The vast majority of wind turbine capacity throughout the world is on land, with only approximately two percent being offshore.
Much of this offshore capacity – over 90 percent – is in Europe where the driving force behind the growth in wind energy investment is an European Union regulation which requires 20 percent of the EU’s energy to come from renewables by 2020.
The advantage of offshore locations, particularly in northern Europe, is that they have strong, persistent winds throughout most of the year so that they are able to match the energy performance of conventional power stations fueled by gas or coal.
However, the offshore wind turbines also require coatings able to withstand enormous stress such as the massive impact of rain and hail drops on blades at tip speeds of 300-500 kilometers per hour (200-300 miles per hour).
The current trend is the development of larger offshore turbines with a higher energy output so that the coatings, particularly those on the blades, have to be even more resilient.
“For coatings companies the wind turbine market, particularly offshore, is a big opportunity since a typical wind farm uses more coatings than an average conventional power station,” said Anders Voldsgaard Clausen, group power generation segment manager, Hempel, Denmark.
“Wind turbine coatings are sophisticated, complex chemical products, which require highly qualified staff to develop and formulate,” he added.
At the end of 2012, Europe had offshore wind turbine capacity of approximately five gigawatts (GW) of electricity or 92 percent of the world total with much of it in the northwest Atlantic and North and Baltic Seas. The UK accounted for close to 60 percent of the European total, Denmark 18 percent, Belgium eight percent and Germany six percent, according to figures from the World Wind Energy Association (WWEA), Bonn, Germany.
Over the last few years the UK, as part of a big expansion in wind energy, has accounted for two thirds to three quarters of the growth in global offshore wind capacity. One in three of wind turbines installed in the UK are now offshore.
Nonetheless, the center for offshore wind turbine production and development has been Denmark where wind turbines were first introduced in the 1980s. The country is the wind turbine manufacturing base of Vestas Wind Systems of Denmark and Siemens AG of Germany, the two global leaders in wind turbine production. It also has a number of major centers for testing new prototypes and their materials.
The leading coating companies serving the sector are Hempel, Jotun of Norway and Netherlands-based AkzoNobel, which are already the world’s major suppliers of marine coatings.
There are also a number of niche players working on specialist coatings, particularly for the new generation of rotor blades.
Some companies, like BASF and Bayer MaterialScience (BMS) of Germany, have a strategy of being a supplier of a wide range of materials, including coatings, for wind turbines.
Coatings for the machines’ steel-framed towers and foundations make up the largest segment of the offshore turbine coatings market in volume terms. It tends to be dominated by marine coatings producers who can take advantage of their expertise for coatings for maritime conditions.
“The easiest method of gaining the confidence of the relevant parties (in the offshore wind industry) is by demonstrating a track record in the offshore environment,” said Clare McDermott, business development manager for wind energy at AkzoNobel.
A priority for OEM turbine manufacturers and wind farm operators is that the coatings should need minimum maintenance during their average 20-year life cycle.
“Correct coatings specification, preparation and application will help protect offshore wind assets for a specified time without the need for major maintenance,” explained McDermott. “Long-term costs can increase substantially if assets are not protected correctly from day one due to the offshore environment being so harsh.”
Coatings producers are having to respond to changes like the building of large turbines, new foundation types and positioning of wind farms in deeper waters with even harsher conditions.
“(Existing foundations) may not be the foundations of choice as turbines increase in size and move into deeper waters, with jackets, and even floating turbines becoming more prevalent,” said McDermott. “These structures are, in general, more complex to coat.”
Much of the cost of mainly polyurethane-based coatings for towers and foundations is in their application during the manufacturing process, which involves three coats—the primer, middle barrier and top coat. These can take six to ten hours per coat to dry, according to Hempel.
“One way of cutting the costs of the three-coats application is to merge the primer and middle phases into one so, with the top coat, there are only two,” said Clausen.
Another option is a change formulations to speed up curing rates by combining polyurethane with polyaspartic systems.
“Polyaspartic systems are very reactive and might be applied in their 100 percent solid content, so they are especially suitable for applications where high film thickness and fast curing are necessary,” said Maria Almato Guiteras, a BMS coatings specialist. “Besides their use in blade coatings, polyaspartics are the technology of choice for protective coatings applied on the towers of the wind turbines.”
The biggest technological challenge in wind turbine coatings, however, remains the protection of the blades, which not only need a high level of resistance to erosion by rain, hail, snow and ice as well as UV light. They must also have durability while maintaining the aerodynamics of a variety of blade shapes.
The polyurethane and related coatings materials have to be compatible with materials within the structure of the blades, usually comprising epoxy resins and glass or carbon fibres.
Unlike with coatings for towers and foundations in offshore turbines which are governed by standards such as Norway’s Det Norsk Veritas, there are no agreed standards for coatings blades.
“The turbine manufacturers have adopted their own systems to differentiate themselves,” explained Clausen. “You normally have to customise your coatings to meet the different needs of manufacturers.”
With blades becoming even bigger with lengths of as long as 90-100 meters, the necessity for standards is even more pressing. “The longer the blades the higher the wing tip speeds and levels of stress and erosion,” said Clausen. “There are no perfect solutions to this problem yet.”
The reward for coatings companies in developing successful high tech products for the European offshore wind turbine market is gaining a head start in the fledgling global offshore sector. Once the technologies for offshore conditions have been proven to work there is likely to be within a few years a large world demand for them.