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HeatSpring PV Course Blog

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STC Voltage Higher than PTC, CEC and NOCT because ambient vs. cell temperature

Posted on 4 April, 2016 at 21:46

Q:

Hi Sean,
I have just taken the test and question 25 seems to be off. Perhaps I require further clarification. Anyways, the question is:
Which test conditions have higher voltage?
a. STC, b. PTC, c. CEC, d. NOCT
According the the textbook STC (option 'a') is the correct answer and it explains that the reason is because the other test conditions have higher cell temperatures and therefore lower voltage.
Now, STC (option 'a') temperature is 25C, whilst the other are 20C. So, actually all the other options have 'lower' temperatures and hence should generate more voltage... This is the opposite to the answer noted as correct. I am missing something here, or there is an actual error?
Thanks!

A:

STC is the cell temperature and the problem with STC is when the sun beats down on PV, the PV will heat up considerable, often 30C over ambient.

NOCT, PTC and CEC test conditions are 20C ambient temperature.

NOCT (Nominal Operating Cell Temperature) is also 800W/square meter and 1 meter/second wind speed and will often be about 47C (about 27C over ambient).

PTC (Performance Test Conditions) and CEC (California Energy Commission Test Conditions) are the same as NOCT, except the irradiance is 1000W per square meter, so I would expect a no wind day at 1000W per square meter to be a bit more than 30C above ambient.

When I did a class at SAIT College in Calgary, it was freezing 0C ambient and we measured the PV module with no wind and it was 38C (100F)!

Another interesting thing about PV and temperature is that thermodynamics comes into play. If you have a PV module that is working and sending power to the grid, it will not heat up as much as a module that is not sending power back. In theory, if you had a 100% efficient module, it would not heat up at all from sunlight and a module at PTC would have a higher voltage than a module at STC. Since Einstein says we can not get over 50% efficient, then STC will be lower voltage than PTC due to heat. Einstein has been proven wrong a few times now. What do you expect from a patent clerk anyway.

Thanks,
Sean White

Categories: PV Boot Camp and NABCEP Entry Level Exam Prep Course

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