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Richard Reid, senior vice-president of Majid Al Futtaim Properties, elaborates on the application of district cooling at the company’s flagship Mirdif City Centre, due for completion by early 2010.
What was the specific district cooling requirement?
Sustainability was always on the agenda, and pursuing LEED meant efficient HVAC systems were a must.
Essentially the brief required the designer to provide a central cooling plant with a high degree of efficiency and flexibility. Due to the nature of shopping malls, flexibility is critical, as tenant loads can vary significantly, depending on the type of use – that is, electrical goods stores, jewellers, etc. – that have very high-density heat gains compared to high-end boutique clothes stores that have relatively low-density loads – and at the time of design the actual tenant is often unknown and may change in future.
As a result, the final load required can vary significantly at the shop-unit level, as well as at the plant level, where the peak load varies greatly over the year due to climatic variations. Therefore ensuring a high degree of control and good efficiency at part loads, as well as limiting possible tenant interference with systems, were key requirements. In addition, it was necessary to have a design that could be commissioned quickly and would be flexible in nature as well as open to future expansion.
What size plant was specified? Who was the main technology vendor?
The total cooling plant capacity is 53,4 MW at present, which is expandable by up to 62,3 MW with the addition of a future chiller. There are 6 No. 8900 kW duty York high-efficiency HV (11 kVA) chillers installed. The main chiller features are: the use of a safe refrigerant, namely R134a, which has no ozone-depletion potential and no phase-out schedule; the chillers have very good efficiencies, with a peak full load COP of 6,2 at ARI conditions and 5,2 at actual Dubai conditions, with a NPLVs of 6,37 and 7,01 respectively, exceeding ASHRAE 90.1 requirements.
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In addition to efficient HV chillers, the heat rejection system comprises highly-efficient induced-draft, contra-flow cooling towers which were optimised with the chiller selection to provide best aggregated COPs, which resulted in a condenser flow of 2,5 gpm/ton. The cooling tower capacity is controlled via variable speed fans, and the condenser water circuit is provided with variable speed pumps to ensure minimum motor energy consumption at part load.
The chilled water distribution system comprises a variable primary, variable secondary system with VFDs provided throughout. The system can also operate in a coupled mode, which allows chilled water zones close to the energy centre to benefit from residual pump head available from the primary pumps.
What were the sustainability and energy efficiency considerations?
As the project is targeting a LEED rating, it is important that these requirements were taken into account carefully. The use of high-efficiency chillers using R134a refrigerant ensured both the fundamental refrigerant management and enhanced refrigerant management were complied with. Furthermore, the energy centre equipment exceeded the energy efficiency requirements of ASHRAE 90.1 at ARI conditions, and ensured a good basis for energy performance credits under LEED. The HVAC system efficiencies, in particular, at part load have been reinforced with the widespread used of VFDs on pumps and fans. VFDs have been utilised for the cooling tower fans, the condenser water pumps, the chilled water primary pumps, the secondary pumps and on the air-handling plant.
The project also has an extensive metering strategy, with remotely-monitored energy and electrical meters integrated with the BMS to ensure the LEED Measurement and Verification credit is attained, as well as providing the operator with a powerful tool to assess where and when energy is being used to facilitate an energy management plan.
Finally, the project employed a dedicated commissioning authority company, which is based permanently on-site, and has been involved from the early design stages to ensure the energy-consuming systems can be, and are, commissioned carefully to perform as designed. This important additional service also facilitates the attainment of commissioning and enhanced commissioning LEED credits.
How are the district cooling plant and sub-station being integrated into the project as a whole?
The energy centre is situated at the south-east edge of the development, and is essentially located next to one of the car park structures. The cooling towers are hidden from view by an acoustically-treated parapet, which should ensure that the resultant noise levels at nearby areas are no higher than that experienced previously from the local road infrastructure.
Both the district cooling plant and the sub-station are integrated into the main mall architecture and surrounding city plan. The district cooling plant occupies a key corner of the site, allowing for good plant access and the opportunity for a bold design using mass and height. The sub-station, situated on a main arterial road into Mirdif, has been designed to meet the local planning restriction that requires all new buildings to have ‘folkloric’ façades, in keeping with traditional Arabic architecture.
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