calculating nighttime radiant cooling

jp_2

after some thought, I realized this can be approximated from the same methods "you out there" use for snowmelt applications.  you know the fluid temp, the outside temp, the sq footage of the collector/emitter.



after all cooling is heating from the "other" direction.  its all just moving heating around. a boiler heats the tile floor, and the tile floor cools the boiler supply.



I'll admit, the idea of radiant cooling of a surface below the ambient is an interesting idea, I have yet to see any papers on the subject.    my bet, a case of good beer, is that any cooling of a surface below ambient is really due from evaporation.





jp

Royboy

geez, that's tempting

a case of good beer ...



now, I'd have to do some digging to document this, but I clearly recall reading (back in the long-ago) accounts of flat plate collectors freezing on nights where ambient air temps remained well above freezing. and it was explained in terms of the collectors "seeing" the much colder radiant temp of the clear night sky and thus radiating heat out into the cold void to a point where the absorbers became considerably colder than ambient air. and froze! (which I'm sure was a major bummer).



evaporation not really involved - rather radiation.



in theory this principle could be used to chill a case of good beer to below ambient air temps. ;-) at least on a clear night ...

Royboy

amazing

first dusty tome I took off the upper solar shelf, I found this.



from ASES's Progress in Solar Energy, vol 6, Engineering Division. I bet there's considerably more out there with a little digging. I probably haven't cracked this volume in 25 years. there was a ton of good & interesting research done in the late 70s, early 80s.

jp_2

Haaa

looks like i owe you a case of beer!!!?



i've looked for the longest time and found nothing, hel[ps to use the exact wording sometime during searching......



thanks



this  should be easy to test though.  any surface say, above the trees, should feel this effect, a metal roof should be a good test subject.



I've felt this during the winter, evening feels dam cold compared to the temp, then theres been real clear winter nights that seem warm too?



I've cooled small surfaces well below room temp with  H2O evaporation, around to 40F if i remember correctly.





jp

jp_2

roof temp not work

to mimic the froze collector, place a thin metal plate on a foam board.  i'm guessing a thin metal plate to reduce heat capacity of test specimen. put this on the roof.



insulating  the plate from a heat source(roof) gets rid of any heat conduction from other sources, leaving  you with mainly radiation pointed up. 



the steel roof most likely can replenish its lost heat from the structure below, keeping it more less equal with ambient air temps.  so that test should fail.



this now makes some sense, the question is how large of a delta T is possible,  this could answer Marks question of how many btu's can you get rid of.  thus an equation could be born!





jp

Royboy

another piece of research

from googling the first part of the title of the initial find:



http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1502-05.pdf



looks like this one may have done some of the calculating for you, jp



in the live world, there's frost on my metal roof this morning. its 25° out, though, so we don't have to invoke any non-intuitive mechanisms.



I'm guessing the shipping on that beer is going to be a killer ...

jp_2

HOW Embarasing!!!!!

well all along it was right in front of my nose!



I was too lazy to do as I suggested with the metal plate and foam board, but last night I walked around with my $60 IR temp gun.  the sun had just gone down, still a little light out, no suggestion of sun still showing though.



this is what i found.   the red truck:

door 36F

hood 30F

roof  25F



little red garden tractor trailer   21F



hard packed dirt (ground)   31F



damp ground (bare)  40F



$3 outside thermometer 28F *****CORRECTION was reading 38F, not 28F at  9pm.



this morning, before sun up, about the same darkness as last night:



truck door   25F

hood        12F

roof           6F



little trailer   15F



dry ground 12F



plastic tot in the yard  8F



thermometer   24F



IR gun can give funny readings though, the galvanized steel cabin roof reported -5F last night, but my hand didn;t stick to it.





jp



next, see what happens on a.) warmer nights, b.)hazy to cloudy nights



added:



roy, i quickly glanced at that last pdf file, tons of interesting stuff in it, thanks again.

Royboy

cool temps

in your measurements make me wonder if that beer is perhaps closer than I thought ... ;-)



15° here this morning. went out to cut some daffodils for my wife's earthday birthday bouquet & they were froze solid. cutting frozen flowers is a little odd.

jp_2

nightime sensor

mark,



years ago you wanted a sensor to detect this.  ever find one?



seems to me just take the differential between a surface temp pointed up and the air temp.



not sure what good it would be for?



jp