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Abstract: The purpose of this work is to characterize the sky relative radiance and luminance distributions from the analysis of sky images acquired using a monochromatic CCD wide angle lens camera. The proposed methodology is based on the reduction of the number of the gray levels from the original image, in order to sharpen the different regions of the sky hemisphere, resulting in low level gray scale maps. In these maps each gray level is related to an intensity level and each pixel is associated to a sky direction. An experimental system that is able to simultaneously supply images from the sky and solar irradiance data was developed using a commercial CCD camera. The sky relative radiance and luminance distributions for different sky conditions were also determined. The resultant maps agree with observations of the main characteristics of sky radiance and luminance distributions. [Copyright &y& Elsevier]

• The mesh calibration achieved the required precision with minimal sample size. • The transmissivity of mesh screens was measured with <1 % error with 5–7 samples. • Layer arrangement in mesh screens affected their transmissivity by up to 0.3%. • The dependence between layers influenced mesh screen transmissivity by up to 1%. • Interlayer dependence and arrangement must be considered to achieve <2 % error. Mesh screens can be installed over a photovoltaic (PV) module to provide varying degrees of outdoor solar irradiance for I-V curve measurements. In such tests, it is often not possible to cover the PV module and the irradiance sensor with the mesh screen at the same time, requiring the transmissivity of the mesh to be accurately measured beforehand. This paper describes a measurement method and evaluates the uncertainty in determining the transmissivity of mesh screens used to attenuate solar radiation on PV modules. The method involves modeling the short-circuit current of the PV module as a function of irradiance and temperature indoors and measuring the transmissivity of mesh screens outdoors. Two mesh screens with nominal transmissivities of 50 % and 70% were assembled and used in single-layer and double-layer configurations, resulting in four different mesh filters. The transmissivity of each configuration was measured under four different conditions obtained by combining two angles of incidence for solar radiation – normal and 30° - and using two PV modules, one with monocrystalline and the other with polycrystalline silicon cells. The proposed measurement method succeeded in measuring the transmissivity and determining the optimal sample size for all mesh screen configurations, taking 5 to 7 measurements to obtain an uncertainty of less than 1%. The work highlights the importance of measuring the transmissivity of mesh screens and provides valuable results for future research using this method to study the performance of PV modules at low irradiance levels. [ABSTRACT FROM AUTHOR]

The performance of a photovoltaic (PV) system depends on several factors, such as the solar radiation availability and its spectral distribution, the PV module temperature, soiling, cable losses, PV power degradation over time and so forth. An important factor that also affects the PV array power is the mismatch loss due to the differences between single modules, since is inherent to the manufacturing process certain variability in the I – V curve parameters. The manufacturing technology of PV modules has improved considerably, resulting in higher efficiencies and better quality control process, which enabled a lower maximum power tolerance range of PV modules available in the market. The actual shape of the statistical distribution of the main electrical parameters is necessary to evaluate the mismatch losses using simulation software, and also to verify if a new selection of PV modules besides the one performed by the manufacturer is relevant. In order to analyze these topics, a statistical study was carried out based on data obtained from I – V curve measurements of 105 multicrystalline PV modules with the same nominal characteristics. The measurements were performed in a pulsed solar simulator in standard test conditions. The descriptive statistics were obtained for each main electrical parameter and the best probability density function that describes the parameters dispersion was determined. The results show that the maximum power, the maximum power voltage and the open circuit voltage are preferably represented by a Burr probability density function, however a normal distribution is adequate as well. The short circuit current, the maximum power current and the fill factor are actually described by a two parameter Weibull distribution. In order to analyze the effects of the mismatch losses in arrays, several I – V curves of strings with 10 PV modules randomly selected from the sample were synthesized and compared to strings of modules sorted by the maximum power current value. The advantage of performing a new selection of PV modules with better current match was not relevant in comparison to random strings. The selection performed at the factory for a PV module with the same nominal power is sufficient to prevent considerably mismatch losses considering that the PV modules were sorted using standard procedures. [ABSTRACT FROM AUTHOR]