Sunday, April 20, 2008

Keeping the Burj Dubai Cool

Cool runnings

Keeping over 160 floors of residential, hotel and commercial premises constantly at their desired temperatures is no mean task. Add the need for energy efficiency to the list and the MEP consultants and contractors had a large task on their hands. How do you provide cooling to a mixed-use building of more than 160-storeys while maintaining energy efficient standards? The MEP consultants at the Burj Dubai have opted for a district cooling system with ice thermal storage and a mix of air handling units and fan coil units to serve local zones within the structure.

A 13,000TR district cooling plant will serve the Burj Dubai and Dubai Mall that lies within the Downtown Burj development. A total of 45.7MW of cooling will be needed for the Burj Dubai tower itself. The supply of chilled water to the tower is carried out by a forced pump system. The primary chilled water is pumped in 750mm flow and return pipes from the Emaar district cooling plant to the tower basement, which is the heart of the building from the mechanical services viewpoint.

It is then distributed to the upper levels of the building via 600mm diameter pipes within a main riser in the central core of the tower; the pipes reduce to 450mm diameter at height due to the reduced amount of water needed in the smaller, upper-level floors. "The district cooling plant is a self-contained unit with its own chillers and cooling towers," explains Greg Sang, Emaar assistant director - projects. "[It] has a primary chilled water loop and from this a secondary chilled water loop is routed through a service culvert in Dubai Mall, with connections to the Mall before entering the basement level of Burj Dubai.

There is a set of primary pumps at the point of entry to the main chilled water distribution plant within the Burj Dubai. These pumps are on the primary side of a number of heat exchangers sets whose secondary sides feed the various different cooling circuit zones in podium and tower areas via secondary pumps. The basement heat exchangers provide hydraulic separation between the district cooling plant distribution system and the high pressures that exist in certain parts of the tower that are created by the extreme height of the building.

From basement level the chilled water is pumped to plantrooms at levels 17, and thereafter to further plantrooms at levels 40, 73, 109 and 139 prior to local distribution to air handling units and fan coil units. Supplied from the district cooling plant at a flow temperature of 3.3°C, the temperature of the chilled water increases as it is distributed vertically in the building. It returns to the district cooling plant at 12.1°C. The peak chilled water flow rate through the system is 1,245 litres/s (1.245m3/s).

"There are a number of different zones within the building operating at different flow rates and pressures," explains Sang. "The highest pressure, largest capacity set of four duty pumps in the tower are located on level 17 and operate at a flow rate of 185 litres/s, each at a working pressure of approximately 28 bar," he adds. At a local level, the hotel rooms and private residential apartments will all be provided with air conditioning via fan coil units. The boutique offices located at the higher levels of the building will be served by a combination of fan coil and variable air volume (VAV) units.

Meanwhile, the public areas such as the podium and restaurants will be fed via central air handling units that are zoned in relation to the different uses of each individual area. The use of fan coil units is ideal for this project due to the zoning of the overall floor areas that is necessary to maintain the building's structural integrity, explains Hyder Consulting senior mechanical engineer Alastair Mitchell.

Preparing for failure

Back-up systems have been provided throughout the chilled water distribution system. Standby pumps have been included on every circuit and, unusually, a spare heat exchanger has been included in every heat exchanger set. In addition, preparations have been made to ensure a certain degree of cooling can be maintained in the event of a mains power failure. The building's standby generation system will supply power to some of the chilled water distribution pumps.

And the potential failure of the district cooling plant has not been forgotten: "If there were ever to be a significant failure [in the district cooling plant] there is a changeover arrangement whereby we can connect the tower to an alternative district cooling plant and run at reduced capacity," explains Mitchell. "We would have to revise our primary pumps [for the task]; they operate in parallel [during normal operation], but there's a special bypass arrangement being installed where we can operate these in series," he adds.

There's four pumps in parallel, but we can change this to two sets of two pumps in series to generate the additional head needed to get the water from the alternative district cooling plant," Mitchell explains. Installation of the chilled water system is at an advanced stage, with main riser pipes already beyond level 109 and work underway within the level 109 chilled water plantroom. There is one further plantroom to undertake, that located at level 136.

Ice cool operations

To further increase the energy efficiency of the Burj Dubai cooling system, an ice thermal storage system is being employed within the district cooling plant serving the tower. This helps to reduce power consumption in the daytime and high-load conditions, explains Greg Sang, Emaar assistant director - projects.

The system involves the use of a store of ice slurry to reduce the temperature of the chilled water. The ice is created during off-peak periods and through the night-time and held within a thermal storage unit for use when required.

The system is now commonly used in the USA and Europe for commercial and district cooling applications; it is believed that the system at the Burj Dubai development will be the first to be employed in the Middle East. In general, the use of an ice storage system can reduce the total installed chiller capacity by up to 35%.

The latent energy stored in ice is eight times greater than that possible with a similar volume of chilled water, which makes such systems favourable for large district cooling plants. Further benefits include a reduction of 30-40% in the size and cost of all auxiliary equipment needed and the ability to deliver chilled water at lower temperatures than would normally be possible from chillers, enabling the use of smaller equipment such as pipes and pumps, hence lower capital costs.

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