[RE-wrenches] Mixing and Matching PV modules--Error Corrected

[Bill Brooks]

Peter,

Although last time we discussed, you didn't seem to agree with me, I guess
I'll try responding to this question.

The best situation is to keep the two strings of modules in their own
strings. 7 of the older in series, paralleled with the 7 of the newer in
series. The operating voltages, and open circuit voltages are for all
practical purposes--identical. It appears that the only difference is the
size of the cell--producing a higher current as expected--with little or no
change in voltage. Operating voltage is the key--as in our last discussion.


Mixing the strings will result in lower output overall. Perhaps trying it
both ways would prove it to you. An I-V curve tester will clearly show the
benefit of not mixing the modules within the string. As I teach my classes,
you have to think like a "PV". The I-V curve is limited by the lower current
modules in both strings if you wire as you suggest. If you wire the modules
with the like modules, each string will produce their maximum current.

Draw out the I-V curves and it may make more sense. 

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: Monday, March 30, 2009 3:50 PM
To: 'Peter Parrish'; 'RE-wrenches'
Subject: Re: [RE-wrenches] Mixing and Matching PV modules--Error Corrected

I think I understand all the dictums about single-strings of PV modules: all
the same modules, all the same azimuth and tilt, but I have a "special
situation".

We are starting to accumulate a number of "second hand" PV modules. Some
have frames slightly damaged by the shipper; some have come from warranty
replacements; and some are odd lots that we haven't been able to sell.

What I want to do is to set up a demo system composed of two different types
of modules. This is my initial design:

Total of fourteen modules. Two strings of seven modules each. Each string
consisting of four modules of one type (type A) and three modules of a
second type (type B).

Both module types have 50 cells and the type A module has 155mm-sq cells and
the type B module has 150mm-sq cells. So you would think that the
performance specs would match pretty well. However, since the type B modules
are an earlier vintage, they probably don't have the latest and greatest
surface treatment, etc., so the current specs are lower than one might
expect.

Specs on type A: Voc = 30.8V, Vmp = 24.5V, Isc = 8.70A, Imp = 8.16A. Specs
on type B: Voc = 30.6V, Vmp = 24.6V, Isc = 7.38A, Imp = 6.93A. 

So here's my question: would putting together the strings of four type A and
three type B cause any problems? I know the string current would be limited
by the type B modules, but above and beyond that, what would be the
operating point of the type A modules and what would be the stress if any on
the type B modules? I'd like this demo system to last a while and I
certainly don't want to see scorch marks on the type B modules after a few
months of use. 

Any thoughts?

- Peter

Peter T. Parrish, 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