Experimental assessment of reverse electrodialysis in closed loop configuration fed by NH4HCO3-water solutions

Closed loop reverse electrodialysis is a novel technology for converting low-temperature waste heat into electric energy. This innovative heat engine consists of two units: (i) a reverse electrodialysis unit where power is produced exploiting the concentration difference between a diluted and a concentrated salt solution and (ii) a thermally driven regeneration unit where low-grade waste heat is used to re-establish the original salinity gradient between the two salt solutions. Among all the possible salt solutions suitable as working fluid in such application, Thermolytic salts solutions, in particular Ammonium bicarbonate solutions, may be promising thanks to their characteristic thermally degradation properties. The thermolytic salt ions dissolved in water are converted into ammonia and carbon dioxide when heated up above a certain temperature (around 60°C). Then, dissolved thermolytic salt can be removed nearly completely from the dilute solution by means of a thermal desorption process and absorbed again in the concentrate solution, thus restoring the salinity gradient exploited by the reverse electrodialysis unit. In this work, for the first time a lab scale prototype of a Closed-loop reverse electrodialysis fed by ammonium bicarbonate aqueous solutions was built and tested. The proposed prototype consists of a conventional reverse electrodialysis unit and a regeneration unit constituted by (i) a vapour stripping column and (ii) a barometric column. The process performances in terms of power output, thermal power consumption, thermal and exergy efficiency were experimentally assessed. Also the influence of operating conditions, e.g. solutions concentrations was investigated. First experimental results demonstrated the feasibility of the process.