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Re: [EnergyPlus_Support] Re: weather file radiation
The IWEC document I referenced starts with Duffie & Beckman but goes
on through all the models used...justifying the one they used to create
the solar data on IWEC.
Dru
"Edward Lyon"
<eglyon@xxxxxxx> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
cc:
02/06/2003 01:09 Subject: [EnergyPlus_Support] Re: weather file radiation
PM
Please respond to
EnergyPlus_Suppor
t
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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