Power generation using profiled membranes in reverse electrodialysis

Reverse electrodialysis (RED) is a technology to obtain energy from the salinity difference between salt water and fresh water. Traditionally, ion exchange membranes, separated by non-conductive spacers, are used in this technology. As an alternative for these non-conductive spacers, in this work, heterogeneous ion exchange membranes were hot pressed in the dry state to create a profiled membrane comprising 230–245 μm ridges (in wet state) on one side of the membrane. Stacking such profiled membranes creates channels for the feed water, thus make the use of spacers obsolete. The performance of a RED-stack with such profiled membranes was compared for the first time with a RED-stack with traditional, non-conductive spacers. The ohmic resistance was significantly lower for the stack with profiled membranes compared to that with spacers, whereas the boundary layer resistance was higher. This resulted in slightly higher power densities for the stack with profiled membranes. Despite this small improvement, profiled membranes have a strong future development potential. Experimental data show that the hydraulic friction is much lower for the stack with profiled membranes and hence higher Reynolds numbers are possible than in a stack with spacers. Furthermore, profiling membranes allow much freedom to create new profile geometries where a hydrodynamic flow can be combined with efficient mixing in the boundary layers.

► Profiled ion exchange membranes make the use of spacer obsolete in reverse electrodialysis.
► The ohmic resistance is lower using profiled membranes compared to spacers, whereas the boundary layer resistance is higher.
► The hydraulic resistance is much lower using profiled membranes compared to spacers.
► The (net) power density obtained for stacks with profiled membranes is higher than that obtained for stacks with spacers.