Concentrated Solar Power

A few years ago I got the idea to make a mirror to reflect light to a point. It’s the same concept as a magnifying glass in which you focus the sun’s heat energy.

Making the solar mirror is simple. The materials you need are:

· A rotating platform on top of which a container can rotate
· Plaster-of-paris (roughly 2.5 litres or 2500 mL)
· A container with inside diameter roughly 10.5 inches
· A large potato chip bag (with shiny metallic surface on the inside)

The liquid plaster-of-paris will be placed inside the container, and the container will be made to rotate about its center at roughly one revolution per second. Due to the nature of the physics, this will cause the surface of the plaster-of-paris to form a parabolic shape. Upon hardening, the plaster will retain this shape.

Mathematically, the parabolic shape is defined by:



where w is the rotation rate in radians/sec, and g is the gravitational constant = 9.81 m/s².

To make the rotating platform you may have to get creative. One way is to get an old turntable and place the container on top, or use a motor-gearbox-pulley combination to rotate the container at one revolution per second. I don’t have pictures of the turntable I made as it was a few years ago and I have since dismantled it.

Once you’ve made the plaster shape, it’s time to create a mirror surface on it. This is where potato chip bags come in handy. It’s a cheap way to make a shiny reflective surface.

Clean out the inside of the chip bag with detergent. Then cut pie shaped pieces out of it and glue them onto the surface of the dish, shiny side up. This will create the mirror surface. See Figure 1.

Figure 1: Parabolic dish

When the sun is out, orient the dish so that it directly faces the sun. The focal point will be roughly 5 inches from the bottom. Mathematically, the focal distance is given by:



You can find the hot spot by moving your finger along the space above the center of the dish until you feel it. That’s your focal point. It can get very hot so be careful. Using a digital kitchen thermometer I measured a temperature of over 200 degrees Celsius! See Figure 2.


Figure 2: Temperature of sun's focus

This is a small-scale example of concentrated solar energy. Clearly, there’s serious potential for future energy needs.

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