The origins of these innovations can be traced back to 2000 when Vestas introduced a variable-speed V80-2.0MW turbine featuring an 80-metre rotor diameter and doubly-fed induction generator (DFIG). Following this, the V90 and V100 models were introduced, with increased 90-metre and 100-metre rotor diameters and a choice between a 1.8MW or 2MW power rating. To date, more than 7,800 2MW units have been manufactured, making it one of Vestas' most popular and one of the industry's most-sold series.
The V100-1.8MW set an important trend. It was one of the first turbines with a low specific-power rating (specific power is the power rating divided by the rotor-swept area), which aimed to achieve high capacity factors at low and medium wind-speed sites. Now, with the 1.8MW/2MW GridStreamer turbine, Vestas seeks to target the market for grid access and behaviour-friendly low wind.
The GridStreamer models come with similar rotor diameters to previous versions. However, they also include a liquid-cooled Vestas-designed permanent-magnet generator (PMG), and full power electronic converter is now fitted as standard. Also included is a new gearbox developed by German automotive transmission specialist ZF Friedrichshafen.
The turbines use a Vestas Cooler Top radiator unit first used on the V112-3.0MW, which has natural water circulation and a minimum number of moving parts.
Explaining the GridStreamer's development, Finn Strom Madsen, president of Vestas Technology R&D, says the changes were not only technological. "The V100-2MW GridStreamer model also moved up from IEC class IIIA to class IIA while the power rating increased from 1.8MW to 2MW," he explains. "These changes resulted in higher loads and increased demands on structural strength and stiffness."
IEC classes are defined by the International Electrotechnical Commission. Class I turbines are designed to operate at average wind speeds of 10 metres per second (m/s), Class II at 8.5m/s and Class III at 7.5m/s.
Madsen says that, because of the changes, a single cast-main shaft housing with integrated bearings was introduced instead of the original two individual bearing blocks. "The switch to a PMG was prompted by our belief that this contributes towards meeting future grid requirements, including wind markets with a high wind-power penetration," he says.
Vestas' long-term design philosophy has been to keep the nacelle's mass as low as possible. PMG's offer both superior partial-load efficiency and weigh significantly less than DFIGs.
Technically, the new V100-2.6MW turbine is similar to the lightweight V90-3.0MW. It includes a compact drive system with flanged gearbox plus integrated rotor bearing and DFIG. The V100-2.6MW concept, with its enlarged rotor and reduced power rating, follows common engineering up-scaling practice and is based on the principle that power is a function of torque and RPM.
Madsen says Vestas has learned from the past in developing its turbines. In 2003, the company set new standards by designing its V90-3.0MW product with a 104-tonne top-head mass (nacelle plus rotor). But soon afterwards, serious gearbox-related issues emerged and additional design imperfections had to be addressed. The V90-3.0MW was withdrawn from the offshore wind market for one year in 2007.
At least five gearbox retrofits were carried out, the number of yaw motors was increased from four to six and the main chassis structure was reinforced. Nacelle mass was increased to 111 tonnes.
Madsen says Vestas wanted to be in control while resolving the gearbox issue. "In close cooperation with our gearbox suppliers we found and implemented a solution, which created a sound basis for platform continuation and expansion," he adds. "Customers also responded positively and we believe that, in fact, it impacted positively on our reputation in the market."
With the V100-2.6MW, Vestas focuses on the market for Class II turbines with a power rating of more than 2MW, including sites with megawatt or tip-height restrictions.