A thesis submitted m partial fulfillment of the requirements for the degree of Master of Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 1994

Knowledge and understanding of the thermal interactions in an air-conditioned building have become a necessity in the aspect of providing comfort as well as for rational use of energy. The heat transfer occurring in a building is a dynamic phenomenon influenced by many factors. Heat gain through the building envelope is highly variable with time due to the fluctuations of solar radiation intensity, ambient temperature and other environmental conditions. This heat gain is controlled by the thermal properties of the building envelope material. Similarly, other heat sources within the building contribute to the dynamic behavior of heat transmission and removal. The thermal characteristic of the building envelope and content serves as a modulator of the rate of heat transfer into the space and the heat removal as well. The building interior structure and furnishings have finite thermal cairacitance and therefore have the ability to store heat. The instantaneous heat input into the space for instance may not most of the time be equal to the rate of heat removal by the air conditioning equipment. Some of this heat is absorbed and stored by the building interior elements. After some time this heat is released and becomes a cooling load of the space. The effect is a time delay of the maximum rate of heat transfer at the same time reduction in the magnitude of the peak load. This phenomenon is highly prevalent in structures of heavy construction than in lightly constructed ones. In this study, an attempt is made to develop a model to represent this dynamic behavior in a building. Simple electrical analog networks were used to characterize the thermal network. Submodels were developed for each heat load factors and elements. These submodels were integrated forming a thermal system describing the dynamic thermal interactions. Data collected from an office building in Metro Manila, Philippines were used to simulate the model using a finite difference procedure. The simulation results show that the instantaneous heat gain into the building is highly responsive to the variation in solar and ambient conditions. Peak heat gain is experienced in the early afternoon hours where solar radiation and ambient temperature are at there maximum. This is due to the fact that the building facades are covered mostly with glass. The results also show that the building interior mass significantly affects the cooling load of the space. Almost half of the total cooling load during the day time where there is air conditioning comes from the heat stored in the interior building mass. The model developed was applied to determine the cooling load of the building under a selected design condition. Results indicate a substantial difference in the magnitude and time of the peak cooling load.