A novel method of converting thermal energy into mechanical work is presented, using semi-permeable membranes to convert osmotic pressure into electrical power. This method, a closed cycle pressure-retarded osmosis (PRO) process known as an osmotic heat engine (OHE), uses a concentrated ammonia-carbon dioxide draw solution to create high osmotic pressures which generate water flux through a semi-permeable membrane against a hydraulic pressure gradient. The depressurization of the increased draw solution volume in a turbine produces electrical power. The process is maintained in steady state operation through the separation of the diluted draw solution into a re-concentrated draw solution and (nearly) deionized water working fluid, both for reuse in the engine. The use of deionized water working fluid has been shown to allow for high membrane water flux and efficient mass transport, as internal concentration polarization effects are eliminated. Modeling of the engine indicates that membrane power density may exceed 200 W/m(2), given appropriate operating conditions. The thermal efficiency of the engine is predicted to approach a maximum of 16% of Carnot efficiency (maximum theoretical engine efficiency), with practical efficiencies most likely in the range of 5-10% of Carnot efficiency. The temperature of heat used for the engine may be very low (40 degrees C with a 20 degrees C ambient temperature), allowing for the production of potentially low cost, carbon neutral power from waste heat, low temperature geothermal reservoirs, or other non-combustion thermal energy sources. This combination of a highly concentrated NH3/CO2 draw solution and a deionized working fluid may allow for highly effective power generation from osmotic pressure gradients. (c) 2007 Elsevier B.V. All rights reserved.