Advantages and Application Status of Pultruded Plates for Wind Turbine Blades

 

Since 2021, the Wind Power Industry has Officially Entered the Era of Parity.

 

In order to further reduce the weight and cost of wind turbine blades and improve competitiveness, the entire wind power industry has accelerated the development and application of new materials or new technologies. By using reinforcing fibers with better mechanical properties, such as high modulus glass fiber or carbon fiber, the mechanical properties of composite materials can be significantly improved, but at the same time it also brings higher costs.

 

By adopting more advanced composite material manufacturing processes, the mechanical properties of reinforcing materials can be fully utilized to achieve the purpose of reducing blade weight and cost.

 

 

Wind Power Generation

 

Pultrusion technology began in the 1950s and has developed into one of the main molding methods for fiber-reinforced composite materials by the 1980s.

 

In recent years, pultrusion technology has been paid more attention by various industries. Its products are developing in the direction of complex shapes, extreme cross-sectional dimensions, efficient production, diversified types, and high performance, and are suitable for various engineering fields. For example, from wires as thin as 1mm to rods of 70mm, and then to 800mm×280mm hollow profiles, the pultrusion speed of some products is as high as 6m/min.

 

 

EN161 Carbon Pultruded Beam Blade Source: China Jiangsu Network

 

Pultruded board is a composite material board with a certain cross-sectional shape and continuous length made by pultrusion process after multiple bundles of reinforcing fibers are impregnated with resin. The pultrusion process gives the fibers a certain tension, which can significantly improve the mechanical properties of the product along the fiber direction, so its key performance is better than that of infused fabric and prepreg composite products.

 

Vestas, a world-renowned wind power manufacturer, was the first to adopt the technology of assembling the main beam with pultruded plates, splitting the original integral main beam into pultruded plate standard parts and assembling them into shape. This revolutionary and innovative design has greatly promoted the application of pultruded plates in the field of wind power blades.

 

 

This article introduces the development history, technical advantages and application status of pultruded plates for wind turbine blade main beams, and proposes the development direction of pultruded plates for wind turbine blade applications in the future.

 

1. Development of Pultruded Plate Technology

 

Carbon Fiber Pultruded Plates

 

20 years ago, Europe used hand lay-up, vacuum infusion, and prepreg to produce wind turbine blade main beams, but some manufacturing defects of carbon fiber were difficult to avoid.

 

In 2003, Vestas took the lead in applying for a patent for using pultruded plates to manufacture wind turbine blade main beams.

 

In 2015, Vestas disassembled the integrally formed main beam into efficient, low-cost, high-quality pultruded plate standard parts, and realized assembly and infusion molding.

 

Through a large number of tests, verifications and improvements, Vestas took the lead in the world to use carbon fiber pultruded plates on a large scale on wind turbine blades. All blades above 2.0MW developed later used this product, which greatly promoted the application of carbon fiber pultruded plates in the wind power field.

In recent years, foreign blade manufacturers such as GE-LM, Siemens-Gemesea, and Nordex have successively broken through Vestas’ patent restrictions, designed their own carbon fiber pultruded plates, and applied them on large blades.

 

Taking 2021 as an example, the amount of carbon fiber used in wind turbine blades is 33,000 tons, and the main application form is pultruded main beams. Therefore, pultrusion molding has become the mainstream process for the production of carbon beams for wind turbine blades.

 

In China, after gaining development experience in the manufacturing and application of glass fiber pultruded plates, some complete machine factories, blade companies, and material suppliers have turned to the development of carbon fiber pultruded plates, and gradually applied them to large offshore wind turbine blades over 100 meters, such as Shanghai Electric’s 102, Yunda Wind Power’s 110, and Goldwind Technology’s 123.

 

Figure 1 Carbon Fiber Pultruded Board

 

In the early stage of carbon fiber pultruded board development, foreign carbon fiber was used. With the advancement of domestic carbon fiber technology and price advantages, it has gradually become the main supply channel for domestic carbon fiber pultruded board. By adjusting the match between fiber and resin, the modulus of carbon fiber pultruded board can be increased from 130GPa to 145GPa, further improving the lightweight level of blades.

 

Glass Fiber Pultruded Board

 

Europe and the United States have studied the use of glass fiber pultruded board to manufacture blade main beams. For example, wind turbine manufacturer Senvion uses NEPTCO’s RodPack pultruded board and UD fabric mixed infusion to form blade main beams, but this solution has not been used in batches. In recent years, some European blade design and manufacturing companies have successively developed glass fiber pultruded boards, but there are still no reports of application cases.

 

 

Figure 2 Glass Fiber Pultruded Board

 

Different from the order of developing pultruded boards abroad, China first developed and applied the more cost-effective glass fiber pultruded board. In 2014, Zhongfu Lianzhong Composite Materials Group Co., Ltd. (hereinafter referred to as Zhongfu Lianzhong) took the lead in applying for a patent for the application of pultruded boards on wind turbine blades.

 

Zhongfu Lianzhong LZ84-6.25 blade

 

In 2019, Zhongfu Lianzhong and Zhongfu Carbon Core Cable Technology Co., Ltd. (hereinafter referred to as Zhongfu Carbon Core) successfully broke through the technical system of high-strength glass fiber pultruded board manufacturing and its integrated infusion on large wind turbine blades, and developed megawatt-level blades using pultruded main beams.

 

Subsequently, more and more domestic complete machine factories, blade companies, and material suppliers have devoted themselves to the research and development of pultruded boards and their application technologies, and have produced a large number of patented technologies involving pultruded board cross-sections, pultruded board manufacturing processes, and pultruded board applications.

 

In the early stages of domestic pultruded board development, the industry first focused on the research and development of low-cost glass fiber pultruded boards and their application technologies, and developed high-performance pultruded boards by using ultra-high modulus glass fiber (dip modulus of about 95GPa) and high resin content.

 

After mastering a certain amount of experience, in order to achieve blade cost reduction, a lower level of high modulus fiber (dip modulus of about 90GPa) and low resin content were used to further improve the glass fiber stiffness and strength conversion efficiency, and successfully developed pultruded boards with comparable performance.

 

The entire domestic market has quickly accumulated and shared manufacturing and use experience, so pultruded plates have been widely used in large wind turbine blades.

 

At present, some companies are trying to develop higher-performance pultruded plates using ultra-high modulus glass fiber and low resin content, hoping that the design modulus of pultruded plates will reach 65GPa-68GPa.

 

Domestic research has been conducted on pultruded resins of various systems, such as unsaturated polyester, polyurethane, epoxy resin, etc., and the epoxy system has become the mainstream system in the industry because of its mature process, high performance and stability. In addition, domestic companies have broken through the concept design of thin and wide sheet sections defined by Vestas, and innovatively developed narrow and thick sections, which greatly improved the universality of pultruded plates for blade geometry and the quality of blade main beam infusion.

 

Carbon-glass Mixed Pultruded Plate

Currently, carbon fiber materials are expensive, and the main beam of pultruded plates manufactured by them is costly, so they can only be used for 100-meter offshore wind turbine blades. In order to reduce material costs while retaining higher performance.

 

Domestic blade material manufacturers have turned their development direction to carbon-glass mixed pultrusion technology. This technology combines the high strength, high modulus, and low density of carbon fiber with the high elongation and low cost of glass fiber, breaking through the technical barriers of low modulus and heavy weight faced by traditional glass fiber large blade design.

 

By changing the ratio of carbon yarn and glass fiber, the modulus can be linearly changed from 60Gpa to 120Gpa. It provides more possibilities for blade design, optimization, weight reduction, and cost reduction, and finds the best fit between performance, weight, and cost.

 

Figure 3 Carbon-Glass Pultruded Board

 

In July 2021, the first carbon-glass pultruded blade in China, Mingyang Intelligent MySE11-99A1, was successfully launched. The blade is 99 meters long. At present, many pultruded composite material suppliers such as Huamei, Fengdu, and Zhongfu Carbon Core provide Mingyang with carbon-glass pultruded beam products, which have been applied to a variety of blade shapes.

 

 

Mingyang Smart MySE11-99A1 blade Source: Mingyang Smart

 

Mold-Free Pultruded Board

 

Traditional pultruded boards have a layer of mold release cloth on both the upper and lower surfaces, which mainly forms a rough bonding surface, absorbs mold release agents, and protects the surface of the board from contamination and damage. However, the use of mold release cloth has increased the cost of materials, production equipment and quality control. For this reason, since 2022, many domestic pultruded board manufacturers have successively developed mold-free pultruded boards to improve the competitive advantage of their products.

 

Figure 4: Unreleased Cloth Extruded Board

 

Currently, Sany has used unreleased cloth sheets in batches on multiple blades and plans to further expand the scope of application; Sinoma, Times, Chongtong and Shuangrui have all entered the trial or small batch stage.

 

Polyurethane Pultruded Board

 

In the past two years, the price of epoxy resin has risen by more than 34%, causing the cost of blades to rise by more than 10%. Therefore, it is urgent to find a low-cost and high-performance resin to replace epoxy resin.

 

Polyurethane resin provides a new idea for reducing the cost and improving the performance of blades due to its low cost, high toughness and fatigue resistance.

 

On the other hand, among the production cost factors of pultruded board, the pultrusion speed has the greatest impact. Polyurethane has a lower viscosity and faster curing speed than epoxy resin, which can effectively increase the pultrusion speed and thus reduce production costs. Compared with epoxy system, it can save up to 20% of production costs.

 

Figure 5 Polyurethane Pultruded Board

 

In 2020, Sinoma Blade was the first to try out polyurethane pultruded beams. In February 2023, the first test blade of the TMT95 blade of Times New Materials used glass fiber reinforced polyurethane pultruded board beams.

 

However, since the blade has not yet completed full-scale testing and verification, subsequent batch production blades still use pultruded boards with epoxy resin systems. Therefore, there are currently no cases of mass application of polyurethane pultruded beams in the industry.

 

 

Times New Materials TMT95 polyurethane pultruded beam blade Source: Times New Materials

 

1. Pultruded board technical advantages and application status

Technical advantages

 

• Excellent mechanical properties

The fiber volume content of the pultrusion process is as high as 70%, which is significantly higher than that of vacuum infusion (fiber volume content ranges from 55% to 60%); and the pultrusion process achieves higher fiber straightness. The combination of the two greatly increases the proportion of fiber modulus and strength converted into composite materials.

 

Table 1 Performance of Various Unidirectional Composite Materials

 

Item	Fiberglass Pultrusion board	Infused Fiberglass Cloth	Carbon Fiber Pultrusion Board	Carbon Fiber Prepreg
Fiber Mass Content %	83	76	75	68
Fiber Density(g/cm3)	2.6	2.6	1.8	1.8
Resin Density(g/cm3)	1.15	1.15	1.15	1.15
Composite Material Density(g/cm3)	2.15	2.00	1.60	1.59
Modulus Ex GPa	62	50	135	125
Specific Modulus [GPa/(g/cm3)]	29	25	85	79

 

Note: Ex- elastic modulus in the fiber direction of the composite material. As can be seen from Table 1, the modulus of glass fiber pultruded plate can reach 62GPa, while the modulus of traditional infused glass fiber cloth can only reach 50GPa, and the modulus of the former is 24% higher than that of the latter. Comparing the modulus of unit mass composite materials, glass fiber pultruded plate is about 16% higher than that of infused glass fiber cloth. The modulus of carbon fiber pultruded plate can reach 135GPa, while the modulus of carbon fiber prepreg can only reach about 125GPa, and the modulus of carbon fiber pultruded plate is 8%-16% higher than that of prepreg. Comparing the modulus of unit mass composite materials, the modulus of carbon fiber pultruded plate is about 7% higher than that of prepreg. Taking a domestic 84m blade as an example, the weight of the glass fiber pultruded plate main beam blade is about 500kg lighter than the infused main beam blade, and the weight reduction is about 2.5%; while the weight reduction ratio of carbon fiber pultruded plate blade compared with glass fiber main beam blade can reach 15%-30%.

 

(2) Stable Quality

Stable pultrusion process parameters, production environment, glue content and fiber tension keep the fiber straightness in the composite material at a high level, effectively improving the stability of product quality and performance. It solves the problem of blade main beam wrinkles caused by vacuum infusion and prepreg molding processes, reducing production quality costs.

 

(3) Modular production reduces blade production investment. The modular production of pultruded main beams is to cut and assemble the pultruded sheet coils according to the required length to form a pultruded main beam, which is then hoisted into the blade shell mold and infused with the shell as a whole. This process saves the main beam mold and space investment required for blades of different models. The unified cutting and assembly tooling can flexibly respond to the needs of different blade models, saving a lot of fixed asset investment.

 

Application Status and Challenges

 

(2) Application status In 2019, China’s wind power blade self-use pultruded sheet business started. After three years of rapid development, the pultruded sheet main beam has basically replaced the traditional glass fiber vacuum infusion beam and carbon fiber prepreg beam in the current mainstream blade types in China. According to the latest statistics, there are more than 1,420 pultruded sheet production lines for wind turbine blade main beams in China, with a total estimated capacity of 360,000 tons, of which carbon sheet capacity is estimated to be 18,000 tons. At present, pultruded sheet has formed a mature, stable and large-scale supply chain. Pultruded sheet for domestic wind turbine blades is completely provided by the domestic market, and foreign blade manufacturers (such as Vestas and GE-LM) also purchase a large number of pultruded sheets from China.

(2) Challenges Pultruded sheet has been used in large quantities on blade main beams, but it still faces many challenges on the application side.

 

• Defects Introduced by Release Cloth.

For example: incomplete drying of release cloth affects the curing of epoxy resin, quality defects and loss caused by overlap, wrinkles on release cloth, release cloth residue caused by tearing, and tearing of the sheet body.

 

• Straightness/Flatness.

With the pursuit of high modulus and high production efficiency, the increase in fiber content and extrusion rate will further increase the internal stress of the pultruded sheet, thereby affecting the straightness and flatness of the pultruded sheet, and causing problems such as non-conformity or offset during beam assembly and use.

 

• Infusion Defects and Repairs.

In the assembly and stacking of pultruded plates, stacking may form resin-rich plates; bubbles may remain during the infusion process or resin deficiency may occur. In addition, when infusion defects occur in the pultruded main beam and shell, how to effectively repair them to ensure that the blade structure safety is not affected is also a key issue currently concerned by the industry. In summary:

 

(1) Currently, the only pultruded plates used for wind turbine blades abroad are carbon fiber pultruded plates; while domestic pultruded plate technology has developed rapidly and diversified in the past two years, and has developed more cost-effective glass fiber pultruded plates, carbon-glass mixed pultruded plates, demolding-free cloth pultruded plates and polyurethane pultruded plates, which have helped the large-scale development of domestic wind turbine blades.

 

(2) Compared with traditional UD fabric vacuum infusion and prepreg composite materials, pultruded plates have obvious advantages, with higher mechanical properties, better quality stability and lower production input costs.

 

(3) Carbon fiber pultruded board technology is becoming more mature and gradually being applied in batches. With the development of offshore wind power and the acceleration of the localization of carbon fiber, carbon fiber pultruded board will have a broader development and application prospect, and at the same time, it also puts forward higher requirements for its quality stability.

 

(4) Glass fiber pultruded board has been applied in batches in the wind power blade industry. Due to its huge advantage of low cost, it will still occupy the mainstream in the future onshore wind power blades and will continue to develop in the direction of higher modulus and low cost.

 

(5) In the context of weight reduction, cost reduction and environmental protection in the wind power industry, carbon glass mixed pultruded board, demolding-free pultruded board and polyurethane pultruded board came into being and have been applied or tried in batches on some blade shapes.