Continuous Selective Reverse Electrodialysis Process Based on Salt-Lake Brines and Its Thermodynamic Analysis

Salt-lake brines contain a large amount of clean and sustainable salinity gradient energy (SGE), which can be theoretically converted into electric energy through reverse electrodialysis (RED). Due to the weak infrastructure, fragile ecological environment, and harsh environmental protection requirements in the salt-lake areas, it is very necessary to utilize the SGE contained in salt-lake brines to make effective supplements for energy. In this work, we introduced a continuous selective RED (SRED) process based on salt-lake brines to realize the conversion of SGE to electric energy. A lab-scale SRED stack was utilized to investigate the effects of operation parameters systematically, and thermodynamic analysis was conducted to find the main sources of irreversibility in the process under a constant discharging current density. Results show that when the salinity ratio between high-salinity feeds (HSFs) and low-salinity feeds (LSFs) is highest as 300, the open-circuit voltage (OCV) and the stack electrical resistance are highest as 0.976 V and 11.82 Ω, which results in the lowest maximum power density (Pd,max) (0.101 W m–2), exergy efficiency (ηT) (2.12%), and energy efficiency (Y) (0.21%). Although increasing the Mg2+ concentrations in feeds can reduce the OCV slowly, the stack electrical resistance reduces from 28.35 to 11.82 Ω dramatically; hence, Pd,max increases from 0.060 to 0.101 W m–2 inversely. However, ηT and Y decrease from 12.69 to 2.12% and from 0.83 to 0.21% over the Mg2+ concentrations in feeds, respectively. Moreover, the Mg2+ concentration in the LSF of 81.02 ppm can result in the lowest stack electrical resistance (7.93 Ω) and the highest Pd,max (0.122 W m–2); increasing the Mg2+ concentration in LSFs to 139.30 ppm can increase ηT to 6.21%. In addition, the results of the cations’ concentration variations in LSFs indicate that forced discharging can offset the uphill transport effect of multivalent ions. This work provides some useful and meaningful guidance for the industrialization of the salt-lake brine-based continuous SRED process.