Salinity gradient power (SGP) represents a viable renewable energy source associated with the mixing of two solutions of different salinities. Reverse electrodialysis (SGP-RE or RED) is a promising technology to exploit this energy source and directly generate electricity. However, although the principle of this technology is well known since several years, further R&D efforts are still necessary in order to explore the real potential of the SGP-RE process. With this regard, the aim of the REAPower project (www.reapower.eu) is the development of an innovative system for power production by SGP-RE process, using sea (or brackish) water as a diluted solution and brine as a concentrate. The use of sea or brackish water (instead of fresh water) as diluate allows reducing the electrical resistance of the diluate compartment and increasing the achievable output power. This work presents the R&D activities carried out so far within the REAPower project, particularly focusing on the relevant progresses in membranes development, stack design and process modelling. An extensive experimental campaign has been performed on a lab-scale unit, allowing to reach a power density among the highest, so far presented in the open literature. These results provided useful information for the final goal of the project, i.e. the construction of the first SGP-RE system on a small pilot-scale, in order to demonstrate the feasibility of the future scale up for this technology.

In the examined heat engine, reverse electrodialysis (RED) is used to generate electricity from the salinity difference between two artificial solutions. The salinity gradient is restored through a multi-effect distillation system (MED) powered by low-temperature waste heat at 100 °C. The current work presents the first comprehensive economic and environmental analysis of this advanced concept, when varying the number of MED effects, the system sizing, the salt of the solutions, and other key parameters. The levelized cost of electricity (LCOE) has been calculated, showing that competitive solutions can be reached only when the system is at least medium to large scale. The lowest LCOE, at about 0.03 €/kWh, is achieved using potassium acetate salt and six MED effects while reheating the solutions. A similar analysis has been conducted when using the system in energy storage mode, where the two regenerated solutions are stored in reservoir tanks and the RED is operating for a few hours per day, supplying valuable peak power, resulting in a LCOE just below 0.10 €/kWh. A life-cycle assessment has been also carried out, showing that the case with the lowest environmental impact is the same as the one with the most attractive economic performance. Results indicate that the material manufacturing has the main impact; primarily the metallic parts of the MED. Overall, this study highlights the development efforts required in terms of both membrane performance and cost reduction, in order to make this technology cost effective in the future.