The global wind turbine industry has transformed from a collection of small companies in Denmark to corporations pulling off enormous feats of engineering.

Prototype wind turbines whirl at a testing site in Osterild, near the northern end of Denmark’s Jutland peninsula. By Rasmus Degnbol

OSTERILD, Denmark — At the northern end of Denmark’s Jutland peninsula, the wind blows so hard that rows of trees grow in one direction, like gnarled flags.

The relentless weather over this long strip of farmland, bogs and mud flats — and the real-world laboratory it provides — has given the country a leading role in transforming wind power into a viable source of clean energy.

After energy prices spiked during the 1973 oil crisis, entrepreneurs began building small turbines to sell here. “It started out as an interest in providing power for my parents’ farm,” said Henrik Stiesdal, who designed and built early prototypes with a blacksmith partner.

The initial windmills made by small operations had quality problems. Blades — then just 15 feet in length — would break or fall apart.

Now, they are giants, made by global players pulling off enormous feats of engineering.

Technicians reach the roof of these enormous wind turbines either via an internal elevator or, if the turbine is installed offshore, by helicopters that lower them into the fenced-off area. By Rasmus Degnbol

The biggest turbines in Osterild stretch more than 600 feet high. The largest rotor blades can reach 270 feet in length, comparable to the wingspan of an Airbus A380, the world’s largest commercial plane. The price tag: Up to 10 million euros, or more than $12 million.

The monstrous scale has helped turn wind into a mainstream form of power.

Larger turbines harness more wind, creating more energy. The biggest modern offshore turbines produce nearly 20 times as much power as ones developed three decades ago.

The larger the size, the lower the cost of generating energy. In parts of northern Europe, wind is now a major power source. It accounts for 4 percent of overall global energy supply, according to the International Energy Agency.

Blades for wind turbines lie outside a factory, waiting to be transported to wind farms. By Rasmus Degnbol

From those early Danish innovators, the industry has grown to be dominated by companies like Vestas Wind Systems and Siemens Gamesa Renewable Energy.

The heart of the Siemens Gamesa business lies in Brande, a small Jutland town. It was there in the early 1980s that an entrepreneur named Peter Sorensen founded a wind business called Bonus with a couple of workers from his father’s irrigation company.

Siemens bought Bonus in 2004, and today, Brande is home to large engineering, training and maintenance hubs.

Staff sitting at consoles there can monitor wind farms around the globe. Often, when a problem shuts a turbine down, they can restart it electronically without needing to send a maintenance team.

Siemens Gamesa staff can check the performance of turbines around the world, and fix problems remotely, often from their desks in Brande. Carsten Snejbjerg for The New York Times What began as the offshoot of an irrigation company in the Danish town of Brande is now a major industrial enterprise. Carsten Snejbjerg for The New York Times

At a cavernous workshop, technicians build custom turbine models and facilities for testing whether components are robust enough to last two decades or more. Inside, the towers are so big that elevators haul engineers up and down. Passengers must wear a climbing harness in case they fail.

At a Siemens Gamesa training center, technicians learning how to lower themselves with ropes and harnesses in case elevators inside the turbines fail. Carsten Snejbjerg for The New York Times

The rotors are connected to the windmill tower by a nacelle — a large enclosure the size of a trailer, with plenty of room inside to walk around.

Large enclosures, or nacelles, connect a wind turbine's rotors and house its generator and controls. Carsten Snejbjerg for The New York Times

Looming over the top deck outside are the rotors. When they whirl, the whole column sways like a ship at sea.

A technician walks inside a prototype wind turbine in Osterild, Denmark. These test models help turbine makers boost performance. Carsten Snejbjerg for The New York Times

Making these blades is difficult and labor-intensive.

Teams of workers gradually fill a mold with strips of fiberglass interlaced with balsa wood for strength. They then inject resins and other chemicals into the container to form the hardened structure.

The huge size of the blades, and the complexity of the process, mean completely automating it does not make economic sense. Around 1,300 people work in the factory, and making a single blade can take about three days.

It’s a difficult balance for manufacturers to achieve both size and efficiency.

The largest blades already weigh around 30 metric tons, and making them longer adds to their weight, fast. Overweight blades might lead to turbines being worn down faster, and would put enormous stress on other components.

A group of workers prepare a mold for a wind turbine blade. They fill the mold with fiber glass cloth in a process that loosely resembles making a surfboard. By Rasmus Degnbol

Designers are testing tweaks to the shape and size of a blade, experimenting with the changes in wind tunnels or on computers. They have found that gluing various add-ons to the blades can substantially boost performance.

One fix — a combination of serrated teeth and combs that reduced the sound of the blades — was inspired by the wing feathers of owls.

Many of the big turbines that are designed at the center will eventually make their way to the open water, where there is more space and the wind is more powerful.

Their huge components, difficult to transport on roadways, can be loaded at ports onto special boats, which take them to sprawling wind farms in the sea.

The enclosures which connect the blades to the wind turbine’s tower are readied to be loaded onto ships. By Rasmus Degnbol

The first offshore wind farm was built using a barge that had a crane mounted on a truck. Companies today have developed specialized vessels to carry these offshore turbines to their floating platforms.

They must contend with a variety of challenges, including the corrosive impact of saltwater. To service wind farms far from the shore, maintenance crews sometimes live on special ships.

It’s a complicated calculus.

In the early years, building an offshore wind farm was fantastically expensive, and governments offered generous subsidies to help the industry develop. But prices have been falling, and government support has “melted away,” according to Andreas Nauen, the chief executive of Siemens Gamesa’s offshore wind division.

Lower costs, though, have also made wind power more appealing elsewhere. Mr. Nauen is optimistic that new markets for wind power, which was once mostly concentrated in northern Europe, will emerge in Asia and the United States.

“This is real,” he said.