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Reverse Electrodialysis Heat Engine (REDHE) is a promising technology to convert waste heat at temperatures lower than 100. °C into electric power. In the present work an overview of the possible regeneration methods is presented and the technological challenges for the development of the RED Heat Engine (REDHE) are identified. The potential of this power production cycle was investigated through a simplified mathematical model. In the first part of the work, several salts were singularly modelled as possible solutes in aqueous solutions feeding the RED unit and the corresponding optimal conditions were recognized via an optimization study. In the second part, three different RED Heat Engine scenarios were studied. Results show that power densities much higher than those relevant to NaCl-water solutions can be obtained by using different salts, especially those based on lithium ion (i.e. LiBr and LiCl). Results on the closed loop show efficiencies up to about 15% corresponding to an exergetic efficiency of about 85%, thus suggesting that the RED Heat Engine could potentially be a promising technology, with applications mainly in the industry where low-grade heat that has no alternative use can be converted into electricity

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The coupling of Reverse Electrodialysis with Membrane Distillation is a promising option for the conversion of waste heat into electricity. This study evaluates the performances of the integrated system under different operating conditions, employing validated model and correlations. This work provides a detailed description of the behaviour of a real RED-MD heat engine and indicates the set of inlet concentrations, velocities and equipment size which returns the highest cycle exergy efficiency. These operating conditions were selected for the pilot plant developed within the EU-funded project RED Heat to Power. For the first time, a perspective analysis was also included, considering highly performing RED membranes and future MD module. Relevant results indicate that technological improvements may lead to interesting system performance enhancement, up to an exergy efficiency of 16.5%, which is considerably higher than the values reported in literature so far.

A B S T R A C TSalinity 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.

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