Hybrid Car – More Fun with Less Gas

Black roof tiles or black metal sheet best for collector. - Page 2

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Posted by Jeff on June 20, 2008, 7:29 pm
AJH wrote:

   Solar Mike in NZ did something like this with water. Unknown how that
worked for him. I don't know what the pressure drop would be with air.

   I think the best/easiest air collectors are flow through, where the
air flows through the absorber.


Posted by gary on June 22, 2008, 1:51 pm

An interesting idea,
If I am understanding correctly, you want to use the triple wall
polycarbonate as the air channel for the collector, with the back
surface painted black for absorption, and placed right against the
rigid insulation?
I think that the air channels in the polycarbonate may be too small,
and offer too much resistance for any significant length of collector?
The usual rule of thumb is that you want about 2 cfm per sqft fo
collector area.

I guess if turns out to be too much flow for the airspace inside the
triple wall, you could space the triple wall above the rigid
insulation board, and run the air in a channel behind it (a backpass
collector).  You lose the advantage of the channels in the Polycarb
directing the flow, but backpass collectors are usually more efficient
(lower loss).

I'm not clear on whether you want airflow in the topmost channel of
the triple wall?  It seems like this would not be very effective in
picking up heat from the absorber (back wall), and might tend to
increases losses out the front wall?


Posted by AJH on June 22, 2008, 5:25 pm
 On Sun, 22 Jun 2008 06:51:33 -0700 (PDT), gary@builditsolar.com wrote:

That's correct but I wondered about getting the matt black *inside*
the bottom polycarbonate channel.

Thanks for that another rule of thumb is that air speeds needs to be
under 10 metres/sec to keep pumping losses low. It looks like your
figures give 600 litres/minute per square metre which is 0.01M^3/sec
per M^2, canceling through that's 0.01m/sec, so not a lot of power
lost. I'll have a 6m length but I do realise the 1cm by 1cm channels
will give some resistance.

Actually I'm intrigued by the idea of a plenum behind a porous
collector, so the air passes though the collector, as mentioned by
another responder to my post. This triple wall polycarbonate is quite

No I was just looking at the top channel as being an insulating air


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