There are two main approaches to manufacture wind turbine blades: infusion and prepreg. Overall, infusion is perceived to be the cheapest of the two but prepreg manufacturing gives better resin properties and thus a higher quality of processing. As I discussed in the previous post, the length of the blade is a very important parameter since it determines the area covered by the blades and thus the amount of wind power it can extract. But as blades get bigger, they also get heavier and thus the load on the bearings and the generator become higher. The best solution seems use materials with high specific strengths and stiffness, e.g. : stiffness of carbon fiber is around 4 times higher than glass fiber’s (Gurit’s Wind Energy Composite Handbook) – but it’s also much more expensive. For small wind turbines the design is stiffness driven, while bigger blades suffer significantly more of fatigue.
This technology creates a vacuum environment around a stack of reinforcing fibers and the resin is sucked into it. On the one hand the viscosity of the resin is determining in how fast this process will be and if the whole stack will be penetrated. On the other hand the permeability of the stack is equally determining the rate of infusion. Generally, the smaller the fiber, the closer they are packed and the lower the permeability.
The hardening process of the resin is mostly initiated with a catalyst or a hardener. A precise timing is needed, since the hardening may only begin after the resin is infused and has penetrated deep enough into the laminate. The mixing process creates the danger of introducing air into the resin system, which would deteriorate the mechanical properties. It can be avoided by degassing it in a vacuum chamber.
The infusion process itself is challenging. Any leaks will result in an inflow of air which will spread across the stack of fibres. A widely used solution is to apply a priming gelcoat to the mould which will keep air from being sucked in.
After the fiber laminate is penetrated, it should be cured with a heat treatment to develop optimal mechanical properties. Normally temperatures of around 50°C are enough to complete the cure in less than one day. The cure is mostly done before the piece is taken out of the mould.
How is this process going to be used to build the blades of a wind turbine? As we discussed last post, there are two main structural designs, both using a central part functioning like an I-beam, and the shell which gives the blade its wanted aerodynamic properties. The structure is not to complex and thus infusion should be possible. Instead of manufacturing the whole blade as one part, the central part, the “spar box” is mostly created in a parallel process. The shell is a sandwich structure: two high strength skins which have a lightweight core material in between. The core fulfills the role of the vertical piece in the I-beam, resulting in a lightweight construction with a high stiffness thanks to the skin layers of unidirectional fibers. Mostly, this sandwich structure is infused as whole.
The second technology to produce the blade is prepreg, short for pre impregnation. A layer of fibers is impregnated with resin, stored and later used to build a composite component. This technology has the advantage of having a highly controllable resin content, better quality of resins and is easier automated. On the other hand the storage of the prepreg is a challenge.
Resin is first applied on a paper carrier, fiber strings are laid onto it, a second paper is applied, it all goes in the heating oven to reduce the viscosity of the resin, it is rolled to improve impregnation of the fibers, the paper carriers are removed and backers applied to secure it for storage.
The prepreg technology depends on the same properties as infusion: viscosity of the resin, permeability of the fibres and the applied pressure gradient. The viscosity is mostly very high (solid at room temperature) and to be able to impregnate the fibres the viscosity should be lowered significantly by applying heat to it. A few other properties are important to be familiar with.
Tack is the amount of stickiness of the surface of the prepreg and is important since it determines how well it sticks to the mould surface and how difficult it will be to remove after the impregnation process. Drape is the ability of the prepreg to follow the curves of the mould. This is important because it determines if volumes of air are created, which could damage the structure. Another aspect to keep in mind is the reaction of latent catalysts which will harden the resin until it looses all its drape and tack. It has then reached its “out-life”.
But how is a composite made out of this prepreg? This happens by making a lay-up of prepreg plies, if needed with core material in between (in case one is manufacturing a sandwich structure). The layering should happen with great care and speed, reducing the risk of contamination of the surface of the layers and to avoid bumps between the layers. When the desired stack is build, a nylon peel ply is placed on the upper surface to prevent any contamination. Then, a perforated film is added to gain control over the flow or the resin in the curing process of the component. A polyester breather and finally a vacuum bag are applied to make it ready for vacuum treatment.
To consolidate the prepreg and to remove the air, vacuum is now applied. During this process, a lot of heat is released (consolidation is exotherm for common resins). This can get trapped inside the component, resulting in damaging heat peaks. Therefore the temperature should always be monitored carefully. Another problem is that water or other chemicals used during processing can become gas, trapped inside the component and thus building up damaging pressure.
Overall, the prepreg method is percieved as being of higher quality but is more costly. But technologies are widely used for blade design.