The amount of energy used for cooling buildings has doubled worldwide since 2000, making it the fastest growing end-use for energy in buildings.
So with the planet warming, every effort must be made to keep cool without increasing climate change.
In Part 1 of this series, The Fifth Estate explored passive (no energy needed) ways of cooling, depending on the climate.
Part 2 investigates the active ways of cooling. By definition, these cost money to operate so first apply all the possible passive techniques listed in the previous article.
Electric fans are a common form of cooling, ranging from portable table and pedestal types to window, box and ceiling fans.
If you have a lightweight building in a warm temperate climate, fans in all living rooms can certainly reduce effective cooling energy use. They typically use less than 10 per cent of the energy of a packaged or split-system air conditioner for an equivalent space. But they aren’t as effective because all they do is stir the air around, cooling you with a draught rather than the surrounding space, and they are the least effective option in times of extreme heat.
In general, DC fans and smaller fans are more efficient than AC fans and larger fans. Always check the power consumption as the cost will soon mount up. Do this by dividing the airflow rating by the wattage (stated on the energy label) for each fan you are comparing: the higher the number the more efficient it is.
A few other points about choosing fans:
- Some fans can turn back and forth, so cooling a wider area
- Ducted systems with no visible blades are generally more expensive
- Check how noisy a fan is before purchasing
- If you’re choosing a ceiling fan, its diameter should be around a quarter to a third of the smallest room dimension – for example, the room width is four metres, the fan blade should be at least one metre
- Most ceiling fans are reversible to drive down warm air from the ceiling in winter
- Fans are more effective in lower humidity environments, as body sweat can evaporate faster, providing a better cooling effect
Most airconditioners used in homes are based on vapour compression refrigeration or “reverse cycle” technology, the same as a fridge, except it chucks the heat outside. It’s basically an air source heat pump.
This means you can reverse it in the winter, to heat the home, although it will not be as efficient as a ground source heat pump.
They also do a certain amount of work to dehumidify the house – if it’s airtight (it’s worth mentioning here that if the space you’re conditioning is not airtight, there is no point in using an airconditioner).
Some units let you adjust the level of humidity control; others dehumidify as a by-product, so consider, for your own local conditions, how much control you want over this function.
You can even use the water that condenses to water your plants.
All air conditioners come with an Energy Efficiency Rating (EER). You can use this to compare the models. An EER of 5 would mean that it will take 1 kW of electrical power to move 5 kW of heat.
The energy rating codes for different cooling technologies in Australia are explained on the government’s energy rating website.
Airconditioner labels are a little bit different to labels for other household products and have some product specific information on them. Airconditioners can currently be rated up to 10 stars, and the more stars the better.
The labels also show the capacity output and the power input. If two products have the same star rating and same capacity output, choose the one with the lower power input.
The label also shows whether the unit has a variable output compressor, also known as an “inverter” airconditioner. These adjustable units with variable speed fans will be able to accommodate a wider range of operating conditions more efficiently.
Even better is if the unit can cooperate with a voluntary peak electricity demand management program, which will reduce the chances of a power cut when everybody is using power at the same time.
But bear this in mind: all air conditioning units and air source heat pumps are tested under specific conditions, which are rarely the same as the conditions under which they frequently operate, which means that in practice they can be much less efficient.
The bigger the difference between the temperature of the air in the intake space compared to the temperature of the air at the output space, the more work it has to do, and the less efficient it becomes.
In the test conditions for airconditioners, the temperatures are intake: 27°C dry bulb (19 °C wet bulb), and output: 35°C (24 °C wet bulb), which is only an 8° dry (5°C wet) difference. You can therefore make up your mind by comparing this to your local operating conditions how efficient a given unit would be in practice.
While we’re on this subject, the system will be more efficient at cooling if the outdoor unit is sited in the coolest, shadiest place possible.
However, the opposite will be true in the winter if you are using it for heating. So if possible find a space that is shaded (perhaps by a tree) in summer but receives low angled sun in winter.
As with fans, check how noisy and the relative power consumptions they have before purchasing.
Make sure you choose a solution that is appropriate to the volume of space you are treating, its aspect, the typical indoor temperature, and the amount of insulation and other forms of passive cooling you have; for this you may need specialist advice and be sure to get at least three estimates. It is common for systems to be over specified and therefore use a needless amount of energy.
A rough idea can be gleaned from a ‘Fair Air’ calculator provided by the Australian Institute of Refrigeration, Air conditioning and Heating.
But there is another kind of air conditioner – evaporative cooling. This doesn’t need an energy guzzling compressor or condenser, and so can use less energy.
However, it does need a climate that’s hot and dry and won’t work in areas of high humidity.
It works by evaporating water – a PCM (see above). But they can require a large amount of water and therefore should not be used if your supply is metered and you live in an area of water scarcity.
“Indirect” systems are available which are more efficient then direct systems; these cool the incoming air and then let it absorb heat from a separate air stream, which then flows inside without the humidity of a direct system.
Perth-based Phase Change Products have a High Efficiency Thermal Air Conditioning (HETAC) system regulated by phase change materials (could be water) in a storage tank connected to an airconditioner, which they claim runs on half the energy of conventional air conditioning.
The airconditioners on the market in Australia are among the least efficient in the world; the best available airconditioning equipment is up to five times more efficient than the least efficient ones currently available, so it really pays to shop around. Don’t necessarily buy the cheapest model, but look at the overall cost over the time of its operation.
You can also compare the models online.
The energy efficiency of some appliances can depend on their location, usage patterns and climate, so the EEAT is looking at a zone-based energy efficiency labelling system but it’s not ready yet.
The proposed new ratings would provide star ratings and energy use figures for three zones—hot (Brisbane and Darwin), average (Adelaide, Sydney and Perth) and cold (Canberra, Hobart, Melbourne and New Zealand).
If it’s hot and humid many people reach for the chiller/dehumidifier, but these beasts are terribly inefficient. This is because to make moisture condensed out of the air something needs to be cooled to below the dewpoint.
If humidity control is separated from temperature control, and more efficient fans and pumps used in the system, this can save up to 40 per cent of the energy. So this means you should choose a desiccant humidity control technique and sensible heat should be removed with a high-temperature cooling source.
Dehumidification on its own without lowering ambient indoor temperatures can often improve comfort in buildings at a far lower cost than using ACs for dehumidification.
Splitting a building into different heating and cooling zones means you don’t have to cool all of it if it’s not being used. Room systems (mini-split airconditioners) or variable volume controls, dampers and valves are used to achieve this.
In areas that require winter heating and summer cooling, heat pumps are ideal for new builds and major retrofits – all you do is reverse them as the season switches over. They use similar technology to refrigerators and can triple the effectiveness of the energy they consume.
Ground source heat pumps work best for this purpose. They are even more cost-effective if you’ve hired an excavator to dig a hole for another reason, like landscaping – use the opportunity to bury the pipes.
Properly integrated heat pump-connected systems can provide heating and cooling, support zoning and integrate with renewable electricity.
Several Australian suppliers exist and they can provide three benefits:
- In cooling mode, the heat sucked out of the room can be diverted to produce hot water that can be stored for when it is needed.
- In heating mode, the cold generated can be used to chill water, again to be stored for when it is needed.
- They can be operated at the optimal time to maximise the efficiency of the system and avoid operation at very low or very high outside temperatures.
Using the sun’s heat to power cooling is a great idea, because when you need it most it’s always there. Many types of solar airconditioning technologies exist, but only a few are available in the domestic market.
There is much work going on to develop solutions for the Australian market.
Some examples include:
- Adsorption and absorption chillers that use heat (from solar thermal collectors or waste heat from other processes, for example), mostly found in commercial buildings.
- Solar desiccant cooling – perfect for areas of high humidity since it uses a desiccant material to absorb moisture from the air. Solar heat is then used to dry the desiccant material in a continuous process. The CSIRO has successfully deployed this technology commercially.
- Solar cooling using a thermally driven compression process, but it’s still in the development stage, with some interesting research going on at the Australian National University.
- Photovoltaic (PV) panels directly powering the compressor of conventional electric vapour compression cycle air conditioners (but this isn’t always as great an idea as it seems, since peak cooling demand load is often in the evening: a battery is required here).
- Solar thermal collectors supplying heat directly to the refrigeration cycle of conventional electric vapour compression cycle air conditioners. Most solar cooling systems make use of flat plate or evacuated tube collectors.
The future of cooling
Watch out for the coming cooling revolution aimed at reducing the energy demand for cooling substantially. New tech will include:
- vapour compression heat pumps with ground, air or water as the source
- heat activated and integrated heat pumps
- adsorption heat pump
- thermos-elastic or membrane cooling systems.
And finally, here’s one to lobby your local government about. District cooling networks supply chilled water produced in a central plant through a network of insulated pipes.
Qatar has the world’s largest district-cooling system. Paris boasts two district cooling systems, and holistic urban planning and district cooling is being piloted in Tokyo, Hong Kong and Singapore.
It enables multiple energy efficiency and renewable energy opportunities at system level. While not necessarily more efficient than high-efficiency stand-alone air conditioners, if powered by renewable energy and using thermal storage it can be highly efficient and cost-effective.
Multiple benefits include increased usable area in the building, less noise and less maintenance.