Formulation of Daylight Factor Based Metrics Under All Sky Conditions for Building Daylighting Design


Student thesis: Doctoral Thesis

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Awarding Institution
Award date19 Sept 2022


Daylighting is an important sustainable development strategy for building assessment. The calculation of the daylight illuminance for a given position in a building is a key step in daylighting schemes. In order to predict the daylight performance of room space and set a scale for architects to use when comparing aspects of various daylighting schemes, there are a number of rules of thumb and design metrics. The traditional method for evaluating daylighting performance in a building is the daylight factor (DF) approach, which is assumed under the conventional overcast sky without sunlight. However, such metric is not flexible enough to predict dynamic daylight variations caused by the movement of the sun when the sky is clear. Climate-based daylight metric (CBDM) which considers the real climatic data for the location has been adopted to evaluate the dynamic daylight performance. The usual method to calculate the CBDM is full scale computer simulations. However, computer simulations are quite time demanding and designated skills are required. Architects and building practitioners prefer simple method to assess daylight performance particularly during early design stage when various building schemes and concepts are being appraised.

Recently, daylight factor calculations have been extended for all sky conditions. It means that daylight factor based metric (DFBM) can be a dynamic metric and it can take into account the building directions, solar positions and the effects of direct and reflected sunlight. The DFBM covers different forms of DF such as point daylight factor (PDF), average daylight factor (ADF) and vertical daylight factor (VDF). The main advantage is that different forms of daylight factor are strongly correlated with the external environments, building orientations and room parameters including window area and visual transmittance. It could be easily to vary these parameters to meet the criteria, particularly during the initial design stage when different building options and design schemes are analyzed and evaluated. After the preliminary design has been made, sophisticated lighting simulations can be conducted to get more accurate results. Spot checks of the calculated results with the simulated findings can be carried out. As such the findings and the objectives of the proposed research will provide more comprehensive studies on these daylighting aspects which could be globally adopted and useful to practitioners engaged in visual comfort, fenestration and daylighting design and evaluation.

The first part reviews the DFBM and gives an overview of the current daylight metrics. And then, daylight performance of high-rise building is analyzed. Two calculation approaches namely the configuration factor approach (CF) and the Nomograph and Waldram diagram approach (NWD) are proposed to predict sky component (SC) and externally reflected component (ERC) at low floors where are heavily blocked by surrounding buildings. The influences of external shading devices on daylighting performance at top floors of high-rise buildings facing unobstructed skies are also conducted.

The second part firstly proposed the simple correlations between DFs under unobstructed 15 CIE Standard Skies. The daylight glare index (DGI) and uniformity (Uo) under various sky conditions and times of a day are also computed. Subsequently, the correlations between DFs under obstructed skies are also elaborated. It is quickly to get the required DFs from the proposed models under complex obstructed conditions.

The third part finds the correlations between the CBDM and DFBM based on different case studies. A series of simple mathematical models were established based on the computed results in order to predict various DAs and UDI from the traditional DFs (Sky 1). The results showed that DAs and UDI can be computed by using the DFs of a particular point with high accuracy. In addition, various DAs and UDI can be obtained by DF approaches and modelled CIE Standard Skies including the Hong Kong representative sky. The findings could be applied where the data is not easily measured.

The last part analyses the electric lighting energy reductions and switching frequency of an atrium building under different daylight-linked lighting controls based on measured daylight illuminance data in HKUSPACE. The correlations between DA and standard on-off control, DAcon and dimming control also be estimated according to measured data. The results show that the performance of individual daylight-linked lighting controls strongly replies on the interior daylight availability which is important for architectural and daylighting designs.

    Research areas

  • point daylight factor, average daylight factor, vertical daylight factor, sky component, externally reflected component, CIE Standard Skies, visual comfort, dynamic daylight metrics, useful daylight illuminance, daylight autonomy, daylight-linked lighting controls, external shading devices, building daylighting design