Spectral correction

What this step does

The row measures how the colour of the sunlight changes the module output. Low sun shifts the spectrum toward the red. Humid air absorbs infrared light, which shifts the spectrum toward the blue. Each shift helps some cell technologies and hurts others. The row acts on the array energy. The Model tab's Spectral correction step sets the spectral modifier, and the Model tab's Where the correction acts step sets the power the row is measured on.

The math

1. Running total
2. Loss at a timestep

$$E_{after} = E_{before} - E_{row},\qquad E_{row} = \frac{\Delta t}{1000\,P_{DC}}\sum_i L_i \tag{1}$$ $$L_i = N\left[P_s(G_i) - P_s(M_i\,G_i)\right] \tag{2}$$

where

Symbol	Meaning	Unit
$E_{before}$	the running total of the loss diagram before this row	kWh/kWp
$E_{after}$	the running total after this row	kWh/kWp
$E_{row}$	the row taken as a loss	kWh/kWp
$\Delta t$	the length of one timestep	hours
$P_{DC}$	the DC nameplate of the system	kWp
$L_i$	the power lost at timestep $i$ over the whole system	W
$N$	the number of modules in the system	—
$P_s(g)$	the module power the row is measured on at irradiance $g$: the module's maximum power at the cell temperature, or its nameplate power scaled by the irradiance, as the Where the correction acts step sets it	W
$G_i$	the irradiance the module converts at timestep $i$: the effective irradiance on the front plus the bifaciality factor times the rear irradiance	W/m²
$M_i$	the spectral modifier of the Spectral correction step at timestep $i$, a factor near 1	—

Reading the number

Negative for a loss and positive for a gain; the row is a gain when the modifier exceeds 1. The cell technology, the sun height and the water vapour at the site set it, the same way on utility and rooftop systems. Cadmium telluride gains about 1 % at humid sites; crystalline silicon moves by a few tenths of a percent.

References

1. Lee & Panchula 2016, Spectral correction for photovoltaic module performance based on air mass and precipitable water, IEEE 43rd Photovoltaic Specialists Conference

API

loss.spectral

Symbols follow the symbol registry.