Abstract

This work investigates the process of pressure retarded osmosis (PRO) for salinity gradient energy conversion in power production applications. A mathematical model of the PRO process is developed with consideration for non-ideal effects including internal concentration polarization, external concentration polarization, and spatial variations that are caused by mass transfer and by pressure drop along the length of the membrane. A mathematical model of the osmotic power plant is also developed with consideration for pre-filtration and pick-up head, and for mechanical and electrical equipment efficiencies. A distinction is made between the gross power developed by the PRO process, and the net power available to the grid after parasitic loads are accounted for. This distinction leads to observation of a trade-off that exists between the different non-ideal effects. A method is developed for adjusting operating conditions in order to minimize the overall impact of non-ideal effects and to achieve maximum net power. Important improvements in net power densities are realized as compared to results obtained when general rules of thumb are used for operating conditions. The mathematical model is validated by experimental investigation of PRO at the bench-scale. It is found that test conditions generally used in the literature may not be appropriate for power production applications. Test conditions which strike a balance between pressure drop and other non-ideal effects may provide more realistic results.
An analog electric circuit is developed for a simplified PRO process and osmotic power plant. The analog circuit is used to develop strategies for controlling operating conditions of the system, including by control of the load and by control of a flush valve. Both of these provide satisfactory tracking of the desired operating conditions and can also be used for tracking the maximum power point. The proposed strategies respond quickly to changes in source and load.

The osmotic power potential is evaluated for remote micro-grids in Quebec. The osmotic power potential of selected rivers is calculated and compared against peak power demand of nearby communities. In each case, only a small portion of river flow is needed to satisfy the peak power demand of the micro-grids. This suggests that osmotic power can serve as a reliable source of electricity in such applications. An osmotic power plant prototype is designed for Quebec and its potential for power production in remote communities is evaluated.