Continuous Selective\nReverse Electrodialysis Process\nBased on Salt-Lake Brines and Its Thermodynamic Analysis

Salt-lake brines contain a large amount of clean and\nsustainable\nsalinity gradient energy (SGE), which can be theoretically converted\ninto electric energy through reverse electrodialysis (RED). Due to\nthe weak infrastructure, fragile ecological environment, and harsh\nenvironmental protection requirements in the salt-lake areas, it is\nvery necessary to utilize the SGE contained in salt-lake brines to\nmake effective supplements for energy. In this work, we introduced\na continuous selective RED (SRED) process based on salt-lake brines\nto realize the conversion of SGE to electric energy. A lab-scale SRED\nstack was utilized to investigate the effects of operation parameters\nsystematically, and thermodynamic analysis was conducted to find the\nmain sources of irreversibility in the process under a constant discharging\ncurrent density. Results show that when the salinity ratio between\nhigh-salinity feeds (HSFs) and low-salinity feeds (LSFs) is highest\nas 300, the open-circuit voltage (OCV) and the stack electrical resistance\nare highest as 0.976 V and 11.82 Ω, which results in the lowest\nmaximum power density (<i>P</i><sub>d,max</sub>) (0.101\nW m<sup>–2</sup>), exergy efficiency (η<sub>T</sub>)\n(2.12%), and energy efficiency (<i>Y</i>) (0.21%). Although\nincreasing the Mg<sup>2+</sup> concentrations in feeds can reduce\nthe OCV slowly, the stack electrical resistance reduces from 28.35\nto 11.82 Ω dramatically; hence, <i>P</i><sub>d,max</sub> increases from 0.060 to 0.101 W m<sup>–2</sup> inversely.\nHowever, η<sub>T</sub> and <i>Y</i> decrease from\n12.69 to 2.12% and from 0.83 to 0.21% over the Mg<sup>2+</sup> concentrations\nin feeds, respectively. Moreover, the Mg<sup>2+</sup> concentration\nin the LSF of 81.02 ppm can result in the lowest stack electrical\nresistance (7.93 Ω) and the highest <i>P</i><sub>d,max</sub> (0.122 W m<sup>–2</sup>); increasing the Mg<sup>2+</sup> concentration\nin LSFs to 139.30 ppm can increase η<sub>T</sub> to 6.21%. In\naddition, the results of the cations’ concentration variations\nin LSFs indicate that forced discharging can offset the uphill transport\neffect of multivalent ions. This work provides some useful and meaningful\nguidance for the industrialization of the salt-lake brine-based continuous\nSRED process.