Model based evaluation of exterior daylight illuminance distribution

Energy conservation in buildings is greatly influenced by natural daylight in tropical region. A vital step towards development and promotion of daylighting technology in buildings is a prior study on estimation of exterior daylight availability for illuminating its interiors. In view of this, the present communication depicts the preliminary work progress carried out to arrive at a comprehensive idea on assessment of daylight availability and its characteristics. The study is reported taking the representative case of Bangalore (India) (latitude 12.97° N, longitude 77.56° E),which in future work facilitates to device suitable interior illuminance models and lighting controls for a daylight-artificial light integrated scheme. The objective of this paper is to document the estimation of spatial exterior daylight parameters like exterior horizontal as well as vertical global and diffuse daylight illuminance in addition to their respective luminous efficacies computed using an established analytical model. The paper also highlights user friendly computer simulation tool developed, for the detailed estimation of daylight availability at a particular region by the inclusion of corresponding solar radiation data.     

用太阳辐射转化法计算室外照度值

针对中国光气候数据资料不足、太阳辐射数据丰富以及资金有限的现状,在分析多种光气候数据采集方法的基础上,选用太阳辐射转化法获得光气候数据.引入发光功效的概念作为室外照度值和太阳辐射量转化的基本关系,并分析对比了已有的发光功效模型,运用天空晴朗指数,以重庆1991-1992年的实测数据为基础进行拟合,建立了总发光功效模型和...     

An analysis of climatic parameters and sky condition classification

In modelling solar radiation, of the required data daylight illuminance, luminous efficacy, radiance and luminance sky distributions, are always analysed under various sky types. Sky conditions are frequently categorised into overcast, partly cloudy and clear using some common climatic data including cloud cover (CLD), sunshine hour (SH) and solar radiation. This paper presents the study of these climatic parameters and their applications in the classification of sky condition. Frequency of occurrence and cumulative frequency distribution of each sky indicator have been established to interpret the prevailing sky conditions in Hong Kong. Special features on defining the sky clearness are highlighted and the implications for energy-efficient building designs are discussed.     

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.     

Zenith luminance and sky luminance distributions for daylighting calculations

We derive an equation for zenith luminance as a function of turbidity and solar altitude based on analysis of large quantities of luminance and illuminance data measured in San Francisco, California, between September 1979 and August 1982. Using only average turbidity values to predict hourly zenith luminance as a function of solar altitude can produce large errors. We compare the equation derived from our data, which is valid over a wide range of turbidity and solar altitude, to other published models. We also compare the relationship between horizontal illuminance and zenith luminance from the clear sky and conclude that, when ideal clear days are compared, this relationship is similar to earlier work based on measurements in European climates. Finally, we compare our sky luminance distribution measurements to previous published luminance distributions using the diffusion indicatrix. Our results are intended to help improve daylight availability prediction techniques and define additional requirements for data collection.     

Useful daylight illuminances: A replacement for daylight factors

This paper describes the application of a new paradigm, called useful daylight illuminance (UDI), to assess daylight in buildings. The UDI paradigm is designed to aid the interpretation of climate-based analyses of daylight illuminance levels that are founded on hourly meteorological data for a period of a full year. Unlike the conventional daylight factor approach, a climate-based analysis employs realistic, time-varying sky and sun conditions and predicts hourly levels of absolute daylight illuminance. The conventional approach produces a single number – the daylight factor as a percentage – for each evaluation point in the space. In contrast, a climate-based analysis results in an illuminance prediction for every daylight hour of the year for each point considered. The UDI paradigm offers a way to reduce the voluminous time-series data to a form that is of comparative interpretative simplicity to the daylight factor method, but which nevertheless preserves a great deal of the significant information content of the illuminance time-series. The UDI paradigm informs not only on useful levels of daylight illuminance, but also on the propensity for excessive levels of daylight that are associated with occupant discomfort and unwanted solar gain. In a conventional analysis of daylight provision and solar penetration, the two phenomena are assessed independently using methods that are idealised (daylight factor) and qualitative (shadow patterns). The UDI paradigm offers a simple methodology whereby daylight provision and levels of solar exposure are quantified using a single evaluative schema. Thus, it is also well-suited for teaching purposes. Application of the UDI paradigm is demonstrated using an analysis of design variants for a deep-plan building with a light-well. Comparison is made with the conventional daylight factor approach, the LEED daylight credit and measures of daylight autonomy.     

Classification of daylight and radiation data into three sky conditions by cloud ratio and sunshine duration

An analysis of daylight and solar radiation data in low latitude/tropics area has been done. This analysis classified three sky conditions: clear sky, intermediate sky and overcast sky. There are two methods have been used to classify these sky conditions. Firstly, cloud ratio method that is the ratio of the diffuse illuminance or irradiance to the global illuminance or irradiance. This method based on data measured at the IDMP Station at Hasanuddin University during 1995–2000. Another method is sunshine duration, which is based on the data measured at meteorological observatory in Makassar during 1995–2000. This study aims to investigate the accuracy of daylight and solar radiation data gathered during 1995–2000 by comparing data measured from two different sources above. The results of both methods are relatively similar, which indicate that daylight and solar radiation data gathered in Makassar are accurate and useful for various daylight designs and applications.     

根据光气候数据确定隧道洞外景物亮度的方法

具体某地区隧道洞外景物亮度参考值的确定一直缺乏方便可靠的方法。为了解决这一问题,根据隧道照明的实际需求,以重庆地区一年的光气候实测数据为依据,首先以垂直面照度映射洞外景物亮度,分析它的各影响因素,然后建立了不同朝向垂直面转换发光功效和晴朗指数及太阳高度角的数学模型。经过与重庆地区其他年份实测数据的对比,证明该模型具有较高的精度。基于光气候数据中长期连续的水平太阳辐射及天气状况的观测,该方法可快速且相对精确地确定洞外景物亮度参考值。     

北京天然光总照度变化趋势分析

基于北京连续54年太阳辐射数据,结合中国辐射光当量模型,获取北京天然光年总照度值和月总照度值。应用Mann—Kendall检验方法分析北京天然光年和月总照度值年际变化趋势,结果表明：北京天然光年总照度基本变化趋势为明显减少,且在1977年突变发生后其下降趋势明显增加;各月月总照度值基本年际变化趋势为明显减少,其中7月月总照度值下降幅度最大。11月月总照度值下降幅度最小;除5月在1967年发生突变外,其它月份的突变均发生在1975—1981年之间;各月天然光月总照度值年际变化在突变前为不明显波动变化趋势,突变后均为明显增加的下降趋势。云量及由大气污染所产生的气溶胶光学厚度是造成北京年与月总照度值持续下降的重要因素。     

Daylight availability data for San Francisco

This paper analyzes solar radiation and daylight measurements taken during a four-year period in San Francisco, California. Horizontal and vertical surface measurements were taken by nine sensors at 15-minute intervals under all sun and sky conditions. The data base from which results were derived exceeds 400 000 measurements.Equations are derived for clear sky global, direct, and diffuse illuminance and irradiance on a horizontal surface as a function of solar altitude, and for overcast sky horizontal illuminance and irradiance. We present the standard deviations for all parameters in our equations to show the scatter in our data. The average illuminance on horizontal and vertical surfaces by hour and by month are presented as isolux contour plots. These data are also displayed as probability distributions, showing the percent of time in a year that a given irradiance or illuminance value will be exceeded. Monthly average values of sunshine probability are determined and compared to long-term NOAA data.     

A design tool for predicting the performances of light pipes

Light pipes are simple means of directing daylight (diffuse and direct light) into interior spaces. Previous work by the authors described the initial work on a luminous flux and illuminance predictive model for straight light pipes, using a basic equation for illuminance distribution as a function of horizontal distance. Further work has now produced a model that uses the cosine law of illuminance to describe the distribution of light from the light pipe diffuser as well as takes into account pipe elbow pieces or bends. The resulting illuminance model can be described as a quartic cosine model. By producing a “luxplot” prediction for any given light pipe application, it is possible to maximise the potential of these daylight providers and design their configuration to suit any given need. As part of this study, wide-ranging illuminance and luminous flux data were collected both for the formulation of this model (as the formula is semi-empirical) and its validation.     

Overcast sky conditions and luminance distribution in Hong Kong

In studying solar energy and daylight availability, particularly the sky luminance distributions, the required data are always analysed under various sky types which are categorised by some climatic parameters. Overcast days are important because they are used in more general sky models and appear quite frequent in some places. This paper studies some common climatic parameters including cloud cover, sunshine hour, solar irradiance, daylight illuminance and sky luminance and their applications in the classification of sky conditions. Frequency of occurrence of each parameter under overcast skies in Hong Kong has been established. It has been found that no single parameter can effectively describe the fully overcast skies and hybrid indices should be adopted. The general luminance distribution formula for the overcast sky with various luminance distribution parameter values has been used to test the selected sky luminance databases. The analysis indicates that the CIE standard overcast sky shows a good agreement with the overcast sky luminance distributions obtained in Hong Kong.     

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.     

Analysis of atmospheric turbidity for daylight calculations

A large set of illuminance and irradiance data has been collected for four years at 15-minute intervals in San Francisco. This data set has been used to investigate the impact of atmospheric turbidity on daylight calculations. Existing predictive formulae for Linke turbidity, $\text{T}{}_{\mathrm{<mtext>l</mtext>}}$, provide moderate agreement to measured values of $\text{T}{}_{\mathrm{<mtext>l</mtext>}}$ when using nominal design values for the Ångström scattering coefficient, β, and precipitable water vapor, w. When average measured values for β and w are used, the agreement improves. We suggest the use of an illuminance turbidity, T_il, to calculate direct normal illuminance directly. We derive a simple approximate solution, T_il = 1 + 21.6 β. T_il appears to be a better parameter to describe atmospheric conditions since, unlike $\text{T}{}_{\mathrm{<mtext>l</mtext>}}$, it is insensitive to air mass and thus solar altitude or time of day. We present and compare plots of T_il and $\text{T}{}_{\mathrm{<mtext>l</mtext>}}$ vs. solar altitude, time of day, and month. Finally, we examine and compare several alternative pathways to derive direct normal illuminance from irradiance and luminous efficacy (dependent on β and w), or directly from β.     

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.     

An analysis of measured and simulated daylight illuminance and lighting savings in a daylit corridor

Lighting control integrated with daylighting is recognised as an important and useful strategy in energy-efficient building designs and operations. Prediction of the internal daylight levels is a key stage in daylighting designs. With the advances in computer technology, the computation of daylight illuminances can be conducted via lighting simulation program. This paper presents a study of the daylight coefficient (DC) approach using RADIANCE lighting software in simulating the indoor daylight illuminance of a corridor. The interior daylight illuminance data measured in the corridor were compared with the simulated results based on the computer software. It was found that the DC approach could give satisfactory results especially for the sun-shaded surface and sun-facing surface receiving a large amount of direct sunlight. Further, the daylight illuminance detected by the photosensor was also simulated in conjunction with measured daylight illuminance, dimming ratio and electric lighting power to predict the lighting energy savings. The findings suggested that the measured and predicted data showed a good agreement when large electric lighting savings resulted. The probable reasons causing the discrepancies were discussed.     

The energy performance of the Central Sunlighting System

This article presents a simulation study to predict the energy performance of the Central Sunlighting System (CSS) installed in open-plan offices. Several simulation tools are combined to conduct the simulations. SkyVision calculates the daylight luminous flux and the lighting and solar heat gains of the CSS. A set of coefficients pre-calculated using Radiance relates the desktop illuminances to the CSS luminous fluxes. DaySim is used to compute the daylight illuminance from the perimeter windows. ESP-r is used to compute the heating and cooling energy use of the office spaces. The results show that the CSS may save a significant amount of energy in North American climates. Energy savings from the combination of daylighting from windows and the CSS for typical, four-cubicle, open-plan offices range from 44% to 57% for lighting, 8% for cooling and from 14% to 23% for the total (lighting, cooling and heating) energy.     

Estimation of total equivalent temperature difference values for multilayer walls and flat roofs by using periodic solution

A theoretical methodology was developed to find total equivalent temperature difference (TETD) values for multilayer flat roofs and walls of buildings. The TETD values based on time lag and decrement factor are estimated using measured hourly solar radiation on a horizontal surface and outside air temperatures. The time lag and decrement factor are obtained numerically using periodic solution of one-dimensional transient heat transfer problem for the building structures. The problem is initially formulated using a differential equation and the appropriate initial and boundary conditions. It is then converted into dimensionless form and solved by application of complex finite Fourier transform (CFFT). The solution is used to find hourly variation of temperature inside the structure, the heat gain to the building through the structure, the highest and lowest temperatures at the inner and outer surfaces of the roofs and walls, and the time periods involved in reaching these temperatures. Sol-air temperatures for different directions as well as time lag and decrement factor are obtained, and these are used to determine the TETD values of the structures.     

Climatic requirements and their accomplishment by building design illustrated by three dwelling-houses in the Black Forest (Federal Republic of Germany)

A luminous efficacy algorithm is applied to U.S. Weather Service solar irradiance (SOLMET) data to determine direct and diffuse beam illuminances incident upon the horizontal. SOLMET typical meteorological year (TMY) data is searched and the number of hours in a year for which the total horizontal illuminance exceeds a given level is determined for each of 15 selected illuminance levels and for each of several SOLMET cities. Knowing the transmittance of a skylight, and other important characteristics of the application, the number of skylights required to produce a given average interior illuminance level is calculated for each of the 15 exterior illuminance levels. The result is a table of performance (percent of time the desired illumination level is achieved) versus the number of skylights required.     

A computer model for predicting the daylight performance of complex parallel shading systems

In tropical areas where the exclusion of solar heat from buildings is a paramount consideration, shading systems consisting of adjustable parallel multiple slats have evolved which exclude direct solar radiation whilst permitting some daylight penetration. The modelling of daylight illuminance from these systems is not easy using conventional graphical or computational quantitative techniques. This paper sets out a computer-based model for the analysis of the general case of parallel shading systems. The model is illustrated by reference to the Rowshan shading system used in the Hedjazi architecture of Saudi Arabia.