[RE-wrenches] Module Degradation - voltage or current?

Bill Brooks billbrooks7 at yahoo.com
Wed Jun 4 15:53:16 PDT 2014


Folks,

 

Voc does not degrade as much as Isc, but neither is key.

 

Both Vmp and Imp degrade. We call this a degradation of Fill Factor. This
happens with all PV systems and the degradation rates for current and
voltage are very similar.

 

Imp and Isc degrade because less light gets to the cell from etching glass,
yellowing encapsulant and IR coating on cells changing.

 

Vmp changes because of increased contact resistance of conductors on the
cells and other contact points. Voc is unimpacted by contact resistance
since there is no current flow.

 

Bill.

 

From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Ray Walters
Sent: Wednesday, June 04, 2014 11:30 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Module Degradation - voltage or current?

 

I agree that Voc doesn't seem to degrade as much.  I've had a chance to
measure several modules over 20 years old, and the Voc is only dropping
about 1/2 the rate suggested by others here.   I think a more appropriate
number might be 0.25% V drop/ year.  



R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer, 
Licensed Master Electrician
Solar Design Engineer
303 505-8760

On 6/4/2014 11:36 AM, Nathan Charles wrote:

So I read this paper, Smith, R. M., Jordan, D. C., & Kurtz, S. R. (2012).
Outdoor PV Module Degradation of Current-Voltage Parameters, which seems to
seems to suggest that Voc doesn't degrade like Isc degrades. Based on that
and other data I've seen it seems like panels that degrade at 0.5%/year are
outliers on the low side. 

 

Thoughts from other wrenches?

 

 

 

On Tue, Jun 3, 2014 at 11:59 PM, Bill Hoffer <sunengser at gmail.com
<mailto:sunengser at gmail.com> > wrote:

Dana 

 

Bill Brooks, passed on past data, ( correct me Bill if I am out of date
here) recommends 1% Power loss per year with approximately 0.5% current and
0.5%voltage.  That is also what Solar Energy International Best Practices
curriculum recommends.  Based on some newer data from larger projects
requiring testing to actually determine those values for the Bankability
requirements ( PVsyst PAN files),  I believe we are seeing lower numbers
than that now.  In general, quality of modules are higher than the older
modules.  We of course are usually not privy to those results and may not
even be getting the same level of binning for small residential projects as
the MW projects do.   0.5% V loss per year seems like a like a good
conservative number to me, especially if you add a little buffer (1-2%) in
there.

 

Bill

 

On Tue, Jun 3, 2014 at 3:41 PM, Dana Brandt <dana at ecotechenergy.com
<mailto:dana at ecotechenergy.com> > wrote:

Hi Wrenches,

Does anyone know what percentage of module degradation over time is voltage
and what is current? I'm wanting to make sure we give ourselves plenty of
breathing room on the minimum Vmp of the inverters over the long term so
need to try to anticipate how much voltage we'll lose on the array over 30
years. 

Thanks,



Dana

Dana Brandt
Ecotech Energy Systems, LLC
www.ecotechenergy.com <http://www.ecotechenergy.com> 
dana at ecotechenergy.com <mailto:dana at ecotechenergy.com> 
360.318.7646 <tel:360.318.7646> 


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-- 

William Hoffer 

161 SE Fourth Ave

P.O. Box 1823
White Salmon, WA 98672-1823
sunengser at gmail.com <mailto:bhoffer at sunergyengineeringservices.com> 


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-- 

Nathan Charles 

Engineer

NABCEP Certified PV Installation Professional #042013-20

Paradise Energy Solutions

(717) 283-2021 direct






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