Sea water desalination by reverse osmosis: the true needs for energy

Almost half the exploitation costs of sea water reverse osmosis plant is affected to the electricity costs. Therefore, it's crucial to control those, and firstly to evaluate it. Each case should be treated independently, since the need in electricity depends on different factors: sea water characteristics, values taken for RO sizing parameters, electromechanical components efficiency . . . and, last but not least, choice of the system for hydraulic energy recovery of the concentrate. Energy recovery devices are an important part of any sea water reverse osmosis system, and any future decrease in specific energy consumption is dependent upon the further development and improvement in the performance of such devices. Recent technological innovations, such as energy recovery turbine, are required to reduce and improve the energy consumption. One of the simplest methods to operate a sea water RO unit is to run the pump at constant speed, to obtain the required membrane feed pressure through a feed throttle valve, and to recover the remaining brine energy via a Pelton turbine. Increasing hydraulic performances of Pelton turbines requires the optimisation of all components, taking into account their coupling. Until now the development of Pelton turbines has relied almost entirely on experimental and empirical knowledge and has not benefited from numerical flow simulation. This is mainly due to the fact that until now CFD simulations of free surface flows have not been fully validated and were impractical in an industrial environment, mainly due to the large computing time and resources that are needed. The numerical codes are still under development and, as with all CFD methods, have to be evaluated and validated before they can be used as standard tools in design. The highly successful use of CFD in Francis and Kaplan turbines has brought benefits of improved performance, more rapid and less risky design processes, large reduction in the need for empirical methods and reduced need for model and prototype tests. It is natural that these modern sophisticated design tools should now be adapted for use in Pelton turbines.