The windy city
The first large-scale building integration of wind turbines is being completed at the Bahrain World Trade Centre. How has this impacted on the MEP servicing?
The use of wind power has taken on a new form in Bahrain. Wind was traditionally used in the Middle East to cool buildings through natural ventilation wind towers, but in the newly constructed Bahrain World Trade Centre (BWTC) it will instead contribute to the electrical supply and operation. And in a world-first, the turbines used to capture the wind's energy have been fully integrated into the building structure.
Three 29-metre diameter wind turbines form the central feature of the BWTC. These are connected by nacelles to horizontal bridges, effectively tying together the two office towers of the BWTC. And at 50-storeys and 240m tall, the sail-shaped towers complete the dramatic and distinctive form of the building.
The BWTC forms the focal point of a masterplan to regenerate an existing hotel and shopping mall in the Manama business district of Bahrain. The use of wind turbines in the project follows over three years of intensive research and development by international multidisciplinary design practice Atkins and Danish partners Ramboll and Norwin.
The elliptical plan and sail-like profiles of the towers have also been carefully designed to maximise the potential of the wind turbines. The towers are orientated to face the dominant prevailing onshore wind and their shape acts as aerofoils, effectively funnelling and accelerating the airflow between them and into the path of the turbine blades. The towers taper towards the top, their changing shape balancing with the increasing wind speed at height to ensure virtually equal wind speeds at all of the turbines.
Shaun Killa, chief architect with Atkins, proposed the design solution. "From the outset the project had as the basis of design the utilisation of conventional technologies and the development of a built form that would be sympathetic to receiving wind turbines," explains Killa. By adapting standard available wind turbines the premium on the project price of including the turbines was under 3% of the total project value. This compares favourably to Atkins' researched figures, which showed that this value could be 30% when using special turbines on large-scale building integration projects.
Wind tunnel testing was carried out to confirm the movement of the air around the towers and the availability of power. The structural shape will create an S-shaped airflow, with the centre of the airflow remaining almost perpendicular to the turbine. This increases the potential for creating power with the turbines and reduces fatigue on the blades. These tests were followed by computational fluid dynamic (cfd) modelling.
Engineering predictions by Atkins show that the wind turbines can operate in wind directions of 270-360Ãƒâ€šÃ‚Â°; however caution has been applied and initial operating regimes are based on a more limited range of 285-345Ãƒâ€šÃ‚Â°. At all wind directions outwith this range the turbines will automatically switch to standstill mode.
"Vertical axis turbines are not new technology waiting to mature, but have been known for a long time. There were a number of turbine manufactures that could supply 29/30m turbines," stresses Atkins senior project manager Simha LytheRao. The turbine blades and associated equipment were supplied as a complete package by Denmark-based Norwin. And in a truly international effort, the blades were manufactured by Danish firm LM Glasfiber in their India facility; the main gear and cooling system was from Germany; the generator from Finland; the main shaft is from the Czech Republic; and the control systems were from Denmark.
"[Norwin] was involved in the project from the very start and close co-ordination took place between the designer, supplier and contractor throughout the entire construction period and this work is still ongoing," stresses LytheRao. "The basic components of the turbines are identical to the ones you would find on any normal-mounted turbine, but the key structural components have been strengthened to increase safety and to ensure building user comfort," he adds.
Norwin and the main contractor Nass Murray & Roberts (NMR) carried out the installation of the turbines. "Large-scale turbines had never been placed at this height or between buildings before, which created new challenges," stresses LytheRao. The nacelles were lifted onto the bridges using a tower crane; this was carried out in parallel with the installation of the three bridges. The installation of the rotors followed this process. "The three blades used for each rotor were assembled on the ground and the entire rotor was lifted by tower crane into its final position and bolted to the main shaft," explains LytheRao.
"The lifting was NMR's scope, but under the supervision of Norwin; Norwin did the actual installation [of the turbines] when they were lifted into place," adds LytheRao. Cabling was within the scope of the main contractor, with the connection of cables between turbines and controllers carried out by Norwin.
"The main co-ordination issues relate to the services between the turbines and the buildings, including power cables, control cables, fire detection cables and chilled water pipes. As part of the ongoing co-ordination, the different interfaces were addressed and handled with the main MEP contractor," explains LytheRao.
All MEP installation services within the buildings and bridges, including all power cables for the wind turbines, is being carried out by the main MEP contractor, an EMCO Mercury joint venture. The lead-time for the turbines was 14 months, with the installation of the final blade being carried out in March 2007. Testing and commissioning of the turbines is scheduled to be undertaken within the next six months and they are expected to be operating on a daily basis after a further six months.
The electricity produced by the turbines will feed into the general electrical supply of the development. "Comprehensive ground tests indicate that the turbines will generate 11-15% of the energy requirements of the two towers," states LytheRao. "We have assumed that the turbines will operate on average for approximately 50% of the time over one year. This is under the provision that the buildings are able to consume the produced power," he adds.
To interface electrically with the building, each nacelle has a 225kW nominally-rated, 400V, four-pole induction, 50Hz asynchronous generator that is connected to a generator control panel inside each tower. From the control panels, separate low voltage (lv) feeders connect to the interfaces on the main lv switchboard at three substations which supply electricity to the landlord areas of the development.
"The length of the lv feeders from the generator control panels to the building electrical system interface points required careful study in order to avoid excessive voltage drop and to ensure there were no problems with harmonics and voltage disturbances," explains LytheRao.The generators are designed to start and run in an asynchronous mode in parallel with the mains electrical grid. In an outage or reduction in voltage or frequency from the power supply the turbines will be shut down.
The main plant is located in basement and ground levels and on service floors at levels 5, 25 and 39 within the towers. A new 11kV intake substation has been constructed for the development. Primary chilled water is being provided to basement-level plate heat exchangers by district cooling firm Tabreed; secondary chilled water is pumped from here to fan coil units (fcu) and air handling units (ahu) throughout the towers. "The secondary chilled water system is based on variable flow utilising modulating and on/off two-port control valves at the ahu and fcu with variable speed primary pumps," elaborates LyntheRao.
In addition to the wind turbines the building includes a number of features intended to reduce its carbon footprint. These include significant shading on the external glass facades; openable windows to allow mixed-mode operation in winter; thermal buffer zones between the outside and the air conditioned spaces; heat recovery; grey water recycling; and solar powered road and amenity lighting.
Construction of the BWTC began in June 2004, with substantial completion scheduled for the end of 2007. And following the success of the project, the concept of large-scale turbine integration in buildings is already being taken up on further developments. Atkins has been commissioned to design a 400m-high tower in Dubai which includes the technology as well as photovoltaics. Ramboll and Norwin is also currently engaged with a project in London, UK, where three smaller wind turbines will be installed at the top of the residential building Castle House.