Sky model blends for predicting internal illuminance: a comparison founded on the BRE-IDMP dataset

The BRE-IDMP validation dataset contains simultaneous measurements of sky luminance patterns and internal illuminances in two full-size office spaces. This benchmark dataset has been applied previously to test the illuminance predictions from a lighting simulation program under real sky conditions. Sky luminance patterns were mapped into the lighting simulation so that the absolute accuracy of the program could be evaluated without the uncertainties that are introduced when sky models are used. For this follow-on study, the BRE-IDMP dataset is now used to quantify the divergence between the sky model generated luminance patterns and the actually occurring conditions based on the resulting internal daylight illuminances. The internal illuminances were predicted using three ‘narrow-range’ models (CIE overcast, CIE clear and intermediate) and the Perez All-Weather model. Predictions from the narrow-range models were used to investigate formulations for sky model blends. The illuminance effect of arbitrary sky model blends is reproduced in a post-process of the illuminance predictions from the ‘narrow-range’ sky model types. The determination of an optimum sky model blend is described. The findings show that relatively simple blends of just two pure sky models (e.g. CIE overcast and intermediate) may be adequate for the prediction of time-varying illuminances founded on climatic test reference year data.     

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.     

天空亮度分布研究

本文提出了似阴天空亮度分布表达式,经验证表明,它与天空亮度实测结果吻合较好。在本文中还采用一个式子表示了各类天空亮度分布,这对于垂直面照度计算等无疑是方便的。     

An analysis of all-sky zenith luminance data for Hong Kong

Sky luminance distribution data are of great importance to estimate internal daylight illuminance for building energy modelling. Sky luminance of a particular sky patch is always expressed as a ratio of the corresponding zenith luminance. For places where measured zenith luminance is not available, generating the required data from conversion models would be an appropriate alternative. This paper describes the sky luminance measurements made in 1999 at the City University of Hong Kong and reports the findings. A simple model relating zenith luminance and horizontal diffuse illuminance has been proposed. An evaluation of the proposed model and the Perez zenith luminance model has been carried out. It has been found that the proposed model gives reasonably good agreement with the measured data and outperforms the Perez model. The simplicity nature of the proposed model provides a convenient and reliable approach for building designers in estimating the zenith luminance as well as the whole sky luminance distribution.     

天然采光设计中的参考天空分类

天然采光的基础是天空亮度分布,而参考天空分类是确定天空亮度分布的前提。结合IDMP重庆光气候观测站1 min光气候观测数据,通过对影响天空亮度分布的气象参量研究分析,确定容易被肉眼观测识别的云量、日面状况、水平区域到天顶亮度渐变状况等气象参量是影响参考天空分类的关键因素。以此为基础,将参考天空划分为6类天空类型。     

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.     

用模糊数学研究天空类型

本文利用图像数字化处理系统对天空亮度分布进行了定量实测,提出了除下雨等天空特别阴暗的、一般需要人工照明的特殊天气现象外,根据天空亮度分布规律,将天空划分为CIE晴天空、似晴天空、似阴天空和CIE全阴天空,并由模糊数学证明这种天空分类是合理的。     

Lowering the outdoor temperature in high-rise high-density residential developments of coastal Hong Kong: The vegetation influence

This paper investigates the impact of on-site variables on the influence of vegetation in lowering outdoor temperature. The study uses six critical on-site variables such as surface albedo, sky view factor, altitude, shrub cover, tree cover and average height to floor area ratio to carry out analysis in five data sets. The climatic parameters and physical characteristics were measured and surveyed, respectively in 216 stations in high-rise high-density residential developments of coastal Hong Kong. The data set is mainly categorised into peak summer clear sky days (PSCS-days), peak summer partially cloudy days (PSPC-days) and late summer days to address the issues related to cloudy sky conditions in Hong Kong. The trend analysis shows that at a lower sky view factor, ranging from 0.1 to 0.25, shrub cover as low as 10–15% in a 1000 m² open area is more effective in reducing outdoor temperature than the same level of tree cover in a location with a higher sky view factor (0.4 or above). The regression models used in this study were able to explain the influence of on-site variables on vegetation in lowering the outdoor temperature within the respective urban settings. In high-rise high-density environments, on-site variables such as sky view factor and altitude have a substantial impact on the influence of vegetation in lowering outdoor temperature. However, off-site variables such as high urban density and anthropogenic heat could negate the behaviour of sky view factor and altitude. The study suggests that increasing the tree cover from 25% to 40% in the pocket parks in coastal area residential developments of Hong Kong could reduce daytime urban heat island intensity (UHI) by further 0.5 °C.     

A dynamic sky recognition method for use in energy efficient lighting design based on CIE standard general skies

This paper innovatively proposes an “active green lighting model” for control and optimization of building luminous environment. It is expected to optimize the utilization of daylight dynamically in energy efficient lighting design from the viewpoint of intelligent control. As one primary work of implementing the overall model, the luminance distribution of CIE standard general Skies is addressed, and a dynamic sky recognition method for utilization optimization of daylight is proposed based on swarm intelligence. The experiment results of instance field data show the proposed intelligent sky recognition method is feasible and effective.     

天空类型分维研究

天顶亮度转换系数是反映天空亮度分布变化特征的参数，它与地域无关。由天顶亮度转换系数变化过程的分维值证实，除特殊天气现象外，可把天空分成ＣＩＥ标准晴天空，似晴天空，似阴天空和ＣＩＥ标准阴天空４种天空类型、这对于研究天空亮度分布是合理的。     

三种标准天空分量叠加的天空亮度模型构建

天空亮度分布是研究建筑采光的基本资料。研究建筑动态采光要求量化年周期内天空亮度动态分布,本课题提出了通过三种标准天空亮度分布模型的分量进行叠加以近似拟合某时刻天空实际亮度分布的构想,并以此为基础建立动态天空亮度模型。本文主要介绍了三种标准天空确立的研究方法、分量叠加方式及该模型校验测试方式。     

A simplified procedure for determining indoor daylight illuminance using daylight coefficient concept

The first step in evaluating the visual performance and energy efficiency provided by daylight requires an accurate estimation of the amount of daylight entering a building. The actual daylight illuminance of a room is mainly influenced by the luminance levels and patterns of the sky in the direction of view of the window at that time. The daylight coefficient concept, which considers the changes in the luminance of the sky elements, offers a more effective way of computing indoor daylight illuminances. Recently, Kittler et al. have proposed a new range of 15 standard sky luminance distributions including the CIE (International Commission on Illumination) standard clear sky. Lately, these 15 sky luminance models have been adopted as the CIE Standard General Skies. This paper presents a graphical method to calculate interior illuminance for the CIE standard clear sky using the daylight coefficient approach. The simplified techniques in the form of a nomograph and Waldram diagram were established and described. The performance of the proposed approach was evaluated against the results obtained by an independent calculation approach and a computer simulation program. It was shown that the daylight illuminances estimated by our graphical tool were in reasonably good agreement with those produced from the other two methods. The findings provide building professionals and students a reliable and simple alternative that incorporates the daylight coefficient concept to estimate the interior daylight illuminance and assess daylighting performance.     

Computer program for calculating the daylight level in a room

A computer program has been developed that calculates the total quantity of daylight provided to an arbitrary place in a room by direct incident daylight, by reflected daylight from opposite buildings and ground, and by interreflected daylight from walls, ceilings and floors. Input data include the geometry of the room, the reflection coefficients of walls, ceiling, and floor, and the dimensions of the building itself and surrounding buildings.The program calculates daylight levels at all relevant places in the room. The calculation of reflected light is based on view factors and total exchange factors between rectangular surfaces, and we assume that all light is reflected diffusely from all surfaces. Using this approach we can compute the influence of reflectances from surfaces whose colors differ from that of the wall (such as doors).The program contains the formulae describing the distribution of sky luminance for CIE Standard Overcast Sky [1] and CIE Standard Cloudless Sky [2]. For a cloudless sky, the program computes the shadow patterns of the surrounding buildings. It is also possible to calculate the illuminance provided by artificial lighting.     

Verification tests of a mirror box type artificial sky without and with building scale model

A mirror box type artificial sky for simulating the CIE standard overcast sky has been designed and constructed, while the verification method has been developed and tested. Ratio between the mirror height above the workplane and the room length is 60:133 or 0.451, whereas the ratio between the mirror height above the workplane and the room width is 5:9 or 0.556. Taking the CIE standard overcast sky as reference, indoor illuminance ratios at various elevation angles relative to the zenith is found to be more consistent than luminance ratios. The largest error of illuminance and luminance ratios are respectively 10% and 43%, obtained at 0° elevation angle. Horizontal workplane illuminance values are on average 11,400lx, with illuminance uniformity U₀ and U₁ of respectively 0.92 and 0.86. Based on test results with a building scale model, four out of nine measuring points inside the model have small errors, four have medium error, and one has a large error of 25%. Most of the obtained errors are within the tolerable range of ±21% from the ideal values. Based on the conducted tests, the constructed mirror box type artificial sky is considered appropriate to be utilized for its purpose.     

天空散射光照度转换成天顶亮度

本文建议了一种天顶亮度转换系数,它与室外无遮挡水平面上的天空散射光照度相乘,就能获得该天空类型的天顶亮度值。     

夜空公园：演变﹑实践与启示

夜空公园是拥有卓越星空质量和自然环境的地域,具有科学研究、生态保护、科普教育、保护和利用文化遗产,以及公众休闲等作用和意义。梳理了夜空保护活动的发展历程及夜空公园的研究进展与实践现状,分析归纳了国际夜空公园建设的前期准备﹑夜空保护计划﹑夜空公园教育与拓展活动,以及夜空公园的可持续发展等内容,介绍了国内保护区类型和主题公园类型夜空公园的实例及实践困境,提出了夜空公园本土化发展建议。     

Urban design factors influencing heat island intensity in high-rise high-density environments of Hong Kong

Previous research on Urban Heat Island Intensity (UHI) in Hong Kong was limited to 4 weeks of field measurements during the summer in 3 major coastal housing estates. The current study extends this work to 6 months enveloping 3 “seasons” and 7 different locations within the coastal area. Variations in UHI in the range −1.3 ° to 3.4 °C were recorded. The study reveals seasonal changes are in general more influential on UHI than changes due to geographical characteristics. Among the seasonal models, the peak summer clear sky day's daytime and nocturnal models examined separately were found to provide the clearest indicators of the impact of urban design variables on UHI with R²value of 0.7 and 0.8, respectively. Sky view factor, surface albedo, altitude, vegetation above 1 m in height, average height to floor area ratio, location quotient and proximity to sea are critical variables in mitigating both daytime and nocturnal UHI. Combining daytime and nocturnal data dilutes the impact of extreme values on UHI, by up to 50%, and is not useful for design solutions. Ideally design solutions may respond to the adverse impacts recorded in peak summer clear sky daytime at the concept design stage, and then have the design manipulated and validated for other seasons.     

多病床非典病房的气流分布研究

到2003年8月7日止，在全球34个国家和地区相继出现了严重急性呼吸系统综合症(SARS或者非典)，其中，共报告有8422例疑似案例，916例死亡案例。2002年11月到2003年6月间的非典流行中，医院医护人员受感染情况最为严重，世界范围内20％的感染群体为医护人员，香港地区22％的确诊案例为医护人员。因此，为减少交叉感染，加强有关工程控制，特别是在非典病房里进行通风设计是非常必要的。本文总结了非典病房气流分布的最新研究。这项研究由非典特工队于2003年4～7月间完成。非典特工队是由香港工程师协会组织，来自7个专家协会的工程师组成的研究小组。在对香港治疗非典医院的现有空调系统进行了解后，非典特工队首先运用计算流体力学模拟确定了在6个床位的病房里减少交叉感染和改善污染物稀释的通风设计；接着，对新设计在香港大学屋宇设备实验室里的全尺寸非典病房里进行了测试。测试表明，新设计方案在这个接近现实的全尺寸非典病房里运行良好。采用床头回风口回风设计可有效地对病人产生的含病毒颗粒进行局部捕获。根据这项研究，作者提出了原则性的设计建议。2003年里，香港政府和医院管理局医院采纳了非典特工队提出的设计基本原理建造了1200多个床位的可用于非典治疗的新病房。另外，气流分布是复杂的紊流过程，送风格栅结构如导流器或散流器的微小改动以及送风参数如风速、温度和风向的微小变化都会引起气流的改变。适当的设计对于减少病人间、病人与医护人员之间的交叉感染以及有效稀释和排除含病菌颗粒都是至关重要的。     

阴天空与最大亮度元

利用大气光学和几何光学的成果，讨论了阴天天空亮度与粒子散射和大气质量的关系，从微观机理上说明了阴天空最大亮度元的存在性，并得到了有趣的结果。