Carbon Fiber in the Wind
Is there a market for carbon composites in wind turbine blade construction? Yes, but the real question is, how big will it be?
By Ginger Gardiner, Contributing Writer | July 2007
Source: Gamesa
Turbine manufacturer Gamesa Eolica’s technicians uses continuous carbon fiber and glass fiber to fabricate a spar for one of its 42.5m and 44m (139.5-ft and 144.3-ft) blades for its G8X 2-MW turbines. Blade production currently takes place at Gamesa factories in both Spain and the U.S.
Much has been said about the potential for carbon fiber use in wind energy systems. The driver for its use is the need to optimize stiffness-to-weight as wind turbine designers increase blade length (and rotor swept area) to make turbines more cost-effective. Historically, blade length has increased 10 percent annually and doubled approximately every seven years. In the mid-1990s, the largest turbine had an output of 800 kW. Today, a turbine of average size is rated between 1.5 MW and 2.0 MW, and at least three of the top 10 manufacturers have 5.0-MW prototypes installed or in development. Of the top 10 global wind turbine and wind blade manufacturers, the largest two, Vestas Wind Systems A/S (Randers, Denmark) and Gamesa (Zamudio, Spain), publicly acknowledge that they currently use carbon fiber. Vestas and Gamesa accounted for 44 percent of the total wind turbine market in 2006, but each uses carbon fiber as the primary reinforcement in the spars of all blades above a certain length. DeWind Ltd. (Lübeck, Germany, part of EU Energy in Milton Keynes, U.K., in turn a subsidiary of Composite Technology Corp. in Irvine, Calif.), also uses carbon, but inserts a precured carbon prepreg spar into a shell preform, which it then infuses using VARTM. By contrast, GE Energy (Atlanta, Ga.), Suzlon Energy Ltd. (Aarhus, Denmark), Nordex AG (Norderstedt, Germany) and LM Glasfiber Group (Lunderskov, Denmark) all use some form of resin infusion, rather than hand layup, to improve the fiber-to-resin ratio of their all-fiberglass blades. Additionally, these companies credit sophisticated refinements in blade design and drivetrain for significant weight, cost and efficiency improvements, without resorting to the use of carbon.
Still, analysts believe carbon fiber has a more significant role to play, and blade manufacturers are researching where and how to use the material — and where to get it until the present supply crunch eases. As one major carbon supplier has stated, “We’re talking about many companies and a significant amount of material.” In fact, at the current rate of growth, carbon fiber consumption in the wind industry could be as much as 6,804 metric tonnes/15 million lb per year.
Solving the cost/benefit equation
Predicting the future of carbon use in wind systems is difficult, in part, because of the markedly different approaches that blade manufacturers have taken to the same engineering problem.
In Europe, Vestas manufactures blades at its factories in Lem and Nakskov in Denmark and in Lauchhammer, Germany. Historically, its blades have been made using fiberglass/epoxy prepreg. Each blade consists of two shell sections, which are bonded onto a precured spar, with the spar determining the strength of the blade. As Vestas built larger turbines, however, it explored new designs and new materials. When Vestas began European production of its V90 1.8-MW, V90 2.0-MW and V90 3.0-MW turbines in 2004 — the 44m/144-ft blades for the V90 3.0-MW turbine are the longest Vestas currently has in production — carbon fiber replaced fiberglass in several key areas, including the spar cap, making the 44m blades lighter than their 39m predecessors. The new blades also incorporate an advanced aerodynamic profile that helps to minimize the loads on the nacelle and the tower. Reduced weight means reduced loads, which improves efficiency. The 1.8-MW and 2.0-MW V90s also use the new 44m blade, with a 27 percent increase in swept area at the same weight as the 37m/121-ft blade used in the V80 wind turbines.
Vestas also is manufacturing blades from wood and carbon fiber/epoxy composites. In a presentation at the SAMPE UK 6th Annual Technical Conference in 2006, John Rimmer of Vestas Blades (Isle of Wight, U.K.) described the company’s technology for making blades of 40m/131 ft and longer: Finnish beech, balsa, carbon fiber and a small amount of fiberglass reinforcement are laid up dry and then processed by resin infusion. According to Rimmer, this combination can produce a finished blade in 24 hours. Vestas acquired this technology in 2003 when it merged with NEG Micon (Randers, Denmark), which, in turn, obtained it in 1998 from Aerolaminates (Newport, Isle of Wight, U.K.), which had been supplying Wood Epoxy (WE) composite blades to U.K. companies.
At the time of its development, NEG Micon R&D manager Ole Gunneskov explained the reasoning behind the WE composite technology. “Apart from its natural strength, the clear advantage we get from wood is that it has a natural attenuating effect, and that it is an extremely stable material in its own right. The combination of wood and epoxy has already been proven through application over many years in wind turbine blades.” He continues, “The combination of aerodynamic design, use of carbon fiber and dry layup of materials means a weight reduction of around 30 percent, compared with the traditional wind blade made from fiberglass-reinforced polyester. Another ad-vantage over the traditional fiberglass blade is that we achieve a slimmer and lighter blade design, reduced load on the main components and lower costs.”
The WE blades were used in NEG Micon’s NM82 1.5-MW turbine, which was competitive with Vestas’ V80 models and thus phased out after the merger. However, according to an independent financial analyst’s report, Vestas has built a 59m/194-ft WE composite blade that has a total weight of 12.6 metric tonnes (27,778 lb) and a total head mass (THM) of only 210 metric tonnes (462,970 lb), comprised of the nacelle and rotor. REpower’s (Rendburg, Germany) new 5-MW turbine, which uses LM Glasfiber’s 61.5m/202-ft blades, has a THM nearly double this at ~400 metric tonnes (881,849 lb).
Like Vestas, Gamesa uses carbon fiber in its largest blades: the 42.5m and 44m (139 ft and 144 ft) blades for its G87 and G90 2.0-MW turbines, respectively. These are made in both Spain and Gamesa’s Ebensburg, Pa., plant, which has produced roughly 400 blades since production startup in mid-2006.
Gamesa interleaves carbon fiber prepreg with glass fiber prepreg on the upper and lower surfaces of the tubular spars. Some carbon fiber also may be used on both of the spar beam sides, again interleaved with fiberglass.
Source: Shi Pengfei, China Hydropower Engineering Consulting Group
A Vestas V80 2-MW turbine installed in Jiangsu province.
Gamesa’s technical specification for the G87 wind turbine says, “Glass fiber blades are dimensioned by maximum deflection; in long blades this would mean an important increase in weight.” It goes on to say that the use of carbon fiber allows the blades “to be dimensioned by stress, optimizing the quantity of material …. The carbon and glass fiber combination is an agreement between structural stiffness and cost.”
The technical specification also describes how the manufacture of this blade is automated using prepreg tape placement and tape winding. The outer shell is fabricated in two pieces, each comprising a glass fiber/epoxy and PVC foam core sandwich. Once the shell halves and spars are cured, they are adhesively bonded together.
