On top of the world
The ship approaches Tower 91. When it is completed, Horns Rev will resemble a slightly tilted matrix of 10 rows of eight towers each—a rhombus pointed northwest. (The numbers denote the turbine's position in the grid; 91 will sit atop the first tower in the ninth row.) We drop anchor at around 2:15 p.m., and it takes the ship about 90 minutes to settle into a position between 20 and 21 meters from the tower; only then can the crane lift its 92-ton payload.
Then the men suit up. There's a tension and (dare I say it) an electricity in the air. Poul Møller and Rolf Kirk, who are about to climb the tower and await the nacelle, are smiling. There's a certain urgency, too: because nacelle 91 was loaded last, it will have to be installed first. But 51 will be the first tower plugged into the transformer station on a platform about 800 meters away, in just a few days. The towers will be wired serially, in five groups of 16 each, using 34-kV, lead-sheathed, three-phase cable, with conductor cross sections of 95 or 150 mm2, depending on load. (Power is produced at 690 V and converted to 34 kV by a transformer in each nacelle.) Thicker 400-mm2 cable will carry current from the five lead turbines to a central transformer, which will step it up to 165 kV. A sea cable with a conductor cross section of 630 mm2 carries that current to shore, following a natural channel.
Small reports ripple off in the distance, as a pile driver pounds heavy tower foundations some 20 meters into the seabed. Horns Rev is being built in stages, and several crews are working out here today: a couple of ships are laying foundations; another is placing transition pieces over the foundations; and the Ocean Hanne is erecting wind turbines—in all, about 150 people are involved. The work follows the wiring plan: the first row of turbines will be finished first, then the first two columns, then the next two, and so forth. So far, only the first row of towers has been erected.
Supplies for the two men heading up the tower are brought up by crane: buckets of bolts, tools, and Vitell bottled water. But in the last hour, the weather has turned. Suddenly, a thick fog has enveloped the ship, and there are reports of thunderstorms heading this way. It's 13 šC out, but it seems much colder. "That's just the moisture in the fog," A2Sea supervisor Svend Hansen says up in the wheelhouse. He's calling for weather reports every few minutes now. "You always feel it like that." At about five o'clock, just as the crew is readying the lift gear, Svend walks out on deck and calls a 30-minute rain delay.
The men file into the mess for a subdued dinner. They say little as they eat. But then Svend appears after just 15 minutes to announce the postponement has ended early. There is an immediate clamor of scraping knives and forks and stacking dishes, and within a few minutes, the mess is empty.
The weather looks grim, but the winds are still and the nacelle is hoisted and placed atop of the tower in just 15 minutes. The choreography of the ropes—which involves scurrying first to the tower and then up to the bridge deck as the suspended nacelle rotates 180 degrees—is executed flawlessly. While four men on the boat hold the guylines tight against the railings, the crew up top provisionally bolts the nacelle down—I can hear, from way up there, the rat-tat-tat of the impact wrench—and within an hour, the nacelle has been unleashed from the crane—my cue to don a harness and climb 62 meters up the tower's ladder and take a look. This is exhausting, though earlier in the day I got some good advice. "Just lean back against the tower wall," Lars suggested. "And don't use your arms too much, use your legs." A few minutes up the ladder and I can see neither where I began nor where I will finish.
At the top of the tower, Rolf and Lars are using a hydraulic tool set at 1600 meter-newtons to tighten each bolt. Theis and Poul are up in the nacelle. To get to the hub, where Poul is doing finishing work, I must slide headfirst over a giant ball at the nacelle's front end, a back-up hydraulic pump to stop the blades in case of emergency. It's easy to imagine sliding over the ball and then plunging out the hole at its bottom. Inside, the rotor hub is hardly big enough for two hunched-over men. Through the hole, and through the mist beyond it, the ship looks very small. "You do not think about it," Poul says. A few minutes later, he unhooks his harness from a bar on one side of the hub and steps over to the other and re-hooks. Astonishingly, for the briefest of moments, only surefootedness keeps him from tumbling 70 meters down to the sea—or so it seems to me.
The threat of another storm sends us back down to the ship, and work stops for an hour or so before Poul, Theis, and Rolf head back up to install the third rotor blade. Vestas has built a special lift gear to clasp the rotor, one that won't damage its fragile skin, and to send it straight up to the hub. Poul and Rolf will be inside to guide it, having removed the eye-shaped panel that comprises most of the hub floor. Theis will help guide the rotor from the nacelle's roof.
At 11 o'clock a.m., the crane rumbles to life. The crew has trained a spotlight on the nacelle, and against the fogged-up sky, it's really beautiful. Periodically the mist lifts and the transformer station appears, shimmering on the water like a ghost ship. Then the mist descends and even the tower, just 20 meters away, is barely visible.
Back on the ship, I can make out two men up in the hub, shadows crouching inside the gaping hole. The lift gear and the rotor it holds rise tentatively, lurching up a few meters, then stopping, then lurching again. It seems to take a lot longer this time, but maybe that's just the cold—the moisture just gets you in the nose. I watch through binoculars as the round end of the blade moves seemingly into position and then out, exposing a crescent of light from within the hub, almost like the moon passing through its phases in speeded-up time. Then, at 12:40 a.m., René and Thomas tie their rope to the railing and walk over to tell me that it's done.
It had taken these men less than eight hours to finish a task they would have been lucky to complete in twelve. Yet René and Thomas hardly seem jubilant as they walk down to the main deck, where their crewmates are now milling about. But then, there is more to do. There are 90 bolts on blade 1 to tighten, first by hand, then by hydraulics, which will take six or seven hours. There is a whole other nacelle to install tomorrow. And then there are 77 more after that.
About the Author
ROBB MANDELBAUM is based in New York City, has contributed to The New York Times Magazine, Discover, and Worth. This is his first article for IEEE Spectrum.
To Probe Further
The progress at Horns Rev may be tracked at http://www.hornsrev.dk, while the Danish Wind Industry Association supplies detailed technical background information in five languages at http://www.windpower.dk. Meantime, the American Wind Energy Association (http://www.awea.org) keeps up-to-date on wind energy topics in the United States and abroad.
The U.S. Department of Energy's National Renewable Energy Laboratory posts information on the latest government research—including state wind maps—and funding at the Web site for the National Wind Technology Center (http://www.nrel.gov/wind).