Large-scale power production by pressure-retarded osmosis, using river water and sea water passing through spiral modules

Abstract

In principle a very large quantity of electric power could be produced by the worldwide application of pressure-retarded osmosis (PRO) to the osmotic pair, river water/sea water. The utility of the process depends on the economics, i.e., whether the produced energy cost, dollars per kilowatt hour, and the plant capital cost, dollars per kilowatt, can be adequately low. The study was limited to spiral modules, i.e., originally flat sheet membranes. A very important cost item was the “Yuma” specific plant capital cost of 1000 dollars per daily cubic meter of permeate. This value was derived from consideration of the world's largest RO plant, that in Yuma, Arizona, and was used in PRO calculations with modification as required for differences in flux and for economy-of-scale effects. Within these limitations, the key parameters were found to be twofold: First was the K term in PRO. This is the resistance to salt diffusion in the porous substructure and support fabric region of the membrane and must be as low as possible because an increase in K decreases permeate flux virtually exponentially. Second was the size of the PRO plant, characterized by the flow rate of the river utilized. The larger the PRO plant, the more important the economy-of-scale factor becomes in minimizing the energy and power costs mentioned above. A key assumption in the comparative plant cost calculations was that half of such costs would be independent of changes in plant flux and the other half proportional to it. Based on previous PRO tests and some optimism, K terms of 10 and zero were considered. A “moderate” river flow rate of 3 million m 3/d flow rate was considered as well as a “large” river size, that of the Mississippi, 1,500 million m 3/d flow rate. The following was found: A “moderate” flow rate PRO plant with an optimistically low K term of 10 d/m (permeate flux 0.29 m 3/m 2d) would give unacceptably high energy and power costs as would a moderate plant with K = 0 (flux 0.725 m 3/m 2d). A Mississippi river plant with K = 10 would produce marginal energy and power costs, i.e., higher than expected from conventional existing power plants and perhaps acceptable under certain circumstances but with a K value of zero would produce adequately low energy and power costs. If the specific plant capital cost estimate could be reduced from 1000 to 500 dollars per daily cubic meter of permeate, as reported by some RO investigators, all PRO costs would be reduced by about half, thus rendering the moderate flow rate PRO plant with K = 0 marginally acceptable and both Mississippi PRO plants acceptable in terms of low energy and power cost. In view of these possibilities and the tremendous amount of benign and renewable energy and power potentially available, it is believed that river water/sea water PRO should be seriously investigated.