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Fri 10 Jul 2009 04:00 AM

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LEED position

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.

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.

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.

What were the main logistical and technical challenges?

The servicing of a large building from central locations is always a challenge to the architecture; the impact on visual aesthetics both inside and outside the mall from exposed services needs to be addressed by screening and co-ordination. The concept design philosophy was to use the roof for services distribution, causing the primary structure to be increased to allow for the considerable load from the primary header services.

How has passive design played a major role in reducing the cooling load?

Natural ventilation has been achieved for all the main car parks surrounding the development. Natural ventilation could not be employed economically for the mall internal areas, due mainly to the limited period of the year that climatic conditions allow its use. Other issues with this strategy for the mall were the noise pollution form the neighbouring Emirates Road and the ingress of dust and insects that could result (particularly in food and beverage) areas, and the attendant hygiene and maintenance implications.

However, due to the HVAC control systems employed in the mall, free cooling will be facilitated via mechanical means to all main mall areas when outdoor enthalpy is below the return air enthalpy. In addition, there is widespread use of thermal wheels, heat pipes and VFDs on air-handling plant to recover energy and minimise energy consumption related to supply air-conditioning and its distribution.Furthermore, to minimise cooling loads, vertical roof lights have been specified with high-performance glazing and overhangs in certain areas for shading. The roof is also specified with high levels of insulation and a high albedo roof to minimise heat absorption, which reduces cooling loads and facilitates the attainment of a LEED credit for minimising the heat island effect.

Finally, the daylight harvested from the roof lights will help reduce the artificial lighting power in the internal mall, and therefore cooling loads during daylight hours. A significant number of LED lighting fixtures have also been selected, particularly for effect lighting within the mall, and these also contribute to reducing the lighting power and cooling load. Is district cooling flexible enough for a shopping mall environment?

The cooling design incorporates a number of important features ensuring flexibility, which is one of the key requirments for a shopping mall. In particular, the chilled water distribution system is flexible and efficient at part load conditions.

Firstly, the primary chilled water distribution ring, which distributes chilled water from the roof to the entire development, is a variable volume system with VFD operated pumps. There are also eight secondary chilled water variable volume pumping stations using VFD operated pumps, located strategically around the roof of the mall to serve eight main internal distribution zones.

To ensure stable control on all terminal units and on AHU coils at all load conditions, a significant investment has been made to provide pressure-independent control valves for every cooling coil. The use of pressure-independent control valves will ensure that a valve authority of one is provided under operating conditions, which will lead to very stable control.

The valves are factory-set at the design flow, and therefore the system is balanced automatically, and only verification of the pressures across valves is required during commissioning. This removes the complicated and time-consuming process of trying to proportionally balance a variable flow chilled water system.

The use of pressure-independent control valves will allow opening or closing of tenancies without any impact or effect on the balance of the chilled water system throughout the development. Essentially, as units are added or removed, the valves will compensate automatically and maintain valve authority, and no manual balancing will be required.

The use of these valves should also assist in maintaining the design delta T achieved by the system overall, thereby reducing the risk of overflows caused by unstable control valves and high or differential pressures.

This project sets something of a benchmark in terms of district cooling?

There are a number of new and efficient district cooling plants coming on-line that are achieving very good kW/ton. The Mirdif City Centre plant also utilises the latest systems, by using HV chillers, contra-flow cooling towers to condenser water flow, variable primary distribution systems, etc., and can be considered on par with the latest plants. One of the major advantages of this plant, however, is that it also gave control over the design of the secondary systems (or building systems).

The extensive measures that have been incorporated into the design of the HVAC building systems will therefore greatly contribute to realising the actual efficiencies of the district cooling plant, which is something that the large district cooling plant companies are most often deprived of because of poor building HVAC system design or operation from third-party developers/operators. The building systems often connected by the district cooling plant companies often provide poor system delta Ts (i.e. low return temperatures) and therefore result in poor overall efficiencies for the district cooling plant.

Energy-efficiency measures at Mirdif City CentreHere is a breakdown of strategies to ensure maximum flexibility in use, efficiency in operation and care in ensuring Ts are maintained for the entire system:

• Use of pressure-independent control valves on all cooling coils, including tenancy FCUs;

• Use of intelligent control strategies such as differential pressure reset control on all secondary (building) chilled water systems;

• Valve position limitation based on chilled water return temperature for all AHU coils, ensuring high return temperatures or a warning of operational issues;

• Night-time set-back operation for AHUs, reducing the number of AHUs in operation and allowing a rise in internal temperatures;

• Use of enthalpy control on air-handling systems to facilitate free cooling during times of advantageous outdoor air conditions;

• High T cooling coils - additional heat exchange area is provided on all coils to assist design T achievement, even if chilled water is delivered 1°C higher than design, or as coils become fouled in between cleaning;

• The use of variable speed drives on all chilled water (primary and secondary) pumps and on condenser water pumps;

• Contra-flow, induced-draft, low-flow cooling towers with VFD control of fans;

• Variable air volume supply AHUs to all main mall areas;

• Demand-controlled ventilation systems for minimisation of outdoor air cooling based on indoor air-quality sensors;

• Use of heat-recovery wheels and heat pipes on various HVAC systems;

• Use of automatic condenser tube brush cleaning systems. These cycle daily and keep chiller condenser tubes in a clean condition, preventing fouling and therefore keeping them at maximum efficiency;

• In addition, condenser and chilled water systems have been provided with high-efficiency bag filtration systems to help minimise fouling, as well as the usual treatment systems;

• Extensive energy metering has been provided on the chilled water system - i.e. at each chiller, main primary ring, each secondary pump station and all mid to large internal tenancies to assist in energy management. These are in addition to extensive electrical metering on main energy-consuming plant at all the above plus for every tenancy.

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