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Posted by on April 16, 2007, 5:01 am
>... some day it's going to warm up and I'll want to open my in-ground
>swimming pool. In order to extend the season (I live in Maryland),
>I'm thinking about either a solar blanket or a "pill".
NREL says 1550 Btu/ft^2 of sun falls on the ground on an average 53.4 F
April day in Baltimore. The average humidity ratio w = 0.0052, with
V = 10.4 mph average windspeed, which makes the airfilm conductance
about 2+V/2 = 7.2 Btu/h-F-ft^2.
>The latter is apparently a substance that slows down evaporation,
>which presumably is the main form of head loss.
ASHRAE says an uncovered A ft^2 pool loses about 100A(Pw-Pa), where Pw
and Pa are pool and air vapor pressures. Pw = e^(17.863-9621/(460+Tp))
"Hg (100% RH at Tp (F)), approximately, and Pa = 29.921/(1+0.62198/w).
>The pills are not supposed to be as efficient as the blankets (perhaps
>the blankets have a slight insulating value...
Maybe R1 with 80% solar transmission, for a clear vs blue blanket.
>... I'm leaning towards the pill, but I'm skeptical about how well
>they work. And it's hard for me to do a scientific comparison,
>since I only have one pool :-).
You might weigh a pill and chop it up or dilute it several times,
homeopathic-like, to treat water in one dark plastic bucket (does
the coverage formula include depth?), and put a pool cover sample
over another bucket of the same kind...
If the pill completely eliminates evaporation and you wrap both buckets
with a few layers of bubble wrap with black plastic on the outside, the
covered one might gain 1240 Btu/ft^2 = 24h(Tc-53.4)1ft^2/R1, which makes
Tc = 105 F :-) The pill version might have 1550 = 24h(Tp-53.4)1ft^2x7.2,
which makes Tp = 62 F.
These will both work better if you paint your pool black
or let it get muddy, like a natural lake in the sun.
Nick
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