[RE-wrenches] Two strings of Different Orientation

[Bill Brooks]

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-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at 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.
 
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