Wind is a emerging as a reliable and inexpensive source of renewable energy. Globally, the average cost of wind is $83 per megawatt-hour compared to averages for coal and gas being $84 and $98 respectively. “In the USA, gas is slightly cheaper than wind but this is the only large economy where that is the case. As a comparison, solar photovoltaic energy averages $122 globally for each MW-hour,” said Giles Dickson who is CEO of the European Wind Energy Association (EWEA). Wind power’s costs will tumble by 16% as capacity doubles over the next 33 years, while the cost of solar PV is set to fall by 18% over this timescale.
Composite materials are used typically in blades and nacelles of wind turbines. Generator, tower, etc. are manufactured from metals. Blades are the most important composite based part of a wind turbine, and the highest cost component of turbines. A wind turbine blades consists of two faces (on the suction side and the pressure side), joined together and stiffened either by one or several integral (shear) webs linking the upper and lower parts of the blade shell or by a box beam (box spar with shell fairings)
A major trend in wind turbine development is the increase in size and offshore placements. Increasing size is motivating by the desire to reduce of the leveraged cost of energy. There are two ways to produce more power from the wind in a given area. The first is with bigger rotors and blades to cover a wider area. That increases the capacity of the turbine, i.e., its total potential production. With increasing size, the weight of the rotor blades increases, so that gravitational loads become design drivers. Also longer blades deflect more, so that structural stiffness (to ensure tip clearance, i.e., to avoid the blade to hit the tower) is of increasing importance. Thus, from a materials perspective, the stiffness-to-weight is of major importance. In addition, with the turbine designed to be in operation for 20–25 years, the high-cycle fatigue (exceeding 100 million load cycles) behavior of composites and material interfaces (bondlines, sandwich/composite interfaces) is of major importance.
The second is to get the blades up higher into the atmosphere, where the wind blows more steadily. That increases the turbine’s “capacity factor,” i.e., the amount of power it actually produces relative to its total potential (or more colloquially: how often it runs). So the third engineering challenge is to find designs and materials that can stand up to the stresses that come along with height and higher winds. Engineers are coming with many innovative solutions to meet these challenges.
Glass and carbon fibers.
The stiffness of composites is determined by the stiffness of fibers and their volume content. Typically, E-glass fibers (i.e., borosilicate glass called “electric glass” or “E-glass” for its high electric resistance) are used as main reinforcement in the composites. With increasing the volume content of fibers in UD composites, the stiffness, tensile and compression strength increase proportionally, yet, at high volume content of fibers (after 65%), there might be dry areas without resin between fibers and the fatigue strength of the composite reduces. Typically, the glass/epoxy composites for wind blades contain up to 75 weight % glass.
Carbon fibers are considered to be a very promising alternative to the glass fibers. They show much higher stiffness and lower density than the glass fibers, thus, allowing the thinner, stiffer and lighter blades. However, they have relatively low damage tolerance, compressive strength and ultimate strain, and are much more expensive than the E glass fibers. Carbon fiber reinforced composites are sensitive to the fiber misalignment and waviness: even small misalignments lead to the strong reduction of compressive and fatigue strength. Carbon fiber composites are used by the companies Vestas (Aarhus, Denmark) and Siemens Gamesa (Zamudio, Spain), often in structural spar caps of large blades.
Aramid and basalt fibers. Further, an interesting alternative is using non-glass, high strength fibers first of all, aramid and basalt fibers. Aramid (aromatic polyamide) fibers demonstrate high mechanical strength, and are tough and damage tolerant, but have low compressive strength, low adhesion to polymer resins, absorb moisture, and degrade due to the ultraviolet radiation.
Basalt fibers show good mechanical properties, are 30% stronger, 15–20% stiffer and 8–10% lighter than E-glass, and cheaper than the carbon fibers. The application of basalt fibers in small wind turbines have been demonstrated and the results were very encouraging.
Hybrid composites. Hybrid reinforcements (E-glass/carbon, E-glass/aramid, etc.) represent an interesting alternative to the pure glass or pure carbon reinforcements. Ong and Tsai demonstrated that the full replacement would lead to 80% weight savings, and cost increase by 150%, while a partial (30%) replacement would lead to only 90% cost increase and 50% weight reduction for 8 m turbine. The world currently longest wind turbine rotor blade, the 88.4 m long blade from LM Wind Power is made of carbon/glass hybrid composites
Typically, thermosets (epoxies, polyesters, vinylesthers) or (more seldom) thermoplastics are used as matrices in wind blade composites.
Thermosets. Thermosets based composites represent around 80% of the market of reinforced polymers. The advantages of thermosets are the possibility of room or low temperature cure, and lower viscosity (which eases infusion and thus, allowing high processing speed). Initially, polyester resins were used for composite blades. With the development of large and extra-large wind turbines, epoxy resins replaced polyester and are now used most often as matrices of wind blade composites.
Thermoplastics. Thermoplastics represent an interesting alternative to the thermoset matrices. The important advantage of thermoplastic composites is their recyclability. Their disadvantages are the necessity of high processing temperatures (causing the increased energy consumption and possibly influencing fiber properties) and, difficulties to manufacture large (over 2 m) and thick (over 5 mm) parts, due to the much higher viscosity.
Nanoengineered polymers and composites. In several works, the possibilities of improvement of composites properties by adding nanoreinforcement in matrix were demonstrated. Additions of small amount (at the level of 0.5 weight %) of nanoreinforcement (carbon nanotubes or nanoclay ) in the polymer matrix of composites, fiber sizing or interlaminar layers can allow to increase the fatigue resistance, shear or compressive strength as well as fracture toughness of the composites by 30–80% . Loos, Manas-Zloczower and colleagues developed various wind turbine blades with secondary carbon nanoparticles reinforcement (vinyl ester, thermoplasts, epoxy composites containing CNTs) and demonstrated that the incorporation of small amount of carbon nanotubes/CNT can increase the lifetime up to 1500%
Composite Sandwich Technologies Lighten Components
Leveraging its private resources with several Small Business Innovation Research (SBIR) contracts with both NASA and the U.S. Department of Defense, WebCore Technologies LLC, of Miamisburg, Ohio, developed a fiber-reinforced foam sandwich panel it calls TYCOR that can be used for a wide variety of industrial and consumer applications. Testing at the Ballistic Impact Facility demonstrated that the technology was able to exhibit excellent damage localization and stiffness during impact.
The TYCOR fiber-reinforced composite is strong, lightweight material ideal for structural applications (Spinoff 2004). The patented and trademarked material has found use in many demanding applications, including marine, ground transportation, mobile shelters, bridges, and most notably, wind turbines. One of the most recent applications for TYCOR is in the field of wind turbines, where, according to Sheppard, “TYCOR offers a compelling value proposition which stresses not only competitive pricing but also weight savings, resin savings and overall manufacturing cost reduction when compared to other sandwich core materials.”
The lightweight composite allows builders of turbines to make larger, more efficient turbines. And with its low price, TYCOR allows users of wind energy to recoup their costs more quickly. “TYCOR’s largest market, today, is wind energy. WebCore supplies our core material to wind blade manufacturers around the Nation and around the world,” says Sheppard.
Breakthrough Magnetic Alloy Could Lead To Cheaper Cars, Wind Turbines
Scientists have created a promising new magnetic material that could lead to cheaper cars and wind turbines. The new magnetic alloy is a viable alternative to expensive rare-earth permanent magnets, the U.S. Department of Energy and Ames Laboratory reported. The material could eliminate the need for one of the “scarcest and costliest” rare Earth elements, dysprosium, and replace it with abundant cerium.
The alloy is composed of neodymium, iron and boron “co-doped “with cerium and cobalt. Recent experiments demonstrated the cerium-containing alloy boasts intrinsic coercivity (the ability of magnetic material fight demagnetization) that is even greater than dysprosium’s containing magnets of high temperatures. This material is also between 20 and 40 percent cheaper than magnets containing conventional dysprosium.
“This is quite exciting result; we found that this material works better than anything out there at temperatures above 150 [degrees Celsius],” said study leader Karl A. Gschneidner. “It’s an important consideration for high-temperature applications.” Past attempts to use cerium in rare-earth magnets were unsuccessful because the element reduces the Curie temperature (the temperature at which an alloy loses its magnetic properties). This new co-doping method coupled with cobalt allowed the scientists to substitute cerium for dysprosium without reducing the magnetic properties of the material.