Design Optimization of Batteryless Photovoltaic-Powered Reverse Osmosis Water Desalination in Remote Areas

Reverse osmosis (RO) is one of the main technologies for water desalination, which can be used in locations with water resources that have high salinity content (such as saline ground water or seawater) to produce fresh water. Energy requirement for RO is less than other desalination processes, but is in the form of electric power, which can be scarce as fresh water in in remote areas not connected to the grid. Fortunately, many areas with fresh water shortage due to lack of rainfall have abundant sunshine. The combination of solar power and RO desalination is attractive, but remote areas usually requires small modular units, which favors photovoltaic (PV) solar energy harvesting. It is important to consider the net cost-effectiveness of the system when designing the PV-RO desalination plant. Adding battery storage to a PV-RO system has the advantage of steadier operation, but is an additional cost whose real benefit is only realized with a larger PV array that can harvest more energy during daytime. This paper compares the net unit cost of fresh water for realistic scenarios of PV-RO systems with and without battery storage. A multi-level optimization approach previously developed by the authors for time-variant power PV-RO systems is adopted; a “sub-loop” optimization determines the operating pressure and flow rate given a fixed system configuration and instantaneous power input, while an “outer loop” optimizes the configuration of the desalination plant. The sub-loop optimization is done via an enumeration approach, while the outer loop is optimized via a mixed real-coded genetic algorithm (GA). A demonstration study shows a batteryless system being approx. 30% more expensive per unit fresh water production than a fully optimized battery-backed system. However, most of the cost of a batteryless system is in initial investment, which with 7% less annual operating cost, can present a plausible design choice for remote areas.