Membrane-Based Osmotic Heat Engine with Organic Solvent for Enhanced Power Generation from Low-Grade Heat

We present a hybrid osmotic heat engine (OHE) system that uses draw solutions with an organic solvent for enhanced thermal separation efficiency. The hybrid OHE system produces sustainable energy by combining pressure-retarded osmosis (PRO) as a power generation stage and membrane distillation (MD) utilizing low-grade heat as a separation stage. While previous OHE systems employed aqueous electrolyte draw solutions, using methanol as a solvent is advantageous because methanol is highly volatile and has a lower heat capacity and enthalpy of vaporization than water. Hence, the thermal separation efficiency of a draw solution with methanol would be higher than that of an aqueous draw solution. In this study, we evaluated the performance of LiCl–methanol as a potential draw solution for a PRO–MD hybrid OHE system. The membrane transport properties as well as performance with LiCl–methanol draw solution were evaluated using thin-film composite (TFC) PRO membranes and compared to the results obtained with a LiCl–water draw solution. Experimental PRO methanol flux and maximum projected power density of 47.1 L m^–2 h^–1 and 72.1 W m^–2, respectively, were achieved with a 3 M LiCl–methanol draw solution. The overall efficiency of the hybrid OHE system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages under conditions with and without heat recovery. The modeling results demonstrate higher OHE energy efficiency with the LiCl–methanol draw solution compared to that with the LiCl–water draw solution under practical operating conditions (i.e., heat recovery <90%). We discuss the implications of the results for converting low-grade heat to power.