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re: solar pool heating
10 oct 1996
why not add a plastic film greenhouse onto the south side of your house, or
build yourself a plastic film greenhouse in the sun near your house, within
20' say, with a couple of underground pipes for warmwater heat transfer from
a strawbale tank lined with epdm rubber for space and water heating?

if the greenhouse were very near the house, say within 10', some of the
solar "waste heat" from heating the water tank could heat the house with
warm air. a greenhouse might be a fine place to put your central air outdoor
unit, if it can work as a heat pump in the winter. 

stuppy (800) 877-5025 sells 20' wide x 10' tall greenhouses for 64 cents
per square foot, up to 48' long, 57 cents if longer, plus about $200 for
shipping from kansas city, and a 30' wide x 100' long x 15' tall 3,000 ft^2 
greenhouse can be put up by 3 people in one day, from scratch, including
the ground stake "foundation." the standard way to keep the plastic film
from fatiguing in the wind in the winter is to use two layers and inflate
it to 1/4" water column pressure with a $50, 50 watt blower. you might only
turn on the blower when it is windy, or just use 1 layer of plastic and some
sort of ridge venturi to make a slight vacuum inside when the wind blows.

as an alternative to those standard quonset hut or gothic arch-shaped
greenhouses made with curved galvanized pipes, you might spend another
day making the 18 curved beams you'd need for a 16' wide 8' tall x 32'
long greenhouse out of 36 12' 1x3s costing $2 each, bending two of each
to an 8' radius, with some 1x3 spacers and purlins and ridgepole every 2',
held together with deck screws, for a $200 512 ft^2 structure to collect
and store about 200k btu of sun per day, about 2 gallons of oil's worth,
on an average december day.

these 1x3 beams fail in horizontal shear, slipping when you put the ends on
the ground and push on the curved top side. so they could use some glue, and
a solid middle 1x3, or a little chicken wire on each side of the 1x3 spacer:

     -----------------------------------------------------------

                            12' 1x3

     -----------------------------------------------------------
                     |    chicken wire   |
                     |   1x3x3x3 spacer  |
                     |     every 2-3'    |
                     |    chicken wire   |
     -----------------------------------------------------------

                            12' 1x3

     -----------------------------------------------------------

it might look something like this:

     ---                  .
                   .      c      .            the 10' x 32' r10 insulating 
               .               c     .        and reflective cover is shown  
      8'     .                      c  .      in the raised position.
           .                            c.
          . plants,       b "solar" pool  b   ---
         .  picnic table, a    cover      a.
        .   etc.          l  106 f water? l .  4'
     .....................e...............e..................
        |                16'                |

                          |       11'       |
        .....................................
        .                 . . . . . . . . . . ---
        .                 . .      8'     . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . . 30'         . . 32'
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . .             . .
        .                 . . . . . . . . . .
        ..................................... ---

the tank might be 8' wide x 4' deep inside, made with about 80 strawbales
and a few 2x4s and dacron rope and ground stakes, and a single piece of epdm
rubber, 20' wide x 44' long. this piece of rubber might drape over the top
of moldy old $1 strawbales, with the sides uncovered, all natural-like, with
a plastic film vapor barrier underneath. (yuppies and architects might use 
bright yellow $2 strawbales, with accent pumpkins.)

the cover might be

1. rigid as shown, weighing 300 pounds at 1 lb/ft^2?, made with $160's worth
of 4x8'x2" styrofoam sheets and a few 2x4s, painted white underneath and dark
on top, with a counterweight, small reversible motor and photocontrol, or

2. a $24 flat polyethylene pillow, filled with tiny cold bubbles at night, and
air during the day, using a $50 wet/dry shop vac with a bubble sensor in the
return line (how do we make that?) to make bubbles out of a soapy water puddle
at the bottom of the plastic film and collect them at the top, for a few
seconds every few hours. the cover might be on the bottom if people were in
this large hot tub, and they would want to be careful to neither slip nor
puncture the plastic film, nor crawl underneath, as one solar pool heating
professional suggested. the north half of the poly film overhead might be
painted white, to reflect more sun down into the water on a winter day.

3. another way to make a cover is to assemble $30 worth of 4" solid thinwall
pvc drainage pipe on the floor of the tank, into a 30x8' picture frame, lay
some plastic mesh on top for reinforcement, place an inch of concrete over
that, then $120 worth of 2" styrofoam on top of that, and some very thin
thin reinforced concrete slabs, pieces of slate, or 1/2" of pebbles over that.
if all goes well, when you fill the pvc pipe with a little air at 2 psi from
a 340 cfm, 150 mph $49 sears leaf blower (?) to make a maximum bouyant force
of about 640 pounds, the cover might magically rise up to the top, and people
could sleep on it at night. a guest room. with a klaxon and automated
"dive! dive! dive!" announcement a few seconds before the cover sinks at dawn
the next morning to absorb sun into the water through a single layer of
plastic film on top?

4. or, you might lift the neutrally-bouyant slab with a polyethylene film
cover on top, by inflating that. more at night than during the day, when
the floor would be awash. or you could haul the cover up out of the water
with a rope harness and a $149 sears garage door opener, which some people
use to haul boats out of the water.

this could become a serious hobby. or even a business.

what would the average water temperature be in december? where i live,
the south wall might receive about 1000 btu/ft^2/day of sun, of which 90%
might make it through the outer layer of plastic film and 80% be reflected
from the white surface overhead, and 90% of that might fall into the water,
so the average daily solar gain into the water might be 1000x256x0.9x0.8x0.9
= 166 k btu. the greenhouse might be 70 f on an average december day in the
philadelphia area, with some sun, or warmer if there's a reflecting surface
to the south. 

the pool would mainly lose heat through the "solar pool cover," over a 6 hour
solar collection day, so if there were no useful heat output from the pool,
it might have an average steady-state stagnation water temp t such that
166k btu = 6(t-70)240 ft^2/r1 ==> t = 70 + 166k/(6x240) = 185 f. warm enough
to cook lobsters?

the thermal conductance of the pool with the cover in place would be about

    320 ft^2/r10 = 32 for the cover,
  + 320 ft^2/r50 =  6 for the walls,
  + 320 ft^2/r10 = 32 for the bottom,
    for a total of 70 btu/hr-f,

and the average temp inside the greenhouse might be 50 f in december, so if
the pool temp were 130 f, say, the top and sides of the pool might lose
24(130-50)70 = 134 k btu/day, and the 4x8x32x64 = 65,536 pounds of water
might lose 134k/66k = 2 f on a cloudy day, and another 2 f if it supplied
a modern house with the heat equivalent of a gallon of oil per day.

and another 1 f if it were supplying hot water for the house, via some hot
water plastic pipe running around the top edge... how much plastic pipe area
do we need to heat 3 gpm of water from 55 f to 110 f in 130 f water? we need
to transfer 3x8x(110-55) = 1320 btu of heat in a minute, or about 80 k btu/hr
(23 kw), which we might do across a liquid-liquid interface with an r0.1 thin
pvc pipe wall between (copper pipe might make this r0.03, and tend to inhibit
algae growth in the pool), with an average temperature difference of something
like 50 f, so we'd need a pipe surface area a such that 80k = 50 a /r0.1, so 
a = 0.1x80k/50 = 160 ft^2. a $4 10'x4" pvc pipe has an area of pix4/12x10
= 10.5 ft^2 and a volume of 3.14x(1/6)^2x10 = 0.87 ft^3 (which would displace
$1.77 worth of concrete at $55/yd^3.) so, do we need 16 of these 10' pipes?
i don't think so, since we do not or should not use 3 gpm of hot water for
more than about 20 minutes at a time, ie 60 gallons or 7.5 cubic feet, or
about 8 pipefuls of water, or 80 linear feet of pipe, once around the top
edge, just under the water surface. 

now, how can we keep this pipe half full of air, to float the pool cover?

nick

nicholson l. pine                      system design and consulting
pine associates, ltd.                                (610) 489-0545 
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email: nick@ece.vill.edu

computer simulation and modeling. high-performance, low-cost solar heating and
cogeneration system design. bsee, msee. senior member, ieee. registered us
patent agent. solar closet paper: http://leia.ursinus.edu/~physics/solar.html



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