UAE - A wind power world first in Bahrain

For centuries, people have harnessed the power of the wind. Windmills are a regular feature of the flat and low-lying Dutch landscape, and turbines in the North Sea off Britain’s east coast are now being used to generate offshore power. With energy consumption increasing at a rapid pace worldwide, the need to provide renewable energy solutions is more important than ever. So when British architect Atkins decided to incorporate large-scale wind turbines into the twin-tower Bahrain World Trade Center, currently under construction in Manama, all eyes were firmly trained on the project.

“I had always wanted to incorporate energy-generating features on our designs, says Shaun Killa, head of architecture, Atkins & Partners Overseas. “A number of years before we started the WTC, I started researching the possibility of incorporating large-scale wind turbines into the tower design. I was acutely aware that it had never been done before so the first challenge was to understand the constraints on how they work. “This then had to be combined with a building form that created the most favourable wind patterns, and at the same time was efficient and economically driven.” Killa says that his attention was drawn to the possibility of incorporating the feature into this particular project around three years ago, when the WTC client asked Atkins to master plan the plot which at that time was home to the existing Sheraton hotel, a small office building and a shopping centre.

“The client’s requirements were for Atkins to extend the existing shopping centre, add commercial office towers and combine the whole development, including the Sheraton Hotel and existing facilities into one cohesive development.” The team began looking at the existing access of the shopping centre, and found that extending it created a central mall axis, which was perpendicular to the Arabian Gulf. “When considering the location of the commercial towers, the natural tendency is to have two towers on either side of this axis.” Another factor that helped clinch the decision was the site’s exposed location next to the Arabian Gulf. “Because I am a keen sailor, I also noticed on my first few site visits to Bahrain how windy it was, with the constant onshore breeze blowing perpendicular to the sea-facing site. “It struck me that this would be a perfect opportunity to create a twin tower which could also incorporate wind turbines.” The result: Three 29 m-diameter horizontal axis turbines mounted between the WTC’s 240 m-high triangular-shaped twin towers. Killa says that undertaking a project of this scale was compounded by the fact that there was no precedent to go by — it had literally never been done before. “When we embarked upon this design there was no authority that we could speak to, and when we started talking to turbine manufacturers, at some points they simply couldn’t answer our questions.”

As with any unique project, the team faced a myriad of design challenges. Research by Atkins has shown that the large-scale integration of turbines into buildings mostly failed because of the excessive cost (up to 30% of the project value) associated with the adaptation of the building design, and also as a result of high research and development costs for special turbines. “Where designers have gone wrong in the past is that they haven’t fully appreciated the constraints of wind turbines — they have designed buildings with turbines that are either of a size that is not commercially available, or not type-tested which results in years of expensive research and development,” explains Killa. “The success of the Bahrain WTC is that it incorporates standard type-tested wind turbines which have been adapted to this application with fairly minimal modifications, helping to reduce the cost of integrating them with the building.” The result is that the premium on this project for including the wind turbines was less than 5% of the project value. A key aspect of the project focused on understanding the nature of horizontal axis turbines. “In the greenfield site or open sea, they’re allowed to turn to face the direction of the prevailing wind. But if you fix them on bridges supported between two towers then this is a different regime and you have to understand how to shape the building to direct the wind perpendicular to the turbines.”

This meant that the two towers had to be designed in such a way that they would funnel the onshore wind between them, as well as creating a negative pressure behind, in order to accelerate the wind velocity between them. The funneling of the towers has the effect of amplifying the wind speed at the turbine location of up to 30%. “Even when the wind blows from 450 on either side of the turbines’ central axis, the shape of the towers actually carves the wind to strike the blades at right angles. The negative pressure on the leeward side of the building creates an ‘S’ form which further corrects the wind direction,” says Killa. Extensive wind tunnel testing was carried out to ascertain the precise effects of the building on the turbines. This helped Atkins to tweak the building design and prevent a yawing effect on the blades, so as to reduce their stress and fatigue. According to Atkins’ research, the projected energy yield from the turbines is predicted to be between 1,100 and 1,300 mWh per year, and will amount to approximately 11 to 15% of the building’s electrical energy consumption.

In carbon emission terms this equates to an average of 55,000 kgC (UK electricity basis). But since this project is a world first, and because wind turbines have never been placed 160 m above ground level and between buildings, the yield may be even higher. The three 31.7 m-span bridges (upon which the turbines will be fixed) are complex structures incorporating specialised bearings which will allow the towers to move 0.5 m relative to each other. In addition, the bridges have been designed to withstand and absorb both wind- and turbine-induced vibrations. “The concrete structure is a few months from being topped out and then the steel works for the top will commence,” says Killa. “The bridges are due to be erected at the end of April 2006, and the turbines are likely to be lifted into position a month-and-a-half later. We expect the overall project to be finished in autumn this year.” When completed, the Bahrain WTC will help to reinforce sustainable design on the conscience of architects, designers and potential clients worldwide. “Many people were nervous about doing something like this because it was so new — it’s always easier when someone else has done it before,” says Killa. “But this can be used as a precedent. People will be able to visit the project and have the confidence that the technology is available to make this work.”