(725b) Design & Construction of a Pilot-Scale Osmotic Heat Engine

Pressure retarded osmosis (PRO) is a method used to capture the energy stored in salinity gradients.  While open-loop PRO using freshwater and seawater has been considered, using a dilute feed solution and highly concentrated engineered draw solution, a closed loop has been shown to yield higher power densities.  This closed-loop process, known as the osmotic heat engine (OHE), can produce electricity while using low-grade or waste heat to recover the draw solution concentration.  Applications of such a technology include waste heat recovery and grid storage for intermittent renewable energy sources.

The majority of previous work on the OHE has been largely theoretical, but in this study a pilot-scale OHE was constructed using an ammonia-carbon dioxide draw solution.   A previous study was conducted on the bench-scale to understand the power density of membranes at elevated temperature similar to OHE conditions.  A spiral-wound membrane element was used for the membrane step, with draw solution pressures ranging from 0 to 20 bar.  Diluted draw solution was sent to a bank of falling-film ammonia strippers and condensers to recover the concentration of the draw.  Both streams used a nitrogen sweep gas to remove ammonia in the stripping section while providing ammonia to the condenser.  Altering the temperatures of the heating and cooling fluids could also dictate the overall temperature of the OHE.  Actual stripping effectiveness will be compared to model-based predictions.  This platform allows for future optimization of the OHE for overall energy efficiency.