Old wind power generators have to be disposed of – whether due to material fatigue or simply because they are being replaced by larger and more efficient systems. A study by the Fraunhofer Institute for Chemical Technology ICT predicts that the 15,000 rotor blades that will have to be discarded in 2024 will be joined by another 72,000 in the subsequent three years. We already have environmentally friendly methods for disposing of the steel and concrete in the wind power generators, but recycling the rotor blades remains problematic.

Firmly bonded and nearly impossible to separate

Rotor blades are not made of steel. “That would be too heavy and inflexible. They are made largely of glass-fibre-reinforced plastic (GFRP) and balsa wood bonded with epoxy or polyester resin,” says Peter Meinlschmidt, project manager at the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut, WKI in Braunschweig. This bond is extremely strong. It has to be: the rotor blades reach top speeds of more than 250 kilometres per hour, subjecting them to an enormous force. For single-origin recycling, however, this is precisely the problem, as it is very difficult to separate the individual components of the composite material.

A rotor blade contains around 15 cubic metres of balsa wood, which is not only one of the world’s lightest woods, but also extremely resistant to pressure. “That’s the key advantage of balsa over most plastic foams,” explains Meinlschmidt. Previously, there was no possibility to recover it when disposing of the old rotor blades. “Although it has hardly any energy content, it is burned as a composite material, usually in cement factories. The cement raw materials have to be heated up to about 1500 degrees Celsius before they coalesce and form cement clinker, so these factories require a great deal of energy. In addition, the melted glass fibres and the ash can later be added to the cement and replace portions of the quartz sand that would otherwise have to be input into the process.” But the number of cement plants in Germany is limited (there are 53 in total), and so is their need for rotor blades as combustion material.

Disassembling rotor blades with a water jet lance

But there is still hope for getting the impending flood of rotor blades under control: Meinlschmidt and his team – Fraunhofer ICT colleagues and industry partners – have developed a new recycling technology. To recover and recycle the balsa from the rotor blades, the detached blades are disassembled on the spot. “The conventional approach is to use a band saw to cut the rotor blades into thirds or quarters, but this is a relatively complex process. That’s why we came up with the idea to try it with a water jet lance instead. And what do you know: it was much faster and better,” says an enthusiastic Meinlschmidt. The lance can be mounted on a special vehicle and controlled from there. “The tremendous thrust would make it extremely difficult to guide the lance by hand.” Then, while still on-site, the 10- to 20-metre-long rotor blade segments are fed into a mobile shredder that breaks them into pieces about the size of the palm of a hand.

Finally, the research team uses an impact mill to separate these pieces into their individual components. To this end, they are set in rotation and hurled against metal at high speed. As Meinlschmidt explains, “The composite material then breaks apart because the wood is viscoplastic, while glass fibres and resin are very hard.”

Insulating with rotor blades

At Fraunhofer WKI, the balsa pieces are processed to make, for instance, ultra-light-weight wood-fibre insulation mats. “Currently around 10 per cent of building insulation materials are made from renewable resources – there’s room for improvement here.” With a density of fewer than 20 kilograms per cubic meter, these mats are so far unique on the market and provide similarly good insulation to common polystyrene-based materials.

The recycled balsa can also be used to produce a novel, elastic wood foam. For this, it is ground to a very fine powder and combined with a foaming agent. The foam’s stability is created by the wood’s own cohesive forces, which render synthetic adhesives superfluous. The foam is suitable for use as an environmentally friendly insulating material, but also as a packaging material that can simply be disposed of in the paper recycling container.

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Using layers made up from compounds such as carbon bound together using a tough resin are being increasingly used in

Paul Hooper from Atkins said:

We believe that there is potential to significantly reduce the costs of overhead line systems by utilising advanced composites in the support structures as an alternative to traditional metallic structures. Additionally these materials have the potential to reduce the amount of equipment required and can be readily shaped to blend into the environment and thereby reduce the visual impact.

For this project the group will support the copper ‘contact wire’ from a high tensioned carbon-fibre reinforced plastic catenary cable. The cable itself would also be supported by glass-fibre reinforced plastic poles and support cantilevers.

Glass-fibre reinforced plastics are electrically insulating and the carbon fibre reinforced plastic does not expand or contract as the temperature changes. Due to these properties the proposed new design will mean the spacing between pylons can be increased by approximately 40% which would therefore require fewer poles and less cable tensioning equipment. It would also eliminate the need for electrical isolation and reduce the amount of electrical bonding required.

The glass-fibre is also more resistant to corrosion, meaning that it will last longer and be less subject to environmental damage and thereby cheaper to maintain. As a result it is believed that inspection and maintenance will only be required every 15 years, rather than every five. The anticipated life of the pylons will also increase with replacements required approximately every 40 years rather than every 25. Although elements of the proposed design are more expensive than existing options, the estimated cost savings for the project are substantial, ranging from £50k-£100k per kilometre for a typical track where pylons are used. A similar cumulative maintenance saving is also expected after seven years of operation.

Paul Griffiths at UK Tram said:

This exciting project forms part of our Low Impact Light Rail initiative which was launched to encourage new ideas within the industry that help to reduce costs in track and energy usage on the UK’s tram networks. Through projects like this we aim to improve the cost effectiveness of light rail, thereby assisting in bringing the service to more of our towns and cities.

Stage one of the Composite Overhead Line Structures project is expected to be completed by the end of January 2015 with stage two focussed on a demonstration of the pilot structure later this year.

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The new spring, which Audi developed in collaboration with an unnamed Italian supplier is thicker than the standard steel spring and has a slightly larger overall diameter with a lower number of coils.

What the spring gains in size it dramatically looses in weight. Where a steel spring for an upper mid-size Audi weights nearly 2.7 kilograms, the new composite spring with the same properties weights around 1.6 kilograms. Together the four GFRP springs thus reduce the weight by roughly 4.4 kilograms (9.7 lb), half of which pertains to the unsprung mass.

Dr. Ulrich Hackenberg, Member of the Board of Management for Technical Development at Audi said;

The GFRP springs save weight at a crucial location in the chassis system. We are therefore making driving more precise and enhancing vibrational comfort.

The core of the springs consists of long glass fibres twisted together and impregnated with an epoxy resin system. A machine wraps additional fibres around this core — which is only a few millimetres in diameter — at alternating angles of plus and minus 45 degrees to the longitudinal axis. These tension and compression plies mutually support one another to optimally absorb the stresses acting on the component. In the last production step, the blank is cured in an oven at temperatures of over 100 degrees Celsius.

The GFRP springs can be precisely tuned to their respective task, and the material exhibits outstanding properties. It does not corrode, even after stone chipping, and is impervious to chemicals such as wheel cleaners. Last but not least, production requires far less energy than the production of steel springs.

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