Impact of temperature on power density in closed-loop pressure retarded osmosis for grid storage

Abstract

Closed-loop pressure retarded osmosis (PRO) has been recently proposed as a means of transforming unusable forms of energy, such as waste heat, into valuable electricity. The process, which is also referred to as an osmotic heat engine (OHE), also enables a form of osmotic grid storage for intermittently available renewable energy sources, where available energy is stored as an osmotic potential and that energy is released via PRO when energy demand is high. The OHE has the potential to generate greater power than conventional open loop PRO because the draw solution can be engineered to have very high osmotic pressures, via enhanced temperature, solute concentration, or a combination of both. These variables change fluid properties and the performance of the membrane, which may or may not be beneficial to overall OHE operation. Using a custom-built, bench-top PRO system, a commercially available forward osmosis membrane from Hydration Technology Innovations™ (HTI) was evaluated for water flux and power density at two temperatures (20°C and 40°C) and three draw solution concentrations (0.5, 1.0, and 1.5M sodium chloride) that are similar to temperatures and draw solution osmotic pressures capable in an osmotic heat engine. In general, power densities increased with the increasing draw solution concentration and system temperature. The highest observed power density (18.0±2.3W/m2) was measured at 20.7bar (300psi) using a 1.5M sodium chloride draw solution at a system temperature of 40°C. Experimental data compared favorably to predicted performance using previously published governing equations for PRO water flux and power density.