Correction of the Uneven Brightness Coefficient for the Tropical Sky Conditions

Abstract

In the design of buildings and structures, the calculation of natural lighting plays an important role in providing visual comfort, rational use of the daylight resource and achieving energy efficiency. Calculation methods and parameters are improved in the research process to imitate/simulate as much reality as possible. Until now, the overcast sky CIE is mainly used in the daylight calculations as the most unfavourable condition. This sky condition is characteristic of a long winter period of temperate climate and is not typical of the tropical sky, where diffuse horizontal illuminance values are very high. For these reasons, an update of theoretical studies in the daylighting calculations and design the daylighting systems must be completed. Accordingly, this study proposes modern methods of analysing the firmament luminance distributions to determine the coefficient of uneven brightness when calculating the Daylight Factor for more realistic sky conditions in the tropical climate of Vietnam. For this, the sky types have to be defined according to the locations. Fifteen international standard types of the firmament with their descriptors are provided by Kittler et al. and a technique using a relation of diffuse and total solar illuminance levels named the cloudiness coefficient Ko are considered to confirm the sky type for Hanoi and Ho Chi Minh City. Parameters were found to computing the sky luminance distributions and the coefficient of uneven brightness for Hanoi and Hochiminh City. A comparison of the results shows the differences of using classical Overcast sky condition and the proposed sky conditions. The method offered and verified in this study showed that, it has potential to be used for difference climate area.