Making wind work
The soon-to-be-built World Trade Centre in Bahrain will showcase the first large-scale integration of wind turbines in a building. John Haven describes the design evolution.
The Bahrain World Trade Center forms the focal point of a plan to rejuvenate an existing hotel and shopping centre on a site overlooking the Arabian Gulf in the central business district of Manama. The design of the towers was inspired by the traditional Arabian Wind Towers in that the shape of the buildings harness the unobstructed prevailing onshore breeze from the Gulf, providing a renewable source of energy for the project.
The two 50-storey sail-shaped office towers taper to a height of 240m and support three 29m-diameter horizontal-axis wind turbines. The towers are integrated on top of a three-storey sculpted podium and basement, which house a shopping centre, restaurants, business centres and car parking.
The elliptical forms and sail-like profiles act as aerofoils, funneling the onshore breeze between them as well as creating a negative pressure behind. This accelerates the wind velocity between the two towers. Vertically, the sculpting of the towers is also a function of airflow dynamics. As they taper upwards, their aerofoil sections reduce. This effect when combined with the increasing velocity of the onshore breeze at increasing heights, creates a near equal regime of wind velocity on each of the three turbines.
Understanding and using this phenomenon has been one of the key factors that has allowed the practical integration of wind turbine generators in a commercial building.
Wind tunnel testing has confirmed how the shapes and spatial relationship of the towers sculpt the airflow. This creates an S-flow, where the centre of the wind stream remains nearly perpendicular to the turbine within a 45° wind azimuth either side of the central axis. This increases the turbines’ potential to generate power. And it reduces fatigue on the blades to acceptable limits during wind skew across them.
The impetus for this innovative design came entirely from Atkins chief architect Shaun Killa. The wind in the Arabian Gulf, with its dominant sea breeze characteristic, allows designers to move away from the more conventional omni-directional solutions and consider uni-directional wind turbine options. These lend themselves to the large-scale integration in buildings.
Research by Atkins has shown that the large-scale integration of turbines into buildings mostly fails 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.
The premium on this project for including the wind turbines was less than 3% of project value.
This building is not intended to be a low-carbon-emission solution by European and other world-wide standards. However, aside from the wind turbines, it does include a number of other design features that are of interest and reduce carbon emissions when compared with other buildings in the Middle East. These include:
- Buffer spaces between the external environment and air-conditioned spaces
- Deep gravel roofs in some locations that provide kinetic insulation
- Significant proportion of projectile shading to external glass facades
- Balconies to the sloping elevations with overhangs to provide shading
- Low-leakage, openable windows to allow mixed mode operation in winter months
- Enhanced thermal insulation for opaque fabric elements
- Dense concrete core and floor slabs presented to the internal environment in a manner that will level loads and reduce peak demand with associated reductions in air and chilled water transport systems
- Low pressure loss distribution for primary air and water transport systems that reduces fan and pump power requirements
- Dual drainage systems that segregate foul and waste water and allow grey water recycling to be added at a later date
Three wind turbines have been integrated into the building to generate electricity. Horizontal axis wind turbines are normally pole-mounted and turn to face the direction of the wind thus maximising energy yield.
The practical application of such turbines to buildings in variable direction wind climates is therefore difficult. The majority of architectural studies deploying building-integrated, horizontal axis turbines deploy the principle of a fixed turbine as in the Bahrain World Trade Center.
Development for vertical-axis wind turbines is encouraging and of course they benefit from the advantage of being truly omni-directional. However, at the time of design development for this project, large-scale proven vertical-axis turbines were not available for building applications.
The fixed horizontal turbine suffers the drawback of only being able to operate with wind from a limited azimuth range, if problems with blade deflections and stressing through excessive skew flow are to be avoided. From the outset of this project, the shape of the towers has been designed to capture the incoming wind and funnel it between the towers.
The fixed, horizontal axis wind turbines on this project comprise the following key components:
- Nacelle, including enclosure with gearbox, generator, cooling system and associated control systems
- Control, monitoring and safety systems
- Electrical Building Interface
The nacelle is the term used for the cowling containing the gearbox, brake, controls, etc. And in addition, there is the rotor. Nacelles have been designed to sit on top of the bridge, rather than within it, to portray the functionality of the turbine. The turbine is a simple and robust “stall controlled” type. The stall control is a passive way of limiting power from the turbine. The rotor blades are bolted on to the hub at a fixed angle. And the profile has been designed to ensure that, the moment the wind speed becomes too high, it creates turbulence on the leeward side of the rotor blade and prevents lift. This stalls the blade so that the power output stabilises at a maximum output.
The full power of about 225kW will be achieved at 15-20m/s depending on air density. In the event of extremely high wind speeds under operating or standstill modes, the tip of the blade extends by the force of rotation and acts as a self-regulating governor brake through the exertion of drag.
A key part of the design is the determination of loads on the rotor, through the nacelle and on to the bridge and buildings, so that structures can be analysed for strength and fatigue.
The load calculation approach for this project has been made by the bridge design consultant in conjunction with the wind turbine manufacturer using a specially adapted version of the industry-best wind turbine simulation tool, Flex4.
The tool has been adapted to take account of the influences of the buildings and the bridges. A total of a 199 different load cases has been modelled for each turbine and validating calculations or operational processes prepared to theoretically demonstrate that the turbine and bridge would survive without excessive fatigue. During the early stages of operation, this theoretical analysis will be validated and appropriate adjustments made to the operating regime that may increase or decrease energy yield.
The projected energy yield from the turbines, taking into account wind and availability, amounts to 1,100-1,300MWh per year and will amount to about 11-15% of the office towers’ electrical energy consumption. In carbon emission terms, this equates to an average of 55,000kg of carbon (UK electricity basis). These figures are conservative.
Since this is a world first and because wind turbines have not been placed 160m above ground level and between buildings, the yield may be higher.
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