Model step
Spectral correction
Spectral correction to module power
Inputs used: Spectral mode, Spectral model, Spectral coefficients, Technology, Precipitable water, Relative humidity, Altitude
What it is
Sunlight arrives with a spectrum, a mix of colours, that changes with the length of its path through the atmosphere, the air mass, and with the water vapour in the air. A solar cell converts some colours better than others, so two timesteps with the same irradiance can give different module power. The spectral correction accounts for this with a modifier near 1 on the irradiance the module converts. The size of the effect depends on the cell technology: it is largest for thin-film cells such as cadmium telluride and small for crystalline silicon. Lee and Panchula's correction needs the precipitable water or the relative humidity from the weather file; the air-mass modifier needs only the sun's position and the site elevation. The automatic mode, the default of the API, runs the correction for cadmium telluride modules only.
Options
- Lee and Panchula: a spectral modifier from the air mass and the precipitable water, with published coefficients for each cell technology
- Air-mass modifier (King 2004): a polynomial in the absolute air mass with De Soto's coefficients, applied to the effective irradiance
- Off: no spectral modifier
References
- Lee & Panchula 2016, Spectral correction for photovoltaic module performance based on air mass and precipitable water, IEEE 43rd Photovoltaic Specialists Conference
- Gueymard 1994, Analysis of monthly average atmospheric precipitable water and turbidity in Canada and northern United States, Solar Energy 53(1) 57–71
- King, Boyson & Kratochvil 2004, Photovoltaic Array Performance Model, Sandia report SAND2004-3535
API
spectral.mode = auto | on | off, spectral.model = lee_panchula | king