Reverse electrodialysis (RED) is a technology to generate electricity using the entropy of the mixing of sea and river water. A model is made of the RED process and validated experimentally. The model is used to design and optimize the RED process. It predicts very small differences between counter- and co-current operation. It was decided to focus on co-current design because co-current operation causes smaller local pressure differences between the river and seawater compartments-hence smaller risk of leakages and the possibility to use very thin membranes with high fluxes and very open spacer structures with low hydrodynamic resistance. Segmentation of the electrodes proved to increase the power density by about 15% under realistic operational conditions. The model shows that with smaller systems - in terms of length of the flow path - higher power densities are possible. This effect is rather dramatical and poses a challenge for designing improved RED stacks on large commercial scale. It is suggested to reduce the flow path length by applying a fractal structure design of the spacers. Such structures can be made by profiling the membrane. (C) 2010 Elsevier B.V. All rights reserved.