Monday, January 08, 2007


Recently there has been some really hot weather in Melbourne that made me search for alternate methods of cooling.

The first and easiest method I discovered is to keep a 2L bottle of water in my car. After it's been parked in the sun on a hot day I pour the water over the windows. The energy required to evaporate water is 2500 Joules per gram, this means that the 500ml that probably evaporates from my car (I guess that 1.5L is split on the ground) would remove 1.25MJ of energy.from my car - this makes a significant difference to the effectiveness of the air-conditioning (the glass windows being the largest hot mass that can easily conduct heat into the cabin).

It would be good if car designers could incorporate this feature. Every car has a system to spray water on the wind-screen to wash it, if that could be activated without the wipers then it would cool the car significantly. Hatch-back cars have the same on the rear window, and it would not be difficult at the design stage to implement the same for the side windows too.

The next thing I have experimented with is storing some ice in a room that can't be reached by my home air-conditioning system. Melting ice absorbes 333 Joules per gram. An adult who is not doing any physical activity will produce about 100W of heat, that is 360KJ per hour. Melting a kilo of ice will abrorb 333KJ per hour, if the amount of energy absorbed when the melt-water approaches room temperature is factored in then a kilo of ice comes close to absorbing the heat energy of an adult at rest. Therefore 10Kg of ice stored in your bedroom will prevent you from heating it by your body heat during the course of a night.

In some quick testing I found that 10Kg of ice in three medium sized containers would make a small room up to two degrees cooler than the rest of the house. The ice buckets also have water condense on them. In a future experiement I will measure the amount of condensation and try and estimate the decrease in the humidity. Lower humidity makes a room feel cooler as sweat will evaporate more easily. Ice costs me $3 per 5Kg bag, so for $6 I can make a hot night significantly more bearable. In a typical year there are about 20 unbearably hot nights in Melbourne. So for $120 I can make one room cooler on the worst days of summer
without the annoying noise of an air-conditioner (the choice of not sleeping due to heat or not sleeping due to noise sucks).

The density of dry air at 0C and a pressure of 101.325 kPa is 1.293 g/L.

A small bedroom might have an area of 3M*3M and be 2.5M high giving a volume of 22.5M^3 == 22,500L. 22,500 * 1.293 = 29092.500g of air.

One Joule can raise the temperature of one gram of cool dry air by 1C.

Therefore when a kilo of ice melts it would be able to cool the air in such a room by more than 10 degrees C! The results I observe are much smaller than that, obviously the walls, floor, ceiling, and furnishings in the room also have some thermal energy, and as the insulation is not perfect some heat will get in from other rooms and from outside the house.

If you have something important to do the next day then spending $6 or $12 on ice the night before is probably a good investment. It might even be possible to get your employer to pay for it, I'm sure that paying for ice would provide better benefits in employee productivity than many things that companies spend money on.


John Hughes said...

Christ, you'd better get those 25 nukes built soonish.


Anonymous said...

Since evaporating water needs energy, having water *condense* will in turn free up energy... so I'm not convinced having water condense on your ice buckets is that useful. Basically it means you are "melting ice to condense water from the air".

But in general, having some open water in the room (i.e. ice buckets) is likely to increase humidity in the room. So I wouldn't bet your solution does on overall decrease humidity.

OliBlogger said...

Nice to have some theoretic background on your solutions to these problems. What I'd like to know next is their effectiveness in practice. Does the 2L of water really help to get your car bearably cool?

etbe said...

Anon: I expect that the amount of water condensed from the air is greater than that which is lost to evaporation. I plan to test this at some future time but have not done so yet.

Oli: They are all effective in practice. I did it in the Mythbyster manner and performed the tests before doing the theoretical work. The water on the car windows does not make the car cool (it's still filled with hot air), but it does mean that the air-conditioner can immediately cool the air. When the windows are hot the cool air from the air-conditioner just goes to cool the windows.

Zed said...

why not spend $500 on a portable a/c unit. They're not that noisy on low fan and its way more cost effective.

Anonymous said...

Best thing Mazda did on the older 929 was include a fan system powered by solar cells in the sun roof, once the car got to a certain temp whilst parked the fans kicked in to extract the hot air and keep the car significantly cooler once the owner returned.

etbe said...

Zed: Prices are around $700AU for a portable A/C unit. Fitting one of them into a window is a pain, they take up space, and they ARE that noisy. $700 == six summers of ice (or more if you count the cost of electricity).

Anon: That Mazda feature sounds good, however it will only keep the inside of the car at slightly above the ambient temperature. After dumping some water on the windows they are slightly cooler than the environment, it makes a difference. The ideal solution would be what Mazda does plus water sprayers. Maybe with the air being circulated you could make do with only water sprayers on the windscreen and the rear window (which are standard on hatch-backs anyway).

John Hughes said...

Oh, by the way, forgot to mention that here in France you used to be able to buy a "room chiller" - a box with a fan that you loaded with ice and used instead of an airconditioner.

Basicaly they disapeared around the time of the great heatwave a couple of years back, everyone realised that real airconditioners were cheaper (in the long run) and worked better.

etbe said...

John: That room chiller sounds like a good idea. Sure it won't be as good as a real A/C but it does have some advantages (such as not requiring a window for hot air exhaust).

In Australia evaporative cooling devices are on sale at more places than A/C devices. Melting ice is a much better cooling method than evaporating water as it doesn't increase the humidity.

I have heard of people in Australia putting ice in evaporative cooling devices and using them like a French room chiller. I haven't seen an evaporative device on sale that had space to add ice though.

Anonymous said...

You might be in trouble with theCash washing requirements of level 3 water restrictions.

etbe said...

Anon: The web site about car washing restrictions that you cited says that "a bucket, filled directly from a tap can be used to clean windows, mirrors and lights". For the purpose of this legislation I'm sure that a bottle filled from a tap would be equivalent to a bucket.