Too Hot To Sleep?

[Swelters]

If you are not an aircon user, the proverbial hot tin roof may be just what you need at night. Or this is the surprising result of some recent studies by my brother Ron, which we posted in our blog today.

http://www.thaivisa.com/forum/blog/swelters/index.php?

[chownah]

Just a random thought I've been having about tin roofs radiating to the night sky:

Background: It is my understanding that a reflective metal surface will reflect any visible light and especially important for this discussion it will reflect infra-red (radiant heat).....and that for infra-red not only does this reflective ability keep heat from entering the object with the reflective metal surface but it will also keeps heat from leaving the object via infra-red radiation from its surface....that is to say that a shiny metalic surface blocks infra-red transmission IN BOTH DIRECTIONS.

If this is true (maybe your brother knows about this) then the shiny metalic surface of your sheet metal roofing would not be a very good radiator to the night sky. I have not direct experience with this....maybe you could do an experiment....get two piece of galvanized roofing (cut one piece in half) and give one a thin coat of black paint (I suppose flat black and not shiney black would be best but don't really know). put a piece of insulation under each of them and orient the black painted one so the UNpainted side has the insulation......then at night set these out so the uninsulated side (shiney side on one and black painted side on the other) have a clear shot at the sky (no trees or other obstructions) and then see if one becomes noticeably cooler than the other. Black anodized metal would be better than black painted I think but I'm trying to go with things more readily available.

Of course my memory may be defective and the effect I'm describing might just be a fantasy and not real at all....I think I'll go out now on the internet and see if I can find out.

Chownah

[chownah]

Emissivity is the key word.....you want high emissivity for a surface to be able to radiate....you want high emissivity....there are special high emissivity paints....they are black....don't know if they are readily available in Thailand....a link:

http://evitherm.athena.as/Default.asp?ID=6...p;mode=html#aq6

containing:

E9. What is emissivity?

All objects at temperatures above absolute zero emit thermal radiation. However, for any particular wavelength and temperature the amount of thermal radiation emitted depends on the emissivity of the object's surface. Emissivity is defined as the ratio of the energy radiated from a material's surface to that radiated from a blackbody (a perfect emitter) at the same temperature and wavelength and under the same viewing conditions. It is a dimensionless number between 0 (for a perfect reflector) and 1 (for a perfect emitter). The emissivity of a surface depends not only on the material but also on the nature of the surface. For example, a clean and polished metal surface will have a low emissivity, whereas a roughened and oxidised metal surface will have a high emissivity. The emissivity also depends on the temperature of the surface as well as wavelength and angle. (Further info)

[Swelters]

Yes, Chownah is right, a shiny roof would both reflect heat from sun and also be a poor emitter at night. I didn't Ron had a shiny roof in mind, rather the more typical grey color which has an emittance of about 0.9. This is enough to drop the temp a couple of degrees below air temp at night, as you can see in our roof (which happens to be tile, not metal, see picure below). The question is, can this temperature drop be exploited?

Jack Swelters

[chownah]

Can it be exploited? I think so. I believe I remember some indegenous people making ice in covered pits which they opened to the night sky every night and closed up during the day...probably in a desert region. My comment was to try to indicate what might be done to improve the efficiency. My guess that a flat horizontal plate with a few centimetres of water on it, no glass cover, and an insulated bottom, built high enough to get a clear view of the sky and with reflectors to block any view it might have of ground based items (buildings and trees) might work as a "cold collector"......but you would have to keep it from getting hot during the day probably...... at least that is what I would try first if this was a project I wanted to be active with...I've thought about this alot but I'm really busy with my organic farming and I also live in an area where the heat is not sooooo bad (northern Thailand)...

The reason I think it would work is that the reason it cools off at night is heat radiation to the night sky...isn't that right?...and it just seems like studying the effect should lead to some enhancement....don't you think?

Chownah

[Swelters]

Can it be exploited? I think so. I believe I remember some indegenous people making ice in covered pits which they opened to the night sky every night and closed up during the day...probably in a desert region. My comment was to try to indicate what might be done to improve the efficiency. My guess that a flat horizontal plate with a few centimetres of water on it, no glass cover, and an insulated bottom, built high enough to get a clear view of the sky and with reflectors to block any view it might have of ground based items (buildings and trees) might work as a "cold collector"......but you would have to keep it from getting hot during the day probably...... at least that is what I would try first if this was a project I wanted to be active with...I've thought about this alot but I'm really busy with my organic farming and I also live in an area where the heat is not sooooo bad (northern Thailand)...

The reason I think it would work is that the reason it cools off at night is heat radiation to the night sky...isn't that right?...and it just seems like studying the effect should lead to some enhancement....don't you think?

Chownah

Organic farming sounds more interesting, but you can find some tools to figure night radiation at:

http://www.nationmultimedia.com/webblog/vi...52&bid=2537

[chownah]

Can it be exploited? I think so. I believe I remember some indegenous people making ice in covered pits which they opened to the night sky every night and closed up during the day...probably in a desert region. My comment was to try to indicate what might be done to improve the efficiency. My guess that a flat horizontal plate with a few centimetres of water on it, no glass cover, and an insulated bottom, built high enough to get a clear view of the sky and with reflectors to block any view it might have of ground based items (buildings and trees) might work as a "cold collector"......but you would have to keep it from getting hot during the day probably...... at least that is what I would try first if this was a project I wanted to be active with...I've thought about this alot but I'm really busy with my organic farming and I also live in an area where the heat is not sooooo bad (northern Thailand)...

The reason I think it would work is that the reason it cools off at night is heat radiation to the night sky...isn't that right?...and it just seems like studying the effect should lead to some enhancement....don't you think?

Chownah

Organic farming sounds more interesting, but you can find some tools to figure night radiation at:

http://www.nationmultimedia.com/webblog/vi...52&bid=2537

Didn't see any tools at your 'brother's' blog....just a formula and a mention of a cheap radiant thermometer....are these the tools you were speaking of or did I miss something? He did indicate that his calcs showed that you could probaby get from 50 to 80 watts of cooling per sq. metre....he didn't say how many hours of the night one could expect to get this...anyway, a 100 sq. m. roof area should be able to get you 5,000 to 8,000 watts of cooling....not too impressive since it would be spread out over the entire roof. I guess what you need is some way to collect all that coolness and put it in one room.

Maybe the best way to utilize night sky radiation is to circulate night air (which is cool because of night sky radiation) and blow it into the house....there will be a point where this effort won't be worth the return which according to what I posted before seems to be around the 8 deg. C difference in day/night temperature range. What if already cooled night air was circulated through your radiator where it would be given a couple degree boost in cooling and then brought into the house? This does add a layer of mechanical complexity which is absent in your internally exposed roofing idea. Your internally exposed idea is sort of elegant in its simplicity....maybe you could add a horizontal "curtain" to close to insulate the house from the heat during the day and then open at night.

Chownah

[Swelters]

OK, we've reached the point where we think maybe we can cool night air from it's excessive 32 degrees by allowing it to be exposed to the underside of an uninsulated roof which is a radiator with say a temperature of 28 degrees.

However cooling the air at constant moisture content also raises its humidity from say 70 percent to about 80 percent, and people even at rest don't seem to like such "heavy" air, I can say myself that it produces a kind of unpleasant suffocating feeling. It's the same thing that often happens at sundown, it gets cooler but "muggier." And if you engage in any physical activity the 80 percent air is going to be less 35% effective at evaporative cooling.

You can look at this by plotting the change on a psychrometric chart as I've done above. It appears that by cooling from the locus of the red dot to the blue dot you are moving within a zone of discomfort without any movement across the gradient of improvement.

Conclusion: In the tropics, cooling the evening air may not help much. What you would really like is drier air more than cooler air.

The radiant cooling directly from 28 degree roof underside to your 34 degree body will however presumably help some.

[chownah]

I've been wondering for quite awhile if it is possible to cool the air all the way to the dewpoint and then cool it some more to extract some of the moisture and then let it ambiently warm to a moderate temperature with a lowered relative humidity...the warming might be done by simply mixing with ambient air or by running the cool moist air through a heat exchanger that cools the incoming air to the process. This will clearly not always be possible but the farther north one goes in Thailand the greater the chances it might and since I live way up in Chiangrai this is the approach that I would try to take if I had the time to invest in this type of project.

Chownah