Investigating the effect of temperature and concentration on the performance of reverse electrodialysis systems

Reverse electrodialysis (RED) is a membrane-based technology proposed for harvesting electricity from a salinity gradient. In this study, a detailed examination of the effect of temperature and concentration on RED membrane properties is undertaken. Modelling of the co-ion concentration as a function of temperature allows the Donnan potential to be estimated, while membrane resistance and ion diffusivity can be modelled by an Arrhenius relationship with temperature. Membrane resistance is best modelled using a reciprocal relationship with concentration, while permeability and diffusivity show a power law dependence upon concentration. Using these results, the effect of increasing the temperature of the entire RED system is compared to the case where only the temperature of the diluate solution is increased. The model is in good agreement with experimental results across temperatures from 10 to 40 °C. It shows that a comparable increase in gross power density can be achieved when only the temperature of the diluate solution is increased, relative to increasing the temperature of the entire system. However, increasing the temperature also reduces the pumping energy required for each stream. In the present case, this reduction in pumping energy with temperature is more significant than the changes within the stack itself. Thus, an increase in temperature of the entire system from 20 to 40 °C results in a 27 % increase in net power density as compared to an increase of the diluate solution alone. It is thus concluded that increasing the temperature of the entire system (rather than just the diluate stream) provides a pathway to increasing the power density of the system if waste heat is available, promoting its adoption for energy harvesting.