Precipitable Water Method

The spectral shift factor for CdTe is primarily driven by the amount of precipitable water in the atmosphere, and consequently, the primary input to the calculation is precipitable water. There are several viable sources for precipitable water data: TMY3 data, free online websites, and approximations derived from either relative humidity and ambient temperature data or dewpoint temperature and ambient temperature data.

PlantPredict automatically chooses the best input data source for you, depending on what fields are available in your weather file. If precipitable water data is directly available in your weather file, PlantPredict will use that as input to the calculation. If precipitable water is not available, PlantPredict will estimate precipitable water from relative humidity and ambient temperature. Finally, if no relative humidity data is available, PlantPredict will estimate precipitable water from dewpoint temperature and ambient temperature.


Precipitable Water Method Inputs





1.) If neither relative humidity nor precipitable water data is available in the weather file, first estimate the relative humidity from dewpoint and ambient temperatures using the following approximation:

Precipitable Water Method Algo82
2.) If relative humidity is available in the weather file or if dewpoint was used to estimate relative humidity, estimate the precipitable water using the following approximation, where the air temperature is given in Kelvin, and the relative humidity is given as a percentage:

Precipitable Water Method Algo83
Precipitable Water Method Algo83-1
3.) Find the spectral shift factor:

a.) For First Solar Series 2, Series 3, and Series 4 modules:

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b.) For First Solar Series 4-2 and newer modules:

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