When you think about solar energy, you may automatically relate it to heat or warmth created by the sun. So, what is heat? Heat is actually a form of kinetic energy. It is the motion of molecules that travel by radiation when the sun shines on an object. Heat that is transferred through solids is called conduction, heat transferred through moving gas or liquid is called convection, and heat transferred through space is called radiation. Another word for the transferring of heat is called absorption.
Heat transfer example
All three types of heat transfer can be demonstrated with one cup of tea. Imagine you are warming up a hot cup of delicious tea. First, you would warm up a cup of tea in a microwave. A microwave oven heats water through radiation. This is how the sun’s radiation reaches the earth. Radiation is solar energy that travels as electromagnetic waves. In this way, energy from the sun reaches the earth. The water molecules vibrate with the waves, and the friction from the motion heats up the tea inside the cup. When the cup of water is removed from the microwave, heat is transferred from the tea to the surrounding air through convection and radiation. In convection, heat travels by actual flow of the warmed fluid (either a liquid or a gas). Convection causes warm air to rise and cool air to fall. If a metal spoon is placed in the tea, the spoon is heated through conduction. Heat travels from the hot to the cold end of the spoon because the hotter molecules are moving at a greater speed. They bump into the cooler molecules and cause them to move faster. As the fast molecules jostle the slow ones, heat moves up the spoon.
Can you think of ways that other materials are used to capture heat energy or protect us from it? What is different about these materials? Can you think of some examples?
Heat absorption experiment
Let’s experiment with different colors and objects below to determine which materials will absorb or reflect heat.
- One sheet of black construction paper
- One rock
- One sheet of white construction paper
- One glass of water
- One sheet of aluminum foil
Place one sheet of black construction paper, one rock, one sheet of white construction paper, a metal spoon, a glass of water, and one sheet of aluminum foil out on a table on a bright sunny day (making sure all are under direct sunlight).
Leave objects out for 60 minutes.
Go back outside, pick up each item and feel each object (be careful: some may be hot) and evaluate the amount of heat that each material absorbed or reflected and come up with some conclusions why.
As you can see from this experiment, different materials have different specific heat capacities. Materials with low specific heat such as metals require less heat to raise their temperature than materials with high specific heat, such as water. You will also find that solar energy can be absorbed, transmitted, or reflected from a material. Below are examples that illustrate each of these properties:
- Smooth metal will reflect light (for example, a mirror). Reflection is defined as heat, light, or sound that is thrown back without absorbing it.
- Glass will transmit energy.
- Darker colors will absorb energy.
- Aluminum foil may act as a good insulator. Insulators retain (or keep) heat in.
Solar stadium challenge
Use what you have learned about solar heat transfer and heat absorption in materials to complete the following challenge:
Imagine that you are a structural engineer and your company desires to bid on a construction project to design and construct the shell of a new football stadium which will be housed in Anchorage, Alaska. As a potential location for a new National Football League (NFL) team franchise, the Alaskan Wolverines, this stadium will be primarily heated by solar power. Solar power will be the most important part of the design because it is a renewable energy source; the sun has maximum exposure in this geographical area, and will be more cost effective to operate. Your solar stadium will need to keep fans warm during the harsh winters and will need to use the sun’s strong rays to retain that warmth. Because the goal is to keep fans warm, the solar stadium cannot be an open-air stadium like Levi’s® Stadium—it must be enclosed.
After you design and construct a prototype (model) of your solar stadium, you will test its ability to absorb and retain heat. After your stadium is exposed to light for a specific period of time, you will test how well your design retains heat by putting one chocolate chip inside the stadium. The goal is to slightly melt the chocolate chip to ensure it is retaining heat and that your stadium design will keep fans warm. If your chocolate chip fully melts, that means it has absorbed too much heat. If your chocolate chip doesn’t melt at all, that means your design reflected more light than it absorbed or that heat escaped in some manner. Take your time with your prototype design, because your design will determine the comfort of thousands of fans coming to enjoy the game.
A. Design and construct a model solar stadium that will absorb and retain heat for fan comfort.
B. Your goal is to first make the stadium hot enough to warm up the inside of your solar stadium to test. You will use a 200-watt lamp in place of the sun for testing. Your model must meet the following design goals:
- The base of the stadium model should be no more than 15 cm by 15 cm.
- The lamp must be 30 cm above the roof of the model.
- There should be a way to insert a thermometer into your model for testing.
- Your goal is to let light/heat into the box and store it for a specific time period
- You should make a window so you can place your testing material (which is a chocolate chip--that is mentioned above) into the solar stadium after the initial 5 minute heating to see the results of heat retention (refer to testing procedures below).
- Packing Foam
- Clear plastic (saran wrap, transparency, or report covers)
- Aluminum foil
- Construction paper (black)
- Masking tape
- Lamp with 200-watt light bulb
- Watch with second hand or stopwatch
- Chocolate Chips
Step #1: Identify the problem and generate ideas
Understand the problem, test different materials options that you can use included in the material list and see how well they absorb, reflect, and insulate. Come up with initial design solutions. Your goal is to let heat and light into your solar stadium and trap it there. You will test different materials to see what works best, and you will record the temperature inside your stadium while testing your design solution.
Predictions about materials
- What properties do you want the outside walls of your solar stadium to have?
- What properties do you want the inside walls to have?
- The materials available to you are foam, aluminum foil, sand, clear plastic, water, dark paper, and light paper. What materials will work best and support the best solution? These are your predictions. Now you can do some testing and find out if your predictions are right.
Step #2: Design a prototype
Sketch out your design for the solar stadium keeping to the criteria of the project.
Step #3: Build a prototype
Gather your materials and make a prototype (model) of your solar stadium.
Step #4: Test your design
A. Test your solar stadium design. You will use a chocolate chip (to represent how the heat is affecting the fans) to determine whether or not the inside of the stadium is comfortable enough for fans to enjoy the game.
B. Set up the lamp (be very careful not to touch hot surfaces). Use a 200W lamp as your sun. Design your stadium structure so that the chocolate chip will melt slightly. When testing, remember that the goal is to have your stadium retain some heat.
C. Record room temperature.
D. Insert the thermometer in your solar stadium. Place your solar stadium under the 200-watt light bulb and turn on the light. Place the lamp 30 cm above the roof of the stadium.
E. Heat your stadium with the 200-watt lamp for a maximum of 5 minutes.
F. Record the temperature every 30 seconds during that time. You can keep track of your data on a separate sheet of paper.
G. Turn off the lamp after 5 minutes. Add your chocolate chip (fans) and let sit inside the stadium and record the temperature for the next 10 minutes. Remember, you do not want the chip to melt too much, since this would make the stadium too hot for fans.
H. After the 10 minutes are up with the chocolate chip inside your solar stadium, record results and try to improve design (timed melting results).
Questions to ponder:
- Which materials worked best for the outer walls during heating? Why?
- Which material worked best for the inner walls during heating? Why?
- Did you use any different materials for your roof?
- Did your predictions about materials work out?
- Do your results agree with your predictions?
- Which material is good absorber? Reflector? Insulator?
- Were you able to retain heat in your solar stadium to keep your fans warm, or did your fans melt because it was too hot?
- Did you redesign?
Hopefully you will appreciate the knowledge you gained in this exercise and continue to build upon it. Solar power is continually changing the framework of how we use energy and it will be up to you and future generations to find innovative ways to capitalize on all the benefits it offers.