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re: house cooling
20 may 2001
the q wrote:
>>out here in some areas of saudi arabia and the gulf states old houses
>>used to have small towers on them, through which the wind could blow
>>at the top. so drawing air up through the house and cooling it... i've
>>always wondered whether a black metal tube on the south side of the
>>house would have the same effect. sun heats tube hot air rises...
a "tube" wouldn't have a lot of thermal mass or sun-gathering surface or
heat exchange surface compared to a flatter south-facing surface, and
heating all the chimney air at the bottom would make for a higher average
chimney temp and more airflow than heating it as it rises...
>we don't want to fill the house with warm air during the day, but the
>chimney might have some thermal mass to store solar heat to increase
>nighttime airflow, with some sort of gadget that only allows airflow
>at night...
if we store q btu of sun during the day, and cfm = 16.6asqrt(hdt)
and q = 6hxcfmxdt over 6 hours at night, then cfm = 3.6(a^2hq)^0.33
= 115q^0.33 with h = 16' and a = 8 ft^2, eg a 2'x4'x16' tall chimney.
nrel says a level surface in phila gets an average of 1890 btu/ft^2 of
sun on an average july day, and 820 falls on a south wall. an 80% ground
reflector adds 0.5(0.8-0.2)1890 = 567, so a linear foot of 16" tall x 2'
wide masonry bench over a south wall receives 2x1890+16/12x(820+567)
= 5629 btu/day, or more, with a light colored wall above it, or more,
with a linear parabolic reflector above it (which might turn it into
a solar barbeque :-)
if all this heat were stored and used for nighttime chimney air heating,
an l' bench might produce 2046l^0.33 cfm for 6 hours every night, if air
enters the chimney at the outdoor temp, or more, with warmer house air.
making 4k cfm of airflow (like a good 20" box fan in a window) requires
l = (4k/2046)^3 = 7.47'. the bench might be 2 8" hollow blocks high and
3 wide, with lots of heat transfer surface inside and a smooth surface
on the outside, with 22 btu/f-ft and a 5629/22 = 261 f temperature rise.
it could use more thermal mass, eg a concrete top, or 6 4" pvc water
pipes threaded through block holes for 55 btu/f-ft with a 103 f rise.
nick
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