Air conditioning: Cold comfort
Air conditioning is responsible for the biggest slice of the energy bill in many buildings, which makes cost a central issue for these systems. However, they can also damage the environment, so it’s vital for building owners to take account of the energy efficiency of their AC, says Ian Vallely
Many of the numbers that surround air conditioning (AC) are disturbing, if not downright alarming. The government, for example, estimates that cooling in AC systems might account for around a tenth of total UK electricity consumption (in 2010, that equated to a whopping 29trn Watt hours of energy).
And, a report published last June by the Building Research Establishment, revealed that roughly 65% of offfice space and 30% of retail space in the UK had some form of AC in 2012 – a figure that is likely to be considerably higher today.
What’s more, air conditioned buildings use roughly twice as much energy as naturally ventilated ones, according to the Royal Institute of British Architects. However, the good news is that there are ways to mitigate the costs of operating an AC system, and they don’t need to be expensive. Ray Gluckman, director of Gluckman Consulting, has this advice if you are starting from scratch: “The first question you need to ask is: do I need air conditioning at all? If the answer is yes, then do you really need it all year round?”
Graham Wright, legislation and compliance manager at Daikin, adds: “Buy the most efficient equipment you can. Sometimes you have to compromise because of limitations within the building, but you should aim for the best.
“Next, you should make sure you maintain the equipment effectively… There is a legal requirement to have the system inspected every year to check that all the refrigerant gas remains contained in the system, and that filters and outdoor units are clean to work at their peak efficiency.”
Neil Hitching, sales manager of Toshiba Air Conditioning, agrees that maintenance should be a priority: “This involves ensuring the system is still operating within its parameters and has no faults, checking for leaks, and making sure that filters are clean (badly maintained filters can increase energy consumption by 20% to 25%).
“The period between cleaning will depend on the environment – a hotel has a lot more traffic in and out of rooms than, say, an office, so the system is working harder and cleaning will be [need to be] more regular. Outdoor units [such as air handling units and condensers] also need cleaning; here, the location is important. For example, the environment near an airport will be harsher, so they will have to be cleaned more often.”
Gluckman sees AC as a means to control a space at a comfortable operating temperature of around 20°C. However, for him, this temperature is only a guide: “I think there should be quite a wide temperature band between the need for heating (if the temperature is, say, below 18°C) and cooling (if it’s above, say, 25°C). You don’t always have to control the internal temperature at precisely 20°C.”
The Carbon Trust has produced an excellent interactive guide to energy efficiency in air conditioning systems (bit.ly/2u2Un8D), which offers recommendations to tackle poor energy efficiency:
Heat sources, which an AC system must neutralise, include the general ambient temperature outside, which can leak into the building through a ventilation system. Another is solar gain, especially inside buildings clad with a lot of glass (a well designed structure will have shading installed on south-facing windows).
A third common source of unwanted heat is lighting (although this is becoming less of a problem as traditional lighting is replaced with LEDs, which generate less heat).
Other heat sources include people; even just sitting at our desks, each of us produces heat equivalent to a 60W light bulb. Lastly, there is all the heat-emitting equipment in modern offices, including computers, printers and telephone exchanges.
There are essentially two types of air conditioning – water-chiller-based and direct expansion (DX). Which is installed will depend on the application. A big office building, for example, will generally employ a water chiller, which pumps cold water around the building to heat exchangers in ductwork associated with each room.
Water chillers have become far more efficient in recent years. However, the pumps that propel the water around the system also contribute to the heat load. Gluckman explains: “If you have a 10kW pump, where does that 10kW go? It goes into the water…
“You will pay for those pumps twice in energy terms, because you pay to run the pump itself and you create heat load, which has to be removed through cooling. That’s why controlling pumps with variable speed drives, for example, can be an effective energy-saving measure… particularly under part load conditions.”
This brings us to AC design. For Hitching, it’s vital to remember that no two structures are the same: “Even if there are two buildings next door to each other, the AC will be differently applied and used. Manufacturers publish nominal energy efficiencies for their equipment, but true efficiency depends on the design and times of usage, so it’s important, when looking at energy efficiency of the system, that you gain a true picture of how it is to be applied and the design parameters of the particular building.”
You should, he says, also look at how the system is operating: “For example, if it is not controlled by a central system, people can just leave it running all the time. A centralised controller will enable a responsible person to switch the AC system off or optimise its control with on/off timing and so on to save energy.”
Martin Fahey, head of sustainable solutions at Mitsubishi Electric, agrees: “You can have the most modern system possible, installed and commissioned to run in an incredibly energy efficient way. Without an effective control system, though, it could be wasting energy.
“Modern air conditioning controls offer advanced remote monitoring and reporting to help with energy scheduling, trend tracking and individual billing. They can also allow for both individual room control and centralised management, and can help plan maintenance and report faults while showing energy use throughout the life of equipment.”
AC design must also take account of pipe runs, because, as you add length to the pipe run, performance drops. Every manufacturer publishes a nominal duty based on pipework 5m in length and a common set of design conditions to allow comparison with competitor systems. However, these are not so useful in ‘real life’ situations.
Legislation concerned with energy use by AC
And, as Gluckman points out: “Air conditioning is definitely one of those technologies where people design for the worst condition – the hottest day with the building fully occupied. That might mean you need, say, 1,000kW of cooling. However, nobody instructs the designer to think about the fact that, for most of the year, the heat load might be as low as 200kW.
“So they should actually optimise the design, not necessarily for the peak. It has to deliver the peak, but it doesn’t have to be very efficient because it is a relatively rare occurrence. It is important to design for a range of operating conditions and that has a lot to do with control. You can’t control well if the design is poor.”
So, if there is a load of 1,000kW, you could buy one chiller that would deliver 1,000kW. However, a big chiller on part load is not efficient. So you could buy four 250 kW chillers, which is much more flexible and efficient.
The other category of air conditioning – DX – includes single and multi-split systems, and variable refrigerant flow (VRF). In DX systems, the refrigerant itself travels to the room being cooled, so there are direct expansion evaporators, either in each room or in ductwork where air is ducted to several rooms. DX AC is typically used in shops, supermarkets, small office buildings, hotels, restaurants, and so on.
Wright adds: “Most split systems will last for 10 years, and other more competent systems such as VRF normally last 15 years. You should have a plan for each of your systems in terms of the gas that is in it, how long it is going to last, and when you plan to change it out.”
A big innovation in recent years in DX systems is the use of variable speed compressors. Gluckman explains: “In a big VRF system – say, in a hotel – you could have 50 rooms each with a DX cooling unit in a duct, but they might all feed back to one outdoor unit.
“Variable speed drive (VSD) on the compressors allows for much better control. They also now tend to have VSDs on the fans in the room, partly to keep the noise to a minimum, but also for energy efficiency.”
Ian Vallely is from Beltring Communications Ltd