Diffuse Irradiance Values Research Articles

Diffuse irradiance on tilted planes in urban environments: Evaluation of models modified with sky and circumsolar view factors

Numerous studies have analyzed existing irradiance models for estimating sky diffuse irradiance on tilted planes. However, few have evaluated the suitability of these models for estimating irradiance in obstructed environments, specifically considering the effects of obstacles. In this study, three irradiance models —one of them with five variants— for estimating diffuse irradiance on tilted planes located in urban environments were evaluated. All models have been adapted to consider the effect of an urban canyon on the diffuse irradiance received on a tilted plane through the sky view factor and the circumsolar view factor. All the models were evaluated against the diffuse irradiance values obtained by the ISO 15469:2004(E)/CIE S 011/E:2003 angular distribution model. Therefore, it was necessary to determine the standard sky type corresponding to each record of the measurements performed by a sky scanner at the radiometric station of UPNA (Spain). A total of 4,864 scenarios were considered to occur from the combination of different orientations and inclinations of the plane, and different orientations and aspect ratios of the urban canyon. The results revealed that the Perez model considering a 35° half-angle circumsolar region has the best performance, followed by the Perez model considering a 45° half-angle circumsolar region.

Empirical Models for the Estimation of Solar Sky-Diffuse Radiation. A Review and Experimental Analysis

Accurate solar radiation data are essential for the development of solar energy application systems. The limited availability of solar radiation data, and especially diffuse irradiance values, makes it vital to develop models to estimate these data. The development of estimation models has been the objective of many studies. This paper presents an extended review of the diffuse ratio (k) vs. clearness index (kt) annual, monthly, daily, and hourly frequency regression models. It is however interesting to note that there is a dearth of such knowledge for diffuse ratio–clearness index regressions that are based on averaged data. Monthly-averaged daily global irradiation data are now easily available from the NASA website for any global location. Using existing models, it is possible to decompose the daily to averaged-hourly global irradiation values. The missing link so far has been hourly averaged diffuse irradiation. This article presents regression equations which could be used to estimate that information. For this purpose, hourly global and diffuse irradiation data was pooled from 19 different locations to obtain three latitude-dependent regression models relating the monthly-averaged hourly diffuse ratio ( k ¯ ) to the clearness index ( k t ¯ ). The results show a high relationship between both variables. These regression equations could be used to estimate the averaged diffuse irradiation values from averaged global irradiation values, which are more easily available.

Comparison of different models for estimation of diffuse solar radiation in Jharkhand (India) region

The main objective of this paper is to estimate the monthly average diffuse solar radiation for the Jharkhand region (India). The measured data for the region is not available; in this paper the monthly average of solar radiation is calculated. Estimated values of diffuse radiation to total solar radiation using different correlation equations developed by Kreith, Gupta, Liu and Jordan and Page are presented. These models are then compared by estimating diffuse irradiation values for different locations over Jharkhand. As the Kreith model is most suitable for diffuse solar radiation in India, taking it as the base model the percentage error is calculated for all the other models.

Estimation of direct normal irradiance from measured global and corrected diffuse horizontal irradiance

DNI (direct normal irradiance) can be calculated from global and diffuse horizontal irradiance measurements. However, the diffuse irradiance values need to be corrected because the pyranometer's shadowband does not obstruct only the solar disk but also a larger part of the sky vault. In this study, we use four diffuse correction models (Drummond [1], LeBaron et al. [2], Batlles et al. [3], Muneer and Zhang [4]) and, considering the importance of DNI for several solar energy projects, we try to assess the performance of the models when the calculated DNI is compared to the measured one by the pyrheliometer. Based on 1-min averaged measurements covering a one-year period in Athens, Greece, it is concluded that the empirical approach of Batlles et al. performs best. It presents the lowest residuals (±1%) with no dependence from the clearness index. The estimated annual DNI, derived from the synergetic use of this model with measurements of horizontal global and diffuse irradiance, is lower by 0.4%, while the calculated values from the global and uncorrected diffuse irradiances are overestimated by 7.7%.

Wait-free shared-memory irradiance caching.

Parallelizing rendering algorithms to exploit multiprocessor and multicore machines isn't straightforward. Certain methods require frequent synchronization among threads to obtain benefits similar to the sequential algorithm. One such algorithm is the irradiance cache (IC), an acceleration data structure that caches indirect diffuse irradiance values. In multicore systems, the threads must share the IC to achieve high efficiency. A proposed wait-free mechanism for accessing the shared IC doesn't use the common blocking or busy-waiting methods, thus avoiding most serialization and reducing contention. A comparison with two classic approaches—a lock-based mechanism and a local-write technique—on two systems with up to 24 cores shows that the wait-free approach significantly reduces synchronization overhead, thus improving performance. The Web extra PDF augments the article. The Web extra video is an animation that demonstrates the wait-free IC system running at close to interactive rates on an 8-core machine.

Comparative study of various models in estimating hourly diffuse solar irradiance

This work presents a comparison among seventeen different proposals for estimating the hourly diffuse fraction of irradiance. Twelve of them are polynomial correlations of different orders, two are based on a logistic function and the three last ones consider the diffuse irradiance values in the previous and posterior hour to that of the calculation. In general, the proposals showing the more favourable statistics indexes are those that consider the process dynamics, as they behave better than the rest of the models even when the polynomial correlations and the logistic function are calibrated for the experimental data used in this work. The models Dirint and BRL are the ones recommended for the data of this study, as they exhibit the highest precision and generate a series of hourly diffuse irradiance values of which the distribution functions are very similar to those of the experimental data.

Experimental and theoretical investigation of diffuse solar radiation: Data and models quality tested for Egyptian sites

In order to obtain reliable irradiation data for the design, operation and economic assessment of solar power stations, eight widely used standard models predicting diffuse irradiation values are tested. A statistical analysis of hourly, daily and monthly correlations is carried out, using a 5 yr data archive (1999–2003) of hourly global and diffuse solar irradiances obtained at five selected meteorological stations over greater Egypt area. The accuracy of the candidate correlations are performed in terms of the two widely used statistical indicators, mean bias and root mean square errors; standard deviation, standard error and F-test, were also introduced. The results indicate that the correlations relating the diffuse fraction ( K D) with both clearness index ( K T) and the sunshine fraction ( S/ S O) are more reliable for diffuse irradiation predictions in the Egyptian environment than using each variable separately.

A numerical method to estimate time-varying values of diffuse irradiance on surfaces in complex geometrical environments

The estimation of diffuse irradiance that impinges on a window or photovoltaic panel is of major importance for the determination of their performance. The present paper introduces the concept of angular shading factors (ASF) for the calculation of time-varying diffuse irradiances to be performed without the need to repeat time consuming intereflection calculations at every time step. For the calculation of ASF, a Monte Carlo backward ray tracing technique was used. As a result, geometrically complex scenes consisting of surfaces with diffuse and specular reflectances, while transparent and translucent surfaces can be modeled as well. After that, the calculation of diffuse irradiance can be performed in a dynamic way, by taking into consideration the sky radiance distribution variability.

A simple clear skies model for the luminous efficacy of diffuse solar radiation on inclined surfaces

In the present work we study the luminous efficacy of diffuse solar radiation incident on vertical surfaces for a clear sky and mean hourly values of diffuse irradiance and diffuse illuminance. We develop a model easy to use, similar to a model previously obtained for horizontal surfaces. To develop the present model for vertical surfaces we assume that the slope of the surface influences diffuse illuminance and diffuse irradiance in the same way. As a consequence of this hypothesis, the luminous efficacy of diffuse solar radiation is assumed to be the same for both horizontal and inclined surfaces.

Modeling the luminous efficacy of diffuse solar radiation on inclined surfaces for all sky conditions

In the present work, we have undertaken a study of the luminous efficacy of diffuse solar radiation incident on vertical surfaces facing north, east, south and west for all sky conditions and mean hourly values of diffuse irradiance and diffuse illuminance. We have developed luminous efficacy models, of the type previously obtained for a horizontal surface, assuming that the surface slope will qualitatively influence illuminance and irradiance in the same way. Taking into account that the models obtained show that there is little variation of the luminous efficacy with the independent variables, an approximation to these models has been proposed that considers a constant value for the luminous efficacy of diffuse solar radiation. Statistical assessment shows that this approximation offers reasonable results for north, east and west vertical surfaces. When statistical assessment of the models is performed, it is observed that the values of the statistical indicators largely depend on the possibility for the surface to “see” the solar disk, as determined from geometrical considerations. For both types of models, better values of the statistical indicators are obtained when the surface sees the solar disk than when it does not.

On shadowband correction methods for diffuse irradiance measurements

Diffuse irradiance, G d, is an important variable in solar resource assessment. The diffuse irradiance can be worked out from global, G, and direct, G b, irradiance measurements, but this method involves the use of relatively expensive tracking mechanisms. Alternatively, a widely accepted technique uses a pyranometer with a shadowband. Because the shadowband screens the sensor from part of the diffuse radiation coming in from the sky, a correction must be made to the measurements. However, because of the anisotropy of diffuse radiation it is difficult to compute an exact theoretical correction. In this study we use two data sets registered in two locations in Spain. The first one consists in coincident hourly values of global, direct, and diffuse irradiance; the latter by means of shadowband. The other data set includes the same variables but as 5-minute values. Our goal is to study the necessary correction factor for diffuse irradiance measurements obtained by means of shadowband. After testing several well-known correction methods, we have developed two different correction models, using two-thirds of the hourly data set, while the remaining one-third and the whole 5-minute data set have been used for validation purposes. The last validation test suggests that our anisotropic models provide reliable corrections for conditions different than the ones where they have been developed. The results obtained by the developed models show a negligible mean bias deviation. Approximately 55% of cases present deviations lower than 5% over the mean value of diffuse irradiance.

Monthly maps of daily diffuse solar irradiance for Italy

Since measurements and long-term records of diffuse solar irradiance for most parts of Italy are not available, the existing correlation relations and models are used for estimating these values for the Italian stations. In this study we used the correlation relations of Page [ Proc. UN New Sources of Energy (1961)] and Liu and Jordan [ Solar Energy 4, 1–19 (1960)] for calculating the monthly average of daily values of diffuse irradiance for 140 stations distributed over all Italy, by feeding the monthly values of global and extraterrestrial solar irradiance as given by Lavagnini et al. [IFA R.I. 89/17 (1989)] for these stations. Monthly maps of daily diffuse solar irradiance are produced for the whole country, taking into account the values obtained from the correlation relations and some considerations regarding topography, climatology and quality of data. It is found that the values of daily diffuse solar irradiance in Italy range between 2.0 MJ/m 2 (in winter months) and 8.0 MJ/m 2 (in summer months), whereas for global irradiance the values are between 4.0 and 30.0 MJ/m 2 in winter and summer months respectively.

A simple procedure for correcting shadowband data for all sky conditions

A model is presented that corrects the horizontal diffuse irradiance measured using a polar-axis design shadowband. Ratios of true diffuse to uncorrected diffuse measurements are categorized and averaged according to parameters describing the associated geometric screening and current sky conditions. The screening parameter is a geometric calculation for the amount of the sky hemisphere eclipsed by the band. The sky condition parameters account for the anisotropic distribution of the diffuse irradiance across the sky hemisphere. An independent set of shadowband diffuse data is corrected using this parameterization technique. Comparison of these corrected values with concurrent true diffuse irradiance values gives a root mean square error of 6.9 W/m 2 and a regression slope of 0.99. The anisotropic contribution to the total shadowband correction is examined for seven different levels of sky clearness and indicates the importance of its consideration in the model's development. Since the correction ratios are developed from parameters that are not site specific, the model should be generally applicable.

Diffuse irradiation contours for southern Africa

Abstract Diffuse irradiation contours are drawn for the southern African region from the monthly mean daily diffuse irradiation data of 61 locations spread all around southern Africa. The distribution of average daily diffuse radiation throughout the subcontinent is compiled in the form of 12 monthly maps. Yearly mean daily diffuse irradiation values are calculated and isoradiation lines are presented. The iso-radiation maps show N-S and E-W gradients during most of the months of the year. The maximum radiation zone is observed to move southward in summer and to the north in winter. The results are due to the seasonal variations of the subcontinent.

The distribution of global and sky radiation throughout Lesotho

The global and diffuse irradiation values are estimated, for 12 locations in Lesotho, using the procedure recently suggested by Gopinathan. Angstrom type correlation with the regression coefficients estimated from the elevation of the locations above sea level is used for estimating global radiation. Diffuse irradiation values are calculated from the equation Hd / H = 0.697 –00.577 ( S / S 0 ). Insolation values, throughout the country, are very high during all the months of the year. The results show that Lesotho has ideal conditions for solar energy utilization.

Solar irradiance in NW Italy

An analysis is made of the monthly mean global and diffuse irradiance values recorded by the Genoa University Geophysical Institute radiation network to derive characteristic elements of the solar climate in four typical NW Italy locations. Diffuse irradiation was estimated for the stations where diffuse irradiation data were not recorded, by using correlations between measured data with clearness parameter and sunshine index. Local factors which affect the attenuation of global irradiance under cloudless skies are discussed.