Wind turbines are devices that extract the kinetic energy from wind and convert it into mechanical energy, which is then further converted into the more usable form of electrical energy.
Energy harnessed from the wind currently provides approximately 10% of the world’s energy supply, with its presence in the renewable energy sector only projected to increase as its potential is further realized. In order to remain competitive with existing technologies, the optimization of wind turbine efficiency is crucial and dictated by engineering design complemented by judicious choice of materials. Additionally, the materials should be durable, ideally recyclable, and low-cost in terms of manufacture so as not to offset the positive environmental impact and economic advantages of wind energy.
A wind turbine consists of three main components: the tower, nacelle, and rotor blades.
Fig 1. Denmark is a strong proponent of wind power, with wind turbines even appearing on the Faroe Islands.
The tower provides structural support upon which the nacelle and rotor blades stand and is made of tubular steel, concrete, or steel lattice. Naturally, the materials must be strong and robust in nature to withstand harsh environmental conditions and strong winds.
The nacelle houses the inner machinery including the generator, which converts the mechanical energy to electrical energy. As the nacelle contains mostly mechanical parts of the wind turbine’s operation, the materials are not particularly subject to many deviations and variations.
Fig 2. Inside mechanical components of the nacelle in a wind turbine.
The rotor generates aerodynamic torque from the wind with its rotating motion as the blades spin. Optimization of the shape and material of the blades should allow for the blade to spin faster and capture wind at lower velocities to increase turbine efficiency. The shape of the rotor blade must be aerodynamic, much like the wings of an airplane. The material of the blades must enhance rather than hinder their aerodynamics and fulfill the following criteria: high stiffness for optimum aerodynamics, low density to reduce gravitational forces, and long fatigue life to reduce material degradation. A 20-year lifespan is usually the industry standard for long fatigue life, which sustains 108-109 stress cycles the material can handle before failure.
In evaluating the broad categories of materials available, foams, polymers, and rubbers are eliminated due to their inadequate stiffness and density for a cantilever beam serving as a model for the rotor blade. Ceramics don’t hold up well against long-time fatigue loads, meaning that they can fracture easily. This leaves woods and composites remaining that satisfy these material requirements. Wood is an environmentally friendly option with the advantage of having low densities. However, its low stiffness makes the material susceptible to bending and deflections in the wind, severely compromising overall turbine efficiency. Composite materials remain the most practical and prevalent choice. Within this family of materials, a rich variety of innovative possibilities is explored.
Fig 3. Rotor blades preparing to be assembled.
Fibrous materials are characterized by the fact that they significantly longer than they are wide. The exceptional strength and stiffness of fibers make them excellent candidates for turbine blade materials, where the long fibers provide longitudinal stiffness when aligned parallel along the blade length. Fibers are often brittle and can snap easily, so they are not used alone as a material but rather as additive reinforcements.
Carbon fibers have superior mechanical properties with high stiffness, high strength, and low density, albeit along with higher costs. They are composed of pure carbon atoms as hexagonal repeating units in a crystallographic lattice arranged on top of each other in planes, with strong forces within the plane and weak forces between. This gives rise to high anisotropy with high stiffness and thermal expansion properties. The low density of carbon-fiber blades offer increased length without the burden of increased weight, thereby increasing turbine efficiency. Additionally, the lighter blades reduce the overall weight and strain the nacelle carries.
Glass fibers are available at a lower cost compared to their carbon counterparts, and are thus more prevalent in industry. They are composed of mainly SiO2 and Al2O3, with other oxides present in small quantities. Because there is no crystallographic order, the material has an amorphous structure with isotropic properties. This means that its properties such as stiffness and thermal expansion are consistent along and across the fiber. Glass fibers are 10-20 μm in diameter and are of moderate stiffness, high strength, and moderate density. Learn more about aluminosilicate glass here.
- E-glass, or electric-glass made of alumino-borosilicate characterized by its high electrical resistance.
- S-glass, or high strength-glass made of magnesium aluminosilicate but with higher costs.
Aramid fibers are synthetic fibers that are highly heat-resistant, making them suited for wind turbines that operate in temperature extremes. The fibers are composed of aromatic polyamide chains held together by strong hydrogen bonds that contribute to the toughness of the fiber.
The polymer matrix provides structural support by binding the fibers together and consist of two main classes: thermosets and thermoplastics. The main physical difference between them is their behavior in different temperatures. You can learn more about the differences here.
Thermosets contain polymers strongly cross-linked together in irreversible chemical bonds. This makes them resistant to high temperatures and remain in a permanent solid state once cooled. This can possibly give rise to internal stress in the composite structure. Examples of thermoset polymers are as follows:
- Unsaturated Polyesters: General Polyethylene terephthalate, amorphous
- Vinylesthers: General Vinyl ester (VE)
- Epoxies: General Epoxide; Epoxy (EP+GF25+MD45), (EP+GF30+MD20)
Thermoplastics contain polymers that lack these strong chemical bonds so that interactions are reversible. They soften when reheated, allowing for the possibility of remolding and repairs when necessary. However, this property also causes them to melt under high temperatures, making them impractical for some of the harsh conditions wind turbines must endure.
When combined together, the fibers and polymer matrix make up a composite material with different chemical and physical properties than their individual constituents. The resulting material is reinforced with complementary properties making up for deficits in the other. Common fiber-containing composite materials used in turbine blades are with glass and carbon. The long fibers provide stiffness and strength while the polymer matrix supports the fibers by providing out-of-plane strength, flexibility, fracture toughness, and increased stiffness. In an optimized composition and combination, the resulting blades are lightweight with excellent mechanical properties.
- General Polyamide 4T (PA4T+GF30), composite material reinforced with 30% glass fiber