This project is based on the design of a hydraulic-wind-mill powered reverse osmosis (RO) desalination plant (Freshwatermill), focusing on increasing the energy efficiency of the system. While RO membranes are generally not designed for variable operation, wind energy has a characteristic unpredictability and variability. In particular, the project focuses on enabling the combination of RO and wind energy by finding a system configuration that will enable increasing the operating range in order to maximise the production of fresh water with the available wind energy without the use of energy storage. The proposed solution is to use the strategy of unit switching, in which multiple equally smaller RO membrane pressure vessel units can be switched on or off depending on the power supply in order to maintain the membranes within their optimal operating conditions of As a first step, an experimental set-up is designed to test the behaviour of the RO system with a varying power supply and to measure how much fresh water production can be expected with the selected equipment depending on wind power. The test will then define whether the strategy of unit switching proves to be advantageous over a more traditional layout with a fixed numbers of units. After testing this strategy it is decided to reject it and opt for a fixed number of RO units as this is more advantageous in terms of fresh water quantity produced as well comprising a simpler and more robust RO system. Although the fixed RO system slightly lowers the fresh water quality at low powers, the level of salts remains within an acceptable range for drinking standards. Furthermore, with the fluctuating wind energy source, the lower quality water produced at low powers will combine higher quality water produced at higher powers resulting in an averaged out water quality in the long term. In a second part a computer model will then be developed to carry out simulations on a complete Freshwatermill system (including the windmill and hydraulic drive-train) to test its performance as a whole. This allows determining the capacity of RO to couple to the hydraulic windmill in order to exploiting the full range of available wind power and maximise the fresh water production. A financial analysis is finally performed to evaluate whether increasing the RO capacity is worth the investment of additional components.