Non-Single Charge Ions Effect on Reverse Electrodialysis Technology for Salinity Gradient Power Generation

Abstract

Salinity Gradient Power (SGP), also known as Blue Energy, is gaining attention in the recent years due to its global availability and renewable nature [1,2]. SGP is based on the physico-chemical potential existing when two water streams of different salinity are mixed. The most important and well-known technology to harvest the SGP energy is the Reverse Electrodialysis (RED), which converts the blue energy into electricity through ion exchange membranes (IEMs). Usually, seawater has been targeted as high concentrated solution and river water as low concentrated solution. Nevertheless, other water bodies are possible, for example, industrial brine solutions, groundwater or treated urban wastewater streams. The presence of the non-single charge ions presents in common water streams can affect power performance due to an increase in the internal resistance of the system [3,4], and thus their effects need to be thoroughly studied.This work aims to model the RED performance providing a robust and comprehensive mathematical tool to predict power output under real conditions. The model takes into account different variables (flowrate and temperature), the RED system characteristics (effective area, thickness of both membranes and spacers and number of cell pair) as well as different scenarios in term of streams composition. In this sense, the concentration of the main compounds presents in the typical water sources was studied achieving different correlations as function of the cationic or anionic type of membrane.For this purpose, electrochemical impedance spectroscopy (EIS) technique was employed to measure the resistance of IEMs in contact with NaCl solution including mixtures of non-single charge ions (Ca2+, Mg2+, SO4 2-) in order to find suitable mathematical correlations to be integrated into the mathematical model [1]. EIS measurements were performed by placing the membranes in a nylon cell system previously submerged in the different solutions for seven days to ensure impregnation.The results obtained were validated with different experimental conditions in terms of water composition. Theoretical and experimental results fit well with errors below 5%, showing that the presence of divalent ions has a negative effect on power performance due to the increase in membrane resistance in comparison to pure NaCl solutions. Nevertheless, the maximum decay in power output, for a wide range of real scenarios studied, was not higher than 16%, which can still be considered acceptable. Acknowledgements: The authors want to acknowledge financial support from the Spanish Ministry of Economy and Competitiveness through the projects: CTQ2015-66078-R, and CTM2017-87850-R. This research is also being supported by the Project “HYLANTIC”-EAPA_204/2016, which is co-financed by the European Regional Development Fund in the framework of the Interreg Atlantic program.