Innovation: The ingenious is always simple

As a company strategy Vensys Energy AG of Germany aims at entering new wind markets by issuing wind turbine production and geographical sales licence packages to local partners. Eize de Vries evaluates Vensys’s direct drive wind technology and licence partner CKD NOVE Energo.

On the back of the latest Vensys Energy AG product brochure appears the quote: ‘The ingenious is always simple’ from the famous German poet, writer and playwright Johann Wolfgang von Goethe (1749–1832). The wind turbine developer and industry newcomer itself is characterized by innovative high-tech products at 1.2 MW, 1.5 MW and 2.5 MW, and a focus on systems robustness and overall design simplicity. The company’s major shareholder is Hugo Denker who acquired 70% of Vensys equity at the end of 2002. Denker was, until then, a minority stakeholder in German turbine manufacturer REpower, before selling his stake to Portuguese construction company Martifer. Vensys has financed development of its wind turbine technology through stakeholder input, licence fees and research grants from the state of Saarland. The company currently employs 28 people, 22 of whom are engineers.

The technology roots of Saarbrücken-based Vensys Energy AG, founded in 2000, date back more than a decade to the Genesys wind turbine development project. The project raised considerable attention and product development took place in an academic environment at the Saarbrücken University of Applied Sciences, the Saarbrücken Hochschule für Technik und Wirtschaft des Saarlandes. The project, by the Forschungsgruppe Windenergie – a specialized eight-person design team headed by wind veteran Professor Friedrich Klinger – resulted in the erection of a 600 kW direct drive prototype, named the Genesys 600, in early 1997. Plans to enter series production in 1998 never materialized for largely non-technical reasons, say insiders. With its 45.9 metre rotor diameter and a number of novel built-in technology features, this modern pitch-controlled variable-speed wind turbine concept stood as the model for the later 1.2 MW Vensys 62 and its successors.

A cool feature

One of the distinct features of the early Genesys 600 is an unusual 620 V air-cooled permanent magnet (PM) type generator, in which the multi-pole external generator rotor rotates outside the stator with the permanent magnets facing inward. Incorporated in the design is a patented passive generator air-cooling system, where electrical fans and temperature control equipment could be eliminated. Design objectives focus on efficient cooling and fail-safe operation under all operational conditions.

The innovative Vensys 1.2 MW type 62 turbine

The airflow that powers the turbine rotor is guided directly over hot generator parts such as the stator with the aid of specially shaped cooling ducts. According to Vensys’ technical information, the degree of air-cooling provided corresponds directly to power output, which it says guarantees a favourable and relatively constant coil temperature level. The generator stator coils are made of preformed copper wires, and are completely sealed against harsh environmental conditions such as moisture and silt particles in the passing air. The generator concept itself is known as ‘Aussenläufer’ in German and the specific design configuration is said to enable a substantial reduction in the outer generator dimensions. Reduced generator dimensions, particularly a smaller outside diameter, can substantially ease road transport logistics. In more common radial-flux type electric motor and generator configurations, the generator rotor turns inside the stator, while either the permanent field excitation or external field excitation magnets face outward. The advantage of a PM-type generator with permanent field excitation is that there is no need to generate the direct current normally required for field excitation, which results in a slightly higher partial load efficiency. A related advantage is the elimination of slip rings that are otherwise required for field current passage. However, some potential disadvantages attributed to PM generators are the loss of the field current strength control variable, a more complex assembly and disassembly process, and the high cost price of the (NdFeB) magnetic material.

Toothed belt

Another major Vensys innovation first introduced in the Genesys concept is a pitch-control system that employs near wear-free toothed belts instead of the more common hydraulic cylinders or geared electric pitch motors. One key advantage claimed for the toothed belt drive is its insensitivity to shock loads, as such impact forces are distributed over multiple meshing teeth pairs. A second advantage is that the drive system does not require grease lubrication and is therefore regarded as virtually maintenance free. Toothed belts themselves may be new as power transmission elements for wind turbine pitch drives, but applications for industrial and automotive purposes are widespread. Among highly demanding successful applications is the rear wheel drive of all Harley Davidson motorcycle models, replacing the traditional steel chain. And, like all modern pitch-controlled turbines, triple redundancy in Vensys turbines is provided by means of three separate AC-type pitch control motor drives. Each pitchable rotor blade is therefore fitted with either a battery-powered or maintenance-free, double-layer, capacitor-type energy storage back-up system. Both independent fail-safe systems provide sufficient power to the pitch system, enabling a safe stop in case of an event such as a grid failure. Vensys turbines are fitted with the Ultra Caps system designed for about 100,000 charging cycles, which under average operating conditions is said to cover the entire installation lifespan.

Detail of passive generator cooling system


Toothed belt pitch drive

Each blade system also has the built-in capacity to act as an independent aerodynamic brake. The 1.2 MW Vensys 62 mechanically builds on proven Genesys design principles and operational experiences, but it also features a newly designed 620V 88-pole PM generator. The generator design again incorporates a passive air cooling system and the turbine a toothed belt type pitch drive. Also the mechanical drive train layout again comprises a proven combination of a stationary main pin and a rotor hub that rotates on two slow-speed taper-roller bearings with grease lubrication. The generator rotor is directly flanged to the rotor hub. The mechanical drive solution is largely similar to Enercon’s direct drive turbine generator and rotor support concept (except the initial 230/280 kW E-30). The Vensys 62 prototype was erected in May 2003 at Sitzerath, a town 50 km from Saarbrücken. However, a fire in the surprisingly spacious nacelle, which contains the cast main pin carrier, a turbine control unit and four yaw motors, severely damaged the machine within months of its commissioning. After extensive repairs the prototype was put back into service and has been operational since.


Thermal reserves

A second Vensys 62 wind turbine has been operating in China since June 2005. It is owned and operated by Goldwind, the first local licensee for the Vensys 62. In addition, the Chinese firm markets 600 kW and 750 kW REpower turbine models under licence.

Based on the Vensys 62, the German company developed a low-wind-speed version with the same capacity but an enlarged 64 metre rotor. Extensive testing revealed that built-in thermal generation reserves allowed a capacity up-scaling from 1.2 MW to 1.5 MW, with only minor modifications required to the generator. In practice, even during elongated full-load periods the generator coil temperature should never exceed the maximum design temperature during normal operation. Excessive coil temperatures are known to drastically shorten generator life and in PM generators excessive magnet temperatures result in a permanent loss of magnetic properties. On the other hand, when a direct drive generator runs with relatively low operating temperatures, this offers room for the power output to be increased without sacrificing generator operational lifetime. The reserves found in the 1.2 MW Vensys generator resulted in two scaled-up 1.5 MW sister models, the Vensys 70 (Diameter (D) = 70 m; IEC WC II) and Vensys 77 (D = 77 m; IEC WC III). Both the 70 metre and 77 metre rotor diameters were selected as these sizes are common in the volume wind market and it enables Vensys to purchase semi-standard rotor blades from multiple suppliers. Similar rotor blade sizes are, for instance, applied at the 1.5 MW REpower MD70/77 series and GE’s 1.5 MW range. LM Glasfiber of Denmark and Germany’s Abeking & Rasmussen supply the blades for the company’s 1.2 MW and 1.5 MW wind turbine types. The Vensys 70/77 is today’s volume model. Power output is about 30% more compared to the smaller Vensys 62/64 sister product, while the price difference is only €200,000.

Naked ambition

At the Saarbrücken HQ, Vensys engineers are now developing a new 2.5 MW Vensys 90/100 series that will become available with either a 90 metre or a 100 metre rotor. Among the technology features is a new PM generator with integrated cooling system, and a different power converter technology. Also new is a single main rotor bearing, a technology choice that is in line with a major wind industry trend. As part of the latter design approach, the traditional main shaft has been eliminated and the rotor hub is connected directly or with an intermediate flexible device to this single large-diameter rotor bearing. The solution is analogous to a comparable layout found in recent wind turbine designs like the 3 MW Vestas V90-3 MW, 2.5 MW Fuhrländer FL 2500, 1 MW and 3 MW WinWinD turbines, and the 5 MW Multibrid M5000. The single main bearing concept was first applied for in a 250 kW gear-driven HSW turbine in 1988. This was followed in the mid-1990s by a gear-driven 1.2 MW Autoflug prototype, and the direct drive Lagerwey 750 kW.

The new Vensys drive train design applied for the Vensys 90/100 is designed to substantially reduce the combined mass of the nacelle and rotor, the so-called top head mass (THM). The objective is to bring the new multi-megawatt class Vensys wind turbine from a THM point of view in line with comparable state-of-the-art gear driven turbine types. Besides substituting the ‘traditional’ main pin and the cast bearing housing with its two bearings by a single rotor bearing, additional mass savings have to come from a relatively lighter generator. That design objective is a Vensys 90/100 generator mass that is only about 20% more compared to the mass value of a Vensys 70/77 generator, achieved despite a 67% increase in power rating. However, reaching a THM for direct drive wind turbines that is in line with modern geared equivalents is by no means easy. Direct drive concepts are inherently at a disadvantage due to the square-cube rule for mass against volume, where the relatively large ring generator dimensions negatively impact on total mass. With improved modelling tools, stronger and lighter materials and clever design solutions, wind turbine designers attempt to address the issues. A proven mass-reducing product design strategy is the application of spokes and hollow structures in disc-shaped objects, instead of developing solid components.

Licence to thrill

While Vensys has issued a number of licences to partners around the world, the total number of wind turbines running, and therefore the operational track record, is still relatively limited. Goldwind of China has progressed most in terms of numbers turned out of its factories and put in active service. It holds a Vensys 62/64 and Vensys 70/77 licence valid for the Chinese market only. The company recently completed a new manufacturing plant in Beijing and boasts a total order volume of more than 600 units in 2008, subdivided into both 1.2 MW and 1.5 MW turbine types.

A second Vensys licence is held by Spanish company Eozen, formerly named El Marquisado Eolico. It bought 1.2 MW and 1.5 MW turbine manufacturing licences for the Spanish market. The Madrid-based project developer earlier this year completed a brand new manufacturing plant in Andalucia, a region in the south of the country. Among the plans is to develop a 38 metre-long rotor blade and to build these in-house as well as manufacturing Vensys-type wind turbines.

In Canada a newly established local subsidiary is active under the trade name Vensys and is responsible for local sales and marketing. The third Vensys 62 prototype was installed in Springhill, Nova Scotia, in Canada during December 2005 by Ottawa-based developer Vector Wind Energy Inc. Three more Vensys 62 machines followed at the same location in 2006. Of late a new licence has been issued to Indian company ReGen Powertech Ltd that is valid for the local Indian market, and a second licence to Enerwind of Argentina that is valid for Brazil.

Most recently, the large Czech industrial engineering group CKD NOVÉ Energo acquired a 1.2 MW Vensys 62/64 licence that is valid for the Czech Republic and Slovakia. The CKD conglomerate, sometimes referred to as the ‘Siemens of the Eastern-block,’ is active in the manufacture of railway equipment, components and systems for the gas industry, electric machines such as generators and motors and power plants up to 100 MW. In addition, CKD has decades-long experience with manufacturing heavy-duty electrical machinery for hydropower plants. The Prague manufacturing facility is still in an early production development stage and had, up to August 2007, turned out 12–15 Vensys 62 and Vensys 64 units. Total production in 2007 amounts to 18 1.2 MW turbines and this volume is to be expanded to 35–50 units in 2008, says Vensys-CKD general manager Karl Navratil. Wind turbine production currently takes place in a small confined section at one factory building and amidst other mechanical construction activities. Navratil comments, ‘Our plan is to gradually up scale production in Prague to an annual volume of 500 MW per year from 2010. This includes two assembly lines for generator production, which is already one of our key competence areas. We are in addition expanding co-operation with local component sub suppliers, as part of ramping up the wind turbine production supply chain.’ CKD plans to establish a centre for new energy technologies including biomass, solar energy and hydropower in a refurbished part of the existing facilities and Vensys-CKD will be housed in this technology centre. Navratil adds that in order to meet demand, CKD should manufacture many more units, but faster expansion has been curbed by insufficient working capital.

MainWind, which buys, sells, reconditions, and re-erects used wind turbines all over the world, recently erected the first two 1.2 MW Vensys turbines manufactured by CKD in Poland.

The Dutch company’s Managing Director Robert de Knecht observes: ‘Due to continuing gearbox problems with conventional turbines and perhaps also other reasons, many of our potential clients are keen to buy a direct drive turbine.’ He points out that their choice is currently limited to only one major supplier, Enercon, and a couple of new wind market entrants. And like all other equipment suppliers, Enercon customers face long turbine delivery periods.

‘What we therefore need most at the moment is a much larger quantity of Vensys turbines to serve our clients. Equally important is the building of a sound track record with multiple Vensys installations operating in different wind climates and geographical conditions over a prolonged period,’ concludes de Knecht.

Eize de Vries is Wind Technology Correspondent with Renewable Energy World

Direct drive, no easy success

Over the past three to five years a number of newcomers have tried to make their mark in the direct drive market segment, without exception aiming to seize a share of the rapidly expanding global wind turbine market. Like many of its competitors, Vensys aims to exploit competitive advantages of modern, high-yield direct drive wind installations to fuel its growth. By eliminating the trouble-prone gearbox from the wind turbine drive train, superior systems reliability and elongated operational lifetimes are claimed.

The gearbox is a crucial component in geared wind turbines with a fast generator, a proven concept that in multiple design variations continues to dominate the world market. Its main function is to step up slow rotor revolutions in the speed range of around 8–20 rpm for a 2 MW turbine, say, to a nominal generator speed usually of 1000 or 1500 rpm. By contrast, in a direct drive turbine, generator speed is identical to that of the rotor. Since the launch of the 500 kW Enercon E-40 direct drive turbine in 1992 competitors have been keen to enter this promising market segment. Often these new direct drive contenders have developed clever and highly innovative products, but their commercial success has generally been limited. Some developed only designs while others succeeded in building a prototype or even a small series. Among many direct drive wind pioneers are names like Jeumont Industrie of France, US-based Kenetech, and Heidelberg, MBB, Neptun, and Seewind – all of Germany. Unfortunately they haven’t had the success they initially hoped for. And, during the past decades the share of direct drive turbines in the world’s supply mix has remained at 13%–15% on a MW basis. Enercon has successfully maintained its leading role as a semi-monopolist, market and technology leader, and achieved an impressive cumulative operational track record of over 11,400 turbines with capacities from 100 kW to 6 MW. Lagerwey of the Netherlands ranked second in the commercial direct drive wind turbine market segment with a total track record of about two hundred 750 kW installations. However, the Dutch wind company filed for bankruptcy in 2003.

Recent direct drive initiatives

Scanwind of Norway–Sweden developed a 3–3.5 MW turbine with a PM type generator and a 90 metre rotor diameter.

M. Torres of Spain developed the 1.5 MW TWT 1500/72 with a 72 metre rotor. The turbine was later scaled up and optimized into the 1.65 MW TWT 1650/70 and a 1.65 MW TWT 1650/77 sister model, featuring rotor diameters of either 70 metres or 77 metres.

Leitner AG of Italy developed the 1.2 MW Leitwind LTW 61 with a 61 metre rotor. The turbine was turned into the 1.5 MW LTW 70/77 that is available with rotor diameters of either 70 metres or 77 metres.

Emergya Wind Technologies BV (EWT) of the Netherlands during 2003 acquired Lagerwey’s Intellectual Property Rights. The deal included the 750 kW direct drive LW50/52 turbine series, which was scaled up into the 900 kW Directwind 900 with an enlarged 54 metre rotor. The company earlier this year got a new main shareholder. Short-to-medium-term plans include the development of a new multi-megawatt class turbine, and a stock listing before the turn of the year.

Before Lagerwey’s bankruptcy, Zephyros became an independent company but also went bankrupt again. The current owner is Harokasan of Japan that operates from a production location in the Netherlands.

Unison of South Korea most recently developed a new 750 kW direct drive turbine U-series featuring a water-cooled multi-pole generator, and rotor diameters of 50 metres, 54 metres, and 57 metres for different wind climates. The turbine is a series product.

Finally, DarWinD of the Netherlands aims to develop a 4.7 MW offshore wind turbine with a two or three-blade rotor. According to company sources, a DD 115 prototype with a rotor diameter of 115 metre with these or other specifications is planned for late 2008 or 2009.

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