re: cooling swimming pool in high temp
10 apr 2006
>i often visit the desert rose baha'i institute near tucson during the
>summer, and the sun is so hot that the pool water gets uncomfortably
>warm to swim in.
nrel says 2240 btu/ft^2 of sun falls on the ground (610 diffuse) in tucson
on an average 86.6 f july day with a 99.4 f daily max and a humidity ratio
w = 0.0109 and an 8.9 mph windspeed. the ground gets about (2240-610)/250
= 6.5 hours of direct sun. the deep ground (water) temp is 68.4 f.
ashrae says pools lose about 100(pw-pa) btu/h-ft^2, where pw and pa are "hg
vapor pressures near the pool and in air. pa = 29.921/(1+0.62198/w) = 0.515
and 2240 = 24x100(pw-0.515) make pw = 1.449 = e^(17.863-9621/(460+tw)), so
tw = 90 f. tw = 75 f makes pw = 0.887, and 24x100(0.887-0.515) = 892, so
shading 100(1-892/2240) = 60% of the pool would lower its temp to 75, with
less pool evaporation.
>there are two buildings near the pool -- the kitchen and the library.
>i'm looking for an affordable way to recommend siphoning off this heat,
>simultaneously cooling the pool water and using the heat in one of
>these buildings effectively.
keeping 1 ft^2 of pool 75 f requires dumping 2240-892 = 1348 btu/ft^2 of
heat, eg 135 btu/ft^2-h over 10 hours at night. if 135=100a(0.887-0.515),
a = 3.62 ft^2, so we might cool a 36'x48' pool by pumping 36x48x1.35/8.33
= 280 gallons of water up over a 6267 ft^2 roof to evaporate it at night,
about the same as the daily average per capita us water consumption.
if the day temp is 93 f and we pump 75 f water over the roof when it hits
140 f and the average roof temp is 117, a square foot of roof might lose
6.5h(117-93)(2+8.9/2) = 1006 btu/day, so we might heat (2240-1006)/(140-75)
= 19 pounds or 2.3 gallons of pool water per day from 75 to 140 f, which
might heat about 2 gallons per day of 68.4 f fresh water to 135 in a grey
(pool) water heat exchanger in the kitchen, eg 3 $20 100'x1" pe pipes
inside a $30 100'x4" black corrugated plastic drainpipe spiral. during
the day, the pool water would return to the pool at about 75 f.
since water is scarce in arizona, some pool shading might be in order,
with minimal wetted roof area.