While many engineers may seek to design out air conditioning there are still many buildings where this remains the only option for maintaining comfort levels. So the important thing is to ensure that it's efficient, says Dean Ward.
For some years now, air conditioning has been seen as the bête noire of energy efficiency in buildings, and building services engineers have been encouraged to avoid mechanical cooling wherever possible. The fact is, though, that high internal heat gains from office equipment and increased staff density means that natural ventilation simply won't maintain comfort levels in most commercial premises.

Clearly, this presents a dilemma for those who want to increase energy efficiency in buildings and reduce carbon emissions. And the air conditioning industry has been working hard to develop newer technologies to help achieve acceptable comfort levels with minimum energy consumption. Building services specifiers, therefore, need to be aware of the technologies that are now available and how they can be combined with well-designed systems for maximum energy benefits.

Leveraging benefits

In this respect it's important to stress that the design of the system is essential in taking full advantage of new technologies, so that the two work in harmony. For example, a chiller may be designed to facilitate free cooling, but it is the design and control of the system that leverage the potential benefits and turn them into reality.

So, for instance, there are now chiller designs that enable some free cooling at ambient temperatures as high as 15 deg C, giving a greatly improved ESEER (European Seasonal Energy Efficiency Ratio). In fact, where chillers are operating 24 hours a day, the energy savings through free cooling can be as high as 35 per cent over the course of the year.

Here, the set point temperature in the space makes a significant difference to the number of hours that free cooling can be exploited through the year. Thus, raising the set point in the conditioned space by as little as 1 deg C, which is unlikely to be noticed by occupants, will greatly increase the hours of free cooling available.

Similarly, raising the flow temperature of the chilled water facilitates free cooling and though higher volumes of air will be required to achieve set point temperature, the savings on cooling in a properly configured system will more than compensate for the extra fan power required. And the higher return water temperatures may enable heat recovery for use elsewhere in the building - perhaps to increase the efficiency of an air-source heat pump.

Another relatively new technology that building services engineers can use to good effect is the Turbocor centrifugal compressor, which is now being used in some air-cooled and water-cooled chillers. This compressor is variable-speed-controlled and highly responsive to variations in cooling demand, so that the ESEER can be greatly improved - an ESEER of up to 9.0 has been demonstrated during field tests.

In addition, Turbocor compressors use magnetic bearings, so there is no physical contact between moving parts, which eliminates wear and tear and reduces noise and vibration. This characteristic greatly reduces maintenance requirements and overall life cycle costs.

In other circumstances, scroll compressors may be the preferred choice as they also offer efficient part-load performance. In the past, scroll compressor chillers have been restricted to relatively small projects but multiple scroll compressor chillers are now able to offer cooling capacities as high as 1MW, with very precise step adjustment.

Facilitate free cooling

Alternatively, semi-hermetic screw compressor chillers may be the most appropriate solution for a particular project and, again, some of these are able to facilitate free cooling and efficient operation at part-loads. So, as noted earlier, the key thing is to choose the compressor technology that is best suited to the particular requirements of the project.

Looking beyond the compressors, there are other recent developments that can further contribute to the overall efficiency of the system, so it really is important to take a holistic view. A case in point is the efficiency of heat exchange in the chiller and there are now new micro-channel condensing coils that use parallel flows to increase heat exchange performance by up to 35 per cent compared to traditional coils. So if these are combined with efficient compressors, such as Turbocor, the overall efficiency of the system will be greatly improved.

Of course, there are many other considerations, such as dc or ec fan motors, control options and energy-saving defrost cycles - all of which serve to improve overall efficiency and reduce carbon emissions. And, with modular chillers, it's often possible to 'mix and match' key features in a tailored solution that gives the best performance within any constraints that may apply, such as budgets.

However, if you're unaware of all of the options that are available you may well miss out on some of these opportunities to add value for the end client while also minimising environmental impact. Keeping abreast of developments, and working with suppliers that are able to offer a wide range of technologies backed by specialist expertise is the best way to deliver a system that meets all the criteria.