Development status of composite wind turbine blades

The application of composite materials in wind power generation is mainly to manufacture rotor blades, nacelle covers and fairings. Relatively speaking, the technical threshold of nacelle cover and fairing is low, so it is not difficult to produce and develop. The rotor blade of wind turbine is one of the key components of wind turbine, and its design, material and technology determine the performance and power of wind turbine. In the history of wind turbine 100 years, blade materials have experienced wood blades, cloth blades, aluminum alloy blades and so on. With the appearance of networked wind turbines, wind power generation has entered a period of rapid development, and some properties of traditional blades can no longer meet the requirements when used in more and more large-scale wind turbines, so composite blades with high specific strength have been developed. At present, almost all commercial blades are made of composite materials, and wind turbine blades have become one of the important application fields of composite materials.

The main advantages of using composite blades are: ① light weight, high strength and good stiffness. As we all know, the performance of composite materials can be designed, and the strength and stiffness can be designed according to the mechanical characteristics of blades, thus reducing the weight of blades; ② If the design life of the blade is 20 years, it will experience fatigue alternation of more than 108 weeks, so the fatigue performance of the material is good. Composite materials have low notch sensitivity, large internal damping, good seismic performance and high fatigue strength; (3) Wind turbines are installed outdoors, and offshore wind farms have been developed in recent years. Due to the influence of various climatic environments such as acid, alkali and water vapor, the composite blade has good weather resistance and can meet the use requirements; ④ Convenient maintenance. Composite blades generally don't need major maintenance except that they are painted on the blade surface every few years. 2. 1 composite blade material system

Wind turbine blades are thin-shell structures made of composite materials, generally consisting of roots, shells and stiffeners or beams. Composite materials generally account for more than 90% of the total wind turbine blades. At the beginning of the development of composite blades, the cheap glass fiber reinforced unsaturated polyester resin system was used, and it is still the material used by most blades today. With the continuous increase of blade length, this system can no longer meet the requirements in some occasions, so naturally, carbon fiber, a reinforced material with better performance, has entered the field of vision of blade manufacturers. The effect of adding carbon fiber on composite blades is discussed in the literature. Generally speaking, the blades below 22m are made of glass fiber, and the blades above 42m are made of carbon fiber or carbon-glass hybrid fiber [8]. In terms of resin matrix, polyester resin has low price and good molding process, but its properties are average, while epoxy resin is just the opposite, with superior properties but high price and poor process operability. So at present, some blade manufacturers generally use vinyl resin with cost and performance between them.

In view of the high price of carbon fiber in the world, some people think that carbon fiber for blade production is too expensive, but it is not. On the one hand, due to the increase of blade length, its requirements for stiffness are more stringent. When making larger blades, simple glass fiber can no longer meet the requirements. The stiffness of carbon fiber is about three times that of glass fiber, and the stiffness of composites made of carbon fiber is about twice that of FRP. In this sense, the introduction of carbon fiber is necessary and necessary. On the other hand, due to the increase of blade size, its mass is becoming more and more huge. The introduction of high-performance carbon fiber can greatly reduce the weight of the blade. With the reduction of the blade weight, the rotor blade shell, transmission shaft, platform and tower cover can also be reduced [9], thus reducing the cost of the fan as a whole and offsetting or partially offsetting the cost increase caused by the introduction of carbon fiber. With the manufacture and operation of large and super-large offshore wind turbines, the era of large-scale application of carbon fiber in wind turbine blades is not far off.

2.2 Manufacturing process of composite blades

At present, the blade forming process is generally to form the blade skin, main beam and other parts on each special mold, and then glue the two skin, main beam and other parts together on the main mold, and then make the whole blade after clamping, pressurizing and curing. The specific molding process can be roughly divided into seven types [10]: ① hand paste; ② Vacuum resin molding (VIP); ③ resin transfer molding (RTM); ④ Siemens resin impregnation process (Scrimp); ⑤ Fiber winding process (FW); ⑥ Epoxy saturation process of wood fiber (West); ⑦ molding. Among the above processes, ①, ④, ⑤ and ⑤ are open mold forming processes, and ②, ③ and ⑤ are closed mold forming processes.

Traditional blade production generally adopts mold opening technology, especially manual pasting, which will produce a large number of volatile toxic gases such as styrene, which will bring harm to operators and the environment. On the other hand, with the increase of blade size, in order to ensure the stable operation of the generator and the safety of the tower, it is necessary to ensure that the blade is light in weight and evenly distributed in mass. This promotes the development of blade production technology from open mold to closed mold. Closed mold technology, such as the popular vacuum resin introduction molding method, can not only greatly reduce the volatilization of styrene in the molding process, but also easily control the resin content accurately, thus ensuring the uniformity of the mass distribution of composite blades and improving the quality stability of blades. With the continuous increase of blade size, some new problems that have never been encountered in the production of small and medium-sized blades in the past have appeared in the manufacturing process.

3. 1 big mold problem

The shape and size of large composite blades are closely related to their manufacturing dies. In order to ensure the design shape and dimensional accuracy of composite blades, the longer the blade length, the higher the requirements for the rigidity and strength of the mold during molding, and the weight and cost of the mold will also increase greatly. In order to reduce the die weight and die cost, the manufacturing die of large composite blades is gradually changed from metal die to composite die, which also means that the blades can be made longer. The main advantages of using composite die are as follows: ① In order to achieve the best aerodynamic effect, the blade has a complex aerodynamic shape, and the chord length, thickness, torsion angle and airfoil of the blade are different at different radii of the wind wheel. If it is made of metal, it is very difficult to realize these changes on the mold, and the use of composite mold can greatly reduce the technological difficulty; (2) Because the mold and the blade are made of homogeneous materials, and the thermal expansion coefficient of the mold is basically the same as that of the blade material, the precision and dimensional stability of the manufactured composite blade are better than those of the blade products manufactured by the metal mold; ③ The use of compound mould can also greatly shorten the manufacturing cycle of mould and improve the production efficiency.

3.2 Resin curing time in vacuum resin introduction molding method

The advantages of vacuum resin injection molding (VIP) in many blade forming processes are gradually emerging, with many advantages such as less investment, simple operation, good working environment and good product performance. At present, VIP is widely used in the field of blade manufacturing. In the traditional VIP process, resin, accelerator and curing agent are generally mixed in proportion first, and then vacuum suction injection is started. As long as the dosage of accelerator and curing agent is well controlled, there is no problem when this method is used on parts of general size. However, in the manufacture of large-size composite components such as blades, due to the long time of glue absorption and glue injection, it is easy to gel before glue injection if it is not well controlled. In addition, when a large amount of glue is used, the glue solution prepared in the bucket may also explode. In order to prevent this situation, we can consider designing a mixing device of resin and curing agent. The resin and curing agent are in different containers before suction and injection, and the resin and curing agent are mixed in real time during suction and injection, so as to avoid excessive explosion and gel, increase production safety and save raw material consumption.

3.3 Curing of blades

In the process of blade production, due to the huge mold size, it is generally impossible to cure by traditional external heating methods such as oven, and the production is generally only carried out at room temperature, which leads to a long curing cycle of blades and makes it difficult to carry out more continuous production. The solution is that the blade is basically formed on the mold and then demoulded, and then cured after illumination outdoors. At present, many enterprises adopt this blade production method, such as AVIC Baoding Phaeton, a leading domestic blade enterprise. However, this method also has its inherent shortcomings. The production is affected by the weather, and the product stays in the mold for a long time before demoulding, which will affect the production efficiency. Therefore, it can be considered to build a heat source in the mold, such as laying a fluid heating pipe or electric heating cloth, and realize the rapid solidification of the blade by heating the mold with the built-in heat source, so as to realize continuous production without natural conditions. Moreover, because the baking method after illumination is seriously restricted by climate factors, most of the current leaf production bases are built in the north with sufficient sunlight. After the blade mold with built-in heat source is adopted, the climate requirement of blade production is greatly relaxed, so we can seek to establish a blade production base in the south, so as to build a more reasonable blade industry pattern in the country.

3.4 Long-distance transportation of blades

At present, all the blades of wind turbines in the world are produced by integral molds, which are huge in volume and weight, and the blade production can only be carried out in the production base, so the problem of blade transportation is becoming increasingly prominent. On the one hand, for safety reasons, railway and highway administrations all over the world have restrictions on the length and height of goods. For example, the blades and towers of wind turbines are tens of meters or even longer, and the nacelle covers are generally three meters or even higher, and the diameter of the lower flange of the tower exceeds. On the other hand, China's wind farms are widely distributed, with remote locations and inconvenient transportation. When building a wind farm, the transportation cost of large blades is very high, and some areas can't even deliver them at all. It can be said that the problem of long-distance transportation has increasingly become a bottleneck restricting the development of wind power generation. In this regard, we can consider using the combined mold to manufacture blades, that is, designing the fan blade forming mold into a detachable and convenient combined mold, transporting the mold, tooling, important parts and raw materials to the vicinity of large-scale wind farms by ordinary roads or railways, and quickly building a simple workshop to manufacture blades at the wind farm site; Another idea is to use combined blades, that is, to make the blades into several sections, so that their size is within the maximum allowable range of highway transportation, and then to assemble them after they are transported to the wind farm. However, whether this idea can be applied to practice remains to be verified by experiments, and there is no report on it at present.

3.5 Disposal of Scrapped Blades

Wind power generation is one of the sustainable industries, but the composite blades currently used are not recyclable, which has become the biggest worry of composite blades. At present, it is difficult to recycle composite blades made of thermosetting resin, and the general treatment is only to pile them in the open air. With the increasing size and number of wind power blades, the impact of these blades on the environment after retirement can not be ignored, which is also contrary to our current purpose of advocating sustainable development.

In order to solve this problem, the current development trend is to improve the reinforcing materials of blades, such as using biomass materials, that is, using wood and resin to make blades by lamination. It is reported that the specific modulus of gradient laminated bamboo materials has exceeded that of glass fiber reinforced composites, and the specific strength has reached the same order of magnitude. However, although the resin consumption of laminated bamboo materials has decreased, thermosetting resin is still needed, which can only cure the symptoms rather than the root cause. The most thorough solution is to develop recyclable thermoplastic composite blades, and some achievements have been made in this research. Gaoth Tec Teo Company of Ireland, Mitsubishi Heavy Industries of Japan and Cyclics Company of the United States signed a cooperation agreement to develop thermoplastic composite blades, glass fiber reinforced low viscosity thermoplastic CBT & reg； of Cyclics Company; Resin made the world's first 12.6m recyclable wind turbine blade. It is said that after this kind of blades are retired, the average material recovered by each set of blades can reach 19t, which is an unprecedented data. However, the current performance of this thermoplastic resin may not be ideal when manufacturing larger blades. It is said that the above companies are currently developing blades over 30 meters. Whether this "green blade" can be widely used in large-scale fans remains to be verified by time.

3.6 Other issues

At present, the overall situation of wind power generation is excellent, but there are also voices of opposition. For example, some animal protectionists think that wind power will endanger the survival of some animals, while others think that wind power has noise pollution and affects the landscape. In addition, although the price of wind power generation is decreasing at a rate of 3-5% every year, the on-grid tariff of wind power generation in China is still high [14]. It should be pointed out that any technology is not perfect, and there may be flaws. As a cause that can benefit future generations, wind power generation generally has more advantages than disadvantages. On this issue, on the one hand, the government needs to increase publicity to correct the public's views on some problems of wind power generation; On the other hand, the government can also consider increasing policy support and guidance for wind power, improve the price competitiveness of wind power, and realize the sound and rapid development of China's wind power industry. The development of wind power generation depends on the production and manufacture of a large number of wind turbines, which are inseparable from blades, and the manufacture of blades needs the support of composite materials industry. Wind power generation is a rare opportunity for China composite industry. It is our goal to choose the best material system and manufacturing technology to manufacture high-quality composite blades to meet the needs of the rapid development of wind power generation. At present, some new ideas and new technologies, such as large composite combined die with built-in heat source, improved vacuum resin molding process, recyclable thermoplastic blade resin matrix, etc., may develop into new hotspots leading the research and manufacture of wind turbine blades in the future.