FirstLook question [RE-wrenches]

[Ray Walters]

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Do they also ask the panel orientation relative to azimuth and tilt?
I've used NREL's PVWATTs calculator that lets you program in the  
panel orientation and computes (I believe) total KWh/ day based on  
direct normal + diffuse radiation.
I think you would need to look at both for flat plate, while  
concentrators would only use the direct normal number.

Ray


On May 1, 2008, at 6:25 PM, Allan Sindelar wrote:

>
>
> 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
>
>
> [Non-text portions of this message have been removed]
>
>
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R. Walters
Solarray.com
NABCEP # 04170442	




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