Concrete solution for next generation of windfarms
A new report calls for the greater use of concrete to help deliver the development and construction of the next generation of windfarms.
Windpower is a growing industry and with the Government committed to generating 20% of electricity from renewable sources by 2020, it is set to grow further still with a significant programme of offshore and onshore windfarm development. However, this growth has decreased the number of prime sites with high wind availability and good access at a time when the demand for more energy output from windfarms is increasing. As a result, the next generation of windfarms will require larger wind turbines placed at greater height and located at more inaccessible sites. The report, Concrete Towers for Onshore and Offshore Windfarms, by consultant Gifford & Partners for The Concrete Centre, believes that this increase in turbine sizes, rotor diameters and tower heights makes concrete construction a very competitive option for windfarms.
The publication of the report is well timed as the Crown Estate, which owns the seabed around Britain, is under pressure to progress with the next phase of windfarm development. Described as ‘Round Three’, this next generation of windfarms envisages offshore developments large enough to supply millions of homes with power. Four years ago in ‘Round Two’, the Crown Estate announced the construction of 15 offshore windfarms, such as the Thames Array, which would generate up to 7GW of electricity, enough to power four million homes. The Crown Estate met developers at the British Wind Energy Association offshore conference in July.
There are a number of reasons why the report advises the wind industry to realise the potential of concrete. There is a growing emphasis on using towers above 100m in height, to maximise the potential wind energy. Currently most onshore UK wind towers typically have a rotor diameter of 40m with a tower height of 70m and power outputs of up to 2.75MW. However, larger turbines are predicted in the future, each with an output of up to 4.5MW. Turbine rotor blades will have to be some 60m long and be supported on towers over 100m tall.
Taller wind towers need to have greater structural strength and stiffness in order to carry both the increased turbine weight and bending forces under wind action on the rotors and the tower and also to avoid damaging resonance from excitation by the forcing frequencies, resulting from the rotor and blades passing the tower. Concrete has higher damping properties than other construction materials. Prestressed concrete, in particular, has high fatigue resistance that provides more tolerance and less risk of dynamic failure. In addition to its benefits of strength and stiffness, concrete offers a low maintenance and long-life solution of up to 80 years. This is especially relevant for wind towers located in harsh onshore or offshore environments.
There is also an environmental benefit that is particularly relevant for windfarm projects: the low environmental impact of concrete. Not only is reinforced concrete 100% recyclable, but its embodied CO2 and energy content can be much lower than that of competitor construction materials. For instance, in a typical 70m high onshore wind tower configuration, relative to steel, the embodied CO2 content of a prestressed concrete design option is approximately 64% lower.
Although some large concrete wind towers have been constructed in Germany and elsewhere in Europe, they represent a small fraction of the total number. But the demand for taller towers means that this should change as the wide-ranging benefits of concrete wind towers are increasingly recognised.
However, the report warns that if the concrete industry is to capitalise on the material’s advantages for taller onshore and offshore wind towers then further design and development effort to optimise concrete tower design is required. Key issues for attention are minimising material content and structural weight and reducing construction times. There should be further examination of the use of in-situ slipform construction methods to overcome the problems of transporting large tower rings/sections and remove the need for large cranes for erection. The development of ‘craneless’ methods for the erection of the turbine nacelle and rotor using lifting platforms climbing up the tower itself could offer further significant cost savings. The concrete tower shell is well suited to supporting the local concentrated loads that would be imposed by such devices.
The demands placed on the next generation of windfarms will be more arduous than before. The move towards taller wind towers is shifting the balance of advantage towards concrete design solutions. Concrete tower solutions are adaptable and durable and offer long life performance with minimum maintenance. The concrete industry is ready to work with the electricity sector to develop optimum solutions.
* Further information: Copies of Concrete Towers for Onshore and Offshore Windfarms can be downloaded from: www.concretecentre.com/ publications
ALAN BROMAGE, THE CONCRETE CENTRE
Copyright The Concrete Society Sep 2007
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