|Title:||Study of optimal commercial building envelope design in cooling-dominant climates|
|Subject:||Exterior walls -- Design and construction.|
Commercial buildings -- Energy conservation.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Building Services Engineering|
|Pages:||xxi, 233 leaves : ill. ; 30 cm.|
|Abstract:||Energy consumed by buildings is an important part of total energy consumption. Energy-efficient building design and retrofitting have attracted the attention of building professionals all over the world. Heat transfer through the building envelope is a major factor that affects the air-conditioning energy consumption of a building. It is believed that a well-designed building envelope could significantly contribute to the energy efficiency of a building. Due to the relatively complex climate factors, building envelope designs in different areas are quite different. The primary aim of this study is to develop a comprehensive evaluation of the energy properties of common building envelope designs for commercial buildings in cooling-dominant climates, so as to provide a practical reference for energy-efficient building design and retrofitting. The evaluation takes into consideration both thermal and daylighting performance. A series of simulation works were conducted. Insulation against wall area, transparent insulation against window areas as well as applications of shading devices were investigated and discussed. Four cities with different latitudes, namely Singapore, Hong Kong, Houston and Maimi were selected. The impact of envelope orientation and building type were also considered. In cooling-dominant climates, heat gain through window areas is mainly caused by the transmission of solar radiation, thus reducing unwanted solar radiation is a promising way to reduce energy consumption. First, an experiment was conducted to study the actual thermal performance of the interior blind in Hong Kong. An office room facing east was selected. During a clear sunny day when the tilt angle of the blind louver stayed at 90°, heat gain through the envelope was reduced by around 9%. Secondly, an EnergyPlus simulation based on the same office room was also finished. A gerenally satisfactory agreement was achieved between the experiment and the simulation results. Thirdly, the two most popular shading designs, namely the interior blind and the overhang on an office building were investigated. A series of simulation tests were carried out to further analyze the thermal performance of the interior blind and the overhang shading designs. The results show that with the application of the internal blind, annual solar radiation heat gain through the window areas can be reduced by at least 20%. With the application of an overhang, a reduction of over 40% can be expected. The orientations and geographic locations have a significant influence on the devices' shading effects. Shading devices on the east and west facades reduce the most solar heat gain, while devices on the north facade reduce the least. In cooling-dominant climates, the contribution of the north facing shading device cannot be ignored. As the latitude rises, interior and exterior shading devices on the north facade made a smaller difference. Occupants' behaviors could significantly affect the performance of the blind. Setting the slat angle at 90° could only reduce window heat gain by about 15% to 20%, while setting the slat angle at 30° could achieve a reduction of around 40% to 50%. It was also discovered that a further increase of overhang depth does not affect the performance of the overhang significantly when the overhang depth exceeds half of the window's height. A series of simulation studies were also carried out to research the practicality of different advanced glazing materials in cooling-dominant climates. Single-layer glass with different thicknesses, double-layer glass with different gas layers and low-e glass with different thermal parameters were considered. Results show that when the thickness of a single-layer glass changes from 6mm to 12mm, the thermal performance of the single-layer glass remains almost unchanged. Double-layer glass could reduce heat gain through window areas by 15%~20%. Due to its low solar transmittance, low-e glass performs better than double-layer glass. When low-e glass with a solar transmittance of 0.59 is used, solar heat gain could be cut by around 40%. If low-e glass with a lower solar transmittance is selected, an even larger reduction can be expected. Besides, orientation would affect the performance of glazing materials significantly. Advanced glazing materials applied on the east and west window areas could reduce more solar heat gain by around 20% than those on the south and north window areas. In order to obtain a comprehensive evaluation on the overall energy performance of energy-efficient building envelope designs, daylighting simulation work was also carried out to study the lighting energy reduction when a daylighting strategy was applied to commercial buildings, and based on the same building model for the thermal simulation. Results indicate that energy savings from daylighting on low-e glazing and double-layer glazing are almost the same on all orientations (around 1.7% in office buildings, 0.8% in hotel buildings). Considering its better thermal insulation effect, low-e glazing is generally the best choice among all glazing materials. Daylighting performance of overhang shading is not different to that of glazing materials. With respect to the interior blind, lighting energy saving is significantly lower (0.7% to 1.6% in office buildings). Orientation and geographic locations also have clear affects. All the envelope designs perform better on the east and west orientations. In areas with low latitude, the difference among orientations is significant, and energy-efficient designs on east and west facades can reduce solar heat gain by more than 50% than those on the south and north facades. As latitude rises, the difference is only around 10%.|
For the assessment of energy-efficient retrofitting projects, apart from the annual saving of actual values and economic payback time, energy and CO₂ emission payback periods should also be taken as indispensable references. A case study involving a life-cycle and pay-back period analysis of the energy and CO₂ emission was conducted for the addition of external overhang shading as energy efficienct retrofitting in a university campus in Hong Kong. Results indicate that although introducing an overhang shading system could reduce almost half of the cooling load in the related area, the energy and CO₂ emission payback periods of the project were still unrealistically long due to the requirements of structural strength under typhoon situations. This case study presents an example of a multi-disciplinary approach being not only important to energy-efficient retrofitting projects but also necessary for policy making in different climatic and geographic regions. Finally, another series of simulation work was also practised to investigate the performance of the two most popular insulation methods on wall surfaces, namely thermal insulation and high-reflectivity coatings on commercial building walls in cooling-dominant climates. Results show that in cooling-dominant climates, the implement of thermal insulation on wall areas could receive an at least 80% reduction in solar heat gain through external walls. The effects of insulation measures vary with the wall orientations. Insulation on the east and west walls can receive around 10% more reduction than that on the south wall. Wall insulation on the north orientation could only achieve an about 50% profit compared with those on the east and west orientations. In climates with little diurnal temperature change in the summer season when air-conditioning systems operate during the daytime only, internal insulation performs 20% better than external insulation in reducing the air-conditioning load, whereas in climates in which there is a free cooling period during the night, a 20% more reduction can be expected from external insulation. In conclusion, in this thesis, a comprehensive study was conducted to assess the thermal and daylighting performance of popular energy-efficient building envelope designs, so that an optimal envelope design can be achieved for commercial buildings in cooling-dominant climates. The result arising from the thesis further emphasizes the significance of building envelope design in cooling-dominant climates. In cooling-dominant climates, energy-efficient building envelope designs on the east and west orientations can achieve a better profit than those on the north and south orientations. In low-latitude area, the difference of performances among envelope designs on the different orientations is quite signifincat. As the latitude rises, the difference become smaller. Internal shading device is also a practical shading alternative. In many cases, internal shading can perform as well as external shading. The adjustability of internal shading enables occupants to retain a more comfortable thermal and visual environment, which cannot be achieved by external shading. Especially in the area like Hong Kong where typhoon strikes a lot in summer, internal shading is prefered. Appropriate insulation on external wall can also achieve a better thermal performance in cooling-dominant climates.
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