# Precipitable Water & Air Mass Spectral Shift Method

Lee et al, showed that the spectral shift of CdTe and c-Si are both dependent on precipitable water and absolute air mass. In CdTe PV modules, precipitable water is the primary driver of spectral shift, with air mass being of secondary importance. For c-Si PV modules, the opposite is true, and air mass is the primary variable effecting spectral shift.

## Algorithm

Compute the spectral shift factor.

Note that this equation becomes unstable as UW approaches 0 and the air mass gets large near sunrise & sunset, so the author suggests clamping these values as follows:

If monthly spectral shift factors are desired (e.g. to automatically populate monthly factors for a site), compute the irradiance-weighted monthly values from hourly or sub-hourly values. The subscript n denotes the month of the year; subscript i denotes the hourly (or sub-hourly) value within each month.

For example, if the January spectral shift factor is desired, sum the product of all hourly spectral shift factors and global horizontal irradiance for that month, and divide by the sum of the global horizontal irradiance for that month.

## References

Accurate Measurement, Using Natural Sunlight, of Silicon Solar Cells by William M. Keogh and Andrew W. Blakers, http://cses.cecs.anu.edu.au/files/Natural_sunlight_PiPV.pdf

L. Nelson, M. Frichtl, A. Panchula, Changes in Cadmium Telluride Photovoltaic System Performance due to Spectrum by, IEEE Journal of Photovoltaics, Vol. 3, No. 1, January 2013.

M. Lee, L. Ngan, J. Sorenson and A. Panchula, Understanding Next Generation Cadmium Telluride Photovoltaic Performance due to Spectrum, 42nd IEEE Photovoltaic Specialists Conference, June 2015.

Lee, M, Panchula, A., Spectral Correction for Photovoltaic Module Performance Based on Air Mass and Precipitable Water, 43rd IEEE Photovoltaic Specialists Conference, June 2016