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, lets 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 dont 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. Lets ignore the bypass diodes. The external circuit might have a resistance of 5 milliohms. With the MC connectors mated, lets make our current measurement with a clamp-on ammeter. Lets 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.parr...@calsolareng.com Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 ________________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Greg Egan Sent: Saturday, February 05, 2011 11:30 AM To: re-wrenches@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 re-wrenches-requ...@lists.re-wrenches.org wrote: Send RE-wrenches mailing list submissions to re-wrenches@lists.re-wrenches.org To subscribe or unsubscribe via the World Wide Web, visit http://lists.re-wrenches.org/listinfo.cgi/re-wrenches-re-wrenches.org or, via email, send a message with subject or body 'help' to re-wrenches-requ...@lists.re-wrenches.org You can reach the person managing the list at re-wrenches-ow...@lists.re-wrenches.org When replying, please edit your Subject line so it is more specific than "Re: Contents of RE-wrenches digest..." When responding to posts within the Digest, be sure to restore the Subject: line to the original, and please edit out any extraneous lines from the quoted message. 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 <solarepiph...@gmail.com> To: re-wrenches@lists.re-wrenches.org Subject: Re: [RE-wrenches] PV Encapsulant Quality Message-ID: <AANLkTi=faxbtrhlxor9wqalejn8e7v_gygpdyepus...@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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachme nts/20110204/799c5d79/attachment.html> ------------------------------ Message: 2 Date: Fri, 4 Feb 2011 13:21:09 -0800 From: "Brian Sipp" <brian.s...@firstsourcesolar.com> To: "'RE-wrenches'" <re-wrenches@lists.re-wrenches.org> Subject: [RE-wrenches] I Need a Sharp SunVista Manual Message-ID: <8B7DB7ABA13B4A05A1E69FCEA166F07B@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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachme nts/20110204/5c558a63/attachment-0001.htm> ------------------------------ Message: 3 Date: Fri, 4 Feb 2011 16:31:41 -0500 From: "Jeff Irish" <j...@hvce.com> To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org> Subject: Re: [RE-wrenches] I Need a Sharp SunVista Manual Message-ID: <86FA2C8AA80F3C49AE21B7EC7BD559B8798995@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 j...@hvce.com Jeff Irish Hudson Valley Clean Energy and Adirondack Solar ________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@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/> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachme nts/20110204/6a2faf4d/attachment-0001.htm> ------------------------------ Message: 4 Date: Fri, 4 Feb 2011 14:05:03 -0800 From: "Brian Sipp" <brian.s...@firstsourcesolar.com> To: "'RE-wrenches'" <re-wrenches@lists.re-wrenches.org> Subject: Re: [RE-wrenches] I Need a Sharp SunVista Manual Message-ID: <F23DB5CB2F37496092CD7CCC9A263EC8@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-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@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 j...@hvce.com Jeff Irish Hudson Valley Clean Energy and Adirondack Solar _____ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@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/> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachme nts/20110204/43b16ce8/attachment-0001.htm> -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: image/gif Size: 3648 bytes Desc: not available URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachme nts/20110204/43b16ce8/attachment-0001.gif> ------------------------------ Message: 5 Date: Fri, 4 Feb 2011 16:05:45 -0600 From: Jesse Dahl <dahlso...@gmail.com> To: RE-wrenches <re-wrenches@lists.re-wrenches.org> Subject: Re: [RE-wrenches] PV Encapsulant Quality Message-ID: <69420d91-5072-41e1-af12-4a9fe7d1b...@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 <solarepiph...@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 _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachme nts/20110204/7214d211/attachment-0001.htm> ------------------------------ Message: 6 Date: Fri, 04 Feb 2011 23:22:56 -0700 From: Allan Sindelar <al...@positiveenergysolar.com> To: Kent Osterberg <k...@coveoregon.com> Cc: RE-wrenches <re-wrenches@lists.re-wrenches.org> Subject: Re: [RE-wrenches] Matching modules into Xantrex GT Message-ID: <4d4cecc0.8000...@positiveenergysolar.com> Content-Type: text/plain; charset="us-ascii" An HTML attachment was scrubbed... URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachme nts/20110204/a351e6c6/attachment-0001.htm> ------------------------------ _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org End of RE-wrenches Digest, Vol 4, Issue 49 ****************************************** _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
