The Promise of High Power Density Closed Loop Pressure Retarded Osmosis Via the Osmotic Heat Engine

Pressure-retarded osmosis (PRO) is a means by which the energy contained within salinity gradients can be captured and converted into useable energy via a hydroturbine. Saline water solutions can be used as a means of energy storage, and this stored energy can be released using a closed-loop PRO process known as an osmotic heat engine (OHE). In this study, a cellulosic FO membrane was evaluated in closed-loop at a variety of temperatures, draw solution concentrations, and hydraulic pressures to determine how these parameters impact the flux of water and salt through the membrane. Furthermore, the impact of these test conditions on membrane power density, and thus the maximum energy captured per unit of membrane area, was also examined. While the membranes were only tested up to 21 bar, it is clear that increasing salt concentration increases peak power density. This observation is appropriate with respect to theory. The experimental results also suggest that for constant draw concentration, increasing temperature or decreasing hydraulic pressure will increase water flux and power density. Under these same conditions, salt fluxes were reduced by up to a factor of three, suggesting that significant external concentration polarization is impacting the performance of the membrane. Maximum power density seen throughout the trial was 18 W/m², which was seen at 40 C with a 1.5 M sodium chloride draw solution.