Energy use for most building-related end-uses is falling as average insulation levels and equipment efficiency improves – in terms of carbon emissions, these gains could be negated by growth in air conditioning energy use. Of course, the future is not necessarily a continuation of present trends. Here three possible futures for air conditioning are examined, and the potential impacts on the environment.
Possible futures
The number of possible futures that could be considered is large. A few easily definable, reference scenarios illustrate the range of possibilities and can act as benchmarks against which to compare the consequences of alternative sets of assumptions. These are:
- business as usual (continuation of historical trends)
- minimal air conditioning (avoiding air conditioning wherever possible)
- energy-efficiency regulation (introduction of assertive regulation)
Business as usual
Conceptually, the simplest future to describe is more of the same: market drivers stay much as they are; technology does not alter significantly; the area of cooled space develops along a classic S-curve, with first-time sales eventually declining as a level of market saturation is reached. In time, a replacement market builds, and is assumed to consist of like-for-like replacement.
It is assumed, rather arbitrarily, that the maximum possible market share is around 60%. It is difficult to see expectations for cooled space declining unless there were to be, for example, a major health scare. It has been said in the context of US offices, that once 20% of the office space in a city is air conditioned, this sets the rental value and uncooled space can only be let at a discount. With concern about – and perhaps evidence of – global warming, uncooled space may well look like an increasingly risky commercial option.
The business as usual model predicts that sales accelerate gradually from current levels, reaching a peak of over 2000 MW pa around 2010. By 2020, about 40% of commercial floorspace is air conditioned. The average compound growth rate of sales over the period is 6·5% pa, compared to around 16% pa for the period 1991 to 1996. (But 7% by value in 1997 though some analysts put it lower).
On a per capita basis, the projected figures are only around 15% of the current level for the US and around 7% of that for Japan. The implied growth of peak electrical demand by 2005 is around 9 GW – similar to the projected growth in winter peak demand.
On a business as usual scenario, individual system efficiencies do not improve. However, the average efficiency of all systems does change slightly because of the changing mix of central and packaged systems.The growth in carbon emissions associated with this expansion of electricity consumption depends on assumptions about the future fuel mix of the electricity generation system.
Figures have been used from Energy paper 65 and interpolated where necessary. These assume that the carbon intensity of power generation stays close to current levels for the next few years, before rising slightly as magnox nuclear stations are phased out and replaced by fossil fuelled generation.
From about 2005 to 2020 there are relatively small fluctuations in carbon intensity around the values typical of the mid 1990s – but tending to rise towards the end of the period. Because the assumed carbon intensity is higher in 2020 than now, the increase in carbon emissions is even higher than for electricity consumption, rising to over 9 MtC pa. Of course, this result is dependent on how the generation mix actually changes – for example, whether a significant proportion of renewable generation is added. The effect of changes in electricity demand – whether due to air conditioning or other mechanisms – on carbon emissions also depends on which types of power station are required to operate more frequently.
Minimal air conditioning scenario
The minimal air conditioning scenario is not considered likely in the short to medium term. Market drivers are such that it would be impractical to limit the use of cooling to this minimal level. Although acceptable comfort may be definable technically in terms of internal temperature frequencies, the view is taken (perhaps pessimistically) that the majority of the market – at least for the foreseeable future – will be driven by less rational expectations.
But theoretical analysis of typical offices suggests that it would be possible, technically and economically, to provide acceptable summer conditions in 77% of office space without using air conditioning. The remaining spaces are mainly deep-plan spaces for which technical solutions exist but are uneconomical. The existing penetration of air conditioning into offices already exceeds this level.
In this scenario it is assumed that:
- progressive removal of air conditioning from a proportion of existing buildings on refurbishment, until only 23% of offices are air conditioned
- pro rata reductions in other sectors
- no further increase in the proportion of new buildings that are air conditioned increased energy efficiency for all new and replacement systems
- the most demanding requirements of the minimal use scenario are introduced from 2001
Building services engineers have the power to influence the use of air conditioning.
Energy efficiency regulation scenario
In this scenario, it is assumed that the demand for cooled space is unchanged from the business as usual scenario and that regulatory energy efficiency measures are introduced over a period of time. It must be emphasised that this is an illustrative scenario, but one that is believed to be feasible. The crucial barrier to more energy efficient cooling is market pressure that puts first cost ahead of energy efficiency.
If all players in the market have to comply with (reasonable) requirements, there are no significant technical obstacles. Two regulatory mechanisms are assumed: mandatory energy efficiency performance levels for packaged equipment, and mandatory building-level requirements imposed through Building Regulations. Minimum standards are already in place in Canada, the USA, Mexico, the Philippines, Taipei and South Korea and are being considered in Australia, China, the EU, New Zealand and Thailand. Japan has voluntary standards.
Studies of the CEC have shown that many products on the European market would fail to meet these minimum standards. This scenario assumes a phased introduction of energy efficiency standards for packaged equipment. From 2001, new systems (including replacements) are15% more efficient than the average current system; from 2005 they are 25% more efficient and from 2010 they are 33% more efficient (in all cases relative to current systems). A system life of ten years is assumed.
A number of studies – and evidence from the US market – show that these levels of efficiency improvement are feasible. One recent UK paper suggested that 50% reductions could be realised by optimising system design, while a separate study concluded that 50% reductions are technically possible, but the economic optimum would be a 25% improvement.
Technically, there is certainly scope to improve the energy efficiency of central system air conditioning. Mandatory efficiency standards for chillers in central systems would have a similar impact to standards for packaged systems. More significantly, in UK conditions the main energy consuming component of all-air systems is the fan. Analysis has shown that, typically, 70% energy savings can be achieved with a five-year payback by the use of lower (but not low) design air velocities.
There are obviously space implications for larger ductwork, but these are not insuperable. This scenario assumes a phased introduction of such regulations. Specifically, it is assumed that from 2001 all new installations reach the typical performance of the EEBPp Energy Consumption guide ECON 19. It is assumed that, by eliminating worse than typical new installations, energy savings of 25% accrue. From 2005, more stringent requirements enforcing performance at ECON 19 good practice levels are introduced, resulting in savings of 33% from current levels.
Since fan energy use is largely determined by duct sizing and duct replacement is only likely in the context of major refurbishment, assuming a replacement period of 20 y. In consequence, the impact of these regulations in the next 20 y is almost all through new installations. Under this scenario, energy consumption by air-conditioning in 2020 is 35% lower than in the business as usual scenario – but is still more than two and a half times that in 1997.
In the earlier years of the scenario, most savings come from improvements to packaged plant (because of the more rapid stock turnover assumed), but by 2020, 70% of the savings are associated with central plant. The reduction in carbon emissions is also around 35% – equivalent to just over 3 MtC pa. The absolute values are subject to the same caveats that have been outlined under the business as usual scenario.
Feet on the ground: where next?
If current trends continue, energy use and the consequent carbon emissions associated with air-conditioning will increase substantially by 2020 – on the estimates a business as usual scenario would quadruple current levels. Regulatory measures could substantially reduce this growth – the energy efficiency regulation scenario reduces consumption and emissions by 35% in 2020.
It can be argued that more extreme limits on the use of air conditioning could reduce energy consumption below current levels, but it is believed that while these may be technically feasible, market demand for air conditioning would make them very difficult to implement.
Building services engineers have the power to influence the use of air conditioning. Questions that engineers can ask themselves include: can this building be designed to stay comfortable without mechanical cooling? For example is night cooling appropriate? If not: is it possible to limit or contain the heat gains to a level such that only local cooling, or one of the less familiar low-energy cooling systems can be used? For example is ground-water cooling feasible? If not, can a system and its design parameters be selected with energy use in mind? For example if an all-air system is used, keep the velocities down.
Successes – and shortcomings – need to be disseminated honestly and impartially. Professional institutions and governments both have roles to play here. CIBSE already provides tools through its Guides and Application manuals. The governments' Energy Efficiency Best Practice programme is a source of case study information, general guidance and – through its 'Design advice' activity – of subsidised strategic design advice.
Source
Building Sustainable Design
Reference
This article is based on a report: Local cooling, global warming? UK carbon emissions from air conditioning in the next two decades, by E R Hitchin CEng BSc MCIBSE MIGasE and C H Pout BSc D Phil of the Building Research Establishment which was delivered at the CIBSE 2000 Conference in Dublin. You can find a full copy of this at the CIBSE website: www.cibse.org
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