Reducing pumping energy by using different flow rates of high and low concentration solutions in reverse electrodialysis cells

• A constant power region existed in the middle range of flow rates.
• High net power output was obtained in the constant power region.
• Different flow rates were used for high and low salt concentration solutions.
• A lower flow rate can be used for HC than the LC solution in the RED stack.

Energy use for pumping affects both net energy recovery and operational costs of reverse electrodialysis (RED) systems. In order to reduce the energy needed for pumping, electrical performance and hydrodynamic power losses in a RED stack were investigated by simultaneously (2–140 mL/min) or independently varying the flow rates of the high concentration (HC, 35 g/L NaCl) and low concentration (LC, 0.35 g/L NaCl) solutions. Power was not consistently reduced at lower flow rates due to trade-offs between increases in diffusion boundary layer resistance and decreases in solution resistance of the LC channels. The maximum net power output (~0.04 W) was obtained with both LC and HC flow rates at ~20 mL/min. Separately varying the flow rates of the HC and LC solutions indicated that the optimum flow rate of the HC solution (10 mL/min) was much lower than that of the LC solution (20 mL/min) due to the more substantial impact of the LC channel on power production. The use of these two optimized flow rates minimized hydrodynamic power losses (pumping energy) while producing comparable power to that achieved with the two higher flow rates (50 mL/min of both HC and LC solutions).