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6-MW Direct Drive Wind Turbine Begins Offshore Trials

Construction on DONG Energy's Gunfleet Sands demonstration project, 8.5 km off the English coast in 13-metre waters, is now complete following the installation of two 6-MW Siemens wind turbines, the company has announced.

Work on commissioning the direct drive turbines has now begun and the demonstration project is expected to start generating electricity in spring 2013.

Two prototypes have already been installed onshore in Høvsøre and Østerild, Denmark, at DTU's national wind turbine test centre, but this project is the first time the 6-MW turbines have been tested offshore.

Renewable Energy World spoke with Henrik Stiesdal, chief technology officer at Siemens Wind Power, about the demonstration project, Siemens’ contract with DONG Energy, and the turbine development process.

The Tests

When asked how long the demonstration project will run, Stiesdal said the two turbines are "intended to be energy-producing machines – so their lifespan will be as long as the rest of the wind power plant", projected for 20 years. The 6-MW turbines are designed for a 25-year life.

Testing will proceed during the first two years of installation, said Stiesdal, and will measure how the larger machines perform in real-world conditions offshore. Stiesdal noted that the machines will be relatively closely spaced, so the wind conditions that arise when one turbine is downwind of another will be tested, as well as offshore wake conditions which are more pronounced than onshore. Tests will measure the turbines’ responses to wind and wave load patterns that cannot be replicated onshore, and how they will function when connected to an offshore grid.

Another important area for trial will be the working environment on the turbine, Stiesdal said. As wind farms move further offshore, the people who maintain and repair them are increasingly up to 100 km away from medical assistance and comforts. "In the wind industry there is often an element of hardcore mechanical engineering determining the layout of the machine, and then the working-place consideration is an add-on. We want to do things differently on the 6-MW," said Stiesdal.

The two Gunfleet turbines also test various changes Siemens has made to the prototype machines. For example, the air-to-liquid cooler atop the turbine was moved from the back of the nacelle to the front. The cooler was originally installed at the back end of the machine "mainly for traditional and cosmetic reasons," according to Stiesdal. But it was moved after tests showed that it could potentially restrict the ability of a helicopter pilot to make quick evasive maneuvers if necessary. Stiesdal also mentioned a change in hand rails to make it easier for workers to climb, and the elimination of the hydraulic crane as it was found that an overhead rail and a small hoist would do the job "smarter and easier".

The Rollout

Siemens had previously signed a framework agreement with DONG Energy to deliver 300 of the 6-MW turbines for use in DONG’s projects in the U.K., many of which are proposed for deeper waters. The turbines are to be delivered between 2014 and 2017. "The agreement with Siemens will enable DONG Energy to install a significantly larger turbine from 2014 compared to the turbines we use today," said Paul Childs, DONG Energy Wind Power’s U.K. communications manager.

Stiesdal projects that a pilot series of between six and 10 machines will be installed this year in Denmark, Germany, the Netherlands and the U.K. Asked about previous projections of up to 50 turbines, he explained that "If you have too many you end up with a large fleet of machines that are not completely identical to the final serial ones." At the heart of Siemens’ development process, Steisdal said, is to make one to two prototypes and then a pilot series, the main purpose of which is to "get some operating hours" in order to gather comprehensive statistics on any operational issues. Delivery of the turbines will start in 2015-16, he said.

He also noted that the framework agreement is a master agreement which identifies a number of projects for which DONG expects to place orders with Siemens. It specifies only projects in the U.K., but "that is a coincidence, a timing issue," and he emphasised that Siemens will still have machines available for other companies.

The Future

When asked whether Siemens expects to make any major revisions to the 6-MW turbine’s design, Stiesdal was emphatic: "No. We have not seen anything in the prototypes that justifies significant changes." He said the company will make tweaks only, such as ergonomics or changes to the heating and cooling systems. 

So is Siemens leaning toward using direct drive technology in all its future offshore designs? Stiesdal says unambiguously, "Yes. It’s on the drawing board because that’s how the world is: with a bigger machine, direct drive becomes even more attractive. It scales very well, which yields a big benefit. Bigger is better in terms of economy."

The U.K.’s Round Three projects are in deeper waters and will have much larger capacities than existing wind farms, so larger turbines will be needed. There are only two currently operational wind turbines with a higher-rated capacity than the Siemens 6-MW turbine: a 7.5-MW turbine from Enercon and a 6.15-MW turbine from REpower. Neither is a dedicated offshore machine.

Japan’s Marubeni Corporation owns a 49.9 percent stake in Gunfleet Sands. The wind farm's total capacity is 172 MW. 

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