reveals that the main contributors to entropy generation are heat transfer, activation overpotential and ohmic overpotential in order of magnitude. Furthermore, the high efficiency of EBC compared with other technologies, its broad application space, especially its advantages of integration with energy storage indicate its feasibility and potential.