Towards the Carnot efficiency with a novel electrochemical heat engine based on the Carnot cycle: Thermodynamic considerations

The efficiency of emerging continuous electrochemical heat engines is limited by large irreversible losses associated with non-isothermal heating and cooling processes. To fundamentally address this limitation, this study proposes for the first time an electrochemical quasi-Carnot cycle (EQCC), in which the temperature of electrolytes is changed by adiabatic processes. Specifically, the EQCC is implemented by four electrochemical reactors with flow battery structures operating under two different conditions. The performance of the ideal EQCC is derived mathematically, and the non-ideal EQCC considering irreversible losses is realized by adjusting the current distribution in the system. Results show that the thermal efficiency of the ideal EQCC is the Carnot efficiency, and its power density could reach 945 W m−2 under a temperature difference of only 18 °C. The optimized non-ideal EQCC system performs a relative efficiency to the Carnot efficiency of 55.2% corresponding to the maximum power density of 12.6 W m−2, which can reach 81.8% by reducing the power density to 7.0 W m−2. Furthermore, the application potential of EQCC is demonstrated by its competitive performance compared with the thermally regenerative electrochemical cycle and electrochemical Brayton cycle. The EQCC proposed in this study provides a novel idea for low-grade heat harvesting.