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. © 2015 Elsevier B.V. All rights reserved.

The focus of this work is to determine an optimal operating strategy for a given design that maximizes system utility subject to constraints on water quality. An appropriate optimization problem for operation of an RO desalination plant driven by solar power without batteries subject to input variability has been formulated and solved for two cases, one with a product buffer tank and the other without the buffer tank. Two different strategies are presented and optimal solutions corresponding to both are presented. We demonstrate that given the same design and incident radiation profile, the two operating strategies differ in levels of productivity and ease of implementability. The permeate tank acts as a surrogate energy storage device, storing energy during high power availability as lower concentration permeate. Operating the RO plant in this manner addresses the twin challenges of maximizing the power availability and energy storage. The question of designing an appropriate system that is optimal or even feasible under varying power availability or water consumption patterns has not been addressed in this work. However, this analysis is also suitable at the design stage. If a particular design configuration and choice of operating strategy does not result in feasible operation, either the design or operating strategy has to be modified. Hence, it is worthwhile to incorporate different operating policies along with different equipment sizes and configurations in the design stage itself.