How Dubai is keeping its cool

Burj Dubai

There is one thing you can be sure of: when it is completed in 2009, the Burj Dubai will be the world’s tallest building. Just how tall it will actually be is still a mystery; its developers Emaar Properties PJSC of Dubai are not saying – it’s going to be a surprise. There is, however, talk of it reaching as high as 150 stories.

What the developers will say is that it will hold the record in all four tall building categories: highest structure, roof, antenna and occupied floor. On a clear day the tip of the spire will be visible 95 km away.

As the tower rises from the desert, setbacks will occur in an upward spiralling pattern, decreasing the mass of the tower as it rises. It will be clad in a high performance curtain-walling system containing 84,000 m2 of glazing and 28,000 m2 of metal.

The building is effectively a small vertical town; residential, commercial, hotel, leisure and retail occupants are stacked one on top of the other. “It’s difficult to design the mechanical, electrical and plumbing services for such a tall building in such a harsh environment,” says Roger Frechette, director of MEP at Skidmore, Owings & Merrill, and the man heading the tower’s MEP design.

At its peak cooling load, the building will need 53 MW of cooling. This will be supplied from a district cooling plant serving the Burj and the surrounding development. The building’s height means the cooling system will be at high pressure so a heat exchanger will separate the system in the tower from the district mains.

Dubai’s hot and humid environment, when combined with the cooling requirements of the building, will produce an enormous amount of condensation on the air-conditioning units’ cooling coils. This condensed water will be collected and piped to a holding tank in the building’s basement where it will be used to irrigate the tower’s landscape planting. “It is estimated that over the course of a year over 50 million litres of water will be produced – enough to fill 20 Olympic swimming pools,” says Frechette. However, even this figure will fall short of the 1 million litres of water a day needed to satisfy the domestic demand.

Like its cooling requirement, the building’s electrical needs are also large. “The electrical load will be close to 45 MVA,” says Frechette.

And the tower will hold another record: the public will be whisked to the viewing gallery at the top of the building at more than 18 m/s (40 mph) in the world’s fastest lift. The Burj Dubai will also be the first high rise in which the lifts will be programmed to permit evacuation for certain fire or security events.

Key facts

• Completion date 2009
• Cooling capacity 53 MW
• Electrical load 45 MVA

Dubai Marina

This £125m development, completed in 2004, consists of six residential towers on a two-storey podium and the related infrastructure. The tallest towers are 42 stories high, measuring 190 m. The fully landscaped podium accommodates 64 luxury villas, six swimming pools, 2800 m2 of sports facilities, 200 m2 of restaurants, 4000 m2 of retail and a 200 m2 mosque. And this 10 ha site is just one small part of a 3.5 km long marina basin .

Connell Mott MacDonald, the M&E consultant on the development, recognised early on that one of the major challenges it was facing was the predicted 47 MW of cooling needed to keep the residents comfortable in a region where temperatures can reach 46ºC in the desert sun. The usual approach in Dubai is to locate cooling plant on the roof of each individual building. However, CMM realised that on a project of this scale, this approach would be inefficient, as well as increasing the structural loading on each building. Instead, the firm decided to use a single, centralised plant space to serve the whole development and “optimise” energy consumption. CMM say this is the first time such a system has been used in Dubai.

The cooling system was designed with centrally located chillers and condensers. By moving the condenser plant from the roof, CMM was able to use more efficient water-cooled condensers, located 200 m from the development, in place of the usual air-cooled plant. The system allowed the engineer to take advantage of load diversity across the site. Centralising the seven, 7.5 MW chillers enabled CMM to reduce installed cooling from 47 MW to 37.5 MW, as well as providing the opportunity to install larger, more efficient cooling plant.

The system is designed to be robust. The chilled water system is zoned using heat exchangers to provide pressure breaks and prevent the whole site being affected if a pipe fails. Zoning the system also allowed the primary circuit to be maintained at a constant volume to ensure stable operating conditions are maintained in the plant.

The chillers are staged to maintain maximum operating efficiency through varying loads. Secondary and tertiary circuits are both variable volume to reduce chilled water circulation and pump power.

The total electrical load for the project is 52 MW, which includes the load for the 11 kV chillers. CMM says each chiller includes a 1 MVA/11 kV compressor motor in lieu of 400 V machines to save space and energy.

Key facts

• Completion date 2004
• External design conditions 46ºC db/30ºC wb
• Internal design conditions 23 ºC db 50% rh
• Cooling capacity 37.5 MW
• Cooling load 125 W/m2
• Chilled water circuits 7ºC flow/15ºC return
• Refrigerant R134a
• Installed electrical capacity 52 MW

City of Arabia Mall

The statistics are staggering: 1000 shops, 1 million m2 of retail space, parking for 10,000 cars, 22,000 m2 of plant space and a dinosaur theme park, all in this £1.15bn development. There is even a 4 km long artificial waterway containing 130,000 m3 of water where a fleet of water taxis will take people from residential areas to the mall and theme park. Given the statistics, it will come as no surprise to be told that when it opens in 2008, the City of Arabia Mall will be the biggest shopping mall anywhere in the world.

“This is a huge space to keep cool and the air-conditioning will be by far the biggest energy bill once the buildings are operational,” says WSP director Paul Hockings, the engineer in charge of the mechanical, electrical and plumbing services for this massive scheme. His statistics for the mall’s services are mind-boggling too: 700 m3/s of fresh air, 500 m3/s of extract air and 200 m3/s of kitchen extract.

Even the canal – the Wadi Walk – will have to be fed with a constant supply of almost pure water. It will also have to be cooled. ”In the Dubai heat the system must be capable of dealing with the problems of constant filtration, evaporation and the elimination of stagnant water,” explains Hockings.

In total 120 MW of cooling will be provided by an off-site district cooling system to keep the development cool.

The district cooling scheme will also keep the mall’s big attraction – a dinosaur theme park – cool. The park has been devised by the Natural History Museum and Jack Rouse Associates. Called Restless Planet, the development aims to transport the visitor back in time to a period long before retail malls were invented and dinosaurs ruled the earth.

Thirty-four different species of dinosaur programmed to move, roar and even walk, will bring the prehistoric world to life. The exhibits are designed to follow the evolution and demise of the dinosaur.

The dinosaurs will inhabit a 50,000 m2, fully air-conditioned park housed in two buildings designed by multidisciplinary design practice Furneaux Stewart and architect YRM. One is a 65 m high ETFE-clad dome, the other, called the cloud building, will be used for exhibitions; it will also house three theme park rides.

The environmental consultant for this part of the mall is Cameron Taylor and the primary design consideration for the plastic-clad dome is solar gains in the occupied spaces. To minimise solar gain and glare within the building, both static and dynamic shading is used. Fortunately, a 12 m diameter structural “cap” at the top of the building provides shade from the midday sun. To protect against early morning and late evening sun, an external shade has been designed, which will be programmed to follow the sun’s path through the day. It will be supplemented by internal shading devices.

Displacement ventilation will keep the occupied area within the dome cool without having to cool the entire volume of the building.

The system will cool up to 3 m above the floor; the temperatures within unoccupied areas will be allowed to rise.

The challenge in air-conditioning the cloud building is to keep the interior clear of services to allow construction and fit-out of the rides. Solar gain has been minimised by the “cloud” shading structure and a heavyweight roof. The cloud provides shade for both the roof and the roof-mounted plant. In addition it helps hide the plant from the adjacent hotel.

To ensure the park is always supplied with chilled water, the district cooling mains will feed the scheme from two different locations, each with its own dedicated plantroom. The plantrooms will house energy metering equipment, heat exchangers and controls.

Key facts

• Completion date 2008
• Total cooling capacity 120 MW

The Palm Jumeirah

Architect and engineer RMJM was commissioned in 2004 by Palm District Cooling to design four district cooling energy centres to meet the needs of properties on the crescent of the Palm Jumeirah development (above), writes RMJM’s Paul Allen.

District cooling schemes are becoming more common in Dubai as a way of deferring the initial capital cost of the cooling plant, freeing additional lettable floor area, reducing maintenance call-outs and removing noise pollution potential. The initial cooling load was estimated at 352 MW but escalated to 478 MW after the developer finalised its requirements.

The crescent forms the outer breakwater of the Jumeirah Palm. It is about 12 km long, 275 m wide and consists of three segments with two bridges and a vehicular tunnel to the trunk. It has 40 land parcels that have already been sold to developers and construction is under way.

In the early stages of design, the use of seawater for condenser heat rejection appeared an obvious contender for both direct and indirect seawater cooled centrifugal chillers, with and without cooling towers. However, there were concerns about the high salinity of the seawater and problems with corrosion. Other major considerations were the high sea water temperatures of 39°C during the peak summer months and the impact of the return water quality on the marine environment.

The solution lay in the method chosen for the disposal of sewage effluent from the hotels and resorts. On the crescent sewage is treated then polished to potable quality for irrigation purposes. The excess is allocated as make-up water to offset cooling tower evaporation and drift losses. Evaporation losses during peak summer conditions will be about 750 m3/hr. The cooling towers have an ambient wet bulb temperature of 31°C, providing a water leaving temperature of 34.5°C, a significant improvement over seawater.

The process cooling plant is managed on a commercial basis and so must be economical, efficient and reliable. The most efficient configuration for chillers was found to be duplex chillers in a counter-flow arrangement. The capacity of duplex chiller modules varies but typically will provide about 20,000 kW of cooling with a leaving water temperature of 4.5°C and a system temperature difference of 8.9°C.

The choice of refrigerant is still open to discussion. The main manufacturers of seriously sized chillers use either R134a or R123 refrigerants. As an HFC, R134a is more acceptable environmentally than its competitor R123, an HCFC. However, the difference is marginal and the global warming potential of R134a is actually higher than that of R123.

In its favour, R123 is about 10% more efficient than R134a, leading to lower running costs, and it is also a low pressure refrigerant, which means that any leakage would run into the chiller, not from it. The R123 machines also consume less electricity: 0.66 kW per tonne of refrigeration, compared with R134a’s 0.73 kW.

Chilled water is distributed to the end users through GRP pre-insulated pipework, using variable speed secondary pumps, to building energy transfer stations, which contain plate heat exchangers and energy meters for tenant billing. Usually, when district cooling is used, the provider assumes control of the tenant circulation pumps to ensure the temperature difference is maintained, providing more efficient operation. However on the crescent many hotels and resorts have crucial cooling requirements, such as aquatic life-support systems, and wish to retain control of their own systems.

The contract is about to be awarded and the first plant will be producing process chilled water in October next year. Progress can be viewed on www.thepalm.ae.

Key facts

• Completion date 2008
• Cooling capacity 478 MW
• Chilled water circuits 4.5ºC flow/13ºreturn
• Refrigerant yet to be decided

Mall of the Emirates

When it opens later this year, this giant 267,000 m2 mall will be the largest shopping mall outside North America. But it is not just the 140,000 m2 of retail space, arranged over three levels, that will keep visitors entertained. If shoppers fancy a change of pace then developer Majid Al Futtaim Developments has also provided an in-house ski resort, complete with a 400 m ski slope covered in real snow (see “Sun, sea and snow”, page 43). There will also be a 400-suite, five-star hotel and a 515-room four-star hotel, a multi screen cinema, an amusement park and parking for 6000 cars.

The major feature of the shopping mall is a fully glazed, 26 m wide, 42 m high central spine, which runs north–south dividing the building in two. In a country where outside temperatures often exceed 50ºC, project engineer WSP was concerned the spine’s glazed barrel-vaulted roof would introduce excessive amounts of heat into the core of the space. “The obvious solution to the heat problem would have been to shade the glazing,” says Rob Gregory, technical director of WSP and lead mechanical, electrical and plumbing engineer on the project. However, this was rejected on aesthetic grounds. Instead, special tinted double-glazed panels were selected to limit the amount of heat transmitted to the mall. To further limit heat transmission fritting was applied to the panels, covering 70% of their surface at the top of the vault had and decreasing as you move down the arch.

Designing an-air conditioning system capable of keeping a mall this size cool has been quite a challenge for WSP. Computational fluid dynamic modelling established temperatures at the higher reaches of the galleria could be as high as 65ºC during peak summer months. “This would obviously have a negative effect on occupant comfort within the upper levels of the space,” explains Gregory.

In all, 130 roof-mounted air-handling units cool the mall, each supplying about 10 m3/s of conditioned air. Two types of units are used: recirculation units and fresh air units. The fresh air units are fitted with variable frequency drives controlled by air quality sensors to allow the air-handling unit’s operation to respond to changes in demand. Drum diffusers jet air to the space and exhaust air is taken through the back of the shop units to ensure air is distributed throughout the public spaces.

In areas where high temperature air will accumulate, such as the top of the mall, smoke ventilators will vent hot air to outside. Make-up air will be introduced through a series of fresh-air natural ventilators at the base of the vault.

The 52 MW of cooling needed to keep conditions in the mall at the optimum temperature for shoppers is provided by 13 centrifugal chillers. There are two chilled water circuits: a 900 mm diameter primary circuit, operating at 12ºC return/6º flow, connects to 16 risers at the building’s perimeter. A set of secondary pumps then distribute the chilled water to air-handling units and retail units in that part of the building.

The pressure in the secondary circuit controls the speed of the secondary pumps. As the temperature in the space approaches design, the chilled water requirement of an air-handling unit will decrease. Valves on the air-handling unit cooling coil will close increasing the circuit resistance. As the resistance increases, the pumps will slow down to reduce the volume of water pumped to save energy.

Temperature is also sensed on the primary return pipe to the chillers. As the secondary pumped volume decreases, the temperature in the return pipe will fall below a set point and a chiller will be taken off line.

Key facts

• Completion date end 2005
• External design conditions 46ºC
• Internal design conditions 23ºC
• Cooling capacity 52 MW
• Chilled water circuits 6ºC flow/12ºC return
• Installed electrical capacity 95MW