|
|
re: swimming pool solar heaters (what we need is a good cheap collector.........)
25 may 2000
lutz wrote:
>nick, thanks for your comments.
you are welcome.
heating a pool without a good insulating cover (eg us r20) is like
heating a house in the winter with all the doors and windows open.
you might say "the furnace works fine" if it's warm enough indoors,
but something is wrong with that picture.
>...might it not be more simple to put at the same place below the pool
>where are now this black leaking collector a ramp of dark colored
>concrete which has at the upside a pipe with some holes in it and
>at the downside a tube like a gutter which collects this warm water
>and pumps it up in the pool ?
it seems even simpler to darken the concrete coping (the "sidewalk"
around the edge of an ordinary outdoor in-ground pool) and run water
from a pipe with some holes around the outer edge of the coping, then
back into the pool by gravity.
>only problem: really much chlorine is lost and logically the
>water will be polluted with dust and alga.
then maybe the water should run through pipes in the concrete.
on the other hand, exposing more pool water surface to air lets
it absorb more oxygen which can burn off impurities and reduce
or eliminate the chlorine requirement. (i don't like chlorine.)
>>hmmm, $7.40 per square foot plus shipping from say, sao paolo,
>>versus $0.14 per square foot for a solar cover with a 5 year
>>guarantee--53 times cheaper.
>you need to pay here directly to the reseller, if you can’t buy
>a whole roll with 250 qms, $0,60 per square foot for a solar
>cover with a 6 month guarantee only – still 12 times cheaper.
water warehouse (www.waterwarehouse.com, (800) 574-7665) in kenosha, wi
might sell you cheaper solar covers. their $109.99 12 mil (0.012 inch)
20x40 foot solar blanket has a 5-year guarantee. but the blue color
reduces the solar transmission by 50% (my measurement), and it's only
us r1 at best, and it's work to remove. we can do better than that.
>but you never have a differential of temperatures by heating an
>outdoor pool of 5°c per day, haven’t you ?
can a solar pool cover can raise the temperature of an outdoor pool 5 c
per day? a 6' deep pool with 384 btu/f-ft^2 of thermal capacitance and
a solar cover that transmits 2500x0.5 = 1250 btu/ft^2 of sun (a clear
may day where i live) gains at most 1250/384 = 3.3 f on the first day,
if it's perfectly insulated, less on the 2nd day, and so on.
(i measured a sample of the deluxe solar-cell sunblanket (tm) from
"the pool cover experts," century products at 171 medford street/
malden, ma/02148 (800) 225-3472. "saves energy! raises water temp
10 degrees or more in season, without running your heater.")
can a "solar pool cover" make an outdoor pool 5 c warmer over, say,
two weeks than the same uncovered pool? maybe, with enough sidewall
and bottom insulation, in a low-humidity climate. a 24' above-ground
circular pool with 48" sidewalls has about 450 ft^2 of top surface
and 300 ft^2 of sidewall...
ashrae says a square foot of exposed pool water loses 100(pw-pa) btu/h,
where pw and pa are partial pressures of water in air near the pool and
outdoor air in inches of mercury. at 80 f, pw = exp(17.863-9621/(460+80))
= 1.05", approximately, and the average humidity ratio w = 0.0036 pounds
of water per pound of dry air in phila in march (when the average outdoor
temp is 42.4 f), so pa = 29.921/(1+0.62198/w) = 0.17", so a square foot
of exposed pool water loses 88 btu/h or 2107 btu/day, more than the 1200
btu/ft^2-day average solar gain. an r20 cover would reduce this by about
50x, to 24h(80-42.4)1ft^2/r20 = 45 btu/day.
here's a computer program to estimate pool temps using monthly average
philadelphia weather data for january through may:
10 data 30.4,620,.0025
20 data 33.0,870,.0027
30 data 42.4,1200,.0036
40 data 52.4,1520,.0050
50 data 62.9,1760,.008
60 rvs=1'sidewall and bottom us r-value
70 gs=750/rvs'thermal conductance of sidewall and bottom (btu/h-f)
80 for m=1 to 5
90 read ta,i,w'ambient temp (f), sun on horiz surf (btu/day) and
humidity ratio (#h20/#dry air)
100 pin=.96*450*i'solar power input without cover (btu/day)
110 pa=29.921/(1+.62198/w)'vapor pressure of water in outdoor air ("hg)
120 tp=ta'initial pool temp est (f)
130 tp=tp+.1'pool temp estimate (f)
140 pw=exp(17.863-9621/(460+tp))'vapor pressure of air near pool ("hg)
150 pout=24*(100*(pw-pa)*450+(tp-ta)*gs)'pool heat power output (btu/day)
160 if pout>can't those good german engineers come up with a metric r2
>>solar cover that's easy to slide off the pool?
>...you can motorize it controlled by a solar sensor.
>but this isn’t even cheap, isn’t it ?
no. one local engineer with a house on a small sloping lot built
a tennis court over a pool, with motors and tracks to slide the
tennis court off for swimming, but that wasn't cheap.
>in france they have... a system with plastic balls (like ping-pong)
>floating on the surface at night and with sun on it (or if you want
>enter) they are stored by the water flow in a chamber...
sounds like a start, but empty ping pong balls are only us r1 at best,
vs an r20 house wall, and they would leave 9.3% of the water surface
exposed to air at night if they were half-submerged, or more if they
floated higher or lower, so lots of pool heat would be lost by evaporation
at night, and a lot more during the day with no balls on the surface.
"coverite" soft plastic baggies can insulate better, since they can
cluster together like soap bubbles, with less exposed water.
>...nothing is simple or even cheap.
but maybe simpler or cheaper or better-performing. 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?
nick
|
|
