Finding better draw solutes for osmotic heat engines: Understanding transport of ions during pressure retarded osmosis

The osmotic heat engine (OHE) is a process by which engineered salinity gradients can be utilized as source of energy generation and storage. The OHE collects low grade thermal energy to concentrate a salt solution and then releases that chemical potential energy via pressure retarded osmosis (PRO). The closed loop feature of the OHE allows the user to select any number of solutes for the draw solution so long as they can be thermally regenerated. The potential to generate energy from a particular solute is dependent on the concentration and chemistry of the solute since both diffusivity of the solute and selectivity of the membrane for that solute determine transport. In this work we evaluated three inorganic draw solutes (sodium chloride (NaCl), magnesium chloride (MgCl2) and magnesium sulfate (MgSO4)) on their potentials to be used in OHE. Power densities over 13 W/m2 and 14 W/m2 were achieved with MgCl2 and NaCl respectively. However, NaCl generated much larger salt flux than MgCl2. Substantially lower power densities were achieved by MgSO4. Differences in power density could be partly attributed to differences in hydrated ion radius which had substantial impacts on reverse solute flux and the subsequent development of internal concentration polarization.