The Experts below are selected from a list of 267 Experts worldwide ranked by ideXlab platform
Christian A. Gueymard - One of the best experts on this subject based on the ideXlab platform.
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Parameterized transmittance model for direct Beam and circumsolar spectral Irradiance
Solar Energy, 2001Co-Authors: Christian A. GueymardAbstract:Abstract An upgraded spectral radiation model called SMARTS2 (Simple Model of the Atmospheric Radiative Transfer of Sunshine) is introduced. The solar shortwave direct Beam Irradiance is calculated from spectral transmittance functions for the main extinction processes in the cloudless atmosphere: Rayleigh scattering, aerosol extinction, and absorption by ozone, uniformly mixed gases, water vapor, and nitrogen dioxide. Temperature-dependent or pressure-dependent extinction coefficients have been developed for all these absorbing gases, based on recent spectroscopic data obtained either directly from the experimental literature or, in a preprocessed form, from MODTRAN, a state-of-the-art rigorous code. The NO2 extinction effect, in both the UV and visible, is introduced in detail for the first time in a simple spectral model by taking into account temperature-dependent absorption coefficients. Aerosol extinction is evaluated using a two-tier Angstrom approach. Parameterizations of the wavelength exponents and single-scattering coefficient for different aerosol models (proposed by Shettle and Fenn, Braslau and Dave, and also in the Standard Radiation Atmosphere) are provided as a function of both wavelength and relative humidity. Moreover, aerosol turbidity can now be estimated from airport visibility data using a function based on the Shettle and Fenn aerosol model. SMARTS2 also has an optional circumsolar correction function and two filter smoothing functions which together allow the simulation of actual spectroradiometers. This facilitates comparison between modeled results and measured data. Preliminary performance assessment indicates that the direct-Beam Irradiance predicted by the proposed model compares well to published reference spectra obtained with rigorous radiative codes, and to measured spectroradiometric data.
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Analysis of monthly average solar radiation and bright sunshine for different thresholds at Cape Canaveral, Florida
Solar Energy, 1993Co-Authors: Christian A. GueymardAbstract:Abstract Data for Beam, global horizontal, and global normal radiation measured at Cape Canveral were analyzed for the period 1986–1991. Some empirical correlations between these parameters are proposed. A detailed analysis of the Beam Irradiance data also led to the derivation of sunshine fractions corresponding to different radiation thresholds. It is shown that the current WMO threshold of 120 W/m 2 corresponds to optimal radiation/sunshine correlations. Finally, improved monthly irradiation empirical fits using sunshine data are proposed.
Pierre Ineichen - One of the best experts on this subject based on the ideXlab platform.
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Clear sky and all-weather global and Beam Irradiance models: long term validation
2016Co-Authors: Pierre IneichenAbstract:The meteorological satellite images as data source to evaluate the ground Irradiance components become the state of the art in the field of solar energy systems. The strongest argument is the high spatial coverage, and the fifteen minutes temporal granularity when using images from MSG. They also have the advantage to provide «real time» data used for example to assess the proper operation of a solar plant. On the other hand, long-term ground data are very scarce concerning the Beam Irradiance. The use of secondary inputs such as polar satellite data and ground information increases significantly the precision of the algorithms, mainly for the Beam component. Following a paper from Zelenka concerning the nuggets effect, the interpolation distance to the nearest ground measurement site is limited to 10 to 30 km, depending on the Irradiance parameter. This strengths the satellite derived data argument. The use of data derived from models or interpolated between nearby measurements sites are strongly related to the quality of the ground measurements used in the deriving process. This means that the preparatory steps are essential to ensure the quality of the data to be used as input to system simulations. Our approach is to first apply a stringent quality control, including time stamp of the data, absolute and relative calibration coefficient of the sensors, long term stability, components coherence etc., on the both the ground and modeled data.
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Long term CM-SAF satellite global and Beam Irradiance validation
2014Co-Authors: Pierre IneichenAbstract:The meteorological satellite images as data source to evaluate the ground Irradiance components become the state of the art in the field of solar energy systems. The strongest argument is the high spatial coverage, and the fifteen minutes temporal granularity when using images from MSG. They also have the advantage to provide «real time» data used for example to assess the proper operation of a solar plant. On the other hand, long term ground data are very scarce concerning the Beam Irradiance. The use of secondary inputs such as polar satellite data and ground information increases significantly the precision of the algorithms, mainly for the Beam component. Following a paper from Zelenka concerning the nuggets effect, the interpolation distance to the nearest ground measurement site is limited to 10 to 30 km, depending on the Irradiance parameter. This strengths the satellite derived data argument.
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Long Term Satellite Global, Beam and Diffuse Irradiance Validation
Energy Procedia, 2014Co-Authors: Pierre IneichenAbstract:In the field of solar energy applications, the use of geostationary satellite images becomes crucial, since they allow the retrieval of Irradiance at the surface, with the best possible spatial and temporal coverage. This study, conducted on data from 18 European and Mediterranean sites, over 8 years of data shows that it is now possible to retrieve hourly global and Beam Irradiance data with a low uncertainty, typically 17% for the global, and 34% for the Beam component, with a negligible bias.
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Five satellite products deriving Beam and global Irradiance validation on data from 23 ground stations
2011Co-Authors: Pierre IneichenAbstract:Models converting satellite images into the different radiation components become increasingly performing and give often better estimation of the solar Irradiance availability than ground measurements if the station is not situated in the near vicinity of the application. Five different satellite products deriving both global and Beam Irradiance are validated against data from 23 ground sites. The main conclusions are: * the global Irradiance is retrieved with a negligible bias and an average standard deviation around 16% for the best algorithm. For the Beam Irradiance, the bias is around several percents, and the standard deviation around 35%; * the main deviation comes from the knowledge of the aerosol optical depth; * the high latitude sites give not poorer results than the other sites, The interannual variability of the Irradiance conditions, the lack of independent ground measurements such as aerosol data, the difficulty to assess the exact calibration of the ground data, and the choice of a specific year to carry out the validation, conduct to results that give good indications, but from which it is difficult to draw general conclusions.
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Aerosol quantification based on global Irradiance
2010Co-Authors: Pierre Ineichen, Richard PerezAbstract:The atmospheric aerosol content has the highest influence on the solar Irradiance attenuation by the atmosphere. In the field of solar energy modelization, this parameter is mainly extracted from climatological data banks or interpolated maps with a low grid resolution. It can be evaluated from Beam Irradiance measurements, but these data are rarely acquired by national meteorological institute in a routine basis. This paper presents a method to retrieve the atmospheric turbidity from hourly global Irradiance measurements with the help of a clear sky model by back-calculation. Even if the accuracy of the result is varying from one station to the other and is highly dependent of the data quality, it is of great help to solar Beam (or direct) Irradiance evaluation from geostationary satellites.
Josephin Enz - One of the best experts on this subject based on the ideXlab platform.
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Microstructure by design: An approach of grain refinement and isotropy improvement in multi-layer wire-based laser metal deposition
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2020Co-Authors: Martin Froend, Volker Ventzke, F. Dorn, Nikolai Kashaev, Benjamin Klusemann, Josephin EnzAbstract:Abstract The additive production of metallic components with high-throughput is usually associated with high process temperatures and slow cooling rates. This typically results in strongly oriented columnar grain growth along the building direction of the structure having exceedingly large grain sizes. As a result, such structures show typically low strength and anisotropic mechanical behaviour in as-deposited condition. Consequently, post-processing is commonly performed to homogenize and eventually increase the mechanical properties of the deposited structures. In this regard, precise control of the applied process energy allows a modification of the local temperature distribution and cooling conditions during the additive manufacturing process, which strongly influence the resulting solidification microstructure. The aim of the present study is the development of an approach that allows to influence the solidification conditions in wire-based laser metal deposition of an Al-Mg alloy through specific adjustments of the laser irradiation. It was found that significantly different solidification microstructures in as-deposited condition can be achieved by adjusting the laser Beam Irradiance within a range resulting in conduction mode welding conditions while keeping the heat input constant. The application of high laser Beam Irradiances, close to the transition to keyhole mode welding, results in structures with a homogeneous large-grained solidification microstructure exhibiting a degree of anisotropy of around 12 % between building direction and the direction of deposition. In contrast, the use of low laser Beam Irradiance close to the lower limit of stable melting, results in structures with a significantly refined microstructure. Consequently, an increase of yield strength of up to around 20 % and microhardness of up to 13 % , as compared to structures processed with high laser Beam Irradiance, could be obtained. Moreover, the anisotropy of the as-deposited structure was reduced to a degree lower than 2 % .
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Thermal analysis of wire-based direct energy deposition of Al-Mg using different laser Irradiances
Additive Manufacturing, 2019Co-Authors: Martin Froend, Volker Ventzke, Nikolai Kashaev, Benjamin Klusemann, Josephin EnzAbstract:Abstract The wire-based direct energy deposition of metallic lightweight materials such as titanium or aluminium alloys has recently received increasing attention in industry and academia. However, high-throughput deposition is mostly associated with process-limiting phenomena such as the development of high temperatures resulting in poor surface quality as well as coarse and unidirectional solidification microstructures. In this regard, laser systems, which are already widely used in industrial processes, allow for a great variety in the controllability of energy inputs, thereby enabling the control of process temperatures and resulting microstructures. The subject of the current study is the detailed elucidation and evaluation of important features such as the development of temperature gradients, resulting cooling rates and thermal cycles for different laser Beam Irradiances. Significant heat accumulation and process instabilities as well as inhomogeneous thermal profiles along the length and height of the parts were observed at a high laser Beam Irradiance. In contrast, lower laser Beam Irradiance resulted in a more stable process with increased cooling rates, which favourably influenced the refinement of the solidification microstructure.
Martin Froend - One of the best experts on this subject based on the ideXlab platform.
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Microstructure by design: An approach of grain refinement and isotropy improvement in multi-layer wire-based laser metal deposition
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2020Co-Authors: Martin Froend, Volker Ventzke, F. Dorn, Nikolai Kashaev, Benjamin Klusemann, Josephin EnzAbstract:Abstract The additive production of metallic components with high-throughput is usually associated with high process temperatures and slow cooling rates. This typically results in strongly oriented columnar grain growth along the building direction of the structure having exceedingly large grain sizes. As a result, such structures show typically low strength and anisotropic mechanical behaviour in as-deposited condition. Consequently, post-processing is commonly performed to homogenize and eventually increase the mechanical properties of the deposited structures. In this regard, precise control of the applied process energy allows a modification of the local temperature distribution and cooling conditions during the additive manufacturing process, which strongly influence the resulting solidification microstructure. The aim of the present study is the development of an approach that allows to influence the solidification conditions in wire-based laser metal deposition of an Al-Mg alloy through specific adjustments of the laser irradiation. It was found that significantly different solidification microstructures in as-deposited condition can be achieved by adjusting the laser Beam Irradiance within a range resulting in conduction mode welding conditions while keeping the heat input constant. The application of high laser Beam Irradiances, close to the transition to keyhole mode welding, results in structures with a homogeneous large-grained solidification microstructure exhibiting a degree of anisotropy of around 12 % between building direction and the direction of deposition. In contrast, the use of low laser Beam Irradiance close to the lower limit of stable melting, results in structures with a significantly refined microstructure. Consequently, an increase of yield strength of up to around 20 % and microhardness of up to 13 % , as compared to structures processed with high laser Beam Irradiance, could be obtained. Moreover, the anisotropy of the as-deposited structure was reduced to a degree lower than 2 % .
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Thermal analysis of wire-based direct energy deposition of Al-Mg using different laser Irradiances
Additive Manufacturing, 2019Co-Authors: Martin Froend, Volker Ventzke, Nikolai Kashaev, Benjamin Klusemann, Josephin EnzAbstract:Abstract The wire-based direct energy deposition of metallic lightweight materials such as titanium or aluminium alloys has recently received increasing attention in industry and academia. However, high-throughput deposition is mostly associated with process-limiting phenomena such as the development of high temperatures resulting in poor surface quality as well as coarse and unidirectional solidification microstructures. In this regard, laser systems, which are already widely used in industrial processes, allow for a great variety in the controllability of energy inputs, thereby enabling the control of process temperatures and resulting microstructures. The subject of the current study is the detailed elucidation and evaluation of important features such as the development of temperature gradients, resulting cooling rates and thermal cycles for different laser Beam Irradiances. Significant heat accumulation and process instabilities as well as inhomogeneous thermal profiles along the length and height of the parts were observed at a high laser Beam Irradiance. In contrast, lower laser Beam Irradiance resulted in a more stable process with increased cooling rates, which favourably influenced the refinement of the solidification microstructure.
H. D. Kambezidis - One of the best experts on this subject based on the ideXlab platform.
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Investigation about the dependence of spectral diffuse-to-direct-Beam Irradiance ratio on atmospheric turbidity and solar zenith angle
Theoretical and Applied Climatology, 2007Co-Authors: D. G. Kaskaoutis, H. D. Kambezidis, Z. TóthAbstract:The modifications of the solar spectral diffuse and direct-Beam Irradiances as well as the diffuse-to-direct-Beam ratio, E_dλ/E_bλ, as a function of the aerosol optical depth, AOD, and solar zenith angle, SZA, is investigated. The E_dλ/E_bλ ratios decrease rapidly with wavelength and exponential curves in the form E_dλ/E_bλ = aλ^−b can be fitted with a great accuracy. These curves are strongly modified by the solar spectrum distribution, which is affected by the aerosol loading, aerosol optical properties and SZA. The spectral dependence of the above E_dλ/E_bλ ratios in logarithmic coordinates does not yield a straight line, while a significant departure from the linearity is revealed. The reasons for this departure are investigated in detail and it is established that the aerosol physical properties such as single scattering albedo and size distribution along with the effect of SZA are responsible. These parameters strongly affect the scattering processes in the atmosphere and as a consequence the diffuse spectral distribution. The E_dλ/E_bλ ratio, which is an indicator of the atmospheric transmittance (King, 1979), exhibits a strong wavelength and aerosol-loading dependence. The observed differences between turbid and clear atmospheres constitute a manifestation of contrasting air properties and influence solar Irradiance spectra. The present work aims at investigating the effect of atmospheric turbidity and SZA on the E_dλ/E_bλ ratio. For this reason, two distinct cases are examined: one having different atmospheric turbidity conditions but same SZA and a second having different SZAs and same atmospheric turbidity levels.
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Checking the validity of the Ångström's formula with spectral data of direct-Beam Irradiance obtained in Athens, Greece
Atmospheric Research, 2006Co-Authors: Dimitris G. Kaskaoutis, H. D. KambezidisAbstract:Abstract This work analyzes the ability of the Angstrom's formula to correctly estimate the spectral aerosol optical depth. For this purpose, direct-Beam spectra were obtained by a LICOR spectroradiometer in the 300–1100 nm wavelength range at several sites in the Athens basin under cloudless skies and different atmospheric conditions during May 1995. The total optical depth was retrieved by means of the SMARTS radiative transfer code. The objective of this study is to compare the aerosol optical depths (AOD) derived by the Angstrom's formula with the spectral aerosol optical depths (SAOD) retrieved through the SMARTS model and, therefore, to check the validity of the former. From this comparison, it is found that the correlation coefficients between SAODs and AODs are higher than 0.9 in most cases. The greatest SAOD − AOD differences are found to take place at the shorter wavelengths. A further analysis of the above differences at 500 and 1000 nm with SMARTS shows very low values, especially at 500 nm, indicating the accuracy of the Angstrom's formula fitting. The differences at 1000 nm are much higher due to the water vapor absorption parameter that causes uncertainties in the water vapor optical depth determination. On the other hand, the sole contributors to the total optical depth at 500 nm are aerosols and ozone; as the ozone optical depth can be estimated with a great accuracy, the Angstrom's formula gives an accurate fit to the SAOD values estimated by the SMARTS model.
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Variability of the Linke and Unsworth-Monteith turbidity parameters in Athens, Greece
Meteorology and Atmospheric Physics, 2000Co-Authors: H. D. Kambezidis, A. K. Fotiadi, B. D. KatsoulisAbstract:This work studies the variability of the Linke ( T _ L ) and Unsworth-Monteith ( T _ U ) turbidity parameters in the urban atmosphere of Athens. Beam Irradiance observations performed at the Actinometric Station of the National Observatory of Athens are used in the period 1975–1995. This study examines (i) the inter-annual variation of T _ L and T _ U , (ii) their mean seasonal variability, (iii) their monthly average variation, and (iv) their mean daily variation. Also, for various air-mass origins, the mean seasonal variations of T _ L and T _ U are given. The frequency of occurrence of the parameters in various ranges is also shown. An innovative interpretation of the impact of the Saharan dust on the radiative properties in the atmosphere of Athens is attempted.