[RE-wrenches] PV voltage at Isc
Peter Parrish
peter.parrish at calsolareng.com
Sun Feb 6 09:28:53 PST 2011
Dear Greg and others,
The answer to your “paradox” is actually quite simple. It is also a great
example of how a paradox can stimulate further thought until a more refined
answer can be achieved.
First, let’s set up the problem. You have a PV module and an external
circuit. In the case of an Isc measurement, the external circuit is some low
resistance #10 or #12 wire and a pair of MC-4 or MC-3 connectors. #10 wire
is rated at approximately one milliohm (0.001 ohm) per foot. I honestly
don’t know the resistance of a well-crimped MC connector but lest say it is
in the range of one milliohm.
So, if we call the “external circuit” everything outside of the j-box on the
back of the module (the wire whips and the two MC connectors) and the “PV
module” everything inside the j-box, the series string of the PV cells and
their interconnections. Let’s ignore the bypass diodes.
The external circuit might have a resistance of 5 milliohms.
With the MC connectors mated, let’s make our current measurement with a
clamp-on ammeter. Let’s make our voltage measurement at the terminal block
inside the j-box.
If we measure Isc to be 8.0 amps. If we actually had a j-box with an
accessible terminal block, I would expect something on the order of 40
millivolts. One would need to have a more sensitive DMM than wrenches
normally carry.
In general, the equation would be Vterm=Isc*Rext where Rext is the
resistance of the external circuit and Isc is the current flowing in the
external circuit.
The resolution to the paradox (at least at this level) is that there is some
resistance in
The external circuit, albeit a relatively low value.
But the argument might take the following tack: what if I use larger and
larger gauge wire and a more massive connector (like a utility-scale splice
block)? The answer is that there will always be some resistance in the
external circuit, however small and the voltage will always obey the
equation Vterm=Isc*Rext.
We have neatly side stepped one possible question, "Where does Isc come
from? And what is the equivalent circuit model for a PV cell?"
I'm not sure we need to drill down to that level (unless some one wants to
contact me off line), but the quick answer is that there is a fairly good
model for a PV cell that consists of a "current source", and two resistances
(a series resistance and a shunt resistance). In an Isc measurement, the
series resistance does contribute to an "internal voltage drop", but we
can't directly measure that "series resistance" with a simple Isc
measurement. With a full I-V trace we can determine all of the most
important parameters: Isc, Rser, Rsh and Voc.
- Peter
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
________________________________________
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Greg Egan
Sent: Saturday, February 05, 2011 11:30 AM
To: re-wrenches at lists.re-wrenches.org
Subject: [RE-wrenches] PV voltage at Isc
Fellow Scientists,
I got in a discussion with a wrench the other day. He is under the
impression that when a PV module is at short circuit current there is no
voltage. My understanding of electricity is that if there's no voltage
there's no current because current needs voltage in order to flow. I'm
seeing it as a difference in potential, voltage that is. If there's no
difference in potential I don't understand why current would flow.
I googled it and the few links I checked out were on his side - zero voltage
at Isc. All this is of course irrelevant to the business of installing PV
so if you don't care I don't blame you.
So I still think that if you a precise enough meter that if you stuck the
test leads into the cables sticking out of the back of a shorted PV module
that you'd read some small voltage that was approaching zero. Any
explanation of why I am right or wrong in plain English (I'm not really a
scientist) would be welcome.
All the best,
Greg Egan
Remote Power Inc.
Fairbanks, AK
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Today's Topics:
1. Re: PV Encapsulant Quality (Eric Thomas)
2. I Need a Sharp SunVista Manual (Brian Sipp)
3. Re: I Need a Sharp SunVista Manual (Jeff Irish)
4. Re: I Need a Sharp SunVista Manual (Brian Sipp)
5. Re: PV Encapsulant Quality (Jesse Dahl)
6. Re: Matching modules into Xantrex GT (Allan Sindelar)
----------------------------------------------------------------------
Message: 1
Date: Fri, 4 Feb 2011 10:33:27 -0800
From: Eric Thomas <solarepiphany at gmail.com>
To: re-wrenches at lists.re-wrenches.org
Subject: Re: [RE-wrenches] PV Encapsulant Quality
Message-ID:
<AANLkTi=fAxBTRHLxoR9WQaLEJn8e7v_gygpdYepusjb6 at mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Here is a great report that goes over advanced life cycle
testing of modules and their reasons for failure.
http://www.nrel.gov/docs/fy11osti/47755.pdf . It is a recent study and there
is some good info regarding encapsulents. I can tell you that the high
ranker on the results chart is the Silicon Energy Cascade module. With its
glass on glass encapsulent and integrated wireway, its the most robust
module I've ever seen. We installed the largest WA residential system
utilizing these modules in Dec of last year. They are nice to work
with...not without their challenges of course, but as far as warranty..I
think they will last the test of time.
Eric Thomas
Solar Epiphany
(206)919-3014
www.solarepiphany.com
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Message: 2
Date: Fri, 4 Feb 2011 13:21:09 -0800
From: "Brian Sipp" <Brian.Sipp at FirstSourceSolar.com>
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Subject: [RE-wrenches] I Need a Sharp SunVista Manual
Message-ID: <8B7DB7ABA13B4A05A1E69FCEA166F07B at FirstSource>
Content-Type: text/plain; charset="us-ascii"
I just got off the phone with the worlds worst customer (dis)service rep. at
Sharp. It really calls into question whether I want to sell any more of
Sharp modules, if this is how they treat their past customers.
Does anyone have a PDF of the Sharp SunVista manual? The only thing I find
online, or Sharp would provide, is a 2 page cut sheet.
Brian M. Sipp
First Source Solar Systems
<http://www.firstsourcesolar.com/> www.FirstSourceSolar.com
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Message: 3
Date: Fri, 4 Feb 2011 16:31:41 -0500
From: "Jeff Irish" <jeff at hvce.com>
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] I Need a Sharp SunVista Manual
Message-ID:
<86FA2C8AA80F3C49AE21B7EC7BD559B8798995 at HVCE-SERVER.hvcegroup.local>
Content-Type: text/plain; charset="us-ascii"
Brian,
I have one but it's a big file and I don't want to overload everyone's
mailboxes. Email me off-line at jeff at hvce.com
Jeff Irish
Hudson Valley Clean Energy
and Adirondack Solar
________________________________
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Brian
Sipp
Sent: Friday, February 04, 2011 4:21 PM
To: 'RE-wrenches'
Subject: [RE-wrenches] I Need a Sharp SunVista Manual
I just got off the phone with the worlds worst customer (dis)service
rep. at Sharp. It really calls into question whether I want to sell any
more of Sharp modules, if this is how they treat their past customers.
Does anyone have a PDF of the Sharp SunVista manual? The only thing I
find online, or Sharp would provide, is a 2 page cut sheet.
Brian M. Sipp
First Source Solar Systems
www.FirstSourceSolar.com <http://www.firstsourcesolar.com/>
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Message: 4
Date: Fri, 4 Feb 2011 14:05:03 -0800
From: "Brian Sipp" <Brian.Sipp at FirstSourceSolar.com>
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] I Need a Sharp SunVista Manual
Message-ID: <F23DB5CB2F37496092CD7CCC9A263EC8 at FirstSource>
Content-Type: text/plain; charset="us-ascii"
Wow, that was fast! Thank you to everyone who replied on and off line. As
always the wealth of knowledge on the forum, and folks openness to share
always impresses me!
This made my day!
PS: Sharp - Start taking notes...
Brian M. Sipp
First Source Solar Systems
<http://www.firstsourcesolar.com/> www.FirstSourceSolar.com
Office: 530-554-1925
Mobile: 916-342-2630
_____
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jeff Irish
Sent: Friday, February 04, 2011 1:32 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] I Need a Sharp SunVista Manual
Brian,
I have one but it's a big file and I don't want to overload everyone's
mailboxes. Email me off-line at jeff at hvce.com
Jeff Irish
Hudson Valley Clean Energy
and Adirondack Solar
_____
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Brian Sipp
Sent: Friday, February 04, 2011 4:21 PM
To: 'RE-wrenches'
Subject: [RE-wrenches] I Need a Sharp SunVista Manual
I just got off the phone with the worlds worst customer (dis)service rep. at
Sharp. It really calls into question whether I want to sell any more of
Sharp modules, if this is how they treat their past customers.
Does anyone have a PDF of the Sharp SunVista manual? The only thing I find
online, or Sharp would provide, is a 2 page cut sheet.
Brian M. Sipp
First Source Solar Systems
www.FirstSourceSolar.com <http://www.firstsourcesolar.com/>
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Message: 5
Date: Fri, 4 Feb 2011 16:05:45 -0600
From: Jesse Dahl <dahlsolar at gmail.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] PV Encapsulant Quality
Message-ID: <69420D91-5072-41E1-AF12-4A9FE7D1B492 at gmail.com>
Content-Type: text/plain; charset="us-ascii"
Wrenches,
I live in northern MN where Silicon Energy is building a new manufacturing
plant. I also teach at the local community college. I have met with silicon
energy and have seen the tests. Great module. Looks nice installed too.
Mounting options are limited but the glass on glass is a really good idea.
Jesse
Sent from my iPhone
On Feb 4, 2011, at 12:33 PM, Eric Thomas <solarepiphany at gmail.com> wrote:
Here is a great report that goes over advanced life cycle
testing of modules and their reasons for failure.
http://www.nrel.gov/docs/fy11osti/47755.pdf . It is a recent study and there
is some good info regarding encapsulents. I can tell you that the high
ranker on the results chart is the Silicon Energy Cascade module. With its
glass on glass encapsulent and integrated wireway, its the most robust
module I've ever seen. We installed the largest WA residential system
utilizing these modules in Dec of last year. They are nice to work
with...not without their challenges of course, but as far as warranty..I
think they will last the test of time.
Eric Thomas
Solar Epiphany
(206)919-3014
www.solarepiphany.com
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Message: 6
Date: Fri, 04 Feb 2011 23:22:56 -0700
From: Allan Sindelar <allan at positiveenergysolar.com>
To: Kent Osterberg <kent at coveoregon.com>
Cc: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] Matching modules into Xantrex GT
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