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

Abstract: The angle-dependent spectral response is measured for six differently textured silicon solar cells before and after encapsulation. Deviations from Lambert's cosine law and differences due to the textures can be clearly determined by means of an uncertainty analysis. This effort in highly accurate angle-dependent measurement is needed for reliable prediction of annual performance. Energy rating using common simulation models for three different climate zones shows differences of more than 1% in yearly energy yield solely due to the different angle-dependent behavior of the textures. Modules featuring solar cells with random upright pyramids exhibit the largest angular losses of the studied textures.

Abstract: JRC ESTI and PTB performed a comparison of spectral irradiance measurements on various light sources at the JRC premises. The measured sources included four standard lamps (two calibrated by PTB and two calibrated by NPL), global natural sunlight at 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 rotating grating spectroradiometer is composed by a single stage monochromator and a Si-PbS sandwich detector in order to cover a wavelength range from 300 to 2500 nm. All instruments were calibrated in house by the respective owner institution using respective procedures and traceability chains. The measured spectra will be compared and deviations will be discussed with respect to the given uncertainties. Finally, exemplary spectral mismatch calculations will be shown in order to evaluate the impact of the deviations on PV device calibration.

Abstract: The prediction of the short-circuit current and hence the efficiency of solar cells is one of the main tasks for the energy rating (IEC 61853). For the complex simulation of the short-circuit current in the course of a day a comprehensive characterization of solar cells and a detailed knowledge about the spectral distribution of the natural sunlight are necessary. PTB recently developed a multi-functional laser-based Differential Spectral Responsivity (DSR)-facility for characterization and primary calibration of solar cells with lowest uncertainties. Furthermore PTB developed an outdoor facility for secondary calibration of solar cells and spectral irradiance measurements. We investigate the spectral responsivity, the linearity, the I-V-curves and the angular dependence of a reference solar cell using the multi-functional DSR-facility for several irradiance levels and temperatures. The obtained experimental results are combined with calculated solar spectra for a simulation of the short-circuit current and further the efficiency for a course of a day. These calculations are done for several orientations of the solar cell (roof: east, south, west and flat). The algorithm takes the orientations of the solar cells towards the sun and the spectral distribution (SMARTS, direct and diffuse light, including albedo) into account. For validation of these simulations we measured the short-circuit currents for the same orientations with the outdoor facility over a complete day. The results show a good agreement of the measurements and the simulations. This study demonstrates, that a realistic simulation of short-circuit current and hence the efficiency can be obtained for various kind of solar cells at any outdoor or indoor conditions, if typical properties of the solar cells (spectral responsivity, linearity, the I-V-curves, angular dependence) and the spectral distribution are known.

Abstract: Calibrated World Photovoltaic Scale (WPVS) reference solar cells are widely used for calibrating the irradiance of solar simulators as well as the determination of spectral mismatch correction factors. WPVS reference solar cells should provide long term stability of the short circuit current under standard test conditions < 0.3% over typical recalibration intervals of 1-2 years. The UV-irradiance of the utilized light sources (natural sunlight or solar simulators) can lead to degradation effects either in the solar cell itself or in the encapsulant. In this study different types of WPVS reference solar cells were exposed to elevated UV-irradiance in order to investigate the long term stability of the devices over a period of several hundreds of hours of permanent irradiation. Prior and after the UVexposure high precision differential spectral responsivity (DSR) calibrations were performed in order to investigate the impact of the UV-aging on the spectral responsivity and hence the short circuit current of the individual devices. In order to identify small relative changes < 0.2% of the spectral responsivity a DSR-facility is needed with very high reproducibility. This could be realized at the DSR and Laser-DSR facilities at PTB. Nine different types of reference solar cells were prepared by Fraunhofer ISE. They were developed to spectrally match different solar cell technologies. The BG38 and BG40 filtered devices should spectrally match a-Si solar cells, the OG590 and RG610 filtered devices should spectrally match μ-Si solar cells and the GaAs solar cell should spectrally match CdTe solar cells. The three unfiltered c-Si devices show different spectral responsivity in the IRregion. In conclusion these reference devices represent a thoroughly set covering all possible spectral responsivities needed in the PV-calibration and testing community.

Abstract: In recent years, satellite-based solar radiation data resolved in spectral bands have become available. This has for the first time made it possible to produce maps of the geographical variation in the solar spectrum. It also makes it possible to estimate the influence of these variations on the performance of photovoltaic (PV) modules. Here, we present a study showing the magnitude of the spectral influence on PV performance over Europe and Africa. The method has been validated using measurements of a CdTe module in Ispra, Italy, showing that the method predicts the spectral influence to within ±2% on a monthly basis and 0.1% over a 19-month period. Application of the method to measured spectral responses of crystalline silicon, CdTe and single-junction amorphous silicon (a-Si) modules shows that the spectral effect is smallest over desert areas for all module types, higher in temperate Europe and highest in tropical Africa, where CdTe modules would be expected to yield +6% and single- junction a-Si modules up to +10% more energy due to spectral effects. In contrast, the effect for crystalline silicon modules is less than ±1% in nearly all of Africa and Southern Europe, rising to +1% or +2% in Northern Europe.

Abstract: This paper presents a method for spectral response determination of photovoltaic devices using a commercially available pulsed source solar simulator and broadband filters. A fitting algorithm which is an iterative process is developed to model the spectral response curve. The method is tested on two different technologies of photovoltaic modules and the result shows that a fair agreement between the modelled and calibrated spectral response could be achieved with the improvement in the quality of measurement.