Experimental and numerical studies on improving the indoor thermal environment in a disaster-relief temporary prefabricated house located in the subtropics

Abstract

Natural disasters have an increased occurrence rate in recent decades, and hence providing disaster victims with a temporary housing has always been one of the key problems to be addressed in disaster management. Prefabricated houses (PHs) can be very effective in quickly providing disaster victims with a temporary living place whenever there is a natural disaster. Currently, most existing research and development efforts related to indoor thermal environment and comfort mainly focused on providing occupants with a comfortable indoor thermal environment and improving energy use efficiency in conventional permanent buildings. However, temporary PHs are much different from conventional permanent buildings due to their inherent characteristics. Unlike in conventional buildings, environmental control systems are normally not installed inside PHs so that unbearable indoor thermal environment can be often experienced by their occupants. On the other hand, although temporary PHs for disaster relief are intended for temporary purpose, unfortunately, they are increasingly used for a prolonged time period of up to several years. Therefore, it has become highly necessary to study the thermal environment inside temporary PHs used for disaster relief in order to provide their occupants or disaster victims with a more acceptable thermal environment. Consequently, a programmed research work on studying the thermal environment inside PHs used for disaster relief has been carried out and is reported in this thesis. This thesis begins firstly with reporting a study on measuring the actual yearly indoor thermal environments in an experimental PH located in the subtropical Chengdu City, China. The measured results suggested that at closed door and windows condition, the air temperature inside the experimental PH was very high at daytime in summer, but very low at nighttime in winter. Therefore, the actual thermal environment inside the experimental PH was highly unacceptable if it was used on a long-term basis. The variation of the air temperature inside the PH appeared synchronized with that of outdoor air temperature at daytime in summer, suggesting the insignificant thermal mass of its envelope to store solar heat gain. The measured results also indicated an indoor air temperature stratification inside the PH in both winter and summer. At daytime, indoor air temperature was increased with the height but at nighttime, decreased with the height. Besides, at nighttime in both winter and summer, the external surface temperature of PH's roof was lower than the outdoor air temperature, reflecting the sky cooling effects for the experimental PH. Secondly, the thesis presents a numerical study on both the thermal performances of the experimental PH's envelope, and the effectiveness of applying various passive cooling measures to the PH for improving its indoor thermal environment in summer. A numerical model for the experimental PH was firstly developed using Energyplus and validated using the measured results from the experimental PH. Using the validated model, a detailed breakdown of summer heat gain by the PH was then numerically evaluated. The results showed that PH's windows were responsible for the lion's share of the total heat gain, followed by the roof and east wall. Thereafter, the effectiveness of applying four passive cooling measures, individually or in combination, to the PH was numerically studied using the validated model. The study results suggested that adding a thin movable fabric layer of 0.9 reflectance to the walls and roof of the PH, and applying external window blinds would lead to a very high percentage reduction in unacceptable hours for occupants, without however the need to implement all the four passive measures, so as to save the implementation cost. Thirdly, using the validated model, a further numerical study on various design issues for a disaster relief PH, including orientation, natural ventilation strategies and window to wall ratio (WWR), to achieve an improved indoor thermal environment, is presented. The numerical study results demonstrated that under the climate condition of Chengdu City, the orientation for the PH would have an obvious effect on its indoor thermal environment, with a south-facing orientation being the most favourable for the PH. In addition, in contrast to that in conventional permanent buildings, the use of daytime ventilation was more effective than the use of nighttime ventilation in the PH for improving its indoor thermal environment. Lastly, the study results suggested that there was an optimum value of ~20% for WWR for the PH, at which its indoor thermal environment was the least unacceptable for PH's occupants. Finally, a study on developing an alternative general method (AGM) which is valid for all climate conditions and easy to be implemented, to evaluate the atmospheric downwelling radiation (ADR) values is reported. The AGM was developed based on the heat exchange of long wave radiation between a horizontal upward facing surface and the sky. Using the AGM developed, the ADR values in Chengdu, China, were evaluated based on the roof of the experimental PH, and its validity was demonstrated by comparing the ADR values evaluated using the AGM developed and that using the widely used Clark and Allen's formula, which is based on a water pond. Since the AGM developed was based on a roof surface, and no water was involved, the AGM developed appeared more advantageous. On one hand, this could eliminate any potential influence on evaluation accuracy when water was used in different climates. On the other hand, the application of the AGM was relatively easier as only a suitable existing roof system was required. Therefore, the use of AGM can provide a more reliable and low-cost alternative to evaluate ADR values, when compared to using Clark and Allen's empirical formula.