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An A-frame solar water heater concept

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Posted by Nick Pine on July 21, 2005, 9:31 am
                                    .                       ----
                                   . .
                                  .   .       30 F
                                 .     .
                                .       .
                               .   pond  .
                              .-----------.         ----    
                         12' . s    4'     . 12'            10.4'
                            . s             .
            <--S           . s      T        .      
                          . s                 .     6.92'
                         . s                   .
                        . s                     .
                       . s                       .
                      .duct                   duct.

We might build a 12'x16' equilateral A-frame with a 4'x12' shallow pond
at the top (view above, in a fixed font) and 2 poly film water ducts
along the north and south edges to avoid wind sliding and overturning...
20 psf makes 208 lb and 1082 ft-lb on each 1' EW slice of the greenhouse,
so we need 12W = 1082 ie W = 90 pounds of water in each foot of duct, eg
1.44 ft^3 of water in a 16" duct.

We could make each of the 10 slightly curved "half-bows" on 4' centers
with 2 12' 1x3s with 1x3 spacer blocks every 2' and a hinge at the top,
and use 3 horizontal 1x3 purlins.

The south side could have 80% shadecloth to make hot air rise under the
pond (which could be poly film over EPDM over foil over welded-wire fence.)

If we can somehow arrange that most of the greenhouse stays cooler while
the airpath between the shadecloth and glazing and under the pond is temp
T during the day, and the space above the pond is also temp T, we might
figure 0.9xsqrt(1000^2+620^2)12' = 12,712 Btu enters a 4' slice of south
glazing on an average 30 F Jan day in Phila, and 0.81^2x4x1177 = 3813 of
that enters the pond. At 130 F, it might also gain 6h(T-130)4ft^2x1.5
= 36T-4680 Btu/day from the bottom, and more, if the ground to the south
is reflective.

If the glazing loses 6h(T-30)12ft^2/R0.8 = 90T-2700 Btu/day and the daily
energy that flows into the slice equals the energy that flows out, 12712
= 3813+(36T-4680) + 90T-2700, so T = 129 F, and the pond slice gains about
3813 Btu, and 50K/3813 = 13.1', so a 16' greenhouse might provide most of
the heat in January. With about 3813x16'/6h = 10.2K Btu/h during solar
collection, 5 gpm (2400 Btu/h-F) would rise 4 F, and 400'x1/2" PE pipe
with 75 ft^2 of U30 surface would rise 10.2K/(75x30) = 5 F.

A row house with a flat roof might have a $8 12' diameter x 3' tall
EZ-Set pool in the basement with a $0 300'x1" fresh water pressurized
PE pipe heat exchanger near the top of the pool under floating Styrofoam
and a low-head pump with a $0 400'x1/2" PE pipe heat exchanger in the
pool bottom. With lots of insulation and 0.8xPi(11/2)^2x3x62.33 = 14216
pounds of water at 110 F after 5 cloudy days, after supplying 5x50K Btu,
it needs to be 110+250K/14216 = 128 F on an average day. Then again, it
might melt :-) It might need reinforcing, eg a tarp tied up around it.
Or maybe we need a different kind of pool.

This might also be a standalone structure in a yard, with the filter pump
that comes with the pool.


Posted by George Ghio on July 21, 2005, 12:45 pm

Nick Pine wrote:

You used the word "Might" 8 times, "Could" 3 times, "If" 3 times, with a
"Maybe" and "Somehow" once each.

"Could" it be that you "Might" be proposing that "If" "Somehow" you
"Maybe" actually built one you could prove your point without using
those words that only confirm that you are in fact just guessing.

Posted by News on July 21, 2005, 1:41 pm



LOL, that was good.  Nothing personal Nick. :-)

Posted by nicksanspam on July 21, 2005, 2:17 pm

Thank you for counting.

The only big risk is the plastic pool losing strength at 130 F, and there
are solutions for that. I go overboard on the "mights" to avoid ignorant
people like you raising angry challenges to 300-year-old settled physics.


Posted by Ed Earl Ross on July 21, 2005, 4:06 pm
 nicksanspam@ece.villanova.edu wrote:

Perhaps a scale model would help settle the mights, coulds and maybes.


"If the man doesn't believe as we do,
we say he is a crank, and that settles it.
I mean, it does nowadays, because now we
can't burn him."  (Mark Twain)

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