Enercon's direct drive evolution

WORLDWIDE: Direct-drive pioneer Enercon has added the E-92 2.3MW IEC Class IIA turbine, a low-to-medium-speed wind turbine, to its range. The company claims the machine, with a 92-metre rotor, has a 10-15% higher yield when compared with the earlier E-82 2.3MW turbine.

The E-92 is the latest offspring of a volume series that originates back to 1995 and the 1.5MW E-66. It is a process Enercon describes as "continuous technological development".

The E-66 grew into the 1.8/2MW E-66 (1999), the 2.0/2.3MW E-70 (2003) and the 2.0/2.3/3MW E-82 volume series (2005). These models are all semi-standard fitted with an electrically excited air-cooled generator featuring an identical 5.3-metre outer diameter. The exception is a strong-wind 3MW E-82 E3 model version, which features a water-cooled generator. Rotor diameters since 1995 have grown in four steps from 66 metres to the current 92 metres, which represents a near doubling of the rotor swept area. At the end of March 2012 more than 10,000 turbines have been developed from the birth of the E-66 up to the E-82.

The E-92 shares many traits with the E-82 series, including the cast main carrier, ring generator, power electronics and turbine control system. Standardising components enables reduced lead times in bringing a product to market and reduced economies of scale in manufacture. As an example of this, Enercon has, in recent years, placed greater focus on replacing welded steel components with redesigned loadand shape-optimised cast equivalents suitable for semi-automated mass production in its own foundry.

While the diameter of all Enercon in-house built generators has largely remained unchanged, generator depth was increased with the E-82. As the turbine's rotor diameter is 11 metres larger than the E-70, rotational speed has to be reduced in order to maintain a comparable tip speed. This has a direct link to aerodynamic noise production, which typically increases with tip speed.

For direct-drive, maintaining an unchanged power rating via reduced rotor speed could cause an increase in generator torque and thus internal heat generation. "Scaling up the E-92 allowed us to reuse the original 3MW E-82 generator - without water cooling - and without significant other changes because extensive tests showed sufficient thermal reserves for tolerating the slight reduction in rotational speed," said Enercon sales director Stefan Lutkemeyer.

Enercon airfoils are - apart from a distinct blade tip winglet - characterised by a spoiler that gradually widens towards the rotor centre and extends to the nacelle spinner/cover. This yield-boosting feature was first introduced in the E-70 turbine in 2003 and is now a standard fitting on all models from 330kW to 7.5MW.

The 43.8-metre E-92 blade's airfoil is more slender when compared with the shorter E-82 blade. This measure is aimed at minimising machine stress and optimum load distribution while achieving reduced mass. Another visible change between both blade designs is the E-92 spoiler width that has been reduced and now stretches over a substantially longer distance towards the blade tip.

The E-92 offers 26% more rotor swept area than the 2.3MW E-82. This translates, according to company statistics, into a 13% higher energy yield at common wind sites with 6.5m/s average wind speed. The yield performance claim by Lutkemeyer is interesting. "The new E-92 will definitely defy one or another of our competitors' turbines with a 100-metre rotor diameter," he claims.

The E-92 will come with hub heights between 85-138 metres, including two concrete-steel hybrid towers of 104 and 138 metres. A prototype is planned for the fourth quarter of this year and will be commercially available in 2013.

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