Tall Building case studies - Tall Buildings: Structural Systems and Aerodynamic Form - page 92

consumption of the building. The premium for including the wind turbines cost less
than 3 per cent of the project value but on the other hand 11-15 per cent of the
electrical energy consumption of the building is expected to be generated. Besides
the wind turbine renewables, the building includes some other sustainable features,
such as buffer spaces between the external environment and air conditioned spaces,
shading devices for sun control, energy efficient lighting system with zonal control
and dual drainage systems for water recycling etc.
Located in the downtown central business of Manama, BWTC has an impressive
site overlooking the Arabian Gulf. At the beginning of the site analysis, Atkins'
principal architect and keen sailor Shaun Killa realised the potential of the prevailing
onshore wind having direction almost perpendicular to the site ( Figure 4.2 ). After
the analysis of the wind, the architect has decided to convert this condition into an
advantage by emphasising a well-designed sustainable solution.
According to the design inspiration idea, for harnessing the prevailing north-
westerly onshore breeze from the Gulf and using its energy, the architect formed the
twin towers and integrated the horizontal-axis wind turbines with the help of the
bridges between them ( Figure 4.3 ).
The fact that the wind turbines can generate energy depends on the consistent
wind energy with the capability of the horizontal wind turbine blades adjusting
themselves to the prevailing wind direction. However, in the BWTC, the blades of
the turbines could not adjust themselves to changing wind directions due to their
standstill positions on the bridges. To cope with the problem, the architect focused
on the shape and the position of the towers directly and decided to design the
building in such a way to obtain optimum onshore wind flow among the towers. In
this context, the design process is based on an “aerodynamic architectural design
approach” ( Section 6.1 ) by using wind tunnel testing together with Computational
Fluid Dynamics (CFD) Modelling.
The elliptical plan forms funnelling the onshore breeze augment the wind flow
and thus accelerate the wind velocity (up to 30 per cent amplifying the wind speed)
and create a negative pressure behind the towers. The tapering shape of the towers
towards the top reduces the effect of the augmentation of the wind flow. This effect,
together with the velocity profile of the wind being lowest at the ground level, allows
all the three wind turbines to rotate almost at the same speed and thus generating the
same energy. Wind tunnel tests show that when the towers are subjected to onshore
wind not perpendicular but with oblique angles of 45º in direction, the centre of the
wind stream remains nearly perpendicular to the turbines.
Between the BWTC towers, to support wind turbines, three steel V-shape bridges
(173º to avoid blade strike during extreme conditions) spanning 31.7 m in length
are connected to the towers at different levels. They allow the towers to move 0.5 m
towards each other. Three 29 m diameter horizontal axis wind turbines are located at
bridge levels 60 m, 96 m and 132 m respectively above the ground ( Figure 4.4 ).
In plan, on one side of the towers, there is a service area placed close to the wind
turbines to prevent undesirable noise and vibration in offices and the shopping mall
area. On the other side of the towers, there are two different types of columns: one is
circular in cross-section and angular in both directions and the other is rectangular in
cross-section; both are inclined almost 15º ( Figure 4.5 - 4.6 ).