[EnergyPlus_Support] Re: weather file radiation

["Edward Lyon" <eglyon@xxxxxxx>]

I would refer you to "Solar Engineering of Thermal Processes" second edition by Duffie and Beckman for a more detailed review of this topic.

The clearness numbers are selected for different times of the year to account for moisture air density at a location, not cloud cover.  Duffie and Beckman present factors that are slightly different than the ASHRAE clearness number for their equations.

A cloudy day reduces direct beam radiation, but increases diffuse radiation.  I have written some code for other simulations that looks at reported cloud cover and makes proportional changes to direct and diffuse calculations.  Even a cloudy day has some component of direct beam solar (40% or so).  The problem is that the cloud data reporting is so uncertain...typically one observation per hour and 50% coverage could be a front covering 50% of the sky or moving puffy clouds covering 50% of the sky in the viewer's estimation.

Ned Lyon

Senior Staff Engineer
Simpson Gumpertz & Heger Inc.

41 Seyon Street
Building #1, Suite 500
Waltham, MA  02453

Phone:  781-907-9000
Fax:  781-907-9009


>>> edhugar@xxxxxxxxx 02/06/03 12:50PM >>>



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