Wind turbines turn full circle – from wood to carbon fibre and back again
EngineerLive.com
Paul Stevens
http://www.engineerlive.com/features/18634/wind-turbines-turn-full-circle-from-wood-to-carbon-fibre-and-back-again.thtml

Wind energy is a renewable and environmentally-friendly alternative to fossil fuels but wind turbines must operate efficiently if their benefits are to be maximised. One crucial aspect of their design that has a direct bearing on both their sustainability and efficiency is the materials of construction.
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Three hundred years ago a millwright had little choice of materials for constructing a windmill, except that different types of wood were available from which to make the various parts. Wood was in plentiful supply, it could be readily worked to fabricate anything from gear teeth to cladding, and the parts lasted well and could be repaired or replaced if they became damaged. Indeed, numerous examples of seventeenth century windmills are still in existence, though the earliest horizontal-axis windmills in North Western Europe were built in the late twelfth century.

With industrialisation and, later, the construction of electricity plants powered by fossil fuels, wind had become a little used source of energy by the mid-twentieth century. However, by the end of the twentieth century and the start of the twenty-first century, wind was being recognised again as a useful source of renewable energy. Once installed and operating, wind turbines do not generate greenhouse gases, carbon dioxide or other pollutants, and the energy supply is free and cannot be depleted (though wind speed is certainly not constant in Northern Europe).

Instead of millwrights, today we have multinational companies that can take on the complete design, construction, installation and maintenance of wind turbines. Danish company Vestas, for example, started manufacturing wind turbines in 1979 and has concentrated exclusively on wind energy since 1987. Vestas is now a global high-technology group with more than 13,000 employees, supplying turbines capable of generating nominal power outputs of 850kW to 3.0MW, with corresponding rotor diameters from 52m to 90m.

Blade technology has played a crucial role in the development of modern wind turbines. Glass-fibre reinforced composites are the material of choice for many turbine manufacturers, but carbon-fibre composites offer a higher stiffness-to-weight ratio. This enables the blades to be lighter and stiffer (depending on the detailed design), which helps to improve the turbine's performance and reduce the costs associated with transport to site and installation. Given that each blade is only manufactured once yet is required to operate for 20 years, a small increase in manufacturing cost for an improvement in operating performance is an attractive proposition. So this is the approach adopted by Vestas for its latest V90-3.0 MW turbine.

Vestas has also gone to considerable lengths to optimise the design of the support tower in its V90-3.0 MW turbine. Not only has a higher-strength steel been used to reduce the volume required (and to reduce weight as well), but the company says that it has pioneered the use of magnets to fasten internal components to the tower walls, thereby helping to improve fatigue strength compared with traditional welding or fastening methods.

Wood and bamboo

While the ancient problem of how to extract useful energy from wind has resulted in the application of some high-technology solutions, it should not be forgotten that natural materials still have a lot to offer for turbine blades. Wood - which is a natural, renewable fibre-reinforced composite - has superb fatigue characteristics and an unbeatable strength-to-cost ratio. Twenty years ago khaya (African mahogany) was used, but this was superseded first by poplar and, more recently, by Finnish birch.

In the UK, wind turbine blades have been manufactured from wood since the early 1980s. Thousands of these have been produced, and the early blades have completed 20 years of trouble-free service. These blades are constructed with a hollow shell moulded in two halves with a shear web bridging between the two halves. Wood represents approximately 70 per cent of the blade mass, with the remaining 30 per cent made up of glass cloth, resin and metal inserts for connecting the blade root to the rotor hub.

However, there is another natural alternative that is currently being developed, especially with a view to wind farms planned for China. Bamboo is often referred to as a wood but, in fact, it is a grass. For turbine blades, good quality bamboo actually offers better properties than Finnish birch and, given the vast and sustainable supply of bamboo available in China, it is an excellent material for wind turbine blade construction. Dr Jim Platts of Cambridge University, who established a company called Composite Technology (now part of Vestas) over 20 years ago, has been helping China to develop its wind turbine industry.

Horizontal versus vertical axis

This 'propeller' type of wind turbine, referred to as a horizontal axis wind turbine (HAWT), has a number of benefits. These include the ability to vary the blades’ angle of attack to maximise the amount of energy extracted from the wind, the turbine’s self-starting characteristic, and the way the turbine can be mounted on a tall tower to access stronger winds higher above ground level. Disadvantages include the difficulty of transporting and installing the towers and turbines, and unpopularity due to their obstruction of views and high noise levels.

Vertical axis wind turbine (VAWT) designs are also available. These are generally claimed to be easier to transport, install and maintain, and they are quieter and do not need to be turned into the wind. A higher aerofoil pitch angle also provides improved aerodynamics as well as decreased drag at low and high speeds. Overall efficiency, however, is approximately 50 per cent lower than for HAWTs, partly because of the additional drag caused by the blades rotating into the wind, and VAWTs are not as good at self-starting as HAWTs.

Nevertheless, one Darrieus-type VAWT that has entered production is the Quietrevolution QR5, designed by XCO2 and marketed by Quietrevolution of London. The QR5 is designed for use in urban environments, being quiet, compact (5m tall by 3.1m in diameter) and capable of extracting useful amounts of energy from the turbulent wind conditions encountered in the vicinity of buildings. Carbon fibre and epoxy resin are used for the three helical blades, spars and torque tube, with a 6kW direct-drive inline generator incorporated at the base of the turbine. Quietrevolution says the QR5 can either be mounted as a standalone generator or in groups within a Y-shaped frame.

The Darrieus wind turbine is probably the most well known design of VAWT, but another is the Savonius wind turbine. Although these are less efficient than other types of wind turbine, they are simpler and cheaper to manufacture, and require very little maintenance – any one of which could be a deciding factor in a limited number of applications.

Llumarlite, a company best known for lighting products, is a distributor for the Ropatec VAWTs that feature aerofoil-shaped vanes (as with the Darrieus concept) and a central cylindrical element that diverts the air (somewhat similar to the way Savonius blades divert air onto each other). The result is claimed to be a low-maintenance turbine that stalls at high wind speeds (to prevent damage), yet at lower speeds delivers an output comparable to that of a HAWT. Llumarlite says the Windrotor can be used either as a source of dc power for applications such as battery charging, or it can be connected to an inverter for delivering ac power or connected to a grid. Being a low-noise turbine, it can be located within urban environments. Interestingly, Ropatec manufactures its Windrotor turbines largely from aluminium.

Back to bamboo

Researchers in India have been investigating ways to improve the design of Savonius wind turbines for small-scale electricity generation. UK Saha et al of the Department of Mechanical Engineering, Indian Institute of Technology, Guwahati, and the Department of Mechanical Engineering, National Institute of Technology, Rourkela, have published a paper Twisted bamboo bladed rotor for Savonius wind turbines in the Journal of the Solar Energy Society of India (SESI), Vol. 4 (2005). This follows on from earlier work that led to the development of a twisted-blade Savonius rotor fabricated from sheet metal.

Experiments with the bamboo-bladed rotor showed a slightly lower rotational speed than the twisted metal blades, but the researchers believe the low cost and the ease of fabrication could make this type of design useful for small-scale power generation in rural areas. The design proposed by the researchers uses sheets made out of woven bamboo, cut to size and stitched at the edges for strength and durability. A paper covering gives an improved surface finish and aerodynamic performance.

Whether the application is a small-scale, low-cost Savonius turbine or a full-size aerofoil turbine blade, there is an additional environmental benefit in using natural materials. Vestas says it has performed a life-cycle assessment (LCA) on its V90-3.0 MW offshore wind turbine and established that it has to generate electricity for just 6.8 months before it has produced as much energy as was consumed throughout its design lifetime. However, carbon-fibre composites require considerable amounts of energy to manufacture, which will have slightly increased this pay-back time. By using natural materials such as wood or bamboo for a turbine's construction, the environmental benefits can be even greater.