Energy and exergy analysis of heat to salinity gradient power conversion in reverse electrodialysis heat engine

Environmental issues have attracted greater concerns, leading to more efforts being devoted to industrial waste heat and renewable energy utilization fields. The reverse electrodialysis heat engine is an emerging method to improve the efficiency of industrial processes by recovering waste heat to generate electricity. Besides, it can also be used to harvest low-grade heat in the renewable energy field, such as solar power, geothermal and ocean thermal energy. Its predominating advantage is to harvest low-grade heat with temperatures below 100 °C, which other power generation techniques can hardly utilize. However, the low efficiency of heat to salinity gradient power conversion and poor property of conventional solution has constrained its application. A cycle using unconventional working fluid LiBr solution is developed and experimentally validated to study the heat to salinity gradient power conversion. Influences of the condensation temperature, generation temperature, temperature of reverse electrodialysis cells, and charge concentration were investigated through energy and exergy analysis. The generation process is divided into two sub-stages during the analysis, namely the solution preheat stage and the solution separation stage. It is concluded that an increase in the energy consumption in the solution separation stage and a decrease in the energy consumption in the solution preheat stage can enhance both the energy and exergy efficiencies of the system. The maximum energy efficiency of 6.05% and exergy efficiency of 56.55% are obtained at respective conditions. Moreover, recovering 49.22% condensate heat for solution preheating would increase the energy efficiency by 173.74% at the basic working condition.