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Three series 60 cell panels on Ford Transit RV w/150V MPPT in southern SoCal, AZ, NM, or TX

Van Gogh

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Jan 5, 2022
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Is anyone successfully running an array at home with three series 60 cell panels (e.g. Rec Alphas) with a 150V MPPT in southeast California, southern Arizona / New Mexico, or southwest Texas?

I'm about to install one on my Ford Transit RV, but I had a sales guy warn about the higher irradiance we see in the desert southwest, so I got my local station to provide 10 years of data and sure enough we routinely hit 1200W/m2 (1.2 x STC) on some days of the year.

If the batteries were fully charged so the MPPT wasn't pulling any current from the array (commonly occurs) and was around noon on a 75-85F very cloudy day (e.g after a short rainstorm) so the panel was near ambient, if the clouds broke and the sun first appeared the math suggests the voltage could reach 44.1 x 1.2 x 3 = ~159V. And the specs actually allow for +/- 3% in panel voltage so technically it could be another 3% higher. On a cold freak hail storm summer day (we do get those every now and then) temps could even be 67F, so now the temp coefficient of Voc could add another 10 x 0.0024 = 2.5%.

But I don't care about the math as much as I care about real user experience, so does anyone living in Phoenix/Tucson, Las Cruces, or El Paso have a three series 60 cell panel array (e.g. Rec Alphas) installed with a 150V MPPT? If so, no issues releasing "the magic smoke" from your MPPT?

I'm fine if my MPPT shuts down when/if the above rare scenario happens, but I don't want to smoke it. If it's that close and others have had problems, I'd rather just used Rec N-Peak 2's that have a lower Voc.

I appreciate conjecture and other related feedback but I really want practical user experience.

Cheers.
 
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I have a 3S 47Voc/panel array in ENE AZ @ 6500ft, so my sun is getting through 14% less atmosphere. I regularly see rated power and higher. Recently saw 3600W (2970W rated) due to the cloud edging you described coupled with cold.

Due to my climate with potentially very cold weather and panels that almost hit 150V @ ambient, I went with a 250V controller.

The existence of light provides the voltage, i.e., You will get the same Voc with 100W/m^2 or 1200W/m^2.

The intensity of light provides amperage, so you multiply your amps by 1.2, not volts.

You will also get more panel heating, so that will offset the output.

Your concerns about Voc are unfounded on an irradiance basis.

132Voc rated vs. 150V PV limit is good down to about -16°C on typical panels.
 
I have a 3S 47Voc/panel array in ENE AZ @ 6500ft, so my sun is getting through 14% less atmosphere. I regularly see rated power and higher. Recently saw 3600W (2970W rated) due to the cloud edging you described coupled with cold.

Due to my climate with potentially very cold weather and panels that almost hit 150V @ ambient, I went with a 250V controller.

The existence of light provides the voltage, i.e., You will get the same Voc with 100W/m^2 or 1200W/m^2.

The intensity of light provides amperage, so you multiply your amps by 1.2, not volts.

You will also get more panel heating, so that will offset the output.

Your concerns about Voc are unfounded on an irradiance basis.

132Voc rated vs. 150V PV limit is good down to about -16°C on typical panels.
Excellent answer. I got a case of your favorite beer or a ctypto transfer for the same amount if you want it. Just shoot me a message. You just saved me a ton of hassle with the clarification that irradiance does not effect Voc. Thank you!!

Cheers.
 
Happy to help. Here's a post I made a while back about that showing PV voltage before and after sunrise:

 
I have a 3S 47Voc/panel array in ENE AZ @ 6500ft, so my sun is getting through 14% less atmosphere. I regularly see rated power and higher. Recently saw 3600W (2970W rated) due to the cloud edging you described coupled with cold.

Due to my climate with potentially very cold weather and panels that almost hit 150V @ ambient, I went with a 250V controller.

The existence of light provides the voltage, i.e., You will get the same Voc with 100W/m^2 or 1200W/m^2.

The intensity of light provides amperage, so you multiply your amps by 1.2, not volts.

You will also get more panel heating, so that will offset the output.

Your concerns about Voc are unfounded on an irradiance basis.

132Voc rated vs. 150V PV limit is good down to about -16°C on typical panels.
Okay, wait a minute, I'm finding data from the manufacturer that seems to suggest more is at play.

NMOT is 800W/m2 at 68F w/Voc = 41.6

STC is 1,000W/m2 at 77F w/Voc = 44.1

That's a 6% increase in Voc with only 5C temperature variance which only accounts for 9 x 0.0024 = 1.2%. So there's a nearly 5% increase in Voc that is unaccounted for. Maybe irradiance does have an effect on Voc, but it's not linear? That's still good news for my project. Any thoughts?

Cheers.
 
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Here's that day it produced > 3600W:

1641410844539.png

Same timeframe for PV voltage/current:

1641410909453.png

While not a direct representation of your scenario, you can see voltage isn't changing much (staying at or below typical 120Vmp), but the current is spiking causing the power spikes - a reaction to solar intensity + cold.
 
Okay, wait a minute, I'm finding data from the manufacturer that seems to suggest more is at play.

NMOT is 800W/m2 at 68F w/Voc = 41.6

STC is 1,000W/m2 at 77F w/Voc = 44.1

That's a 6% increase in Voc with only 9F temperature variance which only accounts for 9 x 0.0024 = 2.1%. So there's a nearly 4% increase in Voc that is unaccounted for. Maybe irradiance does have an effect on Voc, but it's not linear? That's still good news for my project. Any thoughts?

Cheers.

What you don't see in the NMOT test condition is the PANEL temps. The temps listed are AMBIENT.

At NMOT the panels are actually at 45°C, so that's why you see a lower voltage. The panel is at HIGHER temperature than ambient because of solar heating.

At STC, the PANELS are at 25°C as well as ambient, which isn't something you'll actually see in real life.

In short, panels in NOMT test conditions are 20°C hotter than panels in STC test conditions, hence the LOWER voltage.
 
Very good info. Thank you.

Are you 100% confident STC is both panel/module and ambient temp? So they basically expose an ambient panel to 1000W/m2 and immediately record the max Voc? The math says that should yield a ~8% difference in Voc between STC and NMOT, but Rec's datasheet says ~6%. It's pretty close though. And nothing like the 20% irradiance increase I was falsely positing.

Thanks for the chart. Are you able to be certain that is the highest output your array has ever produced at any given moment? The other issue with the chart is the MPPT was drawing current, which inherently reduces Voc, but your other post and your panel's datasheet may settle it for me. Thanks again!

Cheers.
What you don't see in the NMOT test condition is the PANEL temps. The temps listed are AMBIENT.

At NMOT the panels are actually at 45°C, so that's why you see a lower voltage. The panel is at HIGHER temperature than ambient because of solar heating.

At STC, the PANELS are at 25°C as well as ambient, which isn't something you'll actually see in real life.

In short, panels in NOMT test conditions are 20°C hotter than panels in STC test conditions, hence the LOWER voltage.
 
Very good info. Thank you.

Are you 100% confident STC is both panel/module and ambient temp? So they basically expose an ambient panel to 1000W/m2 and immediately record the max Voc? The math says that should yield a ~8% difference in Voc between STC and NMOT, but Rec's datasheet says ~6%. It's pretty close though. And nothing like the 20% irradiance increase I was falsely positing.

While only a partial answer, from the datasheet I provided:
1641414663184.png

STC clearly says cell temp 25°C. NOCT (same as NMOT) ambient temp of 20°C. I had to look up NOCT/NMOT info. 45°C isn't definitive as it depends on panel design, so it varies from panel to panel, but they are typically in that vicinity.

NOCT stands for Nominal Operating Cell Temperature.


Thanks for the chart. Are you able to be certain that is the highest output your array has ever produced at any given moment?

No. I have 6 months of data, but without a download and parsing of the data gathered at 1 minute intervals, I can't be certain.

It's rare that I place demands that require even rated power. Normally, due to my battery bank, I am current limited on charging. On that day, we were recovering from the prior day of very poor solar, so I lifted the current limitations while at lower SoC to expedite charging.

The other issue with the chart is the MPPT was drawing current, which inherently reduces Voc, but your other post and your panel's datasheet may settle it for me. Thanks again!

Agreed. It was just intended to show the increase in current with higher power.
 
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