Reap The Wild Wind

A fresh breeze is blowing at Horns Rev, where the Danes are building the world's biggest offshore wind farm

14 min read

On a beautiful sunny Saturday morning, unseasonably warm for spring, the MS Ocean Adys is getting ready to leave Esbjerg, on Denmark's west coast. Our cargo is stowed safely on board, and now the crew is making final adjustments before taking in the docking lines and pulling up anchor.

Commissioned just days before, the Ocean Adys has the outlines of a small cargo vessel. But unlike the other ships that leave this busy port city for points the world over, piled high with giant sky-blue corrugated Maersk Sealand containers, the Ocean Adys isn't going very far—our trip will last just four hours. Our destination: Horns Rev (Reef), a wind farm now being developed 14 km off shore by Elsam A/S (Fredericia), Denmark's biggest power producer. The ship, too, carries an unusual payload: two boxy white nacelles, 13 meters long and 4 meters high, stacked one atop the other, each housing the generator and other machinery that turn wind energy into electricity. Two rotor blades are attached to each, forming a wide-angled V at one end.

The Ocean Adys carries a second crew, fielded mostly by Vestas Wind Systems A/S (Ringkøbing), the Danish company that's building and installing wind turbines for Elsam. It's an exciting time to be in the business of harvesting electricity from summer breezes and winter gusts. Wind power is the fastest-growing energy sector in the world, and business is booming for Vestas, the world's biggest wind turbine maker. The foreman of the Vestas crew, a young man named Flemming Olsen, has installed turbines across the globe: in Italy, Morocco, New Zealand, and Costa Rica, as well as in Wisconsin, New Mexico, and California. Other crew members have worked in Greece, Taiwan, Japan, and Oregon.

But Vestas has never erected a wind park as large as this at sea, and neither has any other company. The nine wind farms that have been built offshore are mostly tiny and set in sheltered waters. The largest, near Copenhagen, is capable of producing just 40 MW. Horns Rev, when it comes on-line in November, will dwarf it. Eighty towers in 20 km2 of sea will be capable of generating 160 MW of power, or enough for 150 000 households. Elsam expects Horns Rev to make 600 GWh of electricity a year, 2 percent of the country's total electrical consumption.

To build Horns Rev, Vestas has drafted a subcontractor, A2Sea A/S (Fredericia, Denmark), a company that exists solely to transport and install wind turbines. A2Sea in turn took a lease on two freighters, Ocean Adys and a sister ship, the Ocean Hanne, and transformed them into ocean-going construction platforms. It stacked the Vestas crew quarters—which are in fact corrugated-metal containers, grouped in threes or fours and bolted together—just behind the bridge. It added four 60-ton submersible legs with rounded feet, so that the ship could function as a self-propelled platform. It installed two small cranes and one very large one, 91 meters long, to hoist the nacelles and towers that will hold them. It opened two of the hatches, and put a steel frame in the hold, so that the ship could now carry two towers, each in two sections.

The task today, when we arrive at Horns Rev, is to install the two nacelles atop waiting towers. The crew will probably work through the night. They certainly do not expect to get much sleep. Four men will climb 70 meters to the top of each tower to guide the nacelle and then secure it. The men wear harnesses, but still, it's a long way up, and a long way back down.

A technology taking off

Two piers away from the Ocean Adys's berth, excavators claw at an enormous pile of rubble, rumbling over chunks of concrete and grappling with twisted strands of steel I-beams—the remains of an Elsam coal plant that is being torn down because it didn't meet new emissions rules. Denmark has long been a leader in green energy; over the last three decades, it has poured about US $300 million into research and development as well as consumer subsidies into the nascent industry. The latest version of Energy 21, the long-range plan that Denmark established in 1996, calls for renewable forms of energy to supply 20 percent of the grid by 2003; most estimates suggest the actual figure will be closer to 27 percent, virtually all from wind.

Other countries are wiring themselves for green electricity, but none of them has embraced it so thoroughly. Germany, the United States, and Spain each generate more wind power than Denmark. By the end of the year, Germany will have installed nearly 11 GW of capacity, according to Ringkøbing-based BTM Consult ApS. But the green share of the grid in Denmark is much larger. Lately, the European Union and several American states, with Texas leading the way, have followed its cue in setting targets for renewables. But they're not nearly so ambitious—Texas is demanding just 12 percent of consumption by 2010. Still, last year the United States added 1.6 GW. Windmill maker Vestas now is considering construction of a U.S. factory near its Oregon offices that could build 300 turbines a year.

The fact is, for now wind is blowing other renewable energy technologies away. The U.S. Department of Energy reports that the gales of South Dakota alone, properly harnessed, could bathe the entire nation's living rooms in cool air and the warm glow of television. But in northern Europe, and Denmark especially, landlocked wind farms have run out of room. "You don't have many areas where you can put large wind farms with very big turbines," says Vestas project manager Egon Poulsen. "In Denmark you'll rarely be able to build that kind of farm and not put it on top of a house." Even in areas much less densely populated than Denmark, environmentalists and naturalists tend to dislike the idea of building turbine towers the size of Horns Rev's—structures as big as large transmission towers.

Going out to sea to build turbines has other advantages, too. Because the seascape is relatively smooth, wind off the water normally blows faster—and energy content grows exponentially. (The electricity harvest increases as the cube of the change in wind speed.) What's more, since wind at sea generally does not lose speed close to the surface, towers can be 25 percent shorter than on land. And, with less turbulence over the sea, a turbine will last longer.

Of course, there also are disadvantages. Even though the towers can be shorter, it costs a lot more to build a wind farm out at sea—in Elsam's experience, sometimes nearly twice as much. (Undersea power cables and foundations are the main culprits, but having ships idled by bad weather can add to costs, too.) Aware of the risks and skeptical about the rewards, Denmark's biggest power producer balked at building Horns Rev at first, but the government insisted. A combination of mandated prices and premiums paid by consumers will guarantee Elsam a profit over the next decade.

At the mercy of the elements

Weather is the big wildcard in an offshore project. High winds and waves that reach 8 meters roil the North Sea—that's pretty much the point of putting a wind farm out there. If the wind at an altitude of 80 meters is blowing at 45 km/h or more, crews must stop work. So, to minimize construction time at sea, Vestas has set up a mini-assembly plant in a couple of warehouses on the pier to put together the towers and complete work on the nacelles.

The crew loaded the first nacelle, No. 51, on Thursday evening, and now we're waiting for the weather to change for the better. Vestas calls the nacelle a V80, the 80 designating the rotor-blade diameter in meters. It is state of the art. Six years ago, as a concept on a draftsman's computer screen, a predecessor dubbed the V63 had shorter blades, which, in initial versions, would have revolved at only a partially variable speed. In the interim, Vestas engineers found they could outfit the generator's copper-wired rotor with variable resistors.

Now the generator spins at fully varying speeds, permitting the rotors blades to do the same. Less torque on the driveshaft means "materially less weight," says Vestas's Paulsen, "and weight is money." The rotor blades themselves are made of epoxy pre-impregnated glass fiber, which, Paulsen says, makes them robust while further reducing weight and cost.

This kind of technological innovation, along with the economies of scale brought by the industry's growth, is driving the price of wind-generated electricity down. But ordered ranks of slender wind turbines are still a long way from ubiquity, largely because of the wind's vagaries. Breezes generally are stronger during the day when more electricity is used, and they also tend to match seasonal consumption. But, as it is written, "the wind bloweth where it listeth." At times it's fast, at times it's slow, and at times it doesn't blow at all.

To keep electricity supply and demand balanced when wind is overabundant, Denmark's transmission grids sometimes pay regular power plants not to produce. The task is complicated, however, by the fact that most of Denmark's other power still comes from plants that also produce heat for homes and businesses; the grid operators are required to buy much of that cogenerated power, too. As a result, says Birger Madsen, a principal in BTM Consult, up to 5 percent of Danish wind power is wasted or else dumped in European markets at well below cost. Eventually, says Madsen, transmission lines to Sweden and Germany will have to grow larger, and Denmark will need to "make smart alliances between wind producers and, say, hydro suppliers in Norway," which can generate power at will merely by opening reservoir sluices.

For now, Elsam technicians warn that the Danish grid will soon be unable to handle the instability inherent in power that cannot be dispatched on demand. "For the country, we have a fairly strong grid," says Peter Christiansen, a consultant at Tech-Wise A/S, an Elsam subsidiary, "but the amount of undispatchable power is so great that we will see the problems normally associated with weak grids, such as a lack of reactive power." Because of such problems, the jury is out on the question of whether wind can ever deliver a large share of a region's electricity economically.

Perhaps mindful of this, but probably more concerned about the burdens borne by ratepayers, Denmark's newly elected conservative government earlier this year decreed that the country had too much renewable energy capacity. It canceled subsidies for renewable energy and funding for new development. Three of five planned offshore wind farms got axed; only Horns Rev and a project at Rødsand would continue.

A cynic might say the country's biggest power producer has been left twisting in the wind. Its executives are circumspect when asked to characterize the future of wind power at home. "One of the three plants was Elsam's responsibility," says the company's head of wind power development, Flemming Thomsen. "The obligation to build it will be removed, but we believe it will be postponed, not canceled. It will probably be a public tender, and we'll participate, as will other companies."

Even if Denmark is curtailing its commitment to wind, Elsam executives have good reason to expect the global market to remain strong. Demand for wind power will continue to grow both in Europe, notwithstanding the continent's rightward political drift, and elsewhere, too. In the United States, for instance, developers have proposed big wind farms off both Nantucket and Long Island. Company executives think their experience running Horns Rev will prove valuable. Indeed, Elsam is in the running to build a park in the Irish Sea, near Blackpool, England, where 90 turbines are to be installed, each more than twice as powerful as those at Horns Rev.

Anchors aweigh

We've been moored at shore since Thursday, waiting to put to sea. Fortunately, the ship is quite livable. Each of the 14 men—10 from Vestas, two from A2Sea, and the crane operators—gets his own pleasant little room, carpeted and furnished with desk, couch, and bunk bed; each shares a bathroom and separate shower with only one or two others. Comfortable leather-like chairs and footrests face a very big TV set in the lounge. Since the ship is so new—or because it is run by Danes—everything is pristine. Even so, the odor of stale cigarettes is so powerful onboard that even the smelly dockside air comes as a relief.

The work, when it comes, is strenuous and the schedule—14 days on and 14 off, give or take a day—is fatiguing. Yet some of these men have had tougher assignments abroad. René Leed, who's 27, simply scheduled his wedding and honeymoon around this shift. Thirty-nine-year-old Bo Andersen has been home to central Jutland for less than 100 days in each of the last two years. "I have a seven-year-old son, and I have been traveling for five years," he says. "So we are negotiating within the family right now how long I'll continue to do this. This job is two weeks on, two weeks off, so that's no problem. But three months out—it's too much."

The men mostly hail from Jutland. They are sturdy people, not just in physique but in habits of mind. It takes great patience to do a job like this: nothing happens quickly; every task is really a series of smaller tasks, done very carefully, in a complicated ballet. They are nearly evenly divided between a younger group in their late twenties and early thirties, and an older group in their late thirties and early forties. The older men, as a rule, are clearly in charge. They do most of the talking; they have the most important jobs. The younger men, whose main responsibility is to hold the guylines, are quieter.

Finally, after two days of waiting, punctuated by bursts of protracted activity, dawn broke this morning clean and clear. By 7:30 a.m., when I woke up, the second nacelle, No. 91, was already on board. Now, three hours later, we're cruising out of the harbor.

Shortly after one o'clock, Flemming Olsen calls a "tactical meeting." Ten men, on two couches and four chairs, seem to have taken up all the available space in the lounge. There are two pots of coffee and lots of coffee cups. The meeting is basically to lay out the plan of attack—who will do what, and how. On their first outing at sea, it took them 24 hours to install one turbine. Now they spend a lot of time talking about how to work the guylines designed to keep the rotor blades from flailing about as the nacelle is swung around and hoisted to the tower.

"We talked about which mistakes we made last time, and how to avoid them," Bo tells me after the meeting. For example, when the crew installed the third rotor blade last time, says Bo, "the easiest way for the guys on the ship wasn't always the easiest way for the guys on the tower."

Of course, even if installation is perfect, things may still go wrong with the turbines, at least at first. At Middelgrunden, near Copenhagen, for instance, some seven turbines went out of commission, idled when newly upgraded transformers failed. (The wind farm's owners still don't know why.) "That was another manufacturer, and I don't think we'll have the same problem," Søren Vestegaard, Elsam's operations and maintenance manager, says. Elsam has tested prototypes for Horns Rev on land. "We've done a lot of work in new testing, to prepare the turbines at the manufacturing place, to have them running, so we know they're ready to get out there."

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, while the Danish Wind Industry Association supplies detailed technical background information in five languages at Meantime, the American Wind Energy Association ( 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 (

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