CHAPTER 4 THERMAL PERFORMANCE OF BUILDINGS Contents 4.1 Introduction 4.2 Heat Transfer 4.3 Solar Radiation 4.4 Simplified Method for Performance Estimation 4.5 Example 4.6 Computer-based Tools References 4.1 INTRODUCTION The thermal performance of a building refers to the process of modeling the energy transfer between a building and its surroundings. For a conditioned building, it estimates the heating and cooling load and hence, the sizing and selection of HVAC equipment can be correctly made. For a non-conditioned building, it calculates temperature variation inside the building over a specified time and helps one to estimate the duration of uncomfortable periods. These quantifications enable one to determine the effectiveness of the design of a building and help in evolving improved designs for realising energy efficient buildings with comfortable indoor conditions. The lack of proper quantification is one of the reasons why passive solar architecture is not popular among architects. Clients would like to know how much energy might be saved, or the temperature reduced to justify any additional expense or design change. Architects too need to know the relative performance of buildings to choose a suitable alternative. Thus, knowledge of the methods of estimating the performance of buildings is essential to the design of passive solar buildings. In this chapter, we will discuss a simple method for estimating the thermal performance of a building and introduce a few simulation tools used for more accurate calculations. Various heat exchange processes are possible between a building and the external environment. These are shown in Fig. 4.1. Heat flows by conduction through various building elements such as walls, roof, ceiling, floor, etc. Heat transfer also takes place from different surfaces by convection and radiation. Besides, solar radiation is transmitted through transparent windows and is absorbed by the internal surfaces of the building. There may be evaporation of water resulting in a cooling effect. Heat is also added to the space due to the presence of human occupants and the use of lights and equipments. The interaction between a human body and the indoor environment is shown in Fig. 4.2. Due to metabolic activities, the body continuously produces heat, part of which is used as work, while the rest is dissipated into the environment for maintaining body temperature. The body exchanges heat with its surroundings by convection, radiation, evaporation and conduction. If heat is lost, one feels cool. In case of heat gain from surroundings, one feels hot and begins to perspire. Movement of air affects the rate of perspiration, which in turn affects body comfort. The thermal performance of a building depends on a large number of factors. They can be summarised as (i) design variables (geometrical dimensions of building elements such as walls, roof and Fig. 4.1 Heat exchange processes between a building and the external environment Fig. 4.2 Heat exchange processes between a human body and the indoor environment windows, orientation, shading devices, etc.); (ii) material properties (density, specific heat, thermal conductivity, transmissivity, etc.); (iii) weather data (solar radiation, ambient temperature, wind speed, humidity, etc.); and (iv) a buildings usage data (internal gains due to occupants, lighting and equipment, air exchanges, etc.). A block diagram showing various factors affecting the heat balance of a building is presented in Fig. 4.3. The influence of these factors on the performance of a building can be studied using appropriate analytical tools. Several techniques are available for estimating the performance of buildings. They can be classified under Steady State methods, Dynamic methods and Correlation methods. Some of the techniques are simple and provide information on the average load or temperature, on a monthly or annual basis. Others are complex and require more detailed input information. However, the latter perform a more accurate analysis and provide results on an hourly or daily basis. In this chapter, we discuss a simple method that is easy to understand and amenable to hand calculations. Fig. 4.3 Thermal simulation flow paths of a building To understand the process of heat conduction, convection and radiation occurring in a building, consider a wall having one surface exposed to solar radiation and the other surface facing a room (Fig. 4.4). Of the total solar radiation incident on the outer surface of the wall, a part of it is reflected to the environment. The remaining part is absorbed by the wall and converted into heat energy. A part of the heat is again lost to the environment through convection and radiation from the walls outer surface. The remaining part is conducted into the wall; where it is partly stored thereby raising the wall temperature while the rest reaches the rooms interior surface. The inner surface transfers heat by convection and r