A Model and Experimental Evaluation of RO-PRO Low Energy Desalination

Abstract

Reverse osmosis (RO) is currently the most energy efficient desalination technology; however, it requires a great deal of energy to create the high pressures necessary to desalinate seawater. An opposite process of RO, called pressure retarded osmosis (PRO), utilizes a salinity gradient between two solutions of different concentrations to produce pressure, and hence, energy. During this investigation, PRO was evaluated in conjunction with RO, in a system called RO-PRO desalination, to reduce the energy requirement of seawater RO desalination. The specific energy consumption for RO-PRO was modeled using RO conditions at the thermodynamic restriction and a newly developed module-based PRO model. Using a well-characterized CTA membrane, the minimum net specific energy consumption of the system was found to be approximately 40% lower than state-of-the-art seawater RO. A sensitivity analysis was performed to determine the effects of membrane characteristics on the specific energy production of the PRO process in the RO-PRO system. The sensitivity analysis showed that the minimum specific energy consumption using virtual membranes was approximately 1.0 kWh per m<super>3</super> of RO permeate at 50% RO recovery and that a maximum power density of approximately 10 W/m<super>2</super> could be achieved. The experimental power densities for the RO-PRO system ranged from 1.8 to 3.0 W/m<super>2</super>. This is higher than previous river-to-sea PRO pilot systems (1.5 W/m<super>2</super>) and approaches the goal of 5 W/m<super>2</super> that would make PRO an economically feasible technology. In addition, this is the first known system to utilize energy from a volume of water transferred from atmospheric pressure to elevated pressure across a semi-permeable membrane to pre-pressurize RO feed water. In this way, RO-PRO is a feasible future low energy desalination system that should be further investigated in order to improve efficiency and PRO membrane performance.