Summers in Australia are getting hotter. Does evaporative cooling remain a viable option for cooling Australian homes, or is modern refrigerative cooling the way to go?
Spoiler alert: fans of evaporative cooling may not like this article. If you have “evap” cooling in your home – and love it – that’s great, enjoy it! Some people praise the fresh air flow and the way the whole-house fan purges hot air out the windows. But for the modern all-electric Australian home, has evaporative cooling passed its use-by date?
Passive is best
Firstly, steps to passively keep cool in one’s home are the best: shading with vegetation, window coverings, insulation, draught sealing, appropriate ventilation, air movement, such as fans, dressing appropriately. All these should be front of mind as the weather warms up.
But across much of Australia, and with more people working from home, there will probably come the day when active cooling is needed.
Which aircon is best – evaporative cooling or refrigerative?
Two cooling technologies widely used in Australian homes are evaporative cooling and refrigerative cooling, such as a reverse-cycle airconditioner.
Evaporative cooling is a “once-through” cooling process. It involves passing water over pads located in a big box up on the roof. In that big box is also a big fan that blows “fresh” air past those water-laden pads and then into ducts in the roofspace. Humidified and therefore cooled air enters living spaces through ceiling vents. Windows and/or doors must be left open when the evap is running so that the sometimes damp-feeling air can extract heat from the house and then leave the house altogether.
On the other hand, refrigerative cooing is a “closed” process. With this, one keeps windows and doors closed and allows the airconditioner to collect heat into its circulating refrigerant fluid which, pumped by an electrically powered compressor, flows to the “outside unit” where heat is rejected outdoors.
Refrigeration devices are referred to as “heat pumps” because, in the example of space cooling, they pump heat from inside a house to outside. In this closed process, no air leaves the house. It is the rapidly circulating refrigerant that carries heat outdoors.
There are pros and cons when choosing between these two technologies. I’ll describe reasons why evaporative is fading in popularity.
Evaporative cooling might use less electricity – but how would we know?
The main advantage often quoted for evaporative cooling is that under the right conditions (which isn’t everywhere, nor at all times), it can use less electricity than refrigerative cooling.
Theoretically this is correct. Up on the roof, passing warm outside air over water-wetted pads evaporates that water. Evaporation causes cooling of the passing air, with no electricity needed other than to power the air circulation fan.
And so how does electricity use compare? Here are three different views.
Choice Magazine says that “ultimately the cost of evaporative cooling is comparable to [refrigerative] airconditioning”.
For some comparisons, Sustainability Victoria lists that evaporative cooling may use half as much electricity as refrigerative.
Other online sources are far more bullish and say evap might use only one seventh as much.
Which of these is right? Or is any of this right?
There is no way to be sure. Because to date in Australia, there is no standard way of assessing the energy use of evaporative cooling systems.
On the other hand, refrigerative airconditioning systems have been subject to an Australian-government regulated energy-rating scheme for many years.
Government websites scrutinise and rate refrigerative systems down to fractions of stars, and in a recent upgrade, specifically for the full range of Australian climate zones. Were one to buy a certain reverse-cycle airconditioner, the energy-rating website shows an estimate of how many kilowatt-hours of electrical energy the device would consume under standard test conditions, for an average home in a given climate zone, over the course of the cooling season.
Meanwhile, the energy performance of evaporative cooling slips through a crack. Imagine going to buy a car and finding some models carried fuel-consumption labels, but others didn’t have to.
Energy and water consumption rating systems for evaporative cooling are used overseas in countries such as Iran, but not in Australia.
Scouring the internet, we find nowhere is published the actual energy performance of an evaporative cooling system in an actual Australian home, nor with any detailed economic comparison versus refrigerative cooling.
What we do know is that the large air-circulation fans used with evaporative-cooling can draw 1500 watts or more of electrical power when running at full output. That is a lot of electricity and were that applied to a modern efficient reverse-cycle air conditioner, it would produce a lot of cooling.
The energy efficiency of refrigerative airconditioning has improved dramatically over the last decade, which makes some older comparisons with evaporative, that persist on the internet and in the minds of some practitioners, quite out of date.
So, if you are thinking evaporative might be a good choice because it will use less electricity, you may try to, as it’s said, “do your own research”. But sadly, you won’t get far.
Water cost can exceed the electricity cost
We might not know how much electricity evaporative cooling uses versus refrigerative, but one thing we do know is that evap uses a lot of water, whereas refrigerative uses none. How much water exactly? Again, who knows?
Sinks, toilets, showers, and gardens all use water, but on a hot summer day in one’s home, the biggest water-user may be the evaporative cooler. Though there is a government-regulated Water Efficiency Labelling and Standards (WELS) scheme for washing machines, dishwashers, showerheads, taps, and toilets, evaporative cooling again faces no scrutiny.
Over a 12-hour period during a heat wave, some large evaporative cooling systems might use as much as a tonne-and-a-half, or 1500 litres, of water, which includes the volume of water the system must dump down the drain to purge salts. Compare that with the whole-of-house target of 155 litres-per-day that appears on my water bill.
And how much does a tonne-and-a-half of water cost? In Melbourne these days, we pay about $4 per thousand litres including pro-rated sewage disposal charges. So, for that one hot day, even if one is convinced evap cooling doesn’t use much electricity, a household could be running up charges of $6 a day just for water.
The $4 cost for one thousand litres of water compares with the 80 cents paid 17 years ago (from an old water bill on file). With desalination, the world has changed when it comes to water prices – a five time increase! But have we updated our thinking when it comes to the economics of evaporative cooling?
An earlier article here at The Fifth Estate suggested that if one has to pay for metered water, one shouldn’t use evaporative cooling. With current water prices in some of our major cities, this view makes a lot of sense.
And what about the greenhouse-gas emissions associated with water use? If, as is now common, incremental water use comes from an electricity-intensive desalination plant running at the same time as coal is burned to support the power grid, using a tonne-and-half of water adds another six kilograms of carbon dioxide to our Earth’s atmosphere – for just 12 hours of cooling.
And don’t forget these other issues in cooling
Here are some other things to consider when comparing evaporative cooling versus refrigerative.
Evap cooling can’t satisfy under all weather conditions: Evaporative cooling can only reduce the temperature of the air being blown into one’s house by so much, depending on the outdoor weather conditions. High outside air temperature and/or high humidity can render evaporative cooling useless. This is why evap cooling is not recommended for places such as Darwin or Alice Springs and can even be considered inappropriate in Mildura (VIC) or Albury (NSW).
Even in climates such as Melbourne, evap cooling struggles to satisfy on humid days and over some hot nights when the relative humidity level increases and renders evap cooling ineffective.
Contrast this with refrigerative, heat pump cooling which, depending on the equipment’s design, can provide humidity reduction and/or cooling even on a 46 degree day, albeit with greater electricity use because on hot days, heat pumps must pump harder.
Evap cooling and smoke don’t mix: During the terrible bushfires of two summers ago, across eastern Australia “evaporative cooling could not be used because the supposedly” “fresh” air drawn into the house was polluted with smoke. Even during “controlled burns” or a neighbour’s outdoor barbeque lunch, evap cooling can bring smoke into a home. Or if a household burns wood in winter, one must remember to seasonally seal off the ceiling vents, so the home is not unduly polluted by woodsmoke.
A big fan on the roof can make a lot of noise: Thinking of backyard barbeques, our neighbour’s noisy evap cooler high up on their roof sometimes drove us back indoors. Thankfully they’ve recently modernised their house to refrigerative heating and cooling, the operation of which we are unable to hear.
Winter heat loss: Many people don’t realise that evaporative cooling may cost you more in winter than it does in summer. How? Often there are no air-tight seals or dampers preventing precious winter heat from rising through the ceiling vents and ducts and out through that box on the roof. If such vents penetrate your ceiling, seal them up seasonally, and airtight.
One Melbourne client told me they cut their winter heating bill in half simply by covering over the vents in their ceiling. This isn’t surprising because an evap cooler can act as a chimney and suck the air out of one’s house, especially on windy winter days, essentially making your heating also a once-through system.
Once-through evap cooling is inconsistent with draught-proof homes: Many of us are working to draught-proof our homes. This is important for good winter heating performance, for excluding air-borne pollutants and contaminants, and can also be very good for summer cooling with refrigerative airconditioning. Draught-proofing is of less value in a home with evaporative cooling where the windows will need to be opened anyway.
You can’t heat your house with evaporative cooling: A modern and efficient aircon can cost-effectively cool a home in summer and also can, by using the “reverse-cycle” mode, be the cheapest way to heat a home – far cheaper than heating with gas or resistive-electric panel heaters. The all-too-common combination of ducted gas heating and evaporative cooling can lock Australians into leaky houses, high energy bills, and large greenhouse gas emissions, until such time as a conversion to all-electric refrigerative heating/cooling can be done.
Avoid spaghetti in your roofspace: Heating and cooling ducts installed outside of the building’s thermal envelope, underfloor or in the roofspace, even if the ducts are somewhat insulated, are always a source of energy loss. Evaporative cooling means a home will have very large ducts in the roofspace that can inhibit access for other maintenance or upgrades, such as checking insulation. This duct “spaghetti” in the roofspace is something to avoid and becomes unmanageable should there also be gas heating ducts up there competing for space.
Get the most out of your rooftop-solar PV electricity generation: Often evaporative cooling takes up space on the roof better suited to solar PV panels, or the large box can shade solar PV panels. Homes with large solar PV systems may be less cost-concerned about the electricity used by any cooling system, and rather might focus on whether the evap cooling is driving up the water bill.
AND EVEN MORE CONCERNS WITH EVAPORATIVE COOLING
Water leaks: A client had an unexplainedly high water bill. It took some investigating to reveal a stuck valve within the evaporative cooling equipment up on the roof.
From a security perspective, some householders will be wary of leaving windows open when the evaporative cooler is operating.
Evaporative cooling is a ducted whole-of-house solution, meaning satisfactory individual room-by-room comfort control can be difficult to achieve.
Evaporative cooling equipment on the roof can trap burning embers and increase a home’s bushfire risk.
For asthmatics and others with breathing difficulties, evaporative cooling can produce inferior air quality.
In conclusion, for our future in many regions across Australia, the modern all-electric home is likely to feature refrigerative heat pumps for space cooling and heating. Evaporative cooling, like a petrol car or landline phone, will make less and less sense.
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