Waste Heat to Power: Full-Cycle Analysis of a Thermally Regenerative Flow Battery

Abstract

Large amounts of waste heat, below 120 °C, are released globally by industry. To convert this low-temperature waste heat to power, thermally regenerative flow batteries (TRFBs) have recently been studied. Most analyses focus on either the discharging or the regeneration phase. However, both phases have to be considered to holistically assess the performance of the flow battery. Therefore, a dynamic, open-access, full-cycle model of a Cu–NH3 TRFB is developed in Modelica and validated with data from the literature. Based on the validated model, a trade-off between power density and efficiency is shown that depends only on the discharging strategy of the flow battery. For a sensible heat source with an inlet temperature of 120 °C and heat transfer at a thermodynamic mean temperature of about 90 °C, the power density reaches 38 W m^(−2) over a complete cycle, and the efficiency reaches 20% of Carnot efficiency. In a benchmarking study, the power production of the flow battery is shown to already achieve 34% of a fully optimized organic Rankine cycle. Thus, TRFBs require further optimization to become a competitive technology for power production and energy storage from low-temperature waste heat.