Round house [RE-wrenches]

[Eric Smiley]

MPPT will work better when all the modules are oriented the same
direction. Two sub-arrays with different orientations have different
amounts of light striking them. This influences the MPPT voltage in two
ways:
1. The temperature of the modules will be different due to heating from
sunlight. MPP voltage is directly affected by temperature.
2. The MPP voltage decreases slightly as the IV curve "shrinks". While
MPP current changes almost as a direct ratio of  light intensity, the
MPP voltage changes more as a logarithmic function of light intensity -
i.e. as light intensity decreases from full sun MPP voltage decreases
only slightly at first, but then more rapidly at very low light
intensity.

Thus, variations in light intensity cause a only a small change in MPP
voltage, except for temperature effects. Depending on the cell
technology, MPP voltage stays almost constant until light intensity
drops below 200 to 400 W / m^2.

When two (or more) sub-arrays have different orientations, the MPPT
algorithim will find the MPP voltage for the entire array - one
sub-array will be at too high a voltage, the other at too low a votlage.
 As a result  amorphous modules, with their flatter power vs voltage
characteristics and their smaller temperature coefficient for voltage,
will be less affected by the mismatch than crystalline cells.

 If you want to see what happens, Xantrex has a report on ST performance
on their grid-tie website and one of the systems in the report has two
sub-arrays  facing different directions.


----------------------------------------------------------------------
Eric Smiley
Project Leader - BCIT Technology Centre
NE25-3700 Willingdon Ave. Burnaby, BC V5G 3H2
ph: (604)432-8657  esmiley at bcit.ca  www.tc.bcit.ca

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