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solar air heater review and a few questions - Page 3

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Posted by Morris Dovey on September 24, 2008, 4:57 pm
 
gary@builditsolar.com wrote:

<snipped>

Thanks to Gary for posting his test results!

A couple of general observations...

The natural airflow appears to be directly related to the
cross-sectional area of the plenum, and inversely related to the surface
area of the plenum.

What this means is that the "ideal" plenum would be cylindrical. :-)

Given that we're practically constrained to a rectangular cross section,
the best we could do would be a square cross section, which still falls
short of what we want to build - but points us in the right direction:
to incorporate the deepest practical plenum.

For a given design/construction, the operating temperature is directly
related to the height of the panel.

For a given design/construction/height, the amount of energy captured
and the amount of heat delivered will be directly related to the width,
but the temperature is not, in general, affected by the width.

The "ideal" intake vent matches the cross-sectional area of the plenum.
If the intake vent is larger than the plenum, then the plenum becomes
the bottleneck. If the intake vent is smaller than the plenum, then the
intake vent becomes the bottleneck.

The "ideal" discharge vent is likely to be larger than the intake vent
in order to accommodate the same natural flow of expanded air, but I'm
unsure how to generalize this be cause design dependencies may come into
play allowing (or not allowing) the rate of flow to increase through the
discharge vent.

HTH

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/

Posted by Morris Dovey on September 24, 2008, 10:22 pm
 
Morris Dovey wrote:

<snipped>


Hmm - no feedback so I don't know if I managed to communicate anything
helpful or not.

On the off chance that the info might be useful to someone, I've
incorporated it into a(nother) web page at the link below...

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/Astro/Plenum.html

Posted by Steve O'Hara-Smith on September 27, 2008, 4:50 pm
 On Wed, 24 Sep 2008 11:57:38 -0500

    Strangely this did not arrive at my news server until Saturday.


    Seconded.


<snip>    Interesting and at the same time unsurprising observations -
it's very nice to see expectations confirmed by experimental evidence.


    Hmm - contemplates a hemicylinder - nah rectangular it is.


    Yep somewhere diminishing returns will bring a sensible limit to
depth.


    Height too of course - but adding width is more efficient than
adding height since ...


    My inclination is that it's not going to be terribly critical once
the discharge vent is big enough not to be the bottleneck. Expansion is
proportional to temperature (all other things being equal) so the ratio
of discharge vent size to plenum CSA is proportional to height.

<brain engages - whirrr>

    Hmm - ideally the plenum should be expanding in CSA with height to
accomodate the expansion of the warmed air. Anyone care to take a stab at
the appropriate rate ?

--
C:>WIN                                      |   Directable Mirror Arrays
The computer obeys and wins.                | A better way to focus the sun
You lose and Bill collects.                 |    licences available see
                                            |    http://www.sohara.org/

Posted by Morris Dovey on September 27, 2008, 11:04 pm
 Steve O'Hara-Smith wrote:

That may be the most pleasant sound I've heard all week... :-)

If we assume unimpeded/frictionless airflow and constant viscosity with
equal intake, plenum, and discharge vent sizes, then we should see a
higher linear velocity at the discharge vent than at the intake vent
because the discharge volume is greater than the intake volume.

If we choose to design the absorber plenum in such a way that the intake
and discharge linear flow rates are equal, then we need to examine the
ratio of the absolute temperatures at the two vents - and apply that
ratio to determine the reverse taper of the plenum and the vent sizes so
that the discharge vent is as large as necessary, but no larger. :-]


Hmm back at you. :-) If we examine the original assumptions of
unimpeded/frictionless airflow and constant viscosity and consider the
effects of holding more and warmer air in the absorber plenum for a
longer time, I think the results will be heavily dependent on, of all
things, the absorber - since it is at the absorber/heat exchanger
surface that the effects of those factors I was so happy to make
assumptions about will be most pronounced.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/

Posted by Steve O'Hara-Smith on September 29, 2008, 2:57 pm
 On Sat, 27 Sep 2008 18:04:45 -0500


    I rather suspect you're right there, worse yet it's going to depend
on the absolute temperatures of intake and outflow which are not constant.
It might be interesting to make a type 2 collector (your terminology) with
an adjustable sloping middle panel and experiment to see if there's any
appreciable impact on efficiency. My gut feeling is that it's probably not
great.

--
C:>WIN                                      |   Directable Mirror Arrays
The computer obeys and wins.                | A better way to focus the sun
You lose and Bill collects.                 |    licences available see
                                            |    http://www.sohara.org/

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