WindTech: High-speed gearboxes still rule onshore

WORLDWIDE: The decisive switch from geared to direct-drive turbines that was considered inevitable by many just six or seven years ago has not materialised. Eize de Vries describes the latest developments in onshore and offshore drivetrain technology.

High-speed… Nordex has retained its proven three-point gearbox support solution for the N149 4.0-4.5MW turbine
High-speed… Nordex has retained its proven three-point gearbox support solution for the N149 4.0-4.5MW turbine

The cumulative share of direct-drive turbines in the global market continues to vary from 15% to 22%, with Enercon, Goldwind and the Siemens arm of Siemens Gamesa Renewable Energy (SGRE) among the most prominent OEMs applying the technology. But turbines with high-speed geared drivetrains continue to dominate onshore. The latest — and largest — example is GE's 4.8MW machine, featuring a doubly fed induction generator (DFIG) and a rotor diameter of 158 metres.

The leading turbine manufacturers continue to "stretch" their high-speed concepts and are now closing in on the 5MW boundary as they focus on minimising risk and scaling up industry supply chains. Both Vestas and Nordex, for example, have retained their proven three-point gearbox support solutions for their V150-4.2MW and N149/4.0-4.5MW flagships.

GE is likely to opt again for the drive solution first introduced in the 2.X and 2.5xl series - a compact main shaft with two bearings incorporated in a cast housing, which is an integral part of a heavy-duty cast main carrier. The gearbox is flanged to the main shaft rear analogue to conventional four-point support systems with two main bearings in separate bearing housings.

Until recently, high-speed geared technology dominated the offshore sector, too, largely due to the success of Siemens and (MHI) Vestas turbines. Senvion actually holds the record for the most powerful high-speed machine in operation with its 6.3M152, now upgraded to 6.33MW. Bard Engineering installed two prototypes of a 6.5MW turbine with a high-speed Multi-Doured distributed gearbox with two output permanent magnet generators (PMG), but it never made production.

Wind industry expectations a few years ago were that onshore models of 3MW and above would gradually switch from non-integrated high-speed to semi-integrated medium-speed.

The main examples of developments in this area are Winergy's HybridDrive, introduced in 2011, and the functionally comparable Moventas/The Switch's FusionDrive design of 2012.

Winergy says it sold four HybridDrive onshore units to German developer W2E Wind to Energy, which are now running in Germany and Ukraine, and further orders are expected next year.

Offshore

With the levelised cost of energy (LCOE) as the main driver, offshore wind is now showing a clear preference for direct-drive and medium-speed geared solutions.

Medium-speed designs typically incorporate a two-stage or differential planetary gearbox, skipping the "trouble-prone" third stage. Another shared characteristic is a tube-shaped semi-integrated layout with flange connections between the main drive components. Such medium-speed concepts were introduced around 2007 by pioneers Aerodyn (SCD) and Gamesa (G128-4.5MW). Today, the most powerful medium-speed turbine is the MHI Vestas V164-9.5MW. Adwen's 8MW prototype with a record rotor diameter of 180 metres has been discontinued.

Nearly all medium-speed systems in operation are fitted with PMGs. However, Aerodyn's 8MW and 15MW (twin rotors) SCD nezzy concepts, which incorporate electrically excited (brushless) synchronous generators, are also optionally available with HybridDrive.

The Multibrid low-speed geared designs developed by Aerodyn have only had limited commercial success, most notably with the 200 or so Adwen (formerly Areva and Multibrid) 5MW turbines operating offshore. Multibrid is technologically characterised by a compact fully integrated cast structure incorporating a single rotor bearing, a 1.5-stage gearbox and PMG.

Direct-drive models

At the turn of the last decade, some experts thought that PMGs were poised to take over from DFIG. Leading suppliers Vestas and GE both made the move, but then switched back again. Induction-generator (IG) deployment, more recently touted as a likely candidate to succeed DFIG, is still limited to a handful of OEMs.

Enercon launched its groundbreaking 500kW E-40 turbine in 1992, and direct drive has become a mature drivetrain technology in the 25 years since then. All Enercon models, including the latest 4.2MW E-141 EP4, incorporate an electrically excited synchronous generator. Most competitors prefer PMGs, usually more compact solutions designed in outer-rotor or inner-rotor generator layouts.

For many years Enercon and Goldwind formed the backbone of direct-drive success in onshore wind, and both have 4MW-plus machines in operation or development.

Dutch manufacturer Lagerwey is joining the class, having installed two 4.5MW L136 prototypes, while SGRE has recently extended its direct-drive onshore platform to range from 3.2 to 4.3MW, with rotor diameters up to 142 metres.

The strong position of direct drive in the offshore market is largely attributable to the success of SGRE's 6MW and 7MW turbines, with a new 8-9MW machine with a 167-metre rotor diameter now under development. SGRE's next-generation 10-12MW "1X" offshore turbine is likely to be another direct-drive unit, but no details are known.

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