Big Offshore Wind Turbine Tests Well

Issue 7 and Volume 108.

By Drew Robb

Poised to become part of the energy mainstream, wind faces one of its most difficult challenges to date: Scaling output up to the level required by utilities. While it took the industry almost two decades to reach the 1 MW threshold, turbines have eclipsed the 3 MW mark over the past several years.

GE Energy’s 3.6 MW offshore series, for example, has just finished almost a year of rigorous testing and is now being utilized in a 25 MW development project, Phase 1 of what could be a 520 MW offshore windfarm. The Arklow offshore wind park, recently erected off the southeast coast of Ireland, is the first commercial deployment of wind turbines specifically designed for offshore use in excess of 3 MW per unit.

“Developments such as the 3.6 MW wind turbine will play a major role in establishing wind power as a commercially viable method for producing clean energy worldwide,” says Steve Zwolinski, President and CEO of GE Energy’s wind segment. “We plan to make Arklow Bank a showcase of offshore wind power production.”

The 3.6 MW offshore turbine is based on the GE 1.5 MW series, of which GE has installed more than 2,400 units worldwide. But the 3.6 MW series dwarfs its predecessor in many respects. It has a much greater generator size, a rotor diameter of more than 340 feet and a swept area of 27,870 square feet. Rotor speed of the turbine is held stable in the range of 8.5 to 15.3 rpm to prevent serious fluctuations in the turbine’s power output. Annual energy yield is affected by the average rpm over the course of a year — the windier the site, the greater the yield. At 33 ft/s, for example, a turbine would provide 15 million kWh/year.

To optimize cost of ownership, designers also incorporated features including a new blade design, an improved gearbox concept and adjustments to the structure to enhance load absorption and optimize assembly, transport and service logistics. It also includes advanced power electronics technology and a variable speed rotor (photo).

In the summer of 2002, the 3.6 MW prototype was erected in Spain’s Castilla la Mancha region, where wind characteristics closely approximate those experienced offshore. In September 2002, the machine was connected to the network of Spanish energy supplier Iberdrola.

GE’s 3.6 MW offshore wind turbine has a rotor diameter of 340 feet with a variable rotor speed of of 8.5 – 15.3 rpm. Rated wind speed is 49 ft/s with a cut-in wind speed of 11.5 ft/s and cut-out of 82 ft/s. Photo courtesy of GE Energy.
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Tests by Windtest KWK GmbH confirm that the GE 3.6 is well equipped for the high seas. Windtest found the actual power curve for the GE 3.6 to meet or exceed these values in terms of machine performance. Windtest observed a higher output curve at all wind speeds. Secondary tests demonstrated the measured output at the lower end of the power curve was slightly below the expected level, while at the top end of the curve, the 3.6 MW turbine actually performed better than projected in the initial power curve. Overall, measurements indicate that the prototype fulfilled or exceeded engineering expectations.

Although noise emissions in offshore applications are of secondary importance, turbine designers specified an operating noise level maximum at the machine room of 109 dB — a mere 2 dB louder than its 1.5 MW predecessor.

With the prototyping and testing period completed, the 3.6 turbine is now undergoing its most serious challenge to date. Seven turbines of this type have been installed on the Arklow sandbank six miles from the Irish coast. The Irish Sea represents the most difficult sea conditions ever faced by offshore turbines. Previous sites have favored more sheltered waters such as the Baltic Sea. Thus the success of this project against the fury of Mother Nature will probably do much to accelerate the learning and implementation of best practices for future offshore wind development.

Because the 3.6 MW represents the cutting edge of turbine technology, it demands a significant amount of monitoring, piloting and time spent on commissioning. GE is monitoring both the Arklow Bank Wind Farm and the Spanish prototype simultaneously. This concentration on quality has already reaped rewards. During the rigorous commissioning phase at Arklow, extensive work has been done to optimize vibration behavior and add a higher level of redundancy to turbine operations to minimize maintenance trips. Further, engineers are adding further remote operation capabilities to the equipment to eliminate certain hands-on maintenance tasks.

In addition, each of these sites will be a platform for further enhancements allowing rapid validation of configuration upgrades and performance improvements that are scheduled for later in 2004.