insolation = system performance [RE-wrenches]

Keith Cronin keith at islandenergy.net
Wed Feb 2 15:33:24 PST 2005


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Bill

Those other factors can really swing the performance immensely. Where do we
get real world numbers on shading and soiling? Is this an educated
guesstimate?
Has anyone in CA figured out a way to gauge this?

I am at .78 for performance. Perhaps I should revisit this #.

Keith Cronin
President
Island Energy Solutions Inc.
270 Kuulei Rd. Suite 106
Kailua, HI 96734
808-262-3268 Tel
808-263-0338 Fax
keith at islandenergy.net
www.islandenergy.net
 
 
-----Original Message-----
From: Bill Brooks [mailto:billb at endecon.com] 
Sent: Wednesday, February 02, 2005 12:30 PM
To: RE-wrenches at topica.com
Cc: Miles Russell; teelee at davis.com
Subject: RE: insolation = system performance [RE-wrenches]

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Marco,

This is a good question that has some data to back it up. My colleague,
Miles Russell at Ascension Technology, now part of RWE Schott Solar did a
report for Sandia for their 2003 PV conference where he used a simplified
method starting from the PVUSA Test Conditions ac power rating as the
multiplier times the annual solar irradiation (other term for insolation).
The upshot was that it estimated the annual energy performance within about
3-4% of actual measured data for a whole series of sites.

The trick is how do you come up with an accurate PTCac rating (not the CEC
method) and what can hinder the correlation with annual irradiation. Typical
batteryless crystalline silicon systems need a 0.75 multiplier to accurately
adjust from STC to PTC rating. A-si, with a lower temperature coefficient is
more like 0.80. However, the other factors that PTC does not consider at all
are shading, orientation, and soiling (issue for Kona, not Hilo). With these
adjustments, an accurate assessment is fairly straightforward.

Final answer:

Annual Energy = PTC x Annual Irradiation x shade factor x orientation factor
x soiling factor

              = 0.75 x STC x Annual Irradiation x shade factor x orientation
factor x soiling factor




-----Original Message-----
From: Marco Mangelsdorf [mailto:marco at pvthawaii.com] 
Sent: Wednesday, February 02, 2005 1:53 PM
To: RE-wrenches at topica.com
Subject: insolation = system performance [RE-wrenches]


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Dear The E. Bill,

What's your rule-of-Bill's-thumb when it comes to estimating actual AC kWh
performance?  kWh/m2/day X array size DC STC X what % in terms of overall
system efficiency?  I've been using .8 as that multiplier and have been
pretty satisfied with the estimated actual real-world results.

marco
ProVision

Jeff,

Sorry that the URL text wrapped in the message, but the interesting thing
about this site is that it appears that among the 10 sites that the annual
average daily 30 degree tilt irradiation is about 5 kWh/m^2/day (sun hours).
The lowest is about 4.6 and the highest 5.3 for plane of array 30 degrees.
The data on the website is in kWh/ft^2/year and is for global horizontal. As
expected, the 4.6 sites are near the ocean and the 5.3 sites are near the
Bay.

Bill.

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