Atmospheric Clearness Index Research Articles

Comparative Analysis of Photosynthetically Active Radiation Models Based on Radiometric Attributes in Mainland Spain

The aims of this work are to present an analysis of quality solar radiation data and develop several hourly models of photosynthetically active radiation (PAR) using combinations of radiometric variables such as global horizontal irradiance (GHI), diffuse horizontal irradiance (DHI), and direct normal irradiance (DNI) from their dimensionless indices atmospheric clearness index (kt), horizontal diffuse fraction (kd), and normal direct fraction (kb) together with solar elevation angle (α). GHI, DHI, and DNI data with 1-minute frequencies in the period from 2016 to 2021 from CEDER-CIEMAT, in a northern plateau, and PSA-CIEMAT in the southeast of the Iberian Peninsula, were used to compare two locations with very different climates according to the Köppen—Geiger classification. A total of 15 multilinear models were fitted and validated (with independent training and validation data) using first the whole dataset and then by kt intervals. In most cases, models including the clearness index showed better performance, and among them, models that also use the solar elevation angle as a variable obtained remarkable results. Additionally, according to the statistical validation, these models presented good results when they were compared with models in the bibliography. Finally, the model validation statistics indicate a better performance of the interval models than the complete models.

Performance and design optimization of single-axis multi-position sun-tracking PV panels

In this article, the photovoltaic (PV) and sun-tracking performance of single-axis multiposition sun-tracking PV panels (MP-PV) is investigated based on solar geometry and dependence of PV conversion efficiency on the incident angle of solar rays on PV panels. For such PV systems, the azimuth angle of the PV panels is adjusted daily M times from an eastward direction in the morning to a westward direction in the afternoon by rotating the PV panels about the inclined north–south axis (INSA) to ensure that the projected incident angle (PIA) of solar rays on the plane perpendicular to the INSA is always less than the angle θa, the specified maximum PIA. Results show that the annual average sun-tracking efficiency (ηst) and the annual electricity generation (AEG) of the MP-PV system increase as M increases, but this increase is less than 1% when M > 5. The optimal θa of the MP-PV system for maximizing AEG and the ratio (Cp,MP-0) of AEG of the optimized MP-PV system to that of a similar fixed PV system are highly dependent on the annual average atmospheric clearness index (Kt) but only slightly dependent on the site latitude (λ). For an MP-PV with a tilt angle (β) of the INSA that is fixed yearly, the AEG of 3P-, 5P-, and 7P-PV systems is about 92%, 94%, and 95%, respectively, of that obtained from a similar dual-axis tracking PV panel (2A-PV). However, for an MP-PV whose β is adjusted four times a year at three tilts, the AEG of 3P-, 5P-, and 7P-PV systems is about 96%, 98%, and 99%, respectively, of that of a similar 2A-PV system. Empirical correlations for quick estimation of the optimal θa and Cp,MP-0 of the MP-PV optimized for maximizing AEG are suggested and examined based on monthly global radiation on the horizon at 30 sites around China.

Ratio of photosynthetically active to total incoming radiation above a Mediterranean deciduous oak forest

Photosynthetically active radiation (PAR) is an important parameter in ecological research. However, it is not routinely measured and often is estimated as a constant ratio of the incoming solar shortwave radiation (Rs). There are only few reported PAR/Rs values worldwide concerning the Mediterranean climate and even fewer from rural or forest areas, especially at higher altitudes. Hourly PAR and Rs flux densities were measured above a deciduous oak forest in Greece from 1999 to 2005, and their relationship was investigated under various conditions. Results show that the annual mean hourly PAR/Rs is 0.454, ranging from 0.443 in spring to 0.478 in autumn, with intermediate values in summer (0.454) and winter (0.459). The ratio increases with dew point temperature but decreases as solar elevation angle or Rs increases. Atmospheric clearness index, Kt, and actual water vapor pressure, ea, are the key factors determining the ratio; however, relative humidity (RH) also seems to have an indirect effect by affecting Kt and ea values. PAR/Rs changes from 0.468 to 0.455 as sky conditions change from clear to overcast and appears to increase with ea. However, Kt affects the ratio when RH is lower than 60%, while ea has a more obvious effect at more saturated atmospheric conditions.

Climatic and energetic analysis of the atmospheric clearness index in Saudi Arabia

This study aims to explore the effect of anthropogenic factors on solar irradiation in Saudi Arabia. The concepts of the clearness index (CI), solar irradiation polygons (SIP) and solar Angstrom polygons (SAP) are for interpretation of solar irradiation. The solar irradiation records are grouped into two time periods. The SIPs show a decline in solar irradiation in the second time period for all regions except the eastern region. The maximum difference between the time periods, of approximately 10.9 (MJ/M2 day), is found in October at Alhefa station in the southwestern region, while at Khurais station in the eastern region the values of solar irradiation are very close together, within 2 (MJ/M2 day). Finally, CI concept is used in order to compare measured against estimated solar irradiation, where a maximum relative error of 11% was found. Finally, solar irradiation generally declined recently, which strongly indicates anthropogenic factors are at play.

Climatic and energetic analysis of the atmospheric clearness index in Saudi Arabia

This study aims to explore the effect of anthropogenic factors on solar irradiation in Saudi Arabia. The concepts of the clearness index (CI), solar irradiation polygons (SIP) and solar Angstrom polygons (SAP) are for interpretation of solar irradiation. The solar irradiation records are grouped into two time periods. The SIPs show a decline in solar irradiation in the second time period for all regions except the eastern region. The maximum difference between the time periods, of approximately 10.9 (MJ/M2 day), is found in October at Alhefa station in the southwestern region, while at Khurais station in the eastern region the values of solar irradiation are very close together, within 2 (MJ/M2 day). Finally, CI concept is used in order to compare measured against estimated solar irradiation, where a maximum relative error of 11% was found. Finally, solar irradiation generally declined recently, which strongly indicates anthropogenic factors are at play.

Analysis and characterization of photovoltaic modules of three different thin-film technologies in outdoor conditions

The instantaneous performance ratio of thin-film photovoltaic modules of three different technologies is analyzed and characterized using contour graphs for different outdoor conditions. This parameter changes when modules are working in outdoor conditions depending on several variables. The most explanatory parameters we have found are the module temperature, the atmospheric clearness index and the solar spectral irradiance distribution; this latter has been included in the characterization using the average photon energy index as it has been identified as a good indicator of the solar spectral irradiance distribution. Moreover, the variance of instantaneous performance ratio for each studied technology has been analyzed. We can conclude that the joint use of all these parameters in contour graphs allows us to better characterize the performance of modules and to reduce the uncertainty observed in previous proposals that only use two of these parameters.

Parameterization of net radiation in an arid city of northwestern Mexico

During the process of urbanization, different surface properties significantly alter the radiation balance. This paper attempts to quantify this balance over different surface types in an arid city of northwest Mexico over several days in August 2011. The albedo of each surface type, as well as local atmospheric properties such as the atmospheric clearness index (K0) and atmospheric emissivity (εatm), were estimated. The surfaces on which measurements were performed were asphalt, concrete, polystyrene painted with white elastomeric paint (PWEP), clay, and grass. It was found that, for a 24-h cycle of measurement, the highest average value of net radiation was for asphalt (146.1 Wm–2), and the lowest average value was for PWEP (33.6 Wm–2). Estimates of albedo values vary depending on the surface, whereas K0 and εatm are dependent on prevailing atmospheric conditions. From these measurements, preliminary statistical models of net radiation as a function of incoming solar radiation and net shortwave radiation were proposed. For each model, the coefficients of determination were higher than 0.97. We discuss the likely implications of the results found for the urban planning of the city.

Artificial neural network based computational model for the prediction of direct solar radiation in Indian zone

In this paper, a computational model for the prediction of direct solar radiation based on neural network analysis of atmospheric clearness is developed. It considers that the major portion of direct solar radiation reaching the earth's surface is governed by Sun-Earth geometry and atmospheric transmittance factors which are exactly calculable by clear day model. Additional variations are due to climate and weather phenomena characterized by relative humidity, mean duration of sunshine per hour, and rainfall, etc., in the atmosphere. These variations are taken into account with the help of a composite parameter referred to as atmospheric clearness index (CI) which is determined using artificial neural network analysis. The contour maps of CI as a function of latitude, time of the day, and month of the year are then prepared using the meteorological data of eleven stations. Model simulation and test results of the trained network for two typical locations (not used in training the network) are presented and compared with measured values. The deviations are well within acceptable error limits [percentage root mean square error values for Ahmedabad and Nagpur are 3.34 and 2.06, respectively]. The methodology, for predicting the direct solar radiation at an arbitrary location using the well known parameters such as altitude, latitude, time of the day, day of the year, and CI values derived from the contour maps, is discussed. The outcome of the present ANN model for four arbitrary locations is compared with NASA SSE data sets.

국내 대기청명도의 경년변화

Since the atmospheric clearness index is main factor for evaluating atmosphere environment, it is necessary to estimate its characteristics all over the major cities in Korea. We have begun collecting clearness index data since 1982 at 16 different cities and considerable effort has been made for constructing a standard value from measured data at each city. The new clearness data will be extensively used by evaluating atmospheric environment as well as by solar application system designer or users. Yearly mean 64% of the atmospheric clearness index was evaluated for clear day all over, and significant difference of horizontal global insolation is observed between 1982~1986 and 1987~1991, 1992~1996, 1997~2001, 2002~2009 through 16 different cities in Korea.

Taxa de absorção atmosférica sobre as cidades de Botucatu-SP e Rio de Janeiro-RJ

O segundo satélite da Missão Espacial Completa Brasileira (SCD2/MECB) foi colocado em órbita em 23 de Outubro de 1998 e carrega a bordo um experimento de células solares. Célula solar de silício é um dispositivo semicondutor, que pode medir a intensidade da radiação visível e parte da radiação infravermelha (400-1100 nm). O experimento permite medir simultaneamente a insolação direta e parte da radiação solar que é refletida pela Terra para o espaço. Os dados do experimento célula solar são transmitidos em tempo real pela telemetria do satélite e recebidos pela estação terrestre em Cuiabá, MT-Brasil (16°S; 56°W). Este fato limita a cobertura espacial para um círculo sobre a América do Sul. O albedo planetário é obtido dentro desta cobertura e seus valores podem ser agrupados em períodos temporais (anual, sazonal ou mensal), ou podem ser estudados para várias localizações (latitude e longitude) durante a vida do satélite. O coeficiente de transmissão atmosférica ou índice de claridade (Kt), medido em estações meteorológicas na superfície da Terra, junto com o valor medido simultaneamente do albedo planetário, permite calcular o coeficiente de absorção atmosférica (Ka). O método desenvolvido neste trabalho para avaliar Ka considera que o albedo planetário é composto por duas partes: uma refletividade local e uma refletividade não local. Considerando este novo conceito, é definida uma taxa de absorção atmosférica (denominada Ra) que é a razão entre Ka e a potência de irradiância solar líquida, que não atravessou a atmosfera (100%-Kt). A taxa de absorção atmosférica assim definida é independente da cobertura de nuvens. O histograma de freqüência de Ra mostra os valores de 0,86±0,07 e 0,88±0,09 sobre as cidades de Botucatu-SP e do Rio de Janeiro-RJ, durante os anos de 1999 até 2006, respectivamente.

Characterisation of the shortwave radiation regime for locations at different altitudes in south-west Germany

The shortwave radiation regime for 3 REKLIP sites at different altitudes in south-west Germany has been characterised. The study period extended from 1992 to 1995. Climatic variables used for the characterisation include surface albedo, cloud cover, relative sunshine duration, fog fre- quency, atmospheric clearness index and precipitation. Given clear sky conditions, the atmospheric clearness index amounted to 76% at the upper mountain site (1489 m above sea level (a.s.l.)) and 68.5% at the lowland site (212 m a.s.l.). Although the vertical profile of incoming solar radiation exhibited a positive gradient on clear sky days (independent of season), the net shortwave radiation showed a dissimilar trend in winter, when albedo values were significantly high at the mountain sites. In general, net shortwave radiation at the lowland site was 1.1 and 1.2 times that of the inter- mediate and upper mountain sites respectively. For each season, theoretically computed diffuse sky radiation increased with cloud cover, N, only to a maximum of about 5 octa, while the diffuse fraction, Kd, of solar radiation rose steadily with N. From April to October, the estimated monthly Kd increased with altitude, being vice-versa for the other months.

Influences of the clearness index for the whole spectrum and of the relative optical air mass on UV solar irradiance for two locations in the Mediterranean area, Valencia and Cordoba

Ultraviolet solar irradiance (290–385 nm) was analyzed in Valencia and Cordoba, Spain, during 1996–1997. Its relation to the instantaneous atmospheric clearness index kt, the relative optical mass mr, the time of the year, and the total or global irradiance on a horizontal surface was established. The maximum values of UV irradiance for each optical mass exponentially diminished with the optical mass, thus for Cordoba and Valencia, Guv = 62.23 mr−1.34 and Guv = 52.84 mr−1.30, respectively. The UV irradiance in Cordoba represented 4.2% of the total irradiance while in Valencia this percentage was 4.9%. In the case of cloudy days (kt<0.4), these percentages ranged between 5.6% and 6.8% in Valencia and between 5.1% and 5.5% in Cordoba.

Anisotropic sky radiance model based on narrow field of view measurements of shortwave radiance

Abstract A model for the average anisotropic sky radiance (or intensity) as a function of the position of the sun, the diffuse fraction k, and the atmospheric clearness index k1 are presented in this article. The complete range of sky conditions from clear to turbid to overcast is covered. Analysis of the observed data indicates that the model can be used to estimate instantaneous sky radiance values with a mean bias error of −11% and a root mean square error of 65% of the mean. The model is shown to account for 83% of the deterministic part of the variance of the instantaneous sky radiance measurements. The formulation for the sky radiance L(θ, F) in the direction (θ, F) is L(θφ)=G d a 0 +a 1 cos θ+a 2 exp (−a 3 ·Ψ) π(a 0 + 2a 1 3 +2a 2 I(θ s ,a 3 ) where Ψ is the scattering angle from the solar beam, a0, a1, a2, a3 are tabulated functions of k and k1, and I(θs, a3) is a specific function of a3 and the solar zenith angle θs.

Measured shortwave sky radiance in an urban atmosphere

An extensive data set of measurements of the shortwave sky radiance at Toronto, Canada, was compiled by the authors in 1984. A pyroelectric radiometer with a half-angle field of view of 5 degrees was used to make a total of more than 100,000 sky radiance measurements. This paper presents an analysis of almost 40,000 of these measurements in the form of a set of plots showing means and percentiles of sky radiance as a function of position in the sky; atmospheric clearness index, k t ; diffuse fraction, k; and solar zenith angle, θ z . The data show (i) that, in the circumsolar region, the highest observed values of sky radiance occur for partly cloudy sky conditions characterized by intermediate values of k and k t , (ii) that, in the regions of the sky away from the solar position, sky radiance tends to increase monotonically both with increasing k and with increasing k t , (iii) that there is a progressive transition with increasing k, from the horizon brightening for skies with low diffuse fractions to horizon darkening for skies with diffuse fractions approaching unity, (iv) that the shape of the sky radiance distribution is a strong function of the elevation of the sun in the sky, and (v) that the sky radiance is highly variable, with a positively skewed probability density function.

Ultraviolet solar radiation at Dhahran, Saudi Arabia

Measurements of ultraviolet solar radiation for the period January 1985 to December 1987 at Dhahran (26°32′N, 50°13′E), Saudi Arabia, are analyzed. Dependence of ultraviolet radiation on the atmospheric clearness index is investigated. For a clearness index above 0.65, the ratio of ultraviolet radiation to total horizontal radiation is a constant value of 3.45%. For a clearness index below 0.65, the ratio was found to vary significantly with clearness index but it was also a strong function of the meteorological events (i.e., rainfall, clouds and suspended dust) bringing about the clearness index change. A ratio of ultraviolet to total horizontal radiation as high as 4.6% was reached on a rainy day, while a ratio as low as 2.1% was obtained for a dusty day.

Effects of the El Chichon volcanic dust cloud on insolation measurements at Corvallis, Oregon (U.S.A.)

Insolation measurements made on clear days at Corvallis, Oregon, since the last week of November, 1982, have indicated a decrease of about 14% in the atmospheric clearness index, an increase by a factor of about 2.3 in the ratio of the diffuse sky irradiation to the global irradiation and a decrease of about 28% in the transmittance of the atmosphere for the direct normal solar irradiation compared to the normal, clear‐day values. It is felt these anomalies are caused by the increased atmospheric turbidity due to the El Chichon volcanic dust cloud.