Mixing panels with different Vmp/Voc

RoySalisbury

I want to add more capacity to my solar setup.  Since I can no longer get the same panels that I currently have, I need to get different panels. However, they are no longer going to have the same specifications.  I'd try to stay with the same overall wattage of the panels, but the Vmp/Voc are not the same.  Currently I have 6 of the following panels:

Kyocera KD320 (320 Watt).  [Vmp - 40.1, Voc - 49.5]

These are setup with 2 in series, and 3 parallel. Going the other way would put me too close to the Voc max rating of the charge controller.

The closest that I can find form "Arizona Win/Sun" is

Silfab Solar 350 (350 watt). [Vmp - 38.9, Voc - 47.5]

When I setup a new string last time they were so far out of spec (210w vs 320w) that I setup a new charge controller just for the 6 320's.  However, that's not an option now.  I'm using an Outback MPPT charge controller (FM60 and FM80).  

My understanding is that if I put 2 more of the 350's in and hooked them in parallel, then I should be fine .. with the restriction that that all my other panels will now get degraded in voltage to the lower set.  Is that right?

If so, then I guess the 'extra' 100 watt on the new panels will make up for the loss.  Also, since the panels normally operate higher then the Vmp, that as long as the newer panels operate above 40.1, then I won't really loose anything.

this is all very confusing.  Any guidance from some experts would be appreciated.

Thanks.

RoySalisbury

Just found this thread from 2012 that may be what I'm looking for.  

http://forum.solar-electric.com/discussion/16660/panels-in-parallel-same-vmp-different-voc

Seems to indicate that I should be fine as the MPPT will settle on the voltage and the panels will adjust the amp to accommodate.  So really should not lose anything as long as the Vmp of the lower panels does not fall below the Vmp of the higher.  

Raj174

I think it's a good match. The output of the new string should be about the same as the others. Almost as though 320 watt panels were installed.

Rick

MichaelK

With a different controller you'd have more flexibility.  You could wire your Kyoceras in two arrays of 3 panels, with a Vmp of 120.3VDC.  But the open circuit voltage of three would be dangerously close to the 150Voc limit of your Outback controllers.  However, with a Midnight Classic 200 for example, you could match 3 Kyoceras wired in series to four generic grid-tie panels running at 30.1VDC.  So, 3X40.1VDC=120.3VDC≈4X30.1VDC.  One string of kyoceras would be putting out ~8.0 amps at 120.3VDC, while the second string of generics would be putting out about 8.4amps at 120.4VDC.

littleharbor2

Hyper Voc. on a Classic 150 would allow up to 174 volts(open circuit) for a 24 volt system and 198 volts (open circuit) for a 48 volt system.

BB.

Remember that Midnite controllers go into "safe standby" when the Vmp-array is in the "hypervolt" range (controller does not charge the battery bank during hypervolt conditions).

-Bill

littleharbor2

Would that not usually be for the brief moment at daybreak while voltage is still in Voc.? It is called HyperVoc. Granted, it's not recommended to run the controller at it's limit. Just wanted to mention the extra headroom of the classics.

BB.

I would guess that if you had an usually cold snap, then the panel Voc-cold may be too high for the Midnite to "start"--But once the direct sun hits the panel and it begins to warm, the Voc voltage will drop to normal operating range.

I don't think there is anything wrong with designing the system this way (you would not want to design your system to operate this way every day as you will lose some energy generation--And remember if the battery bank is disconnected for service, the Hypervolt range goes to zero until the battery is reconnected)--Just a point of information that the controller goes into standby when in Hypervolt range.

-Bill

mike95490

littleharbor2 said:

Would that not usually be for the brief moment at daybreak while voltage is still in Voc.? It is called HyperVoc. .....

But it's not a brief moment,  if it's cold, and some wind, the panels will stay cold and overvoltage for an hour or so, till the sun warms them up.  That is then lost harvest.

Estragon

Extreme cold plus windy in the morning is pretty rare, at least in my climate. Extreme cold tends to occur when a large high pressure area settles in, which generally brings little wind - it gets windy with lows, which brings cloud anyway. IMHO, the few times cold&windy happens is more than made up for by not having to be overly conservative in array voltage.

mike95490

The last week, I've been tripping the overcurrent on my 60A morningstar, with an array that normally puts out about 45A  

 Cold and wind matter.   My inverter has tripped offline twice, from overvoltage, because the charge controllers can't throttle back fast enough in cold scattered clouds.

The cold had my array delivering an extra 15A   

Estragon

Wow. I've not seen that, but the sun is low and weak in the cold here - I don't think I've ever seen over STC rating, and it has to be extremely cold for me to get near Voc limit.