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Humming Windmills on Skyscrapers

Bahrain World Trade Centre (BWTC)  

Built Expressions exclusively spoke to Mr. Shaun Killa, Design Director, Architecture – Atkins, Dubai on the concept of windmills on skyscrapers and its role in achieving green efficiency.

With the global warming creating a global buzz, a constant effort is being attempted by the construction industry in order to make tall buildings energy-efficient. Due to this, the approach of architects and developers has been completely transformed and they are trying to build structures that are more sustainable in the long-run. One such approach is the integration of wind mills in skyscrapers to generate electricity. The technology is still in the process of gaining its impetus in all urbanised areas.In the recent past, the project that has been highly acclaimed for the successful incorporation of wind mills is Bahrain World Trade Centre (BWTC) located in Bahrain. The concept design of the BWTC was inspired by the traditional Arabian “Wind Towers”. The BWTC is a 240m (787ft) high twin commercial office tower complex located in Manama, Bahrain. The towers are the focal point of a master plan to rejuvenate an existing 5-star hotel and shopping mall on a prestigious site overlooking the Arabian Gulf in the downtown

central business district of Manama, Bahrain.

The towers are well known for their striking architecture.  Elliptically shaped, the towers act as aerofoils, funnelling and accelerating the wind velocity between them, and exploit the unobstructed prevailing onshore breeze from the adjacent Gulf coast to provide a renewable source of energy for the building. 

Whilst the impetus for this innovative design solution came entirely from Atkins’ Chief Architect, Shaun Killa, the client readily embraced the concept to portray to the world that Bahrain is committed to options that reduce demand on fossil fuel energy reserves and will move urban and building design in desert climates in a more sustainable direction.

World’s First

What truly sets the BWTC apart, however, is its status as the world’s first commercial building to integrate three 29m-diameter turbine blades into its design.  Collectively, the turbines are capable of generating an equivalent of 11-15 percent of the building’s total electricity demand each year.Accepting such a huge challenge to erect BWTC marvel, Mr. Shaun Killa, Design Director, Architecture – Atkins, Dubai says, “The complexity of integrating large scale wind turbines in a building structure was not to be underestimated and the client expected a key benefit from this project to be the knowledge and experience gleaned which could then be disseminated to design teams globally. Like many architects around the world, the Atkins design team in the Middle East considered design solutions that had incorporated sustainability and investigated the concept of utilising integrated wind turbines on several previous concept designs.”

The wind climate in the Arabian Gulf with its dominant sea breeze characteristic was favourable to harnessing wind energy and allowed the designers to move away from the more conventional omni-directional solutions and consider unidirectional wind turbine options that in many respects, led themselves to the large scale integration in buildings.

Previous research by Atkins had shown that the large scale integration of turbines into buildings mostly fail because of the huge cost associated with the adaptation of the building design. From the outset, this project had as its primary basis of design, the utilisation of conventional technologies and the development of a built form that would be suitable to install the wind turbines. The premium on this project for including the wind turbines was less than 3.5 percent of project value.

So, with the benefit of a favourable wind climate and a design philosophy that minimised turbine R&D / building costs, Atkins, with a team of world leading technologists moved forward with the design and addressed the key issues of producing technically viable solutions and balancing energy yield / benefit with investment.

The Turbines

The key structural components of the turbines have been strengthened to increase safety and to ensure building user comfort. The basic components of the turbine are identical to the ones which are found on any normal mounted turbine. 

Atkins searched the market for some time before they signed with Norwin in Denmark.  They were assigned the supply of the turbines within a lead time which was approximately 14 months and had to closely co-ordinate with the designer, supplier and contractor throughout the entire construction period.

Turbine Installation

The nacelles were lifted onto the bridges using the main tower crane. This was done in parallel with the installation of the three bridges. When all three nacelles were installed the propellers were installed, again using the main tower crane. The three blades used for each propeller was assembled on the ground and the entire propeller was lifted into its final position and bolted to the main shaft.

Conventional Technology

On the BWTC, the design team used conventional technology to design a building that would be sympathetic to the integration of wind turbines and the premium for achieving this was around 3.5 percent of the project’s value.

Elaborating on the concept of wind turbine, Mr. Killa says, “The wind energy harnessed by three massive turbines provides 11 – 15 percent of the office towers’ electricity needs – equivalent to lighting 200 to 250 homes for a year. The design of the towers is derived from the concept behind traditional Arabian wind towers: The sail profiles of the two towers funnel the onshore breeze between them, as well as creating lift, thus further accelerating the wind velocity between the twin structures. Vertically, the sculpting of the towers is also a function of airflow dynamics: as they taper, their reduced width allows for the culmination of an equal regime of wind velocity on each of the three turbines.  This phenomenon is one of the successes of incorporating the world’s first integration of turbines with a commercial tower structure.”

Supported by bridges and spanning the space between the two towers, the powerful turbines lend a dramatic look to the sail-shaped towers. In environmental terms, the generators are perfectly positioned to capture the prevailing Gulf breeze and therefore, reduce overall CO² emissions during building operation, a sign of Bahrain’s lead in adopting environmentally responsible practices. The wind turbines were successfully installed in March 2007, commissioned in April 2008 and was officially certified by Bahrain’s Electricity Distribution Directorate (EDD) in January 2009.  The three 29 diameter turbines are now set to auto-mode meaning they are supplying power to the office towers.

Challenges Encountered The Bahrain World Trade Centre is a technological precedent that sets new standards in sustainable design. Speaking about the challenges encountered during the execution of the project Mr. Killa says, “For anything which is truly innovative you would expect to meet challenges along the way. During construction and installations of the wind turbines, Rambøll and Atkins’ global engineering teams identified and resolved numerous potential technical problems. Among them the most serious was the vibration, which was handled by inserting rubber bearings at the ends of the three bridges. Our next challenge was lifting the three wind turbines, which weighed 68 tonnes each, and placing them on the three bridges which weighed 11 tonnes each. A 270m crane was installed between the towers to facilitate this.”

Sustainable Design

Sustainability was incorporated from the beginning. In its design, the BWTC takes advantage of the location to maximise the energy that can be captured from the sea breeze. Unique to this building, and rising to the challenge of incorporating renewable energy solutions with sustainable architecture, the design provides for these three 29m diameter wind turbines to be horizontally supported between the two towers. The integration of large scale wind turbines into a building has involved extensive research and

development by probably some of the most capable specialists available.

Atkins architects undertook months of meticulous research, including extensive dialogue with turbine manufacturers before and during the concept feasibility study and design development stages. Technical validation included the incorporation of environmentally responsive design elements, different wind regime analysis of turbine performance and SARM (Safety, Analysis & Risk Management) analysis validation.

The turbines produce between 11 and 15 percent of the total electrical consumption of the building. This phenomenon is one of the successes of trying to resolve the world’s first integration of turbines within a commercial tower structure.

The value of ‘Carbon Critical Design’ is of increasing importance within society.  Green building design can no longer be considered a superfluous extra; it is an essential component of all future design.  The necessity of investing in a greener future is high on the political agenda of governments and peer groups of most countries. Organisations such as Leadership in Energy and Environmental Design (LEED) and Building Research Establishment Environmental Assessment Methods (BREEAM) are setting standards that will no doubt become compulsory.

According to Mr. Killa, buildings that represent the World Trade Center organisation are by nature emblematic of the city and are recognised internationally. Mr. Killa says, “In becoming an associate of such a prominent organisation, it was important that Atkins put forward a concept design that was not only strikingly beautiful, but one that would also break new ground in environmentally friendly design. It was also important to create a highly sustainable building that would become a world first in integrating wind turbine renewable energy in Bahrain, showing global leadership in innovation and future concern for the environment in what is considered as an oil rich region. This also would create a great sense of pride for Bahrain as it would lead the Middle East region into adopting a more sustainable future.”

The BWTC building has other environmentally responsive design elements, which reduce carbon emissions compared to most structures in the region. Bahrain is conscious that being the first building in the region to conceptualize such pioneering architecture, the BWTC stands as a leader in the adoption of environmentally responsible practices for the country and region as a whole.

The environmental aspects of the project create a landmark that is concerned with sustainability and the future, namely the depletion of fossil fuels. As a result, Bahrain has developed the first of its kind within the market, and added to their capital value programmes that will be developed around this focal point on Manama’s coast.

Phases of Construction

The foundations for the towers are a piled raft varying in thickness according to loading.  Beneath the main core, the thickness is three metres, reducing to two metres beneath the raking columns.  Pile sizes are typically 1,200mm diameter beneath the main core reducing to 1,050mm diameter beneath the raking columns.

Pile loads vary beneath the raft with a maximum safe working load of 18MN required beneath the main cores.  The piles are designed to resist loads predominantly in shaft friction.

Ground conditions comprise reclaimed land and superficial deposits over weak carbonate rocks over limestone. The piles are designed to resist loads predominantly in shaft friction through the weak carbonate rocks.

The extreme climate in Bahrain and the presence of chlorides and sulphates in the ground and atmosphere require special precautions to avoid problems with durability of steel and concrete structures.

For concrete structures, durability was achieved by adopting a combination of the following measures:

• Specifying high quality, dense, low permeability concrete
• Specifying adequate cover
• Designing to limit crack widths
• Application of external tanking below ground and external coatings above ground
• Tanking was an SBS modified bitumen system.

Sail Shaped Structure

A deep understanding of wind behaviour gained from 25 years of sailing prepared architect Killa to successfully promote the largest-ever integration of electricity generating turbines into a building, the Bahrain Word Trade Centre.

Explaining in detail about the design of the structure Mr. Killa states, “As mentioned earlier, the design of the BWTC twin towers is derived from the concept behind traditional Arabian wind towers: Tapering to a height of 240m, each tower is visually anchored to the ground by a concertina of curved, sail-like forms. With elliptical floor plans, the towers are linked by three bridges spanning over 30 m with a 29-m-dia wind turbine at their centres.

In plan, the sail profiles of the two towers funnel the onshore breeze between them as well as creating lift behind, thus further accelerating the wind velocity between the twin structures.”

Tapering as they rise, the towers reduce this funnel effect, countering the increasing wind speeds with height, and making conditions around the turbines roughly the same. This phenomenon is one of the successes of incorporating the world’s first integration of turbines with a commercial tower structure.  A Danish team of the design firm Rambøll Denmark A/S and turbine manufacturer Norwin A/S engineering worked together for the three bridge/turbine assemblies.

Overall, the integration of the wind turbines into the building design, while technically challenging, has made a significant impact on the sustainability and clean energy credentials of Bahrain.  It has become an iconic landmark of Bahrain, in the same way that the Opera House is representative of Sydney or the Eiffel Tower is synonymous with Paris. 

Design for Seismic and Wind Forces

Many of the technical challenges in relation to the turbines were analysed during the detailed design stages of the project. Extensive wind tunnel modelling that was latterly validated by CFD modelling, proved that the incoming wind is in effect deflected by the towers in the form of an S-shaped streamline which passes through the space between the towers at an angle within the wind skew tolerance of the wind turbine. Engineering predictions showed the turbines are able to operate for wind directions between 270° and 360°, however, caution has been applied and turbine predictions and initial operating regimes are based on a more limited range between 285° and 345°. At all wind directions outside of this range, the turbine will automatically adopt a “standstill” mode. It is no coincidence that the buildings are orientated to the extremely dominant prevailing wind. The funnelling of the towers has the effect of amplifying the wind speed at the turbine location of up to 30 percent. This amplification, in conjunction with the shape of the towers (larger effect at ground) and the velocity profile of the wind (lowest at ground) has the effect of balancing the energy yield to the extent that the upper and lower turbines will produce 109 percent and 93 percent when compared to 100 percent for the middle turbine.

The full power of about 225kW will be achieved at 15 to 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 centrifugal force and rotates to act as a self regulating governor brake, through the exertion of a drag force.

Elucidating the bridge concept between the two towers, Mr. Killa asserts, “The bridges are ovoid in section for aerodynamic purposes and are relatively complex structures because they incorporate maintenance free bearings where they connect to the buildings to allow the towers to move 0.5m relative to each other. In addition, the bridges that span 31.7m and support a nacelle with a mass of 11 tonnes have been designed to withstand and absorb wind induced vibration and vibrations induced by both an operating and “standstill” turbine. Analysis by the bridge designer has been undertaken to estimate the natural frequency of the bridge and to ensure it does not conflict with the frequency of exciting vibrations of itself or the building.”

He adds, “Further, precautions are included in the design to allow the bridge to be damped, if in practice vibrations are found to be problematic during commissioning. These precautions include the facility in the design to add spoilers to the bridge and to adjust the tuned mass damper.”

The bridge is a shallow V-shape in plan (173º) to take account of blade deflection during extreme operating conditions and to afford adequate clearance and thus, avoid blade strike. Under these conditions, blade clearance to the bridge of 1.12m is achieved. The worst scenario is with blade tips extended giving a factor of 1.35 safety margin, and under this condition, adequate clearance is still achieved. Additionally, a laser blade position monitoring system is incorporated that will set the turbine to standstill if deflections become excessive.

The design process lasted for 18 months, including extensive dialogue with turbine manufacturers before and during the feasibility study and design development stages to guarantee both the safe operation of the system and minimise any structural or vibration impacts on the building. The turbines are a “stall control type” which passively limits power produced as wind speed increases. When wind speeds do exceed safe limits (20m/s), the turbines cut-out and then brake to a standstill. The uni-directional turbines selected use a simple gearbox mechanism that requires simple maintenance every four months, with a more thorough service undertaken once a year. The blades should last for up to 20 years, while the generator and gearbox should last for up to 50 years.

Ultimately, design modelling predicted that the three 225kw horizontal axis wind turbines would satisfy between 11-15 percent of the total electrical demand of the office towers, a reduction of approximately 1,500 tonnes of carbon per year.

Structures

The twin towers are framed predominantly in reinforced concrete, with structural steelwork used for the clad steel tower, panoramic lift enclosure and mezzanine floors.  The total height of the structure is 240m above ground floor level and the single storey basement is at a depth of 4.5 metres.

The primary structure comprises main and secondary reinforced concrete cores, reinforced concrete columns and floor plates typically having a storey height of 3.6 metres.  Where the structure tapers in elevation, raking columns follow the sloping face.

The towers are typically mirrors the axis of symmetry, although there are some differences over the height of the podium.

The wind load on the towers is predominantly resisted by the main concrete core that encloses the four main lifts and the adjacent, escape stairs, plant spaces and risers.  About the weak axis, the secondary core relieves some of the load on the main core. Four raking columns triangulate the cores, resulting in a relatively stiff framework.  The outer pair of raking columns following the arc on plan also has a stiffening effect about the minor axis and tends to attract reversible forces under wind loads.

At the upper levels, the secondary core first terminates and the main core then extends to the height of the highest office floor.  When this also terminates, the panoramic lift core extends further stabilising the duplex offices and viewing gallery.  Above this height, the top clad section of the building is framed in lattice steelwork construction to reduce weight. This plan shape and the mirrored arrangement of the twin towers funnel the prevailing wind in much the same way as a Venturi tube.  Whilst this effect is advantageous in terms of wind turbine operation it does mean that wind loads on the towers are elevated, particularly locally.

The structure is also unusual for a high rise tower in that the centre of gravity, centre of mass and centre of stiffness vary from floor to floor, progressively moving towards the panoramic lifts at height.  This also results in a tendency for dead load sway, although the effects are not excessive because of the stiffening effect of the raking columns and the concentration of piles beneath the main core reducing base rotation.

Aesthetics Element

BWTC’s sleek design and crystal blue ‘sails’ can be seen from anywhere in downtown Manama and have come to define the Bahrain cityscape.

Commenting in detail about the structures design Mr. Killa says, “On the ground level of the BWTC you enter the building via a breathtaking glass-domed entrance court, fitted out with cool marble floors and brushed steel features that are architecturally in keeping with the design of the BWTC’s towers. The cool colours and sleek aesthetics create an ultra modern backdrop against which luxury brands and five-star hospitality meet architectural ingenuity and flair.”

The mall is built around a unique concept that combines the elements of wind, water, motion and the veil. Each corresponds with the distinctive shape, design and technology behind the development’s iconic sail-shaped towers, which overlook the Manama waterfront and have been created to veil and harness the wind’s energy and motion. Carried into all aspects of MODA Mall’s design, these elements set the scene for a truly one-of-a-kind, sensory shopping and dining experience.

There are three sky lit courts the Fashion Court, the Jewellery Court and the North Court providing centrepieces connecting the three key ground level components of the overall development (the existing and new parts of the MODA Mall and the Sheraton Hotel). These focal points provide stunningly finished spaces for relaxation, events and social congregation.

There are also two external water features, one at the ground floor main entrance, with the other at the third floor podium entrance. In addition, there are two reflection pools within the main entrance leading into the Fashion court.

The interior and exterior features are fresh, modern and reflect the high-tech, 21st century spirit of the entire development further enhancing the high profile, world class stature of the development.

Business Hub

Situated on the Manama waterfront and in the heart of the region’s leading financial and business hub, the BWTC project comprises the two 50-storey sail shaped twin office towers, MODA Mall and the five-star Sheraton Hotel. The state-of-the-art development cleverly combines the essential elements of business and leisure with cutting-edge corporate offices and leisure amenities including exceptional shopping, fine dining and casual dining restaurants, cafes, a health spa, a five-star hotel and a range of other services.  Located on the ground floor of the BWTC, the MODA Mall is the Kingdom’s ultra-modern fashion destination comprising a veritable who’s-who of the fashion world. MODA Mall is home to some 150 exclusive brand name boutiques offering the latest fashions some of which  have never been represented before in the Bahraini market.

Having executed the project successfully, Mr. Killa proudly says, “The BWTC provides leading regional and international organizations with a world-class business address. The buildings are highly advanced in design and operation and are the Kingdom’s first intelligent offices, employing the latest SMART systems capable of delivering unsurpassed security and maximum competitive advantages and efficiency in office management. In addition, they are the first of their kind in the world, using wind energy to provide 11 – 15 percent of the electricity needs of the office towers.”

He further adds, “Other benefit enjoyed by local and international businesses occupying space at the development is membership to the World Trade Center’s Association (WTCA). The BWTC offers an exceptional opportunity for connectivity with the global business community. With more than 300 centers in 100 countries, WTCA has a membership of approximately 750,000 commercial enterprises worldwide.”

The sweeping staircases, glass domes and brushed steelwork lend a modern look and feel to the mall. This mixed-use design reflects the current regional lifestyle trends, with today’s modern lifestyle requiring the combination of business and leisure within one convenient location.

Other facilities housed at the mixed-use development is the state-of-the-art, international health club Fitness First, and a range of casual and fine dining restaurants, including, Abd El Wahab, BiCE Ristorante, Espressamente Illy Cafe and Maki.

With his last words on the iconic project, Mr. Killa concludes and says, “BWTC has set a standard in terms of sustainable buildings throughout the world. But, its potential continues to carry in influencing future projects.”

Salient Technical Features

BWTC has some of the ground-breaking renewable energy design in the buildings which incorporates many passive energy reduction measures. These include:  

• Buffer spaces between the external environment and air conditioned spaces – examples include a car park deck above and to the southern side of the mall which will have the effect of reducing solar air temperature and reducing conductive solar gain;
• 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;
• Where shading is not provided to glazing, a high quality solar glass is used with low shading co-efficient to minimise solar gains;
• 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;
• Variable volume chilled water pumping that will operate with significantly less pump power at part loads than conventional constant volume pumping;
• Low pressure loss distribution for primary air and water transport systems that reduces fan and pump power requirements;
• Total heat energy recovery heat wheels of fresh air intake and exhausts to recover “coolth” from the vitiated air and recover it to the fresh make up air;
• Energy efficient, high efficacy, high frequency fluorescent lighting with zonal control.
• Dual drainage systems that segregate foul and waste water and allow grey water recycling to be added at a later date;
• Connection to the district cooling system that will involve sea water cooling / heat rejection and much improved levels of energy conversion efficiency;
• Reflection pools at building entrances to provide local evaporative cooling; and
• Extensive landscaping to reduce site albedo, generate C02 and provide shade for car parks.

Significant Milestones

• The initial planning and research into turbine technology commenced in late 2003, and a detailed design of the project was approved.
• Early 2004, construction works on the project commenced.
• In 2006, the turbine research, turbine feasibility and bridge design were completed.
• In 2007, bridges and turbines were installed.
• In April 2008, turbine operation was formally commenced which followed eight months of daily safety and regulatory testing.
• In January 2009, official certification from Bahrain’s Electricity Distribution Directorate was received. This meant the system could be set to an automatic mode to supply the two towers with electricity.

Fact Sheet

Project Team

Site area: 45,000m² Ground footprint 20,500m² Total Build Area: 121,200sq.m Materials: Reinforced concrete superstructure, steel secondary structure and tower apex.

Cubic meters of concrete = 120,000 m3

Tons of steel = 18,000 Tons.

Facade: Double glazing glass and insulated aluminium spandrels, stone wall and floor cladding. Glass Panels = 29,500 m² Cladding panels (aluminium) = 24,000m² Floors: 42 No Client: Confidential Client Representative: HAJ, Bahrain Project Manager: Atkins Architects / Engineers: Atkins Cost consultants: HAJ, Bahrain Turbine Design: Norwin, Denmark Bridge Design: Ramboll, Denmark Landscaping: Shankland Cox, Dubai Interior Design: Aedas, Dubai Main Contractor: Nass, Murray & Roberts JV Height of the BWTC: 240m, 42-storeys Floor plates: Range from 120sq.m to 820sq.m

  Awards    

• PALME – Best use of External Lighting 2010
• NOVA Innovations Award in 2009
• Building Design Architect of the Year 2008 
• Council Tall Buildings & Urban Habitat - Best Tall Building Award 2008
• BEX Award – Sustainability Award 2008
• EDIE Award for Environmental Excellence 2007
• LEAF Awards 2006 – Best use of technology 

 Shaun Killa

Shaun Killa received his BAS and B. Arch from the University of Cape Town.  He worked for several award winning architectural firms in Cape Town on Waterfront projects, office parks, leisure estates, stadiums and masterplans before moving to Dubai and joined Atkins in 1998. He was part of the team on the Burj Al Arab hotel and from 2000 he became Head of Architecture and later Design Director for Atkins Dubai.  He has won several high-rise commissions such as the 21st Century Tower (270m), Chelsea Tower (250m), Millennium Tower (285m), Al Mas Tower (370m), and a number of others around the Middle East. His design work and management of the architectural studio assisted the expansion of the Atkins Dubai office from 60 to over 800 staff over a period of 8 years.  Mr. Kila’s design specialisation in large projects and high-rise towers and his passion for sustainable design has led him to design the world’s first large scale integration of wind turbines on a building for the Bahrain World Trade Centre. Over the past 4 years, he has focussed his design philosophy on innovative low carbon sustainable buildings with challenging architectural norms to create expressive and timeless form such as the DIFC Lighthouse Tower (400m) with turbines and PV integration, and DNA Tower (260m).  Killa has also been the lead designer on master plans in Egypt, Dubai, Cape Town and Singapore and he participates on the advisory Committees of several Universities for their Bachelor and Master’s programs. He is also involved in the office management, marketing and branding of Atkins in the

Middle East and internationally.