Wind blade composites market grows as offshore wind energy advances

There are a variety of techniques in use for non-destructive testing of wind turbine blades. The University of Stuttgart has used an ultrasonic-echo method to examine the bond between the shell and the spar by taking measurements of a pulse sent through the outer shell and reflected as the material changes. It was also possible to look at bonding on the trailing edge despite the sound damping effect of the foam component, using a through transmission method.

Local resonance spectroscopy on the blade shell detected delamination and air inclusions. Force Technology has applied the pulse echo ultrasound technique to whole blades in an automated inspection procedure. This can be used to verify prototype blades before commencing production.

The AMS-46 cart scanner crawls along the blade surface and is used to detect laminar defects, dry areas in a girder spar, to indicate waviness and wrinkles, and to check adhesive bond quality.

During use wind blades can be continuously monitored using a system such as that developed by Igus ITS, now Bosch Rexroth Monitoring Systems.

Accelerometers are placed in the blade to measure vibrations, which are then transmitted, converted to a frequency spectrum and analysed. As one example, in hull damage with multiple trailing edge crackings, the spectrum is affected between 150 and 300 Hz. In the case of severe lightning damage, the turbine can be switched off in a few seconds and limit the effects. In the case of ice formation on blades, the system is said to detect 5-10kg on a 2MW turbine blade, which may not always be visible from the ground, but formed as a surface layer in icy rain.

Research in Bremerhaven has indicated that around 2020 there will be about 26,000 tons of waste blades in Germany. Disposal of end-of-life blades has been reviewed by Holcim (Deutschland) looking at processes like pyrolysis, incineration and landfill.

The option that Holcim chose to implement involved cutting the blades in the field to ease transportation, size reduction in a closed system and separation of materials like metals for recycling, then homogenization of the shredded blade to provide fuel and raw material in a cement plant. The blade ash forms part of the clinker matrix.

At the end of 2011 Suzlon completed its purchase of REpower Systems.

Offshore is an expanding market for the company and the 5MW/6MW manufacturing facilities are located at Bremerhaven. In 2011, 30 5MW turbines were installed at Ormonde, and in 2012-13 a further 290MW are scheduled for installation at Nordsee Ost and Thornton Bank. Design of the new blades is complex involving over 800 load cases, and the effects of the environment like rain, ice and salt and temperature cycling.

The scale up of blade size leads to far greater demands in manufacturing, for example the 40m blade weighs 7 tons and has a surface area of 180 square metres and the 61.5m blade weighs 22 tons and has a surface area of 470 square metres. Automation is an obvious solution to improve efficiency and quality, and due to the large size of the new blades, manual labour is almost impossible to use for some tasks.

Aerospace has implemented some solutions, but these would be too slow for blade making. The biggest manual job is hand laydown of glass fibre and the difficulties of automating this are that the fabric is limp and difficult to handle, the automated process is currently too slow, the complexity of the task and the huge size of the task, not to mention the high investment cost.