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Solar heating panel evaluation - Page 3

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Posted by Gary on August 3, 2004, 2:05 pm
Morris Dovey wrote:

If I am getting your numbers right, 4759 in^3 of air go through the
collector in 16 secs -- This gives a flow rate of:

Flow Rate = (Volume of air per unit time)
         (4759 in^3)(1 ft^3/1728 in^3) /((16 sec)(1 min/60 sec))
         = 10.32 ft^3/min (cfm)

Heat Output
= (Flow Rate)(air density)(Temperature rise)(specific heat of air)
= (10.32ft^3/min)(0.076 lb/ft^3)(181.0F - 85.3F)(0.24 BTU/lb-F)
=  18 BTU/min  or 1080 BTU/hr or 317 watts
To get a rough idea how efficient the panel is, we would have to know:
1) what is the collection area (glazed area) of the panel
2) how the panel is oriented to the sun (e.g. pointed due south and
titled 90 degs).
2) How much sun is shinning on the panel -- if you don't have a way to
measure the sun intensity, then you could do the test on a clear day,
and we could look up the sun intensity knowing your location, and the
date/time.  Doing the test around solar noon (when the sun is due
South), and with the panel pointed due south would be a good setup.

The efficiency is:

    efic = (panels heat output)/(solar input)

        where heat output is the 1080 BTU/hr from above.

The outlet temperature of 181F seems pretty high to me -- I think you
might get more efficiency (more heat out of the panel) if you figured
out a way to increase the air velocity.  Perhaps larger vents, or less
restriction in the panel.  The effect of this would be to lower your
output temperature, but increase the flow rate  -- you should come out
ahead because your losses out the glazed side of the panel will be
less.  The losses depend on the difference between your collectors
internal temperature, and the outside air temperature.

Another way to get the flow rate through the panel is to measure the
flow velocity at the exit vent.  The little hand held devices Kestrel
(and others) sell to measure wind velocity work pretty well as long as
the velocity out of the vent is 90 ft/min or more.  The one I have
cost about $0.  But, timing the smoke puff through the panel might
actually  be more accurate?

What are you using to generate smoke?  I've been looking for a good
way to do this.



Posted by Morris Dovey on August 3, 2004, 3:04 pm
Gary wrote:

Sorry. My way isn't a healthy one and I certainly won't recommend
it to someone who's just helped me...

I blow a tiny puff of cigarette smoke through an 18" length of
1/4" ID tubing.

I would guess that the airflow though my collectors is uneven
(faster through some paths than others) and that the smoke method
may be accurate only for the shortest/fastest paths.

Morris Dovey
DeSoto, Iowa USA

Posted by Morris Dovey on August 3, 2004, 2:51 pm
 Morris Dovey wrote:


Nick, Ecnerwal, and Gary...

Wow! A usenet first for me. I asked a newby question and got not
one but three informative and totally useful responses!

My thanks to all!

Morris Dovey
DeSoto, Iowa USA

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