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

[Nikos Alexandris]

Frank Reekers wrote:
> 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?

Why not?
(
calcs are done for Germany, ever-cloudy summer-sky, etc. right?
just kidding... :-p
)

Nikos

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