Development of a System Measuring the Solar Radiation Spectrum in 5 Planes for Daylight and PV Applications

Abstract Solar energy is abundant and capable of providing much of our energy needs. It has several uses: photovoltaic, biomass (wood energy, biogas, biofuel), daylighting. For each use, its potential depends on the match between the spectral response of the collector (silicon, retina photoreceptors.) and the local solar spectrum. Increasing the performance of solar technologies by making the most of its spectrum (sun plus sky), requires a better understanding of its long term variations. Radiative Transfer Models (RTMs) could be used to produce routinely the spectral irradiance received in the plane of a solar collector and build this climatology, anywhere on earth. The accuracy of RTMs depends on how well daily atmospheric constituents are known at the site. This information is becoming available from satellite data. Spectral measurements are needed to validate the use of RTMs with satellite derived atmospheric information. Since 1992, ENTPE has been maintaining in Lyon, France, a measuring station specialized in daylight ( http://idmp.entpe.fr/vaulx/mesfr.htm ). This station is part of the IDMP network of the CIE (International Commission of Illumination). In 2012, we have decided to add continuous spectral measurements to our station. We chose to measure the spectral irradiance in 5 planes useful for daylight and photovoltaic applications: 1 direct (perpendicular to the sun), 4 global (horizontal, vertical north, vertical east, south inclined at 45°). We decided to use 5 spectrometers (Ocean Optics USB 650/4000) coupled to optical fibers fitted with diffusers. This paper describes the measurement system, the calibration of its components, the error characterization related to the sun tracker, the optical fibers and the spectrometers. It also provides an outline of the automatic quality control which takes full benefit of the other measurements of the station (global illuminance on horizontal and vertical planes, direct normal irradiance). These measurements will be used (1) to validate atmospheric information from satellites as well as RTMs, (2) to produce long term variations of the solar spectrum in Lyon, France.