Thermally regenerative electrochemically cycled flow battery coupled with radiative cooling for low-grade heat harvesting

Abstract

Low-grade heat sources below 100 °C provide a vast quantity of energy, yet harvesting them has been a longstanding challenge due to the limited and fluctuating temperature difference between the low-grade heat sources and the environment. Through thermo-electrochemical modeling, this work shows that coupling a thermally regenerative electrochemically cycled flow battery (TREC-FB) with radiative coolers can lead to an additional operating temperature difference of 11.8 °C, a 36.9% enhancement in energy density, and a maximum efficiency of 3.7% when the heat-source temperature fluctuates between 20 and 60 °C with a heat recuperation effectiveness of 70%. Our model also captures the coupled effects of mass transfer and electrochemical kinetics in affecting power density and efficiency, and we showed that ohmic losses due to internal resistances, and mass transfer overpotential inside the porous electrode are the major factors limiting the power density and efficiency. Finally, we predict that if the effective thermopower of electrolytes can reach 5 mV/K, a maximum power density of 8 W/m2, and a relative efficiency (ηth/ηC) of 20.5% can be achieved, with ηth and ηC denoting the efficiency and the Carnot limit respectively.