Harnessing the wind
The towers' shape and orientation combined with the world's first integration of wind turbines on a building, offers a glimpse of the future of energy-efficient buildings.
In October 2003, the Dubai office of leading architectural firm, Atkins and its principal architect and head of design, Shaun Killa, was approached by a client in Bahrain and asked to design a building that would be unmatched anywhere else in the world. Upon arrival in Bahrain, they found the site had an existing shopping mall, a hotel and a small office building. The client's brief was to increase the commercial office space considerably, double the size of the shopping centre and increase the parking facilities by incorporating a multi-storey car park into the design. It was asked to preserve the existing buildings and incorporate them into the new development to create a seamless division between the 'old' and the 'new'. On the day it visited the site, for what would become the Bahrain World Trade Center (BWTC), it was an unexpectedly windy day and Shaun Killa couldn't help but think about sailing.
After the requisite photos were taken, details jotted and questions answered, Killa recognised the wind velocity, and more importantly, its direction, with respect to the shoreline site in Manama. He began thinking that his years of research into harnessing wind energy into building designs, could actually become a reality. "I had been researching the integration of wind turbines into buildings for more than two years before going to Bahrain," says Killa. "In 2002, I had considered a project in Dubai where wind turbines were incorporated and although the project didn't materialise, it started to intrigue me, leading me to continue my research into understanding wind turbine constraints."
Although the seed had sprouted in Killa's mind, there were more pressing issues to be dealt with before he would allow himself to consider integrating horizontal-axis wind turbines on to a building. In fact, it wasn't until Killa and colleagues began masterplanning the site and began extending the axis of the existing shopping centre toward the shore, did they realise that the site actually lent itself to a twin-tower development. "As soon as we recognised the axial relationship, I started thinking more about the wind aspects in Bahrain," says Killa. "I reviewed Bahrain's wind rose and found that approximately 70% of the prevailing wind was coming directly off the sea onto the site, which was perpendicular to where the turbines would be, and would create a perfect wind regime for turbines between the buildings."
Having spent a significant amount of time researching this type of integration, Killa knew he had to work with the industry standard, certified horizontal-axis turbines which are generally supported on masts in the ocean or on a greenfield site. He was also well aware of the very lengthy research and development process-often five years or more-of testing a non-standard wind turbine to get it to where it can attain the requisite certification.
Although some designers have attempted to use large scale, non-standard vertical-axis turbines, their projects had not materialised. "The research and development work to certify non-standard turbines would take years and cost a fortune," says Killa. "So it's better to consider standard certified wind turbines and find willing manufacturers."
After contacting as many as 30 manufacturers worldwide, Killa found that many of them were uninterested in what they considered to be a small scale project. They were used to turbine installations that included 90m diametre turbines, generating three to five megawatts and installations of between 10 and 100 turbines. According to Killa, many of the manufacturers didn't see the advantage of being involved in a project that included three 29m diametre turbines supported on the bridges of a twin-tower development-regardless of its potentially groundbreaking nature. After months of communication, they found a design-and-build manufacturer in Norway to install the turbines.
The final challenge was one of economics. Where other projects had failed to properly consider the influence of wind direction and velocity of the location, and a design that created a perfect shape for wind turbines, the costs of integrating turbines into the development rendered it economically unfeasible. To address this problem, Killa and his team devised an unusual concept. "The way to keep costs down was to create separate towers, which by themselves would be efficient, however shaped and orientated to harness the wind and then literally bridge the two structures. This method made installing the turbines relatively inexpensive" says Killa.
"We had to conduct a feasibility study to check the economic and technical viability of this world-first integration. Very early in the study, it became clear that it was perfectly possible to have turbines between the towers because of their shape and they would comprise around 3.5% of the cost of the development," adds Killa.
In early 2004, when Killa and his team went back to the client with their masterplan that included several energy-saving measures, sustainability was very seldom considered in the Middle East. At their meeting, Killa presented computer-generated renderings and animations of the BWTC and despite the project brief not requesting anything sustainable, the client completely embraced the concept and the role the building could play in the Middle East. "They liked the idea of a world-first sustainability initiative coming from the Middle East [mostly known for its oil wealth], and they were also mindful of how this building could project a global environmental agenda," says Killa.
Because a sustainable project of this magnitude had never been done before, Killa was pleased to have the complete support of the client. But even he didn't expect the phone call that came just a week after returning from his presentation in Bahrain. "A week after the presentation, we received a call from the project managers saying that they had bulldozers on site and that they were ready to excavate the basement," says Killa. "Of course, at this point, it became a fast-track project."
The design process
With the BWTC, in many ways Atkins was tasked with creating a building that surpassed the accepted architectural formula. It needed to design an iconic building that would define the skyline of Bahrain; it needed to use groundbreaking technology to serve as a flagship for sustainability; and it needed to fast-track the construction process. Because they were attempting to build something that had never been built, Killa and his team needed to consider three design issues. "The first decision, deriving from the axial extension of the existing mall, was to separate the 'office building' into two towers. The second decision was their orientation to create a 'V' shape to funnel the offshore breeze onto the turbines. The final decision was how to shape the towers so as to best carve the wind onto the turbines," says Killa.
The shapes of the towers are elliptical to closely mimic the shape of an airplane wing or the sail of a yacht. These shapes create a negative pressure behind them, which results in an acceleration of air on their leeward side to balance the air pressure. This combination between both towers creates a 'Venturi' effect which further accelerates the wind velocity onto the turbines. "One of the successes of the elliptical design, is how the towers sculpt the wind, to create an 'S' flow between the towers when the wind shifts off axis. This ensures that the centre of the wind stream remains nearly perpendicular to the turbines, within a 45 wind azimuth on either side of the central axis," says Killa. This increases the efficiency of the turbines and thus, the energy that is generated, and reduces blade fatigue caused by yawing.
In considering the design of the towers themselves and how to achieve the highest turbine efficiency, Killa and his team conducted scores of CFD (computerised wind studies) tests and SARM (Safety, Analysis & Risk Management) analyses. The tests led them through approximately 250 different scenarios, all of which demonstrated how they could comfortably use a 60Ãƒâ€šÃ‚Â° wind envelope to generate approximately 15% of the tower's energy requirements. "These tests also revealed that the buildings shape and orientation still redirected the wind perpendicularly to the turbines over a 120Ãƒâ€šÃ‚Â° wind envelope. This could significantly increase the energy produced by the turbines," says Killa. The turbine manufacturers will establish their exact field of operation during their commissioning in October.
The vertical sail-shaped profile of the towers is also a major component in the success of integrating the turbines. "As wind increases in velocity with height, and with the reduced funnelling effect of the tapering towers, a near equal wind speed is created on each turbine. This causes the turbines to rotate at the same speed and produce the same energy."
Few would argue that the BWTC could very quickly achieve an iconic status in the Middle East and perhaps, throughout the world. Its sleek design and crystal blue 'sails' can be seen from anywhere in downtown Bahrain and will likely come to define the Bahrain cityscape.
More importantly, however, is the standard that the BWTC will set throughout the world in terms of sustainable buildings and the potential it carries in influencing future projects. "Atkins is very optimistic about the future because we have clearly demonstrated that we can create a commercial development, which is underpinned by an environmental agenda," says Killa. This building has the potential to reshape the field of architecture in our evermore eco-friendly world and significantly reduce the demand for fossil fuels on projects of this magnitude.