Reverse osmosis (RO) is one of the main commercial technologies for desalination of water with salinity content too high for human consumption in order to produce fresh water. RO may hold promise for remote areas with scarce fresh water resources, however, RO energy requirements being in the form of electric power have few options in such areas. Fortunately, scarce rainfall is often associated with abundant sunshine, which makes solar photovoltaic (PV) power an attractive option. Equipping a photovoltaic powered reverse osmosis (PV-RO) desalination plant with battery storage has an advantage of steadier and longer hours of operation, thereby making better use of the investments in RO system components, but the additional cost from including batteries may end up increasing the overall cost of fresh water. It is therefore of paramount importance to consider the overall cost-effectiveness of the PV-RO system when designing the desalination plant. Recent work by the authors has generalized the steady operation model of RO systems to hourly adjusted power-dispatch via a proportional-derivative (PD) controller that depends on the state of charge (SOC) of the battery, yet the operating conditions; namely pressure and flow for a given power dispatch were only empirically selected. This paper considers a multi-level optimization model for PV-RO systems with battery storage by considering a “sub-loop” optimization of the feed pressure and flow given power dispatch for a fixed RO system configuration, as well as a “top-level” optimization where the system configuration itself is adjusted by the design variables. Effect of the sub-loop optimization is assessed by comparing the obtained cost of fresh water with the previous empirically adjusted system for locations and weather conditions near the city of Hurgada on the Red Sea.

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