Towards an energy-based parameter for photovoltaic classification, "PhotoClass"

Abstract: JRC ESTI and PTB performed a comparison of their respective reference standard spectroradiometers, aiming at improving knowledge on equivalence of spectral irradiance measurements and resulting uncertainty in calibration of photovoltaic devices. The comparison was conducted at the JRC premises using different light sources in order to test the reference instruments under various real-world operating conditions. The measured sources included four standard lamps (two calibrated by PTB and two calibrated by NPL), global natural sunlight under clear sky conditions, a steady-state xenon-halogen solar simulator for solar cell calibration (Wacom) and a steady-state xenon large area solar simulator (Apollo). The PTB system is composed of three compact array spectroradiometers (CAS) equipped with Si, InGaAs, and extended range InGaAs detectors, respectively, in order to cover the wavelength range from 250 to 2150 nm. The JRC-ESTI used a rotating grating spectroradiometer composed of a single stage monochromator and a Si-PbS sandwich detector in order to cover a wavelength range from 300 to 2500 nm, and, for the natural sunlight measurements, additionally a system composed of three compact array spectroradiometers covering the wavelength range from 300 to 1700 nm. Before the comparison all instruments were calibrated in house by the owning institution using their respective procedures and traceability chains. The measured spectra were compared and deviations were found to generally be within combined uncertainties. Finally, exemplary spectral mismatch calculations were made to evaluate the impact of the deviations in spectral measurements on PV devices calibration. The deviations were mostly less than 0.5%, but in some cases exceeded 1%.

Abstract: This work presents a validation of three satellite-based radiation products over an extensive network of 313 pyranometers across Europe, from 2005 to 2015. The products used have been developed by the Satellite Application Facility on Climate Monitoring (CM SAF) and are one geostationary climate dataset (SARAH-JRC), one polar-orbiting climate dataset (CLARA-A2) and one geostationary operational product. Further, the ERA-Interim reanalysis is also included in the comparison. The main objective is to determine the quality level of the daily means of CM SAF datasets, identifying their limitations, as well as analyzing the different factors that can interfere in the adequate validation of the products. The quality of the pyranometer was the most critical source of uncertainty identified. In this respect, the use of records from Second Class pyranometers and silicon-based photodiodes increased the absolute error and the bias, as well as the dispersion of both metrics, preventing an adequate validation of the daily means. The best spatial estimates for the three datasets were obtained in Central Europe with a Mean Absolute Deviation (MAD) within 8–13 W/m2, whereas the MAD always increased at high-latitudes, snow-covered surfaces, high mountain ranges and coastal areas. Overall, the SARAH-JRC's accuracy was demonstrated over a dense network of stations making it the most consistent dataset for climate monitoring applications. The operational dataset was comparable to SARAH-JRC in Central Europe, but lacked of the temporal stability of climate datasets, while CLARA-A2 did not achieve the same level of accuracy despite predictions obtained showed high uniformity with a small negative bias. The ERA-Interim reanalysis shows the by-far largest deviations from the surface reference measurements.

Abstract: Results from traceable calibrations of PV reference cells are compared. Two primary calibration methods were used, namely the differential spectral responsivity (DSR) method at the Physikalisch-Technische Bundesanstalt (PTB) and the direct sunlight method (DSM) at the European Solar Test Installation (ESTI). Furthermore, secondary calibrations were also made at ESTI against the World Photovoltaic Scale (WPVS). The calibration of c-Si reference cells is well established. Here we investigate the extension to filtered c-Si reference cells. Calibrations were performed on three cells. One unfiltered c-Si reference cell was used for control, confirming the good agreement in the calibration of such devices reported previously. Furthermore, two filtered c-Si PV reference cells were calibrated using the same methods. The agreement between the methods is studied and possible sources for discrepancies are investigated. The spectral mismatch correction is identified as crucial factor. The variation in the spectral mismatch correction with variation in measurement of spectral responsivity is found to be within uncertainty limits, whereas the measurement of spectral irradiance of the simulator can contribute much larger uncertainties due to artefacts in the spectral region around 1050 nm. A correction of the measured data alleviates the effect, so that the equivalence of all methods is demonstrated.