Heat Transfer in Building Elements

Introduction

A key function of the envelope of a building is to act as a passive climate modifier to help maintain an indoor environment that is more suitable for habitation than the outdoors. However, besides providing shelter from stormy weather, the building envelope alone can hardly ensure that the indoor environmental conditions will always be comfortable to the occupants, or be suitable for the intended purposes of the indoor spaces, particularly during periods of unfavourable outdoor conditions, such as in the night or when the outdoor air is stagnant.

The need for building design features that would facilitate use of natural ventilation and daylight has diminished, as active means of environmental control, such as central heating, ventilating and air-conditioning (HVAC) systems and electric lights, can be used instead to maintain adequate indoor thermal and visual environmental conditions, and air quality. This has also helped to remove the restrictions imposed on the design of buildings, particularly to maximisation of the amount of floor area that can be built upon a given piece of land.

The increased reliance on HVAC and lighting systems for active control over the indoor environmental conditions, however, has made buildings the dominant energy consumers in modern cities worldwide. This has not only accelerated the depletion of the limited reserve of fossil fuels on earth; it has also exacerbated global warming and environmental pollution due to the emissions of combustion products resulting from burning of fuels for generating electricity, steam, hot water or chilled water for use in buildings. Buildings also contribute to other environmental problems, such as the use of CFC as refrigerants in HVAC plants and halons as fire extinguishing agents, which are causes of the depletion of the stratospheric ozone layer. Therefore, besides fulfilment of the functional needs and aesthetics, the environmental performance of buildings, which includes energy efficiency, has become an essential attribute of environmentally friendly buildings.

Measures that can be used to enhance the energy efficiency of a building include the adoption of building design features that can help reduce the frequency and intensity of use of the HVAC plants and the lighting installations, and the use of more efficient HVAC and lighting system designs and equipment. For instance, the cooling or heating load due to heat transmission through the building envelope can be reduced through the use of thermal insulation at external walls and roofs, and high performance glazing and shading devices at windows. The use of energy-efficient boilers and chillers, variable speed motor drives in heating and air-conditioning systems, and energy-efficient lamps and electronic ballasts can lead to very significant energy saving.

In the design process, the effectiveness of individual energy efficiency enhancement measures, particularly the possible energy and running cost saving, would need to be quantified. The financial benefit, derived from the difference in the annual energy consumption of the building with and without a particular measure, would be essential input to a financial appraisal for determining whether or not to adopt individual measures, and for selecting the most viable choices.

Quantification of the annual energy use in a building requires prediction of the space cooling loads of individual rooms in the building that would arise at different times in the operating periods throughout the year. This involves determination of the heat and mass transfer through the building envelope that are significant parts of the heat and moisture gains or losses of an indoor space. The other sources of heat and moisture gains include occupants, equipment and appliances present within the air-conditioned spaces, and infiltration.

Methods for modelling the heat and mass transfer through the building envelope is a key starting point in the prediction of the annual energy consumption in a building. In most such analyses, the mass transfer modelled would be limited to the bulk air transport into or out of buildings through infiltration and exfiltration, while the moisture transport through the building fabric elements would be ignored.

Heat and mass transfer processes in buildings

The range of heat and mass transfer processes that would take place in buildings is as illustrated in Fig.1, which shows a perimeter room on an intermediate floor in a multi-storey office building. The room is separated from the outdoors by an external wall and a window, and from adjoining rooms at the sides by internal partitions, and at above and below by a ceiling and a floor slab. The room is equipped with a HVAC system that would supply heating or cooling to the room by circulating air between the room and the air-handling unit via the supply and return air ducts.

Fig. 1. Heat and mass transfer processes involved in building energy simulation.

As shown in Fig.1, the heat and mass transfer processes that would take place in a building include:

(a) conduction heat transfer through the building fabric elements, including the external walls, roof, ceiling and floor slabs and internal partitions;
(b) solar radiation transmission and conduction through window glazing;
(c) infiltration of outdoor air and air from adjoining rooms;
(d) heat and moisture dissipation from the lighting, equipment, occupants and other materials inside the room; and
(e) heating or cooling and humidification or dehumidification provided by the HVAC system.

The conduction heat transfer through an opaque building fabric element, such as an external wall as shown in Fig. 2, is the effect of the convective heat that the surface at each side of the element is exchanging with the surrounding air, and the radiant heat exchanges with other surfaces that the surface is exposed to. For an external wall or a roof, the radiant heat exchange at the external side includes the absorbed solar radiation, including both direct and diffuse radiation.

Fig. 2. Heat transfer at an external wall.

The heat transfer through a window is shown in Fig. 3. The window glass will transmit part of the incident solar radiation into the indoor space. While the solar radiation penetrates the glass pane, some of the energy will be absorbed by the glass, leading to an increase in the glass temperature, which, in turn, will cause heat to flow in both the indoor and the outdoor directions, first by conduction within the glass and then by convection and radiation at the surfaces at both sides. The heat flows through a building fabric element resulting from the absorbed solar radiation and the outdoor to indoor temperature difference are often treated together through the use of an equivalent outdoor air temperature, called ‘sol-air temperature’, that will, in the absence of the radiant heat exchange, cause the same amount of conduction and convection heat flow through the element. A similar parameter, called ‘environmental temperature’, is used to account for the combined effects of the convective heat transfer from the internal surface to the room air and the radiant energy gain at the surface.

Fig. 3 Heat transfer at a window glass pane.

The transmitted solar radiation from the windows will be imparted to the indoor air and become cooling load only after it has been absorbed by the internal surfaces. Consequently, the temperature at such surfaces will rise, leading to convective heat flow from the surfaces into the room air. It is this convective heat flow that will affect the indoor air temperature and constitutes a component of the space cooling load. This cooling load component will differ in magnitude and in the time of occurrence of its peak value from those of the radiant heat gain, as shown in Fig. 4, which is the result of the thermal capacitance of the fabric elements or furniture materials that are subject to thermal irradiation. Besides the short wave radiation from the sun, radiant heat gains from the lighting and equipment and the long wave radiation exchange among the internal surfaces within the space will need to undergo a similar process to become a cooling load.

Fig. 4 Radiant heat gain and the resultant cooling load.

When there are air movements into and out of an indoor space, heat and moisture will be brought into or out of the room if the airs that enter the space are at thermodynamic states different from that of the indoor air. The air movements can be set up by pressure differences between the room and the adjoining rooms and the outdoor, due to wind or stack effect, or imbalance in the supply and extract flow rates maintained by the ventilation system.

The thermodynamic state of the air within the room would vary with the net heat and moisture gains experienced by the room air, resulting from heat and moisture exchanges with the enclosure surfaces, air transport into or out of the room bringing with it heat and moisture, heat and moisture gains from sources present within the room, and heating, cooling, humidification or dehumidification provided by the HVAC system serving the room. These heat and moisture transfer processes would need to be modelled for the prediction of the indoor air condition or the rate of heating or cooling, and humidification or dehumidification required for maintaining the indoor air condition at the set point state.