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re: evaporative cooling
19 apr 2002
eric young  wrote:

>i want to run a swamp cooler off of solar cells. i live in arizona,
>and have used evaporative coolers before...

zomeworks is developing a non-electric alternative with better performance. 
"architectural cool cells" have cool water (a day's worth of thermal mass)
in overhead pipes under the ceiling plumbed to a radiator on the roof. at
night, warm water naturally rises up into the radiators and is cooled and
falls back down into the pipes. 

a variation of this might have large vertical pipes that sit on the floor
and a roof pond that's only flooded at night, when a small pump adds water
to the pond from a separate reservoir. this prevents pond water heating and
loss of water during the day, when the sun cooks the pond surface, which
prevents algae growth. the pond might be made from epdm rubber.

with continuous pumping, you might trickle water at night over a roof from
a ridge pipe and collect it in a gutter or move water into the pipes from
a deeper shaded pond on the ground. filling the pond with rocks can provide
shade and protection from drowning. 

these systems can perform better than swamp coolers in well-insulated
houses because closed systems are more efficient, and they can cool
the air in a house without adding humidity. storing 20k btu of coolth
with a 20 f temp rise requires 20k/20 = 1,000 pounds, or 125 gallons,
or 16 ft^3 of water, eg 2 10'x12" pipes.

nrel says august is the warmest month in phoenix, with a 91.5 f average
91.5 f temp and a 79.2 average daily min and humidity ratio w = 0.0113.
this makes vapor pressure pa=29.921/(1+.62198/w) = 0.534 "hg. at night,
at an average 85.4 f outdoors, the pond can be close to the wet bulb temp
twb=9621/(22.47-ln(85.4+100*0.534-twb))-460 = 68.3, after some iteration. 

if there isn't much wind, it can be cooler, with night sky radiation.


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