Why the blade matters
The blade is the part of a wind turbine that meets the wind, so it largely decides how much energy the machine can capture. Its shape determines how efficiently it uses the moving air. Its length determines how much air the rotor can reach. And its materials determine whether it survives decades of changing loads, rain, ice and lightning. Almost every step in turbine history involved a better blade.
From sails to airfoils
Traditional windmills used sails: wooden lattice frames covered with cloth, or later wooden slats that could be opened or closed like shutters. These worked mainly by catching the wind and being pushed. In the 1700s and 1800s, millwrights made practical improvements, such as Andrew Meikle’s spring sails of 1772, which opened automatically in gusts, and the patent sails of William Cubitt in 1807. They were adjustments to a sail, not a new kind of surface.
A different idea came with aircraft. By the early twentieth century, researchers studying wings and propellers understood that a curved, tapered surface can generate lift, a force at right angles to the airflow, which is much more effective than simple pushing. The Wright brothers, for example, treated their propellers as rotating wings, and the study of airplane propellers gave wind designers the theory they needed. The work of Albert Betz on the limit of wind energy capture and the analysis of the aerodynamics of rotors by others set the foundations for blade design.
Designers drew on tested wing design, and early wind blades often used profiles developed for aircraft. Later, researchers developed airfoils intended for turbines, since a blade operates in conditions unlike a plane’s wing: it must work well across a wide range of wind speeds, tolerate dirt and insects on its surface, and be thick enough near the root to carry heavy loads.
Twist and taper
A blade tip moves much faster than the part near the hub, so the wind meets the blade at a different angle along its length. To work well everywhere, a modern blade is twisted, with a steeper angle near the root and a flatter one near the tip. It is also wide near the root and narrow at the tip. The result looks simple, but the shape is the outcome of careful calculation. Pitch systems then rotate the whole blade around its axis to adjust to the wind.
Materials: wood, steel and glass
Early blades were made from whatever worked. Sails and slats gave way to wood and metal. The Smith-Putnam turbine of the 1940s in Vermont had blades covered with stainless steel, and a blade failure ended its operation in 1945, a reminder that fatigue is a central issue. Steel blades are heavy, and weight adds to the loads on the rotor, tower and bearings.
The change that shaped modern blades was the use of composites, in particular glass fibers set in plastic resin. Fiberglass-reinforced polyester became common in smaller turbines in the 1970s and 1980s, helped by advances in industrial chemistry and by boatbuilding, where similar materials were already in use. Epoxy resins later offered higher strength and better fatigue resistance. Some manufacturers and research programs also tried laminated wood and epoxy, and wood remains in use in some products, though glass fiber became the mainstream choice.
A typical modern blade is a hollow shell. Two halves, formed in molds, are bonded around an internal spar or shear web that carries most of the bending load. Lightweight cores such as balsa wood or foam stiffen the shell. For the longest blades, carbon fiber is often used in key load-bearing parts, since it is stiffer and lighter than glass, though more expensive. Lightning protection, drainage and sometimes leading-edge coatings are added, since rain and dust can erode the front edge.
Why longer blades matter
Blade length has increased more than any other dimension. The 1980s commercial turbines had blades of perhaps 7 to 10 meters, and the largest offshore blades today exceed 100 meters. The reason is geometry. The area swept by a rotor grows with the square of the blade length, so a blade twice as long sweeps four times the area. A 10-meter blade sweeps about 314 square meters, while a 20-meter blade sweeps about 1,257 square meters, which is four times as much. Since the wind carries more energy at greater height too, long blades on tall towers collect far more energy than short ones.
The limits are practical. Long blades flex, so designers must keep the tip from striking the tower. Blades must be transported on special trucks or ships, and some have been built in segments joined on site. Their weight increases with length faster than one would like, which drives the search for lighter materials. Tips of large blades move quickly, which brings noise and erosion concerns.
End of life
Because glass fiber and epoxy are bonded into a single material, blades are difficult to recycle. Many retired blades have been buried or cut up for use in cement or other products. Manufacturers and researchers are developing resins that can be separated and reused, but the long-term solutions are still evolving. The blade, which was a key to the success of wind power, is also one of its remaining waste problems.