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Solar Air Collector with Bottom In/Out Ducting? - Page 2

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Posted by David L. Jones on October 6, 2005, 3:24 am
schooner wrote:

Practically speaking - I'd say not.
It's all a matter of getting sufficient fan power and airflow rate.

Dave :)

Posted by schooner on October 6, 2005, 9:15 am
Is it possible to have too much airflow or is a certain amount best?  Would
say a 200cfm fan be too much or is it just a matter of how much fan noise
you want to stand?

Posted by Anthony Matonak on October 6, 2005, 10:14 am
 schooner wrote:

There are a number of factors to balance. Noise is important, as is
the amount of power required to run the fans. Slow fans take less
power and make less noise than fast fans.

As far as heating goes, as long as there is a temperature rise
between the in and out of the collector then you are fine. In fact,
the less the temperature rise and the more the air flow then the better
the efficiency as less energy will be lost through the (typically
uninsulated) front. If you have too cold a day with too high an air
flow then you'll have a lot of heat loss through the front of the
collector and will actually wind up cooling the air instead of
heating it.

Since home made collectors are so cheap, a little less efficiency
can be easily compensated for just by making them a little larger.


Posted by nicksanspam on October 6, 2005, 2:07 pm

On a 6 mph day, a single layer of front glazing might have an U5 outdoor
airfilm conductance and 2+V/2 Btu/h-F-ft^2 for the inside, with V in mph.

But you wrote above, "the more the airflow then the better"...

If 225 Btu/h of sun enters a 1' cube with 5 perfectly insulated sides
and a single layer of "R1" glazing on a 30 F day and C cfm of airflow
provides about Q = (T-70)C Btu/h of useful heat output, where T(F) is
the air temp in the box and G = 0.2+1/(2+0.01136C/2) is the glazing
conductance, 225 = Q + (T-30)1ft^2G makes T = (225+70C+30G)/(C+G), so

10 FOR LC=1 TO 5'log airflow through cube
15 C^LC/100'airflow through cube (cfm)
20 V=C/88'air velocity near glazing (mph)
30 GA=2+V/2'inside airfilm conductance (Btu/h-F-ft^2)
40 G=1/(.2+1/GA)'entire glazing conductance (")
50 T=(225+70*C+30*G)/(C+G)'box temp (F)
60 Q"5-(T-30)*G'useful heat output (Btu/h)

 cfm           mph         air temp      Q, Btu/h

   0.1         0.001136    179.7847       10.9784
   1           0.01136     138.9877       68.9876
  10           0.113636     84.5502      145.5024
 100           1.13636      71.5451      154.5106
1000          11.3636       70.1035      103.5395

there is an optimal airflow (somewhere around 100 cfm/ft^2), altho
nobody uses that much. On the lower end, it isn't hard to believe
that a small fan can double solar collection efficiency, compared
to thermosyphoning air. Deeper boxes can allow more airflow with
a lower velocity and less heat loss near the glazing. How would
this change with a porous mesh absorber with a large gap between
the mesh and the glazing and 70 F air near the glazing?


Posted by David L. Jones on October 6, 2005, 12:56 pm
 schooner wrote:

200cfm is the reated capacity into a free air, you won't get anywhere
near that when you pump it into a ducting. Try 1/10th of that figure,
and if you have enough ducting resistance the airflow of your 200cfm
fan can drop to zero!

This link has more info:
It shows why you should use series fans for ducting.

In practice you'll probably find it's hard to get too much airflow.
The actual optimum will be (grossly) different for different collectors
and particular installations.

Dave :)

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