Who likes to cook? Who likes to eat? How do you think you could heat water or cook food without a stove, oven or microwave? How did people cook before such appliances? People have been able to cook food over open fires, but have you ever heard of cooking with the sun? Although people have used the sun to dry foods for centuries, solar cooking (cooking with the sun) is a more recent technology. There are several reasons to use the sun to cook as much as possible. Regular cooking techniques require burning wood, other fibrous plant remains or fossil fuels (indirectly, if electricity made from burning fossil fuel is used in cooking). Energy from the sun is free — you only need to build a device to capture it. Most solar cookers are easy to build, and solar cooking reduces fossil fuel and electricity consumption.
In areas without electricity or gas for energy, people often do not have ovens or stoves for cooking, so engineers design inexpensive solar cookers to help them. Usually, the people in these areas use wood to cook, so the use of solar cookers also saves trees, which reduces deforestation. Deforestation is a big contributor to global warming, habitat destruction and erosion of soil (which leads to flooding and landslides). Solar cookers can also be used to purify water without using wood or fossil fuel resources. Using solar cookers reduces pollution and the emission of greenhouse gases. Using a solar cooker, rather than an oven or stove, does not heat up the house in summer and is inexpensive to build. These are all good reasons to use a solar cooker! See http://solarcooking.org/ for photographs of solar cookers being used in places all over the world.
Cooking food with the sun is basically the same as regular oven or stove cooking (not microwave cooking). However, there are three principles of solar cooking that engineers must consider: directing the maximum possible amount of sunlight to the food by reflection, converting sunlight into thermal energy, and holding on to solar heat by using insulation. Also, all solar cookers must be able to absorb as much sunlight as possible. Black pots, pans and dishes are all used in solar cookers to maximize the absorption of light. Some solar cookers have black interior surfaces to maximize conversion of sunlight into thermal energy.
Insulation minimizes how fast heat is lost from the solar cooker. Some models also use a glass or plastic lid to trap the heat inside. Cooking times are decreased if insulation is included, such as cardboard, wood, plastic, glass, a layer of air or commercially-produced, high-temperature resistant foam. Box cookers are the most efficient because they retain more heat than other models through the use of insulation on the sides of the box.
Review the ways heat moves: Conduction = by direct contact of two materials Convection = by the interaction of fluid molecules (such as air or water) Radiation = by the movement of heat waves. Which method of heat transfer delivers heat from the Sun to the Earth? (Answer: Radiation) Radiation is how the heat is going to get into our solar oven. So, how are we going to keep the heat in the oven? How does your house keep heat in during the winter, or keep heat out in the summer? (Answer: Insulation) Insulation is any material that slows down heat transfer. For our solar ovens, we are going to use cardboard as insulation. While cardboard is not the best insulator, it will work well-enough for our solar oven. (What might be a better insulator?)
Can you think of any ways to get more heat into our oven? Can we reflect heat? We can reflect heat just like we can reflect light. This means that if we use something reflective, such as aluminum foil, we can bounce more heat into our oven.
Placing a brick or other high heat capacity object in a solar cooker increases its ability to use solar energy. Although the solar cooker takes longer to heat up, the thermal mass (an object that stores the heat from the sun) stores energy that can be used to cook food after the sun sets. Using thicker pots increases solar cooker efficiency for the same reason.
Solar cooker design considerations: Since the sun's position in the sky varies throughout the day, reflectors and reflecting surfaces are used on most solar cookers to allow them to capture enough light. Reflectors direct light onto the pot. Parabolic reflectors can have either a point or a line focus. The parabolic cooker in this activity directs light to the axis. The cone cooker in this activity works similarly. Parabolic reflectors with a point focus maximally heat one spot so the pan is placed at that spot. Panel cookers are more efficient than parabolic cookers because the panels reflect light toward the middle of the cooker and can be adjusted as needed. Box cookers usually have adjustable reflectors and reflective lining, so they are the least affected by variation in the direction of sunlight. However, if cookers are too deep, the resulting shadows in the cooker reduce its efficiency.
Things to Consider Before Beginning: The Box Panel Cooker and the Modified Box Panel Cooker are the easiest to construct. Collect boxes for the solar cookers. Paint the inside of boxes for Cooker #2 the day before, so the paint has time to dry. Gather the rest of the materials and make copies of the Solar Cooker Worksheet. If making the parabolic solar cooker, make copies of the Parabolic Solar Cooker Pattern (one pattern per team, also see Figure 5) for the ends of the cooker. Organize the materials for each type of solar cooker at team workstations. Materials List Each team needs the following plus the materials for one type of solar cooker: Scissors 250 ml beaker (or large glass jar) to hold 200 ml water A non-mercury thermometer Water (200 ml per cooker) Timer Sunglasses (optional) Solar Cooker Worksheet, one per student Mini-size marshmallows (optional) Thin Hershey-brand chocolate bar (optional) Graham cracker (optional) For a shade area control station: 250 ml beaker (or large glass jar) to hold 200 ml water A non-mercury thermometer Water (200 ml) Each team needs the materials to build one of the four types of solar cookers.
Cooker #1 (Box Panel Cooker) 1 cardboard box, ~12 in (30 cm) on the longest side (ask students to bring boxes from home) Rubber cement (or tape) Aluminum foil, roll or sheets, to line the inside of the box
Cooker #2 (Modified Box Panel Cooker) 1 cardboard box, ~12 in (30 cm) on the longest side (ask students to bring from home) Black paint and brushes (or black construction paper, scissors and glue)
Cooker #3 (Simple Cone Cooker) 1 cardboard box in which to prop the cone formed from the poster board (ask students to bring from home) 1 poster board, 35 in 35 in (90 cm 90 cm) Aluminum foil, to cover the poster board Rubber cement (or tape) 3 brass brads (or heavy-duty stapler) Pencil (to poke holes in the poster board if securing with brass brads)
Cooker #4 (Parabolic Solar Cooker) 1 rectangular cardboard box to make the frame, the longest side must be ~12 in (30 cm) (ask students to bring from home) 1 cardboard box to make the ends of the trough, the shortest side must be at least 9 in (23 cm) (ask students to bring boxes from home) 1 poster board, 11 in 14 in (28 cm 35 cm) Aluminum foil, to cover the poster board and trough ends Pencil (to poke holes in the cardboard) Rubber cement (or tape) Clear or masking tape 2 nuts 2 bolts Parabolic Solar Cooker Pattern 1 unpainted wire coat hanger (optional) Construction of Cooker #1: Box Panel Cooker Cut off the top flaps of the box. (Or, remove the cover, if a shoebox.) Cut the cardboard box edges as shown in Fig. 1, so the box opens up to lay flat, as indicated in Fig. 2. To make the shiny inside surface of the box panel solar cooker, cut pieces of aluminum foil so they completely cover one side of the flattened cardboard box. Glue (or tape) the foil to the cardboard surface. Fold the flattened cardboard into a box shape again (see Figure 3) with the foil surface on the inside of the box, being careful not to rip the foil. The side and front walls are adjustable to help direct sunlight into the cooking area.
Construction of Cooker #2: Modified Box Panel Cooker Follow the directions for Cooker #1, the box panel cooker, but instead of lining the box with aluminum foil, paint the inside of the box with black paint (or line it with black construction paper). Ask students what advantages there might be to having the inside of the oven black. Why not use red or white or yellow paint (or paper)? What happens when you wear a black t-shirt on a sunny day compared to a white t-shirt? (Answer: Darker colors absorb heat better than light colors. So when you wear a black t-shirt in the sun you get hotter than if you wear a white t-shirt. Since we want our oven to get hot, we want to collect as much heat as possible. If we were to make our oven insides white, the heat would be reflected rather than absorbed and our oven would not get as hot.)
Construction of Cooker #3: Simple Cone Cooker Cut aluminum foil to fit the poster board. Glue (or tape) the foil to the poster board. As shown in Figure 4, roll the poster board into a cone with the foil surface on the inside, being careful not to damage the foil. Punch holes through the poster board where it overlaps at the bottom of the cone. Fasten the brass brads through these holes. Or, fasten securely using a strong stapler instead of the brads. Open up the box by cutting off or folding in the top flap(s), or removing the lid. Place stones or other weights in the box so it does not move in the wind. Place the solar cone cooker in the box, with its narrow side down.
Construction of Cooker #4: Parabolic Solar Cooker Referring to the Parabolic Solar Cooker Pattern (see Figure 5), cut the ends of the parabolic trough from the smaller cardboard box. Mark on the pieces of cardboard the locations for the holes. Use a pencil to make the holes on each piece of cardboard. Glue (or tape) aluminum foil to the poster board. Gently curve the poster board with the foil surface to the inside, and use tape to secure the trough-shaped poster board to the two, curved cardboard ends (see Figure 6). Cut the top flaps off the rectangular box. Cut a rectangle from one long side of the box (~4.5 in 12 in [11.4 cm 30.5 cm], as shown in Figure 6. Hold the trough in the frame so the hole to attach to the frame is at the corner of the box (see Figure 6). Mark the hole on both sides. Use a pencil to make the holes in the cardboard frame. Use the nuts and bolts to attach the parabolic trough to the box frame. Make sure the trough is stable, but can rotate easily. (Optional) To cook food, straighten the wire coat hanger and bend one end to make a handle. Push the wire through the hole on one side of the trough and out the hole on the other side. Figure 1. The cardboard edges to cut to create the box panel solar cooker.
Figure 2. After cutting the box edges, the cardboard should lay flat in this shape.
Figure 3. The box panel cooker prepared for cooking.
Figure 4. Simple cone cooker construction diagram.
Figure 5. Construction diagram for cardboard side panels of the parabolic solar cooker.
Figure 6. Parabolic cooker construction diagram, ready for cooking.
Absorb: To be taken into a material without transmission or reflection.
Conduction: The transfer of heat from a region of higher temperature to a region of lower temperature by increased kinetic energy moving from molecule to molecule.
Convection: Transfer of heat in a fluid (liquid or gas) when higher-temperature fluid expands and moves, creating heat transfer.
Deforestation: To cut down and clear away the trees or forests.
Electromagnetic spectrum: The entire range of wavelengths or frequencies of electromagnetic radiation extending from gamma rays to the longest radio waves and including visible light. In order of decreasing frequency: cosmic-ray photons, gamma rays, x-rays, ultraviolet radiation, visible light, infrared radiation, microwaves and radio waves.
Erosion: The process of destruction of a surface by abrasive action (for example, the soil eroded away after the trees were removed).
Fossil fuel: A hydrocarbon deposit derived from living matter of a previous geologic time and used for fuel, such as coal, petroleum oil or natural gas.
Global warming: An increase in the average temperature of the Earth's atmosphere, especially causing climatic change.
Greenhouse gases: The atmospheric gases that contribute to the greenhouse effect by absorbing infrared radiation.
Habitat: The area or environment where a person or ecological community lives.
Insulation: A non-conductive material or substance used to prevent the transfer of heat, electricity or sound.
Parabolic: A specific type of curved shape used in solar trough collectors. The shape focuses the sun at 30 to 100 times its normal intensity, achieving temperatures of more than 400 °C. Having the form of a parabola — a plane curve formed by the intersection of a right circular cone and a plane parallel to an element of the curve.
Purify: To rid of impurities; cleanse.
Radiation: The emission and propagation of energy in the form of electromagnetic waves or particles (photons).
Reflect: To cause to bend back or return upon striking a surface.
Thermal energy: The energy an object has due to the motion of its particles.
Thermal mass: Material used to store thermal energy (heat). Stone, concrete, adobe, brick and water work best. Thermal mass is used in a building to absorb or emit heat, and reduce interior temperature swings.
Transmit: To allow to pass through a material.
Original Website: Teach Engineering http://www.teachengineering.org/view_activity.php?url=collection/cub_/activities/cub_energy2/cub_energy2_lesson09_activity3.xml
Authors Xochitl Zamora-Thompson, Sabre Duren, Jeff Lyng, Jessica Todd, Geoffrey Hill, Jessica Butterfield, Malinda Schaefer Zarske, Denise Carlson