Techno-economic assessment of a closed-loop osmotic heat engine

Abstract

Osmotic power harnesses the energy of mixing between high salinity and low salinity streams to generate mechanical energy. The closed-loop osmotic heat engine (OHE) is a low-grade heat powered, membrane-based energy system that couples membrane distillation (MD), a thermally driven membrane process, with pressure retarded osmosis (PRO), an osmotically driven membrane process. The objective of this study was to evaluate the technical and economic feasibility of an OHE to generate electricity. Experimental data and previously established MD and PRO models were used to develop an OHE system model that calculates system efficiency (a ratio between the net energy output and thermal energy input), net power output, and electricity generation costs. Results show that the levelized cost of electricity generation by an OHE at the current state of the technology is $0.48 per kWh, which is not competitive with wholesale conventional U.S. grid electricity costs of $0.04/kWh [1], nor comparable to low-grade heat-powered Organic Rankine Cycle electricity generation costs ($0.08–0.13/kWh). To investigate the robustness of the OHE model, a sensitivity analysis was performed to evaluate the influence of select model inputs on electricity costs. Results indicate that improving PRO membrane power density has the highest potential benefit to reduce OHE electricity generation costs. Development of highly permeable and selective PRO membranes that are mechanically stable at increased hydraulic pressures is critical for maturation of PRO and OHE. Alternative working fluids capable of producing higher osmotic pressures and having lower reverse solute fluxes may aid in increasing OHE performance, but not substantially. Our analysis shows that substantial improvements to system operation and membrane performance could reduce electricity generation cost of large installations close to $0.10 per kWh.