Full speed ahead with prototypes

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The Danish wind industry is promising cheaper power from its new generation of offshore technology than from similar wind plants sited on land. All of Denmark's major wind turbine manufacturers are in the process of fine tuning designs specifically for offshore use and cost cutting solutions to a number of the critical issues have been applied. Large generators and shorter towers are a common theme.

The first multi-megawatt prototypes will be installed before the end of the year. Government policy in Denmark is for 4550 MW of offshore wind by 2015, with the first four 750 MW wind stations to be installed within ten years. The aim is for 50% of Danish electricity demand to be met by wind power by 2030, 4000 MW of which is to be offshore.

Farthest ahead with its offshore technology is Vestas. It will install two 2 MW prototypes in November or December. At least one of these will be on land. The company's Lars Budtz assures that once at sea the 2 MW unit will deliver cheaper power than it can on land. The 2 MW is a further development of the Vestas 1.65 MW pitch regulated turbine, but it will have just one generator. Budtz says there are too few low wind periods at sea to justify a smaller 300 kW generator, as in the 1.65 MW unit, as well as the large one. The tower heights on offer will be nearly 20 metres shorter than those on land since winds are less turbulent at sea and do not increase as dramatically with altitude as they do on land. The rotor diameter of the 2 MW will remain at the 1.65 MW unit's 66 metres, but Vestas expects a 12% increase in production compared with the 1.65 MW on land. Budtz declines to comment on the likely price of the 2 MW.

The experience gained by Vestas in the operation of its first offshore wind farm of 500 kW turbines at Tunø Knob in Denmark has been put to good use. The best way of dealing with the risk of salt air corrosion is to have a temperature in the nacelle which is constantly 10ยก C warmer than on the outside, says Budtz. Large savings are also being made through the development of new types of foundations, he adds. Budtz says Vestas has taken a deliberately conservative step in further developing its 1.65 MW instead of starting from scratch. But within the next few years he expects the company to have both 4 MW and 5 MW turbines available for offshore sites.

NEG Micon has also decided on 2 MW as the optimum size for an offshore prototype and developed it from an existing design, the NEG Micon 1.5 MW, of which 45 are now operating. The first six offshore units will be ready by the summer, says Jesper Kjær Hansen, head of development. "They will all be erected on land and tested under a variety of conditions," he adds. Once again, hub height will be lower than the NEG Micon 1.5 MW, but rotor diameter will be increased for greater output both on land and offshore. The machine's two-speed generator is to be water cooled, allowing for a closed system which can be dehumidified to avoid salt corrosion problems. A "double-control" system is being developed along with ways of making the turbine as maintenance free as possible over longer time periods than on land. Work is also proceeding on ways of replacing major components without the need for an (expensive) offshore crane.

Wincon, too, has a 2 MW offshore machine on the drawing board. This time, however, an entirely new concept has been developed, says the company's Jørgen Christiansen, though Wincon's so-called "power-stall regulation" system has been incorporated in the 2 MW design. The system allows for a combination of pitch and stall control. Wincon's 2 MW is designed with a 60 metre hub height and 70 metre rotor diameter and the prototype will be ready in 2000. To ward of salt laden air problems, Wincon, part of refrigerator and freezer company Vestfrost, is drawing on its mother company's expertise in operating electronics in difficult climate conditions.

Bonus, the fourth independent wind company in Denmark, is so far keeping its offshore plans under wraps, says technical director Henrik Stiesdal. Wind World, recently bought by NEG Micon, is also wary of giving too much away. A 2.5 MW design which it has been working on for some time is to be upscaled to 3 MW, says Poul Anker Lübker. A prototype of the model is planned for installation on land in 2000.

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