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re: keeping the house cooler
11 apr 1997
how about a roof pond, say a flat epdm rubber roof with standing seams every
16' and a 6" lip around the edge? atlanta has a 24 hour average temperature
of 78.8 f in july, with an average daily max of 88 f and average daily min
of 69.5 (when dew begins to form, so evaporative cooling is nil.) a roof pond
covered with soap bubbles during the day, under 2 layers of poly film with
some shadecloth over that, perhaps, inflated and vented with outside air at
night, might have a thermal conductance of something like 2 btu/hr-f-ft^2,
ignoring evaporation, so it might keep a 1-story 32'x32'house with a shaded
south wall and r20 insulation at temperature t, if the energy flowing into
the house over a day, 24h(78.8-t)1024ft^2/r20 equals the energy flowing out
of the pond, say 6h(t-69.5)1024ft^2x2, so t = 73 f. 

a sunspace containing a solar still (with a reflective cover and concentration
ratio of 2:1, heating a shallow trench of lithium chloride solution?) might
provide summer dehumidification, and additional winter solar heat storage in
the form of a few hundred pounds of concentrated licl solution, ready to
absorb water vapor and releasing some 1000 btu/pound of water. (or maybe part
of the roof pond should be licl?)

the average january temperature in atlanta is 41 f, and an average of 1120
btu of sun per day falls on a square foot of south wall, while an average of
820 btu falls on a square foot of horizontal surface, eg a roof pond. if the
pond is covered with tiny cold soap bubbles at night (almost as good as
fiberglass insulation) it might collect about 32x32x820x0.8 = 672k btu/day
of sun and lose about 6h(t-41)1kft^2/r2 during the day and 18(t-41)1kft^2/r20 
at night, making its average temperature t = 41 + 672k/3.9k = 213 f :-)
(radiation loss would make it closer to 130 f, as would using some of this
heat to warm the house, altho that might better be done via a plastic sunspace
along the south wall, on an average day, with an average amount of sun.)

a pond like this would ensure the house roof never saw less than 32 f in
winter, since it would take about 4400 btu to freeze a square foot of it
solid, at a rate of (32-(-10))1ft^2/r20 = 2 btu/hr when it's -10 f outside,
ie 2,200 hours or 13 weeks with no sun at -10 f. it could also melt a lot of
snow on the roof, especially with wellwater keeping it warmer than 50 f. it
might make a good sprinkler system or gravity-feed rainwater source or large 
winter water heater, storing up to 4,000 gallons of water, and gathering 80
gallons a day where i live, with an average monthly rainfall of 4". 

nick




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