Spectral mismatch models are part of the translation from broadband plane-of-array (POA) irradiance to effective irradiance. Generally, the translation takes the form
where is the spectral mismatch modifier, is the reflection losses for the beam irradiance (i.e., ) , is the diffuse POA irradiance and is the soiling factor. Models for are found here.
Air mass model for M
A polynomial may be fit to concurrent measurements of effective irradiance and absolute air mass to model . The polynomial model has the advantage of simplicity: air mass can be modeled from ephemeris. However, care should be taken when applying this model outside the range of data used to fit the model.
In the SAPM, a 4th degree polynomial is fit to data and the coefficients are normalized so that :
This approach is adopted in the De soto et al. performance model , and is illustrated here for a c-Si module.
Model for M using air mass and precipitable water
An alternative model is presented by M. Lee and A. Panchula (2016), where is modeled using air mass and estimated precipitable water:
where is precipitable water (cm) which can be estimated from ambient temperature and relative humidity using an appropriate model (e.g., pvl_calcPwat). Coefficients for this model are found in PVLib function pvl_FSspeccorr.m.