Estimation Of Global Irradiation Research Articles

Estimating global irradiance on tilted surfaces using a cubic multi-directional pyranometer array

The multi-directional pyranometer array is a method for estimation of the global irradiance on a tilted plane and the direct irradiance based on the simultaneous measurement of global irradiance on a set of stationary surfaces with different inclinations and orientations. Experimental testing shows that estimates of global irradiance on a tilted plane are 50% more accurate than transposing measured global and diffuse irradiance. A simplified layout of five sensors yields comparable results.

Köppen-Geiger climate classification adjustment of the BRL diffuse irradiation model for Australian locations

Numerous mathematical models have been developed to estimate diffuse and direct irradiance components based on global irradiation measurements. The Boland–Ridley–Lauret (BRL) model consists of a single set of parameters for all global locations. There is scope to improve the BRL model to better match local climatic conditions. In this research, the Köppen-Geiger climate classification system is considered to develop a set of adjusted BRL models for Australian conditions. Ground-based and satellite-based irradiation data derived from the Australian Bureau of Meteorology are used to tune and test new BRL models developed at a national level and for each climate zone. Irradiation data are processed through a rigorous quality control procedure before parameter tuning. For ground-based data, a new national model results in an improvement in 96% of statistical indicators over the original BRL model while Köppen-Geiger zone adjusted models show improvement over the new national model in 72% of the statistics. For satellite-based global irradiation estimates, a new national BRL model also results in observed improvements, however, no discernible improvement is observed for Köppen-Geiger zone models.

Improving the calibration of silicon photodiode pyranometers

Reliable measurements of global irradiance are essential for research and practical applications. Silicon photodiode pyranometers (SiPs) offer low-cost sensors to measure direct and diffuse irradiance despite their non-uniform spectral response over the 300–1000 nm spectral range. In this study, non-adjusted linear and adjusted calibrations were applied at different times of the year to determine sources of estimated errors in global irradiance due to the two calibration approaches, calibration time, and sensor age. 16 SiPs, along with two standards, measured incident global irradiance over a 5-year period under a range of sky conditions. Sensors performed best in the months in which they were calibrated when using the linear calibration approach. With the solar zenith angle adjusted calibration approach, certain calibration months provide a defendable validation for the following 12 months [ranging an average of 13.5–17.4 W m−2 standard error (SE)], while other calibration months do not provide consistent results and sometimes result in very poor validation (31.1–242.7 W m−2 SE). Older sensors (greater than 6 years) in general become more sensitive to solar zenith angle and their response drifts over time, while newer SiPs performed better than older sensors. Calibrations which accounted for solar zenith angle effects improved global irradiance estimates for older SiPs. For the Lincoln NE location, the appropriate calibration is in spring or late summer, regardless of calibration approach. These results indicate that solar zenith angle correction is not needed for largely diffuse components under cloudy conditions, so that in the future, a “smart” calibration may be possible, where diffuse radiation fractions are known.

Performance of empirical models for diffuse fraction in Uruguay

Knowledge of diffuse solar radiation is required for the estimation of global irradiation on inclined surfaces or for estimating DNI for CSP applications. Since diffuse irradiance data is comparatively scarce relative to global horizontal irradiance (GHI) data, several methods are used to estimate the diffuse component of GHI. These methods have a local component and most of them have been developed using data recorded in the northern hemisphere, where long-term reliable measurements of diffuse irradiance are available. This work considers ten models for hourly diffuse irradiation and evaluates their performance, both in their original and locally adjusted versions, against data recorded at five sites from a subtropical-temperate zone in the southern part of South America (latitudes between 30°S and 35°S). The raw data has been quality-assessed by using a set of seven sequential filters which preserve the natural spread of the data while removing unphysical data points. The local adjustment and performance evaluation are done using random-sampling cross-validation techniques on an ensemble. The best estimates result from locally adjusted multiple-predictor models, some of which can estimate hourly diffuse fraction with uncertainty of 18% of the mean.

CMSAF products Cloud Fraction Coverage and Cloud Type used for solar global irradiance estimation

Two products provided by the climate monitoring satellite application facility (CMSAF) are the instantaneous Cloud Fractional Coverage (iCFC) and the instantaneous Cloud Type (iCTY) products. Previous studies based on the iCFC product show that the simple solar radiation models belonging to the cloudiness index class n CFC = 0.1–1.0 have rRMSE values ranging between 68 and 71 %. The products iCFC and iCTY are used here to develop simple models providing hourly estimates for solar global irradiance. Measurements performed at five weather stations of Romania (South-Eastern Europe) are used. Two three-class characterizations of the state-of-the-sky, based on the iCTY product, are defined. In case of the first new sky state classification, which is roughly related with cloud altitude, the solar radiation models proposed here perform worst for the iCTY class 4–15, with rRMSE values ranging between 46 and 57 %. The spreading error of the simple models is lower than that of the MAGIC model for the iCTY classes 1–4 and 15–19, but larger for iCTY classes 4–15. In case of the second new sky state classification, which takes into account in a weighted manner the chance for the sun to be covered by different types of clouds, the solar radiation models proposed here perform worst for the cloudiness index class n CTY = 0.7–0.1, with rRMSE values ranging between 51 and 66 %. Therefore, the two new sky state classifications based on the iCTY product are useful in increasing the accuracy of solar radiation models.

DNI Estimation Procedures for the Assessment of Solar Radiation Availability in Concentrating Systems

DNI (Direct Normal Irradiance) is the resource utilized by solar concentrators. Besides, the determination of DNI is needed in the models for the estimation of global irradiance on tilted planes, which is the input to flat-plate systems. This paper describes a study of different estimation procedures for the assessment of the DNI, using experimental data with a time scale of 1min, taken at two different latitudes. The analyzed approaches include measuring techniques and models. The results show that the different estimation methods can lead to quite different conclusions when comparing the solar radiation availability in concentrating and flat-plate systems and this can affect the energy and economic evaluations. Based on the experimental analysis, indications for reducing the uncertainty in the estimation of DNI are discussed.

Comparative study of mathematical models in estimating solar irradiance for Australia

Hourly diffuse and direct solar irradiance data are required for weather files used in building energy simulation as well as Photovoltaic and solar thermal calculations. Access to up to date hourly observation data or satellite derived data for these parameters is currently only available for a selection of the Australian Bureau of Meteorology measurement locations. This study aims to investigate the accuracy of the methods used to estimate solar irradiance data from meteorological observations either in the absence of observed irradiance data or when irradiance observations are limited to global irradiance. In particular the focus of this paper is to investigate the accuracy of the process of coupling together global and direct/diffuse models. Five global and four diffuse/direct irradiance models are presented and compared to experimental data for four locations in Australia. In the case where experimental global irradiance data is available, values are used as input into various models to obtain diffuse and direct irradiance. In the absence of experimental irradiance data, the approach taken is to estimate global irradiance with a separate model and then feed these values into the diffuse/direct models. The errors associated with both of these approaches are investigated by comparing the modelled diffuse and direct irradiance values with known experimental data for four Australian locations over a period of a number of years. This study indicates that no single diffuse/direct irradiance model consistently outperformed the other models at estimating diffuse and direct irradiance whilst the Zhang and Huang global irradiance model with coefficients from Seo and Huang achieved the best estimates of global irradiance for the locations investigated. For the approach where global irradiance is estimated from a model, the resulting direct and diffuse data was found to differ significantly from the experimental data. The results indicate that the individual models that achieved the best estimates of global irradiance did not achieve the best estimates of diffuse and direct irradiance when coupled with a diffuse/direct model.

A method for estimating direct normal solar irradiation from satellite data for a tropical environment

In order to investigate a potential use of concentrating solar power technologies and select an optimum site for these technologies, it is necessary to obtain information on the geographical distribution of direct normal solar irradiation over an area of interest. In this work, we have developed a method for estimating direct normal irradiation from satellite data for a tropical environment. The method starts with the estimation of global irradiation on a horizontal surface from MTSAT-1R satellite data and other ground-based ancillary data. Then a satellite-based diffuse fraction model was developed and used to estimate the diffuse component of the satellite-derived global irradiation. Based on this estimated global and diffuse irradiation and the solar radiation incident angle, the direct normal irradiation was finally calculated. To evaluate its performance, the method was used to estimate the monthly average hourly direct normal irradiation at seven pyrheliometer stations in Thailand. It was found that values of monthly average hourly direct normal irradiation from the measurements and those estimated from the proposed method are in reasonable agreement, with a root mean square difference of 16% and a mean bias of −1.6%, with respect to mean measured values. After the validation, this method was used to estimate the monthly average hourly direct normal irradiation over Thailand by using MTSAT-1R satellite data for the period from June 2005 to December 2008. Results from the calculation were displayed as hourly and yearly irradiation maps. These maps reveal that the direct normal irradiation in Thailand was strongly affected by the tropical monsoons and local topography of the country.

Technical note: An investigation of possible improvements in accuracy of regressions between diffuse and global solar irradiation

Horizontal hourly and sub-hourly diffuse and beam irradiance are required for the estimation of global irradiance on a given tilted surface. This information is used by engineers and architects for various solar energy applications and also to calculate solar heat gain in buildings. Although horizontal global irradiance is a commonly measured parameter for many sites, horizontal diffuse irradiance is not so readily obtainable. For such sites that measure global irradiation alone a simple but reasonably accurate method is required to estimate diffuse irradiance from its global counterpart. The aim of this study is to evaluate the accuracies of estimation of diffuse irradiance by developing annual, seasonal and monthly regressions between sub-hourly diffuse-ratio and clearness-index. An error analysis has been undertaken to compare these regression models. Practicalapplications: Diffuse and beam components of global irradiation are required to accurately simulate solar heat gain in buildings. The UK Meteorological office keeps records of diffuse and global irradiances for 9 stations, but there are a further 84 stations that solely record global irradiance. Hence there is a need to estimate diffuse or beam irradiance for these sites. This paper presents an investigation into the uses of annual, seasonal and monthly regressions between diffuse and global horizontal sub-hourly solar irradiation. These regressions have proven to be a simple but reasonably accurate method of estimating diffuse irradiance from its global counterpart.

06/00198 Artificial intelligence techniques applied to hourly global irradiance estimation from satellite-derived cloud index: Zarzalejo, L. F. et al. Energy, 2005, 30, (9), 1685–1697

06/00198 Artificial intelligence techniques applied to hourly global irradiance estimation from satellite-derived cloud index: Zarzalejo, L. F. et al. Energy, 2005, 30, (9), 1685–1697

Artificial intelligence techniques applied to hourly global irradiance estimation from satellite-derived cloud index

Artificial intelligence techniques, such as fuzzy logic and neural networks, have been used for estimating hourly global radiation from satellite images. The models have been fitted to measured global irradiance data from 15 Spanish terrestrial stations. Both satellite imaging data and terrestrial information from the years 1994, 1995 and 1996 were used. The results of these artificial intelligence models were compared to a multivariate regression based upon Heliosat I model. A general better behaviour was observed for the artificial intelligence models.

Statistical assessment of a model for global illuminance on inclined surfaces from horizontal global illuminance

Olmo et al. [Energy 24 (1999) 689] have recently proposed a simple model to estimate global irradiance on inclined planes, which only requires the horizontal global irradiance and the sun elevation and azimuth as input parameters. From now on, this model will be referred to as the Olmo model. Statistical assessment of this model can be considered as important, taking into account that available models for estimation of global irradiance or illuminance on an inclined surface require information of global, and direct or diffuse irradiance or illuminance on a horizontal surface. The version of the Olmo model for global illuminance is tested in the present work using mean 15 min values of global illuminance obtained with 20 sensors of different slopes (zenith angles) and azimuths. The sensors were placed on a spherical dome located at one of the corners of the roof of the experimental site and ground shielded by black mat painted honeycomb material. Assuming a value of the honeycomb albedo of 0% values of the obtained RMSD go from about 8% for surface slopes of 12° to about 30% for a vertical surface facing east. For a north facing vertical surface, receiving mostly diffuse illuminance, a value of about 52% is obtained for the RMSD. Assuming a value of the albedo of 5%, too high for our experimental set up, similar results are obtained. In general the model over estimates global illuminance on inclined surfaces in Madrid, for experimental global illuminance values higher than about 60 klux.

Solar irradiance, sunshine duration and daylight illuminance derived from METEOSAT data for some European sites

Summary Radiometric ground truth data from seven Norwegian stations (58‐64 N), and from five other European stations (38‐61 N), are compared to satellite-derived data in the present paper. Hourly global irradiance at ground level is estimated by the Heliosat procedure from the ‘‘visible’’ channel of the geostationary satellite METEOSAT. With increasing latitude this satelllite sees the earth’s surface at an increasingly unfavourable angle. Nevertheless, in this paper, global irradiance estimates reproduce high latitude ground truth data with negligible Mean Bias Deviations (MBD) and only minor deviations regarding frequency distributions. Moreover, the Root Mean Square Deviations (RMSD) are comparable to those typically seen between ground truth stations some 20‐30 km apart. Using a number of auxiliary models, a multiplicity of ground level solar radiation data is obtained from satellitederived global irradiance data, and made available at the SATEL-LIGHT www server. The accuracy of the half-hourly data thus derived from Heliosat global irradiances, using models for diffuse fraction, luminous efficacy and slope/ horizontal ratios, is successfully verified against ground truth data.

全天日射量から斜面日射量を推定する各種モデルの比較

The models for estimation of global irradiance on tilted surfaces are evaluated. The evaluation is undertaken on the basis of the comparison between estimated and measured values. Measured values at three of the Japanese IDMP sites; Sapporo, Tokyo and Fukuoka, are employed for the comparison. At first, the values of direct irradiation are estimated from the measured horizontal global irradiation using three direct and five diffuse irradiance models. Root Mean Square Error (RMSE) obtained from the estimations of Perez, Erbs, Chandrasekaran and Reindl models are comparatively small among the eight irradiance models. Next, global irradiation on vertical surfaces is estimated from measured normal direct and horizontal diffuse irradiations using an isotropic and five anisotropic sky radiance models. RMSE from the estimations of Perez anisotropic sky irradiance model is the smallest. Finally, global irradiation on vertical surfaces are estimated from the measured horizontal global irradiation using each of the 48 combinations of the eight irradiance and six sky radiance models. The combination of the Perez direct irradiance and the Perez anisotropic sky radiance models gives the smallest RMSE value.

A climatological approach for estimations of hourly averages of global irradiation

AbstractMonthly mean values of daily and hourly global solar irradiation at eight stations in Spain are analysed on the basis of the similarity between the shape of extraterrestrial irradiance during the day and the Gaussian curve. This shape is found to be the same on the Earth's surface after attenuation through the atmosphere whenever the averaged‐out period is long enough. Bearing this in mind, it is possible to obtain values for the ratio between the hourly irradiance and the daily global irradiation, from one parameter σ, with the normal distribution curve. This curve is found to fit closely with experimental results. The mean of the normal distribution is taken at solar noon and the parameter σ is determined for each month by matching the experimental and the theoretical values at this time. The σ‐values thus obtained are found to bear an excellent linear correlation with N (monthly mean of maximum sunshine duration). The close correlation of the regression constants with latitude, altitude, and relative bright sunshine duration provides a simple technique for estimating hourly irradiance from the daily values within a certain range of latitudes. This technique makes the estimation of global irradiation for any smaller time interval possible. The errors involved in the estimations obtained are less than 10 per cent in most cases.

Improved station-independent correlations between global radiation and sunshine duration

Independent regression fits to Angstrom's relation between sunshine duration and global irradiation have been made for monsoon, pre-monsoon and post-monsoon periods. Long-term data of eight stations situated around 20°N lat in Indea were used to obtain collctives fits for each period separately for stations lying in (1) the zone comprising arid and semi-arid regions and (2) the wet-and-dry zone. The root mean square (r.m.s.) errors are < 3% in each case for the estimation of global irradiance of a month and the maximum error found is 6.3% which is significantly smaller than for previous estimates. The present station independent correlations should give precise estimates of G for locations away from pyranometers in a large part of the Indian subcontinent (Fig. 1). The technique of making collective fits depending on seasons and climatic zones may be applied with advantage to other parts of the world.

Evaluation of selected models for estimating solar radiation on horizontal surfaces

Within Task IX of the International Energy's Solar Heating and Cooling Programme, 12 models which simulate solar irradiance on horizontal surfaces were evaluated with data from seven countries. This paper presents the results of this study. Simple, widely applicable models were considered which use standard meteorological observations. Two models use cloud layer amount and type, three use total cloud amount, four use fractional sunshine, while the remaining four models partition measured global irradiation into direct beam and diffuse components. Data from 15 stations were used; four Australian, four European, three Canadian, and four American. In most cases three years of data were used for each station. Cloudiness regimes from relatively cloud free (Australia and Albuquerque) to relatively cloudy (European stations) were represented. Model performance was assessed from the mean bias error (MBE), the mean absolute error (MAB) and the root mean square error (RMSE). Quantitative ranking of models from these measures showed that cloud layer models provided the best global irradiation estimates (MBE < |0.2|; RMSE < 2.1 MJ/m 2/day) and the partitioning models the best direct beam and diffuse irradiation estimates (MBE < |0.6|; RMSE < 1.6 MJ/m 2day). However, RMSE results for 30-day means showed that layer models performed as well as the partitioning models for direct beam and diffuse irradiation. The performance of the better models showed little variation with either season or cloud cover: thus, they seem to be generally applicable. The performance of the better global irradiation models is probably limited more by inadequate meteorological measurements and observations than by defects in the models themselves.

The utility of the Angstrom-type equation for the estimation of global irradiation

L'auteur affirme que l'equation de type Angstrom fournit la meilleure estimation de l'eclairement global

Global irradiation estimation for Italian locations

The Angstrom equation has been fitted using the least-squares method to the monthly average daily global irradiation and the sunshine duration data of 31 Italian locations for the duration 1965–1974. Three more linear equations, obtained by: (i) incorporating the effect of the multiple reflections between the Earth's surface and the atmosphere, (ii) incorporating the effect of not burning of the sunshine recorder chart when the elevation of the sun is less than 5° and (iii) incorporating both the above effects simultaneously, have also each been fitted to the same data. Good correlation with correlation coefficients around 0.9 or more are obtained for most of the locations with all four equations. Substantial spatial scatter is obtained in the values of the regression parameters. The use of any of the three latter equations does not result in any advantage over that of the simpler Angstrom equation: it neither results in a decrease in the spatial scatter in the values of the regression parameters nor does it yield better correlation. The Angstrom equation using the computed values of the regression parameters yields estimates of the global irradiation that are, on average, within ±4% of the measured values for most of the locations. More simply, a single equation using the mean values of the regression parameters also provides reasonably accurate estimates of the global irradiation over the Italian location.

Comparison of techniques for the estimation of daily global irradiation and a new technique for the estimation of hourly global irradiation

Global irradiation and sunshine duration data recorded at Trieste (CNR, Istituto Talassografico di Trieste) during the 11-year period 1972–1982 are analyzed using the classical Ångström equation H = H 0 ( a + bS / S 0 ) and the equation H ′ = H 0 ( a + bS / S ′ 0 ) suggested by Hay [7] for incorporating the effects of (i) multiple reflections, and (ii) not burning of the sunshine recorder chart for small elevation of the sun. The values of the regression constants and the correlation coefficients are calculated using each yearly data set separately. Correlation coefficients of 0·89 or more are obtained for the 11 years. Substantial unsystematic scatter is obtained in the values of a as well as b for different years. The use of the equation H ′ = H 0 ( a + bS / S ′ 0 ) is not found to either decrease this scatter or to give better values of the correlation coefficients. Hourly global irradiation data is also analyzed. Eleven-year mean values of the ratio hourly/daily are plotted against the solar time for each of the 12 months of the year. The normal distribution curve P(t) = 1 2πr exp − (t−12) 2 2σ is found to fit the data closely. The mean of the normal distribution is taken at the solar noon and the σ values are obtained for each month by matching the experimental and the theoretical values at the solar noon. The σ values so obtained are found to bear an excellent linear correlation ( r = 0·996) with S 0 , viz. σ = 0·461 + 0·192 S 0 . This provides a simple and elegant technique for estimating hourly irradiation from the daily values and may be of universal applicability. The technique enables the estimation of global irradiation for any smaller interval of time as well.