Studies on the Design of Reverse Osmosis Water Desalination Systems for Cost and Energy Efficiency

This paper explores optimal design of reverse osmosis (RO) systems for water desalination. In these systems, salty water flows at high pressure through vessels containing semi-permeable membrane modules. The membranes can allow water to flow through, but prohibit the passage of salt ions. When the pressure is sufficiently high, water molecules will flow through the membranes leaving the salt ions behind, and are collected in a fresh water stream. Typical system design variables include the number and layout of the vessels and membrane modules, as well as the operating pressure and flow rate. This paper presents models for single and two-stage pressure vessel configurations. The models are used to explore the various design scenarios in order to minimize the cost and energy required per unit volume of produced fresh water. Multi-objective genetic algorithm (GA) is used to generate the Pareto-optimal design scenarios for the systems. Case studies are considered for four different water salinity concentration levels. Results of the studies indicate that even though the energy required to drive the RO system is a major contributor to the cost of fresh water production, there exists a tradeoff between minimum energy and minimum cost. An additional parametric study on the unit cost of energy is performed in order to explore future trends. The parametric study demonstrates how an increase in the unit cost of energy may shift the minimum cost designs to shift to more energy-efficient design scenarios.