[RE-wrenches] Inverter with two strings of different orientation

[Bill Brooks]

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-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at 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.parrish at calsolareng.com 



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