Heat to Hydrogen by Reverse Electrodialysis-Using a Non-Equilibrium Thermodynamics Model to Evaluate Hydrogen Production Concepts Utilising Waste Heat

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 degrees 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(H2), m(-2) h(-1) and a waste heat requirement of 1.7 kWh g(H2)(-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) = [0-10] 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%.