Solar Reflector Science Projects
Parallel radiation may be concentrated with solar reflectors. A parabolic trough concentrates light on a line and a parabolic dish concentrates light on a point. Both solar reflectors make excellent science projects.
Parabolic Dish: A practical application of a parabolic dish is a flashlight lens, which is used to transform a point source of light into a parallel beam. The reverse is true. Since sunlight radiation is essentially parallel it may be concentrated at the focal point of the lens. As a matter of fact a tiny flashlight lens may be used as a cigarette lighter by substituting a cigarette for the bulb and by pointing the lens in the direction of the sun. A type of solar reflector dish concentrator may also be made by lining the inside of a cardboard box with aluminum foil. Large experimental parabolic dishes known as heliodynes are capable of melting steel but they operate at a low efficiency and must be continually aligned to be of any practical value.
Parabolic Trough: Solar power plants use parabolic troughs to make super heated steam and generate electricity because their fabrication and tracking equipment is less expensive than the dish. A simple parabolic trough may be made with a sheet of cardboard lined with a piece of aluminum foil. I've heard of people using this device to roast hot dogs. Both solar reflectors can be used as cookers, but we won't be cooking hot dogs with our science projects. Right? To be scientific and collect meaningful data we should control as many variables as possible. For this reason we should construct our solar reflectors with the aid of a carefully constructed template.
MATERIALS: Mylar with mirror finish, Al flashing 20"x 4', 1x8x4, 1/2" copper tube 5'long, drywall nails
TOOLS: jig saw, hammer, drill, 3/4" bit
Cut the 1x8x4 in half so that you have two 1x8x2's. On one half of one of these boards sketch out a grid with one inch increments. Use this grid to plot the positive x values of the curve with a special parabola with the formula y = x2/4p. Any quadratic equation can be used to plot a parabola, but this equation will help you pinpoint the parabola's focal point, P. Before plotting this curve choose a value for P, this will be the focal point of the parabola. I have chosen 2" for the value of P, but you may choose any value you wish. One inch increments should give you all the points you need to plot this curve. When you're done half a parabola should look like this.
Notice that this graph shows 1 inch increments of x, but not of y. I've done this to clarify the y values of the parabola. You should have no difficulty finding these points on your one inch grid.
When you're satisfied that your plot is correct cut it out with a jig saw. Use this first cut to sketch out an identical curve on the other board and then flip the first cut board to finish the parabola. Now cut out the board.
Notice the 3/4 inch hole drilled at the focal point of the parabola. This is where the five foot long 1/2 copper tube will be inserted to collect heat from the solar concentrator.
Now comes the tricky part where you might need a little help holding things. This is where the flashing is nailed or screwed to the end supports. Pre drilling holes spaced about 3 inches apart and about 3/8 " from the ends might be a good idea. This will facilitate the process of joining the end supports to the flashing like this:
With a little care you'll build a fine parabolic trough. Notice the furring strips on either side of the trough. These strips give the flashing extra support and also provide a place to attach glazing. Congratulations on the completion of your first solar reflector science project, but you're not done yet. You should now test solar concentrator for efficiency and maximum temperature.
On a sunny day point the parabolic trough directly at the sun and record the maximum temperature of the boiler/flow tube. Now cover the unit with glass or plastic and see if the maximum temperature improves. An oven thermometer will do if you're unable to get hold of a more accurate device.
Now attach a plastic tube to one end of the boiler/flow tube and pour water of a known temperature at a controlled flow rate. Notice how hot the water gets. It might even boil. If you set up a drip rate of 1 gallon per hour you could calculate the heat gain of the unit and compare this to the solar energy available. Refer to Solar Energy Facts to do this calculation. Do these experiments with and without glazing. Compare and explain your results and suggest things that might improve the collector's performance. How much better could a parabolic trough be if the reflecting surface were better able to focus sunlight? What is the difference between temperature and heat.
The parabolic dish is a more complicated project. To do make this as a science project you'll need to build a parabolic template with a central axis. This is then rotated 360 degrees to plot the solar reflector surface. Other projects are available from the Solar Energy Science Projects
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