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Close up -- XEMC's XD115/5MW offshore turbine

In a ceremony that featured the slogan "Dutch design integrity -- Chinese industrialisation power", XEMC Windpower launched its 5MW XD115/5MW offshore-dedicated direct-drive prototype. Eize de Vries, who was present at an ECN test field in North Holland province, the Netherlands, examines the turbine's roots and main technology features.

XEMC's X115 5MW offshore turbine
XEMC's X115 5MW offshore turbine

XEMC Windpower of China acquired former Darwind in July 2009 from Dutch renewable-energy company Econcern, which had earlier filed for bankruptcy. The company was renamed XEMC Darwind. However, the XD115/5MW dates back to 2000 when former Lagerwey with Dutch government support, established a consortium with key suppliers named Zephyros.

The partners aimed to develop an innovative 1.5/2MW direct-drive turbine for both onshore and offshore application, building up earlier 750kW Lagerwey experiences. The efforts led by chief designer and now XEMC Darwind chief technical officer Cees Versteegh resulted in a patented Z72 turbine design that contained several innovative technology features.

These included an ABB-design passive air-cooled 3kV/4kV permanent magnet generator, a single rotor bearing, and a cast main carrier. The turbine would initially be offered in 1.5MW for onshore use and, as a faster spinning 2MW (4kV) version for strong-wind (offshore) locations without noise restrictions.

A 1.5MW Z72 prototype with 70-metre rotor diameter was installed near Rotterdam in April 2002. Zephyros became independent in 2003, but filed for bankruptcy two years later.

A Dutch insider said it was characteristic that two identical sets of photocopies were made of the Zephyros design plans. One went to new owner Harakosan of Japan, which would concentrate on further development and marketing of Zephyros-based onshore turbines. The second blueprint was for Darwind founded in 2006, for developing an offshore-dedicated 5MW turbine.

XEMC Windpower acquired the rights from Harakosan in 2007 to manufacture and market rebranded Zephyros turbines on the Chinese market and according to its own figures has delivered and commissioned around 800 of these XE-series turbines.

Zephyros technological dissemination did not stop there. Following the acquisition of Harakosan Europe by STX Heavy Industry of South Korea, STX Windpower was established in 2009 and now manufactures rebranded Zephyros-based 1.5MW and 2MW turbines, which have undergone further development.

The XD115/5MW features a modest 115-metre rotor diameter. Its nominal tip speed is 108m/s substantially higher than the more common 90m/s values for offshore operation. Choosing a higher rated tip speed typically results in more aerodynamic noise — XEMC documentation indicates 115dBA (decibels) from rated wind speed, while greater care has to be taken regarding increased blade erosion risk. On the other hand, higher rotor speed enables a boost in power output for a given torque rating.

This could partly explain the quite low 270-tonne head mass (nacelle + rotor) for this IEC WC IC 5MW turbine, which among others positively impacts loads, masses and tower and foundation costs. The XD115/5MW design completion has been largely the responsibility of XEMC Darwind’s Dutch development team, however the permanent magnet generator was built by XEMC in China. The latter industrial group employs over 16,000 staff and could thereby benefit from design and volume manufacturing experiences with hundreds of XE-series generators. The 5MW XEMC generator measures around six metres in diameter with a mass of 130 tonnes.

The XD115/5MW generator temperature management is conducted 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 incorporating a heat exchanger. The two exposed pipes on top of the nacelle form part of this cooling-system loop, which provides up to around 40% of the total cooling capacity required under demanding operational conditions.

A dedicated offshore feature is that the nacelle interior uses overpressure to protect the internals against the harsh marine climate. Another reliability enhancement measure was achieved by locating the majority of failure sensitive components, including the water-cooled power electronic converter and transformer, switchgear, and turbine controls in the tower bottom.

Finally, a second near-shore prototype will be installed in China around 100 metres from shore in Fujian province, a project planned for completion Q1 2012 while turbine certification is planned for Q3-2012.

The next important step for XEMC will be the realisation of multiple XD115 pre-series turbines installed offshore to demonstrate its marine-environment capabilities before commencing large-scale commercial deployment.

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