Conductive reverse electrodialysis for low-salinity waters

Introduction. Salinity gradient energy (SGE) originates predominantly in estuaries, where freshwater rivers meet seawater. The energy available from the mixing of these solutions results from the difference in concentration between them. While oceanic waters typically maintain similar salt concentrations, those confined within semi-enclosed basins exhibit variations influenced by the prevailing conditions of evaporation, precipitation, and freshwater influx. As an example, the salinity of the Baltic Sea is notably lower than the average seawater salinity of 3.5%. Reverse electrodialysis (RED) stands as a membrane-based electrochemical method for harnessing SGE, offering a controlled and preferential ion transfer environment. Nonetheless, its efficacy is restricted by the elevated resistance encountered in feed solutions due to their lower salt content. This study focuses on four estuaries in Sweden—three from the Baltic Sea and one from the North Sea—to assess the actual potential of SGE through RED. To address the challenge posed by high resistance, non-conductive spacers within the river water channels were replaced with ion exchange resins. Experimental/methodology. The selection of estuaries was meticulously conducted following a thorough assessment of rivers across Sweden, taking into account their geographical positioning, salinity gradients, ionic composition, and concentration levels. To replicate the properties of river and seawater solutions accurately, salts representing over 98% of the real ionic content were employed. Additionally, sodium chloride equivalent solutions were prepared as a benchmark. In the case of cRED (conductive RED), non-conductive spacers were replaced with Amberchrome 50 WX8 resins. Polarization curves of both RED and cRED were analyzed to characterize stack resistance, open circuit voltage, and both gross and net power density. Results and discussion. The characterization of RED revealed that the generation of gross power density is rather constrained when considering entire Sweden. Furthermore, when accounting for the pump power, it proves unfeasible to achieve positive net energy from any of the estuaries studied. However, the adaptation of conventional RED to cRED demonstrated promising outcomes. The reduction of resistance in river water through the incorporation of ion exchange resin notably boosted both gross and net power density. This study not only highlights the potential applications of low-salinity water in Sweden but also pioneers the expansion of SGE-RED applications to other locations with comparable conditions. Acknowledgments The authors would like to express their appreciation for the financial support of Swedish Energy Agency, Sweden (ref. 51675-1).