Reverse electrodialysis heat engine with helium-gap diffusion distillation: Energy efficiency analysis

The depletion of energy resources poses a significant threat to the development of human society. Specifically, a considerable amount of low-grade heat (LGH), typically below 100 °C, is currently being wasted. However, efficient utilization of this LGH can relieve energy shortages and reduce carbon dioxide emissions. To address this challenge, reverse electrodialysis heat engine (REDHE) which can efficiently convert LGH into electricity has emerged as a promising technology in recent years. Extensive efforts have been dedicated to exploring more suitable thermal distillation technologies for enhancing the performance of REDHE. This paper introduces a novel REDHE that incorporates helium-gap diffusion distillation (HGDD) as the thermal separation (TS) unit. The HGDD device is highly compact and efficient, operating at a normal atmospheric pressure, which aligns with the operational conditions of the reverse electrodialysis (RED) unit. A validated mathematical model is employed to analyze the impacts of various operating and structural parameters on the REDHE performance. The results indicate that maintaining a moderate molality of the cold stream, elevating the inlet temperatures of hot and cold streams, lengthening hot- and cold-stream channels, and minimizing the thickness of helium gaps contribute to improving the REDHE performance. Especially, a maximum energy conversion efficiency of 2.96% is achieved by the REDHE when decreasing the thickness of helium gaps to 3 mm and increasing the length of stream channels to 5 m.