Numerical simulation and operating strategy of a wind-powered electricity and freshwater production system - The case study of Agios Efstratios island in Greece

Abstract

The rapid increase in global population and rising demands have resulted in a shortage of clean water, depleting several of the Earth's reservoirs, especially those that reserve fresh water. Just 0.007% of the water on Earth is used to hydrate and feed billions of people, as determined by the water balance of the planet. Water-stressed nations are now desalinating salty ocean water to produce fresh water for use in their homes, industries, and fields. Among the processes used to produce freshwater, reverse osmosis (RO) is the most reliable technique for seawater desalination due to the high energy efficiency of the membranes and low desalination-associated costs. The major disadvantage of this technology is its high energy consumption, which is mostly derived from conventional sources like fossil fuels. Therefore, with a view to a sustainable future, renewable energy sources must be integrated to power the reverse osmosis systems.

All conventional wind systems convert mechanical energy into electricity, which is used to power the desalination units. Delft Offshore Turbine (DOT) aims to replace the conventional drivetrain with a hydraulic transmission system that uses seawater as a hydraulic transfer medium. The examined DOT500 PRO system consists of a wind turbine connected to a hydraulic positive displacement pump, which induces seawater flow under high pressure. A reverse osmosis unit can be used to desalinate saltwater using this high-pressure flow and a Pelton turbine generator can be used to produce electricity. The long-term objective of the DOT project is to lower complexity, mass, maintenance, and capital expense to make offshore wind a competitive source of electricity. To further reinforce the concept's techno-economic components, the future strategy calls for the centralized production of electricity.

This study aims to develop an operating strategy for the DOT500 PRO system for freshwater production at an offshore location. The viability of using such a system in a place with offshore wind conditions and predetermined freshwater demand was investigated using a constant operating scheme of a single spear valve. A numerical model is developed in Python to deliver the appropriate desalination unit size under offshore wind conditions in order to cover the water requirements of a given location. The model is applied to a case study of Agios Efstratios in Greece and can be used as a tool to investigate possible applications using the associated datasets. This research has yielded the following results:
1. A simple and robust system operating strategy for maximum water production that will ensure stable turbine function, while active control is used to maintain the system at constant operation when its limits are reached.
2. The direct influence of the spear valve position and reverse osmosis unit size on the system’s rated conditions and a reduced rated wind speed by 21.3% from the one utilized conventionally on the reference wind turbine.
3. The minimum required number of RO pressure vessels and membranes according to site-specific offshore wind profile and freshwater demand, as well as options for reducing the RO unit size.