Adding gusto - wind energy forum

Materials World magazine
,
1 Dec 2010
wind turbine

Wind energy professionals had the chance to vent their varying views at a recent conference in London, UK. Eoin Redahan reports

Some are moved by the majestic posture of the hunting falcon in flight. Others draw creative breath from Chinese bamboo forests, seaweed extracts and earth-moving machinery. Wind industry professionals discussed their various inspirations at the Wind Energy – Challenges for Materials, Mechanics and Surface Science Conference, at the Institute of Physics, in London, UK, on 28 October.

The sector has had very little time to catch its breath. According to the European Wind Energy Association, annual installed wind energy capacity in Europe grew from 190MW in 1991, to 10,163MW in 2009. In the same period of time, the total installed capacity has grown from 629MW to 74,767MW. In short, wind energy is a young industry that has been encouraged to grow up quickly.

Spinning around

Jim Platts, from the University of Cambridge, UK, explained that the wind energy industry followed the example of boat-building and started moulding rotor blades using glass fibre polymer composites. Today, the majority of blades are made using this material. Platts uses bamboo strips, glass cloth, a vacuum bag and an epoxy resin in his blades. He ascertained that making resin infused glass-epoxy blades is more time-consuming and less efficient than his technology. He claimed his bamboo blades are less wasteful and use a rapid cure resin that speeds up the manufacturing process.

He said, epoxy ‘has a tight molecular structure and water molecules cannot percolate through, and so it is a complete vapour barrier’, and added that these blades maintain their high structural properties when sealed and remain dry for the whole life of the wind turbine.

With seemingly endless tracts of bamboo forests in China, where each plant can grow about half a metre per day, Platts alleged that his blades are ‘completely sustainable.’

Earlier, one delegate had mentioned that some contracts now require blade manufacturers to take back the blades after 20 years. This, Platts suggested, is where his material excels, as, ‘At the end of its life, you can make furniture out of it’.

Paul Weaver, of the University of Bristol, UK, meanwhile, spoke about the potential for morphing flaps in wind turbine blades. His team has developed a bi-stable composite material rotor blade demonstrator that changes shape to suit wind conditions. The aim, Weaver says, is to find a lightweight material solution rather than use conventional heavy mechanisms.

Trials and tribology

Despite these material and design innovations, the wind energy industry is still beset by testing procedure challenges and the relentless onslaught of the elements, noted the delegates. Given the increasing size of the structures, full-scale testing is expensive and time-consuming, but essential to ensure all blade designs withstand a set number of fatigue cycles and to discover the failure load.

Professor Don van Delft, of the Knowledge Centre WMC, in The Netherlands, who conducts full-scale fatigue tests on rotor blades, explained that a turbine has a full life of approximately one billion loading cycles, whereas, because of time constraints, fatigue tests use between one or five million cycles.

Corrosion also continues to be a problem. Gearbox failures provide a case in point, said Martin Evans, University of Southampton, UK, who spoke about bearing failures in wind turbine gearboxes. He said the high failure rate of gearboxes is undermining the green image of wind energy and that, ‘it only takes one of the bearings to fail catastrophically to cause the whole gearbox to fail’. He added that these bearings often die within a couple of years due to the ‘butterfly effect’, which has been thought to occur when hydrogen fuses with the steel.

Professor Robert Wood, also of the University of Southampton, underlined the shortcomings of existing test procedures. ‘Butterfly-type damage has been studied for about 25 years, and we still don’t know how it is initiated’. Soil, ice and sand erosion are problematic in onshore turbines.

Wood, who presented on tribology and corrosion, highlighted that environmental forces attack offshore facilities. He added that, ‘Although we can try to control the environment, salt atmosphere and sometimes salt water can get in’.

Solutions have been mooted, including using the active extract in seaweed to combat corrosion and developing a solid lubricant for use in the gearboxes of offshore wind turbines.

Scaling up and trimming down

The importance of using less materials in turbines was also emphasised, given the ever-increasing size of structures and the growing scarcity of raw materials. Wood noted that, ‘There is an issue with the quality of the steel. This becomes more apparent as structures get bigger’.

Dr Larry Viterna, of NASA’s Glenn Research Center, in the USA, advocated the use of a two-blade rotor system to get the weight down. He says that, ‘It is lightweight, and it turns faster at a higher RPM’. He also mentioned that, new sandwich composite materials/structures may reduce blade and tower weight by 25%’.

Viterna’s assertions were indicative of an industry in constant flux. Wind energy continues to grow, but the overriding message from delegates was that R&D, testing procedures and materials advances must keep pace with the increasing demand for wind energy.