[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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