Abstract
We propose a methodological approach to provide the accurate and calibrated measurements of sky radiance and broadband solar irradiance using the High Dynamic Range (HDR) images of a sky-camera. This approach is based on a detailed instrumental characterization of a SONA sky-camera in terms of image acquisition and processing, as well as geometric and radiometric calibrations. As a result, a 1 min time resolution database of geometrically and radiometrically calibrated HDR images has been created and has been available since February 2020, with daily updates. An extensive validation of our radiometric retrievals has been performed in all sky conditions. Our results show a very good agreement with the independent measurements of the AERONET almucantar for sky radiance and pyranometers for broadband retrievals. The SONA sky radiance shows a difference of an RMBD < 10% while the broadband diffuse radiation shows differences of 2% and 5% over a horizontal plane and arbitrarily oriented surfaces, respectively. These results support the developed methodology and allow us to glimpse the great potential of sky-cameras to carry out accurate measurements of sky radiance and solar radiation components. Thus, the remote sensing techniques described here will undoubtedly be of great help for solar and atmospheric research.
Highlights
Interest in the accurate measurements of solar radiation on the Earth’s surface has resumed. It was mainly triggered by the large increase in solar energy plants that have been deployed all over the world as part of the most ambitious strategies defined by different international administrations to mitigate climate change [1,2]
The validation of the RGB sky radiance obtained from High Dynamic Range (HDR) imagery was carried out from February to October 2020 by comparing the sky-camera radiance results to the aerosol robotic network (AERONET) almucantar radiance dataset at 440, 500 and 675 nm
To ensure that both instruments observed the same portion of the sky, the SONA almucantar radiance was extracted from the images averaging within the field of view (FOV) of the Cimel (1.5◦ )
Summary
Interest in the accurate measurements of solar radiation on the Earth’s surface has resumed It was mainly triggered by the large increase in solar energy plants that have been deployed all over the world as part of the most ambitious strategies defined by different international administrations to mitigate climate change [1,2]. There is a need to improve the solar energy projections of these plants for different time scales It is especially important for photovoltaic and thermosolar plants, due to their increasing development and perspectives over the coming years [3,4,5]. The success of these projections highly depends on improving solar radiation measurements, both in accuracy and worldwide availability.
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