Marv,
I thought I might catch the attention of a few folks with my "ambient temperature" comment. It was not a typo. While cell temperature dominates Vmp, irradiance also has a small impact (less than most people think). Since there is a counterbalancing effect of temperature and irrandiance in a two orientation scenario, the only consistent condition is ambient temperature. This is for the people that make the argument that Vmp can't possibly be the same for two arrays in two different orientations. For a given cell temperature, irradiance does not change Vmp much. However, Irradiance does change cell temperature relative to the ambient. Since both strings are in the same ambient temperature conditions, that is one way to differentiate that strings on the same roof will be similar, whereas strings in two different states will have different operating voltages because the ambient temperatures are likely to be different. Just a brain teaser-it was fun teasing your brain. Bill. From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Marv Dargatz Sent: Friday, May 21, 2010 7:26 AM To: RE-wrenches Subject: Re: [RE-wrenches] Two strings of Different Orientation Bill, I agree with everything you're saying, if you substitute "cell temperature" for "ambient temperature". See ya! Marv 707 763-4784 x7016 Bill Brooks wrote: Peter, Not to belabor the issue, but your understanding of the I-V curve and how it responds to changes in irradiance and temperature are flawed. I'm sorry you believe that I am misleading you, but the truth is that the I-V curve moves up and down with changing irradiance (current changes with irradiance), and the I-V curve moves left and right with temperature (voltage changes with temperature). Without these fundamental understandings, we do not have an understanding of the I-V characteristics of a PV device. At lower irradiances, some PV modules will slightly reduce in max power voltage for a given temperature, but the temperature is not constant. As the irradiance reduces, so does the temperature, causing a slight increase in the max power voltage. These two counteracting aspects of the I-V curve make the max power voltage of a PV array nearly identical throughout the temperature and irradiance range for a given ambient temperature. Thus, the ambient temperature is the ultimate deciding process on voltage in a round about way. I hope we can come to a meeting of the minds on this some day. Bill. Peter, The issue is the same as it was last year. [#] But it hasn't been resolved to my satisfaction, yet The operating voltage of each string will be nearly identical throughout the day if the strings are the same length and not shaded. [#] This can't be the case. After all, the SE sub-array will have greater irradiance than the SW sub-array in the AM and the converse will be true in the PM Put them on a single inverter. [#] I'd dearly like to. _______________________________________________ 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
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