(704g) The Osmotic Heat Engine: The Promise of High Power Density Closed Loop Pressure Retarded Osmosis

Pressure-retarded osmosis (PRO) is an engineered osmosis process that operates at pressures between forward and reverse osmosis.  A highly concentrated draw solution is pressurized, and water from a clean feed source will permeate the membrane and increase the volume of the draw.  As the volume changes at constant pressure, work is being done on the draw solution, and this energy can be harnessed by a hydroturbine.  When operated in closed-loop, PRO is can function as an osmotic heat engine (OHE).  If specially-designed osmotic agents are used, both working fluids can be recovered by using low-grade thermal energy, effectively converting this waste heat to useable electricity.

In this study, a cellulosic 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. The selected conditions simulate OHE operation.  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.