temperature coefficient question

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Heimhenge
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temperature coefficient question

Post by Heimhenge » Wed Jul 01, 2020 3:12 pm

The coefficient for my new CSUN panels is given as -0.42%/°C (according to the spec sheet). This is the power coefficient, which I believe would be used to derate the panels for performance on hot days.

So I just started testing the 20yo Astropower panels (AP-7105s) that were replaced. Need to see how they're still doing after all that time. Close to 90% so far, which was surprising. Planning to sell them on the used market. The cell temps measured 57°C today using my IR sensor, and I want to be able to correct for that.

So when I pulled out the spec sheets for these guys I find two different coefficients. See page two here:

http://www.getoffthegrid.com/manuals/As ... 0sheet.pdf

Two questions: Why is the coefficient for short circuit current a positive number? And can I get a simple %/°C value from that number and the open circuit voltage coefficient? Thanks for any insights!

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Re: temperature coefficient question

Post by Mike Curran » Thu Jul 02, 2020 9:07 am

Why is the coefficient for short circuit current a positive number?
This link https://www.pveducation.org/pvcdrom/sol ... emperature says "The short-circuit current, Isc, increases slightly with temperature, since the band gap energy, EG, decreases and more photons have enough energy to create e-h pairs. However, this is a small effect and the temperature dependence of the short-circuit current from a silicon solar cell is; or 0.06% per °C for silicon."

I'm not pretending to understand that! :grin:
http://www.tigoenergy.com/site.php?95b2dca2-ca6c
https://enlighten.enphaseenergy.com/public/systems/Hctc107221

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Re: temperature coefficient question

Post by raysun » Thu Jul 02, 2020 9:21 am

Mike Curran wrote:
Thu Jul 02, 2020 9:07 am
Why is the coefficient for short circuit current a positive number?
This link https://www.pveducation.org/pvcdrom/sol ... emperature says "The short-circuit current, Isc, increases slightly with temperature, since the band gap energy, EG, decreases and more photons have enough energy to create e-h pairs. However, this is a small effect and the temperature dependence of the short-circuit current from a silicon solar cell is; or 0.06% per °C for silicon."

I'm not pretending to understand that! :grin:
I don't recall ever paying attention to that spec. Isc is useful for planning array layouts and wire sizes but has little to do with actual operation of the panels, as short circuiting them would have no useful purpose.

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Heimhenge
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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

Re: temperature coefficient question

Post by Heimhenge » Thu Jul 02, 2020 10:38 am

I suspected that positive coefficient might be due to some heating effect on the electrons, as heat has to alter energy levels at the molecular (crystal) level. But yes, it is a very small effect compared to the Voc coefficient. With +0.2 mA/°C and the 37°C above STC we had during the test yesterday, that would be an increase of only 7.4 mA in Isc.

Voc however would drop by almost 3V. Since the STC Voc is listed as 21.0V, I was expecting numbers around 18V. Surprisingly, the four panels I tested had a Voc of 19.0V ± 0.1V.

The Isc measured 4.5A ± 0.1A. This compares to the rated 4.8A @ STC. No need to make that small 7.4mA correction. That's 94% of what I expected.

So with the Voc down less than expected, it appears that (at least these) solar panels age via a reduction in current output while the Voc pretty much maintains. If these numbers hold for the other 12 panels, I think I'd be able to confidently advertise them as "still performing at 94% rated capacity". The spec sheet notes the rated power tolerance is ±10%. Maybe I just got a good run of panels? Do my numbers make sense? Comments welcome.

[EDIT 1] I should probably mention that the panels were within a few degrees of perpendicular to the Sun. Easy to do. Sun's at like 71° elevation around solar noon, so just had to prop up one end of the rack with a box.

[EDIT 2] Answered my own question. Check out this paper by NREL. Page 13 and thereabouts:

https://www.nrel.gov/docs/fy12osti/51664.pdf

If I read that correctly, physical degradation of the cells take place on two fronts. First is the diffusion of dopants across the PN junction. This brings down the Voc. Second is the formation of micro-cracks in the cells, from thermal cycling and wind loading. This increases the total resistance of a series string of cells, lowering Isc. So for this brand of panels (Astropower AP-7105) it looks like it's the micro-cracks that predominate performance degradation. My numbers look more sensible now.
Last edited by Heimhenge on Thu Sep 24, 2020 2:46 pm, edited 1 time in total.

NaomiHowl
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Re: temperature coefficient question

Post by NaomiHowl » Tue Jul 28, 2020 8:16 am

raysun wrote:
Thu Jul 02, 2020 9:21 am
Mike Curran wrote:
Thu Jul 02, 2020 9:07 am
Why is the coefficient for short circuit current a positive number?
This link https://www.pveducation.org/pvcdrom/sol ... emperaturesite says "The short-circuit current, Isc, increases slightly with temperature, since the band gap energy, EG, decreases and more photons have enough energy to create e-h pairs. However, this is a small effect and the temperature dependence of the short-circuit current from a silicon solar cell is; or 0.06% per °C for silicon."

I'm not pretending to understand that! :grin:
I don't recall ever paying attention to that spec. Isc is useful for planning array layouts and wire sizes but has little to do with actual operation of the panels, as short circuiting them would have no useful purpose.
Any suggestions how can we use that spec?

raysun
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Joined: Tue Jul 26, 2016 5:57 am
My RE system: Flexpower Two: (2) FXR3048A, (2) FM80, MATE3s, FlexNetDC
SimpliPhi 48-3.8 (6 @ 48v)
Outback IBR3 battery enclosure
Suniva 330 watt panels (12 - 6 strings of 2 in series)
Hyundai 355 watt panels (6 - 3 strings of 2 in series)
Honda EU7000is gas fuel generator

Re: temperature coefficient question

Post by raysun » Tue Jul 28, 2020 9:23 am

I can't think of any practical application for the consumer other than testing Isc.

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