A novel high-efficiency integrated system combining a thermally regenerative electrochemical cycle and a flow battery

Abstract

A thermally regenerative electrochemical cycle (TREC) harnesses the temperature effect of electrode potential to achieve efficient heat to electricity conversion but suffers from low power density. The flow battery energy storage system is well-suited for large-scale energy storage, offering the benefits of long cycle life and the decoupling of power and energy, but the energy efficiency remains to be improved. In this paper, a novel integrated system combining a TREC and a flow battery is proposed, which has both energy conversion and storage functions by charging and discharging alternately at different temperatures. An experimental investigation was conducted using an all-vanadium redox flow battery (VRFB) as a case study. The experimental results show that, operating between 10 °C and 40 °C, the integrated system achieves the coulombic efficiency, voltage efficiency, and energy efficiency of 96.65%, 92.22%, and 89.12%, respectively. Additionally, the system exhibits a power density of 523.96 W m⁻², an energy density of 25.81 W h L⁻¹, and a normalized thermal efficiency of 2.54%. A comparison indicates superior energy efficiency gains of 3.5% and 8.2% over the VRFB systems operating at 10 °C and 40 °C, respectively. Furthermore, the integrated system exhibits enhanced energy efficiency with increasing operating high temperature, accompanied by reduced normalized thermal efficiency. Additionally, variations in current density impact discharging and charging voltages differently, with a more pronounced effect observed during low-temperature discharging. The integrated system combining a TREC and a flow battery proposed in this study is expected to provide a potential idea for the integration of renewable energy conversion and storage.