Lagerwey Wind BV of the Netherlands re-entered the wind market backed by an experienced design team that developed a range of innovative lightweight direct drive turbines. At the 2.6-MW prototype in Lelystad, wind pioneer and company founder Henk Lagerwey discussed technology choices and preferences and the formidable challenges linked to building an attractive product portfolio and marketing these in an increasingly competitive global market environment.
Lagerwey Wind’s first commercial project was the development of a 2-MW IEC class IIA direct-drive turbine with 82-metre rotor diameter as a license product for an Indian client. One prototype turns at Germany’s wind turbine-testing site in Grevenbroich since 2009. A second prototype has been in operation since 2010 at a site in Tamil Nadu, southern India. This site is characterised by ± 70 percent capacity factor during the annual six-month period wind seasons, when ambient temperatures are high and operating temperatures demanding.
The initial L82-2.0-MW (82m rotor diameter) served as a blueprint for the larger L93-2.6-MW (2 units) and L100-2.5-MW platform (2 units) successor models. All have a distinct nacelle shape with a prominent ring generator located directly behind the rotor. The L93-2.6-MW, like other Lagerwey turbine models, has all of its power electronic systems including the switchgear and converter located in the tower base. The compact nacelle is accessed via an elevator, stairs, and through the cast main carrier from below. A hollow stationary main shaft with twin taper-roller bearing assembly is attached in front and carries the enclosed generator rotating part plus rotor hub with blades. The hollow main shaft also enables easy service access to the hub internals, whereas service tasks like anemometer exchange can also be performed from a safe internal position.
The heart of each Lagerwey turbine is an in-house designed permanent magnet generator that offers full temperature management through passive or natural air-cooling.
Lagerwey explained its main generator design principles and specific features: “Basis is a classic air-cooled inner-rotor design with the stator directly exposed to ambient airflow. Natural cooling as a main benefit does away with the need for cooling-fans and heat exchangers, and elimination of these moving parts reduces overall complexity and positively impacts generator reliability.”
The use of permanent magnets provides a high torque density and that plus the natural cooling results in a compact generator with 49-tonne mass. “We further eliminated a traditional stator housing. This allows for the shortest possible thermal path with minimized heat barriers between generator air gap where most heat energy is generated and dissipation to the exposed stator cooling fins,” he added.
A third distinct generator feature is the form-wound stator coils that are characterized by wires with rectangular instead of circular cross-section. This wire choice according to Lagerwey enables a high copper-filling factor inside the magnetic steel stator slots, which thus promotes optimized slot area usage, the company said. This in turn curbs stator coil current-flow losses, which boosts electrical efficiency and reduces generator-cooling demand because it dissipates less heat.
On the actual manufacturing process, Lagerwey explained: “We produce these form wound coils in-house by means of an highly accurate determined winding process, which optimizes heat transfer to the stator magnetic steel providing a shortest path. Another main benefit of precision coils manufacturing is that we could eliminate epoxy resin as ‘filler material’, which substantially enhances insulation system long-term reliability.”
The integration of these and other design and manufacturing process-related features has resulted into a maximum generator operating temperature in the 70-105 0C range, according to Lagerwey. “This is well below the 140-degree maximum design temperature, which is a critical factor for preventing premature insulation material degradation and irreversible loss of magnet magnetic properties,” he explained.
A final and likely rather unique generator rotor feature is the application of fully embedded magnets that offer full protection against the external environment, said the company. The assembly involves structural and chemical magnets fixation, coating and hermetically sealing them from ambient air exposure.
During turbine operation temperature status levels of the stator, coils, magnets, and main bearings (oil) are continuously monitored.
For a small emerging supplier to simultaneously develop a new turbine including in-house generator and main shaft solution from scratch definitely deserves recognition. On the other hand, adding more than two major innovations in a single new product is also considered to exponentially increase risks of failure. Clipper Windpower’s faith is just one example where ambition to develop a turbine with unusual in-house gearbox and in-house blades quickly led to costly issues marking the company’s ending.
Lagerwey has so far passed this critical initial product lifecycle phase rather well and its modest turbine fleet is claimed to operate with 98 – 99% average availability. Also in its favor is a recently published turbine platform review report by UK consultancy firm SKM, which concludes that it ‘is well designed and well manufactured, assembled and constructed’ and that the platform has reached the highest ‘technology readiness level’.
A new much larger L132-3.8-MW model for IEC class IIA is in development. Main product specifications include a naturally cooled generator with only 75-80-tonne mass, 132-metre rotor diameter and 133-metre hub height. The flagship builds technologically on the L93 and L100 series, and offers for its IEC class a competitive specific power rating of 233W/m2 (L93-2.6-MW - 383W/m2). The IEC class IIIA L100-2.5-MW is by contrast fitted with a modest rotor size (318W/m2), compared to, for instance, the Nordex N117/2400 low-wind benchmark turbine (223W/m2). The company is currently evaluating options to increase the L100 rotor size, by switching from ‘classic’ to loads-reducing slender blades. The L132-3.8-MW prototype is planned for Q1-2016.
Also new is a patent-pending bolted steel shell tower with 135-metre hub height, first applied with the L100-2.5-MW. It has a nine-metre base diameter that cones upward to 2.3 metres, while the upper tower section is a ‘normal’ tubular steel segment. Lagerwey calls it a cost-effective solution with favourable mass characteristics, ±400T for a 135-metre L100-2.5-MW tower, compared to concrete-steel hybrid tower equivalents.
“Our tower design consists of standardized basic elements, steel sheets each measuring 2.8m x 12 metres. A German metal fabrication specialist pre-bends these individual sheets according our specifications, while low-energy plasma cutting is used for making precision holes for the bolt joints,” he said.
A foldable crane system still in development will be employed for future installations incorporating the bolted steel shell tower, and the company said this will be ideal for space constrained forested and other demanding locations.
The L100-2.5-MW will be Lagerwey’s main volume product for the time being, with currently two turbines operating in The Netherlands and Finland, and firm contracts for another over ten units signed. Additional markets are opening up in Turkey and Ireland. The first bolted steel shell prototype is planned in Lelystad during Q1-2014.
Lagerwey sees the much larger direct-drive pioneer Enercon as its natural first competitor, but its European main competitors include familiar names, especially Nordex and Senvion (former REpower), and occasionally Vestas.