The wind energy industry is growing rapidly because a confluence of economic, political and environmental factors has convinced people and governments that wind should be part of a comprehensive energy solution. Given the potential for continued strong growth, wind turbine blade manufacturers need new composite wind blade design solutions that help them enhance engineering methodologies, so they can keep up with the current pace of demand and develop high-quality products faster and at competitive prices.
Wind turbine blade design is about balancing aerodynamic performance and structural integrity. The goal is for blades to extract as much energy from the airflow as possible over a more-than 20-year lifespan, and with the lowest possible life cycle costs. These blades can weigh up to 20 tons and need to withstand tens of millions of rotations and fatigue cycles at speeds up to 200 miles per hour at the tip of the blade. They must also withstand bird strikes, harsh sun, heavy rain, snow, ice, hail, gusty winds and lightning strikes.
From a design and manufacturing standpoint, wind turbine blades have many similarities to helicopter rotor craft blades. The loading on a wind turbine blade and a rotor craft blade consist primarily of aerodynamic pressure loads. Of course, the speed of a wind turbine blade is much slower, so it experiences much lower aerodynamic pressures. Loading is also applied at the root of both wind turbine and rotor craft blades. The overall pressure field on the blade causes a “bending moment” and torque at the root. A “bending moment” refers to the tendency of wind turbine blades to bend and twist during operation, which effectively alters their angle of attack and in turn has a negative effect on loads and energy production. For these reasons, blades are designed with a high level of bending stiffness. There is also typically a requirement that all the major modes of the blade must be above approximately 20 Hz so that the blades are not excited at normal operating speeds.
Composites dominate the wind turbine blade market because of their superior fatigue characteristics and stiffness to-weight ratio, ability to fabricate complex geometries and potential for aero elastic tailoring. The outermost section of the blade is typically a gel-coat layer, which provides a smooth surface to enhance aerodynamic properties. Next typically comes a layer consisting of Nexus, a soft material that provides a relatively smooth but absorbent surface on which to mount the gel-coat. The next layer is a double-bias stack of composite plies made by twisting unidirectional fibers around a core at 45 degrees in both directions to make a torsion tube. At the blade trailing edge, the double bias laminate splits into two layers to accommodate a core material such as balsa, foam or honeycomb. The core material laminate augments the buckling strength of the trailing edge of the blade.
The technology used in manufacturing wind turbine blades has evolved over the past 20-plus years. Blade making has migrated toward processes that minimize cycle time and reduce both cost and the probability of defects. Early blade building techniques grew out of the boat building industry, using processes that were high in labor and prone to inconsistencies and defects. Vacuum infusion and carbon fiber took blade manufacturing technology to a higher level, with improvements in consistency and performance of a blade. “Prepreg” or “pre-impregnated” technology further enhanced blade performance by combining resins and reinforcements in a more rigorously controlled manner before placement in the blade mold.