The concept of pressure-retarded osmosis (PRO) was first introduced by Norman in 1974 , but later named as PRO  and extended by Loeb [2, 3] and other researchers [4-8] . The main obstacle that hinders the development of PRO and limits power production is the lack of appropriate membrane design¬—specifically, a highly permeable and selective barrier layer and a support layer that produces minimal internal concentration polarization (ICP). Internal concentration polarization results from hindered diffusion of feed and/or draw solutes through the porous support layer of a composite membrane. The extent of ICP is theoretically a function of the porous support layer thickness and (macrovoid) porosity and tortuosity. So far, the highest output reported is 3.5 W/m2 of membrane area from a thin film composite membrane tested in the laboratory using simple salt solutions and converting the measured flux into power using a simple thermodynamic model . In order for PRO to be economically viable, a membrane that generates at least 5 W/m2 is required . This goal of the work to be presented is to elucidate the inter-relationships between the five known PRO structure-performance parameters (barrier layer water permeability and salt permeability; support layer porosity, tortuosity, and thickness) and their influence on PRO power production. In the talk, we will map out various combinations of the five structure-performance parameters required to make PRO economical using the commercial CTA membrane properties as the baseline for comparison. Our preliminary results suggest that for a given porous support layer increasing the barrier layer water permeability directly increases power density; however, if salt permeability increase proportionately the gain in power production is completely lost. Hence, PRO membranes must become more permeable to water, while maintaining the current salt permeability. For a given barrier layer, support layer tortuosity appears the most important parameter; as tortuosity approaches unity, power production is minimally affected by changes in support layer porosity and thickness. These results provide great insight into the structural changes required to produce a viable PRO membrane.
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See more of this Group/Topical: Topical 1: Separation Needs for Energy Independence and Environmental Sustainability