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Why we use cell temperature instead of ambient temperature when calculating the short PV string size


On Question 51 can you explain why we use the high cell temperature over the high ambient temperature?


When we are determining voltage, what is important is how hot or cold the solar cell is, not how hot or cold it is outside. That is why airflow leads to better production. BIPV has less airflow than a rooftop, which has less airflow than a ground mount, which has less airflow than a pole mount. The more airflow, the better the cooling, the better the voltage, power, energy and return on investment!

On of the misleading things about PV is that it is sold by its rating at Standard Test Conditions, which is 25C cell temperature, 1000W/square meter and 1.5 Air Mass spectrum of light. For STC, it does not matter what the wind speed would be, because the test condition calls for a 25C cell temperature. This test is misleading, because when there is 1000W/square meter irradiance, it is probable that the cell temperature will be considerably hotter than 25C, even if the ambient temperature was 20C. This is why we have other testing conditions, such as Performance Test Conditions (PTC), which is 20C ambient temperature, 1 meter per second wind speed, 1000W per square meter irradiance and 1.5 Air Mass spectrum of light. Also, California Energy Commission (CEC) test conditions is the same conditions as PTC, however the conditions are determined differently, so the results are slightly different.

PTC is always less than STC, because at 20C ambient, the solar cells will always heat up more than 25C cell temperature. Often the increase in temperature is about 30C for a quick estimate, so expect your solar cells to be about 55C at PTC with the ambient temperature at 20C.

To convert to F from C take

25C x 9 = 225

225 / 5 = 45

45 + 32 = 77F

In other words...

9/5 x C + 32 = F

To go from F to C

68F - 32 = 36

36 x 5 = 180

180/9 = 20C

In other words...

(F - 32) x 5/9 = C

In other words 1.8 change in F is 1 change in C and 32F = 0C

The reason we use ambient for the cold temperature instead of cell temperature is to be on the safe side and that when it is super cold, often it is windy without direct sunlight. We are just expecting ambient temperature to possibly be cell temperature when it is super darn cold.

Why do they use 25C for STC if it is misleading and makes our PV look better than it really is over time? Because that is probably the temperature of an average solar warehouse and it makes the PV sound better. If they originally made STC 55C, then they would have to heat up the testing area to sauna temperatures and the people at the solar factory would quit their jobs.

Thanks for the good question!!

Sean White

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