[RE-wrenches] One Inverter - Two Strings - Different Orientations

[Peter Parrish]

I thought we thoroughly thrashed this one out. A no less an authority than
Bill Brooks assures us that we can orient two (or more) strings differently,
wire them in parallel and the MPPT algorithms in modern inverters will sort
things out, essentially extracting the maximum (or nearly maximum) power
available from each module. 

I remain skeptical, and I won't become a believer until I can run a
"first-principles" calculation on a simulator like PVSyst.

Nonetheless, we do design and have installed systems consisting of two
strings each with different orientations. Typically, the elevations match
but the azimuths are different (e.g. 160 deg and 200 degrees).

Do we get as much total energy in one day as we would with two inverters
being fed by one string each and with each inverter operating with the same
efficiency? We don't know for sure, but it would appear so within 10%.

Here's the real reason for my post. The customer is so happy with his system
he wants to max out (his PV energy production now covers only about 80% of
his usage). Currently he has two strings of 8 each BP3220T modules on a
Fronius IG4000.

Based on Voc (at 14 deg F) for the BP3220T, we can put 14 ea modules on a
string. Based on conventional Pac limits, we can put only 10 ea modules on
the two strings. But taking into consideration that we have strings with
different azimuths, we should be able to safely exceed this limit of 10 ea.

Has anyone tried to figure out what the practical string limit is based on
differing orientations? Let's assume El=25 deg, AZI=160 deg and 200 deg.

- Peter


 
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885