A Modeling Study of RED-MED Salinity Gradient Heat Engine: the Conventional Scheme and A Modified Scheme

Reverse electrodialysis - Multi-effect distillation(RED-MED) heat engine has received increasing attention in recent years, due to its ability of converting low temperature waste heat into salinity gradient energy, and then extracting electric power from it. In this work, the RED-MED coupled system was studied with a mathematical model, which was validated by our experimental results. The influences of RED channel length and the feed flow rate on the performance of the coupled system were studied. Furthermore, in the literature, only one of the two streams leaving RED, i.e. either the dilute or the concentrate, is split and partly mixed with another stream before being treated in MED. In this paper, a modified scheme is proposed, in which both the two streams were split, i.e. only a fraction of the concentrate solution was mixed with a fraction of the dilute. The purpose of the modification is to further reduce the total flow rate in MED. After the modification, both the energy efficiency and the heat exchange area requirement of MED increase. The optimum value of the split fraction was discussed. Results imply that while the studies reported in the literature mainly focus on the aspects closely related to the RED section, attention should also be paid to the overall scheme design of the RED-MED coupled system.