[RE-wrenches] Two strings of Different Orientation

[R Ray Walters]

I agree with your I-V curve interpretation Bill,  I had never considered the counteracting influences on voltage of reduced irradiance and reduced temp.
Very interesting. Also funny how most manufacturers quit publishing IV curves....

One way to check it Peter, is to turn each subarray on separately, and check the operating voltage, then turn them both on.
If Bill is right, you won't  see enough variation to seriously affect MPPT. (Power subarray A + power subarrayB should = total power with both arrays on)
(My problem is intermittent clouds always come and mess up my experiments like this.......)

Also, BIll mentioned the need for field IV tracing tools, Greenlee recently came out with an IV tracer for under $2500, but unfortunately it only does 60 v max.
(somebody should tell them, we'd be more interested in field measurements of entire strings, not individual modules)

R. Walters
ray at solarray.com
Solar Engineer




On May 20, 2010, at 11:02 PM, 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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