JEC COMPOSITES MAGAZINE - Issue #54 - January 2010 - 29

Epoxy vehicles) and the rotational speed, which is dependent on outwardly acting centrifugal force, it is particularly the blade bending wind forces, which are dependent on wind speed, that can produce flexural moments of some 6,000 kN at the blade root, the equivalent of the leverage of eleven VW Golf vehicles on the blade tip. Epoxy resin systems reinforced with glass-fibre mats are predominantly used to meet these requirements. Rather than being woven together, the mat layers are stacked next to one another or in multiple layers and bound with a few threads in order to minimize fibre bending. Carbon fibres are also used, but in much smaller quantities, as they are not only more expensive but also only available in limited quantities. Other materials such as aramid or basalt fibres have played no part as yet. In the manufacture of large blades, the fibres or fibre mats are soaked with two fluid components, the resin and the curing agent, which react with each other after heating in a mould to form a very strong polymer. The complete system makes a fibre composite material that delivers the necessary mechanical stability. The construction sheme of a typical wind turbine blade New curing agents for optimized composites However, it is not the materials which set the upper limits for size. The real reasons are economic, since the profitability of a turbine only increases to a limited extent with rotor diameter. Larger turbines are more efficient, of course, but also more expensive to transport and erect, so their profitability is influenced by many factors. Offshore expansion plans are now playing an important role in many European countries. The report “Delivering Offshore Wind Power in Europe” by the European Wind Energy Association (EWEA, Bonn, Germany) predicts that wind farms with around 20 to 40 GW of capacity will be connected to the grid by 2020. By the end of 2008, plants with a capacity of 373 MW had been installed offshore, according to Siemens AG, and we can expect to see outputs of up to 1,050 MW by the end of 2009 with continued breakneck growth after that. “We expect new installations with a total capacity of 4,200 MW to be put up in 2014,” says Andreas Nauen, CEO of Siemens Wind Power, the top company for new offshore orders in 2007 and 2008. Against this background, material suppliers are facing clear challenges, particularly for the manufacture of rotor blades. Conventional polyester-resin-based composites are being replaced by more resilient fibre-reinforced, epoxy-based materials to achieve the necessary material properties for rotor spans in excess of 100 m. BASF (Ludwigshafen, Germany) sells a broad range of curing agents,accelerators and additives for professional epoxy resin processing under the Baxxodur brand. To address the challenges, the company has recently developed tailor-made epoxy resin systems for the manufacture of large rotor blades that have since been approved by Germanischer Lloyd. BASF has now succeeded in developing innovative epoxy resin systems that can handle the long production processing phase required by the ever larger rotor blades, yet also deliver short curing times to reduce cycle lengths. In this way, individual cycles can be reduced by around 30%. The forces that act even on a 40 m blade are enormous. Besides weight loads of around 70 kN (the equivalent of five VW Golf Different fibre-impregnating processes There are four main techniques for impregnating the fibres. In laminating, the dry mats are impregnated manually one after the other, using rollers dipped in the epoxy resin systems. Infusion technology uses a vacuum to suck the epoxy resin systems into the dry mats that are stacked in the mould. The third possibility is to soak the mats outside the mould and let them pre-cure before laying them in semi-rigid form in the mould for final curing (prepreg technology). For long rounded components, the fibres are impregnated and wound onto a mould before being cured using thermal energy (filament winding technology). Using these four technologies, it is normally possible to manufacture all of the main components of a rotor, including the thick multi-layer blade root that transfers the force to the hub and the internal structural ribs, spars or beams which take up most of the forces acting along the length of the blade. The third component is the outer layer of the (in principle) hollow blade that fulfils the aerodynamic function, and is either cast along with or bonded to the root and internal structure. The modern infusion systems used in the wind energy sector consist of two parts, a resin and a curing agent. Amine formulations are commonly used for curing since they meet the requirements for mechanical stability and processability. The new BASF system also uses this class of substances. Immediately before the infusion, both components are mixed together in a predefined ratio. At this point, the pot life or processing time clock begins to tick as the reaction between the two components begins slowly, and then accelerates at higher temperatures. However, for infusion to work well, the viscosity of the system has to be correct. For this reason, the formulation includes what are called “reactive diluents”, which reduce the viscosity while forming part of the cross-linking reaction. The actual cross-linking or curing of the resin is achieved by the No54 January 2010 / jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #54 - January 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #54 - January 2010