The reverse electrodialysis heat engine (REDHE) has emerged as a promising technique in the waste heat harvest field due to its ability to convert very low-grade waste heat (below 100°C) into power. A REDHE consists of a regeneration unit and a reverse electrodialysis (RED) cell unit, which are in charge of heat to the salinity gradient energy-conversion process and salinity gradient to the power conversion process, respectively. Despite the fact that a regeneration unit accounts for 60% to 70% of the energy destruction of the overall system, most of the research conducted on RED units has barely reviewed the regeneration units. Therefore, the evaluation of regeneration units is necessary. This work provides a review of the research progress made on regeneration units. In this paper, a systematic review of the operating fluids and regeneration methods employed in a regeneration unit is provided. Also, the effects of solvent and solute characteristics on the performance of a regeneration unit and the overall system and the working fluid selection principle are investigated. Additionally, the disadvantages of the methods employed in existing and future development directions are analyzed. Finally, other applications of the heat engine (apart from power production) are presented. Since the RED technology has been primarily used to recover blue energy, its operating fluid is the NaCl aqueous solution. However, the NaCl solution is not suitable for the REDHE technology. Specifically, it is not suitable for the regeneration process of a regeneration unit because the latent heat of water vaporization is huge, which leads to huge energy consumption in the regeneration module. Also, the power generation performance of the NaCl solution is poor, and the maximum power density is only 1/8 of that of the LiBr solution, resulting in the low efficiency of the overall system. Since the REDHE technology is used to recover low-grade heat energy, its regeneration unit adopts a low-temperature separation technology. The main regeneration methods used are distillation separation and membrane distillation separation. The distillation separation technology is the most mature, but it consumes more energy and has a high maintenance cost. On the other hand, the regeneration temperature of the membrane distillation technology is low, but the required technology is difficult to apply, and the cost of the membrane is high. It should be noted that the biggest difference between the REDHE and traditional separation technologies is that the former aims to build an optimal concentration gradient for the battery cells, rather than to perform the complete separation of components. For example, the seawater desalination process assumes that the separated water is pure water. For an RED cell, if the diluted solution is pure water, the battery performance will decline rapidly due to the decrease in the solution conductivity. Currently, more research work has been conducted on various separation technologies and less on the coupling of regeneration technology with power generation technology. Future research should focus on the coupling of the thermal separation module with the power generation module. Although the objective of the REDHE technology is to utilize the industrial waste heat, the heat-source temperature can be very low (down to 40°C), which means that the technology can also be used for low-grade heat recovery such as geothermal and solar energy. At the same time, although the technical efficiency of the REDHE is low, its power generation process is accompanied by electrode reaction and temperature change. This means that during power generation, the REDHE can also be used for sewage treatment, organic matter degradation as well as waste acid and alkali recovery.