Harnessing Solar Energy

Purpose

To discover the properties of light (radiant) energy from the sun by experimenting with solar collectors, cookers, and calculators.

Context:

This investigation could be the beginning of a unit on the many forms of energy.

Ideas in this lesson are also related to concepts found in the following benchmark:

• 4D The Physical Setting: Structure of Matter, (6-8) #3

Planning Ahead

Materials:

• Cardboard box
• Black paint
• 4 small metal cans
• 4 thermometers
• Sand
• Salt
• Water
• Torn-up paper
• Shoebox (without a top)
• Aluminum foil
• Black construction paper
• Clear plastic wrap
• Scissors
• Tape (clear and masking tape)
• Water
• Small paper cups
• Heavy corrugated cardboard (or foam board)
• Flexible cardboard
• Rulers/meter sticks
• Compass
• Thin wooden skewers
• Hot dogs
• Solar-powered calculator
• Light source (including the sun)

Before class, paint the cardboard box black and let it dry.

Motivation

Start this lesson with a motivation activity that helps introduce students to the ideas in the benchmarks. In this activity, students can investigate the flow of energy by testing four different materials (sand, salt, water, and paper) to see which material can best store the sun’s heat. 

Fill each of the four cans with one of the four materials: salt, water, paper, and sand. Place a thermometer in each can. Then place the four cans in the cardboard box that has been painted black. Ask students to predict which material will hold the heat the longest and to explain why they think so.

Place the box in direct sunlight for a half hour. After the half hour, remove the cans from the box and watch the temperatures fall. Ask students to take turns stirring the contents of the cans occasionally. Also have students take turns reading the temperatures every 3 minutes for 15 minutes. Be sure to record the temperatures up on the blackboard or a large sheet of page. After you have received all of the results, graph them on the blackboard or on the large sheet of paper.

Follow up this activity with these questions:

• Which temperature falls the slowest?
• How do your predictions compare to the results?
• What material stores solar energy the best?
• Do you think other materials exist that hold heat better than water?

Development

Collecting Solar Energy
Divide the class into teams of three or four. Each student team should build a solar energy collector from a shoe box (without a top), aluminum foil, black construction paper, and clear plastic wrap by following these directions:

1. Cut off one of the long sides of the shoebox. Cut the two short sides at an angle.
2. Line the bottom of the box with black construction paper.
3. Line the rest of the box with aluminum foil.
4. Make a cover for the box by taping clear plastic wrap along the top back edge and the sides. Leave a corner untaped, so things can be put into the box.

Discuss what would happen to the water in a small cup placed in the box, if the box is placed in bright sunlight. Remind students of what they observed in the previous class, when they tested four materials to see which would best conserve heat.

Then ask students to think about what would happen to the water in a small cup placed in bright sunlight next to the box. Have each team make a prediction based on past experience and discuss the reasoning for the predictions. These predictions will probably involve changes in the temperature of the water in the two cups.

Now have each team try the experiment with small paper cups of air temperature or tepid water (not cold water). Which predictions are confirmed by the experimental results?

Several teams could work together to investigate questions such as:

• Does it make a difference how long the water samples are in the sun before they are tested for changes?
• Are the results different if the boxes are completely lined with black construction paper?
• Are the results different if the boxes are completely lined with aluminum foil?
• Are the results different if the locations of the black construction paper and aluminum foil are reversed?
• Does it make a difference which direction the box faces?
• Can you make tea or cook instant rice in the box?

Students can be assessed on how consistent their explanations are with the results they obtain and how well they control the variables and use controls in each investigation.

A Solar Grill 
The solar energy collector in the previous activity is mainly designed, like an oven, to heat a volume of air. The solar grill is designed to concentrate the solar energy and heat an object, a hot dog.

Each student team should build a solar grill from heavy corrugated cardboard (or similar stiff, but easy to cut, material like foam board), aluminum foil, and flexible cardboard.

They should follow these steps:

1. Cut five identical 15 x 20-cm rectangular pieces of heavy corrugated cardboard. The exact dimensions are not so critical, but all pieces must be the same size.
2. Also cut four identical 3 x 20 cm strips of the same cardboard.
3. Use a ruler to draw a line 2-cm from one of the long edges and parallel to the edge of each of the rectangular pieces. Locate and mark the exact centers of these lines.
4. On three of the rectangular pieces, use a compass to draw a circle with a radius of 8 cm and centered on the center mark you made. (Only part of the circle can be drawn.)
5. Cut off the edge of the three rectangles along the line you drew above and then carefully cut out the semicircles you drew.
6. In one of the remaining rectangular pieces, punch a small hole through the center mark with a thin wooden skewer. Cut a narrow slot in the last piece from the edge just to the center mark.
7. Use tape to assemble the five rectangular pieces and four strips. If necessary, use another diagonal strip of cardboard on each side to make sure the “box” is rigid.
8. Cut a piece of flexible cardboard into a rectangle just the right size to make a semicircular trough that just touches the ends when it is pushed down into the cutouts in the interior partitions in the box.
9. Cover one side of this rectangle with aluminum foil, shiny side out and as smooth as possible.
10. Tape the rectangle into the box to make a trough with a shiny inside surface.

Now have students test the solar grill by cooking a hot dog. Insert thin wooden skewers about 10 cm deep into each end of a hot dog. Try to keep the skewers in a straight line. Put one of the skewers through the hole in the end-piece of the solar grill and rest the other skewer in the slot in the other end-piece. The hot dog should be suspended on the center line of the trough. Put the grill in bright sunlight and test whether the hot dog can be cooked.

Have students explain (with diagrams) how the solar grill concentrates solar energy on the hot dog. Since research indicates that it may not be clear to middle-school students that some forms of energy, such as light, sound, and chemical energy, can be used to make things happen, discuss each team’s diagram with the class and allow the teams time to revise their drawings based on the discussion.

Have students brainstorm how the solar grill can be improved. Be sure they have a rationale for each improvement, for example, a larger trough to catch more light and cook faster. Try the improvements and assess student performance on the basis of their use of energy concepts and the properties of light (reflection) to explain the grill and possible improvements.

Assessment of student work can be based on the quality of the data they get to answer the question(s) investigated, the use of appropriate controls, and explanations that are consistent with experimental results and principles the students are expected to know and be able to use.

Assessment

Children should have many opportunities to observe and talk about uses of the sun’s energy. The sun is the main source for energy in our world, and even the energy in the fossil fuels we use, such as oil and coal, comes from the sun indirectly, because they come from plants that grew long ago.

Solar-powered devices, however, harness the sun’s energy directly, and take advantage of an energy source (the sun) that is available indefinitely. Because the flow of energy is variable in solar energy applications, very large collection systems are needed.

Show a solar-powered calculator to the class and ask students:

• How does a solar-powered calculator work indoors where there is no sunlight?
• What criteria can be used to tell whether solar (light) energy is being converted to electricity by the solar cell(s) in the calculator?

Questions to investigate further include:

• How much of the solar cell array must be in the light to create enough electrical energy to meet the criteria? Is the answer the same for any light?
• Does the distance from the light to the solar cell array make a difference in the energy reaching the cell(s)?
• Does the color of the light make a difference in the energy reaching the cell(s)? (This is tricky to investigate because the energy reaching the cells depends on the wavelength of the light and its intensity, and the response of the cells is different for different colors.)

These do not exhaust the questions students might want to investigate, but each student team ought to try at least one of these three, since the results will help the class get a better picture of the factors that affect the conversion of light to other kinds of energy.