The blades are a crucial component of wind turbine generators. They transfer wind energy to the generator's rotor, causing it to rotate and cut magnetic field lines to generate electricity. To ensure long-term, uninterrupted, and safe operation in extremely harsh outdoor environments, the requirements for blade materials are: ① Low density and optimal fatigue strength and mechanical properties, capable of withstanding extreme conditions and random loads (such as storms), ensuring safe operation for over 20 years; ② Low cost (more precisely, the cost per kilowatt-hour); ③ Normal blade elasticity, rotational inertia, and vibration frequency characteristics, ensuring good load stability transmitted to the entire power generation system; ④ Good resistance to corrosion, ultraviolet (UV) radiation, and lightning strikes; ⑤ Low maintenance costs.
FRP can fully meet the above requirements and is the best material for wind turbine blades.
1.1 GFRP
Currently, most commercially available large wind turbine blades are made of glass fiber reinforced plastic (GFRP). The characteristics of GFRP blades are:
① The strength and stiffness of the wind turbine blades can be designed based on their stress characteristics. Wind turbine blades are mainly subjected to longitudinal forces, namely aerodynamic bending and centrifugal force. The aerodynamic bending load is much larger than the centrifugal force, while the shear stress generated by shearing and torsion is not significant. Utilizing the stress theory that glass fiber (GF) is the main stress-bearing material, the main GF can be arranged in the longitudinal direction of the blade, thus making the blade lighter.
② Airfoils are easy to form and can achieve maximum aerodynamic efficiency. To achieve the best aerodynamic effect, the complex aerodynamic shape of the blades is utilized. Different blade chord lengths, thicknesses, twist angles, and airfoils are designed at different radii of the wind turbine, which would be very difficult to manufacture with metal. At the same time, GFRP blades can be mass-produced.
③ With a service life of up to 20 years, it can withstand fatigue alternating loads of over 108. GFRP has high fatigue strength, low notch sensitivity, large internal damping, and good seismic performance.
④ Good corrosion resistance: Because GFRP is resistant to acids, alkalis and water vapor, wind turbines can be installed outdoors. Especially for offshore wind farms that have been developing vigorously in recent years, wind turbines can be installed at sea, allowing the wind turbine units and their blades to withstand the test of various climatic environments.
To improve the performance of GFRP, GF can be modified through surface treatment, sizing, and coating. Research in the United States has shown that using radio frequency plasma deposition to coat E-GF can achieve tensile and fatigue resistance levels comparable to carbon fiber (CF).
The stress characteristics of GFRP are that it can withstand very high tensile stress in the GF direction, while the force it can withstand in other directions is relatively small.
The blade consists of a skin and a main beam. The skin has a sandwich structure, with a rigid foam plastic or Balsa wood middle layer and GFRP upper and lower layers. The surface layer consists of a unidirectional layer and ±45° layers. The unidirectional layer can be made of unidirectional fabric or unidirectional GF, generally using 7 or 4GF fabric to withstand the axial stress generated by centrifugal force and aerodynamic bending moment. To simplify the molding process, the ±45° GF fabric layer can be omitted, and a 1:1 GF fabric can be used instead, all laid axially to withstand the shear stress mainly generated by torque, and is generally laid on the outside of the unidirectional layer. The beam structure can be either a sandwich structure or a solid GFRP structure. However, the junction between the skin and the main beam, i.e., the beam cap, must be a solid GFRP structure. This is because this part of the beam interacts with the skin, resulting in greater stress, and the strength and stiffness of the skin must be guaranteed.
1.2 CFRP
With advancements in wind turbine blade design technology, wind power generation is trending towards higher power output and longer blades. Increased blade length inevitably increases blade mass. Statistical analysis of blades ranging from 10 to 60 meters in length shows that blade mass increases with the cube of its length. Blade lightweighting has a significant impact on operation, fatigue life, and energy output. During operation, the blade's gravity generates alternating loads, causing fatigue in both the blade itself and the turbine unit. Reducing blade weight can correspondingly decrease the mass of structures such as the hub, nacelle, and tower.
For large blades, stiffness becomes a primary concern. To ensure the blade tip doesn't strike the tower under extreme wind loads, the blade must possess sufficient stiffness. An effective way to reduce blade mass while meeting strength and stiffness requirements is to use carbon fiber reinforced plastic (CFRP). CFRP's tensile modulus is 2-3 times that of GFRP. Using CF reinforcement on large blades fully leverages its advantages of high elasticity and lightweight. Analysis shows that using a CF/GFRP hybrid reinforcement scheme can reduce blade weight by 20%-40%. A research project funded by the European company EC indicates that adding CF to a ￠120m blade rotor can effectively reduce the overall mass by up to 38%, and also reduce its design cost by 14% compared to GF. Another similar study also indicates that wind turbine blades made with added CF are approximately 32% lighter than those made with GF.
The world's largest CF/GFRP hybrid wind turbine blade is currently the 56m long blade developed by Nodex for a 5MW offshore wind turbine. Nodex has also developed a 43m (9.6t) CF/GFRP blade for use in a 2.5MW onshore turbine. Enercon has developed CFRP blades for a 4.5MW wind turbine. Whether large blades should be reinforced with CF is still controversial. Some argue that introducing CF technology into the wind energy industry is "exotic" and expensive, and should be avoided if possible. However, many structural engineers are convinced that the natural law of scale shows that as blade length increases, mass increases faster than energy extraction. Therefore, using CF or CF/GF hybrid fibers is necessary to suppress the increase in mass. At the same time, developing longer blades with sufficient rigidity is also necessary to reduce the cost of wind energy.
Whether CFRP can be widely adopted in wind turbine blades depends on the price of CF. While CFRP's performance is far superior to GFRP, and it is undoubtedly the lightest material for both blades and the entire wind turbine, it is also the most expensive. Even if the price of CFRP drops to $11/kg, the cost of manufacturing blades with CFRP is still too high. Therefore, in-depth research is currently being conducted on raw materials, processing technology, and quality control to reduce the cost of CFRP.
Smaller blades (e.g., 22m in length) generally use inexpensive and readily available E-GFRP, with unsaturated polyester as the main resin matrix. Vinyl ester resin or epoxy resin can also be used. Larger blades (e.g., over 42m in length) generally use CFRP or CF/GFRP, with epoxy resin as the main resin matrix.
