Multi-stage reverse electrodialysis: Strategies to harvest salinity gradient energy

Highlights: • Strategies for multi-stage reverse electrodialysis (MSRED) were compared and analyzed theoretically. • Serial control strategy is more feasible than independent control strategy in practice. • Thickening HC solution compartments can enhance the performance of MSRED. • Counter-flow arrangement is better than co-flow arrangement for MSRED. -- Abstract: Reverse electrodialysis heat engine (REDHE) is an effective technology to harvest salinity gradient energy (SGE) driven by low grade heat. In order to improve the energy conversion efficiency of it, one practical way is to enhance the capturing SGE ability of the reverse electrodialysis (RED) unit. To achieve that, the promising multi-stage RED (MSRED) method is proposed. In this paper, two strategies for MSRED, i.e., individually control (Strategy I) and serial control (Strategy II) respectively, were theoretically compared and analyzed. The results showed that Strategy II is more suitable in practice, although its performance is slightly inferior compared with Strategy I. After that, the influence of feed solution velocity, thickness of high concentration solution compartments, concentrations of feed solutions and flow arrangement on the relevant key performance indicators of MSRED in the two strategies were investigated. And results demonstrated that thickening compartments of high concentration solution, increasing the concentration of high concentration feed solution and adopting counter-flow arrangement are all more effective for elevating the performance of MSRED than increasing feed solution velocity. Furthermore, increasing the concentration of low concentration feed solution overly has a significant negative impact on the performance of MSRED.