Chris C.S. Lau

A Study of 15 Sky Luminance Patterns against Hong Kong Data

A good knowledge of the sky luminance distribution is essential for efficient day lighting designs and developments. This paper studies a set of 15 standard skies established by Kittler, Perez and Darula using the measured sky luminance data from subtropical Hong Kong. A statistical analysis of the results has shown that the standard sky set is adequate to cover the various sky conditions. A reduction of the standard set to five skies is sufficient to model the luminance distributions. The ratio of zenith luminance to horizontal diffuse illuminance (L/zD) which can be used to define homogeneous sky standards was analysed based on these five selected skies. It has been shown that the trend of L/zDv ratios under different sky types followed the ranges described by Kittler et al.

A Study of Solar Radiation Daylight Illuminance and Sky Luminance Data Measurements for Hong Kong

Solar radiation, outdoor illuminance and sky luminance data are important for accurate cooling bad determinations and daylighting evaluations. Systematic data measurements are regarded as the most effective and accurate method of settingup reliable databases. This paper describes solar data measurements made at the City University of Hong Kong and reports some of the general findings. The daily and hourly average values of solar radiation and daylight illuminance are computed and presented using graphical methods. Sky luminance distributions for different typical sky conditions including overcast, clear and intermediate are studied and described. Implications for energy-efficient building designs are discussed

A new approach for predicting vertical global solar irradiance

Solar irradiance, particularly on vertical surfaces, plays a major role in determining the thermal and energy performance of a building. It is important to the design and analysis of both active solar systems and passive solar buildings. Many mathematical models are mainly developed to predict the sky-diffuse irradiance on inclined surfaces from the measured horizontal diffuse component. This paper presents an approach to estimate the vertical global irradiance based on direct beam and ground-reflected components which can be accurately determined. Hourly data recorded from January 1996 to December 1998 in Hong Kong were used for the model development. The performance of the proposed model and two well-known anisotropic inclined surface models (Muneer and Perez) was evaluated against data measured in 1999. Statistical analysis indicated that the proposed model gives reasonably good agreement with measured data for all vertical planes. Although the new model has been found less effective than the Perez model, its simplicity nature provides buildings designers a convenient and reliable alternative in the estimation of vertical solar irradiance.

Standard Skies Classification Using Common Climatic Parameters

A good understanding of the sky luminance distribution is essential for efficient daylight-ing designs and developments. Sky luminance patterns are always modeled under various sky conditions using climatic parameters as weighting factors to indicate the degree of sky clearness. This paper presents the work on the identification of sky luminance patterns using three common climatic elements, namely cloud cover (CLD), sunshine hour (SH) and clearness index (K t ). Three sky types representing overcast, partly cloudy and clear were selected from the Kittler et al. 15 standard skies. It has been shown that sky conditions classified by individual sky indicators produced acceptable agreements with the measured data. In general, CLD shows the best results, followed by K t , then SH. Better results in terms of the frequency of occurrence and root mean square errors can be found when CLD and SH are jointly utilised to describe the sky conditions.

Evaluation of overcast-sky luminance models against measured Hong Kong data

Sky luminance distribution is one of the most important quantities for predicting indoor daylight illuminance levels. Overcast-sky types are essential because they are used in more general sky models and appear quite frequent in some places. This paper presents the work on the evaluation of six worldwide overcast-sky models against two-year (1999-2000) measured Hong Kong sky luminance data. Overcast-sky conditions were identified using cloud cover (CLD) and a subsequent interpretation the overcast skies into thin and heavy overcast types was conducted in conjunction with the clearness index (Kt). A statistical analysis of the models has indicated that the International Commission on Illumination (CIE) standard overcast sky model performed the best, in particular for the heavy overcast-sky condition. The Building Research Establishment (BRE) quasi-overcast-sky model showed a good agreement with the thin overcast distributions which may include a circumstance component and the sky luminance patterns being orientation dependent.