Dr Ian Knight and Gavin Dunn have been scrutinising the energy consumption of air conditioning systems in offices, with some interesting findings.

The European Directive on the energy performance of buildings, the promise of emissions trading schemes and fears about the security of energy supply. These are all strong drivers that are already changing the way we design our buildings. These factors all have a common theme: they place a much greater emphasis on designing low carbon emission buildings and services.

One area that is marked out for particular scrutiny by current and forthcoming legislation is the energy consumption, and hence carbon emissions, of air conditioning systems.

However there is very little information on actually how much energy air conditioning systems consume when used in the real world, particularly given the inherent imperfections in design and maintenance – factors that can significantly influence consumption.

In April 2000 the Welsh School of Architecture at Cardiff University undertook an independent study that monitored the energy consumption in over 30 air conditioning systems in real office buildings. Table 1 gives an overview of the type of systems which were monitored at 15 minute intervals over a three year period.

The aim of the study was to try to answer the following fundamental questions:

  • How carbon efficient are our current air conditioning systems when producing cooling in real buildings?
  • Are some air conditioning systems inherently more carbon efficient than others when used in real buildings?
  • By how much is it possible to improve the average carbon performance of air conditioning systems in UK offices using currently installed air conditioning technology?

Average weekday daily energy consumption profiles were taken over two consecutive summers in 2001 and 2002 for each of the generic systems. These profiles proved to be relatively consistent between systems, with each showing an early morning start up load and a peak afternoon load, though the chilled ceiling systems consumed substantially less energy per m2.

Work was done to normalise this consumption for the internal heat gains encountered per m2 in the offices monitored. The result is a dimensionless term we chose to call the Internal Load Performance Ratio. It was clearly shown that chilled ceiling type systems still consumed less than half the energy per m2 of the next most efficient system type. This finding is reinforced by the annual energy consumptions for the systems tested, as shown in figures 1 and 2.

Before everyone rushes out to buy chilled ceiling type systems there is still one area that is unaccounted for at present. This is the effect of fabric and solar gains on the load imposed on each system. We have the information needed to assess this load, and are searching for the funding necessary to complete the analysis. However, our belief is that even with these fabric and solar loads accounted for, the chilled ceiling systems will still hold a clear efficiency advantage over the other systems tested. This belief is supported by modelling results of seasonal air conditioning energy performance by other researchers.

The performance of the unitary heat pump type systems also cannot be confirmed by this study due to the sample only consisting of one system. This system is contradictory as it appeared to perform well during the summer period, but returned a relatively poor annual consumption.

One of the other important findings from the study is that control of the systems was patchy. The run-hours of many of the systems, shown in table 2, bore no relation to the times they were actually needed. On average most systems ran twice as long as the most conservative estimate of the occupancies served. The better systems from a carbon emissions viewpoint also tended to have good control. The data suggests that 50% energy and carbon savings appear feasible from effective time control alone.

Analysis of the part-load chiller energy consumption profiles also revealed that virtually all the systems were oversized for the loads they actually encountered in practice. Typically they had at least twice the peak capacity required during the study, since few of the systems ran for any length of time at more than half their rated maximum output.

Even the lowest energy consuming chilled ceiling systems were not immune from this problem, which leads to reduced system efficiency as well as increased capital and running costs. Although further work is required to establish by how much oversizing effects the efficiency and costs of the air conditioning systems one conclusion must be that current load estimation and plant sizing methods are over-estimating the cooling loads found in practice in UK offices.

This finding is widely accepted as being correct within the industry, and the reasons for it are not difficult to find. One of the more important reasons for offfices would appear to be the use of inappropriate design standards for internal heat gains.

Table 3 shows the range of the internal heat gains encountered in the offices studied per unit of treated floor area compared to composite good practice guidance and rules-of-thumb.

It appears that the current guidance figures only apply at the highest occupant density, as would be expected. However, the majority of offices surveyed were substantially lower than this figure. Indeed 40% of the sites studied had internal gains less than the lower current guidance estimates. This observation could therefore help explain part of the current oversizing occurring in the air conditioning systems.

Figure 3 presents the calculated total internal heat gains within the buildings studied, showing indicative maximum and minimum values against occupant density in floor area per person. This graph could be used to more accurately estimate internal heat gains with minimal extra information required. These findings are currently being considered for inclusion in the next revision to Part L of the Building Regulations.

The general findings show that:

  • Chilled ceiling and beam systems appear to be the most efficient way of providing cooling in UK offices where appropriate.
  • Most systems appear to be poorly controlled, and that 50% savings appear feasible from effective time control alone
  • Most systems appear to be sized to be twice as large as required by the loads actually encountered. Further savings are therefore possible from improved system sizing.
  • An improved method of internal heat gains estimation based on occupancy has been presented.

What are the implications of this study for future carbon emissions from air conditioning? Figure 4 shows predicted UK carbon emission from air conditioning up to the year 2020 using expected UK market growth and electricity carbon intensity estimates. Three possible scenarios of the way we currently use air conditioning and the consequences can be summarised as follows:

  • Scenario one – assumes no major change to the way we currently use air conditioning.
  • Scenario two – assumes all new systems from 2005 onwards are as efficient as the most efficient systems currently available.
  • Scenario three – assumes all air conditioning systems, both new and existing, are progressively replaced with the most efficient currently available.

From scenario three we can see that, if all air conditioning systems were able to perform to the level of the current best performers, ie chilled ceiling type systems, then air conditioning system carbon emissions could be reduced by 58% from 2000 levels, despite the projected use in air conditioning systems over this period. However, if air conditioning market growth continues past 2020 then further improvements in air conditioning technology may be required to maintain the carbon emissions at this level.

The overall conclusion must be that air conditioning systems used in UK offices have the potential for substantial improvements in system carbon emissions performance, and that these improvements can be accomplished through existing technology.

Domestic bliss?

Over the coming years the number of domestic properties in the UK with air conditioning is expected to increase significantly. The consequences of such a rise could be extremely detrimental in meeting CO2 reduction targets from buildings. To understand better the implications, the Bartlett School of Graduate Studies at University College London is to carry out research over a two year period into the use and efficiency of domestic air conditioning.

The primary objectives of the Engineering and Physical Sciences Research Council funded study are to establish the general profile and motivation for air conditioning use within the UK domestic sector; establish patterns of use for a range of systems including splits, portables, evaporative coolers and central systems; quantify typical current and annual energy consumption along with CO2 emissions and to test the validity of current domestic energy models for cooling, using the data collected. The management team is looking for project partners and organisations or individuals who own or operate domestic air conditioning. Contact Dr Alan Young on 020 7679 5905 or e-mail: alan.young@ucl.ac.uk