Sea change

Offshore wind turbines could turn seawater into drinking water more efficiently by using kinetic energy instead of electricity. Max Glaskin reports.

A new design that eliminates the losses that arise from energy conversion may successfully couple the two abundant natural resources to produce potable water at an attractive cost. The idea of using wind power to desalinate seawater has tempted engineers previously. Both elements are plentiful, they often coincide and are free. But the costs kick in when they are harnessed. A wind-turbine on the German island of Rögen has been used since 1995 to take the salt out of the Baltic Sea, but now a leading renewable energy consultant believes there is a better way to do it. The WindDeSalter from Aerodyn Energiesysteme of Rendsburg, Germany, does the job mechanically. instead of generating electricity to power desalination equipment, the rotational energy from the rotor is transmitted to pumps that draw salt water up to the top of the column where a mechanical compressor pressurises it. The pressurised seawater can then be purified by either or both of two conventional methods. The first technique uses the pumps to increase the pressure so that the temperature rises and the water evaporates, leaving the solute behind. Then the vapour can condense as clean water. The second method is to use reverse osmosis, forcing the water to diffuse through membranes, again stripping it of solutes. Aerodyn chief executive Sönke Siegfriedsen has been involved in the design of 70 different turbines around the world in association with 30 different manufacturers and reckons the WindDeSalter will fit in neatly with the drive towards sustainable development. He claims that 75 per cent of the desalinator’s energy requirements will be supplied by the kinetic system. A small generator will produce the remaining 25 per cent as electricity to power control equipment. All of the equipment can be housed within the turbine’s supporting column, and gravity will force the fresh water out via pipes to where it’s needed. Much of the equipment will rotate within the column as the nacelle swivels to keep the blades facing the optimum direction. Now Siegfriedsen is looking for partners to manufacture and market his design. ’Partners in the wind energy sector could use it as an added-value product to expand their product line,’ he said. The initial requirement of a partner would be to share development costs. He believes his engineers will be able to perfect the design within 42 months and then erect a pilot plant to demonstrate its potential. The location has yet to be decided, but the system will work best if the column is placed on the seabed, so that its salt water reservoir is continually replenished. The column itself will have to be stronger than those normally used for wind turbines so that it can withstand the extra loads it will have to bear from the desalination equipment and water masses. When the demonstrator is proven, the partner company would be able to exploit the design, either through manufacturing the product or by marketing licences. The technology is not limited to turning saltwater into drinking water. It can also be used at treatment plants that are required to process sewage and chemical waste.