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offshore windmill

Giants on the High Seas

Ever-larger wind power stations are being developed out on the ocean, with rotor diameters now reaching 167 meters (548 feet). There is a good reason for this trend: Larger facilities are more economical and less dependent on subsidies. But the enormous loads place tremendous demands on systems and their components.

The view from the gondola at the dizzying height of 120 meters (390 feet) extends far across the flat terrain, where coniferous forests alternate with snow-covered fields. Heavily layered clouds move rapidly along the horizon and highlight the most important feature that characterizes the small village of Østerild in northwestern Denmark for many guests: Strong winds normally blow on this elongated peninsula between the North Sea and Limfjord – almost as much as on the open sea. That’s why the Department of Wind Energy at the Technical University of Denmark at Lyngby established the National Test Center for Large Wind Turbines six years ago. Since then, manufacturers such as GE Wind Energy, Vestas Wind Systems and Siemens Gamesa have been testing new offshore facilities here. From Østerild, engineers and visitors can take a close look at a trend shaping the market for wind turbines: They are expanding in size to achieve greater energy yields and become more efficient for their operators.

Flying High: The gondolas of modern wind turbines are positioned significantly higher than 100 meters (330 feet).

The current record-holder at the test site is a system from Siemens Gamesa with a rated capacity of 8 megawatts. It succeeds an earlier 7-megawatt prototype with a rotor diameter of 154 meters (505 feet). By extending the new facility’s individual rotor blades to 81.4 meters (267 feet), the designers increased their coated rotor surface – and thus the annual production of electricity – by about 20 percent. The rotor blades, which are attached to the hub of a direct-drive, gearless turbine, create a rotor with a total diameter of 167 meters (548 feet). “Larger turbines and larger wind parks allow greater efficiency for operators,” said Stephan Buller, Portfolio Manager for Offshore Turbines at Siemens Gamesa. “This is because components can be better utilized in relation to energy inputs or eliminated in many instances – for example, fewer foundations, fewer towers and less infrastructure such as connecting cables.” The costs of service and maintenance are also lower for larger systems in terms of the kilowatt hours produced. Another advantage is that tried-and-tested technology from 6-megawatt facilities is largely used in the new systems. Components such as rotors, inverters and bearings are certainly larger, but the facility’s basic architecture is unchanged. Greater efficiency is the top priority in the wind energy market since public subsidies are being scaled back in many countries and the facilities increasingly have to support themselves. For example, fixed subsidies, which differentiated between onshore and offshore facilities, were paid out in Germany until the end of 2016. The initial payments for offshore facilities were 15.4 cents per kilowatt hour. But under the last amendment to the country’s Renewable Energy Law (EEG 2017), offshore projects going into operation in 2021 or later must apply for a subsidy in a bid process. In an auction in April 2017, the bidders were given the green light if they asked for subsidies between zero and 6 cents. The average bid amount was just 0.44 cents per kilowatt hour.

Rotor blades with a total diameter of 160 meters (525 feet) and more are common.

On one hand, the operators of offshore wind parks can offset the lower subsidy levels with the economies of scale from larger turbines and systems. They are also betting that the technology – which is now produced in high volumes – has survived its growing pains. For example, in Europe alone in 2017, Siemens Gamesa installed about 450 offshore wind power facilities with a total output of 2400 megawatts. About ten years ago, the German-Spanish manufacturer turned to permanently excited synchronous generators. Instead of conventional drivetrains with a gearbox, they have a direct-driven generator mounted behind the rotor. The generator’s outer ring, which is equipped with permanent magnets, rotates at the speed of the rotor around the inner ring, which functions as a stator. This makes it possible to do without a conventional shaft and gearbox. The transformer can thus be housed right in the gondola instead of the tower base, which enables pretests of the entire system in port.

Costs for Operators Decline

The declining cost of installation and maintenance is a key reason why offshore wind energy is seeing high growth rates. Installed capacity in Europe – where about 90 percent of the global offshore wind power is generated – grew strongly in 2017 and reached nearly 16,000 megawatts – a respectable 25 percent increase over the previous year. On the high seas, there are especially good conditions for high energy yields. “For one thing, there is enough space available,” Buller said. “For another, the wind often blows evenly from one direction for several hours, so it often reaches the ideal speed of 10 meters per second (32 feet per second) or more for electricity production.” The expansion will continue in coming years. For example, Vattenfall has just ordered 72 8-megawatt systems from Siemens Gamesa for Denmark’s Kriegers Flak offshore project. Its installation is due to begin in February 2021. With a total output of about 600 megawatts, it will be the largest offshore wind power facility in the Baltic Sea. Another 41 systems of the same design are due to be built off Denmark’s western coast in the North Sea in 2020 and provide an additional 350 megawatts.

Sealing rings with a diameter of more than 11.5 feet pose a challenge.

That is 950 megawatts in all, which is enough to power nearly a million households. One 8-megawatt facility alone will cover about 8,000 households. But despite the economies of scale, offshore wind power facilities are not likely to keep growing skyward indefinitely. In March 2018, GE Wind Energy indeed announced a turbine that is expected to have a capacity of 12 megawatts in combination with a direct-drive generator. But the demands on systems and components grow tremendously at the same time. “There is an optimal size for a wind turbine technologically, even if we cannot determine that point with certainty today,” Buller said. “The energy that can be produced with the turbine certainly rises with the increase in rotor surface. But the loads that are exerted on components such as the main bearing or the bearings for the rotor blades grow even more.” The result: At some point, the engineers will have to incorporate a disproportionate amount of material to handle the loads – which would make the system too heavy and too expensive.

offshore windmill
Good Conditions: Out in the ocean, the wind often blows constantly from one direction for several hours, enabling high electricity generation.

The systems in the 8-megawatt class that are now installed put huge demands on individual components such as seals. The job of the seal rings is to keep the lubricant in the main bearing and dirt particles, saltwater and rain out of the interior of the mechanical elements to be protected. “But with the increasing diameters of the torque bearings, the design principle of earlier shaft seal rings is pushing against the limits of scalability,” said Jens Kuhnert, Business Development Manager and wind power expert at Freudenberg Sealing Technologies. In direct-drive wind turbines, like those that Siemens Gamesa has developed, the seals are attached to the outer ring of the main bearing. It in turn rotates around the stator as a rotor along with the seal and the seal lip. Here the sheer size of the seal rings with a diameter of more than 3.5 meters (11.5 feet) is a challenge for the manufacturing process, which uses huge vulcanization presses. Increases in bearing diameters are certain to lead to a flattening of the radial curvature. Since the spring at the seal lip acts radially toward the interior, the contact pressure — which makes it possible for the seal to perform its task reliably — declines as the curve flattens. “That’s why we’ve developed a new seal ring that contains a meander spring instead of the worm spring used to this point,” Kuhnert said. With this seal – which is called the Seventomatic – a slender, elongated seal lip creates a V-shape with the carrier body. A curved, vulcanized-in profile strip strengthens the seal lip and the body. This makes it possible for the seal to act as a pressure spring. It “presses” onto the running surface on its own, replacing the customary worm spring. In this way, the linear force no longer depends on the curvature, and the seal can do its job extremely well even at higher loads. In addition, the direction of the seal’s force can be varied at will. depending on the design of the turbine. Its spring force can act toward the interior as well as the exterior. This gives the developers of wind turbines and their components new freedom of design.

Remote Diagnosis of Windparks

Furthermore, longevity is an important requirement for all the components of a wind turbine. That’s because operators rely on operating periods of 25 years in their calculations. Based on capital costs, operating costs and the desired rate of return, they compute the socalled levelized cost of electricity, or LCOE, which they compare to the expected yield of electricity. The remote diagnosis of entire wind parks offers still other ways to keep operating costs under control. In these systems, individual wind turbines are equipped with numerous sensors at important points such as bearings or housings; they measure vibrations, temperatures or torque and report the data to a control point on land. For example, Siemens Gamesa operates a remote diagnostic center at its main Danish location in Brande, where the data lines from all the wind turbines come ashore. Conspicuous data patterns are reported to the operators in real time so any forthcoming repairs or maintenance can be planned early. That means it is not just size that makes the giants of the high seas more efficient.


Advanced testing procedure offers performance improvements and better longevity in offshore wind energy component applications. Our press release Predicting the Future from October 15, 2020 brings you up to date on the latest research.

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