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• An intuitive methodology for assessment of shading losses in PV systems is proposed. • The methodology proposes tools for shadow prediction and power output estimation. • A study of different shadow pattern impact was performed. • The annual shading losses of a PV plant were evaluated. Frequently, PV systems are affected by dirt, dust or shadowing from surrounding elements, such as trees and buildings. The shadows can cause partial shading conditions, when the irradiance is not uniform on PV installation, which lead to losses of power. Thus, the shadow prediction is primordial for a better output power estimation and to minimize its losses. The aim of this article is to propose a methodology using intuitive and available tools for shading prediction and losses assessment on PV installations. A study about the shadow pattern and module orientation (portrait and landscape) influence and an analysis of the shading losses on a PV plant were performed in order to demonstrate the applicability of the methodology. The shading effect is drastic when the shading is parallel to the short edge, whereas the effect is lower when it is parallel to the long edge. On the other hand, the losses due to shading and the difference in the position of the modules (landscape or portrait) have a lower impact on the photovoltaic plant case. It was concluded that the methodology is feasible and applicable for shading impact evaluation, despite limitations for large systems due to the simulation time. [ABSTRACT FROM AUTHOR]

Several methods for analyzing photovoltaic (PV) systems under partial shading conditions (PSC) can be found in the literature. However, the simplest methods are not very accurate. This article presents (1) a simple and accurate method to model I–V curves for PSC and (2) an improvement of a simplified method, without calculating the I–V curves, to accurately estimate the energy generated by PV systems under PSC. The I–V curves of two PV modules under PSC were measured. The measured curves were compared with modeled curves using two methods from the literature and a new proposed method. Four simplified methods from the literature for estimating the energy generated by PV systems were analyzed. The accuracy of these methods was investigated by comparing the calculated values with measured data from a PV system. For modeling the I–V curves, the new method showed a mean absolute percentage error of 1.5% and proved to be accurate. For the evaluated PV system, using a monthly database and the proposed diffuse shading factor resulted in differences between the measured and modeled results of up to 10% per month and 5% for the entire measurement period. The two proposed methods are simple and accurate. [ABSTRACT FROM AUTHOR]

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]

Numerical and experimental analyses of velocity and temperature fields inside a tank submitted to internal natural and mixed convection are presented in this paper. The numerical analyses were performed with two approaches: one using a two-dimensional model in cylindrical coordinates through the finite volume method and another using a one-dimensional model. A turbulence model for low Reynolds numbers was added to the two-dimensional model in mixed convection regime. The two-dimensional model was experimentally validated and then adopted as reference. Its results were compared to those obtained with one-dimensional models (combined with certain computational artifices described in this work) with a good agreement. [Copyright &y& Elsevier]

A numerical and an experimental analysis of velocity and temperature fields inside a storage tank submitted to natural convection is presented. The analysis was performed in two stages. In the first stage, the temperature profile along the vertical axis of the storage tank was obtained experimentally and numerically, for cooling time ranging from 45 to 60 h. The numerical analysis was carried out using a transient bi-dimensional model in cylindrical co-ordinates. In the second stage, after the numerical code validation, 40 cases of cooling with four aspect ratios, five insulation thicknesses, and two different volumes were simulated. In all simulations, thermal losses for the environment in all tank walls (side, top and bottom) were considered. Two correlations for the Nusselt number, encompassing all the forty cases, were obtained with these results. [Copyright &y& Elsevier]

• Uniform EVF increase the differences between simulated and measured values. • The more detailed 3D model, better the energy forecasting. • Accuracy of the simulated results depends on the shading calculation model. • Shaded PV systems need to be simulated carefully. Software for simulation of photovoltaic (PV) systems is widely used for dimensioning and forecasting electrical production. A factor of losses in PV installations is the partial shading caused by surrounding elements, and these software allow the user to estimate this effect. However, the accuracy of these simulated results for shaded PV systems is not widely studied. The purpose of this article is to investigate the accuracy and quantify the differences between simulated and measured data of partially shaded PV systems, obtained with the widely used tools SAM and PVSyst. Measured data from a PV installation were compared to results from simulations performed using the different shading calculation options available in both tools. The simulated outputs were both underestimated and overestimated in the shading situations. This variation was related to the use of an hourly fraction of shading and, in the case of SAM, due to the limitations of the 3D tools available for representation. Another source of differences between simulated and measured values was the use of uniform shading factors for diffuse and albedo. In addition, the simplification of the 3D model had a significant impact on the predicted energy, mainly on cloudy days. Both software overestimated the electricity production for the entire measurement period, reaching differences between the predicted and the measured energy varying from 9% to 24%. Shaded PV systems must be carefully analyzed, and the simulated results may differ from the measured values, which may even influence the decision on the feasibility of an installation. [ABSTRACT FROM AUTHOR]

• Experimental shunt resistance increases for low irradiance levels on crystalline PV modules. • Experimental series resistance increases for low irradiance levels on crystalline PV modules. • Diode ideality factor does not present temperature or irradiance dependences. • Parasitic resistances do not present temperature dependence on the single-diode model. Five-parameter single diode models are the most used for photovoltaic modeling, and although they usually apply the same basic equation, there are several variations in terms of parameter dependences on irradiance and temperature. Different methods with variations on the resistances' values with the temperature or irradiance have been proposed, and there is also discussion on the treatment of the dark reverse saturation current with the temperature level. On experimental works, there is even more controversy. Several authors reach distinct conclusions about the behavior of almost every parameter of the single diode model when extracting them at each irradiance or temperature levels. For instance, in accordance to different publications, series resistance may either increase, decrease or remain constant under irradiance variations. In this paper, we review previous studies and their proposals, discussing also the possible influence of the extraction method on these results. Later, two experiments are performed in a solar simulator to extract all the parameters on a broad range of irradiance and temperature levels with an analytical extraction method for crystalline silicon modules. The results show that the thermal coefficient used to correct the open-circuit voltage for different temperatures may depend on the irradiance level, a finding that is not present in the single diode models found in the literature. The most common approaches for the diode ideality factor and the diode reverse current, constant ideality factor and a temperature variation for reverse saturation current are seen to fit with the experimental data, while the parasitic resistances both presented a significant increase for low irradiance levels and no significant dependence on the temperature. These experimental findings, reached using a set of 27 commercial PV modules, may help improve the development of methods for solving the single-diode model and improving its accuracy at different operating conditions. [ABSTRACT FROM AUTHOR]

• Detailed description of the large area simulator of class CCB is presented. • Integrated I-V tracer allows to monitor PV parameters over time during light-soaking. • 25-hour light-soaking test show stabilization of a CIGS PV module at first 5 h. • The power change of the CIGS PV module occurs mainly in the first 12 min. • The developed device can be used for the study of metastable effects in PV modules. This paper presents the development, characterization, and testing of a continuous large-area solar simulator for PV modules with an automatic I-V curve acquisition system and the results of a CIGS PV module over 25 h of light soaking. The simulator was developed to perform the IEC 61215 stabilization test and automatic characterization of PV modules during indoor testing. The equipment records the I-V curves of the PV module at programmed time intervals so that the change in the electrical characteristics of the PV modules can be tracked over time. The test plane has an area of 1.2 × 2 m and an irradiance of 800 W/m2. The simulator was installed in a container and consists of 16 metal halide lamps of 1 kW each. The simulator is characterized by temporal instability, spatial non-uniformity, and spectral mismatch, resulting in CCB classification. The I-V curves of the tested PV module were measured every 30 s, with 200 I-V points per curve. The power change of the tested module occurs mainly in the first 12 min and stabilizes after about 5 h under the conditions selected for the test. The developed device can save significant resources compared to commercial continuous solar simulators. The ability to automatically track changes in PV module electrical parameters during light-soaking tests is presented in detail and provides important information for future work related to the development and characterization of solar simulators of this type. [ABSTRACT FROM AUTHOR]

• 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]

• Temperature coefficients (TC) of PV modules were evaluated in different irradiances. • TC of V oc varies logarithmically with irradiance for c-Si PV modules. • A novel empirical function is derived to calculate the TC of V oc in any irradiance. • An improved description of PV module behavior under different irradiance is shown. In PV system performance models, the change in temperature coefficients (TC) as a function of solar irradiance (G) is usually not calculated. Although the variation of the TC of open circuit voltage (V oc) with G is predicted by solar cell theory, most performance models do not account for this behavior. Equations describing the variation of TCs with G are not readily available. The main objective of this paper is to experimentally evaluate the variation of TCs with G of crystalline silicon PV modules. Several I-V curves were measured, with G ranging from 100 to 1000 W/m2 and module temperature ranging from 25 to 65 °C. A matrix of 50 I-V curves was obtained for each PV module and TCs were calculated for different G values. The TC of the short-circuit current (I sc) is practically constant with G , except for two studied half-cell PV modules. The absolute value of TC of maximum power (P m) increases for lower irradiance values for most modules tested, but this trend is not seen for two recently manufactured PV modules where a negligible variation with G was obtained. The main result is a new logarithmic function that fits the experimental data and shows a well-defined increase in the modulus of the TC of V oc for low irradiance values. The equation proposed to describe this behavior is a novelty to improve PV device modeling. The results presented here contribute to a better understanding of TCs under varying irradiance, an important aspect of PV module performance. [ABSTRACT FROM AUTHOR]

Abstract: An efficient and reliable characterization of PV devices is paramount for the establishment of new PV installations and for evaluation of the performance of the existent ones. In order to analyze the electrical performance of photovoltaic devices it is essential the determination of the characteristic current versus voltage (I–V) curve from which important information about several parameters of the PV generator can be obtained. The objective of this work is to present an accurate and reliable post-processing method for measured I–V curves (raw data) of photovoltaic devices. In order to implement this post-processing method in-house software (which is described in this work) was developed to post-process I–V curves of any PV device. This software allows the determination of several parameters from a measured I–V curve, as well as its translation to any irradiance and temperature conditions. The post-processing method has been tested in a wide range of raw data I–V curves related to modules based on different technologies. The results have shown a very good level of reliability and accuracy for traditional crystalline silicon and thin film devices. [Copyright &y& Elsevier]

Abstract: The combination of hydroelectric and photovoltaic sources of energy in a generation system may seem unfeasible due to the still quite high costs of the photovoltaic plants. However in the next few decades, significant reductions in those costs are to be expected. Moreover, this combination may reduce the cost of plants implemented in low hydroelectric potential sites for which main system interconnection costs prove prohibitive. The study of hybrid hydro-PV generating plants allows the evaluation of the use of water reservoirs and battery banks as alternatives for energy storage within a system. It also allows the evaluation of the advantages of generation from complementary energy sources. The objectives of this paper is to propose the concept of a theoretical performance limit for the plants and propose a method for the determination of this performance limit using computer simulations with idealized energy availability functions. Using this method, the effect of different degrees of complementarity in time of the energy resources on the performance of hydro PV plants is studied. It has been observed that, as expected, the smallest failure indexes measuring the energy supply to the consumers are associated to the best complementarity in time indexes. The results of the simulations led to the synthesis of failure index curves, as functions of the different degrees of complementarity in time, with data obtained solely with the utilization of idealized functions describing the energy availabilities, as proposed in this work. The results so obtained are a preliminary set treating the influence of the complementarity characteristics on the performance of hybrid plants based on hydro and solar energies and show how the complementarity characteristics may be used to design hybrid power generation systems showing improved efficiency. The subsequent research work shall proceed to at least two different phases. In the first one, the method exposed in the present work shall be applied to real data and compared to the operation of existing hydro PV plants. In the second phase the obtained relations of the performance of the system to the different degrees of complementarity shall be confronted with design parameters of hydroelectric-photovoltaic plants based on complementary resources. [Copyright &y& Elsevier]

Abstract: A numerical analysis of the three-dimensional temperature and velocity fields in horizontal cylindrical storage tanks was performed. The phenomena of laminar natural convection and vertical stratification of temperature were considered. The developed three-dimensional transient computing code solves the equations of energy and momentum through the finite volume method. The simulation of fluid cooling process inside the tank showed the formation of stratified temperature profiles that matched those obtained experimentally. Based on several simulations, a correlation was proposed for determining the degree of thermal stratification inside the tank regarding thermal and geometrical parameters. From this correlation, an expression was proposed to predict the fluid temperature profiles along the time. This information is very important in many applications, such as in thermosiphon solar water heating systems, where the global efficiency of the system increases with the thermal stratification degree of the working fluid. Another case studied considered that the tank was connected to solar collectors, aiming at investigating the influence of the inlet jet position with and without a baffle plate on the preservation of the thermal stratification. Results showed that the baffle plate modified the velocity and temperature fields close to the inlet jet, allowing a better thermal stratification. Also the suitable choice of the inlet jet position allowed the formation of a more effective thermal stratification. Some other aspects of the internal dynamics of this kind of storage tank are presented and discussed. For the cases studied, the inlet jet next to the top led to a greater thermal stratification. However, it was verified that when the inlet jet temperature remains constant for a long period of time, and thus its temperature approaches the temperature of the water inside the tank, for the same height, the temperature profiles obtained become similar to the case of the inlet located at usual height of 2/3 of the diameter. [Copyright &y& Elsevier]

Abstract: The power plants based on renewable sources face various acceptance difficulties mainly due to high initial costs and low overall efficiencies. A way to improve performance of these systems is to utilize more than one type of source of energy chosen to provide some degree of complementarity. An interesting, albeit improbable, combination is obtained using hydroelectric and photovoltaic energy, taking advantage of the complementarity between these two types of sources coupled to reservoir and/or battery storage. This paper discusses energy complementarity in time and proposes a numerical dimensionless index, evaluating this energy complementarity between two types of energy sources, in the same or in different locations, or between two sources of the same type in different locations. In the end, the results of the application of this index to the solar and water availabilities over the state of Rio Grande do Sul, in southern Brazil, is presented in the form of maps. [Copyright &y& Elsevier]

• Outdoor procedure to determine the reference cell for different PV technologies. • Shows how to identify the smallest junction current, in multi-junction devices. • Presents and validates a new methodology to evaluate the outdoor spectral mismatch. • The average of spectral gains or losses under different cloud cover. This article describes outdoor temperature-controlled measurement of various photovoltaic (PV) modules fabricated using different technologies and using reference cells with spectral filters. Short-circuit current measurements of all PV modules and reference cells were recorded simultaneously using 16-bits Keysight digitizers and high-resolution Agilent multimeters. The spectral incompatibility factor was calculated considering the spectral response of each device and the spectrum measured outdoors. Corresponding spectra were also generated by the simple model of atmospheric radiative transfer of the sun SMARTS2 and were additionally used to compare the spectral incompatibility factors. The validation of the measurement method was demonstrated by comparing the incompatibility factor calculated with variation of the short-circuit current relationship between the device under test and the reference cell over time. Additionally, it was evaluated how the relationship between spectral variation of irradiance and the spectral response of different technologies affects the productivity in thin-film modules. Main observation from measurements (performed in Labsol) was that a-Si/μ-Si, CdTe, and triple a-Si junction converted 0.21%, 2.47%, and 3.65% more irradiation into electric current (in summer), respectively, compared to c-Si technology. The PV CIGS module converted on average 0.26% more irradiation into electric current in winter compared to c-Si technology. The results help to understand the spectral effects on the working of outdoor PV module and the relationships between measured and modeled spectra, as well as errors introduced by the spectral incompatibility factor. [ABSTRACT FROM AUTHOR]

• The degradation rate of a Photovoltaic system at Porto Alegre was 0.7% per year. • Electroluminescence and infrared imagery analysis indicate signs of corrosion. • The photovoltaic modules were individually characterized before their installation. Photovoltaic modules long-term reliability is a key requirement to keep this technology a commercially interesting option for energy production. The investment return on PV generation system depends, among other factors, on the peculiarities of the system on efficiency reduction caused by environmental action. Technical standards suggest laboratory tests for the qualification and a better understanding on module degradation behavior. Nevertheless, some degradation modes can only be observed after years of field operation. This paper presents the main signs of degradation on crystalline silicon photovoltaic modules caused by outdoor exposure after a period of 15 years in an installation in the city of Porto Alegre, the southernmost state capital city in Brazil. The electric analysis demonstrated that the modules kept 90.5% of their initial power, with an average of degradation rate of 0.7% per year. Some techniques helped the evaluation of defects caused by exposure of the arrays for long periods of operation, such as electroluminescence imaging and I-V curve testing. The most frequent defects found were browning (encapsulant darkening) and cell encapsulant delamination, leading to the anti-reflective layer deterioration. The junction box of one of the modules was detached, allowing the entrance of moisture and resulting in the disconnection of some of the module cells. The electroluminescence technique also provided a better understanding on modes of degradation. Despite the observed evidences of degradation along these 15 years, it can be said that the system remains within the expected operation limits. [ABSTRACT FROM AUTHOR]

Modeling and simulation of photovoltaic systems, further than aiding on the project design phase, can be used to emulate the system performance in real time, therefore helping to identify any malfunction that may occur. Among the available performance models for photovoltaic systems, the single diode model is preferred by many authors, since it combines relative simplicity and accuracy. Previous works reported that this model has some limitations on describing the photovoltaic system I – V curves under low irradiances, indicating that the variation of the shunt resistance parameter with the irradiance level can be adopted to minimize this drawback. This paper aims to study the shunt resistance dependence on the irradiance level in order to evaluate some of the usual expressions proposed on the literature. A large area pulsed solar simulator model PASAN SunSim 3C was used to acquire the I – V characteristics of several photovoltaic modules of different brands and technologies under 20 distinct irradiance levels ranging from 75 W/m 2 to 1000 W/m 2 . The shunt resistance parameter was calculated as the inverse slope of the I – V curve in the short circuit region, and fitting equations were derived for each photovoltaic technology. The results in general agreed with previous published works, showing the tendency of an increase of the shunt resistance on lower irradiance levels. Some empirical models tested did not present satisfactory accuracy to reproduce the experimental data. Although simpler, an inverse dependence of the shunt resistance on the irradiance using the measured value at STC as a reference was seen to describe adequately the experimental data. A preliminary study showed that the inclusion of this dependence on the single-diode model indeed increases the model accuracy, reducing the average error on the performed tests by more than half comparing to the original model. [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]

The insertion of photovoltaic solar energy has increased considerably over the past few years, with remarkable growth since 2005. It is essential that the electrical energy delivered by the photovoltaic system to the grid has an acceptable quality level. This paper presents test results of power factor and harmonic distortion content in grid connected inverters for photovoltaic systems. Several tests of ten commercially available inverter models from three different manufacturers were performed at the Laboratory of Solar Energy (Labsol) at the Federal University of Rio Grande do Sul (UFRGS), using a test bench with a power analyzer. The power factor and total harmonic distortion in current curves were mathematically modeled as a function of relative power in order to be used in computer simulation of photovoltaic systems. For power loads close to the nominal values, the harmonic content was found to be low and in accordance with the standard requirements. Nevertheless, at low loading levels the inverters may inject high levels of harmonics into the grid. [ABSTRACT FROM AUTHOR]