Efficiency improvement of an all-vanadium redox flow battery by harvesting low-grade heat

Redox flow batteries (RFBs) are rugged systems, which can withstand several thousand cycles and last many years. However, they suffer from low energy density, low power density, and low efficiency. Integrating a Thermally Regenerative Electrochemical Cycle (TREC) into the RFB, it is possible to mitigate some of these drawbacks. The TREC takes advantage of the temperature dependence of the cell voltage to convert heat directly into electrical energy. Here, the performance increase of a TREC-RFB is investigated using two kinds of all vanadium electrolyte chemistries: one containing a typical concentration of sulfuric acid and one containing a large excess of hydrochloric acid. The results show that the energy density of the system was increased by 1.3Wh L-1 and 0.8Wh L-1, respectively and the overall energy efficiency also increased by 9 and 5 percentage points, respectively. The integration of the heat exchangers necessary to change the battery temperature is readily facilitated by the design of the redox flow battery, which already utilizes fluid circulation loops.