Energy recovery devices are an important part of any seawater 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. We discuss and describe a new promising concept of energy recovery based on the pressure work exchange principle. In this device a rotating member comprising a multiplicity of double-sided free sliding ball pistons performs the pressure exchange between the differently pressurized fluids. The technical characteristics of the rotary work exchanger and the predicted performance after making a qualitative comparison with other types of energy recovery devices are presented.