Applying a special protective coating to prevent ice from adhering to surfaces – a simple, compelling idea. Drivers wouldn’t have to scrape ice from their windscreens in the morning, aircraft wouldn’t have to be de-iced with noxious chemicals, and wind turbines would keep their optimised shape and run with maximum efficiency. Scientists around the globe are currently working on just that – anti-ice coatings. Researchers in Bremen, for example, are producing (super-) hydrophobic surfaces with microscopic or nanoscopic textures. “These cause water droplets to simply run off when outside force is applied – like when they are exposed to wind or when the rotors turn,” says Stephan Sell from Bremen’s Fraunhofer Institute for Manufacturing Technology and Advanced Materials (Ifam). Sell explains that as a result of the special texture of these surfaces, droplets of water contract into a nearly perfect sphere as soon as they make impact. Contact with the rotor blade is thus minimised – and the droplets roll off be-fore they can freeze.
Learning from water bugs
Scientists at Harvard University are trying to develop new ways of getting around adhesion by looking to Mother Nature for inspiration. They are taking their cues from insects known as water striders or pond skaters, which have myriad tiny bristles on their legs that allow them to actually walk on water. Researchers hope to adapt this trick for use on functional surfaces. Their approach involves creating a cushion of air between the surface and water droplets, which reduces the area of contact between the two. The smaller the contact surface, the smaller the adhesive force (that makes water “stick” to the surface) – and without water, no ice. Both the Fraunhofer and Harvard approaches have been tested with some success in laboratory experiments. But these new hydrophobic surfaces still have a way to go before they are ready for use in real-life situations – particularly in terms of durability. The potential of such hydrophobic coatings is not limited to wind turbines. Researchers see overhead power lines, LED headlights, and cooling fins in heating exchangers as possible areas of application for the new technology, because ice build-up means down-time no matter where it occurs, and that always costs money.
Fan blows vs heater mats
When it comes to wind turbines, experience has shown that de-icing systems are vital in certain locations; otherwise the turbines simply have to be switched off too often. Accordingly, virtually all manufacturers offer blade heating systems, of which there are basically two kinds. In one, such as that supplied by Enercon, a high-powered fan blows hot air onto the blades. The other type of system, which is used by Nordex and Siemens for example, employs heater mats that are laminated onto the blades. However, these “active de-icing systems”, as they are known in the trade, not only consume electricity, they also require sophisticated sensors to determine when heating needs to be deployed and for how long.