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Cooling swimming pool in high temp

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Posted by Brent on April 9, 2006, 4:58 pm
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.  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.
Thanks, Brent in New Mexico
attorney at newmexico dot com

Posted by SJC on April 9, 2006, 5:14 pm

  They make pool heat pumps to both heat and cool a pool.
A fluid source heat pump should be able to extract the heat
from the pool and put it into the buildings. It will cost, but
could pay itself back over many years in heating fuel savings.

Posted by nicksanspam on April 10, 2006, 12:41 pm

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.

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 1350A(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 $0 100'x1" PE pipes
inside a $0 100'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.


Posted by DJ on April 10, 2006, 2:39 pm
nicksanspam@ece.villanova.edu wrote:

I'd recommend one of those commercially available rubber mat pool
"heater", but then just run it at night ( when, I would hope, it would
be cooler?).

That's trickier. If you wanted to do that, you could incorporate either
radiant flooring and pump the pool water through it, or look into
hydronic radiators.  


Posted by Ron Purvis on April 10, 2006, 3:21 pm
 Couldn't you use really cheap solar hot water collectors pointed straight up
to the night sky and cool the water that way? According to
http://www.renet-eu-india.com/energysolar.php  :

'Night sky radiation cooling' effect may also be used in various cooling
requirements. Under clear night sky conditions when practically there is no
solar radiation, the earth surface continues to emit infrared radiation to
the sky and is cooled to temperature much lower than surrounding air
temperature. Water in a shallow trough having large surface areas when
exposed to clear night sky may be cooled to about 15C below air

According to http://www.rssweather.com/climate/Arizona/Tucson/  the average
overnight low during the times that you would use a pool varies from a low
of 50.5 F in April to a high of 73.4 F in July  and back down to 57F in
October. If you dropped 15C from that the water would be pretty cold. The
only problem is how much the cost would be to build enough collectors to do
this and the space required. You would have to place this on the roof of the
building since you would not want it taking up huge areas by the pool.

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