Sunday, April 22, 2007

How cold that night sky

People who write learned papers and books about tropical architecture and comfort --Floridians, Singaporeans, Australians, and, God knows why, even some Norwegians -- talk of "dumping" building heat, ie trying to find places where the enthalpy is low to which unwanted building heat can be efficiently transferred.

Alas, this is not easily done in Thailand because there aren't any cool places to be found. With a mean temp over the year of nearly 28 degrees, the earth itself is warm -- about 28 degrees, which does not provide much of a "sink" for heat. (One night I walked into a bar in Pnom Penh and started taking the radiant temperature of the palms of all the girls in the bar. I told them that I was measuring what kind of a wife they would be. I leave what one girl did it to the reader's imagination )

The sky, which cools the overheated brow and roof in the night desert (or in places like Hollywood, where I lived a rather disappointing life, but for other reasons than heat, until recently) is fairly warm, and efforts to radiate heat into the sky from buildings are not usually considered to yield much benefit, especially in the hot season.

But since we have recently seen that we are not asking for much cooling, just a few degrees, I've been taking some night sky temperature measurements* with my little radio Shack radiant thermometer. Here are the results on the left. Evidently the night sky temperature is a function of sky moisture, or dew point. And in the hot season, when there is a lot of moisture in the air, the night sky becomes less effective as a heat sink.

But still. How much can we get from a night sky of 15 degrees?

According to radiant heat theory, we should be able to transfer about

k*(Troof^4-Tsky^4) watts/m2

where k= 5.6697xlO-8 w/m2-°K. T

which comes to about 50 to 80 watts per square meter of roof. This is enough to lower the roof temperture, suppress it as we say, by two or three degrees.

Can we use this natural cooling to some good end?


* I know that this instrument is not really measuring the temperature of anything in the sense of say a thermometer, rather it is feeling how much radiation is being emitted by whatever you point it at. Or whatever you point it's 30 degree "eye" at. It thinks that the emittance of everything is the same, maybe 0.9, which is not true or things like aluminum and skies. But since radiation is what we are talking about here, I'm just going to go ahead and figure the heat transfer as if the "temperature" of the sky was what the radiant thermometer says it is.

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