The 88.4-meter-long composite wind turbine blade produced by Danish LM Wind Energy is the longest blade produced by a global wind turbine blade manufacturer so far, and is similar to the length of a 100-yard American football field. The sweeping area of the 88.4-meter blade fan is equivalent to the size of three football fields, and the output power can be used by a small town.
The blade is named LM 88.4 P, which is 15 meters longer than the longest blade LM 73.5 P produced by LM in the past, but the weight is only increased by 6 tons. This is achieved by introducing lightweight carbon fiber with high strength and high rigidity into the blade main beam laminate.
Carbon fiber has been used in the manufacture of wind turbine blades for more than 10 years. The main beam is carbon fiber reinforced epoxy resin prepreg main beam produced by the pultrusion process, which is proved by the high cost of energy generation per unit of MWh. The reasonableness of the price. Now, LM has explored a different method to embed the main beam of carbon-glass hybrid reinforced composite material along the length of the standard glass fiber composite base shell laminate. In this way, the blade shell and the main beam can be manufactured in the same mold, and the blade shell acts as a mold for the carbon-glass hybrid composite main beam, which greatly saves time and cost. Michael Lund-Laverick, Director of LM Composites Technology Project, said: "LM 88.4 P is the first leaf profile produced with hybrid reinforcement technology. It is manufactured using dry carbon glass hybrid fiber placement and vacuum assisted resin transfer molding." The hybrid enhanced design enables LM to use existing technology to produce LM 88.4 P leaf shape, which significantly enhances its core competitiveness. Fully meet the customer's established design specifications for cost and duration. Lund-Laverick added: "We chose the technical solution closest to the actual application, which significantly reduced the risk and promoted the rapid development of the technology required by the customer."
Adwen, the user of the 88.4-meter blade, expressed that it hopes to use the wind turbine for offshore wind farms under Class I wind conditions. Therefore, the design engineers of the LM 88.4 P blade set the reference wind speed that the blade can withstand as 50 meters per second. It meets the requirements of Class I wind conditions.
The driving force of new product design: uncertainty and risk
Next, the blade design was affected by the uncertainty of LM's operating strategy. Lund-Laverick said: "The hybrid enhancement method first solves the biggest uncertainty problem, and this is also the biggest failure we can think of." LM uses failure mode effect analysis software-a formal analysis tool for evaluating design and process risks , To analyze the feasibility of design and process. Of course, before turning to computer assistance, engineers will first orally discuss possible risks. "In this way, we can manage the progress of the project by controlling risks. With the company's solid professional knowledge, rich development experience and good communication mechanism, we can determine the key problems. Engineers will list the possible problems one by one, and then Avoid these problems through corresponding testing and engineering solutions."
The structure analyzed by FMEA is used to determine what kind of tests and experiments are to be carried out. In the process, prototype and test actual sample parts are used to verify design limits and failure modes before computer modeling and analysis begin. Computer simulation is also based on reality, not on primary theory, no matter how powerful the theory is.
After confirming the wind level, design limit and failure mode of the blade, the engineers used the design program and 3D modeling software package LM Blades developed by the company to simulate the static and dynamic fatigue loading of the large blade within the limit range, and the large aerodynamic panel The buckling, the Puck fracture criterion for the strength of unidirectional fiber reinforced composites, and other mechanical/strength, chemical/environmental requirements.
Carbon-Glass Hybrid Best of Both Worlds
LM company's hybrid technology refers to the use of carbon glass hybrid fiber reinforced composite main beam to strengthen the standard glass fiber reinforced polyester composite blade base shell laminate technology. The glass fiber fabric for the blade shell produced in accordance with LM technical requirements is supplied by many suppliers, and the typical supplier of H glass fiber is Owens Corning. The blade shell laminate is obtained by impregnating LM standard polyester resin in a sandwich structure with Barcelona balsa wood as the core material.
The uniaxial carbon fiber/H glass fiber hybrid fabric required for the main beam of the blade is produced by the Norwegian company Devold AMT AS, which belongs to SAERTEX. The fabric product won the "2017 Best Innovation Partner Award" by LM. The fiber fabric structure required for the blade shell and the main beam is composed of ±45° biaxial fabrics with different areal densities, 0°/±45° combined fabrics and 0° unidirectional fabrics. The new fiber fabric combination used on the 88.4-meter blade for the first time was independently developed by the LM team and used with a LM proprietary unknown resin to obtain excellent blade performance. Although the proprietary resin used to infuse the main beam of the blade has a different main chain structure in terms of molecular structure, it can produce a strong bond with the polyester resin used in the blade shell through a chemical reaction.
Carbon glass hybrid manufacturing strategy
The blade is formed by solidifying the upper and lower parts of the windward side and the leeward side. The windward side and the leeward side each contain a main beam. The carbon-glass hybrid production technology includes a two-stage infusion process: the first stage, the blade shell, contains all structural elements except the main beam, and the manufacturing steps are as follows:
Spraying the gel coat in the 88.4-meter mold: The worker sprays the gel coat on the inner surface of the mold before placing the glass fiber fabric for the blade shell, which can omit the painting process after the blade is demolded.
Place a biaxial glass fiber fabric covering layer on the outer surface of the blade.
Put leaf root and edge strengthening materials, mainly unidirectional fabric materials.
Place the sandwich structure with Barcelona balsa core material and fix it with small fasteners. The core material of Barcelona balsa wood is manually laid and fixed by workers.
Lay the inner biaxial fabric layer.
Use vacuum bag and LM standard polyester resin for vacuum infusion.
According to LM's proprietary operating specifications, the parts are cured at room temperature.
After the curing is completed, the base shell is separated from the vacuum bag, and the inspection is ready to enter the next stage.
In the second stage, the main beam of the hybrid composite blade is directly formed on the solidified base shell using the base shell as a mold:
The dry carbon-glass hybrid unidirectional fabric layer is placed directly on the relatively flat center of the blade base shell along the length of the blade by a semi-automatic placement machine, starting from a position 4 meters away from the root of the blade.
The lightweight protective parts designed by LM are added.
Place ±45° biaxial standard glass fiber covering layer.
The same vacuum bag and vacuum infusion process are used to precisely control the infusion conditions, and only the special resin is impregnated into the mixed fiber layer.
According to LM's proprietary operating specifications, the main beam components are cured at room temperature.
Lund-Laverick said: “We have the ability to scale up and apply the existing production technology to the ever-changing wind turbine blades. On these huge blade half shells, there is a slight pre-bending along the length of the blade, and both sides of the blade There is a little curl, but the center part is quite flat.” In the flat area of each half shell, LM uses internally designed and computer-controlled semi-automatic equipment to lay the wide-width dry mixed fiber fabric according to the programmed layup method. Release to obtain the ideal strength performance.
The curved edge of the leaf has complex geometry, so it usually has to be laid by hand.
Both the base shell and the hybrid main beam require equipment to be laid. Because carbon-glass hybrid fabrics are more sensitive than ordinary glass fiber fabrics, LM has developed a manual placement technology to avoid wrinkles and other defects that may be introduced by manual operations.
The blade rib is a sandwich structure composed of glass fiber composite material and foam core material. It is mainly formed by a biaxial fabric layer and a standard polyester resin through an infusion process.
In terms of quality control, LM adopts the Six Sigma lean management model. Lund-Laverick said: "During the production process, we have a set of strict quality control documents and procedures, including continuous visual inspection of the blades." After curing, ultrasonic inspection and visual inspection ensure the quality of the product. Subsequently, the two blade half shells are glued together using conventional methods.
Mold clamping and post curing
After the two half shells are glued together, the LM uses a deflector to compensate for the relatively poor aerodynamic performance near the root of the large blade. The deflector is specially designed for the geometric shape of individual blades, similar to fins, and is molded by injection process. In the final blade assembly process, the VHB tape produced by 3M is attached to the outer surface of the leeward half shell, usually from The position about 5m from the root of the leaf extends to the middle of the leaf. Doing so can reduce airflow separation and increase lift and energy output. In addition, LM will also use proprietary technology and customized installation of baffles on the inner chord of the blade near the blade root to further increase the lift. It is usually produced by reaction injection molding.
At present, the LM 88.4 P blade has passed full-scale static mechanics and fatigue tests on the LM test platform and the independent blade test center in Denmark, and has passed the certification of the Norwegian DNV GL classification society.
The first commercial LM 88.4 P blade has been installed on Adwen's next-generation wind turbine AD 8-180, with a rated power of 8MW and a rotor diameter of 180 meters. The 3 blades produced by LM are transported by trucks and barges, installed by Adwen, and are currently connected to the grid in the port of Bremen, Germany. Unfortunately, because Adwen was acquired by Gamesa, Gamesa was merged into Siemens, and the new Siemens Gamesa Renewable Energy Company cancelled the AD 8-180 platform, so the product of this project became a victim of industry integration.
Lund-Laverick said that LM will continue to develop the carbon-glass hybrid main beam platform. The 69.3-meter blade is currently being delivered to Siemens Gamesa. This is currently the world's largest onshore wind turbine blade. The blade still uses the same design, materials and production specifications of the 88.4-meter blade, and will also use the dry carbon glass hybrid fabric and resin system to obtain lower energy and power generation costs. He added that the current hybrid reinforced blades are mainly produced according to customer needs. As the length of the blades continues to increase in the future, LM will increase the number of blades produced by hybrid technology.