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, ‘THot’
is the temperature of the hotter material, ‘TCold’ 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.
