A review of current and emergent technologies for wind turbine coatings formulation
At first read, the topic seems particularly specialized and perhaps useful to only a small segment of the population of coatings formulators. There is much more to it than that. In itself, this is a case study of how the industry has managed to address practical differing needs for wind turbines based on offshore, onshore and icy cold climates, how these were addressed initially, and what has been learned to steer the current technology and body of work.
In addition, it has forced the development of several new laboratory tests to quantify differences and improvements in varying approaches and ensure that these relate to actual in-field conditions and results. Since a wind turbine blade is nothing more than a rotating fixed wing blade—as you find on a plane, or a rotary blade on a helicopter—the developments for wind turbine blades have transferrable applications to aircraft.
Other than the offshore market, no other segment is challenged by the severe conditions of in-field paint application. Besides the down time associated with painting a turbine, there is the great expense of mobilizing the heavy equipment needed to access the blades, which are 130-170 meters long in the case of large wind farms.
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Winding through turbine history
The largest wind turbine known in the 1940s, a 1.25-megawatt (MW) turbine that sat on a Vermont hilltop known as Grandpa’s Knob, fed electric power to the local utility network. Wind electric turbines persisted in Denmark into the 1950s but were ultimately sidelined due to the availability of cheap oil and low energy prices.1
The oil shortages of the 1970s changed the energy picture for the U.S. and the world. It created an interest in alternative energy sources, paving the way for the re-entry of the wind turbine to generate electricity, as well as evaluation of solar, tide and geothermal sources,
From 1974 through the mid-1980s, the U.S. government (NASA, NSF, DOE) worked with industry to advance the technology and enable development and deployment of large commercial wind turbines. This research and development program established many of the multi-megawatt turbine technologies in use today. The large wind turbines advanced under this program set several world records for diameter and power output for that time.
Today, the largest turbines are offshore for several reasons. Transportation of long blades and tower sections on highways is very difficult and the population continues to object to the turbines as disrupting the aesthetics of mountains on shore.
The Vestas V164 has a rated capacity of 8-9 MW. The windmill has an overall height of 220 m (722 ft), a diameter of 164 m (538 ft), is for offshore use, and is the world's largest-capacity wind turbine since its introduction in 2014. Each blade weighs over 34 tons and the blade tip speed is 104 m/s or 232 miles/hour.
Wind turbine coatings concerns
In the wind industry, most concerns for coatings are for blades because they are constantly exposed to the weather and moving in air. Blades need coatings because their pitting can roughen the surfaces enough to create unstable harmonics that decrease a turbine’s efficiency, while increasing maintenance and repair costs.
The initial challenges for blade coatings were erosion from airborne sand hitting the coating at high speeds, chemical attack from organic acids due to bug swarms, and ice accretion in cold climates. The first occurred in California and the Midwest, the second primarily along Altamont Pass in California and the third became known in Alaska and Canada, where chunks of ice were thrown from the blades, or became inoperable when the blades became unbalanced. For the most part, heavy industrial coatings found in the oil, gas and marine market segments are utilized for the mast and nacelle.
The initial development of blade coatings was focused on hard, hydrophobic surfaces that were believed to be able to prevent ice accretion, be easily cleaned by rain when bugs or detritus accumulated and would be durable to the abrasion of airborne particulates. This was only partially successful, and some wind farm owners resorted to the use of fluoropolymer tapes for the leading edges of the blades for protection from abrasion, since many of the early coatings failed in that respect.
Wind turbine coatings innovations
Recent advances in blade coatings center on softer coatings that are still tough, but not brittle. In addition, some of the functional and “smart” attributes are finding their way in to the development of turbine coatings.
Colorado State University researchers have invented an environmentally friendly, inexpensive, long-lasting, ice-repellent coating that outperforms today’s best de-icing products and that could keep everything from cars and ships to planes and power lines ice-free. Their innovation, described in the Journal of Materials Chemistry, is a gel-based, soft coating made out of PDMS (polydimethylsiloxane), a silicone polymer gel with already widespread industrial use. Their experiments were supported by careful analysis of ice adhesion mechanics.2
US Patent Application US2017/0002230 A13, “Icephobic Coatings with Temperature-Dependent Wetting,” claims durable, impact-resistant structural coatings that have both dewetting and de-icing capabilities. The application explains that the coating has a very low ice adhesion strength, which is the energy required to dislodge ice. This patent was filed by HRL Laboratories, LLC.
US Patent Application US2017/0015862 A1 filed by Akzo Nobel “Coating Composition”, relates the invention of coatings <250g/L VOC that are fast drying and have excellent erosion resistance, through the use of polyamine(s) resin and specific aliphatic or aromatic esters. This is also useful for rain-erosion resistance which is a concern for aircraft.
In an article in Durability & Design, Stephanie Shira writes about elastomeric, rubbery, soft polymer composites with catalyst and healing agents microencapsulated in the main polymer matrix. The self-healing polymer composites are homogeneous on a macro scale, yet heterogeneous on a micro scale. These micro differences in material properties and inter-material interactions account for the macro-scale physical properties and seeming self-healing ability.4
To passively remove ice (as by wind or travel velocity, or under its own gravity, as on a power transmission line), the ice adhesion strength should be under 20 kilopascal (kPa). Recent research has developed icephobic coating formulations with ice adhesion strengths as low as 0.2 kPa.
A different mechanism for ice shedding are “slugs”. SLUGs (Self-cleaning LUbricating organoGels) are another means of achieving the same end of repelling ice from surfaces. However, they function by a completely different mechanism.
BASF in US 2017/0044397 A1 takes a different approach in “Aqueous Two-component Coatings Compositions and High Erosion Resistance Coatings Produced Therefrom”. The coating composition comprises at least one aqueous polymeric resin, at least one carbonate diol and a hardener component comprising at least one polyisocyanate-modified polyester.
As the reliance on renewable energy increases, there will continue to be new developments in coatings, for not only wind turbines, but also solar panels and equipment for tidal energy farms.
- PCI Magazine: Researchers Develop Ice-Repellant Coating
- USPTO: Icephobic Coatings with Temperature-Dependent Wetting
- Durability + Design: Self-Healing Icephobic Coatings [registration required]
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