His formula has remained unchanged since the beginnings of wind energy: make a start and see how far you get. “And with wind turbines, you got pretty far back then with material that was available anywhere,” recalls Stiesdal, “even if you didn’t have a lot of background knowledge.” He acquired the engineering expertise on his own, while studying biology and physics. With a prototype developed by Stiesdal, the Danish company Vestas grew into the world’s largest manufacturer of wind turbines. When Vestas began suffering turmoil, Stiesdal moved to a competitor, the Danish windmill manufacturer Bonus, which was acquired by Siemens in 2004. Siemens is the first company to commercially employ a 3.6 megawatt wind turbine in large numbers for onshore and offshore applications.

“Wind parks on the open sea have great prospects,” says Stiesdal. But many obstacles must still be overcome in order to substantially increase the proportion of wind energy used worldwide. For instance, all wind turbines operate with a gearbox that converts the rotor’s low rotational speed into the high rotational speed used to generate electricity. “But gearboxes are very complex machines that are liable to break down,” says Stiesdal. Repairing or replacing a gearbox at sea is difficult and expensive.

That’s why Stiesdal had the idea of building a wind turbine without a gearbox. Together with his colleagues, he tinkered around with a completely new design. “We turned the whole thing upside down: how do the generator parts have to work and be arranged so that the turbine doesn’t need any gears?” It was widely known that a solution like this was possible – and that it was very costly. The team working with Stiesdal developed a turbine whose individual components can be disassembled without great effort, “like the wedges of an orange,” says the researcher. At the same time, each part must be sturdy enough to serve its purpose for decades in coastal waters, constantly exposed to the harshest climatic conditions. The new turbine uses a synchronous generator that is excited by permanent magnets. They directly convert the rotor movements into electrical energy.

Formerly, wind turbines were made individually, “handcrafted,” one might say, from high-tech materials. Today Siemens assembles the turbines in serial production based on a standardized design. Production of the blades, which are made of fiberglass, wood and plastics, is an art in itself. After all, the tips of the rotor blades, which weigh up to 16 metric tons, must be able to withstand speeds of almost 300 kilometers per hour. And the new generators used in the new gearless test machines are among the largest permanent-magnet machines ever built. They run with torques on the order of 2,500 kilonewton-meters. By way of comparison, the engine of a powerful car has a torque of significantly less than one kilonewton-meter.

The first turbine using the new technology is currently being subjected to long-term testing in the field. “If our concept works, the wind park operators will be excited,” says Stiesdal. In China, India, and the U.S., the wind energy business is currently growing at rates of more than 40 percent. Denmark is a leader internationally when it comes to the use of wind power, which already meets approximately 20 percent of the country’s energy needs. In Germany the corresponding figure is at about six percent.