| nicksanspam@ece.villanova.edu 2005-10-06, 11:21 am |
| Anthony Matonak <anthonym40@nothing.like.comcast.net> wrote:
>...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.
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.
>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.
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=10^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=225-(T-30)*G'useful heat output (Btu/h)
70 PRINT C,V,T,Q
80 NEXT
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?
Nick
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