Study on Key Parameters of Reverse Electrodialysis (RED) Cell-Stack Performance Based on Equivalent Circuit Model

Reverse electrodialysis (RED) is a direct process to convert salinity-gradient energy to electricity utilizing ion exchange membranes. The current density of a unit cell based on normal area of ion exchange membrane is well related to the membrane resistances, channel structure, flowrates and salinities of dilute and concentrate water feed. Alternate stacking of cation and anion exchange membranes enables directional control of ion migration relaxing the concentration gradient, and it increases the voltage in proportion to stack number. High salinity of concentrate water often causes parasitic currents between adjacent cells, and this complicates the power charateristics of a RED cell stack. In order to evaluate the effects of key parameters and to optimize the efficiency of energy harvesting, various equivalent circuit models are developed for typical structures of RED cell stacks. Input parameters are resistances of membranes and electrodes, structure of flow channels, stack number, and salt concentrations. The electrical potential across a membrane is calculated from Nernest equation and used as electromotive force for each unit cell. Output such as power density, short-circuit current, open-circuit voltage, and the distribution of parasitic currents can be estimated by the model. The internal resistance of a cell stack, which is most affected by dilute water concentration, is the critical parameter influencing the power efficiency. Parasitic currents that degrade the overall power characteristics were also considered. Those effects evaluated by the model agree with experimental measurements with varying concentrations, the thickness of water channels, and stack number. Figure 1