Peter, The more you work with IV curves, the better you will understand them. Since most people don't get the opportunity to use an IV curve tester, the understanding of the curves may need some help. As you pointed out in your "constant temperature" information below, it showed the MPP to be 2 volts different. I would want to see the origin of the data and how it was taken, but it does not agree with many other crystalline products. It could be that Kyocera modules fall off in voltage a little a low irradiance, but if you look at a typical family of IV curves for crystalline silicon, the curve is moving up and down with irradiance, and left to right with temperature.
Even with a small difference in voltage with irradiance, as boB pointed out in his post, there is a counteracting effect in a PV array with two strings in different orientations. The less illuminated string will run cooler and therefore at a higher voltage. This temperature differential effect on voltage is at least as large as any irradiance effect on operating voltage. Also remember that the maximum power "Point" is not a point at all, but a range of voltages that last for about 5% of the voltage range with almost no change in power output. When it is all said and done, it is a negligible effect on performance to combine the two--assuming no differential shading. Since you are so into subject, you seem like a good candidate for PVSYST. It's not cheap ($800), but if you like PV:WATTS ver 2.0, you'll love PVSYST. It basically blows the doors off of most of the rest of performance software these days. If you're looking for an end of year purchase for tax purposes, it is a nice option (I get no commission on sales in case you are wondering). The caution is that it does have a relatively steep learning curve and requires a lot of PV knowledge to apply properly. That being said, they have many programming options that no other software program can touch. All the best with your design, but if you put the strings on the same inverter, without differential shading, you won't be disappointed. A more definitive article on the subject was written by KACO several years ago. I don't have a URL for the report, but you can probably get it from KACO. 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: Sunday, December 28, 2008 3:54 PM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Inverter with two strings of different orientation 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. Let's take the simplest case (constant cell temperature) and what was a fairly popular PV module: Kyocera's KC200GT. Although it is a little tedious to calculate the MPP, I found the following: 200 W/m^2 Vmp=25 V, Imp=1.514 A (37.9 W), 1,000 W/m^2, Vmp=27V, Imp=7.58A (205W) For more realistic conditions (i.e. operating temperatures) the result will be different and you could be right, but as far as constant cell temperature concerned, there is a shift with irradiance. By the way, the way you can see that there might be a real effect here is to notice that Isc is almost perfectly linear from 200 W/cm^2 to 1000 W/cm^2, whereas Voc is linear with irradiance only from 600 to 1000 W/cm^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 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
