re: need advice on swimming pool solar heater SOLAR.KnowledgePublications.com

re: need advice on swimming pool solar heater
26 mar 1999
jpbtkb@aol.com wrote:
 
>i live in virginia and am trying to decide if a solar pool heater would
>work in our climate.  we open the pool in mid may and close in mid sept.
>we would like to get another month on each side and even in the summer,
>the water is relatively cool and uncomfortable.

why settle for another month? sterling is the most difficult place for
solar pool heating of nrel's 5 va cities (lynchburg, norfolk, richmond,
roanoke, and sterling), and the worst-case month is december, with a
long-term average daily air temperature of 35.4 f and an average daily
max of 45.0. the average amount of solar heat that falls on a horizontal
surface is 580 btu/ft^2 per day. 

suppose we start with a $100 10' diameter x 4' tall circular pool
set on 1" styrofoam board on the ground inside a geodesic docecahedron
(this requires imagination, a possible problem for andy m.) made with 25
$3 10'x2" pvc pipes. cut 55 50 cent 6" galvanized metal straps with holes
in them, drill a hole in each pipe a few inches from the end, bolt a strap
inside each end of each pipe with a protruding tab, then use one more bolt
to connect groups of 5 tabs together (with one bolted through the tread of
a tire filled with dirt on the ground) to make 11 hubs in a structure like
the one in my yard that's 17' o.d. and 13' tall, with 15 triangular sides.

let's cover this pretty gazebo frame (rudolf steiner considered this
shape sacred) with clear plastic film, using some pipes ripped in thirds
with sheet metal screws for cap strips, and paint the northerly triangles
white to reflect more winter sun down into the pool, and add a plywood
deck and stuff bags of dry leaves under the deck between the pool wall
and the lower plastic film sides of the enclosure. the flat pentagon on
the ground can be inscribed with a 13.7' circle, leaving 22" min for
sidewall pool insulation, perhaps us r44, using dry leaves in bags. 

the two southerly sides of that pentagon might have some dark painted
steps leading up to the deck, with bags of leaves under the steps, so
the winter sun warms the air more in the enclosure to enhance human
comfort and reduce the heat loss from the pool when collecting sun. 

we might cover the pool with a 12' diameter uv poly film pillow, sealed
around the edges with a couple of cap strips above and below a 2" pvc
ring pipe with some 1/2" holes around the inside. the pillow would be
inflated to about 0.25" h20 during the day to a maximum thickness of
about 2'. at night and on cloudy days it would be filled with tiny
soap bubbles that come from a 4' pvc pipe ring with 1/16' holes in the
bottom that sits in about 4 gallons of 2% firefighting foam solution
on the bottom of the pillow. tiny cold bubbles are almost as good as
fiberglass insulation, so this cover might give r36 insulation with
an average foam depth of about one foot. 

the bubble pipe ring would connect through the outer pipe with 4' of
1.5" flexible tubing that slides through a 4-way cross coupling in the
outer pipe and then connect to the pressure side of a shop vac. this
tubing would also contain a small tube for carrying liquid, which runs
from the bottom side of the bubble ring pipe to a couple of small pumps
which connect via bulkhead barbs to the shop vac sump. the vacuum hose
of the shop vac would connect to the outer ring pipe, and it would have
a small adjustable opening to ensure that the pillow is pressurized.

to fill the pillow with bubbles, pump the soap solution from the shop
vac into the pillow. to remove the bubbles, pump the soap solution back
into the shop vac. the pillow needs a pressure control (eg a microswitch
with a tubing film feeler attached to the ring pipe) and a foam level
sensor (a photocontrol in the shop vac return hose) that turn on the
shop vac when the pillow needs more pressure or foam.

to swim, we might lift the (30 pound?) cover up into the roof pent via 3
ropes that connect to the ring pipe and join another vertical rope above
that runs through a pulley under the peak of the gazebo and down through
another pulley at a roof corner to a counterweight with a handle. 

how hot can we make the pool in december, if there's only occasional
swimming? the sidewall has a thermal conductance of about 126ft^2/r44
= 3 btu/h-f, and the top has a conductance of about 78ft^2/r36 = 2 when
filled with foam and 78ft^2/r1 = 78 when filled with air. it might
transmit 90% of the sun that falls on it, 0.9x0.9x78x580 = 37k btu/day,
and we might not lose much heat to the ground. if the solar energy
that flows into the pool equals the heat energy that flows out, over
an average 6 hour solar collection day in december, when the average
outdoor temperature is 40 f,

  37k  = 6h(t-40)78 + 18h(t-40)2 + 24h(t-40)3 = 576(t-40),
  so t = 40+37k/576 = 104.2 f,

not counting the extra solar reflections or the fact that the air
is warmed by additional solar absorbing surfaces inside the
enclosure when the sun is shining.

this might make a nice hot tub, or an interesting $500 24' circular pool
under a 34' dodecahedron made with 20' pipes, with a winch to raise the
cover. greenhouse poly film comes in folded rolls up to 40' wide.

nick