Experimental comparison of the performance of two reverse osmosis desalination units equipped with different energy recovery devices

Sea water reverse osmosis (SWRO) desalination constitutes a successful technology for covering local fresh water supply shortage in many areas of the world and especially in isolated areas such as islands and coastal regions. SWRO units can be combined with renewable energy (RE) technologies such as photovoltaic and wind generators. Small-scale SWRO units combined with energy recovery devices can decrease drastically the energy consumption of the SWRO units. Furthermore, it is proven that the operation of a desalination unit in part-load conditions can result in lower specific energy consumption compared to continuous full-load operation. This paper presents an experimental comparison between two small-scale SWRO units equipped with different energy recovery devices in order to lower the specific energy consumption. The first SWRO unit consists of a hydraulic energy recovery device of the Clark pump type which plays also the role of the high-pressure pump in a conventional reverse osmosis unit. The second SWRO unit is equipped with two types of Danfoss pumps (a rotary piston pump and a motor pump), based on the axial piston principle. The main objective of the comparison is the identification of the energy recovery device with the lowest specific energy consumption of the SWRO unit. Both units are installed at the Laboratory of Agricultural Engineering of Agricultural University of Athens and they are tested under full- and part-load conditions. The experimental operation of the SWRO units in part-load conditions is achieved by varying the speed of the motor pump assembly, the pressure and the flow rate of the feed water. During the evaluation of the measurements results, an optimum operating window was drawn regarding the operation of the SWRO desalination units in part- and full-load conditions. More specifically, the minimum measured specific energy consumption of the Clark SWRO unit was found to be 5.7kWh/m(3) at a pressure of 44bar while the Danfoss SWRO unit showed a minimum specific energy consumption of 4kWh/m(3) at a pressure of 59bar. With these results, a SWRO unit equipped with an energy recovery device operating under full- and part-load conditions is suitable for future direct connection with RE systems such as photovoltaic and wind turbines.