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Solar Collector Tilt, Effeciency, Sun Radation, etc - Attn Gary

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Posted by schooner on December 6, 2005, 12:28 am
 
Just trying to get a better grasp on the radation calulations for silar
BTUs, collector efficiency, tilt anlges, etc.

Been using Gary's RadOnCol program to get some numbers.  See calcs below.

In terms of the ITotal number, is this the total possible BTUs a collector
could get as a maximum based on the solar radation on that date and time?
Is the ITotal what you would use to compare for efficiency calcs or which
total?
What is the sun total number?
Is 90 degree tilt assumed to be vertical, ie wall mounted?

Here is the calc 1:
Solar Radiation on Collector for day of: 12/ 6

  Collector Area:          32.0 (sqft)
  Collector Azimuth:        0.0 (deg) measured from South, + is to East
  Collector Tilt:          90.0 (deg) measured from horizontal
  Latitude:                46.0 (deg)
  Altitude:                 0.0 (ft) Above Sea Level
  Sun Rise:                 7.7 (hr) Sunrise in solar time

Time    Sun --------------------------------  
   Collector ------------------------------
Hour    Az   Elev  DirNormal  Diffuse  Total    HorzTot IncidAng Direct
Diffuse   ITotal

   4   94.8  -36.5      0        0        0        0      90        0
0        0
   5   84.1  -26.1      0        0        0        0      85        0
0        0
   6   74.0  -15.9      0        0        0        0      75        0
0        0
   7   63.9   -6.2      0        0        0        0      64        0
0        0
   8   53.3    2.7     17        1       18       59      53      329
17      345
   9   41.6   10.4    173       10      183     1327      43     4067
166     4233
  10   28.8   16.4    232       14      246     2537      33     6239
223     6462
  11   14.8   20.2    255       15      271     3316      25     7410
246     7655
  12    0.0   21.6    262       16      277     3584      22     7787
252     8039
  13  -14.8   20.2    255       15      271     3316      25     7410
246     7655
  14  -28.8   16.4    232       14      246     2537      33     6239
223     6462
  15  -41.6   10.4    173       10      183     1327      43     4067
166     4233
  16  -53.3    2.7     17        1       18       59      53      329
17      345
  17  -63.9   -6.2      0        0        0        0      64        0
0        0
  18  -74.0  -15.9      0        0        0        0      75        0
0        0
  19  -84.1  -26.1      0        0        0        0      85        0
0        0
  20  -94.8  -36.5      0        0        0        0      90        0
0        0
                    -----   ------   ------    -----            -----    -----
    -----
Day Total            1616       97     1713    18062            43876
1555    45431

Angles in degrees, Radiations in BTU/hr, DayTotal in BTU/day
1 BTU/hr-ft^2 = 3.152 W/m^2



Calc 2 with 45degree tilt:
Solar Radiation on Collector for day of: 12/ 6

  Collector Area:          32.0 (sqft)
  Collector Azimuth:        0.0 (deg) measured from South, + is to East
  Collector Tilt:          45.0 (deg) measured from horizontal
  Latitude:                46.0 (deg)
  Altitude:                 0.0 (ft) Above Sea Level
  Sun Rise:                 7.7 (hr) Sunrise in solar time

Time    Sun --------------------------------  
   Collector ------------------------------
Hour    Az   Elev  DirNormal  Diffuse  Total    HorzTot IncidAng Direct
Diffuse   ITotal

   4   94.8  -36.5      0        0        0        0      90        0
0        0
   5   84.1  -26.1      0        0        0        0      90        0
0        0
   6   74.0  -15.9      0        0        0        0      90        0
0        0
   7   63.9   -6.2      0        0        0        0      77        0
0        0
   8   53.3    2.7     17        1       18       59      63      251
28      279
   9   41.6   10.4    173       10      183     1327      50     3579
284     3863
  10   28.8   16.4    232       14      246     2537      37     5891
381     6272
  11   14.8   20.2    255       15      271     3316      28     7237
419     7657
  12    0.0   21.6    262       16      277     3584      23     7685
430     8115
  13  -14.8   20.2    255       15      271     3316      28     7237
419     7657
  14  -28.8   16.4    232       14      246     2537      37     5891
381     6272
  15  -41.6   10.4    173       10      183     1327      50     3579
284     3863
  16  -53.3    2.7     17        1       18       59      63      251
28      279
  17  -63.9   -6.2      0        0        0        0      77        0
0        0
  18  -74.0  -15.9      0        0        0        0      90        0
0        0
  19  -84.1  -26.1      0        0        0        0      90        0
0        0
  20  -94.8  -36.5      0        0        0        0      90        0
0        0
                    -----   ------   ------    -----            -----    -----
    -----
Day Total            1616       97     1713    18062            41600
2655    44256

Angles in degrees, Radiations in BTU/hr, DayTotal in BTU/day
1 BTU/hr-ft^2 = 3.152 W/m^2



Posted by Gary on December 6, 2005, 2:37 am
 
Hi,


The very last column (labled ITotal) is the one to use in the
efficiency calcs as the collector input.  A 100% efficient collector
would output all of this number as heat.

Definitions for the columns:
These columns deal only with the sun, and don't change if you change
the collector: Az   Elev  DirNormal  Diffuse  Total

Az -- is the azimuth angle (or bearing) to the sun.  0 is due south,
and + angles are to the east.
Elev -- is the elevation angle (or altitude angle) of the sun.
DirNormal -- is the part of the solar radiation that comes directly
from the sun.
Diffuse -- is the part of the solar radiation that gets scattered from
the air or clouds and arrives from many directions.
Total = DirNormal + Diffuse

HorzTot -- is the total (direct + diffuse) radiation on a horizontal
surface.


These columns are the ones that vary depending on how you orient the
collector: IncidAng Direct  Diffuse   ITotal

IncidAng -- is the incdidence angle of the sun on the collector (the
angle between a normal to the collector and the direction to the sun)

Direct -- is the part of the suns direct radiation (column 3) that the
collector sees.  Its calculated as cos(IncidenceAngle)(DirNormal).
I probably should have labeled the column ColDirect.

Diffuse -- is the part of the diffuse radiation that the collector
sees.  For example, a vertical collector sees half the sky, and
therefore half the diffuse from colum 5.
I probably should have labeled the column ColDiffuse.


-------

As soon as I finish doing a bit more cleanup and commenting, I'm going
to make the source for Radiation On Collector and and its supporting
classes, and some simple simulation software availabe on BuildItSolar
as Open Source.   But, for now, here is the part of Collector Class
that calcualtes the radiation on the collector.

     ' Update radiation on collector values
     Icoldir = Idn * ColDotSun
     Icoldif = Ihorzdif * ((1 + Cos(Tilt)) / 2)
     Icoltot = Icoldir  + Icoldif

The first line calculates the direct radiation on the collector.
Where, Idn is the direct radiation from the sun, and ColDotSun is
cosine of the angle between a normal to the collector plane and the
direction to the sun (ie the incidence angle).

The 2nd line takes the diffuse radiation on a horizontal surface
(which sees the whole sky), and corrects it for the fact that
a tilted collector does not see the whole sky.  If you plug 90 degrees
in you will see that the factor comes out to be one half.




Right, 90 deg tilt is vertical.

Gary


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