FirstLook question [RE-wrenches]

Allan Sindelar allan at positiveenergysolar.com
Thu May 1 17:25:49 PDT 2008



Wrenches,
I have registered and used www.firstlook.3tiergroup.com for the first
time. It looks like a powerful resource, especially as it can serve as a
documentable reference for solar and wind assessment data used in system
designs. One question I need help with, please:

PV-relevant solar data is offered as either "direct normal irradiance",
defined as:  
	"the amount of direct radiation received per unit area by a
surface that is always held perpendicular (normal) to the rays that come
directly from the direction of the solar disk in the sky. By keeping the
surface normal to the incoming radiation, you maximize the amount of
energy received. This quantity of particular interest to concentrating
solar installations and installations that track the position of the
sun."
or "global horiziontal irradiance", defined as:
	"is the total amount of shortwave radiation received from above
by a horizontal surface. This value is of particular interest to
photovoltaic installations and includes both direct radiation and
diffuse radiation. 
	Direct radiation is solar radiation that comes from the
direction of the solar disk in the sky. Diffuse radiation is solar
radiation that has been scattered by molecules and particles in the
atmosphere and comes equally from all directions. 
	On a clear day, most of the solar radiation received by a
horizontal surface will be direct radiation, while on a cloudy day most
will be diffuse radiation."

Which figure should PV designers use? It looks to me as though direct
normal irradiance, which is a higher figure (kilowatt-hours/meter
squared/day) would be best for a two-axis tracking array that always
faced the sun. But global horizontal irradiance would be lower than
accurate, as our typically seasonally-adjusted arrays are not
horizontal, yet neither do they follow the daily sun path.

Who can advise a good compromise, or other approach that is defensible
using this data?
Thanks,
Allan Sindelar
allan_(at)_positiveenergysolar.com
<mailto:allan at positiveenergysolar.com> 
NABCEP certified solar PV installer
Positive Energy, Inc.
3225A Richards Lane
Santa Fe NM 87507
505 424-1112


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