Thermal Physics / Townsville

Thermal Physics

Townsville usually has a beautiful climate, located in a tropical region and a rain shadow it has about 300 rain free days per year. The sunshine, high temperatures, lack of moisture in winter and the prospect of flooding in the summer has a profound effect on all aspects of life and landscape. In order to achieve energy efficiency and climatic performance of housing in Townsville the environment and temperature needs to be carefully considered in planning and development stages. This report will explain and explore the physics of heat transfer in relation to various features of housing, provide recommendations for the construction of houses specifically within the Townsville region with consideration to heat transfer from conduction, convection and radiation; and, explore economic factors related to these recommendations. Keeping houses at a comfortable temperature almost all year round without the use of artificial heating or cooling will help to save money, minimise environmental impacts and enjoy the tropical lifestyle.

Understanding that heat can be moved from one place to another by means of three processes is essential in designing a home with regard to the environment and temperature conditions. These three processes are: conduction, convection and radiation. Each process contributes to the heating/cooling of a house.

Conduction is the process by which heat energy is transferred through solids as a result of the vibration of particles (Classroom, 1996). Particles with greater heat energy collide with other particles and transfer some of this energy. So, conduction is the flow of internal energy from a region of higher temperature to one of lower temperature through this interaction, involving atoms, molecules, ions, electrons, etc. in the intervening space (Elert, 1998). There is nothing physical or material about this interaction as nothing other than a transfer of energy is occurring. In this, conduction involves a loss of energy from particles with greater energy and a gain of energy for particles with less heat energy in a collision. For example, if the outside of a wall is hot, heat will be transferred from the outside surface of a block (the outside of the home), to the inside surface of a block (the inside of the home). A diagram of this can be seen in Appendix 1. The rate at which different materials transfer heat by conduction varies and is measured by the material’s thermal conductivity (W/m/K). A table of common materials and their thermal conductivity can be seen in Appendix 2.

A poor conductor of heat is any substance which does not conduct, transfer or absorb heat well or at all. These materials have a lower thermal conductivity and can be found toward the bottom of the table in Appendix 2. Generally, metals are better conductors than non-metals. Metals are excellent conductors of heat as their particles are so close together that the vibrations are passed on tremendously quickly. The rate in which heat can be transformed is increased dramatically. Furthermore, as non-metals do not possess the same tightly bound structure with a loose sea of electrons, heat cannot be transferred as quickly or with the same amount of energy. Materials that are poor thermal conductors can also be described as being good thermal insulators as an insulator is a material that restricts the transfer of energy. Restricting the heat energy that can be transferred through the process of conduction will be beneficial in keeping the house comfortable and cool.  

The factors that affect the rate of heat transfer need to be identified because of the frequent need to increase or decrease how quickly heat flows between two locations. Decreasing the rate of heat flow through the roof and walls of a house should decrease the heat energy transferred through the roof cavity into the house during daylight hours. The difference in temperature, material, area and thickness or distance will affect the rate of heat transfer. In conduction, heat is transferred from a hot substance to a cold one. This transfer of heat will continue as long as there is a difference in temperature between the two, only stopping one they have reached thermal equilibrium (the same temperature). The second variable of importance is the material involved in the transfer. The measure of this is referred to as the ‘heat transfer coefficient’.  The greater this measurement is the greater the rate of heat transfer. The thickness or distance that the heat must be conducted will also affect the rate of heat transfer. The rate of heat transfer is inversely proportional to the thickness of the material. The rate of conductive heat transfer is given by:

, where ‘k’ is the thermal conductivity, ‘a’ is the area, ‘T_Hot’ is the temperature of the hotter material, ‘T_Cold’ is the temperature of the cooler temperature, ‘t’ is the time taken and ‘d’ is the thickness of the material. Decreasing the thermal conductivity, the difference in temperature and the area; and, increasing the thickness of the material, will decrease the rate of the heat transfer.

The second process, convection, is the transfer of heat by the motion of a fluid (liquid/gas). Convection occurs when the heated fluid (liquid or gas) is caused to move away from the source of heat, carrying energy with it (Nave). The process often occurs above a hot surface because hot air expands, molecules spread out and the air becomes less dense. The hot surface (heat source) transfers energy through particle collision to the liquid/gas. As liquid/gas particles gain heat energy they move faster and bonds between them decrease. The particles spread out and the density of the material decreases. Hot air is less dense and experiences a buoyant force, pushing it upward, the surrounding cooler air replacing it (see Appendix 3 for diagram). In houses, after heat is transferred through walls by means of conduction, it is carried away from the walls by conduction and circulated throughout the house. This circulation will continue until the temperature evens out. It is important to understand this process in order to minimise heat inside the home. Through understanding convection, a number of recommendations for the design of a house can be made to increase air flow and decrease hot air in a house.

The final major process of heat transfer is radiation. Radiation, unlike convection and conduction, does not rely on any contact between the heat source and the heated object. Heat from the sun reaches us as radiation, much as visible light and the rest similar to electromagnetic waves that our eyes cannot detect. Heat energy, carried by electromagnetic waves, can be transferred through empty space by thermal radiation, often called infrared radiation. As with light, infrared heat radiation is actually an example of an electromagnetic wave (Fowler, 2008). Electromagnetic waves are formed when an electric field couples with a magnetic field. As both electricity and magnetism can be static, the changing magnetic field inducing the changing electric field and vice versa. The radiation comes about because the oscillating ions and charged electrons in a warm solid are accelerating electric charges. As the transfer of heat is through electromagnetic waves, where a medium is not necessary, radiation works in and through vacuums such as space and air.

To some extent, radiation from a heated body depends on the body being heated (Fowler, 2008). Considering how different materials absorb radiation will reflect the importance of this dependence. For instance, glass will hardly absorb light, the radiation simply passes through. The electrons in glass are tightly bound to atoms and can only oscillate at certain frequencies. For radiation to be absorbed, the frequencies must correspond. The frequencies obtained by ordinary glass do not correspond with visible light and so little energy is absorbed. However, infrared and ultraviolet frequencies do correspond with the natural oscillations of glass and so some heat energy is absorbed by light. A shiny metallic surface will reflect heat and light because of the structure of a metal. As the electrons are free to move through the entire solid, a metal will conduct both heat and electricity easily. The distinguishing shiny surface of a metal is the result of the reflection of light/radiation. The free electrons are driven into large oscillations by the electrical field of the light wave (radiation) and this oscillating current radiates. So, for a shiny metal surface, almost none of the incoming radiation is absorbed as heat, it is reflected. A black substance will neither transmit nor reflect the radiation. It will conduct an electric current but not as efficiently as a metal. There are unattached electrons that move through the solid however they are constantly colliding. These collisions result in the transfer of kinetic energy to heat energy. The material consequently gains heat energy.

Radiation heat transfer can be described with the use of ‘black bodies’. A black body completely absorbs all thermal radiation and so will not reflect light. Emissivity is the measure of an object's ability to emit infrared energy.  Emissivity can have a value from 0 (shiny mirror) to 1.0 (blackbody). The greater the emissivity of an object is, the greater the ability to emit infrared energy and release unwanted heat energy. An example of heat transfer through radiation is found in an attic. The sun radiates heat to the roof, which in turn heats and radiates heat down toward the ceiling. If the insulation covering the ceiling does not resist this heat transfer then the ceiling will heat up, radiate heat down into the home and the home will become increasingly hotter. Despite the best efforts, some heat energy will be transferred through radiation. So, the use of insulation to limit conduction will also decrease the transfer of heat energy through radiation.

The following section of the report will explore and justify recommendations to increase the climatic performance of houses in the Townsville region. It will focus on recommendations to (1) keep cool air inside the house, (2) keep maximum heat out during the day, (3) release unwanted heat quickly one the sun has set and (4) provide low-cost, effective solutions. There are many factors that need to be considered involving the area and environment when making these recommendations.

As the roof is the leading source of heat intrusion into houses, the first recommendation is in relation to its structure. To keep the house cool, the roof should strongly reflect sunlight and also cool itself quickly by emitting radiation to its surroundings when the sun has set. To reduce the thermal energy transferred into the house through the roof both conduction and radiation processes need to be limited. Understanding both these processes justifies the idea that the material used should have a low thermal conductivity, to decrease the rate of conduction and a reflective surface to reflect the heat energy that may be gained through radiation. Another aspect to consider is the thickness of the material. Primary requirements for roofing materials include: high thermal capacity (to absorb solar heat during the day and release it during the night) and good reflectivity (to reduce heat load and thermal movements). The colour, shape and composition of the roof need to be considered.

It is recommended that a light, reflective colour is used on the roof. As discussed in the theory review, the colour will limit radiation as it reflects the electromagnetic waves more effectively than dark or transparent colours. When an object appears a certain colour it means that it is reflecting light of that colour and absorbing all the other colours. The greater amount of light an object absorbs, the greater amount of heat an object absorbs. A black object absorbs all wavelengths of light and reflects none. Whereas objects that are white reflect all wavelengths of light and therefore absorb the least heat. A colour such as white, cream, light beige or light grey would prove to be optimal. This is supported by the practical component of this report. An experiment was conducted in which two different roof structures and 3 different roof colours where tested. The white roof (compared to the black and silver roofs) obtained the best results in that the difference between the initial and final temperatures was the smallest. The white roof absorbed the least amount of heat in the specific time period. The results from this experiment can be found in Appendix 4. As stated above, these results are supported and can be justified with knowledge of the different processes of heat transfer. Additionally, an experiment was conducted to specifically test a white, reflective roof on an average Townsville home. The experiment supported the theory that a white, reflective roof will absorb the least amount of radiated heat. The results from this experiment can be found in Appendix 5.

It is also recommended that steel roof sheeting, such as corrugated iron, is used as the roofing material. This roofing material will lose heat quickly as the sun sets rather than capturing and retaining this heat. Steel has a higher emissivity than most other suitable materials and will result in the material releasing unwanted heat quickly into the surrounding environment. This can be seen in table available in Appendix 6. Steel roof sheeting is an ideal alternative to roof tiles, for example, which will absorb heat during the day and then slowly re-radiate it into the home at night. Secondary data has been found to support this theoretical recommendation. An experiment was conducted by the Florida Solar Energy Centre in order to improve attic thermal performance. A table of the results can be found in Appendix 7. In summary, the experiment found that a white metal roof was superior to that of various other colours and materials.

The next two recommendations are to specifically reduce and limit the rate of heat transfer through the roof and ceiling. The recommendations are concerning the use of various insulations to combat and limit conduction processes. Thermal insulation is a general term used to describe a product that reduces heat gain or heat loss by providing a barrier between two areas that have a significant difference in temperature (Knauf, 2007). There are two main types of insulation: bulk and reflective; it is recommended that both are used.

It is recommended that a combination of both bulk and reflective insulation is used in the ceiling/attic. Bulk insulation reduces the amount of heat being transferred into the home. It works by resisting the amount of conducted and convected heat flow between the hotter air in the roof-space and the cooler air inside the home. There are several types of bulk insulation including polyester wool, bubble wrap, fibreglass (glass wool), rock wool, cellulose fibre and polystyrene and depending on the type may be supplied as a blanket, batts or blown loosely onto the ceiling to form a layer. The level of insulation a product is able to supply is given as an R-value (see Appendix 14 for table of R-values). An R-value, measured in m²K/W, is calculated using the thickness of the material and the thermal conductivity in the formula: R-Value = , where ‘d’ is the thickness in metres and ‘k’ is the thermal conductivity. Reflective insulation reflects heat away from a surface preventing 95% of radiant heat from entering the house. Highly effective at preventing the entrance of radiant heat, reflective insulation will reflect the heat away before it has a chance to heat up the roof space. Together, these components must achieve an R-value of 2.7 to comply with the building code of Australia; however, it is recommended that an R-value of about 4 is achieved through the use of cellulose wet-spray or loose-fill insulation (see Appendix 13). While natural wool and polyester based insulations provide a higher level of thermal efficiency, the insulation provided by cellulose is sufficient and provides an economic alternative. Additionally, the inclusion of a reflective foil laminate installed directly underneath the roof sheeting will compensate for the compromised R-value.  It is important when installing the reflective insulation that there is an air gap left. Air is a poor conductor of heat (has a low thermal conductivity) and will limit heat transfer to radiant heat between the roof and the layers of insulation.

To further enhance the reduction of heat transfer through radiation, the orientation and availability of shade for a house needs to be considered. To increase the climatic performance of a house, the direct sunlight onto and into the home needs to be limited. By limiting this direct sunlight the radiation from infrared light, ultraviolet light and visible light is restricted. It is necessary to understand the movement of the sun to create effective shading. It is recommended that roof overhangs are utilised on north and south sides of the home. During the middle of the day, when the sun is at its highest points (see Appendix 8), the north and south sides of the home are affected. Overhangs of 900mm will provide the walls with complete shade. As unshaded glass is another major source of unwanted heat in a home, it is also recommended that vertical shading is used on east and west facing windows. Vertical shading in the form of lattice screens, timber batten screens, aluminium batten awnings or mixed height planting of scrubs and trees is essential to decrease the transfer of radiant heat to the outside the east and west facing walls of a home that are affected by low-angled sun in the morning and afternoon.

The orientation of the house is critical in creating a suitable design for the Townsville climate. Using knowledge of the sun’s movement across the sky and the consequent angles (as with shading the house) it is possible to design a home that will minimise heat gain and provide shade where it is most needed: living and dining areas.

It is recommended that a house in Townsville is orientated with the longest side on an east-west axis. This will minimise the surface areas that face the east and west and reduce effects of the afternoon and morning long-angled sun. Orientating the house on a north-south axis will present a large amount of wall and window surface area to the low-angled sun. As discussed in the theory review, allowing direct radiant heat into the home will allow furniture and air in a room heat up. Although the conducted experiment only revealed a marginal difference between a pitched and flatter roof, theoretically a pitched roof would provide a higher level of protection from heat. As well as orientating the house, orientating the rooms will effectively enhance the thermal performance of a house, decreasing the temperature in the rooms (see Appendix 9). Living areas including kitchen, dining, living and family rooms are recommended to face north or north-east, taking advantage of the natural shade protection from the angles of the sun. Additionally, the wind direction distribution chart included in Appendix 10 taken from ‘Wind and Weather Statistics Townsville’ shows that most breezes in Townsville are in a north-easterly direction. Orientating rooms on this axis takes advantage of the cool breezes.

 To further increase ventilation and keep rooms cool and comfortable it is necessary to harness and use natural breezes. Maximising access to breezes, enabling ventilation by convection and creating air movement will all contribute to the cooling of a house. To increase the access to breezes, as mentioned above, it is recommended houses face a north or north-east direction. As well as this, homes should be built with single room depths. This provides optimal cross-ventilation through an entry and exit area in every wall for breezes to pass through. The final recommendation to increase ventilation in the house is to elevate the home and ceiling. This is because access to prevailing breezes increases with height. Elevated houses in Townsville receive faster, cooling breezes (see Appendix 11). Furthermore, this design of house allows breezes to pass underneath the home, in turn assisting to cool the floor and prevent hot air rising up into the home. High ceilings will assist in keeping living spaces cool and comfortable by allowing convection process to take place without affecting living spaces. Hot air will rise and the high ceiling guarantees this hot air will stay above the area intended for living. 

Convection can also be enhanced by devices in the ceiling or roof. These devices are additions that can be made to any house to increase climatic performance of a house. They include roof ventilators, louvered windows, grills, gable vents, open eaves, vented ridges, exhaust fans and raked ceilings (see Appendix 12).

In conclusion, the exploration of heat transfer and the physics involved is detrimental to the effective design of houses that wish to achieve a high level of climatic performance. Through understanding the major processes of heat transfer it is possible to make recommendations in relation to the loss and gain of heat to improve the construction of houses. The report reveals that there are numerous means of decreasing thermal heat energy that can be achieved regardless of financial situation.

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