RE: Re: [EnergyPlus_Support] RE: Calculating direct normal and diffuse horizontal radiation form global horizontal radiation

[<chris@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

To complement what Joe said, the idea of splitting the direct and diffuse components from global irradiance data started with a seminal publication of Liu & Jordan in 1960. Since then a number of methods have been proposed, usually for a specific location only. One of the most popular ones is that of Perez, at least for the US. Other methods are frequently used in Europe. Anyway, these methods are not as accurate or universal as we hope they are. The reason is that they are of an empirical nature, hence more or less tuned to local conditions, and do not include the effect of important factors, such as ground albedo. This effect alone makes the accuracy of the splitting deteriorate significantly when the ground is covered with snow, for instance.

See this paper for more insight:

C.A. Gueymard, Progress in direct irradiance modeling and validation. Solar 2010, ASES Conf., Phoenix, AZ (2010), available from http://www.solarconsultingservices.com/publication.php