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Title: An experimental and numerical study on improving thermal comfort and energy saving measures in a sleeping environment
Authors: Du, Jing
Degree: Ph.D.
Issue Date: 2018
Abstract: Sleep plays an important role in people’s daily life, since human beings spend approximately one third of their life time in sleep and can benefit much from sleep. However, sleeping quality can be affected by a number of factors. Among those, thermal environment and indoor air quality in a sleeping environment are generally provided and maintained by building air conditioning (A/C) systems. As a result of the increased people’s living standard and long-lasting hot and humid summers in subtropics, more and more people tend to use air conditioning in their bedrooms during sleep. This, therefore, results in a significant increase in building energy consumption. Due to the excellent performances in achieving thermal comfort and indoor air quality control and energy saving, task\ambient air conditioning (TAC) can be best applied to sleeping environments, as a sleeping person occupies a small space and is immobile during sleep. However, all previous TAC systems developed and applied to sleeping environments predominantly were convection-based to cool an occupied zone in a sleeping environment and the vent supplying outdoor air at a high velocity and a low temperature had to be close to a sleeping person, resulting in a severe cold draft problem. It is commonly acknowledged that draft can not only cause thermal discomfort, but also result in temporary illness or even chronic diseases for a sleeping person. To deal with the draft problem encountered in previous convection-based TAC (C-TAC) systems applied to sleeping environments, a new non-convection based TAC systems should be developed. On the other hand, energy for air conditioning a bedroom during sleep periods can also be saved by reducing the nighttime cooling load from external envelopes. The cooling load in a bedroom at nighttime mainly comes from occupants and bedroom envelopes, with the latter being responsible for about three quarters of the total nighttime cooling load. Furthermore, since a bedroom is usually not air conditioned at daytime, a large amount of thermal energy can be stored in the thermal mass in its envelope components. At nighttime, the energy stored in the envelopes would be gradually released, contributing significantly to the total nighttime cooling load in the bedroom, in particularly, during the first two hours of A/C operation.
Therefore, in this Thesis, a systematic research program to develop a non-convection based TAC system applied to a sleeping environment and to evaluate its performance in terms of thermal comfort, indoor air quality and energy efficiency, and to investigate the impacts of daytime heat gain/storage of the external envelopes in a bedroom on its nighttime cooling load and the related mitigation measures, is reported. The Thesis, first of all, begins with presenting an experimental study, as the first part of the systematic research program, on the thermal comfort and ventilation performances of using a radiation-based TAC (R-TAC) system applied to sleeping environments. The study results demonstrated that using the R-TAC system could not only result in better thermal comfort and ventilation performances, but also effectively resolve the cold draft problem as compared with using the previous C-TAC systems. In addition, the potential problem of condensation on the surface of a cold radiant panel may be resolved by raising the panel surface temperature, or using a lower fresh air supply humidity. Secondly, this Thesis reports a numerical study on the effects of design/operating parameters of the radiant panel in an R-TAC system on indoor thermal comfort and energy performances. In this study, four panel surface temperatures, three different distances between the panel and a bed, three emissivities and two panel areas were used to numerically investigate their effects on indoor thermal comfort and energy performances. A CFD method was developed and validated using the measured data obtained in the first part of the systematic research program. With the validated CFD method, indoor thermal comfort and energy utilization efficiency were evaluated in a simulated bedroom, and temperature and air flow fields visualized to analyze the differences in thermal comfort and energy performances when using an R-TAC system with different design/operating parameters. The study results demonstrated: 1) increasing surface temperature can lead to a higher PMV value and a higher EUC value; 2) increasing surface emissivity and area of the radiant panel can result in a lower PMV value and a lower EUC value; and 3) reducing the distance between the bed and the panel can give a lower PMV value and a higher EUC value. Finally, the impacts of daytime heat gain/storage in the external envelopes in a bedroom on the nighttime cooling load and the related mitigation were numerically studied, and the study results are presented in this Thesis, as the third part of the systematic research program. Firstly, the cooling load characteristics in the bedroom at nighttime A/C operation, with an emphasis on analyzing the impacts of thermal energy gain and storage in the west-facing external wall of the bedroom at daytime on the resultant nighttime cooling load, were studied. Secondly, the impacts of adding an air gap and ventilating the gap in the west-facing external wall on the total nighttime cooling load were evaluated. The simulation results demonstrated that the nighttime cooling load contributed by the heat flows through the west-facing external wall was the highest among all the envelopes, due to its direct exposure to solar radiation, thus the highest heat gain and storage in its thermal mass. Adding an air gap in the west-facing external wall can help reduce remarkably the hourly total nighttime cooling load for the first hour nighttime A/C operating (21:00-22:00), and the total cooling load for a 10-hour A/C period. Furthermore, ventilating the air gap and adhering aluminum foil can help further reduce the nighttime cooling load.
Subjects: Hong Kong Polytechnic University -- Dissertations
Air Conditioning
Buildings -- Energy conservation
Pages: xxvii, 221 pages : color illustrations
Appears in Collections:Thesis

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