Yes we can. I didn't know that the Aurora product had two separate MPPT circuits. Thanks, I'll look into it.
- 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 Kirk Herander, VSE Sent: Friday, December 26, 2008 11:38 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Inverter with two strings of different orientation Can't you use an inverter with two separate MPPT circuits - ie. Aurora? -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Peter Parrish Sent: Friday, December 26, 2008 2:30 PM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Inverter with two strings of different orientation I was afraid that the ultimate resolution would be another software cost item. I'll look into PVSYST, and consider purchasing it if it is a professionally-developed and maintained product, with pre-and post-sales technical support, a policy of regular upgrades, runs on PCs and MACs, and has a model library containing current commercial PV modules. Am I asking for too much? As for Bill's comments, I have a problem with his reasoning. I agree that Voc is predominantly a function of temperature. Secondarily, temperature is a function of the normalized irradiance (my term, which is the irradiance times the cosine of the angle of incidence), ambient temperature and wind speed (the latter two being constant between sub-arrays). However, Voc is not the important factor; Vmp, or something close to Vmp, is. Vmp is a function of irradiance (otherwise MPPT algorithms wouldn't work). Here is the problem as I see it: Vmp for two otherwise identical sub-arrays with significantly different tilts/azimuths will have different I-V characteristics and different MPPT points -- so long as they are not connected in parallel. In specific, they will have different Vmp and different Imp. It is actually worse than that: looking at the two I-V characteristics, there are no points in common between the two sub-arrays (with the exception of very close to Voc, which is not a practical operating point, so it can be ignored). There must be some sort of dynamic interaction between the two arrays to establish a common voltage. This has nothing to do with inverters, because we could examine this problem by loading up the strings with resistors. When two strings are wired in parallel, the voltage across both strings must be equal, by definition. From a practical point of view, when wired in parallel, no operating point can possibly maximize the power from each string, there has to be some reduction in output power. If PVSYST can produce a correct I-V characteristic of two strings in parallel with different tilts/azimuths, and if commercial inverters have the capability to deliver a MPPT capability under this condition, great, otherwise I remain skeptical. Anyone have a citation for the articles purporting to treat this subject? - 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
