Effect of different aqueous solutions of pure salts and salt mixtures in reverse electrodialysis systems for closed-loop applications

Reverse Electrodialysis (RED) in a closed-loop arrangement is a viable way to convert low-grade heat into electric power. The present work experimentally investigates the use of pure salt- and equimolar two salts-water solutions as feeds in a lab-scale RED unit. RED performances were analysed in terms of Open Circuit Voltage (OCV), stack resistance and corrected power density. The pure salts and the mixtures employed were chosen via a computational analysis. Effect of feed solution velocity and concentration was investigated. Results concerning the pure salt-water experiments show that NH4Cl is the most performing salt in the concentration range probed, while higher power density values are expected with the use of LiCl at larger concentrations. As regards the salt binary mixtures, in some cases, the measured stack electrical resistance was found lower than both the two values measured for the corresponding pure salts, thus resulting into higher power density values for the mixtures. This surprising experimental evidence suggests that it is possible to increase the power produced by a conventional RED unit by adding an equivalent molar quantity of another suitable salt. Finally, among the mixtures tested, the NH4Cl-LiCl mixture appears as the most promising, thanks to the combination of the favorable properties of these two salts.