Sustainable growth

Bamboo: the 'green' option for making blades for wind energy

Wind energy production is now a large global industry, one which last year installed more than 20,000MW of electricity-generating capacity using around 10,000 wind turbines. All the blades which power the world's growing number of wind turbines are made of composite materials. The best blades use wood - nature's own fibre-reinforced composite material - which makes up around 70 per cent of the weight.

In fact, wood has superb fatigue behaviour and an unbeatable strength-to-cost ratio, as well as being a low-energy-input material.

Wood uses less energy input to provide the required stiffness compared with materials such as fibreglass, steel and aluminium. In energy-technology terms, this ranks it as a technical material of the highest order.

Since the early 1980s, the best blades in the world, designed and manufactured in the UK, have used wood as the primary structural material. The early blades of this type have now completed over 20 years of trouble-free service. Thousands of these blades have been produced and all follow the same structural pattern of using a hollow shell structure moulded in two halves, with a joining shear web inserted between them as they are bonded together.

Early blades in the 1980s used African mahogany, moving on to poplar, and then more recently to Finnish birch. But there is one type of wood that is better than all of these in having the most ideal properties for blade construction: bamboo, particularly the 'Mao' variety, also known as 'Moso' bamboo.

For China, which has about 2.8 million hectares of Mao bamboo, much of which grows on mountainous areas unusable for farming, this is an exciting new use of a material that has been employed in many ways for thousands of years. China intends to increase the acreage of bamboo farming and there is a desire to develop bamboo for high-technology applications such as wind-turbine blades.

Mao bamboo is a grass with a vast interconnected root system which throws up new shoots each year that grow to full height in a few months. These are cut at four years old, which is when they have their best properties.

For the high-strength, high-stiffness use in wind turbine blades, it is only the outer 'skin' of the bamboo which is of interest - and this is currently mostly milled off and burnt in the process of producing rectangular-shaped strips of bamboo for use in flooring and other construction and furnishing applications.

So this is currently a hardly-used resource. But if all the 'skin' was collected from the annual Mao bamboo crop, there would be enough to build 280,000MW capacity of wind turbine blades every year.

A few years ago, Chinese material resources were rarely considered in relation to technical matters such as this. But bamboo outranks birch as the natural material with the highest properties available for this kind of application.

The Cambridge Engineering Selector, a methodology that identifies stiffness-to-weight and strength-to-weight properties for a number woods, shows that bamboo is superior even to Finnish birch.

To translate this into energy payback terms, we could cut the bamboo and burn it, and we would get the intrinsic energy value from it as heat. If, instead, we use it as the structural material in a wind turbine blade, during the blade's 20-year operational life it will generate 400 times the intrinsic energy content of the bamboo as high quality electrical energy - and you can still burn the bamboo and get its intrinsic energy as heat after it has done that, if you want to!

But bamboo is not only a low-energy-input material which can give a high technology return. In this application for turbine blades it brings a completely new perspective to issues such as carbon trading.

Work is being carried out in China and at the University of Cambridge, analysing the tensile strength and stiffness properties of bamboo and identifying how best to select and prepare bamboo to provide maximum benefits in blade design. What's particularly attractive about this technology is that bamboo brings a new perspective to carbon trading because bamboo sequesters carbon from the atmosphere, building it into high-strength fibres. Using a naturally-growing carbon-based material to create an energy-generating technology in this way is perhaps unique.

In constructing wind turbine blades from bamboo, the blades have captured enough wind energy to pay back the energy in the bamboo used in their construction after just 20 days - an impressive statistic for a wind turbine with an average life of 20 years.

Windy future

In China, the manufacturing and supply routes for generating this turbine-blade technology are being forged and the first wind-turbine blades are expected to be made by the end of this year.

As China readies itself to develop its own wind-turbine industry, Cambridge University and its Chinese partners have shown that bamboo can be used as an alternative to energy-intensive fibres and polymers for this high-technology application.

The use of bamboo in this way makes wind energy an attractive proposition for future sustainable electricity generation.

Dr Jim Platts is a lecturer in design and manufacturing at the Institute for Manufacturing, University of Cambridge, United Kingdom

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