Module 4: Thermal Physics
PBL Report
Solar Heater Design
Done by:
Caleb Foo (4S1 13)
Ng Wei Bo (4S1 24)
Wong Qin Jiang (4S1 29)
PBL Report Overview
1. Design Features of Solar Heater
2. Application of Physics Concepts
3. Information of Trials
4. Data Collection
5. Reflections
Design Features of Solar Heater
Our solar heater is made out of cupboard box which is recycled from a packaging. The box
was cut diagonally after we tested out various angles of inclination.
In between the outermost layer of cupboard and innermost layer of cupboard, we stuffed
pieces of styrofoam, that were used for the protection of laptops. This styrofoam layer was
made to be thick to act as a better insulator of heat energy.
On the innermost layer of cupboard, we made it black with first a little paint and black tape.
The paint was to cover a small part of cupboard piece not covered by tape and acts as a trial
to see if black tape or paint worked better. In the end, we decided to use the black tape
with rougher surface to act as conductor of heat energy.
We later decided to lay some sides within the internal of solar heater with aluminium foil to
reflect the light when it shines in. The aluminium foil ensures that the light would be
reflected and have a higher chance of more light being focused on the object being heated,
increasing heat energy that the object is exposed to.
We also made use of mirrors and hand lens to refocus the sun’s light rays onto the object
itself. The mirrors are placed at the bottom of the solar heater, whereas the hand-lens was
placed above the solar heater, so that light rays would point directly at the object.
For containing the heat energy within the solar heater and yet, still allowing light energy to
pass through, we made use of a clear plastic sheet. This plastic sheet gave us the flexibility
to place objects in and out of solar heater, and allowed most of the light energy to pass
through.
Application of Physics Concepts
1. Refraction of light
A lens was placed at the top of the system, directly above the chocolate chip. It will
focus the rays from the sun onto the chocolate chip so that the chocolate chip will
receive a more intense radiation.
2. Reflection of radiant energy
Mirrors were placed around the chocolate chip to reflect sunlight onto the chocolate
chip.
Aluminium foil was also used to line the inside of the system. As aluminium foil has a
shiny surface, it will reflect the sunlight that enters onto to the chocolate chip.
3. Absorbers and emitters of radiant energy
Black tape was used to cover the walls of the system. As black surfaces are good and
efficient absorbers of thermal energy, the sunlight that is absorbed by the black tapes
will heat up the air within the system. The temperature of the air will be higher so that
the chocolate chip can heat up faster.
4. Thermal conductors and insulators
Styrofoam is a poor thermal conductor, with a thermal conductivity of 0.033. There are
air spaces in between styrofoam pieces. Air is also a poor thermal conductor, with a
thermal conductivity of 0.024.
Since both styrofoam and air are poor conductors of thermal energy, they trap heat
energy within the system and reduce heat energy loss by the system. Thick walls of
layers of cardboard and styrofoam ensure that thermal energy is trapped within the
system, reduces the heat energy loss by system.
Aluminium is a good conductor of heat energy, with a thermal conductivity of 250. By
lining the inside of the system with aluminium foil, more heat energy can be absorbed
and this is heat up the air within the system.
Information on trials
Prediction
The rate of melting of the chocolate is proportional to the temperature of the air
Trials
Our first prototype did not include the aluminium foils, the mirrors and the hand lens. With
only concepts that made use of the black tape and the insulating styrofoam, we tried to see
if the air within the system felt warmer than the air outside the system.
We purchased two small bags of ice and placed one bag inside our solar heater and the
other outside it. We then used a cling wrap to cover the front of the solar heater so that the
air within the system is trapped. Then we placed our solar heater in a place where sunlight is
intense for some time, and then we observed how much the ice in the two bags had melted.
The ice in the bag inside the solar heater had melted more than that in the bag outside the
solar heater, thus we concluded that the temperature of the air inside the solar heater was
higher than that outside it. However, the difference was not very significant and we decided
that there should be further implementations to be made.
Implementations
Inner walls partly coated with aluminium foil and partly coated black. Aluminium specifically
located to reflect more radiant energy towards the chocolate. Black regions absorb the
radiant energy and are majorly responsible for the increase in temperature of the air in the
melter. The black surface (mainly on the bottom surfaces) heats up quickly and transfers
heat energy to the air above it. The heater air becomes less dense and rises while cooler air
sinks. This sets up a convection current within the system.
We used various implementation to ensure the air in the chocolate melter was hotter than
the air outside the melter, as well as ensure maximum radiant energy reached the chocolate.
Implementations include
-
Thick walls of the melter made up of layers of cardboard and styrofoam. Both
cardboard and styrofoam are bad conductors of heat energy, and the pockets of
trapped air are also bad conductors of heat. The pockets of trapped air also help trap
heat energy within the system. This reduces the heat energy loss by the system.
-
Outer walls coated black as black surfaces are good absorbers of radiant energy. This
ensures maximum radiant energy absorbed by the walls of the system.
-
Inner walls partly coated with aluminium foil and partly coated black.
Aluminium specifically located to reflect more radiant energy towards the chocolate. Black
regions absorb the radiant energy and are majorly responsible for the increase in
temperature of the air in the melter. The black surface (mainly on the bottom surfaces)
heats up quickly and transfers heat energy to the air above it. The heater air becomes less
dense and rises while cooler air sinks. This sets up a convection current within the system.
Data collected from tests
Graph of temperature/ °C against time /s
90
80
Temperature/ °C
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
Time/ s
Time/ s
Temperature/ °C
0
42.12121
10
46.06061
20
51.44444
30
54.33333
40
58.77778
50
66.18421
60
72.58065
70
79.19355
Initial
Final
Temperature
temperature/ °C
temperature/ °C
difference/ °C
42.12121
79.19355
37.07234
70
80
We collected the temperature of the inside of the melter every 10 seconds. This was to
guarantee that the physics concepts (radiation, convection, conduction) were put to actual
use and that there was visible result. The temperature managed to rise above 80 degrees
celsius, which is way higher (more than 40 degrees celsius) than the ambient temperature of
the air outside the melter.
Reflections
Caleb
I felt that our project could have been done better, however availability of the
materials and resources greatly pulled the team progress back. We initially intended for the
shiny surfaces in the inner walls of the melter to be mirrors, however specifically cut mirror
dimensions are almost impossible to come by and instead, we used a less shiny, but more
available, aluminium foil.
Besides that, I feel that I have been able to grasp the thermal physics concepts more
detailedly as well as been able to apply these concepts to actual real life applications. The
APBL allowed me to think of how to more effectively melt the chocolate with these
concepts. Also, the Action PBL design was largely based on the deduction that most of the
energy supplied to the design was the radiant energy from the sun, and not secondary
sources (secondary because the energy originated from the sun to begin with) like
conduction from the ground surface or from the air around the chocolate melter. Hence,
the design greatly revolved around how to trap the radiant energy, and at the same time,
reflect the radiant energy directly to the chocolate melter.
I also realised that many tools or machines in the real world are the product of the
applications of physics concepts as well as countless calculations. The real world is never so
simple and that much brainstorming and ideas are merged with theoretical knowledge (like
how we embarked on our project) to form products such as (chocolate melter, rice cooker,
bicycle, cars, etc. the list is endless). This project not only gave me a greater understanding
of the concepts and formulas, but thought me how the real world operated - every minute
detail has a rationale behind it.
Wei Bo
I feel that our team could not get our hands on quite a number of products, such as
glass, which eventually affected our progress greatly, trying to look for the items in
hardware shops and such.
I feel that I have learnt much from this Action Project-Based Learning (APBL).
Firstly, I have learnt that many products that help us in daily work are made from the
combination of science principles. Many household products such as cooking devices,
refrigerators, make use of combination of science principles.
Secondly, this APBL let me realize that simple home-made products could be
changed to green products, considering they can all be made by recycled material. It is just
the terrible work that we must go through to get the same work done.
This APBL also gave me a better understanding on how thermal properties work and
how thermal physics can be applied to real life. Normally, for physics, I would not be able to
tell when to apply it. However, after the completion of this APBL, I could understand fully
how thermal concepts could be applied to everyday life.
Qin Jiang