CompositesWorld
Issue link: https://cw.epubxp.com/i/517026
JUNE 2015 6 CompositesWorld PAST, PRESENT & FUTURE » In 1987, a 3.2-MW prototype wind turbine was installed in Hawaii. At the time, it was the largest wind turbine in the world. Manufactured by the Boeing Co. (Chicago, IL, US) under a NASA/ US Department of Energy (DoE) development program, the MOD-5B had a rotor diameter of 97.5m and two blades, which featured partial-span, variable-pitch control, that is, the outermost portion of the blade could be rotated to adjust its aerodynamics. Tis NASA program exposed the challenges for the design, manufacture and operation of MW-scale wind turbines. During the 1990s, mainstream commercial wind turbines were kW-scale and gradually growing in turbine power rating and rotor size. Te prevailing rotor confguration was the "Danish" type: three blades with full-span, that is, entire-blade, pitch control. Fast-forward to 2012. Te Chinese company Envision Energy Co. Ltd. (Beijing) and its Danish subsidiary's design team installed a 3.6-MW prototype with a two-bladed, partial-span pitch rotor. Currently, the U.S.-based company Zimitar Inc. (Benicia, CA US) is developing a similar rotor. Are wind turbine rotor designs going "back to the future"? Not necessarily. First, the Danish confguration still prevails. More importantly, the MOD-5B rotor was all-steel and weighed 144 MT. Te Envision prototype uses FRP materials for the blade structure, and incorporates a carbon fber-reinforced polymer (CFRP) main rotor driveshaft — a frst for a commercial, MW-scale turbine. In addition to reducing component weight, Envision reports the CFRP shaft has fexibility characteristics that mitigate loading compared with the steel alternative. Similarly, the Zimitar rotor design employs FRP blades and aerodynamic control devices. It has a 64% larger diameter than the MOD-5B and nearly twice the rated power at approximately 68% of the weight. Tat points to another diference. Over the past two decades, blade length has increased in proportion to turbine rated power, and as rotor size expanded at a given rating. A decade ago, a typical 2-MW rotor had an 80m diameter. Today, to capture more energy, some 2-MW rotors have diameters of 110m and greater. As blade length increased, it was increasingly critical to mitigate weight growth yet ensure adequate stifness. Tese objectives were realized by using FRP materials with increased stifness-/ strength-to-weight. Current strategies fall into three categories: 1) increasing the fber weight fraction (W f ) for glass fber-reinforced plastic (GFRP), using standard E-glass, 2) using intermediate or high-modulus glass fbers, and/or 3) using alternatives with increased stifness-/strength-to-weight, such as carbon fbers. Historically, wind blades have benefted from a shift to manu- facturing by vacuum-assisted resin transfer molding (VARTM) or by using prepreg materials, with increased compaction and W f compared to the earlier wet-layup process. Although this resulted in weight and stifness improvements, there are practical limita- tions for compaction of the thick laminates typical of MW-scale blades. Also, the fatigue strength of blade laminate tends to decrease with increased W f . For GFRP materials, ongoing develop- ments in fber sizing and fabric architecture are improving fatigue strength and infusibility of heavy fabrics in thick laminates. Another trend is the increased use of pre-consolidated elements in blade construction. Tese can include sub-elements that are fabricated and cured, using the same basic processes as the rest of the blade, or other processes, e.g., pultruded "rods" or "slats." Sub-elements are integrated into the structure during layup and infusion of the blade shells and can enable increased W f , improve fatigue resistance and ease manufacture. CFRP has been used for load-carrying blade structure, most notably by turbine manufacturers Vestas Wind Systems A/S (Aarhus, Denmark) and Gamesa (Zamudio, Spain), and, in some recent models, by GE Wind Energy (Fairfeld, CT, US). CFRP has known strength and stifness benefts, but disadvantages include cost, sensitivity to manufacturing variations and electrical conduc- tivity, which can complicate lightning protection. A "middle ground" approach between use of standard E-glass and CFRP is intermediate- and high-modulus glass fbers. Owens Corning (Toledo, OH, US) is an industry leader in this tech- nology, with several fber and fabric products developed for wind blades. Today, the use of these glass fbers appears to be growing somewhat faster than the use of CFRP. Longer blades complicate not only manufacturing but trans- portation as well. Terefore, modular designs are of increasing interest. Gamesa was the frst to commercialize a mid-span joint at the MW-scale on its G128 4.5-MW turbine blades. Wetzel Engi- neering Inc. (Lawrence, KS, US) also is developing a modular blade concept, using "spaceframe" technology. As the trend toward increased blade size continues, the chal- lenges of weight mitigation, stifness optimization, manufactur- ability and transportability will motivate further developments in materials, processes and design. Not "back to the future," but a promising future, nonetheless. ABOUT THE AUTHOR Dayton Grifn, MSc, is a senior principal engineer at DNV GL (Seattle, WA, US), with 20 years of wind energy experience. He is an internationally recognized expert in the area of wind turbine blades, including rotor blade aerodynamics, structure, materials and manufacturing technologies. Wind turbine blades: Back to the future? Blade length has increased in proportion to turbine rated power and as rotor size expanded at a given rating.