The Reverse electrodialysis heat engine (REDHE) combines a reverse electrodialysis stack for power generation with a thermal regeneration unit to restore the concentration difference of the salt solutions. Current approaches for converting low-temperature waste heat to electricity with REDHE have not yielded conversion efficiencies and profits that would allow for the industrialization of the technology. This review explores the concept of Heat-to-Hydrogen with REDHEs and maps crucial developments toward industrialization. We discuss current advances in membrane development that are vital for the breakthrough of the RED Heat Engine. In addition, the choice of salt is a crucial factor that has not received enough attention in the field. Based on ion properties relevant for both the transport through IEMs and the feasibility for regeneration, we pinpoint the most promising salts for use in REDHE, which we find to be KNO₃, LiNO₃, LiBr and LiCl. To further validate these results and compare the system performance with different salts, there is a demand for a comprehensive thermodynamic model of the REDHE that considers all its units. Guided by such a model, experimental studies can be designed to utilize the most favorable process conditions (e.g., salt solutions).

Reindeer (Rangifer tarandus) have evolved elaborate nasal turbinate structures that are perfused via a complex vascular network. These are subject to thermoregulatory control, shifting between heat conservation and dissipation, according to the animal’s needs. The three-dimensional design of the turbinate structures is essential in the sense that they determine the efficiency with which heat and water are transferred between the structure and the respired air. The turbinates have already a relatively large surface area at birth, but the structures have yet not reached the complexity of the mature animal. The aim of this study was to elucidate the structure–function relationship of the heat exchange process. We have used morphometric and physiological data from newborn reindeer calves to construct a thermodynamic model for respiratory heat and water exchange and present novel results for the simulated respiratory energy losses of calves in the cold. While the mature reindeer effectively conserves heat and water through nasal counter-current heat exchange, the nose of the calf has not yet attained a similar efficiency. We speculate that this is probably related to structure-size limitations and more favourable climate conditions during early life. The fully developed structure–function relationship may serve as inspiration for engineering design. Simulations of different extents of mucosal vascularization suggest that the abundance and pattern of perfusion of veins in the reindeer nasal mucosa may contribute to the control of temperature profiles, such that nasal cavity tissue is sufficiently warm, but not excessively so, keeping heat dissipation within limits.

The reverse electrodialysis heat engine (REDHE) is a promising salinity gradient energy technology, capable of producing hydrogen with an input of waste heat at temperatures below 100 °C. A salinity gradient drives water electrolysis in the reverse electrodialysis (RED) cell, and spent solutions are regenerated using waste heat in a precipitation or evaporation unit. This work presents a non-equilibrium thermodynamics model for the RED cell, and the hydrogen production is investigated for KCl/water solutions. The results show that the evaporation concept requires 40 times less waste heat and produces three times more hydrogen than the precipitation concept. With commercial evaporation technology, a system efficiency of 2% is obtained, with a hydrogen production rate of 0.38 g${}_{{\mathrm{H}}_2}$ m${}^{-2}$h${}^{-1}$ and a waste heat requirement of 1.7 kWh g${}_{{\mathrm{H}}_2}^{-1}$. The water transference coefficient and the salt diffusion coefficient are identified as membrane properties with a large negative impact on hydrogen production and system efficiency. Each unit of the water transference coefficient in the range $t_w=\left[0–10\right]$ causes a −7 mV decrease in unit cell electric potential, and a −0.3% decrease in system efficiency. Increasing the membrane salt diffusion coefficient from 10${}^{-12}$ to 10${}^{-11}$ leads to the system efficiency decreasing from 2% to 0.6%.