Gross vs. net energy: Towards a rational framework for assessing the practical viability of pressure retarded osmosis

Although significant technological advances have been made in recent years on pressure retarded osmosis (PRO), its practical viability remains unclear as few studies have been conducted at an integrated system level to quantify the potential of net energy output. In this study, we develop a framework to assess the net energy output of a PRO system by first quantifying the gross energy output via solving the mass transfer equations for a full-scale PRO module, and then incorporating the major energy losses from pretreatment, flow circulation, and inefficient energy recovery. We also propose a novel concept called net membrane power density that is strongly relevant to the capital cost of a PRO system. Finally, we describe an approach, based on the quantifiable specific net energy and net membrane power density, for assessing the economic viability of a PRO system. Albeit using seawater/river water PRO as the context for illustrating our approach, the assessment framework developed is universally applicable to PRO systems with any solution pairing. The results from this study clearly show the impacts of various parameters on the practical performance of a PRO system, thereby providing important guidance to the improvement of its design and operation.