The sun is a tremendous emitter of energy. Most of the energy is in the form of light and heat, which can be collected and used for generating electricity, as well as for heating, cooling and lighting building.
Photovoltaic Conversion Photovoltaic (solar) cells are large-area semiconductors that convert sunlight directly into electricity. Most of us have seen photographs of photovoltaics on roofs or on the ground near homes and buildings. Photovoltaics can meet virtually any electric power need and are used for numerous applications including watches, calculators, satellites, telecommunications, homes, schools, factories and businesses. Because of the cost, photovoltaic systems are not widely used, but advancements in efficiency and cost reduction should make photovoltaics economically competitive with traditional power sources by the end of the decade.
Solar Thermal Systems Solar thermal systems convert the energy in sunlight to heat by using concentrations such as mirrors to focus sunlight onto a receiver. The receiver contains a fluid that absorbs the heat. The heat is then used to generate electricity or to warm buildings, dry agricultural products or destroy harmful wastes. Solar thermal systems use three different types of concentrators -central receivers, parabolic dishes, and parabolic troughs. Central receiver systems use heliostats (highly reflective mirrors) that track the sun and reflect it to a central receiver atop a tower. Parabolic dish systems use dish-shaped reflectors to concentrate and reflect sunlight onto a receiver mounted above the dish at its focal point. Parabolic trough systems use parabolic reflectors in a trough configuration and are the most mature solar thermal technology .
Solar Heating, Cooling and Lighting Energy from the sun is also used to heat, cool, and light buildings. Buildings are heated and cooled with active and passive systems. Active systems rely on mechanical components to collect and deliver heat. For example, active solar water heaters use electrically powered pumps and valves to control the movement of warmed water. In contrast, passive systems use few mechanical components, relying instead on design features, gravity and natural ventilation to heat and cool homes and buildings. Design features such as an atrium or high windows that allow more sunlight to reach the interior reduce the need for electric lighting.
PROJECT #1 HOW DOES A SOLAR BOX COOKER WORK?
Materials You will need 2 large corrugated cardboard boxes with flaps -one fitting inside the other with about 5 cm between them on all sides and bottoms (inner box should be at least 46 x 56 cm); a flat piece of cardboard about 20 cm longer and wider than the larger box, a light piece of glass or Plexiglas about 50 x 60 cm, a thin metal tray painted black about 42 x 52 cm, dark cooking pots, aluminum foil, water- based glue, lots of newspaper for insulation, string (1 foot long), a stick (about 1 foot in length).
Procedure: Build the solar box cooker by the directions in Appendix 1.
Ideas to Study
1. Investigate various kinds of insulation in your solar cooker.
2. Investigate the cooker at different times during the school year to determine when it takes thelongest or shortest time to cook.
3. Design, build and test your solar box cooker.
PROJECT # 2 WHAT IS THE QUALITY , QUANTITY , AND DISTRIBUTION OF THE SUN’S ENERGY?
A. How much solar radiation is available?
Equipment Low-cost pyranometer and recorder
Resources LI-COR, Inc. (402) 467-3576 (Model LI-1200S Minimum Data Set Recording System). Insolation Data Manual and Direct Nonnal Solar Radiation Data Manual 91990, SERm”P-320- 3880, Golden, CO: National Renewable Energy Laboratory) has data summaries for numerous U.S. cities.
Ideas to Study
1. Using the pyranometer system compare measurements with cloud observations (clear day, partly cloudy, overcast, etc.) Compare observations with printed data summaries of the 3o-yr means. There are a number of variable to consider in your study: days of year (hours of daylight), orientation of radiometer (horizontal, tilt angle, azimuth angle), sky conditions.
2. Determine the connection between weather variable, such as temperature, relative humidity, and cloudiness, and the changes in available solar energy .
3. Determine if your pattern of solar radiation through the day (or year) matches the need for air conditioning, heating, cooking, and hot water in your home?
B. Does air pollution affect the amount of light from different portions of the solar spectrum that reaches Earth’s surface?
Equipment Solar cells, light filters (red and blue gelatin), milliammeter (0-50), or resistors and a voltmeter (0-10 volts). Hint Use the solar cell’s output as measured with the ammeter or voltmeter and various resistors. Measure solar energy at the same sun elevation on different days. Air pollution data are available from the EPA.
Ideas to Study
1. Compare direct sun with full sky radiation (see diagram).
2. Sort out the variables and determine the effects of different kinds of air pollution on different light wavelengths.
3. Investigate if the amount of solar energy reaching Earth is affected by air pollution.
Ideas to Study
1. Design and construct a parabolic reflector. This project requires a knowledge of algebra. Draw 3 parabolic curves using equation of a parabola [y=nx2]. Let n = -10, 1, and 1/2. Calculate x and y for at least 6 data points. Discuss what happens to the focal length [f = x2/4y] as the curve flattens out (that is, as n approaches zero). How does the shape of the parabola aI)d its focal point determine the area in which the reflected light reaches maximum concentration.
2. Design a parabola of reasonable size (no longer than you can build or carry) using the equations y = nx2 and F = x2/4y. Graph the parabola on regular graph paper until you are sure of your design. Transfer the design onto a large piece of sturdy corrugated cardboard. Cut out the parabola. To make the reflector, glue aluminum foil to flexible tag board and smooth it with a squeegee until it is mirror like. Attach the reflector to the cardboard parabola securely. Test the parabolic reflector in the sun. Locate the focal point with your hand. Be careful not to get burned. Do not look at the sun. Measure the focal length and see if it agrees with your calculations. Analyze the effect of tracking the sun on cooking time. If you do not want to spend time designing a solar concentrator you can use the design shown below and test its effectiveness in cooking a hot dog. Make the parabolic trough out of cardboard, poster board, aluminum foil, unpainted coat hanger two boxes (one for the concentrator and one for a stand), nuts and bolts.
3. Investigate the advantage of a collector that tracks the sun. You will need any movable solar collector or concentrator, thermometer or thermocouple. Determine the temperature in the center of a piece of black construction paper when it is flat (perpendicular) to the sun’s rays and when it is tipped at an angle to the sun. Determine the performance of a higher performing collector when it is oriented toward the sun. How much energy is collected during the day if the collector is laid flat on the ground, tipped up at various angles, or continuously pointed at the sun?
4. Investigate what materials are best for absorbing sunlight and converting it to heat? You will need materials made of steel, copper, plastic, color, and thermometer. Place different types and colors of materials on the ground in sunlight. Place thermometers on each one. Be careful to cover the bulb of the thermometer with the same color or type of material, can carefully tape the thermometer to the sample. Record temperature versus time. What effect does insulation or glazing or both have on results? Is a smooth surface better than a glossy surface? What about different kinds of paint? Try putting food coloring into ice cubes and timing the rate of melting in sunlight.
5. Investigate the effect of size on the amount of heat a flat plate collector can absorb? Paint aluminum pie pans of different sizes with flat black paint. Add 100 mL of water to each pan. Measure water temperatures at the start and every 15 minutes. What effect is caused by covering the pans with plastic wrap? Try heating the water to the same temperature using a candle and compare the time and cost.
6. Build a flat plate solar water heater. You will need 112 in or 318 in copper tubing (50 cm), plastic tubing or hose that will fit over the end of the copper tubing, cardboard box, plastic cover, thermometer, two buckets, and a clothespin. Bend the tubing into an “S” shape, being careful not to kink it (a tube bender helps but is not necessary). Paint the tubing with flat black paint and place it in a cardboard box with the ends extending through holes in the box. Create a siphon by placing one end of one piece of the plastic tubing over one end of the copper and the other end in a bucket of water. Use the other piece of plastic tubing to connect the outlet of the copper to another bucket. Use a clothespin to control flow.
Cover the box with plastic and set it in the sun. Measure the inlet and outlet water temperatures. Study the number of turns of copper tubing.
7. Investigate what collector (non-concentrating) designs work best for heating air .You will need thermometers, fan, empty aluminum or steel cans, wood or cardboard box. Make a simple air heater from a cardboard box. Paint the inside black and cover with plastic. Make two holes diagonally (at opposite comers). Use a small fan to blow air through the collector or just let the warm air flow out of the upper hole. Measure air temperatures at the inlet and outlet. You can also measure the airflow rate. Cut cans in half, paint them black, and attach them to the bottom of the box to improve heat transfer to the air . Try insulation. Balance the cost of materials, construction time, and durability against performance.
8. Investigate how a solar still works. You will need a large pan or tub, clear plastic food wrap, rock, masking tape or rubber bands, drinking cup or glass. Make some muddy water and measure enough into a pan to fill it about half way. place the drinking cup in the middle. Cover the pan with plastic wrap and seal the edges carefully. Put a rock on the plastic wrap to make it sag in the middle (but don’t let the plastic touch the cup). As the water evaporates, notice the tiny drops that condense on the cool plastic. How much water collected in the glass? How much water was lost from the pan? You can also study using salt water instead of muddy water. Try insulting the pan or blowing a fan over the top of the plastic. Calculate how large a still would be needed for all the water you drink during the day. Design a more efficient still.
9.Make a solar dehydrator. You will need grapes, plastic window screen, cardboard, masking tape, string, food scale. Make a drying rack by taping plastic screen over a 15 cm X 15 cm hole cut into a piece of cardboard and using string to hang the rack in the sunshine. Or build an oven like the one in the drawing. Remove about 12 grapes from a bunch, weigh them, and spread them on the screen. Cover with a second layer of screen and hang the dehydrator in the sun. When the grapes look like raisins, take them off the rack and weight them again. Try drying other fruits such as peaches, apples, and bananas. Compare the cost of fruit brought fresh and dried in the sun to the cost of dried fruit. Make a dehydrator that has two or more shelves and uses solar heat.
HOW TO BUILD A SOLAR BOX COOKER
- Glue foil on the cardboard. Dilute the water-based glue in a bowl, so that it will last a long time and you can brush-apply it. Glue foil completely over: (a) the inside and outside of the smaller box (cut off the flaps), (b) the inside of the larger box, (c) the inside and outside of the larger box’s flaps, and (d) one side of the flat cardboard piece.
2. Add bottom supports and insulation. Cut out 4 cm squares from the discarded larger box flaps. Glue them on top of each other to form eight 2-3 cm high pillars. Glue these pillars inside the bottom of the bigger box to support the inner box. Tear up newspaper sheets in fourths and crumple each piece into a lemon-sized ball. Cover the bottom of the bigger box with these balls.
HOW TO BUILD A SOLAR BOX COOKER
3. Add inner box and side insulation. Place smaller box inside the larger box. Stuff more newspaper balls between sides of boxes.
4. Cut the flaps of the outer box so that they fit in the inner box. Cut them so that they can be folded over, covering the top space between the boxes as Well as the inner wall of the inner box (see diagram). Fold the flaps over and glue them.
5. Put the black tray in the box. Paint it black if it isn’t already. Use nontoxic paint.
6. Make the lid. Take the flat cardboard piece and center it, foil facing down, on top of the box. Fold down what sticks over the edges of the large box. You need to make four cuts in the cardboard to do this. Then, glue the folded edges of the lid together (not to the box). Make sure the lid fits snugly on the box.
7. Glue the glass to the lid. Cut 3 sides of a rectangle in the lid. This rectangle should be slightly smaller than the glass. Turn the lid over and glue the glass, around its edges, to the inside of the lid. Press it flat until the glue dries. If you use plastic wrap, stretch it out around the rectangular opening and tape in around the sides.
8. Make a prop. Bend up the cut-out rectangle in the lid so that it can reflect sunlight into the cooker. Attach a stick with string to the comer of the reflector and the side of the lid. If it is windy, you may want a prop on both sides.
You are now finished with your solar box cooker and are ready to cook!
Solar Box Cooker: Guidelines for Cooking Food – Teacher Information
1. Put your food in covered black pots in the solar box cooker with the lid on.
2 Aim the box so the shiny side of the lid reflector faces where the sun will be in late morning (lunch) or early afternoon (supper). Tie the prop to hold the lid reflector where it shines the most sunlight into the box.
3. Warning: Temperatures inside the cooker can reach 275 degrees Fahrenheit. Do not leave cooker unattended in a place where it could be disturbed by other students.
4. Food cooks better:
• on a warm, sunny day in late spring, summer, or early fall
• if you put it towards the back of the box
• if you adjust the cooker often so that its shadow lies directly behind it
• if you divide the food up into small pots
5. You need not stir the food while it is cooking. H you open the .box during cooking, be careful of the high temperatures inside.
6. Most of all, put the food in early, and don’t worry about overcooking-solar cookers seldom overcook. Cooking times for recommended foods are:
1-2 hours: rice, fruit, above-ground vegetables, pretzels
3-4 hours: potatoes, root vegetables, some beans (including lentils), most bread
5-8 hours: most dried beans