Air pressure energy storage for reverse osmosis: Bench-scale proof of concept

Society faces diminishing access to clean drinking water because of increasing global population and the development of modern industries. At the same time, climate change and other environmental problems caused by the increase in fossil fuel consumption the emphasis on reducing dependency on traditional energy resources in the process of potable water production. To address both problems, a small-scale wind powered reverse osmosis (RO) desalination system with a unique energy storage mechanism was envisioned to provide an energy buffer such that fluctuating and intermittent wind can be utilized. Energy is stored in the form of compressed air in a pressure vessel (PV).;The feasibility of this innovative design was evaluated for both seawater (35 g/l and 45 g/L NaCl solutions were selected for seawater experiments) and brackish water (15 g/L and 25 g/L NaCl solutions were selected for brackish water experiments) desalination. The RO desalination coupled with the PV energy storage device was tested using bench-scale experiments under different operating conditions. It was found that high initial air pressure can help to produce the greatest flux and good water quality (>98.6% salt rejection), while different crossflow speeds did not affect the performance of energy-buffered experiments. The performance was further tested with simulated fluctuating and intermittent wind speeds to mimic real-world conditions. It was demonstrated that the energy storage tank can largely dampen the variability in applied pressure and discharge rate caused by wind fluctuation and significantly improve the desalination performance for both flux and rejection without any pressure control strategy. Since high quality drinking water (>98.6% salt rejection) was produced under applied pressure as low as 700 psi, the applied pressure could be lowered to reduce energy consumption. Under simulated intermittent wind operation, RO desalination with energy storage showed significant advantage over traditional RO desalination (with no energy storage mechanism) in that PV can store the energy when the wind dies down and the stored energy can be used to produce good quality drinking water when a small amount of energy is available to provide crossflow. The effects of dissolved nitrogen under high pressure on the RO process were also evaluated. The experimental results indicated that comparing with conventional RO, the RO process with PV did not show a consistent benefit or detriment in flux using different feed concentrations from 0 to 45 g/L, but caused a slight improvement on rejection for all feed concentrations.