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Solar Air Collector - S run of heatign duct as collector??

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Posted by schooner on October 5, 2005, 6:29 pm
 
Just thinking of various ways on the solar air collector.

What about using a run of heating duct, or similar smaller steel or aluminum
ducting, and have it connected in an S pattern bottom to top as a single run
of closed ducting.  Air flows in the bottom, up through the ducting and out
the top.  The ducting would be painted black and boxed in a collector box
like most setups and glazed.  The ducting then acts as the absorber and the
air channel.

Has this been tried?  Any comments on possible problems with such a design?
Would the pipe need to be under a certain size?

Got the idea from the Cansolair units which basically do something similar
only using pop cans running vertical with air flowing through them with ends
acting as a baffle.



Posted by schooner on October 5, 2005, 8:04 pm
 
Was out looking at what material could use and seen the 3" flexible aluminum
ducting.  Can be easily bent for S turns.  My only concern would be the
airflow over a long length of this due to the rough surface.  Does anyone
know if this would be workable in such a setup or would airflow be too
constricted?



Posted by David L. Jones on October 6, 2005, 5:04 am
 schooner wrote:

The rough surface has good and bad points. Good in that it helps to
break up any laminar air flow (helping to pick up more heat), but bad
in that it increases the overall duct resistance.

3" is likely to be too small.
To fill a decent size collector (say 1.5m x 1.5m) you'd need about 30m
of tubing - ouch!

My "rectangular tube" approach can use a small total diameter like
this, but because it is "flattened" it requires much less total tubing
length to fill up the collector area.

Dave :)


Posted by David L. Jones on October 6, 2005, 4:52 am
 schooner wrote:

Yes it will work. It is basically like my design which is a flat
aluminium plate for the collector and then I rivot on aluminium backing
"channels" to collect the heat from the front plate (which also forms
the top side of the channel. I use 1" square aluminium tubing to form
the channels and the side of the box, so my collector (minus the
dead-air-space and frame) is only 1" thick.

So my design basically has "square" tubing instead of round.

The great thing about aluminium is that it has 4 times the thermal
conductivity of steel, so the entire box gets hot very quickly. The air
therefore has more surface area (top plate, sides, back) to extract the
heat from.

Dave :)


Posted by schooner on October 6, 2005, 9:14 am
 I'm thinking now of using the flexible tubing but making straight runs
bottom to top into a collector box area.  That way flow will be better but
the rough surface should provide some resistance to improve heating.  The
nice thing of the flexible ducting is no cutting, it comes in 3' compressed
and can stretch to 10' so making 6-8' runs is easy.

So the dead space you use is only 1" between glazing and the collector?

What are you using for the channel?  I was looking at aluminum/steel stud
that had that very shape and though it might work well.  But I figured the
rough surface of the tubing plus the round shape would actually give it more
surface area to the sun.

Any pics of your unit?



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