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Whetting the growing global appetite for wind energy has witnessed a revolution in the carbon fibers industry, with the slew of composite manufacturing plants mushrooming around the world. Increasing wind generation capacities have led to developing larger and lighter blades. Offshore turbines have evolved from the conventional 3 MW into next-generation turbines rated at 5 MW and more, with blade lengths for on, as well as offshore systems, exceeding 45 meters (148 feet). Though the negatives of higher cost and paucity of supply continue to plague the carbon fiber industry, it is expected that the wind energy industry would increasingly adopt this versatile material in the manufacture of lighter and stronger blades. The greatest challenge being faced by current wind turbine manufacturers is to prevent longer and thinner turbine blades from hitting the tower when deflected by higher wind loads. This necessitates stiffer composites that have to also be lighter, because with increasing lengths and weights of blades, buckling is very common, wherein the blade collapses under its own weight while standing vertically. A 53 meter long carbon blade has to be 20% lighter than a glass fiber reinforced plastic equivalent, with looser design constraints in areas, such as flutter. Lighter blades enable in reducing root loadings, in addition to those on the remaining structure, which further reduces weight and cost. As per a study, a cost saving of €100,000 could be achieved over the lifetime of a 3 MW turbine if carbon fiber-based blades are used.
Replacing glass with carbon fiber has been an issue on the boil for several years now. Vestas Wind Systems has pioneered the use of carbon fibers in wind turbine blades, with Gamesa, SGL Rotec and DeWind following suit. REpower and LM have also tried to implement this technology, but to no avail. Vestas and Gamesa have adopted carbon fiber technology long back and used the same in select structural parts of blades, which enables in a lighter outfit for the overall turbine system. Lighter blades also translate into less robust turbine and tower components, thereby justifying the reduction in cost that counterbalances the increase of carbon fiber cost.
Opting for carbon fiber has helped Vestas in adding a further 5 meters (16 feet) in blade length, even while not piling on extra weight. The Vestas V112-3 MW turbine has been specifically designed for areas with low to medium wind speeds, and includes three blades of 54.6 meters (179 feet). Though similar in width as the company’s 44 meter (144 feet) blades, the sweeping area of the Vestas V112-3 MW is 55% larger, resulting in a substantially increased energy output. Greenville, South Carolina-based GE Energy has also jumped onto the bandwagon by announcing that the company’s next-generation wind blades, including the 48.7meter (160 foot) blades for its 1.6-100 turbine would be fabricated using carbon fibers. This could very well be a sign for other wind energy companies to adopt this innovative technology, since GE has decided upon a 100 meter (328 foot) diameter rotor on a 1.6 MW turbine, a development hitherto unheard of.
China-based Qingdao Weili Wind and Solar Power Generating Equipment Co Ltd ranks among the smaller manufacturers who has been using 20% carbon fiber by weight in its X600W direct-drive wind generator for marine and terrestrial applications, and V400W wind turbine for use in land and coastal areas. Carbon fiber blades for 2 kw range wind turbines are also being produced. Zytech Aerodyne is another player to use aerospace grade unidirectional carbon fiber for its Lakota wind blades.
The United Kingdom Nova project team has been successful in developing the sails for a fully working 50-kW prototype demonstrator of its new concept 10-MW offshore, double-arm, vertical axis wind turbine system (VAWTS), which would be located on the Cranfield campus and be operational from October 2012. This prototype, manufactured at Cranfield University’s Composites Centre, was assisted in fabrication by Wellingborough, United Kingdom-based Scott Bader Co Ltd, which has provided its Crystic Crestomer 1152PA urethane acrylate structural adhesive for the lightweight structural bonding of the carbon fiber and glass fiber epoxy composite parts for the two 10 by 1.9 rotor sails. This innovative vertical axis turbine design is expected to be commercialized into a 10 MW offshore wind turbine, having two 160 meter arms to support two 80 meter long V-shaped sails. After fabrication, this turbine would become the world’s heaviest composite construction at a weight of about 160 metric tons. It was crucial for finding ways of reducing the total sail design weight, since the turbine sails for a 10 MW wind turbine had to be scaled up to 80 meters. The rotor sail weight of the prototype was considerably reduced with a structural adhesive, which also ensured in reducing manufacturing costs as against a jointed sectional and mechanical assembly design. Structural adhesive has been used to bond the carbon fiber epoxy ribs, spars and skins of the box design together. After this, the leading and trailing edge components that were independently constructed from glass fiber epoxy resin have been bonded onto the central box of the sail.
Onshore and offshore systems both principally use carbon fiber in the spar, or structural element, of wind blades exceeding 45 meters (148 feet) in length, since enhanced stiffness and lower density of carbon fibers enable creating a thinner blade profiles that are stiffer and lighter, hence the growing incidence of using carbon fiber in wind turbine blades. Enhanced lengths and lightness of wind turbine blades demand extensive use of carbon fiber for fabricating them. For instance, a 50 meter long blade installed in a 4 MW turbine would require 1.2 tons of carbon fiber per MW. Vestas, one of the leading wind turbine blade manufacturers, produces over 10,000 of them on an annual basis, and a major fraction of these blades utilize carbon fiber reinforcement, averaging at about 300-350 kg per blade, preferring to use pultruded profiles for blade tip reinforcement. GE Energy has been employing large-tow (≥24K) standard-modulus carbon fiber that is obtained from Mitsubishi Rayon Co Ltd and Zoltek Inc to be used in yet to be specified primary structures on next-generation 48.7 meter (160 foot) blades that would be installed on its 1.6-100 turbine, and 1,600 of these would be fabricated using carbon fiber. For the 2012 year, GE has set a target of consuming 3,000 metric tons of carbon fiber in its endeavor to enhance use of this material. Initiating the construction of such high-stress structures in high volumes is bound to put pressure on composites manufacturers. To take a case in point, demand for carbon fibers to be used in Chinese wind turbine blades is expected to be 5,300 tons in 2020, 10,200 tons in 2030 and 36,700 tons in 2050. However, domestic carbon fiber production in China has been lagging behind when compared to developed countries, which are also unable to satisfy local demand, resulting in an enormous demand-supply gap.