Luminous Efficacy Models Research Articles

Two new models of direct luminous efficacy under clear sky conditions for daylighting in Burgos, Spain

The use of daylight in buildings contributes to energy savings while significantly improving visual comfort and well-being. It is therefore very important to be able to quantify illuminance to take advantage of daylight. Although several models have been proposed in recent years to determine global and diffuse illuminance, the same may not be said of direct solar illuminance, which situates this study in an area of noteworthy scientific and technological interest. Two luminous efficacy models for clear sky conditions are proposed and the results of benchmarking with previous models from the literature are presented. Data collected in Burgos (Spain) were analyzed. Specifically, eight previous models for the prediction of direct illuminance were compared with our two new models. The two new models predicted illuminance more accurately than most of the classic models. Specifically, running the models on the training data yielded Root Mean Square Error (RMSE) values of 2.58 % and 2.76 % for the first and the second model, respectively. Likewise, the test data yielded RMSE values of 3.31 % and 3.49 %, and the Mean Bias Error values with the training data were 0.06 % and 0.11 %, respectively. The models achieved high accuracy levels with both the training and the test data sets.

Predicting Vertical Daylight Illuminance Data From Measured Solar Irradiance: A Machine Learning-Based Luminous Efficacy Approach

Abstract Daylight data are required for energy-efficient building designs. However, daylight is scarcely measured, making the luminous efficacy model an alternative. This paper presents a method for modeling vertical luminous efficacy (Kvg) using measured data from measuring stations in Hong Kong. The artificial neural network (ANN), support vector machines (SVM), and empirical correlations were proposed for modeling Kvg. Machine learning (ML) models like ANN and SVM were used because they offer more accurate daylight predictions and ease in explaining complex relationships between atmospheric variables. Also, ML was explored since it has not been used in earlier vertical luminous efficacy studies. Sensitivity analysis was also carried out to determine the relative importance of input variables used for developing the proposed models. Findings show that scattering angle and diffuse fraction are crucial variables in vertical luminous efficacy modeling. Furthermore, when all proposed models were used to predict vertical daylight, it was observed that the peak relative root mean square error (%RMSE) was less than 18.6%. The obtained %RMSE showed that all models provided acceptable performance when evaluated against the measured daylight data. Finally, the findings also showed that the ANN models outperformed the SVM and empirical models.

Application of artificial neural networks in horizontal luminous efficacy modeling

Harnessing daylight for energy-efficient designs requires the availability of daylight illuminance data. In the absence of measured daylight data, deriving daylight using the luminous efficacy model is an alternative. This paper presents an approach for modeling diffuse and global luminous efficacies using accessible measured climatic data. The methodology explored machine learning, sensitivity analysis and empirical modeling approaches. Twelve (12) luminous efficacy models were proposed. These models consisted of six artificial neural networks (ANN) and six empirical models. All models also cover the all-sky, overcast and non-overcast sky conditions. The intent of using ANN lies in the need for more accurate daylight predictions and its ease in explaining complex relationships between complex atmospheric variables. Findings from the study show that diffuse fraction is crucial in global and diffuse luminous efficacy modeling. Furthermore, the performance of all models was statistically assessed, and the results show that all twelve proposed models could offer acceptable predictions of daylight. In particular, the ANN models outperformed the empirical models.

Application of luminous efficacies for daylight illuminance data generation in subtropical Hong Kong

PurposeDaylight plays a crucial role in the attainment of building energy savings. Harnessing daylight in building designs will require the need for daylight illuminance data. However, daylight illuminance data are scarce due to few measuring stations. Aside from being sparse, illuminance measuring stations can be expensive to set up, thus making the luminous efficacy model a better alternative. Hence, this study attempted to model horizontal luminous efficacies under the 15 Commission internationale de l'éclairage (CIE) standard skies. Therefrom, daylight illuminance was estimated from a proposed vertical luminous efficacy model.Design/methodology/approachMeasured solar irradiance, daylight illuminance and luminance distribution data were gathered from the local measuring station in Hong Kong. The luminance distribution data were used to classify the skies into the 15 CIE standard skies. Next, the solar irradiance and daylight data were used to derive the horizontal luminous efficacies under each standard sky. Furthermore, a vertical luminous efficacy model developed using the measured data was described, and this was used to predict vertical illuminance.FindingsIt was observed that Skies 1, 8 and 13 seem to be predominant in Hong Kong. Also, the result showed that constant luminous efficacies could be used for deriving illuminance data. Furthermore, horizontal luminous efficacy ranged from 40 to 190lm/W, indicating that daylight can provide sufficient visibility during working hours. The vertical luminous efficacy model proves to offer reasonable estimations of vertical illuminance data.Research limitations/implicationsFurther work needs to be done with more measured data to cover for spring seasons. The described model still needs to be fitted with different world climates to ascertain its universal applicability. The evaluations need to be done under obstructed sky conditions to cater for dense and clustered urban centres.Practical implicationsThe discussed luminous efficacy model could be used to derive illuminance data in the absence of measured daylight illuminance data, especially in the subtropical region. Also, the comparative advantage of daylight over artificial lighting was highlighted in this study.Originality/valueUnlike previous studies, this paper discusses the luminous efficacies of global, direct and diffuse components under the 15 CIE standard skies. Furthermore, the described luminous efficacy analysis provides an approach for deriving vertical and horizontal illuminance data. Such vertical data will be required for analysing building lighting requirements, sensible heat from electric lighting, and energy savings from daylighting controls. Also, the information on horizontal luminous efficacies will help evaluate solar roof and skylight designs.

Proposal and evaluation of typical illuminance year (TIY) generation procedures from illuminance or irradiance data for daylight assessment in the long term

When assessing the long-term daylight availability or the performance of natural lighting systems in a given location, it is necessary to have representative data of local daylight conditions. The use of a daylight test reference year (TRY) becomes a good option in these cases. This paper proposes and evaluates a procedure for the generation of a typical illuminance year (TIY) considering illuminance as the only variable for selecting the typical periods that make up the reference year. Two versions of TIY are presented, one composed of 12 typical months selected from the series of observations and another composed of 365 typical days. Each of these versions is used to obtain a global illuminance TIY (TGIY) and a diffuse illuminance TIY (TDIY) from a 27-year dataset corresponding to the Vaulx-en-Velin station (France). Furthermore, 12 luminous efficacy models have been evaluated in order to obtain a TIY from a TRY generated from irradiance data when no illuminance data are available. Thus, a global luminous efficacy model and a diffuse model are selected after benchmarking different models, considering both their original coefficients and those adjusted to local conditions. The results reveal that the monthly version of the TGIY and the daily version of the TDIY show the best overall fit to the long-term dataset. TIYs obtained from illuminance data are also observed to be statistically indistinguishable from those obtained after applying a luminous efficacy model to an irradiance-based TRY.

A new diffuse luminous efficacy model for daylight availability in Burgos, Spain

The determination of optimal illumination conditions in buildings is of great interest both for reducing energy consumption and for exploiting solar resources with greater efficiency and sustainability. The most commonplace method of estimating daylight is the luminous efficacy approach, using the more widely measured solar irradiance. In this present study, a new model of diffuse luminous efficacy over a horizontal surface is proposed. A comparative study of twenty-two classic models is presented, to obtain diffuse illuminance, using both, the original mathematical models and the adapted models with local coefficients, in order to determine the most suitable models for Burgos, a city located in north-western Spain. With this purpose in mind, twelve models are selected for all sky conditions, five models for modelling clear sky, two for partly cloudy sky, and three for overcast sky. These twenty-two models are then compared with the new model both for all sky conditions and for particular sky conditions (clear, partly cloudy, and overcast). The behaviour of the new model showed greater accuracy than most of the classic models under analysis. Hence, the advantage of the diffuse luminous efficacy model that can be applied both to all sky and to particular sky conditions.

Spectral optimization of color temperature tunable white LEDs with red LEDs instead of phosphor for an excellent IES color fidelity index

The optimal spectral parameters and photometric performances of white LEDs with red LEDs instead of phosphor (pc/R-WLEDs) for the color fidelity index (Rf) above 97 at correlated color temperatures (CCTs) of 2700 K to 6500 K have been obtained based on the luminous efficacy (LE) model. We have first reported four real pc/R-WLEDs with Rfs of 96–97 and LEs of 120–124 lm/W at CCTs of 2969 K, 4468 K, 5682 K, and 6558 K by using blue (448 nm) and red (650 nm) LEDs, as well as green (507 nm) and yellow (586 nm) phosphors. As compared with phosphor-converted white LEDs (pc-WLEDs) as well as quantum dots white LEDs (QD-WLED), it was found that the pc/R-WLEDs, not the QD-WLED as widely expected, make strong candidates for excellent color rendition, especially in the low color temperature range, for replacing current pc-WLEDs in the future.

Performance of global luminous efficacy models and proposal of a new model for daylighting in Burgos, Spain

Abstract Daylighting is recognized as an important and useful strategy in the design of energy efficient buildings. Daylight is still the best source of light for good colour rendering and visual comfort. In this study, a new model of global luminous efficacy over a horizontal surface is proposed. A comparative study of eighteen classic models is presented, to obtain global horizontal illuminance, using both, the original formulation and new formulae with local adaptations, in order to determine the most suitable models for the conditions in Burgos (Spain). With this aim in mind, the selected models consisted of six models developed for all sky conditions, five models for clear sky conditions, three for partly cloudy sky and four for modelling overcast sky conditions. These eighteen models were also compared with the proposed model using experimental global illuminance measurements for different sky conditions. It was shown that the proposed model behaved in a better way than most of the classic models selected from the literature; both for all sky conditions and for particular sky conditions (clear, partly cloudy and overcast). The proposed model was therefore generally applicable, with no need to employ a different model for each particular sky condition.

Comparative Analysis of All-Sky Luminous Efficacy Models Based on Calculated and Measured Solar Radiation Data of Four Worldwide Cities

Luminous efficacy model uses solar radiation data to generate illuminance data, and its performance also depends on the quality of solar radiation data. Various luminous efficacy models are reviewed and evaluated to select a universal luminous efficacy model. Since most luminous efficacy models are fitted with specific local climate characteristics, the model that has the least locality as well as accuracy is a mandatory requirement. Three representative luminous efficacy models are selected and evaluated with measured solar radiation and illuminance data from four worldwide cities. It was found that all the evaluated models provide good predictions (over 0.96 R2 value) for both global and diffuse illuminance. Among them, the Perez luminous efficacy model shows the highest performance in terms of accuracy and bias. However, illuminance data prediction based on estimated solar radiation data is more common practice rather than those from measured solar data. The performance of the selected luminous efficacy models is evaluated when recently proposed universal solar radiation model supplies predicted solar radiation data. The result indicates that the quality of estimated solar radiation data has a much deeper impact on the performance of the luminous efficacy model. Within the current limited technology and measured data resource, the consecutive processing of the modified Zhang and Huang solar model and Perez luminous efficacy model could provide the best option to predict both global and diffuse solar radiation and illuminance. But, users of the model-based illuminance data should interpret their simulation results with the error (30%~40% in RMSE and ±6% in MBE) in predicting global and diffuse illuminance.

Monthly luminous efficacy models and illuminance prediction using ground measured and satellite data

This paper presents the derivation of the simple monthly luminous efficacy models to be applicable in daylight design and practice. Illuminance and irradiance data from the CIE IDMP station in Bratislava and climatological data from the Slovak Hydrometeorological Institute as well as data obtained from the satellite representing the climate of Bratislava are used in this study. Three models of luminous efficacy based on statistical analyses are proposed. New models are compared with the Perez model and results are discussed. Finally the study is devoted to predict exterior illuminance using the proposed luminous efficacy models and derived from satellite irradiance data. The advantage of the presented approach is in application of irradiance data regularly measured on the ground and data from satellite images when no measured illuminance is available. After a simple calculation processing the results have shown good accuracy in terms of the RMSE and MBE when comparing the new model with the Perez model as reference.

Model development and evaluation of global and diffuse luminous efficacy for humid sub-tropical region

Abstract Daylight has a significant role for energy conservation in buildings. Luminous efficacy models have been applied by many researchers to estimate the illuminance level from available irradiance data. In the present study, the characteristics of global (Kg) and diffuse (Kd) luminous efficacies are reported based on measured data for the humid sub-tropical region of New Delhi, India. Four existing efficacy models have been compared based on the measured irradiance and illuminance data. Statistical analysis indicates that the existing models performed poorly for the selected location. The performance of the models was significantly improved when the optimized coefficients were adopted using the measured data. Global and diffuse efficacy under clear, intermediate, overcast and all sky conditions, using existing and developed efficacy models were computed and compared. A good agreement has been exhibited for variation of efficacies with the measured values for different sky conditions from cloud to clear skies. Variation of global and diffuse efficacy for sky (clear, intermediate and overcast) w.r.t. solar altitude angle and sky clearness index has been presented. Finally, mean monthly global (Lg) and diffuse (Ld) illuminance were also estimated using the original and optimized version of efficacy models to provide impression of deviation of the models.

Impact of Different Luminous Efficacy Models on the Calculation of Lighting Energy Uses in Non-residential Buildings

Artificial lighting has a relevant impact on the electricity uses in non-residential buildings. The calculation method introduced in the relevant EN standards takes into account, only to some extent, the daylighting role on the lighting performances of buildings. An alternative method is under development for the Italian territory, which introduces a climate based approach to properly estimate the day light contribution. The method requires the implementation outdoor global and diffuse illuminance Typical Meteorogical Years (TMY), which calculated starting from solar irradiation data and luminous efficacy models. Different models can be used for this purpose, consequently these differences may lead to different estimates of daylight availability in buildings during the year. This paper explores the impact of different luminous efficacy models on the lighting energy uses in non-residential buildings. A typical office building is used for the study and calculations are carried out for three localities of the Italian territory. The outdoor luminous environment, the reference indoor visual task and the building daylight characteristics are taken into account. Results allow to compare the energy lighting performance of the building applying the different luminous efficacy models and, as a consequence, to select the most suitable model to be used in a climate based artificial lighting calculation method.

Theoretical Modeling of Luminous Efficacy for High-Power White Light-Emitting Diodes**Supported by the National Natural Science Foundation of China under Grant Nos 51307113 and 51407124, and the Natural Science Foundation of Jiangsu Province under Grant No BK20130307.

The photometric characteristics of high-power white light-emitting diode (LED) devices are investigated. A theoretical model for the luminous efficacy of high-power white LED devices and LED systems is proposed. With the proposed theoretical model, the mechanism of the luminous efficacy decrease is explained. Meanwhile, the model can be used to estimate the luminous efficacy of LEDs under general operation conditions, such as different operation temperatures and injection currents. The wide validity of the luminous efficacy model is experimentally verified through the measurements of different types of LEDs. The experimental results demonstrate a high estimation accuracy. The proposed models not only can be applied to estimate the LED photometric performance, but also is helpful for reliability research of LEDs.

Modeling the luminous efficacy of direct and diffuse solar radiation using information on cloud, aerosol and water vapor in the tropics

This paper presents luminous efficacy models for direct and diffuse solar irradiance using information on cloud, aerosol and water vapor in the tropics. The model is based on five years (2007–2011) of diffuse illuminance and irradiance measurements and two years of direct illuminance and irradiance measurements, April 2010–March 2012. Data are taken at four solar radiation monitoring stations in Thailand, specifically Chiang Mai (18.78 °N, 98.98 °E) in the Northern region, Ubon Ratchathani (15.25 °N, 104.87 °E) in the Northeastern region, Nakhon Pathom (13.82 °N, 100.04 °E) in the Central region and Songkhla (7.20 °N, 100.60 °E) in the Southern region. The models express luminous efficacy as functions of the aerosol optical depth and precipitable water, obtained from the AERONET network, and a cloud index for hourly time scales derived from the MTSAT-1R satellite. The model performance is good when validated against independent data from these stations. Root mean square differences (RMSD) of 9.7% and 6.8% for direct normal efficacy and diffuse efficacy, respectively are obtained. The models compared favorably with most existing models when tested against these independent data.

The trend of natural illuminance levels in 14 Chinese cities in the past 50 years

Maximizing natural illuminance, seeking sustainable ecological development, and saving electricity are very important. The annual total natural light illuminance values of 14 typical Chinese cities in different climate regions were obtained in this study based on solar irradiance data of the cities in the past 50 years (from 1961 to 2010). A luminous efficacy model was integrated in the analysis of data. This study analyzes the overall and inter-annual change trends of total natural light illuminance values in each city. Mann-Kendall test method or nonparametric test method can reveal the changing trend of time series; the method does not require samples to follow a specific distribution and cannot be affected by a few abnormal values. The method is suitable for data with abnormal distribution. In this paper we analyze the trend of natural illuminance levels in 14 Chinese cities by Mann-Kendall trend analysis and Mann-Kendall mutation test. The results show that the overall change trends in the annual total natural light illuminance values in Beijing, Guangzhou, Urumchi, Xian, Xining, Shanghai, Changsha, and Chongqing are extremely obvious; the change trend in Erenhot and Kunming are obvious; and those in Changchun, Yushu, Heihe, and Fuzhou exhibit no change. The inter-annual change trends in the total city illuminance values in Chongqing and Urumchi decreased gradually year by year; those in Beijing, Shanghai, Changsha, Guangzhou, Erenhot, Xian, Xining, and Heihe increased at first and decreased in the later years. The illuminance values in Changchun and Yushu increased initially, decreased, and then increased again; in Kunming, these values decreased initially, increased, and then decreased again. The annual total illuminance values of natural light in some cities significantly decrease, thus affecting the accuracy of architectural daylighting design. The total change trend of natural light illuminance has an insignificant association with Chinese light-climate regions. Many factors influence natural illuminance: the major factors are cloud cover and aerosols from air pollution; the minor factors are water vapor, atmospheric molecular scattering and absorption, and solar activity.

Luminous efficacy model validation and computation of solar illuminance for different climates of India

Three luminous efficacy models namely Perez model, Muneer–Kinghorn model, and Littlefair model have been evaluated against measured solar illuminance and solar irradiance at New Delhi. Both these quantities are measured simultaneously, every half an hour, on a horizontal surface, in clear sky as well as intermittent sky. Among three models, Perez model has been found to perform best, with minimum mean bias error and minimum root mean square error. Experimental results also reveal that diffuse luminous efficacy has higher values as compared to global luminous efficacy and beam luminous efficacy. Further, Perez model has been used to assess daylight availability in summer, winter, and monsoon seasons at six Indian stations: New Delhi, Jodhpur, Pune, Kolkata, Shillong, and Leh, covering all climatic conditions in India. This model is used to obtain hourly values of solar illuminance for both global and diffuse components, in a typical meteorological year for each of the six stations. Analysis also shows that Indian climates have clear or intermittent skies for 60%–80% of day-time in a typical year, revealing immense potential for daylight exploitation for energy conservation. Solar global illuminance level assumes the highest value in summer months of April and May, whereas it becomes lowest in winter month of December. In monsoon season, solar illuminance is around 60–65 klx. It is also found that cumulative frequencies for global illuminance increases linearly with solar illuminance, in the range of 0–100 klx. Cumulative frequency for diffuse illuminance increases exponentially with solar illuminance, in the range of 0–60 klx.

A simple model for the derivation of illuminance values from global solar radiation data

This paper presents a new simple luminous efficacy model for global horizontal irradiance. The objective is to derive values of outdoor global horizontal illuminance data from typical local weather station data including global horizontal irradiance and Humidity Ratio of outdoor air. The proposed luminous efficacy model incorporates, as the main influencing variable, the Clearness Factor, which is an original derivative from the Clearness Index. Two further variables are included in the model formulation. These are the Humidity Ratio and the solar altitude. Moreover, the model includes a location-dependent variable, which may be derived from the latitude information. The paper includes the result of the statistical analysis of the relationship between the model predictions and the measured data. The results of this analysis display a good agreement between predictions and measurements.

Modeling luminous efficacy of daylight for Yongin, South Korea

This paper aims to evaluate the global and diffuse luminous efficacy based on the measured horizontal daylight illuminance and on solar irradiance data and to propose a luminous efficacy model. Thus, the daylight illuminance and irradiance were measured for two years from August 2010 to July 2012 in Yongin, South Korea. In this paper, the luminous efficacy models of Perez, Muneer, Littlefair, Robledo & Soler and Chung are compared with the luminous efficacy of daylight data in Korea. In addition, a global and diffuse luminous efficacy model is proposed using the solar altitude, sky conditions, relative optical air mass, atmospheric clearness parameter, and turbidity coefficient. The performance of all models was assessed two common statistical parameters: the relative root mean square error (RMSE) and the relative mean bias error (MBE). The yearly average global and diffuse luminous efficacy values were 115lm/W and 133lm/W. Also, the proposed model showed the lowest mean bias error (MBE) and relative root mean square error (RMSE) compared to the other models. The luminous efficacy of daylight model under real sky conditions proposed in the study could provide highly reliable, accurate values to researchers and engineers when compared to those estimated by the previous model.

Diffuse illuminance availability on horizontal and vertical surfaces at Madrid, Spain

The overall objective of this work is to provide diffuse illuminance availability at Madrid (Spain) through a statistical analysis of illuminance values corresponding to a long-term data series. The illuminance values are obtained from irradiance measurements by means of different empirical models for luminous efficacy. The values of diffuse illuminance on a horizontal and on vertical surfaces facing the four cardinal points are estimated and the different aspects related to daylight availability in an area with specific climatic conditions are analyzed. The experimental data consist of global and diffuse irradiance measurements on a horizontal surface provided by the National Meteorological Agency in Spain (AEMET) for Madrid. These data consist of hourly values measured in the period of 1980–2005. The statistical results derived correspond to a daylight typical year for the five surfaces considered. This information will be useful to building experts to estimate natural illumination availability when daylighting techniques are applied in building design with the main aim of electric energy savings.

A comparison of luminous efficacy models based on data from Vienna, Austria

To support building design in view of daylight quality, computational (simulation) tools can provide effective support. To perform detailed daylight analysis via simulation, appropriate sky models are needed. In the past, various sky luminance distribution models have been developed. Such models, however, require illuminance data for the relevant location. As measured external illuminance levels are not available for many locations, the more widely available irradiance measurements can be translated, using proper luminance efficacy functions, into illuminance values. The present paper compares five global luminous efficacy models based on a database of measured illuminance and irradiance data from Vienna, Austria. These models typically involve mathematical formulations with multiple coefficients, whose values are derived for a specific location. The results suggest that these models performed rather poorly, once tested against Vienna data. However, the models’ performance improved significantly, once the respective coefficients were modified (calibrated) using the Viennese database.