Do Those Bypass Diodes Work? [RE-wrenches]

[Mark Byington]

Yes, Virginia, they do.
We have done a small experiment by shading various sections of a PV array, and measuring the array output power to convince ourselves that the bypass diodes in the modules do allow the rest of the module or the rest of the array to produce power when one section is shaded.  We thought we would share the results with you all because some of you might be wondering the same thing!  P.S. If you don't want to sort through all the data, just skip down to the Conclusions section at the bottom.  P.P.S. - it works just like Bill Brooks said it would!

Test Bed System:
Two Sunny Boy 2500 inverters, 240V, with display
Each inverter has (11) Sharp 185's in a single string
(These modules are 12 rows x 6 columns of cells, 72 cells total)
Total power 4,070 W STC
Peak AC output power 3,054 W
Latitude 37N
Module tilt 30 degrees
Measurements made in full sun on 3/23/04, 12 noon
Temp 75F, wind 5 MPH, insolation 1,006 W/sq. m.
Modules are somewhat dirty (need a bath!)

Measurements:
AC output of inverter #1 only measured (i.e. half the system)
"No shade" output power:  1,474 W
Shade 1 cell only in 1 module only:  1,420 W
Shade 1 row of cells in 1 module:  1,322W
Shade 2 rows of cells in 1 module:  1,300 W
Shade 5 rows of cells in 1 module:  1,325 W

Shade 1 column in 1 module:  1,426 W
Shade 1 whole module:  1,338 W

Conclusions:
1. Shading any number of rows seems to knock out the output from the whole module (this is expected)
2. Shading 1 column knocks out about 1/3 of the output from the module (this is also expected - I think it is because the modules have 3 bypass diodes ea.)
3. Shading 1 cell only, seems to have about the same effect as shading 1 column.  I think this indicates that a dark cell is an open circuit, effectively blocking the current flow from one whole column.
4. When one whole module's output power is knocked out, the other 10 modules in the string produce about the same power as they did before.  This indicates that the bypass diodes are working and have minimal loss.

Mark
Cobalt Power Systems, Inc.
markb at cobaltpower.com




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