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Title: A multi-criteria daylighting performance assessment method for cellular offices
Authors: Ng, Tsz-ho Roger
Degree: Ph.D.
Issue Date: 2011
Abstract: Daylight is the only natural light source that can provide sufficient illumination in building interiors. Exploitation of daylight to light up indoor spaces has always been an indivisible part of building design through the centuries. Although the importance of daylighting has once been lowered after the invention of electric lights, the interest in the use of this natural light from the sky comes back as a kind of clean energy source from the hundred year reign of electric lighting since climate change reminds people to consume less fossil fuel energy. Daylighting has many advantages: saving electric energy, improving human productivity and enhancing mood and health. It plays an important role in our daily lives. However, it is not an easy task for designers to craft a well performed daylit environment. Many researchers have proposed a number of indicators to ensure the provision of daylight into the room and attempt modelling human preferred indoor daylighting performance, but most of them scantily account for a single criterion focusing on either daylight quantity or daylight quality. There is no doubt that each of them explains a part to the daylighting performance; however, the overall indoor daylighting performance should be evaluated in an ampler measure in terms of occupants' visual satisfaction and electric lighting energy saving potential, which are believed to be affected by a list of decision factors that have to be fulfilled. Previous researchers seldom concern the interrelationships among the daylighting performance criteria and decision factors in their proposed assessment methods, resulting in that there remains a significant knowledge gap on their combined effects giving rise to the criticism of poor daylighting design inside buildings neither satisfying occupant needs and preferences nor effectively saving electric lighting energy. This research study is therefore carried out to develop an inclusive daylighting performance assessment method for buildings taking multiple criteria into consideration. Since an individual can practically have full power to adjust the daylit condition as he prefers in his own cellular office and cellular offices are typically less than 16 m2, this study sets local sidelit cellular offices limited to this floor area as the research target. This thesis is built up on the hypotheses that whether a cellular office environment is of good or poor daylighting performance involves a series of criteria and decision factors, and every office occupant has a similar tendency towards each component of a daylit space. It presents a novel daylighting performance assessment method for local cellular offices using the theory of analytic hierarchy process (AHP). The reason of applying AHP in this study is that it is a multiple criteria decision making technique providing a comprehensive and rational framework particularly suitable for dealing with a problem of complex factors. Because daylighting is the kind of science involving human perception probably the most complicated subject in the world, designing a well performed cellular office is precisely such a problem that is believed to be solvable by AHP. AHP is well known for its use of a hierarchical structure to model the problem. In this study, good daylighting performance was regarded as a goal to be achieved and placed at the top of the AHP hierarchy. Occupants' visual satisfaction and electric lighting energy saving potential were hypothesized to be the two criteria having equal weight of influence in the contribution of the overall office daylighting performance and located at the middle level. Brightness and uniformity on task and surrounding surfaces and perceived glare from the side window are the decision factors that were found correlated with these two criteria with different statistical analyses, and they were allocated at the bottom level of the hierarchy for the study of their individual weights of importance with a series of AHP pairwise comparisons.
In this research study, on one hand, field surveys of physical measurements and user assessment questionnaires were conducted in cellular offices to examine the relationships between each decision factor and quantitative lighting parameters. In this process, a kind of advanced technology named high dynamic range (HDR) photography was applied for the measurement of the luminance distribution within the field of view using a consumer grade digital camera fitted with an ultra wide angle lens in a quick and inexpensive manner. It was believed that the primary success in the application of HDR photography for subjective evaluation of a daylit environment in this research study can lead the future lighting researches to a new technical trend. The study also explains the decision factors in regards of occupants' visual satisfaction and electric lighting energy saving potential with the correlated lighting parameters, with which the probabilities that occupants would feel visually satisfied with their sidelit cellular offices and that they would accept not turning on electric lights in their sidelit cellular offices, expressed as a logistic regression formula, in terms of each decision factor, were then derived. This research study, on the other hand, classifies the hourly sky type of the city in accordance with the official hourly cloud cover data such that the percentages of occurrences of clear, partly cloudy and overcast skies throughout a year were computed for the following investigation. Twelve identical cellular offices but having different window sizes, facing different orientations and with or without external obstructions were modelled respectively under the three sky types by the lighting simulation software DIALux for their hourly daylighting situations such that the multi-influences of these three features inside or out of a cellular office on the overall daylighting performance could be revealed after the numerical values of the correlated lighting parameters accounting for the decision factors in these twelve cellular office simulations were obtained and calculations for the probability of achieving good daylighting performance weighted with the frequencies of occurrence of different sky types taking all the criteria and decision factors with their influential weights into consideration were performed. The result was defined as the multi-criteria daylighting performance indicator of that hour, which in this thesis, is called the daylighting performance index (DPI). The DPI of the working hours on the 21st day of every month throughout a year for the modelled cellular offices were worked out and analyzed. This research study launches a scientific grading scheme for cellular office daylighting performance. It was suggested that a cellular office would be accredited as possessing a certain level of daylighting performance according to the average value of DPI (DPIavg) over a year. For instance, a cellular office would be acclaimed to possess the 5-star daylighting performance if the DPIavg is 4.5 or above while it would be granted 4 stars only if the DPIavg is between 3.5 and 4.5, and so forth. With the application of this newly proposed daylighting performance assessment method, the amount of energy saving for electric lighting systems of cellular offices can be maximized at the same moment without sacrificing the visual satisfaction level of office users. The immediate financial reward in saving lighting energy can also be calculated. This new index offers a more informative performance-based method taking both human factors and energy issues into consideration for the daylighting performance evaluation of cellular offices. Since then, building developers, architects, engineers, lighting designers and environmentalists can be more confident to designing better daylighting performance inside workplaces by applying the DPI grading scheme introduced in this thesis.
Subjects: Daylighting -- Evaluation.
Daylight -- Measurement.
Commercial buildings.
Hong Kong Polytechnic University -- Dissertations
Pages: xxxi, 490 leaves : ill. (some col.) ; 30 cm.
Appears in Collections:Thesis

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