Optimal operation of battery-less solar powered reverse osmosis plant for desalination

Small RO based desalination plants driven by solar power are attractive options, especially in areas with unreliable grid power supply. Batteries or other energy storage devices can be used to buffer against the inherent variability in solar power. However, the capital and maintenance cost of batteries and problems associated with their safe disposal renders this option unattractive. The focus of this work is optimal operation of an RO plant driven by solar power without batteries under continuously varying solar radiation intensity. A model of an RO plant driven by solar power without batteries is developed and demonstrated to have adequate predictive capability. The model is used to determine optimal operation policies under two different methods of operation, namely, (i) without storage of permeate and (ii) with permeate storage. Examination of the optimal solution in the first mode of operation reveals that a simple, robust, and model free implementation strategy using active constraint control is possible. Comparison of results show that accounting for the permeate storage results in a higher permeate production, and the buffer tank can be viewed as a surrogate energy storage device.