The wind energy industry had a record year in 2003. New capacity of 8,344 megawatts (MW) was installed worldwide, representing about $9 billion (USD) in manufactured equipment and construction costs. The growth in new capacity has averaged an impressive 26 percent over the past five years, but a recent market report predicts a slight market decline for 2004 - the first since 1992. A major factor in this downturn is the expiration of renewable energy tax credits in the U.S., and uncertainty over the future of those credits. Nonetheless, the global market is predicted to return to an extended period of strong growth beginning next year, marked by a continued trend toward larger rotors and greater diversity in the designs, materials and processes used in the manufacturing of these large blades.
Typical land-based turbines now range between 1.5 MW and 2.5 MW (blade lengths about 35m to 45m/115 ft to 148 ft). Offshore systems are larger yet, with production units and near-term prototypes in the 3 MW to 5 MW range (blades 45m to 60m/148 ft to 197 ft or more). Offshore applications are expected to become a major market sector in the next few years. The size of these systems will continue to present design, manufacturing and logistics challenges, but there appears to be no convergence in the approaches to blade materials, processes and design.
Wind turbine blades have been described as demanding aerospace performance at an industrial price. Aerodynamic efficiency requires high lift-to-drag ratios and insensitivity of the lift characteristics to soiling or surface roughness, while structural efficiency demands relatively thick airfoil shapes. Blades are designed to withstand 50-year extreme wind events, system fault scenarios and fatigue load cycles on the order of 108. For MW-scale turbines, the cyclic bending resulting from the blade self-weight has become a governing load case for the inner blade, hub and bolted connections.
Historically, wind turbine blades have been constructed using either all-fiberglass laminate or primarily fiberglass construction with selective use of carbon for local reinforcement. For blade sizes up to 30m/98 ft, the most common manufacturing approach has been open-mold, wet layup. The most notable exception is VESTAS Wind Systems (Ringkøbing, Denmark), which has long used fiberglass/epoxy prepreg.
In current production, prepreg and vacuum-assisted resin transfer molding (VARTM) have emerged as the two most common replacements for wet layup. The majority of blade manufacturers presently use VARTM. However, it is noteworthy that two of the top four manufacturers (VESTAS, and the Spanish company, GAMESA) use prepreg.
Similarly, there appears to be no consensus about the need for, or benefits of, carbon fiber in large blades. VESTAS is currently testing several prototypes of its 44m/144-ft 3.0 MW V-90 turbine with carbon-fiber spars. GAMESA has announced that it will include carbon spars for 87m/285-ft and 90m/295-ft rotor diameters, and NEG Micon is building 40m/131 ft blades with carbon-augmented wood/epoxy. The largest currently installed turbines, the 4.5 MW E-112 from the German manufacturer ENERCON, include 54m/177-ft fiberglass/epoxy blades that are relatively heavy at 20 metric tonnes each. In contrast, at 13.5 metric tonnes each, LM Glasfiber's (Lunderskøv, Denmark) 54m/177-ft fiberglass/epoxy blades are remarkably light for all-glass construction. Nonetheless, LM reports "selective" use of carbon fiber in its new 61.5m/202 ft blade, scheduled for installation within the next several months on a 5.0 MW prototype under development by German manufacturer REpower.
The prospect for carbon fiber in large blades remains unclear. The most notable applications are still relatively early in their development cycle and have not yet been fully commercialized. Further, some significant players, such as GE Wind Energy, have not yet announced their intentions.
For large blades, selection of materials and manufacturing processes are interdependent. Carbon's structural performance is sensitive to fiber alignment - particularly for compressive strength - and infusion of thick carbon/epoxy parts is difficult. This has prompted nearly exclusive use of prepreg for carbon spars in large blades. Again, there is a diversity of processes in use. The UK-based company DeWind is using an innovative approach to produce 40m/131-ft carbon/fiberglass hybrid blades. In that process, the spar cap is produced using prepreg carbon. After curing, the spar caps are then placed into a preform and infused into the fiberglass blade skins. BONUS Energy (Brande, Denmark) takes one of the more novel approaches, producing 30m/98 ft to 40m/131 ft blades from dry preforms with a single-shot infusion process that eliminates the need for secondary bonding of the blade halves.
Load-reducing designs - via passive response, active control, or some combination - is an area of ongoing development. VESTAS' V90 turbine is a good example of a design where carbon fiber facilitates both passive and active load-reducing technology, with weight savings that extend to many turbine subsystems (see HPC July 2002, p. 27). Other areas of current R&D include aeroelastic tailoring (bend-twist coupling), blade load and condition-monitoring sensors, active aerodynamic control devices and advanced control methods.
To remain competitive and profitable, however, turbine manufacturers need to consider the availability and price-stability of carbon and other raw materials and look for ways to ease transportation logistics and costs. The rapidly changing dynamic of market opportunity, competition and costs will present challenging questions: Should manufacturing remain centralized or will local labor conditions (e.g., skill level and quality control) permit more advantageous global manufacturing sites, with flexibility to shift production between multiple plants? When and how much can they automate the process? Can the high cost of transporting blades to installation sites be reduced by using portable manufacturing processes and/or developing modular or segmented blade designs?
The wind energy industry is presently in its "teen-age" years - rapidly developing and far from settling into its mature self. Like a teen-ager, the industry has many challenges to face, but almost limitless potential for development. Further advancements in blade design, materials, processes and complementary technologies must play a central role if the wind industry is to realize its full potential.