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

Thu Jun 5 14:16:09 PDT 2014

Thanks for the info everyone. Bill's Solar Pro article notes that min Vmp
on inverters is affected by grid voltage. Is that universally true or just
for some inverters? Does that mean that models that work on 240VAC and
208VAC have different min Vmp? I rarely see inverter DC input specs listed
differently for different AC voltages.

Thanks,

Dana

Dana Brandt
Ecotech Energy Systems, LLC
www.ecotechenergy.com
dana at ecotechenergy.com
360.318.7646

On Wed, Jun 4, 2014 at 3:53 PM, Bill Brooks <billbrooks7 at yahoo.com> wrote:

> 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> 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 Bill Brooks, passed on past data, ( correct me Bill if I am out of date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>
> 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
> dana at ecotechenergy.com
> 360.318.7646
>
>
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> --
>
> William Hoffer
>
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> sunengser at gmail.com <bhoffer at sunergyengineeringservices.com>
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> Nathan Charles
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> Engineer
>
> NABCEP Certified PV Installation Professional #042013-20
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> Paradise Energy Solutions
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> (717) 283-2021 direct
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