Re: [GRASS-user] global radiation calculated with r.sun seems to be very high

[Frank Reekers]

I think, that's not a back-of-the-envelope calculation, it is more a 
good way to verify things and not believe everything...
But, back to my problem: Yes, you are right. PVGIS values are real-sky 
values => http://re.jrc.ec.europa.eu/pvgis/solres/solrespvgis.htm.
So, if I build the ratio of real-sky to clear-sky (PVGIS/r.sun) I get 
for february 0.5628, that means ~56% of the solar radiation doesn't 
reach the earth, right?
Let's do the same calculation for a summer day. If I take mid of july 
(day=196) using r.sun (with lin=4.4) I get a global radiation of 7651 
KWh/m² per day, PVGIS shows an average value of 4730 KWh/m² per day for 
july. The ratio for july (PVGIS/r.sun) is 0.6182....hmm, in summer more 
radiation losses in percent as in winter, could this be right?

Hamish schrieb:
ok, this is probably the worst back of the envelope calc you'll ever 
come
across, but it gives me an idea, maybe the PVGIS value takes average 
cloud
cover into account while r.sun (by default) does not? I don't know if
Germany's winter is anything like England's, if it is this might explain
the difference...   ?



lat = 52.267

day_of_year = 45

% lat_tropic: 23 + 26/60 + 16/3600 = 23.438

% so crudely:
[0   -23.438
 45  -11.719
 90  0
 180 +23.438 ]


sun_angle_noon = 52.267 + 11.719 = 63.986

solar_constant = 1367  % W/m^2

peak_rad = cos(sun_angle_noon * pi/180) * solar_constant
mean_rad = peak_rad * 2/pi   % more crudeness

sunlight_hrs = 7   % rough guess

sunlight_hrs * mean_rad
= 2671.8 Wh/m^2 /day   [Clear sky]

  
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