A System for Wave Driven Desalination of Seawater

Water scarcity is a growing challenge in a large part of the world. Desalination of seawater is an energy intensive process and in coastline areas where water scarcity coincides with a significant wave climate, desalination of seawater is an attractive application for utilisation of wave energy. The bulk of the energy requirement in desalination by reverse osmosis goes into pressurising seawater. Wave energy can be used directly to achieve this thereby bypassing the challenges associated with electricity generation. The present paper describes a floating unit which produces fresh water using wave power as the main energy source. A double-acting combined pressure magnifier and energy recovery device uses the irregular motion between two floating bodies to drive prefiltered seawater through a reverse osmosis membrane. The unit is compact with one floating disc positioned in the moonpool of the main floating body. The total system may be completed in the yard, towed to field and anchored using light mooring lines which have only a minimal impact on the floater motions. The unit has been sized for a typical West Africa wave climate using a linearised but fully coupled radiation-diffraction analysis of the vertical relative motion between the bodies in waves. The linearised results have been compared with results from time domain analyses verifying the vertical motions but indicating that significant energy can be extracted also from the relative roll and pitch motion of the bodies. It is concluded that the unit can deliver desalinated water at a cost of 0.8 Euro/m³ even in a relatively benign North West Africa wave climate.