Thermodynamic characterisation of novel solutions for closed-loop reverse electrodialysis

Abstract

Closed-loop reverse electrodialysis is a novel technology to convert low-grade heat directly into electricity. It consists of a reverse electrodialysis unit coupled with a regeneration unit in which the waste-heat is used to restore the initial conditions of the two solutions. One of the most impor- tant advantages of closed-loop system is the possibility to select an ad-hoc salt solution to obtain high efficiency. In this regard, it is important to assess how the salt solution properties affect the performance of the energy generation and solution regeneration processes. The aim of this study is to analyse the effect of thermodynamic properties of non-conventional salt solutions within a RED closed-loop with evaporative regeneration unit. New data for caesium and potassium acetate, in terms of activity and osmotic coefficients, in aqueous solutions at tem- perature between 20 and 90°C are reported as a function of molality. The data are correlated using Pitzer’s model which is then used to assess the performance in terms of Gibbs free energy of mix- ing, thermal power consumptions, thermal and exergy efficiency for different salt-water solutions (i.e. sodium chloride, lithium chloride, sodium acetate, caesium acetate and potassium acetate) are evaluated and compared considering single and multi-stage regeneration units. Results indicated that lithium chloride, potassium acetate and caesium acetate are the most promising salts among those screened.