Peter, Put 7 type A personality modules in one string and 6 Type B, plus 1 Type A. The Type A in the Type B string will just act like one of the Type B boys. No stress, just chillin' a slight bit off max power current. That will be better than mixing both strings with high and low current modules.
In the old days (1980s) we would current match strings as best we could, so what you describe was a common occurrence. Keep all the modules of one current in one string. If you must mix, one or two higher current modules are always okay, but one or two lower current modules will hold up the whole string. Summarizing: 7 Type A in string 1 6 Type B and 1 Type A in string 2 Happiness will be found in this arrangement. Bill. -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Peter Parrish Sent: Tuesday, March 31, 2009 10:13 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Mixing and Matching PV modules--Error Corrected [Bill: I think our disagreement a while ago had to do with the dependence of Voc and Vmp with insolation. I said there was one and you said there wasn't. The way I resolved it in my mind is that there is a small change in Voc with insolation as well in Vmp, but to zeroth order one could ignore the change for most practical calculations.] I should have mentioned this, but I don't have the luxury of putting together a string of type A and string of type B. I have 8 of type A while I have only 6 of type B. I wish I did have 8+8 but I only have 8+6 and I wanted to use them all (I am guilty of optimizing). It seemed a shame to go the 6+6 route, and I'd guess I'd have to do some research on low voltage, grid tied inverters (I think even a Fronius needs 8 module strings). So, if I accept the lower operating current, limited by the type B modules, in a split string...will this stress out the type B modules in the string? - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.com -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Bill Brooks Sent: Monday, March 30, 2009 5:57 PM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Mixing and Matching PV modules--Error Corrected Peter, Although last time we discussed, you didn't seem to agree with me, I guess I'll try responding to this question. The best situation is to keep the two strings of modules in their own strings. 7 of the older in series, paralleled with the 7 of the newer in series. The operating voltages, and open circuit voltages are for all practical purposes--identical. It appears that the only difference is the size of the cell--producing a higher current as expected--with little or no change in voltage. Operating voltage is the key--as in our last discussion. Mixing the strings will result in lower output overall. Perhaps trying it both ways would prove it to you. An I-V curve tester will clearly show the benefit of not mixing the modules within the string. As I teach my classes, you have to think like a "PV". The I-V curve is limited by the lower current modules in both strings if you wire as you suggest. If you wire the modules with the like modules, each string will produce their maximum current. Draw out the I-V curves and it may make more sense. Bill. -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Peter Parrish Sent: Monday, March 30, 2009 3:50 PM To: 'Peter Parrish'; 'RE-wrenches' Subject: Re: [RE-wrenches] Mixing and Matching PV modules--Error Corrected I think I understand all the dictums about single-strings of PV modules: all the same modules, all the same azimuth and tilt, but I have a "special situation". We are starting to accumulate a number of "second hand" PV modules. Some have frames slightly damaged by the shipper; some have come from warranty replacements; and some are odd lots that we haven't been able to sell. What I want to do is to set up a demo system composed of two different types of modules. This is my initial design: Total of fourteen modules. Two strings of seven modules each. Each string consisting of four modules of one type (type A) and three modules of a second type (type B). Both module types have 50 cells and the type A module has 155mm-sq cells and the type B module has 150mm-sq cells. So you would think that the performance specs would match pretty well. However, since the type B modules are an earlier vintage, they probably don't have the latest and greatest surface treatment, etc., so the current specs are lower than one might expect. Specs on type A: Voc = 30.8V, Vmp = 24.5V, Isc = 8.70A, Imp = 8.16A. Specs on type B: Voc = 30.6V, Vmp = 24.6V, Isc = 7.38A, Imp = 6.93A. So here's my question: would putting together the strings of four type A and three type B cause any problems? I know the string current would be limited by the type B modules, but above and beyond that, what would be the operating point of the type A modules and what would be the stress if any on the type B modules? I'd like this demo system to last a while and I certainly don't want to see scorch marks on the type B modules after a few months of use. Any thoughts? - Peter Peter T. Parrish, President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.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 _______________________________________________ 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 _______________________________________________ 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
