Temperature Coefficients
Etebari
Registered Users Posts: 8 ✭
Quite often manufacturers only place Isc, Voc and Pmp temperature coefficients in their specifications but fail to post Imp and Vmp coefficients. Is there a way to derive those values mathematically without contacting the manufacturer?
Comments

Re: Temperature Coefficients
As far as I knowThese are physical characteristics of the silicon material itself and not really related to the specific manufacturer. Here are some numbers I would use (from a large Kyocera panel):Pmax 0.46 0.45 %/^{o}C
Vmp 0.52 0.51 %/^{o}C
Imp 0.064 0.065 %/^{o}C ["fix" value 0.006x is probably a data sheet typo. Bill B.]
Voc 0.36 0.36 %/^{o}C
Isc 0.061 0.060 %/^{o}C
Operating Temp 40 to +90 ^{o}C
The above numbers are in percentSo you just use the specific Vmp/Imp/etc. of the panel you are calculating the temperature factors for... Notice that even for identical panels except one is 315 watts vs 320 wattsThe numbers are slightly different (0.46 vs 0.45 for Pmax). Just guessing but these are probably identical panels just sorted for rated output.
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Re: Temperature Coefficients
basically you'd be guessing. can you tell us what pv you are trying to find out about as somebody here may already know? worst case is you'd need to contact the manufacturer and while you are in contact with them suggest to them to list more specs. 
Re: Temperature CoefficientsAs far as I knowThese are physical characteristics of the silicon material itself and not really related to the specific manufacturer. Here are some numbers I would use (from a large Kyocera panel):
The above numbers are in percentSo you just use the specific Vmp/Imp/etc. of the panel you are calculating the temperature factors for... Notice that even for identical panels except one is 315 watts vs 320 wattsThe numbers are slightly different (0.46 vs 0.45 for Pmax). Just guessing but these are probably identical panels just sorted for rated output.
Bill
I am finding that most are close like you suggest. I've noticed these values make a big difference when sizing wiring. 
Re: Temperature Coefficients
The PV panels I'm concerned about are the Phono Solar PS230P20/U. 
Re: Temperature Coefficients
Sometimes, certain values do not really matter too much... For example Imp/Isc  Current is not affected too much by temperatureand in any case, most people use heavier copper cable than required to carry the solar panel current because they want to keep losses low (in the 1%3% range). Sothere is not too much reason to worry about "safety factors" for current capacity (check the numbers for fusing/breakers/conduit fill/insulation rating/etc... But, usually you will be using 2x or more thicker cable to keep solar power losses low).
And for parallel solar panel strings (three or more identical panel strings in parallel)There is usually a Series Protection Fuse listedwhich is somewhere around 1.56x Isc (typically). But a lot of vendors seem to leave that requirement off their data sheets. That value (also missing from the Phono Solar data sheets) should be a requirement for the designer / inspector for a system sign off (one or two parallel strings, fusing is not needed or usefulBut numbers should still be there to confirm).
I have seen a few high voltage (Vmp~100 volts or so) thin film panels (Dupont?) that had Isc in the 12 amp range, but a series fuse of ~15 amps. For those panels you can probably put three or four panels in parallel without fusing.
The usual "killers" in a design is Vmphot and Voccold... Vmp hot depresses Vmp voltage and can stop a charger from full charging current or prevent a GT inverter from starting/running. Hi Voccold can damage switching electronics.
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Re: Temperature CoefficientsI am finding that most are close like you suggest. I've noticed these values make a big difference when sizing wiring.

Re: Temperature Coefficients
Thermal Characteristics Phono Solar PS230P20/U
Operating temperature range 45 to 85 °C
Temperature coefficient Pmpp 0.51%/K
Temperature coefficient Voc 0.37%/K
Temperature coefficient Isc 0.03%/K
If the manufacturer does not provide all the coefficients it is best to design the system using Isc and Voc just to be safe. 
Re: Temperature Coefficients
Pmax = 0.46 %/C
Vmp = 0.52 %/C
Imp = 0.0061 %/C
This data doesn't make much sense. Imp temp coefficient is virtually zero. Therefore the temp coeff of Pmax should equal Vmp. 
Re: Temperature Coefficients
Thank you CalBikerI agreeImp should be closer to ~0.061 %/C (I think that there is an extra zero after the decimal point).
I am going to correct/adjust my previous post.
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Re: Temperature CoefficientsThank you CalBikerI agreeImp should be closer to ~0.061 %/C (I think that there is an extra zero after the decimal point).
Bill
That looks lots better. If we want to be exact,
given:
TC_Vmp = 0.52 %/C
TC_Pmax = 0.46 %/C
Then
TC_Imp = ((1  TC_P) / (1  TC_Vmp))  1
TC_Imp = 0.0603 %/C
Cal 
Re: Temperature Coefficients
Thank you Cal!
CoolWe have a new poster that can take over writing some of the equation based answers/estimates I giveThey even have a better chance of being correct too. 8):roll:
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Re: Temperature Coefficients
Thanks Bill, but now looking at my equation, I see an error. Results are the same.
Should be:
TC_Imp = ((1 + TC_P) / (1 + TC_Vmp))  1
Cal 
Re: Temperature CoefficientsThanks Bill, but now looking at my equation, I see an error. Results are the same.
Should be:
TC_Imp = ((1 + TC_P) / (1 + TC_Vmp))  1
Cal
Well everybody makes mistakes. That's why we keep an eye on each other. :roll: 
Re: Temperature Coefficients
Attachment not found.Attachment not found.
Looks like I'm getting close to the values I expected, thanks for the input. TC for Pmp, Voc and Isc are on the PV label. Somewhere I read that the value of TCPmp=TCVmp+TCImp. Can anyone verify this? There will be dual Xantrex Controllers in parallel. Still trying to make sense of all of this so I can speed up design of other systems for myself. 
Re: Temperature CoefficientsTCPmp=TCVmp+TCImp. Can anyone verify this?
TCPmp = TCVmp * TCImp
Power = Voltage * Current
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Re: Temperature Coefficients
Thanks, hit the wrong key. 
Re: Temperature Coefficients
Let V = voltage and dV = change in voltage due to temperature, and use the same notation for current and power
(1) P = VI
(2) P + dP = (V + dV)(I + dI) = VI + VdI + IdV + dVdI
subtract (1) from (2)
dP = VdI + IdV + dVdI 
Re: Temperature Coefficients
As strange as it may look, TC_P = TC_V + TC_I is CORRECT.
Look at it this way, if TC_V is 1 % and TC_I is 1 % then the P changes by zero percent.
TC_P = 1% + 1% = 0%
CalSomewhere I read that the value of TCPmp=TCVmp+TCImp. 
Re: Temperature CoefficientsAs strange as it may look, TC_P = TC_V + TC_I is CORRECT.
Look at it this way, if TC_V is 1 % and TC_I is 1 % then the P changes by zero percent.
TC_P = 1% + 1% = 0%
Cal
Not exactly. If V = 20 and I = 10, then P = 200.
Using your coefficient numbers, if you raise the temp 10 degrees, V = (1.1)(20) and I = (0.9)(10)
P = (1.1)(20)(0.9)(10) = 198 
Re: Temperature Coefficients
Yep, not exactly.
Exactly it's:
TC_P = TC_I + TC_V + TC_I * TC_VNot exactly. 
Re: Temperature CoefficientsYep, not exactly.
Exactly it's:
TC_P = TC_I + TC_V + TC_I * TC_V
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