Composites in wind energy

Fiberline Composites makes subcomponents of blades and has been involved in studies of carbon fibre composites: carbon fibre is very sensitive to wrinkles. There are several methods for measuring fibre misalignment including Multiple Field Image Analysis (MFIA), Confocal Laser Scanning Microscopy (CSLM), X-ray tomography and Fourier Transform Misalignment Analysis (FTMA). Shear properties can be monitored using methods like the Iosipescu shear test combined with Digital Image Correlation (DIC). Fiberline has developed an injection pultrusion processing method that gives highly aligned fibres.

Owens Corning OCV Technical Fabrics is a supplier of glass-based fabrics for the global composites industry including wind blades. The Ultrablade fabric helps to improve modulus and strength in the vacuum infusion process (VARTM) supporting the longer blades, while reducing spar weight by around 17% in a study using the IEC 61400-1 class II blade model.

Headquartered in California, Clipper Windpower is a relatively young company producing 2.5 MW Liberty wind turbines and developing 10 MW Britannia models for the offshore market. It is now affiliated to United Technologies Corp. (UTC) and can develop wind projects. According the American Wind Energy Association it had 66% market share of the turbine market of 2.5 MW and larger in the United States in 2009.

It has a new blade to enter the market in the 3rd quarter of 2011, and is focussing on innovative production processes like mechanisation and automation. Laser projection systems can be use for ply placement, for example. The design process takes into account the manufacturing methods. There are non-destructive test methods for inspecting blades such as DIC, ultrasonics, back scatter X-ray and thermography.

Riso-DTU is one of the top universities in the world for wind composites research and has reviewed problems in blades. Various defects may be introduced in manufacturing such as matrix cracks caused by shrinkage, fibre wrinkles and breaks, poor bonding and voids in adhesive layers. Interface defects are a common cause of failure, for example due to lack of adhesive at bond lines. Some faults are not important, but others should be addressed quickly to prevent blade failure: Riso-DTU is examining crack propagation, buckling-driven delamination, cohesive laws and traction-separation laws to try to determine which issues are critical. There are various types of repairs including plug and patch, or a scarf repair.

On the Isle of Wight, Solent Composite Systems is looking at ways to reduce manufacturing costs including automation and low power curing systems. It has tested automated tape laying (ATL) on blade shells, which entailed adjusting the machine to make sure that the leading edge was covered, along with light resin transfer moulding (LRTM). The mould can be heated with insulated water pipes, air or electrical methods. The SmartCure system applies heat in the mould shell combined with zone temperature monitoring to deliver power only where it is most needed. As the exotherm of the cure develops the power is cut to those areas.