| Eco design - Case Study
Powered by Wind
April 8 was a red-letter day for the Bahrain World Trade
Centre (BWTC), designed by WS Atkins. For, that was the day
when the Centre’s three 29 m-diameter turbine blades
were turned together for the first time. “Having all
three turbines spinning simultaneously represents an historic
achievement for this landmark project and Atkins is excited
to have been a major player in turning the original idea into
reality,” Simha Lythe Rao, Senior Project Manager, Atkins,
told the media in Bahrain.
This project is indeed a dream come true. Incorporating large-scale
turbines onto a building is a world’s first. Not surprising
really for a project that’s master class all the way.
The BWTC forms the focal point of a master plan to rejuvenate
an existing hotel and shopping mall on a prestigious site
overlooking the Arabian Gulf in the downtown central business
district of Manama, Bahrain. The concept design was inspired
by the traditional Arabian ‘wind towers’ —
the very shape of the buildings harness the unobstructed,
prevailing onshore breeze from the Gulf, providing a renewable
source of energy for the project.
Design essentials
The two 50-storied, sail-shaped office towers taper to a height
of 240 m and support three 29-m diameter horizontal-axis wind
turbines. The towers are harmoniously integrated on top of
a three-storied sculpted podium and basement that accommodate
a new shopping centre, restaurants, business centres and car
parking. The elliptical plan forms and sail-like profiles
act as aerofoils, funnelling the onshore breeze between them
as well as creating a negative pressure behind, thus accelerating
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 utilising this phenomenon has been one
of the key factors that has allowed the practical integration
of wind turbine generators in a commercial building design.
Wind tunnel testing has confirmed how the shapes and spatial
relationship of the towers sculpt the airflow, creating an
‘S’ flow whereby 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 while reducing
fatigue on the blades to acceptable limits during wind skew
across the blades.
The specific architectural forms of the Bahrain World Trade
Centre towers were borne from using the nautical expression
of a sail to harness the consistent onshore breeze, generate
energy using wind dyna-mics, and to create two elegant towers
for Bahrain, which would transcend time and become one of
a kind in the world.
Getting it right
Research by Atkins has shown that the large-scale integration
of turbines into buildings mostly fails because of the excessive
cost (up to 30 per cent of the project value) associated with
the adap-tation of the building design, and also as a result
of high R&D costs for special turbines. 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 sympathetic to receiving wind turbines.
The premium on this project for including the wind turbines
was less than 3 per cent of the project value. So with the
benefit of a favourable wind climate and a design philosophy
that minimised turbine R&D and building costs, Atkins,
with a team of world leading technologists, moved forward
with the design and addressed the key issues of produc-ing
technically viable solutions and balanc-ing energy yield and
benefit with investment.
Environmentally responsive design
This building is not intended to be a low carbon emission
solution by European and other worldwide 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 to other buildings in the Middle East.
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 that will have the effect of reducing
sol air temperature and conductive solar gain
Deep gravel roofs in some locations that provide kinetic insulation
Significant proportion of projectile shading to external glass
facades
Balconies to 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
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
Variable volume chilled water pump-ing that will operate with
significantly less pump power at part loads than conventional
constant volume pumping
Low pressure loss distribution for pri-mary air and water
transport systems that reduces fan and pump power requirements
Total heat energy recovery heat wheels of fresh air intake
and exhausts.
Recover ‘coolth’ from 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 wastewater and
allow grey water recycling to be added at a later date
Connection to the district cooling system that will allow
an order of magni-tude improvement on carbon emissions as
efficient water-cooled chillers are not allowed in Bahrain
owing to water shortage, whereas the district cooling solution
will involve seawater cooling and heat rejection and much
improved levels of energy conversion efficiency
Dual flush WC and electronic taps with excess water flow restrictors
Reflection pools at building entrances to provide local evaporative
cooling
Extensive landscaping to reduce site albedo, generate C02
and provide shad-ing to on-grade car parks
Solar-powered road and amenity lighting.
Wind analysis
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 very difficult. The majority of architectural
studies deploying building-integrated, horizontal axis turbines
deploy the principle of a fixed turbine as in the case of
the BWTC. Development for vertical axis wind turbines is encouraging
and they do 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.
Extensive wind tunnel modelling, which was latterly validated
by CFD modelling, has shown that the incoming wind is in effect
deflected by the towers in the form of an S-shaped streamline
that passes through the space between the towers at an angle
within the wind skew tolerance of the wind turbine. Engineering
predictions show that the turbine will be 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 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 per cent.
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 per cent and 93 per cent when compared to 100
per cent for the middle turbine.
System components and control
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); rotor; bridge; control, monitoring and safety systems;
and electrical building interface. 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 onto 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, stalling
the blade so that the power output stabilises at a maximum
output.
The full power of about 225 kw will be achieved at 15 to
20 m/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. For this project, nacelles are a conventional
design with some enhancements to suit the desert application
and to increase structural safety. The guidelines in the Danish
code of practice have been used for increasing the structural
safety to ‘high safety class’. Each nacelle operates
inde-pendently and is not affected by the failure of another
nacelle.
Bridges
A key part of the design is the determination of loads on
the rotor, through the nacelle and thence onto 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 199 different
load cases have been modelled for each turbine and validating
calculations or operational processes pre-pared to theoretically
demonstrate that the turbine and bridge can 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 bridges are ovoid in section for aerodynamic purposes
and are relatively complex structures because they incorpo-rate
maintenance-free bearings where they connect to the buildings
to allow the towers to move 0.5 m relative to each other.
In addition, the bridges that span 31.7 m and support a nacelle
with a mass of 11 tonne 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 that it does
not conflict with the frequency of exciting vibrations of
itself or the building. 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 condit-ions, blade clearance to the bridge of
1.12 m 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 incor-porated
that sets the turbine to standstill if deflections become
excessive.
Control, monitoring and safety
Turbine control, monitoring and safety is delivered through
three systems:
Wind turbine control system (WTCS) that directly controls
and monitors the turbines
Extended wind turbine monitoring system (EWTMS) that is a
separate monit-oring system developed for this project
Building monitoring system (BMS).
The WTCS is an industrial quality control system that has
been specifically evolved to control and monitor wind turbines.
It is robust and reliable and, as well as its control and
monitoring functions, it is able to shut down turbines safely
in the event of adverse climatic conditions or owing to other
factors that threaten life-safety or turbine life. It is an
online system that allows operators anywhere to gain access
to the operating data and grant those with appropriate authorisation
control of the turbines. It
has an in-built, independent, emergency, safety surveillance
system that monitors possible faults in the turbine and the
immediate turbine operating environment and brings it to a
standstill, if required. This system overrides the electronic
control system. The WTCS obtains data relating to the turbine
operating environment via the BMS. Finally, it retains significant
data regarding turbine operation and provides tools for analysis.
The EWTMS is a project bespoke system that works in conjunction
with WTCS to provide monitoring and cali-bration of the control
system operational limits required for this specific application.
In total, the EWTMS has 43 additional sensors. In the event
of a control system failure, the turbine is brought to a standstill
by the tip brake working in conjunction with the hydraulic
brake through a power fail, failsafe mechanism. Meanwhile,
the BMS is the building monitoring system that is used as
a means of providing connectivity from remote sensors to WTCS
and EWTMS.
Electrical building interface
Each nacelle has a 225 kw nominally rated, 400 v, closed,
4 pole induction, 50 Hz, asynchronous generator that is connected
to a generator control panel inside each tower. From each
generator control panel, separate low voltage feeders connect
to the interfaces on the main low voltage switchboard at three
substations.
Generators are designed to start and run in an asynchronous
mode and in parallel with the electricity authority’s
grid, but at this stage it is not possible to export electricity
to the electricity supply authority in the event of a surplus
being available. In the event of an outage or reduction in
voltage or frequency from the board’s power supply,
the turbines will be shut down.
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. Extensive dynamic simu-lation studies were carried
out by the turbine manufacturers’ electrical specialist
partner company to ensure compliance with relevant IEC standards.
The design has been validated using a safety, availability,
reliability and maintain-ability (SARM) analysis by Ramboll
with Atkins Science and Technology in a review role.
Energy yield
The projected energy yield from the turbines taking into account
wind and availability data amounts to between 1,100 and 1,300
mw per hour per year and will amount to approximately 11 to
15 per cent of the office tower’s electrical energy
consumption. In carbon emission terms this equates to an average
of 55,000 kgC (UK electricity basis). These figures are conservative.
As this is a world first and because wind turbines have not
been placed 160 m above ground level and between buildings,
the yield may even be higher.
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