[TRNSYS-users] view factor to sky

Fri Nov 17 12:20:20 PST 2006

Dear all,
We're doing research on solar shading and are comparing new techniques 
with old techniques, such as overhangs above the window and obstructions 
next or in front of windows.
Therefore, we make use of type 34 and type 68, and connect the reduced 
total and reduced beam radiation to the specific shaded surfaces of the 
building.
But I was wondering weither I have to change the view factors of the 
shaded windows in TRNBUILD aswell. Which influence does this parameter 
have ? Changing this the whole time would be hard since it is a 
parameter in the bui-file and you can't connect it to an output in trnsys.

And how do you calcute them ? With type 34 it is easy; you can take 
output 6 (view factor to the sky), but for type 68 I'm not sure; I 
thoughed to take output 4 (fraction of diffuse visible for surface). 
Since I'm using the isotropic sky model for diffuse radiation, this 
should be the same, I thoughed.

However, when I put in no obstruction for a vertical wall looking to the 
east, so with a input-file like
"   090    ! Opening slope
   -90    ! Opening azimuth (0 = facing Equator, East is negative)
-180.0 -157.5 -135.0 -112.5  -90.0  -67.5  -45.0  -22.5    0.0   22.5   
45.0   67.5   90.0  112.5  135.0  157.5    ! View angles - ALWAYS 
(-180:step:180-step)
00.000    ! Obstruction height for opening 1, view angle  1 (i.e. 
between -180   and -157.5 degrees or N   and NNE for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  2 (i.e. 
between -157.5 and -135   degrees or NNE and NE  for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  3 (i.e. 
between -135   and -112.5 degrees or NE  and ENE for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  4 (i.e. 
between -112.5 and  -90   degrees or ENE and E   for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  5 (i.e. 
between  -90   and  -67.5 degrees or E   and ESE for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  6 (i.e. 
between  -67.5 and  -45   degrees or ESE and SE  for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  7 (i.e. 
between  -45   and  -22.5 degrees or SE  and SSE for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  8 (i.e. 
between  -22.5 and    0   degrees or SSE and S   for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle  9 (i.e. 
between    0   and   22.5 degrees or S   and SSW for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 10 (i.e. 
between   22.5 and   45   degrees or SSW and SW  for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 11 (i.e. 
between   45   and   67.5 degrees or SW  and WSW for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 12 (i.e. 
between   67.5 and   90   degrees or WSW and W   for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 13 (i.e. 
between   90   and  112.5 degrees or W   and WNW for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 14 (i.e. 
between  112.5 and  135   degrees or WNW and NW  for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 15 (i.e. 
between  135   and  157.5 degrees or NW  and NNW for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 16 (i.e. 
between  157.5 and  180   degrees or NNW and N   for Northern Hemisphere)"

I get a diffuse fraction of 0.8333 instead of 1 or 0.5.

When I put in an obstruction from East to North, with
"   090    ! Opening slope
   -90    ! Opening azimuth (0 = facing Equator, East is negative)
-180.0 -157.5 -135.0 -112.5  -90.0  -67.5  -45.0  -22.5    0.0   22.5   
45.0   67.5   90.0  112.5  135.0  157.5    ! View angles - ALWAYS 
(-180:step:180-step)
00.000    ! Obstruction height for opening 1, view angle  1 (i.e. 
between -180   and -157.5 degrees or N   and NNE for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  2 (i.e. 
between -157.5 and -135   degrees or NNE and NE  for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  3 (i.e. 
between -135   and -112.5 degrees or NE  and ENE for Northern Hemisphere)
00.000    ! Obstruction height for opening 1, view angle  4 (i.e. 
between -112.5 and  -90   degrees or ENE and E   for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle  5 (i.e. 
between  -90   and  -67.5 degrees or E   and ESE for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle  6 (i.e. 
between  -67.5 and  -45   degrees or ESE and SE  for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle  7 (i.e. 
between  -45   and  -22.5 degrees or SE  and SSE for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle  8 (i.e. 
between  -22.5 and    0   degrees or SSE and S   for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle  9 (i.e. 
between    0   and   22.5 degrees or S   and SSW for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 10 (i.e. 
between   22.5 and   45   degrees or SSW and SW  for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 11 (i.e. 
between   45   and   67.5 degrees or SW  and WSW for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 12 (i.e. 
between   67.5 and   90   degrees or WSW and W   for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 13 (i.e. 
between   90   and  112.5 degrees or W   and WNW for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 14 (i.e. 
between  112.5 and  135   degrees or WNW and NW  for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 15 (i.e. 
between  135   and  157.5 degrees or NW  and NNW for Northern Hemisphere)
90.000    ! Obstruction height for opening 1, view angle 16 (i.e. 
between  157.5 and  180   degrees or NNW and N   for Northern Hemisphere)"

I get a diffuse fraction of 0.6111 instead of 0.5 or 0.25.
Where is my reasoning mistake ? And do you have any tips that could make 
our lives somewhat easier ?

Best Regards, and have a good weekend,
Jeroen

-- 
Ir.Jeroen Van der Veken
Afdeling Bouwfysica
Katholieke Universiteit Leuven
Kasteelpark Arenberg 40
3001 Heverlee
T: +32 16 32 13 47
F: +32 16 32 19 80
@: jeroen.vanderveken at bwk.kuleuven.be

Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm