The production of fresh water directly from a renewable energy source such as wind or wave power has a number of inherent advantages. In particular, the mechanical energy from a wind turbine or wave energy converter can be used to directly pressurise water to be used in a reverse osmosis (RO) desalination plant. This eliminates the costs and loss of energy associated with conversion of the energy first into electricity and then into pressurised water, which occurs in traditional desalination plants powered by renewable energy. However, the direct use of renewable energy for desalination complicates the RO desalination plant as traditionally these have been designed for an unvarying energy supply.