Water-organic systems in closed-loop reverse electrodialysis for lower regeneration requirements

Abstract

Salinity Gradient Power (SGP) is an emerging opportunity to produce sustainable energy. Among the different SGP technologies, Reverse ElectroDialysis (RED) looks very promising. In a RED system the salinity gradient between two different solutions is used to directly produce electricity. Closed-loop RED is an innovative process, combining a RED unit with a regeneration system. If low grade waste-heat is used for the regeneration stage, in which the initial concentrations of the solutions exiting the RED unit are restored, the closed-loop can be seen as a RED-heat-engine able to convert thermal energy into electricity. The aim of this work is to perform a preliminary analysis of a regeneration unit based on an evaporative process, in which water-organic systems are used as solvent solution. The use of organic solvents with water, in fact, results in a reduction of the specific energy consumptions and/or the evaporation temperature compared to the “standard case” in which only water is adopted. This reduction can allow the use of very low temperature heat sources. In this work results are reported for the case of different organic-water systems with a different liquid vapour equilibrium behaviour: (i) water-organic heterogeneous azeotropic systems, (ii) water-organic homogeneous azeotropic systems and (iii) water-organic miscible systems. Advantages and disadvantages of the three different scenarios are highlighted in terms of performance enhancement for the regeneration stage as well as in terms of effect of organic solvents presence within the RED unit.