Reverse electrodialysis heat engine (REDHE)

Abstract Reverse electrodialysis (RED) is a membrane technology for the production of electricity via the “controlled mixing” of solutions at different salt concentrations, i.e., a diluted solution and a concentrated solution. The presence of ion-exchange membranes (IEMs) allows the production of renewable energy by converting the salinity gradient, which would be dissipated during a spontaneous (i.e., uncontrolled) mixing process, into an ionic current and, in a second step, into electricity at the electrodes. RED is the inverse process of the well-known electrodialysis process for salty water desalination, in which an electric field is applied at the electrodes and ionic currents are generated within the stack, thanks to the application of IEMs, to produce fresh water. The first section focuses on the fundamentals of the RED technology, particularly membrane properties, fluid dynamics, and electronic systems. The following sections concern the coupling of the RED unit with regeneration units in a closed-loop fashion to produce electricity from waste heat. The development of these closed systems called RED heat engines is described according to the combinations of different technologies. The choice of a closed loop, where the salt solutions are regenerated and reused cyclically, broadens the range of viable working solutions and operating conditions. In particular, solutions different from natural seawater or river water can be used and some studies have focused on the impact of different concentrations or even different salts on the performance of RED. Finally, the last section reports the state of the art of RED heat engines and the relevant experimental and modeling studies published so far.