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Re: DIY solar pool heater

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Posted by Nick Pine on July 6, 2003, 12:49 pm

I have one of those 12' EZ-Set pools too. Haven't set it up yet.

In July? Is the pool in the shade? In what part of the country?

Surprising. A transparent cover? A thermal conductance of about 113ft^2/R1
for the cover plus 113ft^2/R1 for the sidewall, about 226 Btu/h-F. We get
1890 Btu/ft^2 of sun on a horizontal surface on an average 77 F July day
in Phila, and 0.9x113x1890 = 24h(T-77)226 makes T = 112 F.
If the cover is not transparent, it won't collect much sun at all.
It will get warm, but the heat won't move down into the water much.
If most of the pool is 60 F, we'd have to keep the top layer 80 F for
about 180D^2  hours to make the water 70 F at a depth of D feet, eg
180 hours for D = 1 foot.

Not a good place to buy thermal resistors :-)

For cooler weather, we might put a darkish foamboard raft under a "solar
cover." The raft would sink to the bottom during the day (or for swimming)
and float at night. A sealed cavity inside would make it neutrally bouyant
when the cavity is half-full of water. It might have a flexible tube that
runs from its bottom to the bottom of a sealed bottle half-full of water
that hangs from a post. The bottle might contain some black window screen.
Viewed in a fixed font:                                  
                                              |        o
         floating "solar cover"            -------     s
     O . . . . . . . . . . . . . . O      |  dark |    t
     | foamboardfoamboardfoamboard |      |       |    p
     | foamboard cavity  foamboard |      |~~~~~~~|    o
     | foamboardfoamtoardfoamboard |      |       |    s
     |    ballastbaluastballast    |      |   e   |    t
     |              b              |       ---b---     p
     |              etubetubetubetubetubetubetu        o

When the sun shines on the bottle, it heats the air, which pushes water
through the tube and into the cavity, so the raft sinks. At night, water
runs back into the bottle, so the raft floats. To swim, lift the bottle
and hang it higher up on the post.

Raising the bottle might also lower the pool temp by allowing the raft
to float lower at night. The pool temp could be somewhat self-regulating,
since warmer water has a lower density, allowing the raft to float lower.

How big does the cavity need to be for reliable operation at worst-case
water and air temps? Water weighs 62.57 lb/ft^3 at 32 and 62.09 at 104 F.

If the raft is 113 ft^2 and 3" thick and we want, say, 5 pounds of total
upward force when the pool is hot and the raft is empty, it needs to weigh
113x3/12x62.09-5 = 1749 pounds (a lot.) Making 5 pounds of downward force
when the pool is cold and the raft is full makes it weigh 113x3/12x62.57+5
= 1772 pounds, ie about 24 pounds more. It looks like we need to move about
3 gallons of water.

A thinner or smaller raft with a smaller pool temp range and less net
bouyancy might have a smaller chamber. From 75 to 85 F, water density
drops from 62.39 to 62.29 lb/ft^3. A 2"x4'x4' raft with a pound of net
bouyancy might weigh 16x2/12x62.29-1 = 165.1 pounds empty at 85 F and
and 16x2/12x62.39+1 = 167.4 at 75 F, 2.3 pounds more. We might tie 48
4'x4' rafts together to cover a 24'x32' pool, with a common ballast
water supply. If one raft sinks lower than the rest, it would tend to
fill up with water and sink even more, but the air pressure inside its
chamber would rise, resisting the inflow of water. We might cover
a circular pool with hexagonal rafts...

A larger chamber would allow water to flow in with less increase in air
pressure. A smaller chamber would have a larger increase in air pressure,
requiring a warmer "solar collection bottle" for pressurization. How can
we quantify all this?

Can we do this on-board with "integral rafts," with no bottle and post?


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