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In the EU Windheat project, scientists are developing a so-called carbon-nanotube coating that keeps rotor blades ice-free. The image shows the exposed, black special coating.
© Fraunhofer IPA
Rotor blade coating
15.12.2014

Heating system keeps rotor blades ice-free

In cold climate zones, the wind is particularly strong meaning best conditions for a wind turbine. However, in the cold temperatures there is the danger of ice that can freeze onto rotor blades. This can lead to imbalances, impairment of the rotor blade’s aerodynamics and a drop in the efficiency of the turbine. The so-called carbon-nanotube coating offers a solution.

In frosty storms and sub-zero temperatures, a layer of ice forms on the rotor blades. The aerodynamic properties deteriorate and the turbine produces less energy. The ice changes the loads and imbalances occur, meaning that the rotor blades can be damaged more easily. Furthermore, falling icicles are a safety risk. Until now, the turbines were immediately shut down on the formation of ice. The annual electricity production also drastically sinks as the ice causes losses of performance of 14 to 20 per cent. Therefore, many companies shy away from constructing wind turbines in northern, colder regions, even though the wind is strong.
This is where the EU “Windheat” project comes into play: alongside six companies from four countries of the European Union (EU), researchers at the Fraunhofer-Institute for Manufacturing Engineering and Automation IPA in Stuttgart are developing an energy-efficient ice recognition and de-icing system. It shall help to avoid downtimes due to weather conditions.

Only iced-up zones are heated

Earlier heating systems warm the whole rotor blade. For example, this happens by blowing warm air through the inside of the blades, no matter if the whole blade is affected or not. That requires a lot of energy and reduces the efficiency of the turbine.
In the Windheat project, the scientists divide the rotor blade into different zones. Each zone is equipped with a carbon-nanotube (CNT) coating and its own ice detector. “Our coating of carbon-nanotubes only heats the zones that have actually been iced up. That is mainly the edges of the rotor blade,” says Anne Gerten, scientist at the Fraunhofer IPA.

Special coating makes ice melt

Small sensors constantly measure the temperature and the moisture on the surface, react to the slightest fluctuations and recognise when water freezes. If ice is formed, the detectors switch on the corresponding heating element, i.e. the CNT coating, within seconds. Once the ice has melted, the heating system turns itself off automatically. The aim of the project is to increase the energy efficiency by at least 18 per cent using this de-icing strategy.
The CNT coating, which is a few micrometres thick, can be easily applied to the rotor blade by spraying it on to a self-bonding polymer film. A clear lacquer insulates the coating and gives it additional protection against moisture and mechanical influences. The researchers picked the material because of its excellent electrical properties. “In principle, the carbon-nanotubes are wrapped layers of graphene that touch in places. Electric energy is converted into heat where they make contact,” explains Anne Gerten.

Heating systems are also possible for large wind turbines

“We used cost-effective materials for both the sensors and the heating elements. It is an important requirement in order to produce the de-icing system in mass,” says engineer Sascha Getto, who is responsible for the tests in the wind tunnel. The prototypes were conceived for small wind turbines and designed so that they can be scaled up. “Using the Windheat system on larger turbines is absolutely possible and it would also generate significant energy savings for them,” Anne Gerten adds.
Windheat is a European project and is funded by the European Commission within the 7th Framework Programme.

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Link

Windheat
Website for the EU Windheat project.