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re: totally tubular solar water heater
9 jun 1999
>hi solar heads,

hi toby, 

>why isn't black poly tubing utilized more for solar heat collection?

our esteemed colleagues seem to be saying that it cracks and leaks after a
year or so in the sun because of solar heat and uv exposure, and it gets
very floppy and bursts at temperatures greater than 130 f or pressures
greater than 10 psi. however, i recall a definitive posting from a credible
someone who used one kind (not poly? more money?) outdoors on a roof for
swimming pool heating for over 10 years, with no deterioration.

the uv might not matter if it's under glazing, and the temperature limit
might not be important if it has no selective surface, and always contains
running water when the sun is shining, or if the glazing cavity has some
vents that open up to allow airflow when the temperature rises to more than
150 or so, or if the glazing itself limits the temperature rise. polyethylene
film might do this, since it is 80% transparent to longwave ir (ie it has
little "greenhouse effect.") a shallow stagnated test box covered with poly
film with 2" of styrofoam (us r10) on the back hit 207 f in full sun in my
70 f sunspace. 

>black, water-filled, 1" diameter, poly tubing, which comes in 1000’
>rolls, absorbs 39 times (as i'll show below) more heat from direct
>sunshine (solar radiation) per pound than in an equal amount of water
>in a 55-gallon barrell drum.

so it would gain heat at a faster rate in the sun and wake up and start
producing useful heat sooner in the morning than a drum full of water in
a sunspace. good, if those were the only alternatives... 
$75/83ft^2 = 90 cents/ft^2 sounds nice for a solar collector, vs
$699.95/(4'x20') = $9/ft^2 for a fafco solar swimming pool heater.
of course the problem with swimming pools is the cover.

my 1984 used dodge omni automobile radiator cost $35, with its attached 12v
fan (a more efficient 110 v 20" window fan costs < $20.) that combination
might have an effective thermal conductance of about 800 btu/h-f, like
500 ft^2 of flat surface collecting heat from slowly-moving sun-warmed air
near the top of a sunspace.

>tubes are easier to mount.. on roofs...

sure. just lay 'em out in a spiral... can we space the turns apart and
somehow glue them to a dark roof, so it can act as a heat collecting fin?

>on greenhouse or “sunspace” “ceilings”, or attics

this tubing requires some sort of support, eg a tray under some horizontal
sunspace glazing, a la anna edey. is there a simpler way? slip it through
holes in rafters? make sloppy festoons on hooks? u-clamps? ropes?

>i believe the tubing can collect heat much better than most other water
>tanks/drums/containers” simply because, there is more surface area
>per pound of water compared to all other water containers i can think of.

what does it mean to collect heat "better"?

i'm still thinking homemade big fins under some horizontal sunspace glazing
might be a good way to make domestic hot water. no anti-freeze or heat
exchangers, if the sunspace is kept above freezing, nor pumps or controls,
with a pressurized thermosyponing loop through a water heater above with
a heating element that rarely turns on. these might cost more than $1/ft^2,
and there's more labor, but they don't require separate supports, and they
can withstand higher temps and pressures, with no deterioration in the sun: 
                                                  .  .
                                               .  .  .  .
                   aluminum flashing   ssbss . .  1/2"  . .   
                .........................o... .  copper  . .  
                .........................l... .   pipe   . . 
                                       sstss . .        . . 
    sssss is 1/2" galvanized                   .  .  .  .
              drywall l bead      --- .           .  .
              <-- south            |  . 
...then open it up like this:         .  
                                      .           .  .
                                      .        .  .  .  .
     home depot sells dark-           .ssbss . .        . .   
     brown-painted aluminum       ~5" ...o... .  copper  . .  
     roof flashing for $46.50         ...l... .   pipe   . . 
     for a 50'x2'x0.027" roll.        .sstss . .        . . 
                                      .        .  .  .  .
                                      .           .  .
         [use courier font.]          .  
                                   |  .    these might be
                                      .    60% efficient heating 120 f
                                  --- .    water inside an 80 f sunspace.


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