Modelling Reverse Electrodialysis process via Exergy Analysis

Salinity Gradient Power Heat Engines (SGP-HEs) represent a novel technology to convert low grade waste heat into electricity. Reverse Electrodialysis Heat Engine (REDHE) is one of the possible application of this concept, where a common RED unit is coupled with a thermal regeneration unit supplied with waste heat to restore the salinity gradient of the streams to be fed back to the RED unit. In a RED unit, anion and cation exchange membranes (AEMs and CEMs) are alternatively stacked and interposed between salt solutions at different concentration generating an electric potential difference over each membrane along with a selective transport of cations and anions from the concentrated solution to the diluted solution. The generated ionic current is converted by redox reactions into electric current at two electrodes placed at the end of the membrane pile. The performance of a RED unit may be reduce by some detrimental phenomena as (i) osmotic flux, (ii) salt flux and (iii) co-ions flux (due to a membrane permselectivity lower than one). The present work is devoted to identifying the impact of these irreversibility on the RED unit performance via Exergy Analysis: it represents a useful tool to recognize and quantify exergy destruction. The assessment of exergy destruction due to irreversibility occurring within a component provides fundamental information on the bottlenecks of the process, identifying possible design and operative improvements. RED exergy analysis outputs will be crucial for a reliable design of the whole heat engine. Exergy analysis was carried out by developing an ad-hoc distributed parameter model which calculates all chemical exergy flows by taking into account all beneficial and detrimental phenomena occurring within the RED process. Efficiency indicators including exergy efficiency and dissipation are suitably defined to quantify how much each phenomenon affects process performance. The effect of different operating conditions (i.e. external load, feed solution concentrations and flow arrangement) on power generation and performance indicators were investigated.