met office
Energy-efficient air conditioning for the new Met Office complex near Exeter is provided by Carrier Global chillers providing chilled water to an unerring degree of accuracy. The building has received an ‘excellent’ rating under the BREEAM scheme. However, air-conditioning equipment is needed to ensure that fresh air is supplied to the right places and at the right conditions.

Exploiting the energy-saving potential of modern air conditioning

Published:  14 August, 2005

The scope for reducing the annual energy consumption of air-conditioning systems is some 45%. ANDREW KEOGH discusses a whole range of opportunities.

The energy consumed by air-conditioning systems in commercial buildings is expected to double from current levels by 2020. In a typical office, air conditioning can account for over 30% of annual electricity consumption. In recent years the UK has seen a shift in energy-consumption patterns from a winter peak to a summer peak.

Central theme

When you combine these facts with the Government White Paper’s challenging targets for reducing carbon-dioxide emissions it is clear that energy efficiency will become the central theme of the building-services industry for the foreseeable future. Furthermore, we shall shortly have the introduction of the Energy Performance of Buildings Directive to concentrate our minds on the issue.

The growth in the consumption of electricity by air conditioning does not reflect some failing in the technology. Rather it is the direct response of its greater necessity, acceptance and use. People now demand improved comfort in their working environment, their car and even their home. Current building design, usage and regulation also favour the use of air conditioning. Deep-plan buildings, large glazed areas and high internal loads mean that air conditioning is necessary if occupants are to be comfortable enough to work effectively.

Huge variations

However, it is clear that the industry can do more to reduce energy consumption if you consider the huge variations that exist between buildings and air-conditioning systems. The cooling-related energy consumption for a typical prestige office building is around 131 kWh/m2 per year. The same figure for a standard, air conditioned building that utilises current best practice is 52 kWh/m2 per year, a reduction of 60%. Even in the most demanding buildings, the potential for energy savings is some 45%.

What potential is there for improving the energy efficiency of air conditioning?

One obvious solution is not to install air conditioning, and to rely instead on natural ventilation. This certainly has its applications, but if natural ventilation is to meet all the building’s cooling load Louis Sullivan’s assertion that ‘form follows function’ is particularly appropriate. The constraints this places on the building design and occupancy levels and usage may not be either desirable or practicable.

Design criteria

The first step towards more energy efficient systems is to adopt more realistic design criteria.

Consider the following.

An air conditioning system for a building in London will typically be designed so that the internal temperature is held at 22°C when the maximum ambient temperature is 32°C. Long-term weather records for London show that the temperature reaches 32°C for only a few hours each year, yet our plant and system must be designed for this exceptional condition!

Free cooling

If we allow the internal temperature to exceed the stated 22°C for (say) 10% of the time, we can design our plant and systems for an ambient of around 25°C. Not only is the main plant likely to be smaller and cheaper, it will also run closer to its optimum performance for more of the year, reducing energy consumption significantly. Surely such a modest compromise in comfort would be worth the financial and environmental savings that result?

There are also opportunities for energy savings at the other extreme of the design envelope as well. One of the most attractive, potentially, is free cooling. Most modern buildings have a significant demand for cooling (from people, lights and IT equipment), even when the ambient temperature is low. Weather data shows that for a significant proportion of the year we can reject heat directly to the outside air, with high system efficiency. In the past, such systems relied on additional water–air coils to provide the free cooling. However, such systems have a number of drawbacks, mainly that glycol must be added to the chilled-water circuit to prevent the system freezing at low ambient temperatures when it is idle. Glycol is expensive, and the resultant reduction in heat-transfer coefficient means that the fan-coil and AHU heat exchangers have to be increased in size to compensate.

New technology is on the way to improve the situation. For example, the availability of Carrier’s integrated refrigerant-based free-cooling system eliminates the drawbacks associated with traditional systems and makes cost-effective and efficient free cooling available from a standard packaged chiller.

All free-cooling systems rely on a temperature difference between chilled water and ambient. As the UK climate has a high concentration of average temperatures between 6 and 16°C, slight increases in system chilled-water temperatures can substantially increase in the possible hours of free cooling. Full integration of the free-cooling plant with the internal equipment (fan coils/air-handling units) and the overall building control system is therefore required.

Other, relatively simple changes can further improve the energy efficiency of systems.

Heat recovery

All the time a packaged air cooled chiller is running it is rejecting large quantities of heat to the surrounding air. It is highly probable that at the same time a gas or electric heater is providing hot water in the building for toilets and kitchens. Many chillers are available with heat-recovery exchangers that allow the heat previously rejected to atmosphere to be ‘recycled’ to satisfy all or some of this demand.

Further advances in technology mean that it is cost effective to specify variable-speed drives for key parts of the air-conditioning system. For large buildings, a centrifugal chiller with a variable-speed drive for its compressor can offer part load COPs in excess of 10 — a huge improvement over fixed-speed units. More generally, the use of variable-speed pumps for distributing chilled water pumps can yield tremendous savings — not least because these devices run continuously, whatever the cooling load.

Heat pumps

Finally, a long established energy-saving technology may finally come to into its own — heat pumps, first applied by Carrier in 1932! Combining heating and cooling plant using heat pumps can significantly reduce carbon-dioxide emissions, so maybe the time has arrived to shift the emphasis of building-services design. Heat pumps can provide aeffective and environmentally responsible heating, with the additional benefit of cooling when necessary.

Air conditioning has become a necessity at the beginning of the 21st century. This provides a challenge for manufacturers and specifiers alike, as the scale of use has implications for energy usage worldwide.

Leading manufacturers have taken up the challenge, and the innovations in energy consumption will have an enormous impact on consumption in the next few years. Consultants are increasingly designing systems to meet actual environmental conditions rather than meeting levels of demand that are almost never required. The potential energy savings can be huge.

Andrew Keogh is engineering manager with Carrier Air Conditioning, United Technologies House, Guildford Road, Leatherhead, Surrey KT22 9UT.

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