The reverse electrodialysis (RED) stack-harnessing salinity gradient power mainly consists of ion exchange membranes (IEMs). Among the various types of IEMs used in RED stacks, pore-filling ion exchange membranes (PIEMs) have been considered promising IEMs to improve the power density of RED stacks. The compositions of PIEMs affect the electrical resistance and permselectivity of PIEMs; however, their effect on the performance of large RED stacks have not yet been considered. In this study, PIEMs of various compositions with respect to the RED stack were adopted to evaluate the performance of the RED stack according to stack size (electrode area: 5 × 5 cm² vs. 15 × 15 cm²). By increasing the stack size, the gross power per membrane area decreased despite the increase in gross power on a single RED stack. The electrical resistance of the PIEMs was the most important factor for enhancing the power production of the RED stack. Moreover, power production was less sensitive to permselectivities over 90%. By increasing the RED stack size, the contributions of non-ohmic resistances were significantly increased. Thus, we determined that reducing the salinity gradients across PIEMs by ion transport increased the non-ohmic resistance of large RED stacks. These results will aid in designing pilot-scale RED stacks.

Reverse electrodialysis (RED) generates electricity from a mixture of seawater and river water. Herein, patterned membranes consisting of ultra-thin pore-filling membranes (16-μm thick) were used to determine whether the RED system operates steadily when using natural underground seawater and sewage effluent and if the membranes become polluted by various foulants. The flat stack performances, comprising flat membranes and woven-type spacers, were compared with those of the pattern stack, comprising patterned membranes with mirror-imaged wavy lines. The pattern stack clearly reduced the pressure drop and maintained the power within 40% of the initial value, and the flat stack significantly increased to 3 bar inside the sewage effluent and decreased the power to 20% of the initial value. Both anion and cation exchange-surface membranes showed organic fouling and scaling, with more significant fouling in the flat stack. The patterned membranes used here provide a powerful solution to reduce fouling inside RED stacks.