A review of thermally regenerative batteries: From heat to electricity via concentration gradient

초록

Low-grade thermal energy below 100 degrees C represents one of the most abundant yet underutilized energy resources in industrial processes and infrastructure. Thermally regenerative batteries (TRBs) have emerged as a distinctive platform for harvesting such heat by converting thermally induced concentration gradients into electrical energy through electrochemical discharge. Unlike thermoelectric generators or thermo-electrochemical cells, TRBs can temporarily store chemical energy, thereby decoupling heat input from power delivery. Despite increasing research activity, a unified analytical framework for benchmarking TRB performance and identifying thermodynamic bottlenecks remains lacking. This review systematically analyzes TRBs from four perspectives: (i) concentration-gradient-based working principles and the three-stage operational cycle; (ii) key performance metrics and the inherent trade-off between efficiency and power density; (iii) thermodynamic limits evaluated through entropy generation and temperature-entropy (T-S) diagram analysis, which indicate that sensible heat loss during electrolyte regeneration is a major source of irreversibility; and (iv) a comparative benchmarking index, denoted as ZC, introduced as a preliminary, physics-informed screening descriptor for concentration-gradient-driven systems, for which the conventional thermoelectric figure of merit ZT is not directly applicable. Ammonia-based copper TRBs have demonstrated power densities of 20-80 W/m2 with Carnot-relative efficiencies of 10-20%, suggesting a more balanced trade-off between conversion efficiency and power output than many competing low-grade heat conversion technologies. Promising near-term applications include integrated systems for waste-heat recovery, photovoltaic hybridization, copper resource recovery, and CO2 capture. The analytical framework and new ZC index discussed in this review provide preliminary tools for guiding future scale-up, long-term stability evaluation, and techno-economic assessment.

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