Assessment of alternative draw solutions for optimized performance of a closed-loop osmotic heat engine

Osmotic power harnesses the energy of mixing between high and low salinity streams. The osmotic heat engine (OHE) is a closed-loop, membrane-based power generation cycle that couples pressure retarded osmosis (PRO), an osmotically driven membrane process, with a thermal separation process. In this investigation, membrane distillation (MD), a thermally driven membrane process, was used. High power density in PRO is essential to minimize equipment costs and parasitic pumping losses. Likewise, high water flux is needed in MD to efficiently reconcentrate the diluted draw solution from the PRO process and minimize equipment costs. In this study, several ionic organic and inorganic draw solutions were evaluated as working fluids in the OHE. Their performance was assessed in terms of PRO power density and reverse solute diffusion, and MD water flux and thermal efficiency. Potential pore wetting of the MD membrane was also evaluated. The working fluids were also assessed in terms of their potential for equipment corrosion. Results indicate that sodium formate and CaCl₂ outperform NaCl (commonly used PRO draw solution) in terms of PRO power density and reverse solute diffusion, and that LiCl and CaCl₂ outperform NaCl in terms of MD water flux. Furthermore, there were no signs of MD membrane wetting, even at high feed concentrations. Results were used to perform an economic analysis and make future recommendations on the most suitable working fluid for the OHE. Of the select salts, CaCl₂, MgCl₂, sodium propionate, and LiCl resulted in the lowest OHE electricity generation costs and had the lowest potential for corrosion.