Development and assessment of a novel osmotic heat engine for energy generation from low-grade heat

Abstract

Development of clean energy technologies that maximize efficiency and minimize resource consumption is a necessary component for a clean and secure energy future. The osmotic heat engine (OHE) is a closed-loop, membrane based process that utilizes low-grade heat and salinity-gradient energy between two streams for electrical energy generation. The OHE couples pressure retarded osmosis (PRO), an osmotically driven membrane process, with membrane distillation (MD), a thermally driven membrane process. In PRO, water permeates via osmosis through a semi-permeable membrane from a low concentration feed stream into a higher concentration brine (draw solution). The permeate stream becomes pressurized on the high concentration side of the membrane and a mechanical device (e.g., turbine generator set) is used to convert the hydraulic pressure to electrical energy. The MD process utilizes low-grade heat to reconcentrate the diluted brine from the PRO process and to produce a deionized water stream; these streams are then resupplied to the PRO process in the OHE. High power-density (power generated per unit area of membrane) of the PRO membrane is essential to maximize the efficiency and minimize the capital and operating costs of the OHE. Likewise, high separation efficiency is needed in the MD process to effectively reconcentrate the diluted draw solution. Thus, robust PRO membranes that can support high pressure, have high water flux, low reverse salt flux, low structural parameter, and a good membrane support structure are essential. The MD process must also be able to withstand high operating temperatures (> 60 ºC) and feed water concentrations, and have low pore wetting propensity. Additionally, the use of highly soluble ionic organic and inorganic draw solutions can increase PRO power densities while achieving high MD water fluxes, thus increasing efficiencies and decreasing costs of OHE. Therefore, the objective of this dissertation is to develop and demonstrate at the laboratory scale and through modeling work a novel, closed-loop, hybrid membrane-based system that converts low-grade heat to electrical energy. The performance of several membranes used for PRO and the effect of spacer configuration on membrane performance was evaluated. The performance of MD in concentrating hypersaline brines was evaluated and scale mitigation techniques were investigated to restore water flux and sustain the desalination process. Several ionic organic and inorganic draw solutions were evaluated as working fluids in the OHE, and their performance was assessed in terms of PRO power density and reverse salt diffusion, and MD separation and thermal efficiency and membrane pore wetting. The experimental data was used to develop a system model that evaluates system efficiency, net power output, and costs. Modeling results were used to perform an environmental life-cycle assessment using GaBi, a life-cycle assessment software. Although, at its current state of technology, OHE electricity generation costs ($0.48 per kWh) are not competitive with conventional U.S. grid energy costs ($0.04 per kWh), system environmental impacts are an order of magnitude lower. Furthermore, with future improvements to membrane technology and OHE process performance, electricity generation costs for the OHE as an energy storage device ($0.12 per kWh) could be comparable to on-peak demand charges in Southern California ($0.15 per kWh), thus making the OHE an attractive energy storage device.