by Gerhard Hope www.constructionweekonline.com
The Dubai Metro is the world’s first district-cooled mass transit system, according to Stanley Consultants project manager Hisham Hajaj.
A world-class distribution network provides centralised cooling via pre-insulated buried steel piping to all Dubai Metro stations. Five district cooling plants have been built specifically for the Dubai Metro, with the following installed capacities:
• Al Rigga 10,000 TR
• Al Barsha 7,500 TR
• Jumeirah Island 7,000 TR
• Jebel Ali Industrial 4,400 TR
• Al Rashidiya 7,500 TR
Hajaj cited the design challenges as having to reroute existing utilities, the lack of ‘as built’ documentation, unmarked utilities, and the lack of space in some areas for chilled-water piping. This posed specific challenges for the construction as well.
For example, the main contractor would produce drawings rendering the trial pits in ‘soft’ landscaping. These drawings were then submitted to all the relevant authorities for approval. Once approval was obtained, the trial pits were excavated – only to reveal other services crossing the district cooling corridor.
The construction NOC drawings, based on information obtained from the trial pits, were then submitted to all the relevant authorities. Work could only commence once the construction NOC was in hand. If the work was located on any major road, or would affect traffic flow in any way, then approval for road diversion had to be obtained from the RTA as well.
The NOC applications themselves were complicated by the length of time these took to process, which had a great impact on the construction programme itself. “The other main construction issue was co-ordination with the Dubai Metro main contractor, the DURL consortium,” said Hajaj. A substantial portion of the district cooling works fell within DURL’s own site boundaries, which it had its own deadlines to meet.
In terms of procurement, a total of 52 km of piping was needed for the district cooling network. This required close co-ordination with the piping manufacturers during the design stage, as well as submitting bills of quantities before issuing the relevant tender so that the detailed planning could be finalised.
Hajaj explains that district cooling was particularly suited to the Dubai Metro. Not only did it reduce the electromechanical areas inside the stations, but it also reduced the total power connected and consumed by between 30% to 50%. This, in turn, reduced the total carbon footprint of the project. It also reduced the noise and vibration, as opposed to the alternate standalone solution of having roof-top air-cooled chillers and pumps.
“The initial cost of the district cooling plants with associated piping network is higher than an individual standalone solution, but the payback period is much less,” said Hajaj. In addition, the lifespan of the district cooling plants is 25 years as opposed to 15 years for an air-cooled solution.