Drive and innovation: The DeWind D8.2 with Voith WinDrive

Turbine manufacturer DeWind has had an interesting journey in recent years, passing through a succession of owners before reaching the hands of US company CTC. Now plans are underway to begin production of the D8.2 model, a variation on the D8 design tailored for the US market. Eize de Vries reports.

On 12 December 2006 the nacelle of the prototype 2 MW DeWind D8.2 was transported by road from Lübeck to Germany’s North Sea port of Cuxhaven. Based on the initial 2 MW D8 introduced in 2001, the new 60 Hz D8.2 is aimed at the large and rapidly growing North American market. This is the new product of the fourth DeWind owner, US-based Composite Technology Corporation (CTC), and features an innovative hydrodynamic WinDrive® gearbox developed by Voith Turbo of Germany, allowing grid-friendly generation for direct grid access. But how did the company get were it is today, and what is special about this technology?

Company background

Founded in the German town of Lübeck in 1995 by a small but dedicated team, DeWind was a relative late-comer to the wind-industry. For this reason it decided to bypass the 250-300 kW class and to start instead with the development of a turbine in the then popular 500-600 kW range. The effort resulted in an unusually shaped gear-driven pitch-controlled variable speed 500 kW prototype erected in June 1996, for which DeWind in 1997 received a prestigious ‘iF Product Design Award’ from Industrie Forum Hannover. The heart of this prototype – and of all other DeWind successor turbines from 600-2000 kW- was a trend-setting combination of a double-fed asynchronous generator and a pulse-width modulated IGBT type inverter. Only the former Tacke Windtechnik (now GE Wind Energy) had beaten the small wind pioneer from Lübeck by two months, when it installed a 1.5 MW prototype with comparable generator technology in April 1996.

In November 1998 DeWind launched a 1 MW turbine, the D6, and like most of its competitors in this size range, opted for a so-called 3-point gearbox support. In this arrangement the gearbox is fitted with a ‘long’ rigid input shaft, and a main bearing is connected to the machine bed near the hub. All the main drive components are also composed in a line arrangement. Based on this design principle the company’s next step was to introduce an enlarged and weight-optimized 1.25 MW D6 successor in September 2000, followed by the 2 MW D8 a year later (Table 1).

In May 2002 the UK industrial group FKI plc purchased the entire share stock of German turbine manufacturer DeWind and integrated the company into its Energy Technology division. FKI is a major player in power generation, materials handling, automation, packaging and hardware. The company has a presence in 30 countries. The new owners aimed at tripling turnover within three years, accelerating development of larger 3.5 and 5 MW turbines, and opening new manufacturing plants in both the Netherlands and the UK.

However in July 2005, in a surprise development, UK-based EU Energy Ltd acquired DeWind from FKI after the latter decided to withdraw from the wind business in late 2004.

Then in October 2005, EU and Voith Turbo of Germany announced a joint co-operation agreement for the development of the Voith WinDrive for use in a 60 Hz DeWind D8.2 wind turbine aimed at the US market.

Factory floor in Lübeck, Germany dewind energy / ctc

Early in June 2006 US-based Composite Technology Corporation (CTC) announced the completion of the acquisition of EU Energy Ltd., the owner of the EU Energy and DeWind group of companies. Composite Technology Corporation (based in Irvine, California), develops, manufactures and sells innovative products made of high-performance composite materials aimed at the generation, transmission and distribution of electrical power. Under the new arrangement, EU Energy Ltd will operate as a wholly owned subsidiary of CTC. With the acquisition, CTC benefits from a number of turbine reserve agreements with five separate North American wind farm operators signed between February and June 2006 for the new DeWind D8.2 wind turbines. These agreements are for the delivery of around 1500 turbines by 2012. CTC has also acquired non-exclusive licence agreements with two companies to produce and sell the existing 1.25 MW D6 turbines in China. A third non-exclusive licence agreement with another Chinese company includes the production and sale of the existing D8 turbine in that market. CTC will continue negotiations based on a signed Letter of Intent regarding the creation of a joint venture to produce D6 turbines in India.

DeWind technology

In the last ten years, pitch-controlled variable-speed turbines have become the dominant wind technology, and the DeWind range is no exception. As a built-in feature these systems absorb gust-induced peak loads and temporarily ‘store’ the energy through an instant acceleration in rotor revolutions – the rotor effectively acts as a flywheel. This operational principle substantially reduces drive-train peak loads and reportedly reduces noise at high wind speeds.

The majority of modern variable speed wind systems, including the 2 MW DeWind D8, consist of a gearbox,1 and either a 4-pole (nominal 1500 RPM) or a less common 6-pole (1000 RPM) generator that turns at variable frequency. By far the most popular electrical machines used in wind turbines today are double-fed induction generators. Siemens Wind Power of Germany and WES of the Netherlands are among the exceptions (both manufacturers apply asynchronous generators). In addition, for the last few years permanent magnet-type synchronous generators have been gaining ground on gear-driven wind turbines. Examples include the 2.5 MW and 3 MW GE multi-megawatt series, and the 2.5 MW Clipper Liberty turbine type, which features four 660 kW PM generators. Another minority category of pitch-controlled variable speed wind installations, about 14.5% globally on a megawatt basis, comprises direct driven (DD) turbines (no gearbox) mainly supplied by Enercon. Both variable-speed wind turbine technologies (direct-drive and gear-driven) require AC-DC-AC type frequency converters to feed in 50 Hz or 60 Hz (North America) compliant grid power. In wind systems fitted with a doubly fed induction generator only 25%-30% of the generator power has to be fed through an AC-DC-AC inverter. At the time of first introduction (1995-1996) this operational feature represented a substantial cost advantage, as power electronics were 7-10 times more expensive compared with today. The cost savings currently offered by double-fed generator/converter systems may, judged on market and technology preferences, still be worthwhile. All other variable-speed power conversion combinations with other common generator types require a so-called full converter, whereby 100% of power passes the device.

The WinDrive gearbox plays a central role in the D8.2 dewind / ctc

Classic Stall (fixed blade angle) and Active-Stall operation, both in a combination with fixed-speed or two-speed generators (typically 690V), is now limited to a small number of gear driven makes and models in capacities up to 2.3 MW. This category of wind turbine operates with a fixed electrical frequency (50 Hz or 60 Hz) and is directly grid connected. Another category of directly grid-connected wind turbines operates with semi-variable speed; one example is marketed by Vestas of Denmark under the trade name OptiSlip®. All wind systems fitted with a low-voltage generator require a transformer for medium-voltage grid connection.

WinDrive gearbox

Although fixed-speed/two-speed turbines and OptiSlip® type turbines can be sold without any restrictions in the booming US wind market, variable-speed wind system suppliers like DeWind potentially face a problem. The reason is that most of these technologies violate General Electric‘s so-called ‘variable speed patent’ for the US market (a patent inherited from Kenetech), which focuses on variable speed power electronics, and according to industry sources, will remain valid until 2010 or 2011. The first option for suppliers is to close a (cross) licence deal with GE. A second option is to find an alternative ‘non-violating’ controller technology (i.e. Vestas V90-3 MW), and a third is to develop a new ‘non-patent violating’ wind technology solution. Voith Turbo’s WinDrive technology fits into the third category.2-4

Viewed from the rotor, a D8.2 drive system is made up of a 2-stage mechanical gearbox, a hydrodynamic WinDrive gearbox, and a fixed-speed 13.8 kV 4-pole synchronous generator. This generator is directly grid connected (or depending on grid voltage by means of a medium-voltage transformer), meaning that a frequency converter is no longer needed. And as the WinDrive gearbox hydraulics functionally decouple the mechanical gearbox output shaft from the generator input shaft, drive train vibrations are dampened. Shocks and peak loads are substantially reduced, states Vic Lilly, an electrical engineer and DeWind CTO. In the future, he explains, this technology offers the potential for drive train weight reductions of up to 20% and nacelle weight reductions of up to 10%.

However, for now, the D8.2 prototype nacelle is still slightly heavier than the D8. Lilly continues: ‘The WinDrive system is capable of maintaining the full range of grid quality and grid support features that are becoming compulsory for all wind turbines. Secondly, the fact that our D8.2 generator is directly connected to the grid resembles the power system layout of conventional oil, coal or gas fired power plants. Conventional power plant operators favour our fixed-speed synchronous generator solution as the behaviour of their own generators and the attached industrial power plant grid codes are highly similar.’ These D8.2 control capabilities include the ability to adjust dynamic power factor values from +0.9 (capacitive) to -0.9 (inductive), or more. Equally important argues Lilly is that the D8.2 complies fully with the latest Grid Failure Ride Through (GFRT) demands as formulated first by the pioneering German utility E.ON Netz and (in adapted form) now gradually become a wind industry standard.

The new D8.2 prototype in Cuxhaven, Germany dewind energy / ctc

On the last day before the 12-metre long D8.2 nacelle was due to be move to the Cuxhaven DEWI-OCC test site, DeWind engineers and technicians were busy completing some last remaining jobs inside and outside the structure. The characteristically elegant lines of the world-famous Ferdinand Porsche design did not change in the D8.2 during the extensive modification operation from the initial D8. This is despite the fact that the incorporation of the WinDrive gearbox slightly increased total drive train length. Viewed from the front of the nacelle, one clearly visible modification to the left side of the cover are twin air inlets for the two WinDrive radiators. One of the radiators provides cooling for the WinDrive lubrication oil and the second is for the temperature control of the hydraulic oil. The WinDrive principle itself is based on the combination of a hydraulic torque converter, technically known as a fluid machine, and a twin interconnected planetary gear system designed as a superimposed or ‘functional support’ gear (see textbox 1).

The D8.2 turbine outside its nacelle dewind energy / ctc

The output characteristics of the torque converter are comparable to those of a wind turbine rotor, which according to Voith is ideal for combining the two into one integrated system. A key design aim for the DeWind and Voith engineers was to optimize total systems efficiency of the D8.2 (mechanical, electrical and hydraulics losses) as compared to the ‘conventional’ D8 (mechanical plus electrical losses), with the primary goal of maximizing D8.2 energy production. Lilly is aware of the importance of efficiency and says that a key design objective is to achieve a D8.2 power curve at least comparable or even better to that of a ‘normal’ D8 featuring a double-fed induction generator and frequency converter combination. Table 2 shows a first qualitative efficiency comparison for the two drive train system options from rotor power to the medium voltage grid. More exact quantitative figures will become available as the Cuxhaven D8.2 prototype testing progresses during the next months.

Inside the nacelle, the main shaft rests in a main bearing in front and is flanged to the gearbox input shaft as a common three-point drive train support layout. The slow-speed planetary stage and second (intermediate) gearbox stage are identical to those of a ‘normal’ 3-stage D8 gearbox. However, in the modified D8.2 unit the third and final high-speed spur gear stage has been eliminated. The former intermediate gear stage output shaft has now turned into the modified (2-stage) gearbox output shaft. As a result of chopping off a gear stage the gearbox unit length has become shortened by 100 mm. This implies for a similar power level that output shaft torque increases due to a substantially lower rotational speed [P = ƒ(T x n); P = Power, T = torque, and n = rotational speed]. This torque increase made it necessary for DeWind engineers to fit a larger brake disc with two brake callipers instead of one. Another necessary modification was to turn the position of the new output shaft to its original X-Y position. This enables a rather straightforward line connection with the other drive components. Some kind of a cardan shaft connects the gearbox output shaft with the WinDrive input shaft and this unit itself forms one compact mechanical assembly together with the generator. Vic Lilly: ‘This mechanical system integration solution eliminates misalignment risks and offers additional advantages with regard to more favourable systems dynamics.’

For the future

The DE 8.2 prototype will undergo a comprehensive testing and optimizing programme in Cuxhaven, with type certification by DEWI-OCC is expected for May/June 2007. Next on the product development list is a 2 MW turbine with enlarged 90-metre rotor for IEC WC II wind conditions, but the time of prototype erection is not yet available. Under the terms of the agreement with Voith, CTC (formerly EU Energy) was granted exclusive use of WinDrive units up to 2.6 MW 60 Hz until 2013. A much bigger DeWind turbine with a capacity of around 2.6 MW can therefore be expected sometime in future. However, at this stage Lilly prefers not to comment on potential future product developments but to focus instead at targets that have to be realised within the next years: ‘Production of the D8.2 will commence in the middle of 2007 in Round Rock north of Austin Texas (US). Our North American manufacturing partner is Teco-Westinghouse, a well-established Taiwan owned business. The latest plan is to start manufacturing turbines this year with a staff of about 150 persons. This production volume will be stepped up to 220 units in 2008 and 350-450 in 2009.’

Meanwhile Voith is building a new WinDrive manufacturing plant in Crailsheim, Germany, from where it will deliver the units to the US. Although DeWind’s sales organization will be based in the USA, Lilly does not exclude the possibility that the D8.2 will also be introduced back to the wider world. Lübeck will remain DeWind’s research and development centre, which currently employs about 50 highly experienced wind engineers and other specialists. ‘Germany is one of the leading nations in terms of research effort and wind technology development. And DeWind’s roots were in Lübeck, which as a city is situated within the very heart of the European wind industry. Finally the brand name DeWind is well recognized for a strong engineering reputation, even in countries as far away as the US. CTC has therefore decided to skip EU Energy Wind Ltd as the brand name and return again to the brand name DeWind’, he concludes.

Eize de Vries is Wind Technology Correspondent with Renewable Energy World



  1. E. de Vries. Company Profile: DeWind. Wind Directions, November 2002
  2. H. Müller, M. Pöller, DIgSILENT, and A. Basteck, M. Tilscher, J. Pfister, VOITH Turbo. Grid Compatibility of Variable Speed Wind Turbines with Directly Coupled Synchronous Generator and Hydro-Dynamically Controlled Gearbox. Sixth International Workshop on Large-Scale Integration of Wind Power and Transmission Networks for Offshore Wind Farms, 26-28 October 2006, Delft, NL.
  3. Voith Turbo Windrive® brochure, 5/2006
  4. DeWind D8.2 Technical Brochure ‘…. the alternative’. EU Energy, 4/2006
  5. Concise Encyclopedia of Engineering. McGRAW-Hill. ISBN 0-07-143952-8, USA, 2005.

WinDrive operating principle

The heart of a hydrodynamic torque converter is its hydraulic circuit; consisting of a pump, turbine, and a guide wheel with adjustable guide vanes. This is combined in a common housing filled with hydraulic oil. The mechanical energy of the input shaft is converted into hydraulic energy through the pump wheel. In the turbine wheel the same hydraulic energy is converted back into mechanical energy and transmitted to the output shaft. The adjustable vanes of the guide wheel regulate the mass flow in the circuit. At closed guide vanes (small mass flow) the power transmission is at its minimum. With the guide vanes completely open (large mass flow), the power transmission is at its maximum. The wind turbine rotor drives the pump wheel (input shaft of the torque converter). Because of the change in mass flow (due to the adjustable guide vanes) the turbine wheel speed can be adjusted to match the various operating points of the driven machine (generator). At low speed, the turbine develops its highest torque. With increasing turbine speed, output torque decreases. The WinDrive development was initiated based on a Voith’s proven Vorecon® variable speed gear that has been in operation in multiple industrial applications for decades.

Founded in 1867, Heidenheim-based Voith AG is one of Europe’s largest family-owned companies with a €3.5 billion turnover in 2006, and more than 30,000 persons employees at over 200 locations worldwide.

Voith is furthermore a renowned specialist supplier of hydro power stations (joint venture with Siemens AG), paper machinery (Voith Paper) and industrial services (Voith Industrial Services). Voith Turbo manufactures drive systems that efficiently drive and move machinery, on land and offshore. The new Crailsheim based WinDrive manufacturing plant is part of the group division Voith Turbo.

Figure 1. The WinDrive operating principle. Source: Voith WinDrive

The hydrodynamic WinDrive gearbox applied in the D8.2 is based on the combination of a hydraulic torque converter, and a kind of 2-stage functionally interconnected ‘superimposing’ planetary gear system positioned between the 2-stage main mechanical gearbox and the synchronous generator. In this superimposing gear unit, input power is supplied to the carrier of the left planet gear stage (illustration ‘operating principle’). Simultaneously, a hydrodynamic circuit drives the outer annulus (ring) gear via the control drive. In a ‘normal’ planetary gear system one of the three elements (planet gear carrier, ring gear, or sun gear) is fixed. In the Voith WinDrive by contrast all three elements of the left planetary gear stage rotate. Between annulus gear and fluid-machine it is necessary to adapt speed and direction of rotation by means of an fixed gear stage. The revolving planet gear stage unit leads both power flows via a (inner left) sun gear to the output shaft that connects to the generator. In the hydraulic circuits, control power is taken from the output shaft with a pump wheel and returned to the superimposing gear via the turbine wheel of the converter. Power flow in a variable speed gear unit can vary continuously by an interacting combination of revolving planetary gear, and torque converter. The torque converter is provided with adjustable guide vanes and can thus be used as an actuator or control variable for the power consumption of the pump wheel. The energy content of the fluid and torque generated by the turbine wheel varies with changes in pump wheel power consumption. The fluid-machine finally just takes the regulating power required for speed control from output shaft that – relative to the drive power – is small and therefore ensures high total efficiency.

Fluid coupling

A fluid coupling is a device for transmitting rotation between mechanical shafts by means of the acceleration and deceleration of a hydraulic fluid. Structurally, a fluid coupling consists of an impeller (pump wheel) on the input or driving shaft and a runner (turbine wheel) on the output or driven shaft. The two contain the fluid. Pump and turbine are bladed rotors. Basically, the pump accelerates the fluid from near its axis, at which the tangential component of absolute velocity is low, to near its periphery, at which the tangential component of absolute velocity is high. This increase in velocity represents an increase in kinetic energy. The fluid mass emerges at high velocity from the pump, impinges on the turbine blades, gives up its energy, and leaves the turbine at low velocity.

A torque converter is a type of hydrodynamic drive whose function is very similar to that of a fluid coupling. The principal difference is that whereas a fluid coupling is a two element drive that is incapable of multiplying torque, a torque converter has at least one extra element – the stator or guide wheel – which alters the drive’s characteristics during periods of high slippage, producing an increase in output torque

D8 product development

Former DeWind presented the 2 MW D8 at the international Husum trade fair of September 2001. This pitch-controlled gear driven machine had been developed within a two-year period with some key objectives: a marked mass reduction, and a substantial lowering of Costs of Energy (CoE). The prototype was erected in March 2002. In total 43 units are operational throughout Austria, Germany and Belgium. After first beginning the development of a DW 62 rotor blade for the 1.25 MW D6 series along with external partners, DeWind put this experience into practice by developing a new and longer 39-metre DW 80 blade for the D8 turbine. A new feature with blades of that size at the time was to use carbon-epoxy composite for heavily stressed sections. The advantage of using carbon in rotor blades is that it enables a substantial increase in stiffness without the penalty of major weight increase. Blade stiffness is becoming increasingly important for the new generation of large turbine blades, mainly because they have to be dimensioned for maximum permitted deflection under load in order to guarantee tower clearance by the blade tip. As a result, the DW 80 blade mass could be kept as low as 5600 kg. The blade was optimized for lowest cost of energy (€/kWh/20yr) throughout the entire turbine concept. A major achievement of the project was that by allowing a 0.5%-1% energy yield loss, blade and turbine loads could be reduced by 20%. This in turn translates into substantially lower manufacturing costs for the blade itself, as well as substantial cost savings for the entire D8 installation.1

Table 3 shows main technical specifications of the DeWind D8 and D8.2 turbines.

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