Page 11 - North American Clean Energy March/April 2020 Issue

It’s important to remember that PV plants are inherently spectral; they rely on a spectrally variable fuel, and use spectrally selective solar panels. As the solar industry evolves, spectral correction increasingly stands out as an area in need of advancement. Techniques for spectral correction have existed for decades, but have not been routinely deployed on PV plants due to practical limitations. Today’s spectral pyranometers introduce an effective solution for routine spectral correction on PV plants, with tangible benefits to key performance ratios.
Richard Beal is President and CEO of Spectrafy Inc., a Canadian-based sensor manufacturer that has created a line of solar spectral sensors that enable routine spectral correction on PV plants.
Spectrafy /// www.spectrafy.com
Figure 3. GHI weighted monthly spectral correction factors for a polysilicon solar panels, calculated from measured spectral pyranometer data (solid line) and modeled by Plant Predict (dashed line). For Ottawa, Canada from Jan 2018 – Aug 2019.
Quantifying spectral correction factors on site A number of options exist for quantifying spectral effects for PV plant operations:
Thermopile pyranometers
While Class A thermopile pyranometers can provide accurate, unbiased measurement of broadband global irradiance (as detailed above) they are not capable of quantifying spectral effects. To compensate for this, various models have been developed that use inputs such as relative humidity and/or air mass to approximate spectral correction factors for specific types of solar panels. These relatively simple models often fail to capture all spectral effects.
Reference Cell
Reference cells are essentially mini solar panels consisting of a single silicon solar cell. As such, they can effectively mimic the spectral performance of commercial silicon solar panels. However, they also possess the same degradation properties, which makes it difficult to decouple and track panel degradation over time.
Spectroradiometer
Field spectroradiometers, designed for continual outdoor use, have long been used by the PV research community. Incident light is split by a dispersive element (such as a diffraction grating) into its constituent wavelengths and directed towards
an array detector. Cost, performance limitations, and data load generally
make spectroradiometers impractical for widespread deployment on PV plants.
Spectral pyranometers
More recently, the market has produced spectral pyranometers that provide accurate measurements of total global irradiance, like a thermopile pyranometer, while also using spectral measurements to quantify the effects of local spectra on PV panels. Automated subroutines can convert this spectral information into spectral correction factors for easy use within PV performance models.
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