PV orientation [RE-wrenches]

[Bill Brooks]

William,

Once the operating voltage is set by the dominant portion of the array, the
parallel strings still have a max power voltage that is nearly identical to
the array in full sun. It may seem counter-intuitive, but temperature
changes voltage, not irradiance (accept at very low irradiance).

The critical issue is that all the modules in a series string are in the
same orientation since the operating current needs to match exactly.
Parallel strings can be in different orientations with little negative
affect on performance since the operating voltage of each string is nearly
identical.

Bill.


-----Original Message-----
From: William Miller [mailto:wrmiller at slonet.org]
Sent: Thursday, August 22, 2002 2:58 PM
To: RE-wrenches at topica.com
Subject: SPAM: RE: PV orientation [RE-wrenches]


Now throw in this question:

Both arrays are fed into one GC1000 inverter with MPPT.  How will the MPPT
loading affect the array outputs?

Allow me to speculate:  An MPPT device is basically a variable load
resistance.  The load impedance is lowered gradually while monitoring amps
and volts for maximum power.  In the morning, the east array will have the
most current capability, so the MPPT loads it heavily.  The load is that
same for both arrays, so the array with off axis sun will be loaded too
heavily, further reducing it's output. While mid day sun is equal for both
arrays, afternoon sun will of course be the converse of the morning
scenario.

This problem may well erase any gains made by MPPT tracking, but I'd like
to know for certain.  Could it be that non-MPPT controls might produce more
output in this situation.  And we all know that all MPPT circuits are not
equal.

I was hoping someone some way of monitoring a system such as this, but it
would require tracking both arrays, I imagine.

William

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