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.
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.
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% | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45
| 50 | 55 | 60 | 65 | 70 | 75 | 80
|
| 24°C | 12°C | 13 | 14 | 14 | 15 | 16 | 17 | 17 | 18
| 18 | 19 | 19 | 20 | 21 | 21 | 22 | 22
|
| 27°C | 14 | 14 | 16 | 17 | 17 | 18 | 19 | 19 | 20 | 21
| 22 | 22 | 32 | 23 | 24 | 24 | 25
|
| 29 | 16 | 17 | 17 | 18 | 19 | 20 | 21 | 22 | 22 | 23 | 23
| 24 | 24 | 25 | 26 | 27
|
| 32 | 18 | 18 | 19 | 21 | 21 | 22 | 23 | 24 | 25 | 26 | 26
| 27 | 28 | 28 | 29 | 30
|
| 35 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 28
| 29 | 31
|
| 38 | 21 | 22 | 23 | 24 | 26 | 27 | 28 | 28 | 29 | 30 | 31
|
| 41 | 22 | 23 | 25 | 26 | 27 | 29 | 30 | 31 | 32
|
| 43 | 24 | 25 | 27 | 28 | 29 | 31 | 32 | 33
|
| 46 | 26 | 27 | 28 | 30 | 32 | 33 | 34
|
| 49 | 27 | 28 | 30 | 32 | 34 | 35
|
| 52 | 28 | 30 | 32 | 34 | 36
|
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.
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.
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.
|
|
| Measured power consumption
|
|---|
| Setting | Watts
|
|---|
| Low | 37
| | Medium | 41
| | High | 46
| |
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).
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.
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).
| Ratings in Choice
|
|---|
| Make & model | Rating | Power
(W) | Effic.% | Price
|
|---|
| Convair Magicool 004006 | 72 | 58 | 79 | $259
|
| Convair Mastercool 850238-C | 72 | 66 | 81 | $369
|
| Convair Megacool 009438 | 69 | 66 | 72 | $399
|
| Convair Coolmaster 3000 | 64 | 206 | 43 | $449
|
| IXL Blizzard 42056 | 55 | 238 | 40 | $265
|
| IXL Compact 42055 | 41 | 56 | 39 | $159
|
| IXL Icecap 2, 42054 | 20 | 56 | 10 | $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.
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.
