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solar collector idea

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Posted by Jeff on December 29, 2007, 3:59 am
 
   I need to build a car shelter and was thinking, how I can turn this
into a solar collector?

   Here's what I was thinking, it's my interpretation of Nicks solar
attic, sort of:


  The basic concept is dead simple.

  An absorber plate at a 45 degree angle (or so) with glazing closely
spaced above it. The absorber would be polyisocyanurate insulation with
the aluminized side painted with Solkote selective paint, a fraction
above that would be the glazing.

   Now, hot air 75' from the house does me little good. So, Nick's car
radiator with a fan to blow the heated air through and return the
exhaust to a plenum at the bottom of the collector.

   I think the glazing to absorber spacing should be small to maximize
convective transfer. The selective coating would minimize reradiation.

   Does that make sense as a collector?

   UPDATE ON MY HOT WATER COLLECTORS

   Oddly, it's raining here. I have 5 out of my 6 solar water panels
riveted together. It takes about 3 hours to snap the aluminum on the
copper risers and rivet a 2' x 10' panel together. Adding up the time
for cutting and forming the aluminum, drilling, cutting and soldering
the header copper to the risers, smearing heatsink compound, et all, I'd
say it takes a good 8 hours to make one panel. I'll paint them all when
the weather improves...

   Jeff

Posted by nicksanspam on December 29, 2007, 3:14 pm
 


It might be steeper, for winter house heating. How about a vertical wall,
eg U0.58 ThermaGlas Plus twinwall polycarb with 80% solar transmission?


How about a vertical curtain of 80% greenhouse mesh shadecloth or 1-2
layers of dark window screen, a foot or so from the twinwall?


You might put the radiator near the top of the shadecloth and blow air
north into the shelter. A dark foamboard wall north of the mesh could
raise efficiency. It might also help to tilt the mesh north or use more
mesh layers near the top, to make the mesh airflow more uniform.

Near Phila, with an A = 5x4'x12' 12'x20' twinwall and a 1000 Btu/h-F
radiator, we'd have something like this in full sun, in a fixed font:

  200A Btu/h    Ta
    ---         |     1/1000
|--|-->|--------*-------www----- 140 F
    ---         |             |
                |            ---
      1/(0.58A) |             -
30 F ---www-----              |
                              -
which is equivalent to:

                Ta
      1/139     |     1/1000
   ----www------*-------www----- 140 F
  |                           |
 --- 30+200A/(0.58A)         ---
  -     = 375 F               -
  |                           |
  -                           -

If (375-140)/(1/139+1/1000) = 28.7K Btu/h flows into the water, the air
temp Ta = 140+27.8K/1000 = 169F, with a collection efficiency of about
100x27.8K/(250x240) = 46%. With 2 radiators, (375-140)/(1/139+1/2000)
= 30.6K Btu/h might flow, with 100x30.6K/(250x240) = 51%. A reflective
surface south of the glazing would help.


That would also increase the convective transfer to the glazing.

Nick


Posted by Solar Mike on January 2, 2008, 11:35 am
 

Hi Jeff

I have a similar problem, except rather than use a car shelter roof as the
collector, I was going to build a collector roof (30 sq meters) on poles
above the drive way leading to the car port area. Cannot use the vertical
sides due to shading from winter sun by a neighbors house.

I thought about using air collectors to heat the water in the 3000 liter
storage tank via several car radiators and associated fans. But will this
setup be as efficient as say a trickle water collector, there seems to be
greater potential for losses, with all that air rushing around.

The air collector could be much less costly to build, as a mesh curtain of
black green house shade cloth or open weave felt type material could be used
to extract the heat. But this means lots of extra insulation in insulating
the large top and bottom plenums.
The fans will have to work against a static air pressure in circulating the
air at sufficient rate. Car radiator fans don't have any real pressure
ability, so some sort of centrifugal fans would be required e.g. car heater
fans might work. - If desired to run off 12 volts DC supply. If the fan
motors are inside the air path they may overheat.

I think the glazing needs to be some reasonable distance from the collector
surface to allow sufficient air flow, Six inches at least at the bottom,
less at the top. e.g. slant the material inside the box.

Polyisocyanurate will start to fail at the expected internal temperatures
should the fans stop. I would use a non binder type of  high temp industrial
fiberglass or rock wool insulation between the hot air and the poly. This
sort of rules out using the blackened foil covered poly as a direct heat
exchanger.

Seems easier to make trickle water collectors for long term reliability,
much less bulky, long life brass pump, black powder coated zincalume
corrugate sheet. Same basic materials for case and glazing. Spray coat the
inner water exposed surface of the polycarbonate twinwall with clear two pot
epoxy to prevent deteriation of the inner layer by water absorption.


Cheers and all the best for 2008
Mike Scaife (NZ)







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