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 [email protected] -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Bill Brooks Sent: Friday, December 26, 2008 10:34 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Inverter with two strings of different orientation Peter and Darryl, The right way to do this is using a computer program PVSYST ($800). It will calculate the operating voltage of each string and the overall performance. Not much else out there can calculate it. That being said, the loss of these two orientations will be less than 2% based on the different orientations (orientation issues aside). The other wildcard is differential shading. Shading one or two modules in one of the two strings will cause large losses in the shaded string. Under no shading, the losses will be too small to notice. Array voltage is a function of temperature, no irradiance. High irradiance causes higher operating temperatures, but the inverter will be dominated by the higher producing string in nearly all cases. A good power tracking inverter like the SMA or Fronius would be the best call for such an array. Bill. -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Peter Parrish Sent: Thursday, December 25, 2008 11:04 AM To: [email protected]; 'RE-wrenches' Subject: Re: [RE-wrenches] Inverter with two strings of different orientation Thanks, Daryl. That is what I understand, but mine is a superficial understanding right now. For better or worse, I'm one of those guys who want to see the technical details. Does anyone have a reference to the articles Daryl is talking about? - 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 Darryl Thayer Sent: Thursday, December 25, 2008 7:39 AM To: RE-wrenches Subject: Re: [RE-wrenches] Inverter with two strings of different orientation If each string is composed of only one orientation then there would be little loss. There have been a couple of papers on this at different conferences. I also think John Berdner wrote a paper on it for SMA. (I think the annual losses are in the nature of 2 to 5 %) If however a single string has two orientations then much larger losses occur. Daryl --- On Wed, 12/24/08, Peter Parrish <[email protected]> wrote: > From: Peter Parrish <[email protected]> > Subject: [RE-wrenches] Inverter with two strings of different orientation > To: "'RE-wrenches'" <[email protected]> > Date: Wednesday, December 24, 2008, 1:41 PM > I don't think this topic has been dealt with in this > forum. Ocassionally we > want to use a single inverter with two or more strings, > each string being > loaded by a single sub-array. A sub-array being defined as > having a single > uniform tilt angle and azimuth. All modules are identical. > And each > string/sub-array contains the same number of modules. > > > > The question is: "How different can the orientation be > for the different > sub-arrays? > > > > Let me pose a concrete example: Xantrex GT5.0, two strings > of BP Solar > SX3190s. One string has an orientation of 15 deg tilt and > 188 degrees > azimuth; the other has 23 degrees tilt and 139 degrees > azimuth. For reasons > that are difficult to explain, none of the orientation > angles can be > changed, at least for the purposes of this discussion. To > visualize what I > am talking about: the tilts are fairly close (8 degrees) > and the azimuths > differ by about 45 degrees. > > > > How do I calculate the combined performance? How much power > does one give up > compared to using separate inverters for each > sub-array/string? Are there > rules of thumb as to what works and doesn't? Do > conventional inverters have > algorithms that can navigate the non-ideal I-V > characteristic under load? I > know there was a tech bulletin from Fronius a while ago: I > read it but > didn't come away with any definitive answer. I also had > first hand knowledge > with MPPT algorithm problems with the Fronius inverters > during that time > period, which caused me to discount the tech bulletin. > > > > I am quoting a system as we speak, so my inquiry is not > hypothetical in > nature. Any help would be appreciated. > > > > - 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] > > _______________________________________________ > 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 _______________________________________________ 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 _______________________________________________ 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
