Minimizing RO energy consumption under variable conditions of operation

Abstract Specific energy consumption (SEC) for reverse osmosis (RO) desalination systems has usually been estimated using simplistic analyses that consider an average duty point of operation for a certain plant. Amore sophisticated and comprehensive approach that accounts for the effects of variable parameters of operation on SEC was recently described, introducing the concept of the “hydraulic envelope”. Variable parameters include flow rates at variable recoveries, feed temperature and salinity with their resulting pressure requirements, pressure losses caused by membrane fouling, and pressure losses caused by system controls such as feed throttle valves. This paper will explore in greater detail various energy recovery strategies under variable parameters of operation. Particular attention will be paid to a recently developed, innovative energy recovery configuration that uses a motor-driven booster pump coupled to a Pelton turbine, the so-called ‘PROP’, instead of a single-component high-pressure feed pump. This new energy recovery concept can not only be applied to single-stage RO plants, but also as a highly effective interstage booster for dual-stage Brine Conversion Systems (BCS). The concept has been submitted for patenting. Results of the analysis suggest that the key issue for minimizing SEC is to control the plant over the entire width of the operational range without creating throttling losses. This can only be achieved by using hydraulic equipment that allows for feed pressure adjustment at minimum energy dissipation and eliminates the need for throttling valves. It is shown that the newly developed ‘PROP’ concept provides minimum SEC over the entire range of the hydraulic envelope of a plant, while at the same time allowing for maximum hydraulic dynamic control efficiency of the RO unit. In addition, the ‘PROP’ offers a significant savings potential in terms of capital costs compared to conventional energy recovery strategies.