Development of Binary-Salt Working Solutions for Improving the Performance of Reverse Electrodialysis

Abstract

Reverse electrodialysis (RED) is a promising technology for harvesting salinity gradient energy. However, the limited power density achievable with traditional working solutions has impeded further development of RED-based systems like reverse electrodialysis heat engine (REDHE). This study aimed to develop innovative binary-salt working solutions to enhance RED performance and overcome this bottleneck. Four combinations of monovalent salts, formed by pairing a high-activity-coefficient (HAC) salt with a high-solution-conductivity (HSC) salt, were evaluated based on their behavior in RED. Among these salt combinations, the LiCl-NH4Cl solution exhibited the highest maximum power density (Pd,max) and was selected for further investigation. Subsequently, experiments determined that the optimal molar ratio of LiCl to NH4Cl was 2:8, yielding a Pd,max of 9.16 W·m-2, approximately 26.5% higher than that for the equimolar NaCl solution. Finally, the detailed behavior of LiCl-NH4Cl solution with the optimal salt ratio in RED under various conditions was investigated. The results demonstrated consistently higher Pd,max of RED for the LiCl-NH4Cl solution compared to that for equimolar NaCl solution, due to its enhanced conductivity which significantly reduces the resistance of RED. Especially at a temperature of 45 °C, the Pd,max of RED reached 11.87 Wm-2, demonstrating superior performance potential for replacing traditional solutions and enhancing the overall efficiency of REDHE.

Keywords: Reverse electrodialysis, Binary-salt, Working solutions, Salinity gradient energy, Power Density