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re: cooling is my issue, not heating
22 jan 2000
brentm  wrote:

>i'd be interested to see more ideas about keeping a house cool without
>it being underground or sealed tighter than a submarine.

both help. arid is better than humid, climate-wise, since day-night
temperature swings are larger, so a house that's open at night and
closed up during the day with lots of thermal mass inside insulation
can stay cooler, and shaded ponds are cooler in arid climates.

the average humidity ratio in el paso, tx in august w = 0.0112 (pounds of
water per pound of dry air.) the average outdoor temp is 80.1 f, with
average daily min and max temps of 66.6 and 93.5. the water vapor pressure
in air is 29.921/(1+0.62198/w) = 0.5293" hg = exp(17.863-9621/(td+460)) at
the dew point td (using a clausius-clapeyron approximation), so td = 60.4 f.
at 80.1 f, 100% rh air has pw = exp(17.863-9621/(80.1+460)) = 1.051 "hg,
so the average august relative humidity is 0.5293/1.051 = 50%.

bowen's (1926) equation says the evaporative heat loss from a pond surface
equals the convective gain at the wet bulb temp tp, when 100(pw-pa) = ta-tp,
where pw ("hg) is the water vapor pressure near the pond surface, pa is the
vapor pressure in air, and ta and tp are absolute (r) air and water temps.
ta = 80.1+459.6 = 540.1 and pa = 29.921/(1+0.62198/w) = 0.5293" hg and
pw = exp(17.863-9621/tp), so tp = 9621/(17.863-ln((ta+100pa-tp)/100)) =
= 9621/(22.468-ln(592.6-tp)). plugging in tp = 530 on the right makes tp
525 on the left. plugging 525 back in on the right makes tp = 527, which
then gives 526.1, so the wet bulb temp is about 526.1-459.6 = 66.5 f 
(right paul?), close to the average daily min. 

humider key west is 84.3 f in august, with daily lows and highs of 75
and 93.4, on the average. w = 0.0185, so tp = 9621/(22.468-ln(630.3-tp)),
making a shaded pond about 536.6-459.6 = 77 f.

>it does get hot and humid in the north too so what are you folks doing
>to keep cool in the summer?

last summer i used a window ac, but i'd like to make a licl system
someday. solving the humidity problem means you can evaporate water
to reduce the temperature too...

"unglazed collector/regenerator performance for a solar assisted open cycle
absorption cooling system" by m. n. a. hawlader, k. s. novak, and b. d. wood
of the center for energy system research, college of engineering and applied
sciences, arizona state university, tempe, az 85287-5806 usa, in solar energy,
vol. 50, pp 59-73, 1993 describes: "an ordinary black shingled roof... used
as a collector/regenerator for the evaporation of water to obtain a strong
solution of [lithium chloride] absorbent... experimental results [using a
36' x 36' roof] show a regeneration efficiency varying between 38 and 67%.
cooling capacities ranged from 31 to 72 kw (8.8 to 20 tons)", ie about 1 ton
per 100 square feet of roof area.

in the house "water [the refrigerant] is sprayed into an evaporator, evacuated
to about 5 mmhg of pressure, where it immediately flashes into vapor... cold
water, pumped from the bottom of the evaporator, flows through a fan coil...
that blows cool air into the conditioned space. the absorber acts as a vapor
compressor and condenser for the system. water vapor from the evaporator flows
over the absorber where it is absorbed by the concentrated absorbent. the
continuous absorption of water vapor maintains a low pressure in the system
and permits flashing of water in the evaporator... the product of the 
absorption process, a weak absorbent solution, collects at the bottom of
the absorber to be pumped [up over the roof] for concentration."

"the dilute licl solution was delivered to the collector surface through
a spray header spanning the top of the roof and made from 50.8 mm (2 in)
diameter cpvc pipe fitted with 35 evenly spaced brass nozzles. the
concentrated solution collected at the bottom... in a pvc rain gutter, and
returned via gravity feed to a 1608 l (425 gallon) fiberglass tank... in
the event of of a rain, fluid flowing off the collector could be manually 
diverted to a 946 l (250 gallon) wash tank or to a roof drain. during the
initial phase of the rain, residual salt would be washed from the roof
and collected in the wash tank to be stored for later regeneration. after
sufficient rainfall, the rainwater is diverted to the roof drain."

we might add one or two layers of plastic film spaced up over the roof
shingles. the open system described in the paper worked best in low winds,
about 2 mph. warm humid air rising might provide that wind, and draw in 
fresh air through a counterflow plastic film air-air heat exchanger to
make this thermally more efficient with outgoing condensation.

cyprus foote mineral sells licl for about $4 per pound. it's relatively
non-toxic, and a residential system might require 200 pounds.

>as an aside, these solar furnaces you guys build... how do you "turn
>them off" in the summer?

overhangs (including bris-soleils ("sunbreakers") found outside the
rear windows of some sports cars--these might be reflective underneath,
a la fresnel), trumpet vines, grapes, runner beans, greenhouse shadecloth,
greenhouse whitewash (which is cheap and stays on till it freezes :-),
venting, etc.

downward-reflecting parabolic attic concentrators can shade themselves
from higher summer sun...

nick




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