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Item type:Publication, Impact of measured spectrum variation on solar photovoltaic efficiencies worldwide(Center for Open Science, 2021-09-02)In ratings of solar photovoltaic performance, variation in the spectrum of sunlight is commonly neglected. A single spectrum, AM1.5, is used as the sole basis not only for record laboratory efficiencies, but also for commercial module power ratings, the performance metrics for solar power plants, and warranty claims. Incorporation of solar spectrum variation would improve accuracy and reduce the financial consequences of prediction errors. Ground-level measurements of spectral irradiance collected worldwide have been pooled to provide an extensive – though by no means comprehensive – sampling of the variation. Applied to nine solar cell types, the resulting divergence in solar cell performance illustrates that a single spectrum is insufficient for comparison of cells with different spectral responses. In contrast with single-junction cells such as silicon and cadmium telluride, cells with two or more semiconductor junctions tend to have efficiencies below that obtained under AM1.5. Increases in the degree of sun tracking are shown to decrease efficiency for cells with a narrower spectral response. Of the nine cell types, silicon exhibits the least spectral sensitivity: relative site variation ranges from 1% in Lima, Peru to 14% in Edmonton, Canada, with a mean of 4%. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Impact of measured spectrum variation on solar photovoltaic efficiencies worldwide(Elsevier Ltd, 2022-08-01)In photovoltaic power ratings, a single solar spectrum, AM1.5, is the de facto standard for record laboratory efficiencies, commercial module specifications, and performance ratios of solar power plants. More detailed energy analysis that accounts for local spectral irradiance, along with temperature and broadband irradiance, reduces forecast errors to expand the grid utility of solar energy. Here, ground-level measurements of spectral irradiance collected worldwide have been pooled to provide a sampling of geographic, seasonal, and diurnal variation. Applied to nine solar cell types, the resulting divergence in solar cell efficiencies illustrates that a single spectrum is insufficient for comparisons of cells with different spectral responses. Cells with two or more junctions tend to have efficiencies below that under the standard spectrum. Silicon exhibits the least spectral sensitivity: relative weekly site variation ranges from 1% in Lima, Peru to 14% in Edmonton, Canada. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Predictability and Interrelations of Spectral Indicators for PV Performance in Multiple Latitudes and Climates(Elsevier Ltd, 2023-07-15)When PV is installed in the field, the module technologies are rated according to their output energy yield under local operating conditions rather than at standard test conditions (STC), where the spectrum is set to AM1.5G. Care must be taken as this standard is not optimal for all latitudes and the solar spectral distribution variations are one primary influencing factor on PV performance. In addition, obtaining an accurate estimate of the spectral effects on PV performance, as set out in standard procedures, is hampered by the cost of gathering the inputs and the large amount of spectral data required for such a calculation. In this work, based on measured spectral irradiance data from nine sites of different latitudes and climates, we first show a characteristic trend in the spectral distribution over the year concerning the location latitude. The closer a site is to the equator, the more blue-rich the solar spectrum is and the fewer seasonal spectral variations it will contain. Then, we calculate and correlate the most popular metrics (device-independent and device-dependent) used to describe the influence of solar spectra on PV performance. In particular, the monthly irradiance-weighted Spectral Mismatch Factor for different PV technologies and Average Photon Energy show a global linear correlation for data from these nine sites. We use this global linear relationship to propose PV technology-dependent equations that predict annual and monthly spectral gains/losses within a prediction half-interval of up to ± 1.66% by only inserting the monthly or annual irradiance-weighted Average Photon Energy potentially for any site. Reducing the required spectral data sets for performance estimation through our methodology facilitates a more accessible and less costly communication of databases than complete spectral data sets. Finally, using this spectral data, we demonstrate statistically that the Spectral Mismatch Factor and Integrated Useful Fraction Ratio can be replaced by alternative spectral metrics, which require only averaged spectra and, thus, reduce the computational effort to estimate the above indicators. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Overirradiance Conditions and Their Impact on the Spectral Distribution at Low- and Mid-Latitude Sites(Elsevier Ltd, 2023-07-15)Lately, the photovoltaics community has shown an increased interest in overirradiance conditions as there is the possibility that such conditions might lead to malfunctions in photovoltaic systems. Varying irradiance levels, recurrence, and duration of such conditions have been reported worldwide, but experimental studies on the spectral distribution of overirradiance conditions are still scarce. This work analyses measured spectral irradiance of overirradiance conditions along with spectra under clear and cloudy sky conditions in three different sites at low- (Lima-Peru) and mid-latitudes (Madrid-Spain and Berlin-Germany) collected for two years. The Average Photon Energy (APE) was used as a representative index of the spectral distribution. For each site, taking the APE under clear sky into account as a reference, it could be shown that the spectra under cloudy skies are blue-shifted, and the overirradiance spectra are red-shifted independently of the location. The red-shift is proportional to the irradiance enhancement intensity. In addition, all sites have different degrees of blue–shift for cloudy skies, with Lima, Madrid, and Berlin exhibiting a difference in APE compared to clear sky conditions of 17 meV, 38 meV, and 43 meV on average, respectively. This difference in APE for the overirradiance conditions compared to clear sky conditions is also independent of the location with a mean value of (8 ± 1) meV. These spectral shift observations experimentally confirm prior assumptions that overirradiance conditions predominantly cause an enhancement of the direct spectral irradiance. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Measurement and Analysis of Annual Solar Spectra at Different Installation Angles in Central Europe(Elsevier Ltd, 2023-12-01)The solar spectra in outdoor installations is very seldom equal to that under standard test conditions. This points to the importance of measuring, analyzing and understanding the implications of solar spectra variations with respect to the performance of solar cells. In this work, we present and analyze one year of sun spectra measured at two different angles in central Europe, where the spectrometers were installed at the optimum inclination angle and in vertical orientation, the latter being relevant for building integrated photovoltaics. We report for the first time the differences between the two inclination angles in terms of key performance indicators, such as average photon energy, blue fraction and spectral factor. Moreover, we show the impact of these spectral changes on the maximum current density of ideal single-junction and tandem devices in tilted and vertical orientations. Red-shifted solar spectra were more often found at the vertical installation in comparison to the optimum installation angle, which translated into up to 30% lower current-mismatch losses in idealized tandem devices throughout the year.