Before you can use solar power, you have to be able to collect and concentrate the rays of the sun. Parabolic dishes collect the sun's rays and focus them on a single spot above the centre of the dish. They operate efficiently only when pointed directly at the sun. They require a drive and tracking mechanism to follow the sun's path as it crosses the sky. A trough mirror focuses solar radiation along a horizontal band running the length of the trough and will operate throughout the day without motors or tracking devices.
- Skill level:
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Things you need
- Tape measure
- ¾-inch plywood
- Graph of parabola
- 2-inch finish nails
- Flexible yardstick
- Sabre saw
- ¼-inch hardboard
- 2 2-by-4 pieces of lumber
- Drill with screwdriver bit
- Wood screws
- Spray adhesive
- Aluminium foil or reflective Mylar
Use a tape measure, straightedge and a pencil to lay out a grid on a piece of 3/4-inch plywood.
Transfer the plot points on a parabolic graph (see the link in the Resources section) to the grid on the plywood.
Use a hammer to drive a 2-inch finish nail into each point on the plywood layout. Leave about three-quarters of each nail exposed.
Press the flexible yardstick against the nails on the outside of the parabola to form a smooth arc. Use a pencil to trace along the outside of the arc.
Mark the focus and top of the parabola with a pencil.
Remove the finish nails and cut your marked pattern out of the plywood with a sabre saw.
Mark and cut out a second parabola by repeating steps 1 through 6. These two pieces will form the ends of your parabolic trough.
Measure the outside edge of the plywood forms to determine the width of the material needed for your trough.
Measure and cut your hardboard panel. You can make the trough any length you wish. The width of the panel must match the dimension of the outside arc of the plywood forms.
Place both plywood forms upside down on a hard level surface.
Align the hardboard panel with the edges of the plywood forms. Use a drill to fasten each corner of the hardwood panel to the corner of the form with a wood screw.
Gently bend the hardboard panel around the plywood forms. Secure the hardboard every two inches with wood screws. Turn the trough over when you have fastened both sides.
Cut two 2-by-4 pieces of wood to match the length of the trough.
Line the 2-by-4s up with the long edges of the trough. Place one on each side. These will act as braces to keep the edges of the trough rigid.
Fasten the 2-by-4s to the edges of the hardboard. Drive a wood screw through the hardboard into the 2-by-4 about every three inches.
Spray adhesive on the interior surface of the trough and apply a smooth layer of aluminium foil or reflective Mylar.
Tips and warnings
- If you want to use the trough to heat water, drill a hole on the focus at each end large enough to accommodate a copper pipe. Push copper pipe through the hole, and plumb each end.
- If you wish to use the trough as a cooker, cut a piece of rigid wire mesh wide enough to sit at the level of the focus and place it inside the trough.
- Wear sunglasses while working with the trough outside. The parabola focuses visible light as well as heat.
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