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Rising to the challenge

Concrete engineering Towers twice the height of Nelson's Column will be needed for the next generation of wind turbines.Alan Bromage believes concrete is the right solution.

Wind energy is one of the most commercially developed and rapidly growing renewable energy technologies. In the UK, the installation of onshore and offshore wind farms is an industry that grew by 20% last year alone.

Blessed with the best and most geographically diverse wind resources in Europe, the UK government aims to generate 10% of electricity from renewable sources by 2010, this target is to increase to 20% by 2020. This means that over the next five years there will be a significant programme of offshore and onshore wind farm development.

Demands for higher power output, coupled with a decrease in the number of prime sites with high wind availability and good access, means that there is a need for turbines to have longer blades in order to optimise performance in less windy sites. This and the fact that wind speeds tend to increase with height will require increasingly taller wind towers.

The current trend of using predominantly 1.0MW to 2.5MW turbines, which require 40m long blades and 60-70m tall towers looks set to change.

Several wind farm developers are introducing a new generation of 4.5MW to 5MW turbines with blade lengths of up to 60m and towers up to and beyond 100m.

This change to increased turbine sizes, rotor diameters and tower heights makes concrete a far more competitive option. As a material, concrete's durability is well suited to harsh marine or remote exposed inland environments. This means less maintenance and better whole life performance.

For both foundation and pylon applications, concrete's versatility enables design and construction solutions that can meet a wide range of site conditions and accessibility.

Construction efficiency can be further improved by optimising either insitu or precast concrete construction methods.

High quality structural sections can be precast in factories under controlled conditions and transported in units. Simple jointing details are easily achievable and these result in cost effective formwork solutions and fast construction.

Using state-of-the-art mobile volumetric onsite mixing plant overcomes remote access logistical problems to allow insitu construction.

Further cost savings and efficiency improvements can be realised by setting up dedicated production facilities at coastal sites.

Larger, heavier turbines will inevitably experience longer periods of natural oscillation.

As concrete can accommodate detailed section changes relatively easily, designs can be adapted to larger diameters to produce stiffer towers that combat this potential problem.

The overall stiffness of tower structures depends on pylon, tower stem and foundation performance.

Concrete has higher material damping properties than other materials. Prestressed concrete has high fatigue resistance that provides more tolerance and less risk from dynamic failure. Use of concrete for pylons, foundations or both can generate considerable advantages, offering design solutions that will be more tolerant of occasional resonance with a reduced risk of dynamic problems.

Concrete's prolonged service life and its ability to cope with increase loading allows possible retrofitting of turbines after their initial 20 year design-life.

Three to four next generation life cycles could easily be accommodated in this way, thereby avoiding the financial and environmental costs of reconstruction.

Current thinking in Denmark is that steel tower design life is limited to 50 years. Concrete towers could far exceed this.

This is an important consideration, for the fundamental objectives of wind turbines is to contribute to a more sustainable future.

Reinforced concrete's environmental impact can be easily optimised through conservation of materials.

Waste and supplementary materials can be substituted for large fractions of the newly produced cement in the mix.

Recycled concrete can be used instead of natural aggregate, all with no detrimental impact on structural performance.

In terms of life-cycle design, precast concrete solutions lend themselves to simple deconstruction steps and techniques.

Equally for offshore concrete gravity foundations, the use of established flotation techniques avoid potentially complex decommissioning processes and environmental issues associated with driven monopiles in the sea bed.

To date, the benefits of concrete have not been fully exploited by the wind energy industry. This is set to change if the demand for taller and stronger towers for the next generation of wind towers is to be met. Concrete's design and construction flexibility, excellent dynamic and long life performance, low environmental impact and low maintenance means that it is well suited to encourage the greater use of wind energy as a renewable source.

Alan Bromage is head of civil engineering at The Concrete Centre.

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