Take a load off

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MEP Middle East speaks to Scott Wilson associate director: building services Cathy Crocker.

The “huge proportion” of the electrical load consumed by air-con has resulted in a largescale focus on efficiency measures to try and reduce this consumption. “Over the last eight to ten years, developers have focused on how we can reduce the electrical consumption for cooling. A district cooling solution, in the correct application, with the correct design, can consume up to 55% less energy than the equivalent standalone cooling system,” is Crocker’s main argument.

However, it is also important to try and reduce energy consumption at the source, which means the buildings themselves. This, in turn, means that building services designers play a critical role from very early on. “Basically site selection and the outline design stage are, in my opinion, the key time for the design team to get together to lay a good foundation for a building design.

“This is the time at which you can get the building orientation right, the building form and the building fabric. These three elements have a huge impact on the energy consumption of a building throughout its life. As we move on through the design stage into detailed design and construction, obviously the ability to change the building design diminishes.

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“No one will thank a designer, after a detailed design, for suggesting a change in orientation, even if it does bring about a huge reduction in cooling load,” argues Crocker. “Clients, engineers, architects and sustainability consultants really need to get together from day one to agree on a good methodology for the design of a building or the design of a site, so they have a good foundation for an energy-efficient solution.” What elements need to be taken into account in the early design stages? “Building orientation is obviously the key to reducing energy consumption,” points out Crocker. “Here in Dubai it is the early sun in the morning and the late sun in the evening that has the ability to penetrate deepest into the building, and these are the areas we need to concentrate on. The overhead sun, while it has a strong irradiation, does not penetrate as far because it is higher overhead.”

Crocker refers to a project in Dubai where Scott Wilson worked with the architects to come up with a self-shading solution. The building comprised three wings, and to protect the building against the low morning and evening sunshine, a heavyweight, opaque façade was applied to the east and west. “This fairly simple approach had a huge effect on the heat gain, so the building did not suffer from high temperatures in the morning and evening, and we could really drive down the cooling loads,” explains Crocker.

The next critical element for the design to consider is the building fabric. “If we can get the correct building fabric in terms of thermal mass and UV value, we can dramatically reduce the peak cooling load. This, in turn, will drive down the energy consumption and size of equipment,” says Crocker. A simple reduction in a building fabric U value from .45 to .35, applied throughout an entire building, can have a huge and incremental impact on the overall structure.

“Another element which I think is very important for designers to consider is infiltration – the unwanted passage of air in each building due to external pressures. We need to make sure that buildings are well-sealed so that the hot external air does not come into our building and add to the cooling load. This is a design issue, and also a site issue, to make sure that the buildings are detailed and constructed correctly.

“If we address the U values, the thermal mass and the infiltration, I say we can achieve a minimum 30% reduction in peak load. Over recent years, building services designers in the UAE have become more familiar with energy-efficiency measures. Variable speed drives, heat recovery and other measures are being implemented more and more into buildings now as a matter of course, which is great for energy consumption,” says Crocker.

She says that Scott Wilson’s favoured design approach, wherever possible, is deemed ‘mix match’. “In essence it just means turning off the air-con when you do not need it. You have to have a building that is designed correctly to enable natural ventilation – that is, opening windows and a shallow-plan floor plate. In certain periods of the year in Dubai, we do have the capability of turning the air-con off completely.