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re: solar pool heating
5 oct 1996
     the sea is potentially a source of vast amounts of electrical energy,
     as well as haddock. scientists predict that some day, possibly as early
     as next week, whole cities will be powered by the sea. the key will be
     gigantic undersea electric turbines, whose blades will be turned by
     the relentless, powerful motion of lobsters walking along the sea bed.
     if you live near the sea and own a gigantic electric turbine, you can 
     harness this power today. the trick is to make sure your turbine is
     parallel with the prevailing lobster motion.

         from _the taming of the screw: several million homeowners' 
               problems sidestepped_, by dave barry

moving from the ludicrous to the impractical...

>is it easy to put a $900 30' wide x 48' long plastic film greenhouse over an
>existing pool, and hinge a 300 pound 24' x 36' cover made with $300 worth of
>beadboard and 2x4's to a pipe along the 36' north edge, and haul the cover up
>to a 45 degree angle automatically with a $200 dc winch when the sun shines
>and the pool needs heat, in december?

we might try a smaller version of this first, with some epdm rubber draped
over some strawbales inside a quonset-hut-like structure, or more easily
recyclable haybales, if we are surrounded by horses, which (the bales) are
about 3' long and 16" square. stack up 24 of them like this in 3 layers of 8
to make a 56" square x 4' deep hot tub, 88" square on the outside?

             ~12'                  we would need
..............................        a 16'x16' piece of vinyl or epdm rubber
.              .             .        24 bales
.              .             .        12'x50' of greenhouse uv poly film
.              .             .        6 12' pressure treated 2x4s
. . . . .  b a l e  b a l e b         2 4x8 sheets of 2" styrofoam
.         b    .             a        20 12' 1x3s  
.         a    .             l ~12'   some screws and bolts
.         l    .hot tub      e        a pulley
. . . . . e . . . . . . . .  b        some rope
.         b    .             a        etc
.         a    .             l
.         l    .             e
......... e  b a l e  b a l e

          .    c    .                 the cover, shown in the raised position
 sun   .          c    .              might be painted white on the bottom.
    .                c   .    ~10'    it might be 8'x8'x2" thick, weighing
  .                     c  .          20 pounds, and it might be lifted 
 .                         c.         with a small 12 vdc reversible motor,
.         b floating bubble  b        eg a used windshield wiper motor,
.         a  pack cover...   a        especially if it had a counterweight,
.         l                  l        to collect sun or keep some plants
..........e..................e        from freezing at night, automatically.

how much sideways water load do the bales have to hold in, at the bottom?
a foot of water weighs 0.43 psi, so the wall of the tub, 2' deep on average, 
pushes sideways at 0.86 psi, so if the sides are held in every foot along
the perimeter of the tub, each tie has to hold 12"x48"x0.86 = 500 pounds.
some ground stakes and/or tensioned 1/4" dacron rope every foot, running
across the bottom, holding together the pressure-treated 2x4 frame flat on
the ground that surrounds the bales and some 1x3s on edge every 2', tucked 
inside a flat 1x3 frame at the top of the tub?  

i've built curved half-bows like these with 2 12' 1x3s bent into an 8' radius,
held together with deck screws and 1x3 spacer blocks, purlins or ridgepole,
every 2'. a little chicken wire or stucco mesh on each side of the spacer
blocks would keep them from slipping and make the bows stronger.

or should we have one piece of plastic film inside the bows, and another
on the outside? or a single piece on the outside and a fairly airtight seal
at the ground, with some sort of venturi at the top, to make a slight vacuum
when the wind blows, looking like this, from the side:

       1' wide, 1/8" recycled polyethylene "plywood" ridge cap
             venturi                               venturi
             spacer          . hole cut .          spacer
             block.            in film            .block
             .               .          .              .
                  .              1x3              .      <- upper 1x3
             .               .ridge pole.              .
                  .                               .      <- 1x3 spacer blocks
             .               .          .              .
                  .                               .      <- lower 1x3
             .                                         .
will all of our solar heat leak out of the hole at the top during the day?

do we need those central bows? how about heat-sealing 2 16' x 24' pieces of
poly film to each other with 2" strips every 4', ballasting the film sandwich
to the ground with soil or a pressure-treated 2x4, perpendicular to the heat
seals, and inflating the sandwich to 1/4" water column, with 3 horizontal wire
purlins for wind stability, and just 2 end bows, with internal diagonal braces
or external wires to ground stakes running east and west, to keep it from
folding up towards the middle?

on an average december day, where i live, about 10'x12'x900 = 108k btu of sun
would fall into this structure, about one gallon of oil's worth, or 30% more
with a shallow frozen reflecting pool to the south, made from another piece
of epdm rubber draped over a perimeter earth berm. one layer of poly film
has an r-value of about 0.8, and there's about 500 ft^2 of it here, so the
average daytime winter air temperature inside the structure might be about
43 f + 108k/(6hrx500ft^2/r0.8) = 72 f. 

if the bubble pack pool cover transmits 80% of the sun into the water, and
the white paint under the rigid cover reflects another 80%, we might have 
16 ft^2 x 900 btu x 0.8 x 0.8 = 9k btu falling into the hot tub on an average
6 hour day, with 6 x (t-72) x 16 ft^2/r1 btu leaking out, for a water temp
t of about 72 + 9k/(6x16) = 166 f. heh.

the tub would hold about 76 cubic feet of water, weighing about 5000 lb,
and all buttoned up, with about 100 ft^2 of r50 sides and 50 ft^2 of r10
cover, starting at 110 f, it might lose about 24(110-36)(2+5) = 12k btu
on an average cloudy day, cooling to 110-12k/5k = 108 f.

it would be nice if the structure leaked a bit, with most of the rainwater
from the "roof" ending up in the flower bed in the front, and some of it
flowing through the hot tub to end up there as well.

or... suppose the cover and heater were the same, like a "solar pool cover,"
during the daytime, but a lot more insulating at night. this hot tub inside
the enclosure would want the north poly film painted white, to reflect more
sun down into the tub.  
how much heat does a swimming pool need? the cover is the biggest loser:
a 24x36' pool with an r10 cover might need 24(76f-36f)24x36/r10 = 83k btu/day
of solar heat in december where i live, the amount of sun that falls on 
83 ft^2 of vertical surface or 160 ft^2 of horizontal surface, less than
20% of the pool cover area.

how about just deflating a polyethylene film pillow full of tiny soap bubbles
before swimming, pumping it out and letting it sink to the bottom?

like this?                    poly pillow
                          rr   .   .   .   rr   pool rim
   air pump===hose====>   .                 .
                               . water .  <-- floating screen on a soapy water
                          s        .        s          puddle in the bottom  
                          s    detail a     s          of the poly pillow
                          s                 s
                          s                 s   pool sides
                             bottom of pool                 ^  ^  s is some
                                                            |  |  window 
perhaps 3 modes, for a swimming pool?                bubbles|  |  screen
      day: pillow full of air                   float s s s hose s s s float
      night: pillow full of bubbles      detail a:       soapy water
      swimming: pillow on the bottom.               bottom of poly pillow

it seems simpler to do this with a styrofoam cover, for the hot tub, making
the cover slightly heavier than the water, and lowering it by letting some
water fill a few cavities rather than air. suppose it were 4' square inside,
and the 16 ft^2 x 2" thick styrofoam weighs little, and displaces about
10 pounds of water per square foot when submerged. concrete and stone weigh
about 150 lb/ft^2 in air, or 86 lb/ft^3 when submerged. so we need about
10/86x12" = 1.4" of concrete or stone or gravel or something on top of the
styrofoam cover to hold it underwater. lay some polypropylene twine down on
the cover, recycled from haybales, and place 1.4" of concrete on top of it?
or glue a 2" thick x 4" wide styrofoam edge around the top, and pour some
round pebbles on top? to collect sun effectively in this case, we need a
single layer of clear plastic film on top of the hot tub, to prevent
evaporation loss with the cover submerged. 

we might have 16' of 6" poly duct (eg unvented v4-6 at 22 cents per linear
foot from d & l growers at (800) 732-3509) around the perimeter, underneath,
heat-sealed at the ends, so we can float the cover by slightly inflating the
tubing at night, with an aquarium pump. allowing a little air to escape
underwater would probably keep the tub cleaner. hmmm. tropical fish?

we might use 4x8 cover modules for a full-sized pool, with 12 24' pieces of
16" poly duct underneath, inflated to 6" depth, and attach them together so
these tippy boats could support people on "pool patios." party fun :-) 


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