Diffuse Fraction Of Solar Radiation Research Articles

Shedding light on the increased carbon uptake by a boreal forest under diffuse solar radiation across multiple scales.

Solar radiation is scattered by cloud cover, aerosols and other particles in the atmosphere, all of which are affected by global changes. Furthermore, the diffuse fraction of solar radiation is increased by more frequent forest fires and likewise would be if climate interventions such as stratospheric aerosol injection were adopted. Forest ecosystem studies predict that an increase in diffuse radiation would result in higher productivity, but ecophysiological data are required to identify the processes responsible within the forest canopy. In our study, the response of a boreal forest to direct, diffuse and heterogeneous solar radiation conditions was examined during the daytime in the growing season to determine how carbon uptake is affected by radiation conditions at different scales. A 10-year data set of ecosystem, shoot and forest floor vegetation carbon and water-flux data was examined. Ecosystem-level carbon assimilation was higher under diffuse radiation conditions in comparison with direct radiation conditions at equivalent total photosynthetically active radiation (PAR). This was driven by both an increase in shoot and forest floor vegetation photosynthetic rate. Most notably, ecosystem-scale productivity was strongly related to the absolute amount of diffuse PAR, since it integrates both changes in total PAR and diffuse fraction. This finding provides a gateway to explore the processes by which absolute diffuse PAR enhances productivity, and the long-term persistence of this effect under scenarios of higher global diffuse radiation.

Environmental controls on the light use efficiency of terrestrial gross primary production.

Gross primary production (GPP) by terrestrial ecosystems is a key quantity in the global carbon cycle. The instantaneous controls of leaf-level photosynthesis are well established, but there is still no consensus on the mechanisms by which canopy-level GPP depends on spatial and temporal variation in the environment. The standard model of photosynthesis provides a robust mechanistic representation for C3 species; however, additional assumptions are required to "scale up" from leaf to canopy. As a consequence, competing models make inconsistent predictions about how GPP will respond to continuing environmental change. This problem is addressed here by means of an empirical analysis of the light use efficiency (LUE) of GPP inferred from eddy covariance carbon dioxide flux measurements, in situ measurements of photosynthetically active radiation (PAR), and remotely sensed estimates of the fraction of PAR (fAPAR) absorbed by the vegetation canopy. Focusing on LUE allows potential drivers of GPP to be separated from its overriding dependence on light. GPP data from over 100 sites, collated over 20 years and located in a range of biomes and climate zones, were extracted from the FLUXNET2015 database and combined with remotely sensed fAPAR data to estimate daily LUE. Daytime air temperature, vapor pressure deficit, diffuse fraction of solar radiation, and soil moisture were shown to be salient predictors of LUE in a generalized linear mixed-effects model. The same model design was fitted to site-based LUE estimates generated by 16 terrestrial ecosystem models. The published models showed wide variation in the shape, the strength, and even the sign of the environmental effects on modeled LUE. These findings highlight important model deficiencies and suggest a need to progress beyond simple "goodness of fit" comparisons of inferred and predicted carbon fluxes toward an approach focused on the functional responses of the underlying dependencies.

A numerical framework to predict the performances of a tubular photobioreactor from operating and sunlight conditions

A framework model is proposed to evaluate the actual overall growth rate of microalgae in an outdoor tubular photobioreactor. A Monte Carlo-based radiative transfer modeling approach describes the local distribution of light energy inside the broth as a function of static (reactor geometry, location) and dynamic solar radiation parameters (angle of incidence, direct and diffuse solar contribution, incident radiation intensity). The light fields are coupled to a Lagrangian discrete random walk tracking of the cells to give the light variations experienced by each microalga for different broth flow rates. The cell light experiments are combined with a dynamic biological model to statistically calculate the actual overall growth rate. Using this model, 380 numerical experiments were performed for a wide range of geographic, light, biomass concentration, and broth flow turbulence conditions. Correlations for a normalized growth rate, Γ, relating the actual overall growth rate to its asymptotic behaviors (i.e., the instantaneous response and the full integration response), are proposed. The results clearly show that, for a fixed broth flowrate, Γ does not change with cell concentration variation. Under given light conditions, the level of turbulence linearly manages Γ, and thus the efficiency of sunlight utilization by the photobioreactor biomass can be tuned by the broth flow rate in the tubular photobioreactor. Γ also increases linearly with the diffuse fraction of solar radiation. A simple correlation is proposed for fast calculation of the actual overall growth rate.

Assessment and categorization of empirical models for estimating monthly, daily, and hourly diffuse solar radiation: A case study of Iran

Knowledge of diffuse radiation is essential to design and monitor passive and active solar systems. Measuring diffuse radiation is not common in most meteorological stations. Therefore, developing empirical models for many regions is inevitable. In the current study, a comprehensive review was conducted on all available monthly, daily, and hourly empirical model formats (MFs) to estimate the horizontal diffuse fraction of solar radiation (DFSR). A total of 153 monthly mean daily MFs, 159 daily MFs, and 30 hourly MFs were cited. Monthly and daily MFs were classified into six sub-categories, while hourly MFs were classified into two sub-categories. Eleven new empirical regression MFs were introduced to estimate DFSR over five cities of Iran. Results showed that errors in all 11 newly developed models are not significantly different even at the 10% probability level. The yearly average of DFSR for Bandarabbas, Kermanshah, Mashhad, Tabriz, and Zahedan were 12.41, 9.04, 7.65, 8.99, and 9.93 MJ m−2, respectively. The generality of the newly developed models was proved by fitting them to data of four meteorological stations in the U.S.A. Comparing the newly developed models and the seven existing models showed that the newly developed models were more accurate in fitting the data.

Multi-location model for the estimation of the horizontal daily diffuse fraction of solar radiation in Europe

Hourly and daily solar radiation data are crucial for the design of energy systems based on the solar source. Global irradiance, measured on the horizontal plane, is, generally, available from weather station databases. The direct and diffuse fractions are measured rarely and should be analytically calculated for many geographical locations. Aim of this paper is to present a multi-location model to estimate the expected profiles of the horizontal daily diffuse component of solar radiation. It focuses on the European (EU) geographical area joining data from 44 weather stations located in 11 countries. Data are collected by the World Radiation Data Centre (WRDC) between 2004 and 2007. Different analytic functions, correlating the daily diffuse fraction of solar radiation to the clearness index, are calculated and compared to outline the analytic expressions of the best fitting curves. The effect of seasonality on solar irradiance is considered developing summer and winter scenarios together with annual models. Similarities among the trends for the 4years are, further, discussed. The most adopted statistical indices are used as key performance factors. Finally, data from three locations not included in the dataset considered for model development allow to test the proposed approach against an independent dataset. Obtained results show the effectiveness of adopting a multi-location approach to estimate solar radiation components on the horizontal surface instead of developing several single location models. This is due to the increase of the geographical range of applicability without a significant decrease of the accuracy. The percentage gap between single and multi-location model performances is lower than 20% for the most of the statistical indices adopted for both the model development and ex-post validation through the three independent datasets.

Modeling the Diffuse Part of the Global Solar Radiation in Algeria

The diffuse part of solar radiation is one of the elements necessary for the design and evaluation of energy production of a solar system. However, in most cases, when radiometric measurements are made, only global radiation is available. To remedy this situation, we present in this paper, an analysis of the scattered radiation measured on a horizontal surface for three sites in Algeria namely: Algiers, Tamanrasset and BŽchar. We propose in this article models to determine the hourly ratio diffuse radiation/Global radiation received by a horizontal surface. The correlations established for the chosen sites were compared to the work of Liu and Jordan (1960), Orgill and Hollands (1977), Erbs et Al (1982), DT Reindl (1990) and Chendo (1994). The performances of our models were evaluated by conducting two statistical tests on the estimated diffuse fraction of solar radiation. The first test concerns the simplified comparison of the composite Residual Sum Squares (CRSS). In the second test, we calculate the mean absolute error (MAB). The new correlations presented in this study provide a significant improvement by reducing the standard error compared to existing models in the literature at a rate exceeding 40%.

Plastocrono e rendimento de feijão-de-vagem cultivado sob ambiente protegido e no ambiente externo em semeadura tardia no outono

O feijão-de-vagem (Phaseolus vulgaris L.), ou feijão-vagem, é uma hortaliça que, durante o outono no Estado do Rio Grande do Sul, pode ser favorecida pelo cultivo no interior de estufa plástica (cultivo protegido). A determinação do intervalo de tempo entre a emissão de dois nós consecutivos em dicotiledôneas, denominada plastocrono, é importante em estudos de modelagem e para o manejo dessa hortaliça. O objetivo deste estudo foi determinar o plastocrono e o rendimento da cultura de feijão-de-vagem em cultivo sob ambiente protegido por estufa plástica e no ambiente externo. Um experimento foi conduzido durante o outono de 2009, em Santa Maria, em dois ambientes: em ambiente protegido (estufa plástica de polietileno de baixa densidade, com 100µm de espessura) e em ambiente externo (céu aberto). Foram determinadas a densidade de fluxo de radiação solar global incidente, a temperatura do ar, a soma térmica diária, a emissão de nós (plastocrono) na haste principal até o florescimento e o rendimento precoce de vagens nos dois ambientes. A disponibilidade de radiação foi 26% menor no ambiente protegido, e as temperaturas mínima, média e máxima diária foram superiores. O plastocrono foi maior (velocidade de emissão de nós foi menor) no ambiente protegido, enquanto o rendimento de vagens foi similar nos dois ambientes. A redução da densidade de fluxo de radiação solar global incidente em função da cobertura plástica foi apontada como a causa da menor velocidade de emissão de nós (maior plastocrono) pelas plantas no ambiente protegido, enquanto o crescimento reprodutivo no ambiente protegido foi similar ao do ambiente externo, provavelmente devido a menor exposição das plantas a possíveis estresses, mecânico por vento e hídrico parcial nas horas mais quentes dos dias de maior demanda hídrica, além do aumento da fração difusa da radiação solar, que, em parte, compensou o sombreamento pela estufa.

Estimation of Diffuse Fraction of Global Solar Radiation Using Artificial Neural Networks

Measured daily mean air temperature and relative humidity values between 1998 and 2002 for Abha city in Saudi Arabia were used for predicting diffuse fraction of solar radiation in future time domain using artificial neural networks method. The estimations of diffuse solar radiation were made using four combinations of data sets, viz. (i) day of the year and daily maximum air temperature as inputs and diffuse solar radiation as output, (ii) day of the year and daily minimum air temperature as inputs and diffuse solar radiation as output, (iii) day of the year and daily mean air temperature as inputs and diffuse solar radiation as output, and (iv) time day of the year, daily mean air temperature, and relative humidity as inputs and diffuse solar radiation as output. The measured data between 1998 and 2001 was used for training the neural networks while the remaining 250 days' data from 2002 was used as testing data. The testing data was not used in training the neural networks. Obtained results show that neural networks are well capable of estimating diffuse solar radiation from temperature and relative humidity. This can be used for estimating diffuse solar radiation for locations where only temperature and humidity data are available.

Characterization of sky conditions by the use of solar radiation data

A method is proposed to characterize the sky and cloud conditions (SCC) by using ground based global/diffuse irradiance data. The term SCC is defined for the purpose of this paper as the bulk effect of clouds, and other atmospheric constituents on the values of the diffuse fraction of solar radiation ( D G ), the ratio between, the global to extra terrestrial radiation ( K T) and the transmissivity T r measured at the earth's surface. However, there is a difficulty in using the measured values of K T and ( D G ) for describing the SCC since these values depend on the solar zenith angle and not only on meteorological factors. This difficulty is overcome by the use of a simple semi-empirical procedure by which one can eliminate the seasonal and latitudinal variations (due to variations in the solar zenith angle) in D G and K T, or similar relations, leaving only variations due to SCC. The procedure is based on a paper by Zangvil and Aviv on the effect of latitude and season on the relation between the diffuse fraction of solar radiation and the ratio of global to extraterrestrial radiation, Solar Energy 39, 321–327 (1987) where a numerical simulation of the seasonal and latitudinal (zenith angle dependent) changes of positions of points representing different simulated SCC on a K T - D G diagram was performed. The method is demonstrated on monthly mean D G -K T relations calculated from three years of global and diffuse irradiance measured at Sede Boker, Israel. Results show that the large scatter of the original data points on a D G -K T diagram is, to a considerable extent due to a seasonal effect (variations of the solar zenith angle). After these effects are removed by the use of the proposed technique the transformed monthly data points have much less scatter and they describe the observed seasonal variations of sky and cloud conditions correctly.

Impact of Pinatubo aerosols on the seasonal trends of global, direct and diffuse irradiance in two northern mid-latitude sites

In June 1991, Mt Pinatubo's eruption in the Philippines ejected a staggering 20 million metric tons of SO 2 into the stratosphere which resulted in an aerosol cloud covering most of the Earth within a few months after the eruption. In this article we illustrate how the seasonal trends of global, direct and diffuse solar radiation were modified by the eruption in two mid-latitude sites in Europe and the U.S., totalling about 12 yr of hourly data. A spectacular increase in the diffuse fraction of solar radiation as opposed to a decrease in direct radiation which extended from late 1991 to mid 1993 was observed in both sites and for clear sky conditions. Global radiation was not altered significantly from these data sets. Sunphotometer measurements in one of the sites show the volcanic aerosols tripled the total atmospheric aerosol optical depth at 1 μm and approximately doubled the aerosol optical depth at 0.5 μm.

Estimation of the diffuse solar irradiation from global solar irradiation. Daily and monthly average daily values

Data sets of total and direct solar irradiation at Valencia (Spain) measured during the years 1990–1991 have been used to analyze diverse methods of estimating the daily values and monthly average daily values of the diffuse fraction of solar radiation from the ratio of the total to the extraterrestrial radiation (clearness index k t). In the case of daily values, all the methods described in the bibliography have led to significant deviations concerning the experimental values, with a MAD (mean absolute deviation) close to 20%. For the monthly average daily values, the methods based on the PDF (probability density function) of daily values of k t provide more accurate results than the deterministic methods based on the correlation of monthly average daily values of k t.

Dependence on latitude of the relation between the diffuse fraction of solar radiation and the ratio of global-to-extraterrestrial radiation for monthly average daily values

An approach for the prediction of the monthly average daily diffuse radiation, H d , was proposed by Page in 1961. The Page method is based on the use of the linear correlation H d H = c + d H H o , where H and H o are, respectively, the monthly average daily values of global and extraterrestrial radiation, both on a horizontal surface. The values of c and d are a function of atmospheric conditions. cloud cover conditions/types, as well as latitude. In the present work we study the dependence on latitude of c and d for European locations with 36°N < (γ) < 61°N (longitudes between 29°E and 11°W). The dependence is first studied using 28 values of c and d obtained using experimental values of H and H d . Next the dependence on γ is studied using experimental values of H for 64 European locations, obtained for the period 1966–1975, and corresponding values of H d estimated using the European Community Solar Radiation Model (E.C.S.R.M.). In both cases a minimum for c vs. γ and a maximum for d vs. γ are obtained for similar values of γ. Using the E.C.S.R.M. it is shown that both, the minimum and the maximum can be explained by the way H d H and H H o vary with γ.

On the effect of latitude and season on the relation between the diffuse fraction of solar radiation and the ratio of global to extraterrestrial radiation

Data sets of global and diffuse irradiance at Sede Boqer, Israel, for clear, hazy and overcast conditions have been used to simulate the behavior of the relation between the diffuse fraction of solar radiation and the ratio of global to extraterrestrial radiation ( D/G-K T ) as a function of latitude and season. It was found that in the summer solstice the latitude dependence is very small, and in the winter solstice very large. For the equinox the latitude dependence is closer to that of the summer. At a fixed latitude the seasonal dependence is large for high latitudes and negligible near the equator. Some consequences of these findings are discussed.

Global transmissivity and diffuse fraction of solar radiation for clear and cloudy skies as measured and as predicted by bulk transmissivity models

Measurements of cloudiness and of global, direct, and diffuse radiation taken over a 13 mo period at Davis, CA, are analyzed in terms of global transmission ( K t) and diffuse fraction ( K d) for clear sky conditions and for various cloudiness conditions. A number of global transmission clear sky models are compared with observations for ranges of total water column and turbidity and some are found to give representative values for the global radiation at the ground. The dependence of the diffuse fraction on global transmission is found to be best represented by linear formulae—with different dependencies found for clear and cloudy conditions. Global transmission models are also compared with observations for cloudy conditions and found to give representative values of cloud transmissivities if climatological differences in the cloudiness at the measurement site and those sites used to calibrate the cloud models are considered. These results support the use of routine instantaneous surface meteorological data to calculate the most likely instantaneous global and diffuse radiation on a horizontal surface in the absence of any radiation measurements. These calculated irradiances are best used for solar energy system dynamic modeling in which system responses to typical sequences in meteorological conditions are being examined.