Development of composite wind turbine composite blades
1 Introduction Wind power, as one of the most competitive energy sources in the future, has received increasing attention from all over the world. At present, the total installed capacity of wind turbines in the world is 31,000 MW, and it is still growing at an annual rate of more than 27%. The top four countries in terms of wind power generation in 2001 are: Germany 8000MW, US 4150MW, Spain 3300MW and Denmark 2500MW The world wind turbine market is almost occupied by companies in these countries. It is estimated that by 2020, wind power generation in the world will account for 10% of the total global power, with a total installed capacity of 12MW. China has a vast territory and abundant wind energy resources. The wind energy theory development is 3.226 billion kW, and the actual exploitable capacity is 253 million. kW, equivalent to 67% of domestic water resources, ranking third in the world. In the current situation of insufficient conventional energy, wind energy, as a more realistic supplementary energy source, is developed and utilized, and has important economic and social benefits. Composite fan blades are one of the key components of wind turbines. The structure and strength of the blades determine the reliability of wind turbines. Due to various reasons, there is still a certain gap between the research and development of large wind turbine blades in China and foreign countries. The development of large composite fan blades is imminent. 2 Development trend With the continuous expansion of the world wind energy market, the requirements for wind turbine utilization and power generation costs are becoming more and more rampant. A number of high-power, high-capacity megawatt wind turbines have emerged on the market, and the emergence of these large units has placed even more stringent requirements on composite blades. At present, the general development trend of composite blades is as follows. The composite blades are developed from aeronautical airfoil to wind turbine airfoil. As the fan capacity increases, the length and thickness of the blades increase accordingly. At the same time, in order to improve the efficiency of the wind turbine, the lift-to-drag ratio of the blade is increased during the manufacturing process, and the blade wing shape is more complicated. Two development routes for wind turbine blades, namely flexible blades and rigid blades. At present, the world's wind turbine blades are mainly rigid, and only the United States advocates the development of highly flexible variable-thickness wind turbines. The blades and towers are very soft, and if the structural dynamics can be solved, the power generation cost can be quickly reduced. (3) In the application of the material of the blade, new materials have been applied to the manufacture of large wind turbine blades, such as high-strength carbon fiber (NEG-Miam, Denmark) and tough natural fiber (developed by French ATV). The trend is to move toward low cost and light weight, to improve damage tolerance and reliability. Hot melt epoxy prepreg, rigid foam foaming and multi-axial layup technology. Reliability, fatigue resistance and lightning strike resistance testing and evaluation are required. The large-scale single-unit capacity is applied to large- and medium-sized wind turbines and grid-connected power generation operations (so-called wind farms) to reduce the cost of wind power generation. In recent years, large-scale wind turbines have become the main products in the wind power market, and there have been many models ranging from 1MW to 4.5MW for single-unit children. The trend of the whole industry is to expand to a large scale. 3 Selection of blade material Large composite blade is a large composite structural member, and its specific properties are shown in Table 1. Table 1 Large composite material blade performance Model diameter / m cut human wind speed / m8-1 working wind speed / m-s1 maximum wind resistance / m81 applicable temperature / wind energy utilization factor blade material selection should consider sufficient strength, stiffness and life Good formability and processability. 3.1 Base Materials The base materials widely used in hand lay-up are mainly epoxy resin and polyester resin. Polyester resin has good process performance and low price, and polyester can be used for small and medium-sized blades. However, the general polyester composite has a large curing shrinkage rate and poor mechanical properties, so it is difficult to meet the production process requirements of large blades. Epoxy resin has a relatively good bond strength, and the composite material has good mechanical properties, no low molecular weight during curing, small shrinkage, and good heat resistance and chemical resistance. Epoxy resins can be considered for large blades, but their disadvantages such as cost, high viscosity and poor processability have largely limited their application. Modified epoxy resin (usually vinyl resin) has both good processability and excellent mechanical properties. It can be said that it combines the dual characteristics of epoxy resin and poly-resin resin, often in the production of large blades. use. 3.2 Reinforced material blades have large bending and centrifugal forces under the action of aerodynamic load and centrifugal force, and there are also stresses such as torsion and shear. Generally, the abrasion resistance and folding resistance of glass fiber are very poor, and the fiber is easily injured and broken after being rubbed and twisted; the carbon fiber reinforced resin is similar in structure to the glass fiber reinforced resin, in specific strength (strength/density) and specific mode (mode) Carbon fiber is superior to glass fiber in performance indicators such as volume/density, but its cost is much higher than that of glass fiber, which limits its wide application (see Table 2). Therefore, in order to make rational use of glass fiber, an unmachined one-way woven fabric is selected to withstand the bending and centrifugal force, and a small amount of alkali-free balanced woven fabric is selected to improve the torsional rigidity and shear strength of the blade. Table 2 performance comparison fiber melting point / tensile strength tensile elastic limit value / MPa specific strength / cmx106 than chess / cmxlO6 硪 fiber graphite E glass fiber S glass fiber 3.3 surface coating gel coat tree film) material blade durability Sex depends largely on its surface condition and should not be exposed as much as possible to prevent media erosion. For this reason, the outer surface of the blade should be specially made into a layer of a gel coat having a very high resin content, and this layer of resin is called a gel coat resin. The thickness of the gel coat layer is generally 0.25-0.4 mm. If the gel coat is too thin, the glass fiber under the gel coat will be exposed, failing to achieve aesthetics and protection; but being too thick will easily cause breakage. Therefore, it is especially important to choose a gel coat resin reasonably. 3.4 Sandwich material Foam is a lightweight polymer material that is commonly used as a filler in blades. The foam has a unique closed-cell structure, which makes it more absorbent, gas permeable and thermally conductive than the through-hole structure, and has higher strength and stiffness than the through-hole structure. The gas in the foam contains children and gas uniformity, which has a great influence on the quality. Generally, the pores are fine and uniform, and the tensile and compressive strength of the structure is larger than that of the pores and the pores are not uniform. The bulk density and strength of the foam are related to the gas content. The more the gas content, the smaller the bulk density and the lower the strength. At the same time, the foam used in the blade must be a rigid closed cell foam. Generally used polyurethane foam, which has the advantages of small bulk density, high strength, low thermal conductivity, cold resistance and shock resistance. In addition, it can be foamed in situ at room temperature, which is particularly suitable for its core as a sandwich structure for the blade. 4 blade structure design The structural design of the blade is quite complicated, and many factors should be considered: 4.1 Overall design of the blade At present, the power control of the wind turbine mainly adopts two methods: variable pitch control and fixed pitch stall control. The pitch control reduces the aerodynamic performance of the blade by changing the pitch angle to change the angle of attack of the blade profile, so that the wind power of the wind speed zone is reduced, and the speed limit is achieved. In theory, the pitch is a good control method, because the pitch angle can be artificially changed, and the optimal operation of the wind wheel can be realized under various working conditions. However, pitch control requires complex control systems and variable pitch actuators, which make the wind turbine complex, costly, and reliable and safe. The stall pitch control is based on the stall characteristic of the blade wing itself. Under the condition of higher than rated wind speed, the angle of attack of the airflow increases to the stall condition (a >16°), which causes eddy current on the surface of the blade. Reduced, to achieve the purpose of limiting power, and does not require complex variable pitch systems, simplifying the unit and increasing the reliability and safety of the wind turbine. Large-scale wind turbines abroad have widely used the stall control method for wind turbine power control, so this scheme is generally adopted in the overall design. 4.2 Blade layup design The layup design of the blade is another important part of the composite blade design. The layup of the blade is determined by the external load on the blade. The bending distance, the torque and the centrifugal force gradually increase from the tip to the blade root, so the wall thickness of the blade structure also gradually increases from the tip to the blade root. Due to the high strength and low modulus of elasticity of the composite material, in addition to meeting the strength conditions, the blade still needs to meet the deformation conditions, especially for the longer wind turbine blades, especially the collision of the blade and the tower. The blade body design is arranged as much as possible with equal strength, and a large safety factor is required at the blade root portion. 4.3 Blade profile and root structure The blade profile and root form should be considered. The structural properties, material properties and forming process of the blade should be considered. Wind turbine blades are subject to large aerodynamic loads, and extreme wind loads of 50 to 60 m/s are usually considered. Therefore, most of them are constructed with a main beam and a pneumatic outer casing or a reinforcing rib inside the casing to improve the strength and rigidity of the blade. The main beam is commonly used in the D type, C type, 0 type and rectangular form. The ribs are provided in the fasting blades to increase the rigidity and prevent local instability. The main beam forming in foreign countries adopts the winding process, while the domestic is limited by the process equipment, and the hand lay-up process is commonly used. It is more troublesome to form such a main beam by hand lay-up, and the ribbed profile in the fasting shell is more suitable for the hand lay-up process. The upper and lower half shells and the reinforcing ribs are separately formed and assembled into a whole. Considering the requirements of the process equipment and the ease of operation, it is usually more desirable to use the latter profile. The main beam is mainly subjected to axial loads and is usually laid in the axial direction using a 4:1 or 7:1 unidirectional cloth. In addition to bearing part of the axial load, the pneumatic housing also bears the torsional load. According to the composite material optimization design point of view, some 1:1 cloth layers should be arranged for 45 laying. In order to simplify the process operation, ±45 surface layers can be omitted. A 4:1 layer can be used, or some 1:1 layers can be placed on the outermost layer, all laid in the axial direction. Leaf root design is the key to blade structure design and should be taken seriously. Because the load at the root of the blade is the largest, the root connection is mostly transmitted by the shear strength, extrusion strength or shear strength of the composite material, and these strengths of the composite are lower than the tensile compression. And bending strength. It can be said that the most dangerous part of the blade is at the blade root. When selecting the root end form, care should be taken to prevent large shear stress at the root end, especially to avoid interlaminar shear stress. At present, the root end connection forms of composite blades for large and medium-sized wind turbines mainly include composite flanges, metal flanges and embedded bolts. Among them, composite flanges and embedded bolts are the two most widely used methods. 4 Loads in blade design It is also important to consider the determination of the load in the blade design, both to ensure safety during transportation, installation and operation, and to minimize costs. The wind turbine design standard developed by the European Wind Energy Association defines the load conditions as a combination of design conditions and natural environmental conditions, thereby proposing normal load conditions, abnormal load conditions and accident load conditions. The normal load condition refers to the load on the blades of the wind turbine during normal operation (referred to as normal operation, yaw, start and stop), which can be roughly divided into three categories: aerodynamics, gravity and centrifugal force. Therefore, the shear force, bending distance and torque caused by aerodynamic load should be considered in design; the shear force, tensile pressure, bending distance and torque generated by gravity on the blade; tensile force, bending distance and twist generated by centrifugal force on the blade distance. In addition, the effects of gyroscopic forces and turbulent winds on the blades are also considered. Abnormal load conditions refer to the load on the blades of the wind turbine during abnormal operation (referring to extreme wind loads, installation and transportation, dangerous conditions). In the case of extreme wind load (50 times can reach 3.5 times under normal working conditions. Therefore, the blade design must meet the damage under such a large load, and have a certain residual strength to withstand the normal working condition fatigue load of about 108. In the case of installation, transportation and dangerous conditions, the load situation is very complicated, but the number of alternating changes will generally not exceed 106, and the design strength must also be able to withstand these loads. Accident load conditions refer to accidents (speed, Blade damage) The load on the blade. However, the probability of this condition in the blade is small. Even in this case, the blade can be repaired as long as it is not damaged to the skeleton beam. 5 Lightning protection, vibration and deformation and thermal expansion factors In addition to meeting the above requirements, the blade design must also consider the effects of lightning protection, vibration and deformation, and thermal expansion factors. 5.1 Lightning Protection The biggest problem in the operation of wind turbines is the damage caused by direct lightning strikes, especially the damage to the blades. Even if the blade is made of purely insulating material, the possibility of lightning strikes cannot be ruled out. Because the surface of the blade may be contaminated by seawater salt or industrial dust, electric field concentration may also occur, resulting in lightning strikes. If the current only flows through the surface of the blade, the damage caused is weak. If the current penetrates the blade, the blade material is heated to a very high temperature, causing damage or peeling of the blade. If the tip is made of metal, the blade with the tip to the hub should be fitted with an electrical conductor of sufficient cross-section. When the cross section of the conductor is too small, the metal vapor pressure will be generated when the overcurrent is burned, causing the blade to split. At the same time, any conductor installed in the blade will increase the number of times the lightning strikes the blade. At this point the current is transferred from the blade to the ground without damage to its components. The lightning strike current is transmitted from the blade to the earth through the bearing, the nacelle, the generator, the tower and the control system. Therefore, lightning protection and current conduction should be taken into consideration for each component. S.2 vibration is inevitable due to the cross-change and randomness of the load acting on the blade, and the blade itself is an elastic structure. The form of vibration is bending vibration, torsional vibration and bending coupled vibration. If the natural frequency of the blade is close to a certain integer multiple of the rotational frequency, a large dynamic stress is generated, which makes the blade have resonance properties. The fatigue generated by vibration reduces the strength of the material and reduces the service life. Therefore, the natural frequency of the blade is required to be separated from the resonance frequency by a certain distance during design. S.3 deformation and thermal expansion of the blade under the external load will produce elastic deformation, such as shear deformation, tensile deformation and distortion, which can be determined by calculating the tip displacement and torsion angle. In addition, the deformation caused by temperature deformation and additional stress must be considered in the design. The temperature deformation is caused by the inconsistent thermal expansion coefficient caused by the difference in the latitude and longitude. When the temperature changes, the layers of the blade shrink and expand differently, causing the blade to warp. The radial direction of the blade is inconsistent with the axial direction of the blade, which will generate additional stress. Under the action of centrifugal force, similar temperature difference deformation will occur. 6 blade forming process The development of large composite wind blades is realized by different forming processes of composite materials. Multiple process integrated designs must be implemented in their design and production processes. Different processes are applied to different materials, including SCRIMP, SPRINT, RIM, epoxy prepreg and other low temperature and low pressure molding processes combined with tempering and winding processes such as pultrusion and winding. 7 Conclusions This paper has carried out preliminary exploration and summary of blade material selection, structural design and forming process for large wind turbine composite materials. Since the molding process is multiplexed and complicated, it can only be summarized. Large-scale wind turbine composite blade technology has been quite mature in foreign countries, and the domestic is still in the exploration stage, especially the combination of multiple processes needs to be explored.
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