array temperature question

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Heimhenge
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Trace DC breaker and shunt box
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array temperature question

Post by Heimhenge »

The CSUN panels in my array have a temperature coefficient of -0.42% / °C (according to the spec sheet). This is the power coefficient, which I believe is the proper one to use for de-rating array output. I'm interested in how they actually perform because I'm in Arizona. I get how to do the actual calculation for comparison but have a question about array temperature.

Obviously the air temperature is meaningless. I need to measure the temperature of the panel itself which will usually be way above air temperature. So I just ordered a remote IR thermometer (not surprisingly back-ordered right now) with enough range to be useful in this application. If you're curious, it's this one:

http://www.extech.com/products/42510A

Here's my question: To measure the temperature of the panels should I measure the front side of the array where the cells face the Sun? Seems like I might get excess reflected IR if I do that. Or should I measure the back side of the array, which looks like this:
new-array-under.jpg
I'm sure the back will be cooler than the front, but don't know if it's more representative of the cell temperatures. When the thermometer arrives I'll do some tests both ways and see which better reflects the measured array output compared to rated output and post the results. Just looking for some advice here in case anyone else has done this. Thanks.
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Re: array temperature question

Post by provo »

Heimhenge wrote: Sun May 17, 2020 2:07 pm
Here's my question: To measure the temperature of the panels should I measure the front side of the array where the cells face the Sun? Seems like I might get excess reflected IR if I do that. Or should I measure the back side of the array....
With the glass and the underlying silicon, the emissivity could be weird ... for tricky surfaces (shiny copper is another one) I like to put a 1" square of masking tape down, and keep the laser dots on that when I take a measurement. I'd do that on the top side rather than the underside....
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Heimhenge
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Posts: 133
Joined: Sun Jan 26, 2020 10:28 am
My RE system: 9 CSUN 390W panels for 3.51 kW
UniStrut PV rack on a flat roof
Midnight Solar MNPV3 combiner box w/ breakers
Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
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Re: array temperature question

Post by Heimhenge »

Great idea. Shouldn't take long for the masking tape to reach the same temperature as the glass. Would it make much difference what color tape I use? I mean, as long as it isn't aluminized or something like that.
provo
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Hub 10
Mate3s
FNDC and Trimetric
Honda EU3000is generator
Location: Sierra foothills

Re: array temperature question

Post by provo »

Heimhenge wrote: Sun May 17, 2020 3:02 pm Would it make much difference what color tape I use? I mean, as long as it isn't aluminized or something like that.
I think the temp. of the glass under it won't be very affected by the tape -- tan might be better than the dark blue stuff :grin: . Do some experiments and let us know!
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Heimhenge
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Posts: 133
Joined: Sun Jan 26, 2020 10:28 am
My RE system: 9 CSUN 390W panels for 3.51 kW
UniStrut PV rack on a flat roof
Midnight Solar MNPV3 combiner box w/ breakers
Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
Contact:

Re: array temperature question

Post by Heimhenge »

provo wrote: Sun May 17, 2020 3:14 pm
Heimhenge wrote: Sun May 17, 2020 3:02 pm Would it make much difference what color tape I use? I mean, as long as it isn't aluminized or something like that.
I think the temp. of the glass under it won't be very affected by the tape -- tan might be better than the dark blue stuff :grin: . Do some experiments and let us know!
Count on it. I have the tan tape (which I'll use). Also have some black and blue and white, which I'll run for comparison.
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Hub 10
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Location: Sierra foothills

Re: array temperature question

Post by provo »

Heimhenge wrote: Sun May 17, 2020 3:34 pm
Count on it. I have the tan tape (which I'll use). Also have some black and blue and white, which I'll run for comparison.
Said like a true scientist!
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Re: array temperature question

Post by pss »

Whoa, whoa, whoa. I've already posted significant info on panel temperatures and power output. Search the forum.
Forget tape.
Know the ambient temperature, use a laser thermometer and point it at the panel surface, then the nearby ground. You will see how much hotter the panel surface becomes. I'm is So Cal. I have recorded panel surfaces close to 150 degrees F.

Panel output is significantly degraded in these conditions and there is really no way to cool them safely and efficiently in extreme heat conditions. The possibility of putting a circulating cooling tube system on the back side like an ice rink uses with a compressor will use up all your PV benefit.

Heat kills panels just when your AC systems are working their hardest.
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Heimhenge
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Posts: 133
Joined: Sun Jan 26, 2020 10:28 am
My RE system: 9 CSUN 390W panels for 3.51 kW
UniStrut PV rack on a flat roof
Midnight Solar MNPV3 combiner box w/ breakers
Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
Contact:

Re: array temperature question

Post by Heimhenge »

pss wrote: Mon May 18, 2020 8:33 am Whoa, whoa, whoa. I've already posted significant info on panel temperatures and power output. Search the forum.
Forget tape.
Know the ambient temperature, use a laser thermometer and point it at the panel surface, then the nearby ground. You will see how much hotter the panel surface becomes. I'm is So Cal. I have recorded panel surfaces close to 150 degrees F.

Panel output is significantly degraded in these conditions and there is really no way to cool them safely and efficiently in extreme heat conditions. The possibility of putting a circulating cooling tube system on the back side like an ice rink uses with a compressor will use up all your PV benefit.

Heat kills panels just when your AC systems are working their hardest.
I did search for "panel temperature" and got this:
search.php?keywords=panel+temperature
... none of which answered my question.

I know the panels will be hotter than air temperature. The "nearby ground" is a sealed/white painted wood roof. I'm not interested in adding cooling, but you can see from the photo that the backs of the panels are exposed to great airflow. Much more so than panels mounted flat on a tilted roof.

All I'm interested in doing is comparing the calculated de-rated output to the actual output. I need an accurate measurement of cell temperature to do that.

So you are saying that just pointing the IR thermometer at the business side of a panel gives an accurate reading? Is there no additional IR being reflected from the panels? I need the IR emitted by the panels alone, which is why I speculated about measuring the backside of the panel.
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Re: array temperature question

Post by fcwlp »

Heimhenge wrote: Mon May 18, 2020 9:02 am I know the panels will be hotter than air temperature. The "nearby ground" is a sealed/white painted wood roof. I'm not interested in adding cooling, but you can see from the photo that the backs of the panels are exposed to great airflow. Much more so than panels mounted flat on a tilted roof.

All I'm interested in doing is comparing the calculated de-rated output to the actual output. I need an accurate measurement of cell temperature to do that.
I would point it at the back side EVA polymer. As this is a relatively thin polymer compared to the topside glass, you will get a better temperature of the silicon. You can try the tape, but my hypothesis is that it won't make much of a difference as compared to the EVA.

It is amazing how much cooler a white low emissivity roof is in AZ as compared to a black roof. At our previous house in Scottsdale we had a black roof on the garage that could barely be stood on in the summer and after coating with the white low emissivity coating it was almost cool.
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Heimhenge
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My RE system: 9 CSUN 390W panels for 3.51 kW
UniStrut PV rack on a flat roof
Midnight Solar MNPV3 combiner box w/ breakers
Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
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Re: array temperature question

Post by Heimhenge »

For sure. White is the only sensible color for a roof in Arizona. Here's what mine looks like:
foam.jpg
That's two inches of poly foam with an elastomeric sealant. You can walk on it just fine. As long as you ain't wearing cleats. :)
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Re: array temperature question

Post by pss »

viewtopic.php?f=15&t=14204

viewtopic.php?f=1&t=13243

viewtopic.php?f=17&t=9055

My arrays are ground mounted and the panels have anywhere from 2 feet to 8 feet of space underneath them. I have measured temperatures on the panel surface at 1 PM on the hottest days as well as ground temperatures in the sunlight and shade. Quite interestingly I have found that the hottest surface temperatures I can record are on my turf in out back yard. Hot enough to burn a dog's paw.

I have learned about the degradation of my system due to heat and so far I don't see anyway around it. Water is SoCal cannot be used to cool the panels, not even mist them. The air is so dry it would evaporate right away.

LAST QUESTION: Why do people drive around in black cars in Arizona? In So Cal and even in Vegas, may roofs are not white because heavy insulation in the construction neutralizes the benefit of a white roof. In your case, the roof underneath the solar panels is in the shade because of the solar panels, so maybe try measuring the temperature of a piece of flat black painted wood on the shite roof under the solar panels and see how much warmer that is when shaded by the panels. Maybe white not much help under a panel.
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Heimhenge
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My RE system: 9 CSUN 390W panels for 3.51 kW
UniStrut PV rack on a flat roof
Midnight Solar MNPV3 combiner box w/ breakers
Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
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Re: array temperature question

Post by Heimhenge »

Those were all informative links, thanks. Curious they didn't show up in my search.

Regarding cooling the panels ... I wonder why they don't use a back panel colored high emissivity black? Sure, you'll get some IR from the ground during the day, but if the panel temperature is higher than the ground (white roof in my case) temperature, the net is radiative loss from the panels. But I think if I painted the backs of the panels black it would void the warranty. :)

No idea why people here in AZ drive dark colored cars. They pay for it in AC load on the mileage.
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Heimhenge
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Posts: 133
Joined: Sun Jan 26, 2020 10:28 am
My RE system: 9 CSUN 390W panels for 3.51 kW
UniStrut PV rack on a flat roof
Midnight Solar MNPV3 combiner box w/ breakers
Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
Contact:

Re: array temperature question

Post by Heimhenge »

Took awhile since last post to get this done, but I finally have some answers that might be of general interest. Turns out the recommended method of measuring solar panel temperature (for output derating or whatever) is to take the reading at the center of the panel on its back surface. Easy for me to do with a tilted array on a flat roof. Got myself one of these devices:

https://www.cambeep.eng.cam.ac.uk/equip ... ty/raynger

and did the measurement today. Air temp was 100°F (37.8°C) and the array temp was 148°F (64.4°C). Sky was clear with a slight haze. 7 mp SW wind. Time was solar noon. Sun was 13° off perpendicular to the array.

You can check the attached calcs for details, but it looks like under today's conditions, my 3.51 kW array should be producing 2.85 kW. OpticsRE told me it was actually 2.71 kW. That's a shortfall of about 5%, which I assume can be attributed to BOS losses.

Does that analysis look correct? Anyone else run an analysis like this and get similar results?
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Seasonal-Array-Data.doc
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pss
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Re: array temperature question

Post by pss »

Not to beat a dead horse, but my post of May 18th said panel temps would reach 150. In our latest heat wave, our ambient temperature reached 119!
I didn't measure panel temp, figuring I'd die doing it.
Your figures for power seem right to me. The manufacturer's specs are for an illuminated panel under laboratory conditions indoors, not something you can duplicate outdoors.

We just have to live with the fact that where hot, we get more sun, but less peak power. I love March, April May temps, the systems crank.
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Heimhenge
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Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
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Re: array temperature question

Post by Heimhenge »

Yeah, I totally expected those panels would be hot under a noon Sun. Just had no good way to measure their temperature until I picked up that remote IR thermometer. I ran the measurements again yesterday under near identical weather conditions. Got pretty much the same numbers, so I'm confident in my methodology. My intent now is to get those measurements monthly to see how things change over a year. Because ...

I oriented my array at 22°to horizontal to maximize the output for summer when our energy needs are highest. Of course, that causes higher panel temperatures and subsequent reduced output. There may be a better tilt angle that compromises for the two effects, and a year's worth of data should allow me to find it. With the UniStrut rack it would be easy to replace the three vertical members with slightly longer pieces.

But I'm a little puzzled now by that result of only 5% system loss. After I derated for solar incidence angle and panel temperature I was expecting 2.85 kW. What I was actually producing was 2.71 kW. Where was that missing 0.14 kW going?

Not complaining, just curious.
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Re: array temperature question

Post by pss »

Your post says you are using an FM80. And you said your panels are wired for 143 volts. And it is September. The FM80 has a maximum input voltage of 150 volts. In colder weather, especially in winter, the Voc will increase. In your case, if your Voc is every getting to 135 or more, I would say to change the serial configuration of your panels. For a 48 volt battery bank, a Voc of about or 70 or higher is excellent. And as far as the distances you mentioned, voltage drop over them is negligible. My panels are as far as 300 feet from my charge controller. If your Voc exceeds 150, you could damage your FM80.

As for tilt angle, you really want to optimize annual energy production, so the optimum angle should be set for September 21/March 21. The only way to increase power in the summer is to add more panels.
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Heimhenge
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Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
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Re: array temperature question

Post by Heimhenge »

I quoted the wrong value for the system voltage, sorry. Used the open circuit voltage by mistake. Using the MPPT values it would be more like 115VDC.

I hear what you're saying about maximizing annual production. But our home is earth-sheltered with passive solar gain in winter. Pretty much heats itself. Our biggest energy hit is for summer cooling. So it kinda comes down to "maximize production" vs. "minimize electric bills", and because of our skewed demand and APS seasonal rates the situation isn't symmetric. After a year of data I'll know where the sweet spot is.
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Re: array temperature question

Post by provo »

Heimhenge wrote: Sat Sep 26, 2020 11:38 am
I quoted the wrong value for the system voltage, sorry. Used the open circuit voltage by mistake. Using the MPPT values it would be more like 115VDC.
It's Voc which should be kept below 145V for the FM80 -- 150V is the absolute maximum, not for extended periods of time. 143V seems too close to me...
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Heimhenge
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Posts: 133
Joined: Sun Jan 26, 2020 10:28 am
My RE system: 9 CSUN 390W panels for 3.51 kW
UniStrut PV rack on a flat roof
Midnight Solar MNPV3 combiner box w/ breakers
Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
Contact:

Re: array temperature question

Post by Heimhenge »

provo wrote: Sat Sep 26, 2020 12:48 pm
Heimhenge wrote: Sat Sep 26, 2020 11:38 am
I quoted the wrong value for the system voltage, sorry. Used the open circuit voltage by mistake. Using the MPPT values it would be more like 115VDC.
It's Voc which should be kept below 145V for the FM80 -- 150V is the absolute maximum, not for extended periods of time. 143V seems too close to me...
Is that because in certain states of the charge controller it presents an open circuit to the array? Are you saying that could overload something in the FM80 if something like a cloud edge effect happened? And if so, isn't the FM80 designed with the usual safety factors such that it could take the hit?
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Eight Rolls S-550 (2 strings, total ~800Ah @ 24V)
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One VFXR3524A
Hub 10
Mate3s
FNDC and Trimetric
Honda EU3000is generator
Location: Sierra foothills

Re: array temperature question

Post by provo »

Heimhenge wrote: Sat Sep 26, 2020 2:08 pm
Is that because in certain states of the charge controller it presents an open circuit to the array? Are you saying that could overload something in the FM80 if something like a cloud edge effect happened? And if so, isn't the FM80 designed with the usual safety factors such that it could take the hit?
Any time the loads and battery aren't drawing any current from the FM, the FM isn't drawing any current from the panels either. That means the Voc is being applied to the input of the FM. I figure when they say absolute maximum, that's AFTER they've counted all the usual safety factors.
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Heimhenge
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Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
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Re: array temperature question

Post by Heimhenge »

Wait a minute here. You're quoting specs for the FM80. I have the FM100 which has a Voc rating of 300VDC. So all is cool.
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Hub 10
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FNDC and Trimetric
Honda EU3000is generator
Location: Sierra foothills

Re: array temperature question

Post by provo »

Heimhenge wrote: Mon Sep 28, 2020 12:35 am Wait a minute here. You're quoting specs for the FM80. I have the FM100 which has a Voc rating of 300VDC. So all is cool.
Yup, my mistake -- the FM100 is good up to a Voc of 290V continuous. Sorry for my confusion!
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Heimhenge
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Posts: 133
Joined: Sun Jan 26, 2020 10:28 am
My RE system: 9 CSUN 390W panels for 3.51 kW
UniStrut PV rack on a flat roof
Midnight Solar MNPV3 combiner box w/ breakers
Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
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Re: array temperature question

Post by Heimhenge »

No worries. I see you're running an FM80, so easy mistake to make. But you had me panicked for awhile there. :)
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Re: array temperature question

Post by raysun »

The FM100s have a different architecture than the FM80s to be sure. In either, it's best to stay below the rated maximum input voltage.

"And if so, isn't the FM80 designed with the usual safety factors such that it could take the hit?"

To paraphrase the Outback docs on overvoltage: The Flexmax (60 and 80) charge controllers are designed to shut down if the input voltage is above 150 volts, but that does not protect the circuitry. Damage may occur in this situation.

As explained to me by an Outback engineer, in an overvoltage event, the input circuits are still connected to the source voltage though they present a very high input impedence. If the input exceeds the breakdown voltage of the circuit components, the circuit will be damaged.

There's a second failure mode as well. If the input power exceeds the rated power of the charge controller/battery circuit (essentially the nominal battery voltage x the rated charging current) the circuits can lose control, and dump the input voltage and current to the battery. A properly configured array, and a properly specified output circuit breaker will help avoid such a catastrophic event.
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Heimhenge
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Posts: 133
Joined: Sun Jan 26, 2020 10:28 am
My RE system: 9 CSUN 390W panels for 3.51 kW
UniStrut PV rack on a flat roof
Midnight Solar MNPV3 combiner box w/ breakers
Outback Radian GS4048A-01 inverter
Outback FM100 charge controller
Outback FLEXnet DC monitor
Outback MATE3s system controller
Outback Hub
Trace DC breaker and shunt box
Lifeline GPL-8DL x4 = 12.2 kWh @ 48 volts
Running OpticsRE since March 2020
Location: 30 miles north of Phoenix @ elevation 2200 ft
Contact:

Re: array temperature question

Post by Heimhenge »

Getting back to the topic of my original post, here's an updated version of the file I attached earlier.

The topic was centered around the question of whether optimizing the array tilt for summer production had to be balanced with array temperature derating ('cause it gets pretty hot here in the summer). The answer will depend on the variations in what I called EDL (excess derated loss) and OSL (overall system loss). After a full year of taking monthly data I should have an answer.

I've added more comments and details. If anyone else wants to try this analysis it should be a helpful starting point. Thanks to all for the comments.
Attachments
Seasonal-Array-Data.doc
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Last edited by Heimhenge on Wed Oct 28, 2020 2:46 pm, edited 1 time in total.
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