Luminous Efficacy Models Research Articles

Comparison of Simple Illuminance Model Performances under Clear-Sky Conditions for Athens (Greece)

Solar illuminance has become an important task in recent years as a new way of energy saving within the building sector. Incorporation and employ of new more efficient windows and light pipes in buildings require accurate information of external solar illuminance levels for a proper design. Unfortunately, illuminance data is not measured as often as solar radiation and therefore luminous efficacy models must be used. A large amount of these models are based on the empirical relationship existing between the components of luminous efficacy and the corresponding components of solar radiation, i.e. direct, diffuse and global. However, these relationships seem to be non-linear and site specific. In this article, the performance of five illuminance models to estimate clear-sky solar global and diffuse illuminances is analyzed. Experimental values of irradiance and illuminance have been obtained from a solar radiation station sited at the Institute of Environmental Research and Sustainable Development of the National Observatory of Athens (Athens, Greece). Results have shown that illuminance models based on solar geometry variables alone seem to exhibit a noted locality. Inclusion of radiometric variables is thus needed to improve illuminance estimation. This characteristic is noted for both diffuse and global components.

Using satellite-based methods to predict daylight illuminance for subtropical Hong Kong

Satellite-based methods are proposed to estimate global illuminance from geostationary satellite data under local climate conditions. The data used in this model are global irradiance and global illuminance collected from the International Daylight Monitoring Programme research class station in The Chinese University of Hong Kong and the visible channel data of the Geostationary Operational Environmental Satellite 9 from May 2003 to May 2005. The proposed methods consist of an indirect method, which derives global irradiance from satellite images first and then converts global irradiance to global illuminance by a luminous efficacy model, and a direct method, which derives global illuminance directly from satellite images. The root mean squared errors of hourly global illuminance are 35% and 32% for the indirect and the direct methods, respectively.

Evaluation of global luminous efficacy models for Florianópolis, Brazil

Several global luminous efficacy models have been tested with daylight-measured data obtained for Florianópolis, Southern Brazil. The models have been used with their original coefficients, given by the authors and also with local coefficients obtained when the models were optimized with the data measured in Florianópolis. The evaluation of the different models has been carried out considering three sky categories, according to a higher or lower presence of clouds. For clear sky, the models tested have been compared with a proposed polynomial model on the solar altitude, obtained by the best fit of experimental points for Florianópolis. It has been proved that the model coefficients have a local character. If those models are used with local coefficients, there is no model that works better than the others for all sky types, but that for each sky category a different model could be recommended.

A study on luminous efficacy of global radiation under clear sky conditions in Athens, Greece

The current interest in energy efficient design has led to a demand for methods of predicting daylight availability in buildings. However, basic daylight data, which are the basis of these prediction methods, are not available for most regions of Greece, as well as for many areas worldwide. For that purpose luminous efficacy can be used in order to generate daylight illuminance from solar radiation data, which are much more widely recorded. This paper concentrates on the analysis of global luminous efficacy in Greece under clear sky conditions. The study involves the evaluation of existing models with respect to daylight data recorded in Athens, Greece, as well as the development of new luminous efficacy models for global radiation. The study was based on climatic data provided by the National Observatory of Athens during the years 1996–2000 and tested according to CIE. recommendations. The proposed luminous efficacy models were derived by the analysis, correlation and linear regression of the accepted values of measured data. Statistical analysis techniques were used for the assessment of their performance, as well as for the evaluation of the accuracy in predicting global illumination provided by the proposed and the existing models. The new proposed models can offer engineers and researchers a more reliable and accurate approach for estimating the global illumination levels under clear skies. Nowadays, the knowledge of daylight levels is considered highly important in local architecture, the energy conservation and environmental policies in general, particularly in the case of Mediterranean regions, where clear sky conditions dominate.

Clear sky and all sky direct luminous efficacies for Florianópolis, Brazil

Abstract This work presents a characterization of the luminous efficacy of the direct radiation for Florianopolis, Brazil, for which several direct luminous efficacy models available in the scientific literature were evaluated. In the first part of the work, the evaluation was restricted to clear sky conditions. The models evaluated were used with the original coefficients given by their authors and also with local coefficients obtained for Florianopolis, showing their dependence on the local characteristics. None of the models presented significantly better statistics than the others, although one of them, developed by Robledo and Soler, seems to justify better the physical behavior of the direct luminous efficacy. In the second part of the paper, three models of direct luminous efficacy for all sky types have been evaluated, the first two having been specifically developed for the direct luminous efficacy, whereas and the third was developed for the global luminous efficacy and has been adapted in this paper to characterize the direct luminous efficacy. The statistical evaluation of the three models showed that the last general model has accuracy similar to the two other ones.

Clear sky and all sky direct luminous efficacies for Florian�polis, Brazil

This work presents a characterization of the luminous efficacy of the direct radiation for Florianópolis, Brazil, for which several direct luminous efficacy models available in the scientific literature were evaluated. In the first part of the work, the evaluation was restricted to clear sky conditions. The models evaluated were used with the original coefficients given by their authors and also with local coefficients obtained for Florianópolis, showing their dependence on the local characteristics. None of the models presented significantly better statistics than the others, although one of them, developed by Robledo and Soler, seems to justify better the physical behavior of the direct luminous efficacy. In the second part of the paper, three models of direct luminous efficacy for all sky types have been evaluated, the first two having been specifically developed for the direct luminous efficacy, whereas and the third was developed for the global luminous efficacy and has been adapted in this paper to characterize the direct luminous efficacy. The statistical evaluation of the three models showed that the last general model has accuracy similar to the two other ones.

Testing diffuse luminous efficacy models for Florianópolis, Brazil

This work comprises two parts. The first part presents an evaluation of diverse luminous efficacy models of the solar diffuse radiation available in the international scientific literature. These models were used for the estimation of the luminous diffuse illuminance in Florianópolis, Brazil. This evaluation has taken into consideration not only the precision of the models but also the justification of the physical behavior of the luminous efficacy. The second part of this study presents the evaluation of the three specific diffuse luminous efficacy models developed for clear sky conditions, also available in the international literature. It also presents a new model that is reported for the first time in the present work. These models have not shown clear statistical balance differences among each other when estimating the diffuse illuminance, and except for one, all the studied models can perfectly explain the behavior of the diffuse luminous efficacy against the solar altitude. Furthermore, the evaluation has shown that the models specifically developed for clear skies show no better results than the general all sky models, analyzed in the first part of this work, when estimating the clear sky diffuse illuminance with them. These latter specific models do not show better results in the estimation of the diffuse illuminance for the clear sky conditions, than the ones obtained with some of the general models, analyzed in the first part of this work.

Estimation of global illuminance on inclined surfaces for clear skies

In the present work, we have studied the luminous efficacy of global solar radiation incident on vertical surfaces for clear skies (cloudless and rather clean skies) and mean hourly values of global solar radiation (i.e. averaged over each hour). Luminous efficacy models similar to those previously obtained for a horizontal surface have been developed using solar elevation as the independent variable. Thus, the hypothesis that the slope of the surface does not influence the general formulation of the model has been assumed. However, the final formulation of the models clearly depends on the insolation conditions of the inclined surface and, more specifically, on whether direct radiation is or is not incident on it. As a consequence, two different types of models have been proposed to take both insolation conditions into account. On the other hand, global luminous efficacy models have also been obtained as the best fits of the experimental data. Global illuminance has been estimated for the vertical surfaces considered, facing north, east, south and west, with the models obtained as the best fits of the experimental data and the other two types of models mentioned in the preceding paragraph. Comparison of the performance of models for global illuminance estimation has validated the initial hypothesis that the slope of the surface does not influence the general formulation of the model. The performances of the known Perez model and the proposed models are also compared.

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.

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.

On the luminous efficacy of diffuse solar radiation

Models for diffuse illuminance and irradiance and for the luminous efficacy of diffuse solar radiation are developed, specifically for clear skies, and also for all sky types. All luminous efficacy models reported in the present work have the distinctive feature of being obtained from empirical models developed for diffuse illuminance and irradiance, looking in this way, apparently for the first time, for a mathematical coherence between the luminous efficacy and the two magnitudes that define it. Statistical assessment of all the models shows that those for all sky types developed from two independent variables, solar elevation and brightness index, predict the illuminance for clear skies better than the specific clear skies models developed with solar elevation as the only independent variable.

Assessment of Muneer’s Luminous Efficacy Models in Madrid and a Proposal for New Models Based on His Approach

From luminous efficacy models, illuminance can be estimated if irradiance is known or can be estimated. In the present paper, Muneer’s models for the luminous efficacy of global and diffuse radiation, useful for illumination engineers, are statistically assessed using data obtained at Madrid during a seven-year period. Several other models inspired on the simple approach proposed by Muneer are developed and statistically assessed. Some of those model the luminous efficacy of global or diffuse solar radiation, while others directly model the global or the diffuse illuminance. It is shown that for diffuse illuminance estimation the ratio of diffuse to extraterrestrial irradiance is to be preferred as independent variable to the ratio of global to extraterrestrial irradiance proposed by Muneer. Some models that estimate global illuminance from global irradiance and solar elevation, or diffuse illuminance from diffuse irradiance and solar elevation, perform practically on a par with the corresponding luminous efficacy models. It is also concluded that the striking difference between global luminous efficacy between the United Kingdom and Japan observed by Muneer is not due to a latitude effect.

Models for Estimating Solar Radiation and Illuminance From Meteorological Parameters

Sunshine fraction, cloud amount, temperature, and global irradiation are the most commonly measured meteorological parameters. In this article five models based mainly on the abovementioned variables for the determination of irradiation and illuminance components are presented. These models are divided into three categories. Category 1 includes three solar radiation models: a Meteorological Radiation Model (MRM), developed by present authors based on sunshine fraction and temperature data, b Cloud-Cover Radiation Model (CRM)—modification of an earlier model for the UK by the present authors based on cloud-cover in oktas, and c Page Radiation Model (PRM), developed by Page based on simultaneous sunshine and cloud data. Category 2 comprises of a new Diffuse Ratio Model (DRM), developed by authors, for estimating diffuse irradiation when global irradiation is provided as the input parameter. The third category includes luminous efficacy models that enable estimation of diffuse and global illuminance once the respective irradiation components have been obtained from category 1 models. [S0199-6231(00)02003-7]

Luminous efficacy of direct solar radiation for clear skies

In the present work we have modelled mean hourly values of the luminous efficacy of direct solar radiation for clear skies in Madrid using solar elevation as the only independent variable. Models of the polynomial type, similar to those given by different authors, and a model for the direct luminous efficacy mathematically coherent with the corresponding models of direct illuminance and irradiance, are developed. This last luminous efficacy model can be obtained from the corresponding illuminance and irradiance models in an apparently new approach to the subject, and shows a statistical performance as good as the polynomial models. We have taken the opportunity to show how arbitrary polynomial luminous efficacy models can be, and tested the dependence of coefficients in these models on local climatic conditions.

The simulation of annual daylight illuminance distributions — a state-of-the-art comparison of six RADIANCE-based methods

The present work discusses simulation results of annual indoor illuminance distributions for two office geometries situated in Freiburg, Germany, calculated with six different RADIANCE-based daylight simulation methods. These methods are the ubiquitous daylight factor method [P.J. Littlefair, Predicting annual lighting use in daylit buildings, Building and Environment 25 (1990) 43–54.], ADELINE 2.0 [M. Szerman, J. Stoffel, ADELINE 2.0, Radlink Technical Manual, IEA Solar Heating and Cooling, Task 12.], the classified weather data according to Herkel and Pasquay [S. Herkel, T. Pasquay, Dynamic link of light and thermal simulation: on the way to integrated planing tools, 5th Int. IBPSA Conf., Prague, Sept., 8–10, 1997, Vol. II, pp. 307–312.] and two simulation procedures based on daylight coefficients according to Tregenza, namely ESP-r version 9 series [J.A. Clarke, M. Janak, Simulating the thermal effects of daylight-controlled lighting, Building Performance (BEPAC), (1) (Spring 1998).] and a new accelerated method developed by the authors. The new method calculates 145 diffuse and three ground daylight coefficients in a single raytracing run which considerably reduces the required calculation times for an annual daylight simulation. An explicit calculation of the indoor illuminances under all 4703 annual hourly mean sky luminance distributions from the Freiburg test reference year (TRY) serves as a reference case against which the other methods are tested. The simulation results reveal that the accuracy of an annual daylight simulation method is not necessarily coupled with the required simulation time. The quality of an annual simulation rather depends on the underlying sky luminous efficacy model and whether the method considers the hourly mean direct and diffuse illuminances for each time period explicitly. The two methods relying on daylight coefficients exhibit the lowest relative root mean square errors (RMSEs) for the straightforward office geometry. The results for the advanced office show that internal illuminance contributions due to external ground reflections are only considered by the new method.

A comparison of luminous efficacy models with illuminance and irradiance measurements

Several luminous efficacy models have been tested against simultaneous illuminance and irradiance measurements in Helsinki, Finland (60°11′N, 24°50′E). When compared with the measured values, the Perez luminous efficacy model had the lowest relative root mean square difference ( RMSD) of 6.7%. All tested models had an RMSD within 2% of the constant model which used the measured yearly average of 110 lm/W as the constant. For all models, the error was smaller for high solar altitudes but greater for the low altitudes. The monthly average luminous efficacy was fairly constant between April and October but it was clearly lower during the winter months when the sun is very low.

Development of a meteorological illuminance model for daylight computations

Building simulation programmes need detailed hour-by-hour weather data. In the absence of measured irradiation and illuminance data, validated meteorological radiation and illuminance models may be used. These models are primarily based on the estimation of solar-beam attenuation through the terrestrial atmosphere. Broadly speaking, there are two strands of such computations. One of these strands is based on sunshine, temperature and humidity data and takes into consideration the attenuation due to mixed gases (such as oxygen, nitrogen and carbon dioxide), water vapour, ozone and aerosols. The other procedure requires cloud-cover data to obtain the degree of attenuation of clear-sky beam irradiance due to clouds. The authors have extended the application of the above models to obtain hourly and sub-hourly values of solar illuminance, via an interface with a luminous efficacy model. An account of the physical basis of the presently-proposed models and their validation is presented herein.©

Luminous Efficacy Models—Evaluation against UK Data

Luminous Efficacy Models—Evaluation against UK Data

Luminous efficacy of solar irradiance: Improved models

Daylight measurements were carried out at five locations in the United Kingdom as part of the International Daylight Measurement Programme of the Commission International de l'Eclairage. The reporting frequencies range from five-minute to half-hourly time series. Data were collected at Garston (North London), Sheffield, Manchester, Edinburgh (urban) and Edinburgh (suburban). Three existing diffuse and global luminous efficacy models are evaluated against the UK database. The models range from the sophisticated to the simplest. For instance the Perez model involves the incorporation of global and diffuse irradiation as well as precipitation data, whereas the simplest possible model assumes an average value for the respective efficacies. An innovative procedure is described for the statistical validation of the Perez, JOULE and the average-value efficacy models. A simple model which defines luminous efficacy as a function of the irradiation clearness index is presented. The performance of the proposed model is compared with that of the three current ones.

Evaluation of luminous efficacy models against Japanese data: Technical note

SYNOPSIS This note presents the evaluation of Perez and other simpler luminous efficacy models for Japan using the data now emerging from the CIE International Daylight Measurement Proaramme. The results indicate that the perez model, although exhibiting a low scatter, has a tendency to consistently underpredict global and diffuse illuminance while it heavily overpredicts zenith luminance. The average-values for global and diffuse luminous efficacies for Fukuoka data have been found to be significantly different from the ones reported for other geographical locations.