[GRASS-user] r.sun: A procedure to define coefbh and coefdh for inclined surfaces

Pierluigi De Rosa pierluigi.derosa at gfosservices.it
Thu Sep 29 11:02:27 EDT 2011

Dear all

i need to estimate the real-sky radiation for inclined surfaces (tilt
25° degrees) throw using r.sun.
I followed this procedure to evaluate the two parameters coefbh and
Below I explain how I get these two maps (I hope the procedure is
correct... in negative case please alert me) so other people can use it.

1) go to this page
http://www.soda-is.com/eng/services/services_radiation_free_eng.php and
download data for the interested location using horizontal radiation,
for a long period using HC1month data
2) the ratio between Irradiance (monthly mean) and the clear-sky gives
you the Kc for each month. You should get the mean of Kc using all year
you have (ie using data pilot in libre office).
3) run this script
http://www.grassbook.org/scripts3rd/rsun_crop_grass6.sh that gives you
for each day in year the value of W/m2 for beam ,diffuse, reflected and
global radiation.
4) summarise all daily maps for each month in order to have monthly
global radiation map, beam radiation map and diffuse radiation map (i
prepared a python script to do this if someone is interested...).
5) multiply the global radiation for each month to the corresponding
monthly Kc to have the real sky global radiation for horizontal plane
6) at the same website
http://www.soda-is.com/eng/services/services_radiation_free_eng.php get
the monthly inclined value for beam, diffuse and global for the location
of interest using the tilt you need (25° in my case) (Note that the
website provide only values for year 2005) and make the ratio between
Diffuse horizon and Global horizon for each month (Dhs/Ghs).
7) The monthly diffuse horizontal radiation under overcast condition
(Dh) is computed as the monthly real sky global radiation (from 5) by
Dhs/Ghs (from 6)
8) The monthly beam horizontal radiation under overcast condition (Bh)
is computated as the monthly real sky global radiation (from 5) minus Dh
map (from 7)
9) the monthly coefdh map is the ratio of Dh map (from 7) and the
monthly corresponding map of Diffuse radiation computed from r.sun and
summarised in 4
10) the monthly coefbh as above is the ratio of Bh (from8) and the
direct radiation

If this procedure is correct I can write a better howto in a wiki

I have one more question:
once I have coefbh and coefdh maps in order to evaluate the global real
sky radiation map for tilt 25 degrees in slopein parameters I should
create a raster map with the value of 25?
Is this correct?
Thanks in advance



Ing. Pierluigi De Rosa (PhD)
Studio Associato GFOSSERVICES

Via Tilli 58 - 06127 Perugia (PG) 
tel: 075 7825101 / fax: 075 7823038
cel: 3497558268
web: www.gfosservices.it
skype: pierluigi.derosa 

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