SPECTRAL MEASUREMENTS IN PHOTOVOLTAIC SOLAR POWER PLANTS FOR OBTAINING
THE SPECTRAL CORRECTION FACTORS (SCF)

The photovoltaic (PV) world is spectral, so it is essential to measure its parameters accordingly

The solar spectrum, showing the attenuating effects of various atmospheric components. The overall shape represents the sun’s spectrum in space. The white area represents the remaining parts of sun’s spectrum we observe on Earth.

The solar spectrum is very useful for a large number of scientific studies of the atmosphere, as well as for many other applications in agriculture and other sciences, but it is of special interest in relation to the calculation of the solar energy resource in photovoltaic solar power plants (PV), since each type of solar panel responds differently to the incident photon energy, always depending on the solar spectrum at each moment.

The sun is spectral and solar panels are also spectral, so it was necessary to find an adequate technological solution such as that offered by our SEMS Spectral System, capable of solving in the most advanced way what no other manufacturer can offer, that is, the calculation in real time, of the Spectral Correction Factor (SCF).

GHI weighted monthly spectral correction factors for polysilicon and CdTe solar panels, calculated from GEO-SIM-GPV spectral pyranometer data, measured in Ottawa Canada from Jan 2018 – Aug 2019. A SCF of 1.04 represents a 4% boost in PV panel performance due to local spectral effects.

The Spectral Correction Factors SCF, quantify the changes in the performance of the photovoltaic solar panel due to the difference between the spectrum of the sun at each moment of the day and the reference spectrum under which all solar panels are classified. Spectral effects can approach 15-20% instantaneously and 3-5% on average for a whole year, relative to the energy generated by the plant.

Therefore, the Spectral Correction Factors SCF should be used to calculate the "adjusted or corrected" performance of the plant, that is, the Performance Ratio or PR, taking into account the influence that variations in the solar spectrum cause on the response of solar panels and, consequently, in the energy generated by the plant at all times.

It is not possible to obtain a value adjusted to the reality of the Performance Ratio (PR) of a photovoltaic solar power plant, if the spectral correction factor SCF is not taken into account for its calculation.

For this, the SEMS Spectral System incorporates the new, revolutionary and exclusive, spectral Pyranometer model GEO-SIM-GPV, which provides, first, the measurement of Global Irradiance in the plane of interest and, additionally, the solar spectrum in real time, over the entire range from 280 to 4000 nm, with a resolution of 1 nm.

How does the SEMS Spectral System calculate the value of the Spectral Correction Factor (SCF) according to the data provided by the GEO-SIM-GPV spectral Pyranometer in real time, and then apply it to calculate the adjusted performance Ratio (PR) of the plant ?

The corresponding equations are listed below:

1.) First step, SCF calculation:

Donde:
A = Spectral Response of the PV panel
B = Reference Spectra (AM1.5G)
C = Measured Spectra by GEO-SIM-GPV sensor

2.) Second Step, unadjusted PR value:

Donde:
G = Broadband solar irradiance
A = Area of panels
𝑟𝑇 = Panel efficiency at temperature T

3.) Third Step, calculation of the adjusted PR:

Adjusted prediction of plant power generation:

Donde:
PSTC = Panel nameplate power
δ = Temperature coefficient
Tcell, Tref = Temperature of the cell and reference temperature (25 ºC)
G, GSTC = Site irradiance (from a pyranometer) and reference irradiance (1000 W/m2)
SCF = Spectral correction factor, a scalar to correct for spectral shape variance from AM1.5G