SLIPS Coating Technology against Ice Build-up on Offshore Wind Turbines

What is one of the major problems in offshore wind farm operation? ... The build up of ice on the rotor blades of offshore wind turbines.

Low winter temperatures in offshore locations, especially in Northern Europe can create ice on the blades of wind turbines. The accumulation of ice generates additional weight on the blades that can reduce the energy production of the wind turbine and even completely paralyse it. Moreover, too much weight could even damage the blades. The turbine blades must therefore be capable of preventing the formation of ice in order to guarantee their correct operation and ensure that they do not paralyse the generation of energy. Coatings for ice melting that work as resistors which are heated by a electric current are available. However, as this kind of coatings consume energy and are costly, research on passive antifreeze systems are underway, i.e. research aimed at avoiding ice formation on blades without energy consumption.

Micro/nano coating developments for ice-phobic surfaces can be categorised into two main research fields:

• Hydrophobic nanocoatings imitate the lotus’s hydrophobic surface. Recent papers claim that in lotus based surfaces ice nucleation may happen faster than on smooth surfaces in the same highly humid conditions.
• Antifreeze nanocoatings represent new techniques for the nanostructuration of surfaces with antifreeze effects.

This article looks into a potential application of SLIPS based materials (Slippery Liquid Infused Porous Surfaces) in the offshore wind energy sector. SLIPS are designed to expose a defect-free, molecularly flat liquid interface, immobilized by a hidden nanostructured solid. On these ultra smooth slippery surfaces fluids and solids alike (including water drops, condensation, frost, and even solid ice) can slide off easily.

Ice formation by deep freezing (to -10°C) in high-humidity conditions (60% relative humidity) and
subsequent de-icing by heating

According to the team at Harvard University and the University of Puerto Rico, under the lead of Dr. Eisenberg surfaces are coated with slippery liquid infused porous surfaces (SLIPS), which use gravity to shed any liquid or frozen droplets of water.

“SLIPS is a system-based approach that uniquely combines and matches surface structuring and chemical functionalization with infused liquids to overlay an immobilized ‘sea’ of lubricant on a surface,” said Dr. Aizenberg who co-founded SLIPS Technologies and chairs its Scientific Advisory Board. “This results in self-healing surfaces with almost perfect slipperiness toward practically everything, and our system can be customized for the specific physical, chemical and environmental conditions of a particular application.”

The cross-disciplinary group at both universities had previously looked at an ice-free surface inspired by the water-repellent lotus leaf, but this technique can fail in high humidity as the surface textures become coated with condensation and frost. SLIPS, however, consists of a molecularly flat lubricating film that overlays a nanostructured solid, which holds the lubricant in place. Water droplets find it difficult to gain any purchase on the film so they slide off easily.

The first SLIPS invention disclosure dates back to 2010 and marked a major milestone in the technology's development. As mentioned-above SLIPS was invented to address the performance shortfalls of traditional "Lotus Leaf Effect" surfaces that attempt to achieve omniphobic properties but fail to deliver: they are unstable under extreme temperature and pressure conditions; they are not truly omniphobic (for example, they are not suitable for low-surface-tension liquids); they are not optically transparent; and they are fragile and easily damaged.

SLIPS is a truly omniphobic, highly repellent surface with optical transparency, scratch-resistance, self-repair, and self-cleaning characteristics that can operate under extreme environmental conditions.

The SLIPS design concept was inspired by the carnivorous Nepenthes pitcher plant: it uses rain to transform its solid surface into a microscopically thin, smooth and slippery immobilized "sea" of water. The plant's surface is now friction-free, and any insect crawling on it loses all traction and slides right into the center of the plant to be digested as food.

Learning from the pitcher plant, SLIPS transforms a solid surface into a microscopically thin, smooth and friction-free immobilized "sea" of lubricant (the lubricant is water in the case of the pitcher plant). This is done through our proprietary combination of nanoscale surface textures, surface chemistry, and a liquid lubricant that can then be "woven" and immobilized on almost any kind of surface. The surface preparation and the choice of lubricant are matched to form a system that is customized for the particular material and for its intended application.

SLIPS has been internationally recognized and highlighted since it was first disclosed in 2010 and presented publicly by inventor Joanna Aizenberg in June 2011 at the Nanotech Innovation Forum in Boston.

All in all SLIPS is destined for application in the offshore wind energy sector, which would definitely be a benefit to all offshore wind farm operators. 

For more information on SLIPS, please view the following video:

Photos: Courtesy of Vestas Wind Systems A/S and SLIPS Technology

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