Thanks Bob, these are really great reference articles. The equation for Vmp had four terms, three of which are temperature dependent. So I think we put that issue to bed. The reason that this issue is so important for us is this: if Vmp is temperature dependent, then there must be some loss when combining two strings together with different tilts/azimuths. How much, I don't know. Specifically, I have a client for whom we are designing a system with two strings with different tilts/azimuths (otherwise identical) 18deg/East and 15deg/South. The questions is, "Go with one inverter and wire the strings in parallel, or go with two inverters one for each string." Since the cost for two inverters is considerably more than one inverter of twice the capacity, I would like to go with the single inverter, if the losses are in the 1-2% range.
I am also very skeptical about MMP tracking algorithms. I documented the "saw-toothing" on the Fronius IG-4000 input (July 2006), and we still have an outstanding malfunction with a Xantrex XW6048 with an output power oscillation. Right now I will be telling our clients that "using a single inverter -- with two or more subarrays of widely differing orientations (see above) -- is not recommended, because there may be significant losses (significantly more than 1-2%)". - 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 [email protected] -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of boB Gudgel Sent: Sunday, December 28, 2008 5:07 PM To: RE-wrenches Subject: Re: [RE-wrenches] Inverter with two strings of different orientation Peter Parrish wrote: > Bill, > > I looked into this several years ago, and I was able to get some digitized > I-V data. I can't find that now (but I will). The data supports my claim. > Vmp does change with irradiance. It just changes very little. Depends on your definition of "change" I guess. Negligible I would say... That is, assuming a higher irradiance than "dark" of course. I can see where some confusion might come from from irradiance and PV parameter variation... As the irradiance changes, the ratio of Vmp to Voc changes. Lower irradiance normally makes the Vmp/Voc rise somewhat but only because the Voc is changing. Now, put two or more modules in series, then shade one of them and that's something entirely different. Here's a good paper with some good data on this subject. I also see Bill Brooks contributed to this article, http://photovoltaics.sandia.gov/docs/PDF/King%20SAND.pdf (the %20 is a space) And for temperatue relations, here's another good one: http://photovoltaics.sandia.gov/docs/PDF/IEDFB5~1.pdf boB _______________________________________________ List sponsored by Home Power magazine List Address: [email protected] 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: [email protected] 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
