Evaluation of energy harvesting from discharged solutions in a salt production plant by reverse electrodialysis (RED)

Reverse electrodialysis (RED) is a promising membrane-based process to harvest energy based on the salinity gradient between two solutions. In the RED process, cation exchange membranes (CEM) and anion exchange membranes (AEM) stack alternatively between high and low concentrate compartment (HCC & LCC) which lead to migration of the cations and anions in opposite directions, which can then be converted into electric current by redox reactions at the electrodes. Seawater and river water are typically considered as the high and low concentrate feed solution, respectively, in the RED process. However, a salt production plant has great potential as a source of RED feed solutions because it has a variety of solutions with different salinities. In this study, a typical salt production plant equipped with electrodialysis (ED) and an evaporation system was evaluated for harvesting energy by a RED process. This salt plant has five solutions at different flow rates, ion compositions and concentrations. Both standard CMX/AMX membranes as well as one-sided monovalent selective CIMS/ACS-8T membranes were applied to investigate the effect of divalent ions on RED performance. In this study, we propose the best feed solution combination for producing the maximum possible power from a salt production plant by means of RED. Hence, different membrane potentials, resistance measurement, and RED tests were performed. Generally, the standard membranes showed better RED performance when low amounts of divalent ion were present (< 10%) in the RED feed solutions. In addition, one-sided monovalent selective membrane with the selective layer facing the HCC showed 30% higher performance than that with selective layer facing into the LCC. We also concluded that the properties of low concentrate solution are one of the most important factors in RED performance.