re: swimming pool solar heaters (what we need is a good cheap collector.........) SOLAR.KnowledgePublications.com

re: swimming pool solar heaters (what we need is a good cheap collector.........)
4 jun 2000
>...how can we fill a large poly film pillow or pillows with air
>during the day and tiny cold r20 soap bubbles at night, and
>make it sink to the bottom for swimming?

there are lots of issues in this recurring fantasy, which deserves
some methodical tinkering. what do we do about ladders and steps and
pools with irregular shapes, including bottoms that are not flat?
(more than one pillow, with flexible links, like large translucent
winterizing pillows?) how do we clean a pool with a cover like this
floating on top (a hose gap for inserting an aqua critter?) and keep
people from tearing or puncturing the cover when it's on the bottom?
(stronger plastic, with scrim reinforcing?) will pvc pipe and chlorine
corrode polyethylene film (probably.) how much air pressure inside? 
(greenhouses use 0.25" wc (0.01 psi) for their poly pillow inflation.) 
what do we do with frozen bubbles?

should this also be an astm safety cover or a boca(?) air-inflated
structure with people on top? (if not, what happens if somebody falls in
with the cover in place? make the bottom of strong dark plastic, enough
to hold up a 300 pound person?) should it have a high dome shape with
white paint or aluminized mylar on the north side to collect more winter
sun? what about wind, and stability when partly inflated? (add a few
tie downs around the edge?) what kind of bubble solution? (firefighting
foam, stepanol wax, and sodium lauryl sulfate solutions have been used
in foamed greenhouse walls.)

we might begin with an above-ground circular pool like the 15' x 42" wall
version by general foam plastics ($247 at wal-mart) with a 2" pvc pipe
ring that fits closely inside the liner and a layer of 6 mil uv-greenhouse
poly film attached to the top and bottom with screws and ripped sections
of pipe and a rubber gasket to make a 15' diameter pillow. put 6 4x8'
sheets of 2" (r10) styrofoam under the pool and bend and bolt 12 more 1" 
sheets around the outside of the pool wall before installing the liner.
(this stiffer wall should make the pool easier to fill.)

we could fill the pillow with air during the day and foam at night via
a flexible tube from one side of the ring to the other between the films,
with some small holes in the center portion of the tube. with a 200:1
expansion ratio, filling the 176 ft^2 cover a foot deep with (r36, 1/16"
diameter 50 f) bubbles requires about 7 gallons of soapy water. pump air
into the tube to make bubbles. when the cover is fully inflated, withdraw
some air from a one-way check valve in the ring and see if it contains
bubbles.  if not, repeat the process. after bubbles are first detected,
test their level every half-hour and repeat bubble generation as needed.
the air pump needs to deliver about 3 cfm to replace the bubbles every
hour, or 0.5 cfm with 4-hour bubbles. (we need an air filter. dust-free
bubbles can last a year under a bell jar.)

we might sense full pillow inflation by air pressure or a microswitch
arm that rests on the film or a rope over the top that becomes taut or
by optical means across the top. we might detect bubbles in the return
line optically or by measuring thermal or electrical conductance or
suction or by letting return air flow through a 2" piece of insect screen
that distorts and activates a microswitch when bubbles try to flow
through (this method has been successfully used in foamed greenhouses.)

to swim, pump out the air and let it sink to the bottom. (will air pockets
under the upper film make this difficult?) the air pump needs to deliver
some air at 42"/12"x0.43 = 1.5 psi to float the pillow back up. 

we'd need something like a shop vac, an air pump, a solenoid valve,
two temperature sensors, and a microcomputer to make this work...

nick