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XEMC Darwind Refocuses on Onshore Wind with 4.5- and 5-MW Direct Drive Turbines

XEMC Darwind has ambitious plans to enter the main European onshore wind markets with two new 4.5- and 5-MW direct drive turbines based on its 5-MW offshore XD115/5-MW model. XEMC Darwind’s CEO Hugo Groenemans explained the new turbine’s technology specifics and benefits, which include merging Chinese supply chain strengths with job creation and localized industrialization.

XEMC Windpower in 2012 ranked fifth among China's largest wind turbine manufacturers, offering XE-type 2-MW and 2.5-MW direct drive volume-series with a Dutch technology basis origin. The wind business is part of the state-owned XEMC Group established in 1936 as a manufacturer of electric motors and generators. XEMC is today a medium-sized manufacturer based in China and employs more than 11,000 staff.

Design Principle

During 2011 XEMC Darwind installed a 5-MW direct drive XD115/5-MW prototype at an Energy Research Center of the Netherlands (ECN) test site. This turbine is designed for International Electrochemical Commission (IEC) wind class 1B and technologically based upon similar design principles, but adapted to meet specific demanding offshore conditions. Less than a year later a second similar prototype was installed at a near-shore location in the Chinese Fujian province.

A two-year testing and optimizing programme of the Dutch prototype has now been completed, with a Provisional Type A certificate awarded this July. During the past six months its overall turbine technical availability was 95 percent.

XD115/5-MW turbine technology features include a compact fully enclosed and over-pressurized nacelle, incorporating a “classic” inner-rotor XEMC-design permanent magnet generator measuring around six metres in diameter, and a single rotor bearing.

Generator temperature management is enabled by a combination of passive air-cooling, with external air flow passing over the exposed outer stator body, and a supplementary forced-air cooling system that incorporates a heat exchanger. Two exposed pipes clearly visible atop the nacelle form an integral part of this cooling system loop, providing up to 40 percent of the cumulative cooling capacity required under the most demanding high-wind conditions.

Favourable

All failure sensitive power electronic components, including a water-cooled converter and transformer, switchgear, and turbine controls are located in the tower base. The turbine is further characterised by a modest-size 115-metre rotor diameter combined with a relatively high-rated rotor speed of 18 RPM, and for its class favourable 281-tonne head mass (rotor + nacelle). This low-head mass positively impacts turbine loads, tower and foundation loading, masses and costs.

The XD115/5-MW’s 108 m/s-rated rotor blade tip speed is substantially higher compared to common values for offshore turbines that lie typically in the 80-90 m/s range. A turbine configuration with fast-spinning rotor in relation to a given rotor size and power rating allows for reduced generator size, mass and cost. A resulting higher blade tip speed inherently raises aerodynamic sound intensity level (noise). Choice for a high rated tip speed requires additional measures like a special protective coating applied at the blade’s leading (rounded) edge to prevent excessive erosion-related wear and increased risk of premature failure.

“The initial idea for entering onshore markets with new XD-platform turbines originated from the Dutch office and was approved by our majority shareholder,” said Groenemans. “The product-market refocusing includes a continued offering of the XD115/5MW for European and international offshore projects. In parallel, the XD-platform is expanded by two 4.5-MW onshore sister models each destined to a specific IEC wind class.”

Modified

The original XD115/5-MW product specifications indicate 115dB(A) (calculated) maximum sound intensity. Turbine noise is not considered important for offshore applications, but it does matter in onshore wind, especially in noise-sensitive populated areas where it impacts product market chances. Many suppliers under these conditions strive for a maximum value of around 106dB(A), whereby it should be noted that -3dB(A) represents an intensity of sound reduction by 50 percent.

The first onshore model called XD115/4.5-MW is destined for noise-constrained IEC class 1A/1B high wind speed sites, but for locations without such noise limitations the XD115/5-MW will be supplied. The XD115/4.5-MW is technically largely identical to the XD115/5-MW, and already commercially offered. Among the onshore adaptations is a modified generator cooling system, which partly uses clean ambient air due to less stringent demands for a completely sealed nacelle.

Addressing the potential implications of this high 115dB(A) turbine noise figure for XEMC Darwind’s onshore market opportunities, Groenemans explained: “Our on-site prototype measurements showed much more favourable at 108dB(A) sound intensity, representing a huge reduction compared to the initial calculations. Due to our aim for further reducing turbine noise while leaving generator dimensions and specifications unchanged, we decided with the XD115/4.5MW for a slightly reduced rated rotor speed. This also explains the lower rated power, which is a function of generator torque and rotational speed, but has here only limited yield impact for a majority of potential onshore sites.”

Enlarged Rotor

The second onshore sister model called XD133/4.5-MW is being developed for low and medium IEC class 3A/2B wind conditions and only comes in a 4.5-MW version. This turbine will be equipped with a newly developed generator and an enlarged 133-metre rotor diameter.

“Generator outer diameter and general layout remain largely unchanged, but generator length is increased to compensate for a substantially reduced rotational speed of the bigger rotor,” said Groenemans.

The design team further opted for a state-of-the-art slender blade design with carbon fibres incorporated in highly stressed structural areas. This results in a lower mass compared to the much shorter traditionally designed XD115/5-MW (4.5-MW) blades. Cyclic pitch control is another design feature introduced in the XD133/4.5MW, whereby the angle of each individual blade is continuously adjusted during each full rotation.

“The combination of these new lightweight slender blades and advanced control were a genuine breakthrough in the reduction of turbine loads. This in turn resulted into an almost unchanged head mass, despite 34 percent rotor swept area increase,” he added.

The XD133/4.5-MW will feature tubular steel towers up to about 100-metre hub height, and concrete-steel hybrid towers up to 140 metres in the latter combination adding up to a 206.5-metre total installation height. 

“Crossing the 200-metre ‘barrier’ was enabled by a combination of low head mass and the increased availability of cost-effective high-tower designs, together with a growing visual acceptance for such landscape dominant objects,” said Groenemans. “Most important, for similar hub heights and within IEC wind class boundaries, the XD133/4.5-MW will generate about 20 percent more electricity compared to the XD115/4.5-MW.”   

Key Markets

The XD133/4.5-MW prototype is planned for Q1-2015, and commercial market availability in 2015. Groenemans expects the initial markets for both 4.5-MW turbine models to include the northern and eastern parts of Germany, the Netherlands, the UK and Ireland. He pointed to the fact that Ireland’s local grid network currently cannot absorb more wind power, but a planned interconnector cable with the UK might open up fresh opportunities.

Groenemans finally believes that building a European supply chain is essential, and aims to have turbine assembly and testing close to main markets concentrated in the Netherlands and possibly additional locations. 

For the 4.5-MW XD-series, specific main components will be procured in China (including XEMC-made generators and finished castings), and other components in Europe, including especially blades, towers and rotor bearings.  

“XEMC is well known for building high-quality generators, and we can also benefit from their favourable labour costs and cheaper permanent magnets supply. The quality of Chinese castings and their product finishing is comparable to similar European components,” said Groenemans. “Our main role will be in acting systems integrator, by creating local jobs and in ensuring that our clients are served with quality turbines offering high the highest possible return on investment.” 

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