How to rewire GT PV Array for off grid?

Re: How to rewire GT PV Array for off grid?

Not really... The panels probably have a blocking diode so the "low voltage" string will not pull down the 3 string Voc (when cold)...And the controller may lock on the Vmp of the 2 string panel (when hot).

If you have to use the panels... A small relay across one panel in each string. Connect a thermostat set to 32F (or whatever you like) such that it powers the relay "on" at <32F. When cold, the 2 panel string will have a good Vmp>62 volts or so... When >32F, the relay opens and Voc cold is no longer an issue.

When you short out 1/3 of the panels when cold, you will lose 1/3 of the power output.

If you measure the array temperature itself (glue probe to back of panel with a little insulation (so it measures panel temp)--Once the panels get a good dose of warm sun--they will probably quickly exceed your set point even if the ambient temperature remains cold.

Automating the function would help prevent an "oops". It is OK to short the panels--that does not harm them.

Other options could include a stack of series connected diodes (voltage drop of 5 volts of diodes may keep you out of trouble at the cost of 3% power loss. Take out/short out diodes when weather turns warm.

Watch the specs for the controllers. For example (if I recall correctly) the XW MPPT will run up to 140 VDC. Over Voltage at 150V.

You have to be careful...For example, the diodes will require a little bit of leakage current to draw the 5 volts across them. If the charge controller has very little leakage current, you would need to add a small power resistor to "load" the diodes...

Or ignore me plus the overly complex solutions and run two panel strings when cold and 3 panel strings when >32F/panels are warmed up....

-Bill

Re: How to rewire GT PV Array for off grid?

See if there is an "easy way" to make a sane switching arrangement for groups of 6 panels... 3s*2p summer and 2p*s in winter. I don't want to see you lose power with the simple disconnect:

[FONT=Fixedsys]
=+=====+=====+======== + Bus
 |fuse |fuse |fuse
 |     |     |
---   ---    |
|p|   |p|    |
---   ---    |
 |     |     |
 |     |     |
---   ---    |
|p|   |p|    |
---   ---    |
 |     |     |
 +==+==+     |
    |        |
    *a       *b
    
    *c    *d
    |     |
   ---   ---
   |p|   |p|
   ---   ---
    |     |
    *g    |
          |
    *h    |
    |     |
====+=====+============= - Bus[/FONT]               

3sx2p:
a-c
a-d
g-h

2sx3p:
a-h
c-b
g-d

Switch X:
Pos1 a
Com c
Pos2 b

Switch Y:
Pos1 a
Com d
Pos2 g

Switch Z:
Pos1 g
Com h
Pos2 a

All switches in POS-1 = 3 panels in series
All switches in POS-2 = 2 panels in series

Not sure that this makes sense (3 switches per 6 panel sets)... But I think it works.