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re: solar pool heater
14 mar 1997
wrote:
>i have a above ground pool 24' dia by 52" deep
with 452 square feet of top and 327 ft^2 of sidewall... solar pool heating
should be easy, with a low collection temperature and a large built-in
thermal mass and a rather warm air temp in fall and spring, vs winter.
and we are just trying to keep the mass warm, not export heat in the
form of hot water for showers, space heat for a separate house, etc.
>and i wish to build an effective solar heater for it.
the big problem with solar pool heating is insulation. trying to heat
a pool without good insulation is like trying to bail a bottomless boat.
people who sell solar pool heaters don't seem to recognize this. they
just sell bailing buckets. ("leak in the boat? not my problem.")
>living in the chicago area, our season is short on ideal swimming weather.
like in december, when the average outdoor temp is 27 f and the average
amount of sun that falls on a south wall is 740 btu/ft^2/day? seems to me
it would be nice to keep the pool at say, 90 f on a day like that...
>i have the ability to tig weld aluminum tube.
good. sounds like you are handier than the average person.
>last year i made a reflector(polished stainless) to focus on 4-4'alum.
>tubes 1-1/2 dia welded top and bottom to a 2" manifold type arrangement.
>i painted the tubes flat black.
you don't say how large the sun collection area was, but it sounds like
you were not gathering enough sun to make much difference. it also seems
to me that focusing systems tend to be more expensive than non-focusing
ones, and they are better suited for higher temperature applications, eg
solar ovens or pottery kilns or lead smelters.
>it did work but the flow rate had to be dialed down to a trickle!
seems to me the higher the flow rate, the lower the solar heated water
temperature, the lower the losses from collector to air, and the higher
the solar collection efficiency. you don't need to make high temperature
water. any water temp higher than the pool water temp will heat the pool.
>do i need 3 more of these ?
like a fish needs a bicycle :-)
>should i make a insulated box to contain the heat?
it seems to me you need to somehow insulate the whole pool.
>any help on these ? would be great
ok. each square foot of "solar collector" can gather about 740x0.9 = 666 btu
per day (note evil portent) and might lose about 6hr(90-27)1ft^2/r1 = 378 btu
per day for a net collection of about 288 btu/day. suppose your rigid thermal
cover is held up by an inflated top that is 12' above the pool in the middle
and 24' across, with a south-facing area of 12^2xpi/2 = 226 ft^2, so it can
collect about 226x288 = 65k btu/day of mostly horizontal sun, which is only
about 25 degrees above the horizon at noon on 12/21 in chicago...
you have about 780 ft^2 of pool surface, ignoring the bottom for now, since
the soil is warmer and has a higher thermal resistance than air. keeping a
square foot of exterior pool surface at 90 f when it's 27 f outside takes
about 24(90-27)1ft^2/r = 1512 btu/day, so we need 1512x780/r = 65k, so we
need (us) r = 1.18 million/65k = 18, eg 4" of styrofoam at r5 per inch, as
a rigid floating cover, and slipped between the liner and the sidewall.
the cover might be a 24' diameter disk, held up by a single $30 piece of uv
polyethylene greenhouse film, with the north half painted white to reflect
winter sun down into the pool when inflated. the dark colored foamboard
surface would be just above the water line at night (with the poly film edge
below the water to make a good seal), and just below on a sunny day (to allow
the sun to heat the water on top of the foam surface.) to swim in december,
(for a few hours a week) deflate the cover and let the whole thing sink to
the bottom (alas, poor guido--do not swim under the cover.) as an alternative,
the cover might be two layers of poly film, inflated with air during the day
and inflated with tiny cold soap bubbles at night.
the pool contains about 116k pounds of water, so over a cloudy day the
temperature would drop about 116k/65k = 1.8 f.
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
web site: http://www.ece.vill.edu/~nick
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