performance degradation [RE-wrenches]

[Bill Brooks]

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Marco,

Read the CEC PIER report that is on our PIER data site. It is a recently
posted report that reviews the first 6 months of operation of the 12 systems
under test. It has a section at the end of the report that hypothesizes
about voltage reduction rates. http://pierminigrid.showdata.org/docs.cfm

To specifically answer your first question, all systems reduce in output
about 1-2% on average and there is no clear winner on voltage or
current--assume the issue is shared and that is probably accurate. David
King of Sandia has done some work showing the degradation mechanisms in
current, while field testing clearly shows that Voc and Fill Factor both
decrease significantly over time. Fill factor decrease suggests both current
and voltage decreasing. Both are bad, but as you point out, voltage
degradations are far worse because of operating window. 

Most inverter sizing programs are far too liberal in their willingness to
let you install low voltage arrays because they are in competition with the
other guys who are saying the same thing. If possible, always choose the
highest voltage configuration allowed with a particular module--never the
lowest. Although some inverters work better at lower voltages (the CEC
inverter data will be posted soon and you can actually see it), it is a very
sharp two-edged sword that is destined to cut you bad if you decide to build
an array to run at those lower voltages. Higher voltages also allow for some
shading to occur and still be able to power track the array. Inverter
efficiency is NOT king. Operating voltage range is far more important.

Bill.

-----Original Message-----
From: Marco Mangelsdorf [mailto:marco at pvthawaii.com] 
Sent: Wednesday, May 11, 2005 11:19 AM
To: RE-wrenches at topica.com
Subject: performance degradation [RE-wrenches]


We all know that crystalline Si modules experience a decrease in power
output over time.  Is this degradation a function of both a reduction of amp
output and voltage or a loss of one and not the other?  This could be an
important practical issue for systems that are designed with an operating DC
voltage on the low end of the MPPT range.  If the system is on the low end
with new out-of-the-box modules, how will the system perform in 10-20 years
if the voltage drops enough to drop below the MPPT range of the inverter or
charge controller?

aloha,
marco

Marco Mangelsdorf
President
ProVision Technologies, Inc.
69 Railroad Avenue, Suite A-7
Hilo, Hawaii 96720 USA
(808) 969-3281, fax 934-7462
www.provisiontechnologies.com

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