This is the very old and totally outdated ASHRAE clear-sky model. Coefficients were obtained empirically from observations at one U.S. location during a few very clear days. The 2009 edition of the ASHRAE Handbook of Fundamentals introduced a completely different model, which was even improved for the 2013 edition,

See discussion, critique of the old model, and development of new ASHRAE model in

http://www.sciencedirect.com/science/article/pii/S0038092X09001790

and

"Revising ASHRAE Climatic Data for Design and Standards – Part II, Clear-Sky Solar Radiation Model (1613-RP)" ASHRAE Trans. 2013.

This is the very old and totally outdated ASHRAE clear-sky model. Coefficients were obtained empirically from observations at one U.S. location during a few very clear days. The 2009 edition of the ASHRAE Handbook of Fundamentals introduced a completely different model, which was even improved for the 2013 edition,

See discussion, critique of the old model, and development of new ASHRAE model in

http://www.sciencedirect.com/science/article/pii/S0038092X09001790

and

"Revising ASHRAE Climatic Data for Design and Standards – Part II, Clear-Sky Solar Radiation Model (1613-RP)" ASHRAE Trans. 2013.

Dear Sir, Thank you very much for your guidance and help.

Sir I have tried to download ASHRAE 2013 book but couldn't find and I can't afford to buy this book, can you share a link from where I can download ASHRAE 2013 book. I will really be very thankful to you.

regards,

I don't have it either. I can only supply a preprint of my 2013 ASHRAE Trans. paper if you request it...

Thank you very much Sir,

please send me preprint of your 2013 ASHRAE Tans. paper

You need to do that internally, through https://www.researchgate.net/publication/261179954_Revising_ASHRAE_Climatic_Data_for_Design_and_Standards__Part_II_Clear-Sky_Solar_Radiation_Model_%281613-RP%29?ev=prf_pub

Article Revising ASHRAE climatic data for design and standards - Par...

I think A=I0*e. where I0=1367 w/m² and 'e' is eccentricity factor e=1+0.03344*cos(Np-0.048869). where Np= 2*Pi* N/365.25 where N is day number.

B i think is angstrom coefficient, it can be measured for each site in the world or deduced from Linke turbidity factor. see Attachment paper (in French language)

Article Etude comparative de deux modèles de calcul du rayonnement s...

Amor, It's not that at all.

In the ASHRAE model, A, B and C are numerical coefficients that have a fixed monthly value. See the Solar Energy paper I referenced above for details.

Dear Amor...

Sir Chris Gueymard is correct.. please read the question and previous threads then it will be clear to you that what actually I am asking for... anyway thank you for your time.

Sir Chris again thank you very much and I am referring your valuable articles on solar energy. these are really helpful for my research. Infact I couldn't not understand many points mentioned in your papers but I am trying and hope for better...

Regards,

B. ABOUELKHAYRAT, A. OUBARRA, J. LAHJOMRI, «Etude numérique et expérimentale de l’irradiation globale sur une surface horizontale au niveau de Casablanca» 9ème Congrès de Mécanique, 2009, Marrakech

Global, direct and diffuse solar-radiation in Syria

http://www.sciencedirect.com/science/article/pii/S0306261903002320

Global, direct and diffuse solar-radiation in Syria.pdf

https://www.researchgate.net/publication/223276501_Global_direct_and_diffuse_solar-radiation_in_Syria

Article Global, direct and diffuse solar-radiation in Syria

Thank you Bailek Nadjem