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I recently participated in an online briefing with Dave Lettero and Heidi Eggert from the Bonneville Environmental Foundation (BEF), and several prominent bloggers to discuss how we can all reduce our carbon footprint. Traditionally, fossil fuel is how we obtain most of the energy in our country but BEF is working to change that by helping companies become better environmental stewards. The highlight of the briefing was learning how to create a solar oven and a solar cell. Both of these activities can be completed by children (with adult assistance and supervision). The solar oven project is geared towards younger children, while the solar cell project is geared towards older children. I have included both activities below in case you’d like to try them out with your children:
Activity: Solar Oven
OVERVIEW: This is a two-part activity. In Part I students work with the Sun Oven to cook food and purify water using solar energy. In Part II students make their own solar ovens using common household materials.
GOAL: Students learn the basic principles of solar-thermal and passive-solar heating.
TIME FOR PART I: 30 minutes to 1 hour
TIME FOR PART II: 2-3 1-hour class periods
SETTInG FOR PART I: Outside on a sunny day.
SETTInG FOR PART II: Classroom for oven construction and outside on a sunny day for testing ovens and cooking.
MATERIALS FOR PART I: The Sun Oven and Sun Oven manual are in your science kit. You’ll also need a pot to hold water, water, and food to warm up in the oven (e.g., 1-2 student lunches).
MATERIALS FOR PART II: Pizza boxes, tin foil, clear plastic wrap, tape, black construction paper, a stick (for propping the oven open), thermometer, and food to cook in the oven (e.g., bread with cheese).
Solar Oven Instructions
PART I: Use the Sun Oven and the Sun Oven manual to cook food and pasteurize water in the oven. The oven has a built-in temperature gauge so the process can be monitored.
PART II: Construct homemade solar ovens. Depending on class size, students can work in groups of three to five.
1. Use a box knife or sharp scissors to cut a flap in the lid of the pizza box. Cut along three sides, leaving about an inch between the sides of the flap and the edges of the lid. Fold the flap out so that it stands up when the box lid is closed. Cover the inner side of the flap with aluminum foil so that the rays from the sun will be reflected off.
2. Use clear plastic wrap to create an airtight window for sunlight to enter into the box. Do this by opening the box and taping a double layer of plastic wrap over the opening you made when you cut the flap in the lid. Leave about an inch of plastic overlap around the sides and tape each side down securely, sealing out air.
3. Line the bottom of the box with aluminum foil to reflect heat and then cover that with black construction paper (black absorbs heat).
4. Now your oven is ready to try out! Take it outside to a sunny spot and adjust the flap until the most sunlight possible is reflecting off the aluminum foil and onto the plastic-covered window. Use a stick or a dowel to prop the flap at the right angle.
Pizza-box solar ovens will reach about 200 °C on a sunny day, so they take longer to heat things up than a conventional oven does. Try melting mozzarella cheese over toast with basil and tomatoes, warming leftovers for lunch, or experimenting with any other food you can think of ideas for.
To build a more advanced homemade oven, visit the Solar Cooking Archives online (http://solarcooking.org/).
Activity: Create a Solar Cell
OVERVIEW: In this two-part demonstration, a sheet of copper flashing is transformed into a semiconductor to demonstrate the photovoltaic principle.
GOAL: Students learn that semiconductors can be used to generate current and are introduced to the photovoltaic principle.
TIME FoR PART I: 1 hour to cooker copper flashing and 20 minutes for the flashing to cool
TIME FoR PART II: 30 minutes for class demonstration and discussion
The steps can take place during two class periods, or Part I could be done before class.
SETTInG FoR PART I: Kitchen or science lab with a burner to cook copper flashing.
SETTInG FoR PART I: Outside on a sunny day to perform demonstration.
MATERIALS FoR PART I: 2 pieces of 4 x 4 copper flashing (in science kit), scissors, sandpaper or wire brush, mental tongs, and a stove or single burner.
MATERIALS FoR PART II: Plastic container (3-liter bottle with the top cut off), multimeter (in science kit), alligator clips, and saltwater.
Create a Solar Cell
The cuprous oxide created during the cooking process turns an ordinary piece of copper into a type of material called a semiconductor. A semiconductor is in between a conductor and an insulator. In a conductor electricity can flow freely. In an insulator electrons are bound tightly to their atoms and do not flow freely. In a semiconductor there is a gap (called a bandgap) between the electrons that are bound tightly to the atom and the electrons that are farther from the atom which can move freely and conduct electricity. Electrons cannot stay inside the bandgap. An electron cannot gain just a little bit of energy and move away from the atom’s nucleus into the bandgap. An electron must gain enough energy to move farther away from the nucleus, outside of the bandgap. Similarly, an electron outside the bandgap cannot lose a little bit of energy and fall just a little bit closer to the nucleus. It must lose enough energy to fall past the bandgap into the area where electrons are allowed.
When sunlight hits the electrons in the cuprous oxide, some of the electrons gain enough energy from the sunlight to jump past the bandgap and become free to conduct electricity. The free electrons move into the saltwater, then into the clean copper plate, into the wire, through the meter, and back to the cuprous oxide plate.
To Create a Solar Cell:
1. Prepare the Copper: Wash your hands so they don’t have any grease or oil on them. Use metal sheers or strong scissors to cut a piece of the copper sheeting roughly 6 x 6 inches, about the size of the small burner on a stove. Then wash the copper sheet with soap or cleanser to get any oil or grease off of it. Use sandpaper or wire brush to thoroughly clean the copper sheeting so that any sulfide or other light corrosion is removed.
2. Turn Copper into Cuprous oxide: Place the cleaned and dried copper sheet on the burner and turn the burner to its highest setting. As the copper starts to heat up, oxidation patterns will begin to form. Oranges, purples, and reds will begin to cover the copper sheeting. As the copper gets hotter, the colors are replaced
with a black coating of cupric oxide. This is not the oxide we want, but it will flake off later, showing the
reds, oranges, pinks, and purples of the cuprous oxide layer underneath. The last bits of color disappear as the burner starts to glow red. When the burner is glowing red-hot, the sheet of copper will be coated with a black cupric oxide coat. Let it cook for a half hour until the black coating is thick. This is important because a thick coating will flake off nicely, while a thin coat will stay stuck to the copper. Use tongs to remove the copper sheet form the heat and place it in a sink to cool. As the copper sheeting cools, it will shrink. The black cupric oxide now covering the surface also shrinks, but at a different rate than the copper beneath. Black flakes of cupric oxide will pop off the copper with enough force to make them fly into the air. When the copper has cooled to room temperature (about 20 minutes), most of the black oxide will be gone. Rub the sheet lightly with your hands and run it under water to remove most of the remaining black oxide. You do not need to remove all of the black spots, as doing so might damage the delicate red cuprous oxide layer, which is needed to make
the solar cell function. Carefully bend the two pieces of copper (one that you cooked in Step 2 and one that has not been cooked) and fit them in the plastic container so that they do not touch each other. The cooked copper should have the reddest side facing out towards the sun. Set your multimeter to provide readings in mil-
livolts. When the sun shines on your solar cell, you will begin to generate power. In the photo to the right the solar cell is generating 81.6 millivolts. Notice that if you block the sunlight striking the container the voltage quickly decreases
3. Assemble the Solar Cell: Fill the container with hot tap water so that a top portion of both pieces of copper will remain dry. Mix about two tablespoons of salt into the container, and stir the water until all the salt is dissolved.
4. Monitor the Solar Cell: Connect the cuprous oxide to the positive (red) lead of the multimeter, and connect the clean copper sheet to the negative (black) lead of the meter.
5. Now move your container out of the sun light. Notice that the solar cell still generates an electrical charge even in darker conditions.
If you’d like more information on how to shrink your carbon footprint, check out BEF’s site, Shrink Your Foot. This interactive site has tips/tools on how to reduce your carbon footprint.
Win It! One of you will win a Solar Car Kit. Simply head on over to Shrink Your Foot. Come back here and leave me a comment with something you learned. Comments will close on June 14, 2009 at 10 PM PST. One comment per person, please (unless you do the extra credit). US residents only. Duplicates and comments not including the above information will be disqualified. Comments are moderated. If you don’t see your comment in a reasonable amount of time, send me an email. Bloggers and non-bloggers may enter. If you don’t want to leave your email address, please be sure to check back for my announcement on the winner. Please note that winners must respond within 48 hours of being announced/contacted or another winner will be drawn.
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