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Portable evaporative air conditioning/cooling

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Main sections on this page...
Principle
Energy-efficient and cost-effective
Calculate the efficiency of your cooler
Effectiveness
What temperature should a cooler achieve?
Ice?
Simple air cooler
An acceptable air cooler
Specification
Advice
Article in Choice
New developments
Solar powered evaporative air cooling
Links
Index
 
Written 2008/01/16, modified 2017/01/08
Contact: email daveclarkecb@yahoo.com

Introduction

Evaporative air conditioners or coolers (EACs) use much less energy (electricity) than do the other most common form of air conditioners, refrigerated. However, there are many very poorly designed EAC units on the market. The aim of this page is to help you inform yourself about the advantages, disadvantages, principles and pitfalls of evaporative air conditioning.

The fact that there seem to be many poorly designed EACs on the market should concern everyone who cares about the environment. People who buy the poor units may well come to the conclusion that EACs, as a class, are ineffective and then buy the much more energy hungry refrigerated air conditioners, while had they bought a well designed unit in the first place they would have what they need and would save energy, money and unnecessary greenhouse gas production. The problem is often in the design of the individual unit, not in the evaporative cooling principle.

Any form of air conditioning is an active way of cooling your home. It is generally best to explore all possible ways of passively keeping your home cool before going to active methods. Another of my pages on this site discusses other ways of reducing your greenhouse impact.

The key point to remember about EAC is that the cooling effect is directly related to the amount of water that is evaporated per unit of time (per minute or per hour). The more water evaporated the more cooling effect.



 

The IXL Icecap 2 Portable Evaporative Cooler

IXL Icecap 2
This appliance is of such a poor design that it moved me to write this page. Only once previously have I been so disappointed with the poor design or operation of anything as to write an Internet page on it.

I bought one of these because I wanted to cool a small building (that was already set up with as much insulation and passive cooling as I could arrange).

I found that the air blown out by the air conditioner was only one and a half degrees cooler than the air in the room; the cooling effect is proportional to the amount of water consumed. I measured that it consumed around 250mL of water per hour, even when the air was fairly dry - I can sweat more than that! I used a hygrometer to measure the relative humidity in the room (remember that EACs cool the air by making it damper). I did not detect any increase in humidity over a period of about three hours of 'evaporative cooler' operation.

I hardly need say that I also did not record any drop in the temperature inside the room due to running this EAC.

The efficiency of this air cooler was probably about 10%. Also see the rating achieved by this cooler in the article in Choice.

Evaporative air conditioning principle

Evaporative air conditioners (EACs) work by taking heat from the air and using it to convert water to water vapour. The evaporation of water absorbs a huge amount of heat; it takes about five times as much heat to evaporate a given amount of water as it does to raise the same amount of water from freezing point to boiling point.

The exceptionally high 'latent heat of evaporation' of water is one of the most remarkable properties of this most remarkable of substances.

The reverse process, condensation of water from vapour, is what powers hurricanes.

EACs cool the air at the cost of increasing the humidity. If you are to use them effectively you must manage the humidity in the room or building by bringing in some amount of dry air from outside. I would advise using an evaporative air conditioner in conjunction with a hygrometer (an instrument that measures air humidity).

It follows from the above that if the air is already humid an evaporative air conditioner will not be of much use. They are at their best where the air is hot and dry. They may not be the best choice in many coastal areas of Australia.

Even a simple fan is, to some extent, an evaporative air conditioner. It makes you more comfortable by providing a moving air stream in which your sweat evaporates quickly and with minimal discomfort.

The numbers

It takes one calorie of heat to raise the temperature of one gram of water by one degree Celsius.
It takes about 80 calories of heat to melt one gram of ice.
It takes about 600 calories of heat to evaporate one gram of water.

Note the the Calorie (capital C) used in nutrition science is equal to 1000 calories (small c). The Calorie is also known as a killocalorie.
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Energy-efficient and cost-effective

Because there is little in an evaporative air conditioner to consume electricity other than a fan they can be very energy-efficient. The power consumption can be as little as 60 Watts. Also because of their very simple design they can be highly cost-effective. There is much less machinery in an evaporative air conditioner than in a refrigerated air conditioner (sometimes call a reverse-cycle air conditioner or an inverter). The simple design and few moving parts of the evaporative cooler also means that there is little that can go wrong with them. (I have one that is still useable, with some modification, after 30 years of use.)

In these days of climate change it is important that we all try to minimise our greenhouse gas production. Using EAC rather than refrigerated air conditioning can help with this.

However, some EACs are very poorly designed; see the IXL model in the box at the right.

There are several types of evaporative coolers including some more expensive models that are capable of producing cooler and drier air than can be achieved by the more simple EACs. More is given on these at the Home Energy page (see links below.)




Calculate the efficiency of your cooler

You can use degrees Fahrenheit or Celsius, so long as you are consistent.

Note that you will have to allow active content for this calculator to work.
Enter values in the top three boxes and click on the 'Calculate' button
Temperature of air entering the cooler = degrees (To)
Temperature of air blowing out of cooler = degrees (Te)
Temperature of water in sump = degrees (Ts)
Efficiency of your cooler = %
Example calculation: To=30, Te=20, Ts=15; answer will be 66.66%
Equation
E%=(To-Te)/(To-Ts)*100
Symbols
E% = Efficiency of air cooler as a percentage
To = Temperature of outside air (ie. air entering air cooler)
Te = Temperature of expelled air (ie. air blowing out of air cooler)
Ts = Temperature of water in sump
The sump is the water reservoir at the bottom of the cooler.

Notes on this calculation

This calculation gives an approximate, but very useful value for the efficiency of the air cooler. One of the advantages of this calculation is that you do not need to know the relative humidity of the air.

What this calculation does not tell you is the capacity of the air conditioner. That is, the 'cooling power' of the air cooler will be equal to its efficiency multiplied by the volume of air passing through it per unit time.

I'd suggest that if the efficiency of your air cooler is less than 50% you should seriously consider demanding your money back. A good cooler should be capable of achieving efficiencies of 70% or better.

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Evaporative air conditioning can be very effective

My wife and I have ducted evaporative air conditioning in our house. This can reduce the temperature of the air it takes in from outside the house by 10 to 15 degrees Celsius or even more. (On a day of 44 degrees it can keep the temperature inside the house down to no more than 28 degrees.)

Sweating is another example of very effective evaporative cooling. Mammals would not be able to live in anywhere near the range of habitats that they do if the latent heat of evaporation of water was not as high as it is.




Where not to use EAC

Evaporative air conditioners will not be effective where the humidity is generally high. This makes them unsuitable for much of the Earth's wet tropical areas.
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What temperature should a cooler achieve?

The table below shows the temperatures that should be achieved by a reasonably efficient air cooler for given temperatures and humidities of the air drawn in by the cooler. All temperatures are degrees Celsius.

Humidity
T
e
m
p
e
r
a
t
u
r
e
 2%5%1015202530354045 50556065707580
24°C12°C1314141516171718 1819192021212222
27°C14141617171819192021 22223223242425
291617171819202122222323 2424252627
321818192121222324252626 2728282930
351920212223242526272828 2931
382122232426272828293031
41222325262729303132
432425272829313233
4626272830323334
49272830323435
522830323436

The above data is calculated from Fahrenheit data on a table on the Home-energy site; see the link below.

Find the temperature of the feed air (the air going into the cooler) on the column of dark figures on the left and the humidity of the feed air in the row of dark figures along the top. The temperature that the cooler should be able to achieve is at the junction of the row and column.

For example, if the temperature of the air being drawn into the cooler is 32 degrees and its humidity is 30% then look at the row starting with 32 and the column coming down from 30%. The number at the junction of row and column is 23. So, this is the temperature (in degrees Celsius) of the air coming out of your cooler, if it is reasonably efficient.






To use ice to cool a room is energy-inefficient

Some portable EACs come with a built in pockets for ice and small ice containers that you can put in your freezer.

To use ice to cool a room is, in most cases, a bad idea. Making the ice in a freezer releases more heat than melting the ice takes from the room; and you consume electricity into the bargain.

Buying enough ice to cool a room or a home would be very expensive.

The amount of heat absorbed by converting each gram of ice into water is far less than the amount of heat absorbed by converting a gram of water into water vapour. Evaporating a gram of water takes something like ten times as much heat as melting a gram of ice.

The only situation in which I can imagine using ice to cool a room would be acceptable and wise is the price-responsive-load electricity case. Here the ice could be made using sustainable electricity at a time when power was plentiful and cheap, so that it could be used later to cool the room, when the electricity was in high demand and expensive.

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Simple air cooler

It is possible to turn a fan into a very simple air cooler by placing it so that it blows air over a bowl of water; the bigger the bowl the better, and make sure that the air flows right over the surface of the water.

Even better would be if you could arrange a tunnel for the air to blow through using the Coolgardie Cooler principle. Don't have too high expectations, I have experimented with the idea and was disappointed.

What I did was to place a fold-up chair on a larger table. Then I put a large tray (I used a cat-litter tray, about 50 x 30 x 5cm) of water on the chair and a bath towel so that it was in the tray and hung over each side. Then I put another large tray on the table so that the excess water from the towel could drain into it. The fan was placed so as to blow through the tunnel I'd made and I kept the top tray at least partly filled with water. Water was absorbed by the towel (capillary action) and flowed slowly down into the bottom tray, with some evaporating on the way. The more water you can get to evaporate the more effective your cooler.






An acceptable air cooler

 
Measured power consumption
SettingWatts
Low37
Medium41
High46
After the very disapointing experience with the IXL cooler described above, I bought a Convair Magicool. Using the calculator on this page I found its evaporative efficiency to be 71% (the figure given in Choice was 79%). This cooler typically uses around a litre of water an hour in the relatively dry climate where I live.

I have now used it in three summers in my small (insulated) shack. On the hottest days it does not sufficiently cool the whole room for comfort, but when it was directed on the people in the room, they will be comfortable.

From the rate of evaporation given above and figures in Energy Units we can calculate that this cooler cools a room at about the same rate that a simple 750 Watt heater would warm the same room. Put another way, the cooling effect (750W) is about 18 times the power consumption (41W).






Specification

How do you know whether any particular EAC that you see in a store will cool your room effectively?

An authority (eg. government) should mandate the printing of the efficiency of a cooler on the packaging of all coolers. An authority could also enforce the conspicuous printing of the evaporative capacity of all EACs on their packaging. It could be given as the amount of water that the unit will evaporate at a temperature of, say, 28 degrees Celsius and a relative humidity of, say, 45%.

The cooling effect of any cooler is directly related to the amount of water that the conditioner evaporates each hour. This will depend on the temperature and humidity of the air as well as the efficiency of the cooler.

This would give a prospective buyer a good idea of how effective one EAC was compared to another.

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Advice

If an evaporative air conditioner is to effectively cool a fair sized room it must be capable of evaporating at the very least a half a litre of water per hour. If you buy one that can evaporate less than that, demand your money back (like I did); either buy a better air cooler or save your money and buy a fan.

If you cannot be sure that any unit you buy will be effective and efficient, make sure that the retailer will take it back if you are unsatisfied.

This page mainly refers to small, portable air coolers. Of course a ducted evaporative cooler large enough to cool a whole house should evaporate much more water and will consume more electricity (but will consume much less electricity than a refrigerated air conditioner).






Article in Choice, Nov. 2008

Ratings in Choice
Make & modelRatingPower
(W)
Effic.%Price
Convair Magicool 004006725879$259
Convair Mastercool 850238-C726681$369
Convair Megacool 009438696672$399
Convair Coolmaster 30006420643$449
IXL Blizzard 420565523840$265
IXL Compact 42055415639$159
IXL Icecap 2, 42054205610$169
All ratings are out of a possible 100
The journal of the Australian Consumers Association (ACA), Choice, ran an article on evaporative air conditioners in November 2008. It covered models widely available in Australia: these were only Convair and IXL machines.

The recomended models of EACs in the article were:

  • Convair Magicool 004006 (58 Watts)
  • Convair Mastercool 850238-C (66 Watts)
These both achieved a score of 72 out of a possible 100. The IXL Icecap 2, described above, achieved a score of only 20. All IXL models achieved a rating below all Convair models.

The 'Effic.%' column is the 'Evaporative efficiency score' given in the Choice article. I measured an evaporate efficiency of 71% for my Convair Magicool; the Choice article did not state how they calculated evaporative efficiency.

Unfortunately the Choice article must be held in some doubt because there are indications that the author did not fully understand the principle under which EACs operate. In the 'How we tested' section Evaporative Efficiency is defined as "High evaporation efficiency means the cooler gets a better cooling effect for the water it uses." And in the 'What to look for' section it is suggested that buyers should look for units that have "Low water consumption rates"; exactly the opposite is the case.

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Altered 2009/10/14

New developments

Solar powered evaporative air cooling
Reported on ABC Radio National 2009/01/30

Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) researcher Steven White, of the Solar Cooling Research Unit, has developed a new concept in evaporative air cooling. The system will allow solar energy to be economically used to assist evaporative air cooling in areas with high humidity.

The system uses a desiccant - such as silica gel - to take the humidity out of the air so that the, then dry, air can be used in a conventional evaporative air cooler. A solar panel very similar (or identical) to those used for solar water heating collects heat from sunshine. This heat is then used to drive the moisture out of the desiccant so that it can be recycled and remove more moisture from the air that is to go into the air cooler.


I believe the use of a desicant for this purpose is not new; it has been used by other researchers and I have been informed that it is used in at least one commercially available air cooling system. The combination of desicant and solar heating may be new.





Links

Wikipedia, 'Evaporative cooler'

Home Energy Magazine Online has an informative page on Installing and Maintaining Evaporative Coolers. It gives information on much more than installation and maintenance.






Index

On this page...
Advice
An acceptable air cooler
Article in Choice
Calculate the efficiency of your cooler
Effectiveness
Energy-efficient and cost-effective
Ice?
Introduction
IXL Icecap 2
Links
New developments
Principle
Simple air cooler
Solar powered evaporative air cooling
Specification
Top
What temperature should a cooler achieve?
Where not to use EAC
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