Salt screening analysis for reverse electrodialysis

Abstract

As a renewable energy conversion technology, reverse electrodialysis (RED) can be employed for generating electricity, where two solutions with different salt concentrations are mixed together from membrane-separated streams. In addition to the concentration gradient of streams and membrane characterization, the type and nature of the components dissolved in the streams play an important role in electricity generation by RED. Although studies on the effect of salt type on the performance of RED are available, there is a lack of comprehensive analysis related to salt selection for RED applications. In this paper, a model for screening salt solutions, including 174 monovalent and multivalent ions, is developed. Salts for RED were screened using theoretical open-circuit voltage (OCV_th) calculations and compared with a reference salt (NaCl). The activity coefficients of salts in different concentration ranges were calculated using the Bromley model. The effects of membrane properties, including permselectivity and membrane resistance, on the performance of a RED cell, were analyzed. Furthermore, a model for hazard assessment of salts was employed to provide an analysis for the selection of salts with minimum environmental, health, and physicals hazards. Moreover, the effects of ionic properties, including charge density, ionic radius, and hydration energy, on the theoretical OCV were investigated. Between the investigated salts, potassium formate (KHCO₂), sodium bromide (NaBr), potassium acetate (KC₂H₃O₂), and sodium acetate (NaC₂H₃O₂) were found to be promising salts for RED applications due to high theoretical OCV, low hazard potential, and low cost compared to NaCl.