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re: thermo-siphon solar water heater
9 sep 2002
frank monroe  wrote:

>i plan to build a thermo-siphon water heater into the new roof of my garage.
>each collector will consist of five 1 1/2" abs plastic pipes 8 ft long,
>connected at the top and the bottom, between each rafter...

with rafters on 2' centers, they will only intercept about 100x5x1.5/24 
= 31% of the sun...

>...on each side of the roof peak (30 ea x 8 ft pipes on each side)... 
 
the collector pipes would store about 377 pounds of water. warming it
from 74 f to 120 f every morning would require 377(120-74) = 17.3k btu.

with 2x6x8x2 = 192 ft^2 of r1 glazing with 90% solar transmission and
1180 btu/ft^2 of sun on a horizontal surface in honolulu on an average
74 f december day, you might gain 0.9x0.31x1180x192 = 63.2k btu/day and
lose 6h(120-74)192ft^2/r1 = 53k btu/day at 120 f. this seems inefficient.

>the reservoir will be two 4" abs pipes 15 ft. long  and one 6" abs pipe
>20 ft. long (total storage ~ 50 gal.)...

not much, compared to a typical day's use.

>the 6" pipe will be hung under the ridge line and angled down slightly
>(but still above the top of the collectors) with the 4"pipes hung on
>either side and level. all pipes will be insulated.  see below.
>
>    _______________________________           level 4" pipe
>   |   _____________________________ < from top of east collectors
>   |  |
>___|  |_____________________________________ slightly down slanted  6" pipe
>
>___    _____________________________________ low end to bottom of collectors
>   |  |
>   |  |___________________________      level 4" pipe
>   |_____________________________ < from top of  west collectors

looks ok, if the bottom end of the 6" pipe is "still above the top of
the collectors" and the solar water actually flows through the 6" pipe.

>a heat exchanger of 100 ft of 1/2" or 3/8"  copper tubing will be coiled
>(spring-like) inside the 6" pipe.  both in and out connections will be at
>the high end of the pipe (left end on above sketch).  the feed will be from
>the cold water supply and the outflow will be connected to the cold water
>taps of the washer and the showers (so that hot water can be supplimented if
>necessary to increase shower temperature)..

why not run the solar hot water into the drain fitting on the water heater
with a high temp garden hose?

>should i consider using copper pipe instead of abs for the collectors?

yes, to decrease the amount of water stored in the collectors and
increase the sun-gathering surface.

>i could solder fins on the pipes too. the cost isn't much different but
>the copper would require soldering which is much harder than the solvent
>assembly of abs..

you might make ersatz big fins with brown-painted thick aluminum coil stock,
ie roofing material (approx. 50 cents/ft^2 in 50' coils, a foot wide) bent
into a u around some 1/2" hard copper pipe, then sandwiched and bolted below
the pipe with some drywall l-bead on each side, then spread out flat again,
with the pipe as a bump in the middle, facing the sun, and the brown side
of the aluminum towards the sun, like this, in courier font:

                                                  .  .
                                       s       .  .  .  .
                   aluminum flashing   ssbss . .  1/2"  . .   
                .........................o... .  copper  . .  
                .........................l... .   pipe   . . 
                                       sstss . .        . . 
    sssss is 1/2" galvanized           s       .  .  .  .
              drywall l bead                      .  .

                                  --- .  
              <-- south            |  . 
                                      .   
...then open it up like this:         .  
                                      .           .  .
                                      .s       .  .  .  .
     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 . .        . . 
                                      .s       .  .  .  .
                                      .           .  .
                                      .  
                                   |  .  
                                      . 
                                  --- .

>is it worthwhile to use double glazed panles?

not in hawaii.

>is 100 ft enough tubing for the heat exchanger?  too much? i know it
>kind of depends on the speed of the flow and the teperature of the
>surrounding water.  is there a formula to calulate this?

this might be an ashrae heat exchanger with cmin = 5gpmx8btu/fgx60m/h
= 2400 btu/h-f and z = cmin/cmax = 0 and a = 100x1/2"/12xpi = 13.1 ft^2
and u = 30 btu/h-f-ft^2 and ntu = au/cmin = 0.164 and e = 1-exp(-ntu)
= 0.15 = (tco-tci)/(thi-tci) = (tco-74)/(120-74), so 74 f water would
exit at tco = 74+0.15(120-74) = 81 f, but with copper pipes, you might
use a large pressurized insulated tank with no heat exchanger...

with a heat exchanger or gravity feed or an rv pump, you might consider
storing hot water in a plywood trough with bulkhead fittings and a poly
film duct liner.  a 30" diameter greenhouse ventilation duct (about 60
cents per linear foot) in a square 2'x2'x20' trough could store about
640 gallons of water.

nick




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