Temperature Coefficients

Etebari
Etebari Registered Users Posts: 8 ✭
in Advanced Solar Electric Technical Forum #1
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?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #2
    Re: Temperature Coefficients

    As far as I know--These 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 %/oC
    Vmp -0.52 -0.51 %/oC
    Imp 0.064 0.065 %/oC ["fix" value--- 0.006x is probably a data sheet typo. -Bill B.]
    Voc -0.36 -0.36 %/oC
    Isc 0.061 0.060 %/oC
    Operating Temp -40 to +90 oC

    The above numbers are in percent--So 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 watts--The 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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    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.
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  • Etebari
    Etebari Registered Users Posts: 8 ✭
    #4
    Re: Temperature Coefficients
    BB. wrote: »
    As far as I know--These 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 percent--So 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 watts--The 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.
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  • Etebari
    Etebari Registered Users Posts: 8 ✭
    #5
    Re: Temperature Coefficients

    The PV panels I'm concerned about are the Phono Solar PS230P-20/U.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #6
    Re: Temperature Coefficients

    Sometimes, certain values do not really matter too much... For example Imp/Isc -- Current is not affected too much by temperature--and 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). So--there 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 listed--which 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 useful--But 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 1-2 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 Vmp-hot and Voc-cold... Vmp hot depresses Vmp voltage and can stop a charger from full charging current or prevent a GT inverter from starting/running. Hi Voc-cold can damage switching electronics.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #7
    Re: Temperature Coefficients
    Etebari wrote: »
    I am finding that most are close like you suggest. I've noticed these values make a big difference when sizing wiring.
    Why? Wire sizing is based on current, and the variance in current from a PV module due to changes in temperature is very small.
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  • DLwindsun
    DLwindsun Administrators Posts: 34 admin
    #8
    Re: Temperature Coefficients

    Thermal Characteristics Phono Solar PS230P-20/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.
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  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    #9
    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.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #10
    Re: Temperature Coefficients

    Thank you CalBiker--I agree--Imp 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.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    #11
    Re: Temperature Coefficients
    BB. wrote: »
    Thank you CalBiker--I agree--Imp 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
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #12
    Re: Temperature Coefficients

    Thank you Cal!

    Cool-We have a new poster that can take over writing some of the equation based answers/estimates I give--They even have a better chance of being correct too. 8):roll:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    #13
    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
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
    Re: Temperature Coefficients
    calbiker wrote: »
    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

    Well everybody makes mistakes. That's why we keep an eye on each other. :roll:
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  • Etebari
    Etebari Registered Users Posts: 8 ✭
    #15
    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.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #16
    Re: Temperature Coefficients
    Etebari wrote: »
    TCPmp=TCVmp+TCImp. Can anyone verify this?

    TCPmp = TCVmp * TCImp

    Power = Voltage * Current

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Etebari
    Etebari Registered Users Posts: 8 ✭
    #17
    Re: Temperature Coefficients

    Thanks, hit the wrong key.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #18
    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
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  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    #19
    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%

    Cal
    Etebari wrote: »
    Somewhere I read that the value of TCPmp=TCVmp+TCImp.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #20
    Re: Temperature Coefficients
    calbiker wrote: »
    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%

    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
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  • calbiker
    calbiker Banned Posts: 50 ✭✭✭✭
    #21
    Re: Temperature Coefficients

    Yep, not exactly.

    Exactly it's:

    TC_P = TC_I + TC_V + TC_I * TC_V
    ggunn wrote: »
    Not exactly.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #22
    Re: Temperature Coefficients
    calbiker wrote: »
    Yep, not exactly.

    Exactly it's:

    TC_P = TC_I + TC_V + TC_I * TC_V
    When I get a chance I will test that with a real world example and see if it makes sense.
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