Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
Have you looked at the DOE2 model? When hourly solar data isn't available, DOE2 estimates the direct normal and diffuse radiation based on geometric data, a clearness number, and a cloud cover factor. It roughly corresponds to the algorithms SUN and CCF described in ASHRAE's Procedure for Determining Heating and Cooling Loads (1975). The algorithm is described on page III.21 of the DOE2.1A Engineer's manual. Not sure if it qualifies as simple. Parts of the model are also described in Chapter 14 of the 2013 ASHRAE Fundamentals handbook. You'll probably be able to find an appropriate model in Duffie and Beckman's Solar Engineering of Thermal Processes as well.
