Thermal mass numerical study - analysis of some factors involved and their importance

Abstract

This study aims to show, from a theoretical point of view, the influence of various factors on dynamic thermal behaviour of buildings. This type of analysis requires the consideration of many factors that vary with the type of building construction, type of occupation and climatic conditions. With the increasing concern with energy consumption, it is important to understand how these factors interact with each other. From all the factors involved, thermal mass is the main focus of this study. It is a very important aspect for south European climate conditions, due to the fact that it can reduce peak cooling and peak heating loads and indoor air temperature variations during the day, but its efficiency depends on the combination with all the other factors. In this context, numerical simulations of generic simplified building envelopes with different thermal mass were performed to evaluate the influence of several factors considered independently and combined. These factors were the geographic location, the envelope thermal insulation, the glazing orientation, the type of occupation and the ventilation of indoor air. The software Energy Plus was the tool used in numerical simulations. The model was a simplified building construction, a closed box, where the walls, the roof and the ground floor had the same composition. This model was used to simulate three different envelopes: heavyweight elements with exterior insulation, heavyweight elements with interior insulation and a lightweight elements thermal insulated. All those three different envelopes had the same thermal resistance. In the parametric simulations the next aspects have been considered: local weather, thermal resistance, thermal mass, glazing orientation, occupation and ventilation. Concerning the factors considered in the simulations, the weathers from two Portuguese districts were chosen in order to compare the thermal behaviour in two different climates. To study the relation between thermal resistance and thermal mass, the insulation layer was increased in similar rate for each envelope. In order to evaluate the importance of glazing in thermal behaviour, windows were applied in North and South walls. To determine the importance of occupation, the permanent presence of 5 people within the indoor space was considered. The last studied factor was the implementation of two different ventilation rates of indoor air. The results obtained were the indoor temperatures for each of the described factors, which were translated in 3 parameters: average temperature, temperature damping and thermal lag. These parameters were calculated for the coldest and hottest months, which in Portugal coincide with January and August, respectively, and allowed to determine thermal mass influence in indoor temperature and its relative weight when combined with the other factors considered.