Does you family head to the kitchen table every time they hear a fire siren? Do you get even with the neighborhood bully by inviting him over to dinner? Does Dad refer to the fire detector as the “oven timer”? It sounds like your family might need cooking lessons. Well Reeko can’t (or won’t) provide cooking lessons but he sure knows how to make a cool little oven – out of a pizza box! And while we’re at it we’ll learn how thermal collectors are able to collect sunlight and convert the solar radiation into heat.
The sun is an ultimate form of energy and it has produced that energy for billions of years. Solar energy, or solar radiation, can be converted to other forms of energy – electric or heat. Almost 200 years ago, a man named John Herschel invented a “solar thermal collector box” that used sunlight converted to heat to cook food. Today, solar thermal collectors, or solar ovens, are of great use in countries where electricity is not available. In some cases, they are used to heat and purify water which means that this great invention actually saves human lives.
Thermal collectors come in many types but each of them works in the same basic manner. First, they concentrate sunlight usually using a mirror or reflective metal surface. They attempt to collect or reflect as much sunlight as possible into a smaller, concentrated space. They also use some sort of black surface to convert the sunlight to heat. Black absorbs sunlight (hence the reason it is black) and when it does, it gets very hot. And lastly, after the sunlight is collected and turned into heat, the heat that is produced is trapped, usually using a clear plastic or glass covering to hold the heat in (while still allowing the sunlight to get through).
- Glue black construction paper to the bottom of a pizza box.
- Glue or tape aluminum foil to the inside top of the pizza box. Keep the foil as flat as possible. Any wrinkles will decrease the amount of sunlight we collect.
- Position the lid so it reflects the sunlight onto the black construction paper.
- Put a glass of water in the box
- Cover the entire box with plastic wrap
The box will be able to reach temperatures in excess of 150 degrees. With a mirror like reflective surface the box would be able to reach even higher temperatures. With a glass covering the box could get even hotter and could actually replace a regular household oven.
Parent/Teacher/Advanced Notes [click to expand]
A parabolic thermal collector is the most powerful type of thermal collector. It concentrates sunlight at a single, focal point, via one or more parabolic dishes -- arranged in a similar fashion to a reflecting telescope focuses starlight, or a dish antenna focuses radio waves.
There are two key phenomenena to understand in order to understand the design of a parabolic dish. One is that the shape of a parabola is such that incoming rays are parallel to the dish's axis and will be reflected toward the focus, no matter where on the dish they arrive. The second key is that the light rays from the sun arriving at the earth's surface are almost completely parallel. So if dish can be aligned with its axis pointing at the sun, almost all of the incoming radiation will be reflected towards the focal point of the dish -- most losses are due to imperfections in the parabolic shape and imperfect reflection.
Losses due to atmosphere between the dish and its focal point are minimal, as the dish is generally designed specifically to be small enough that this factor is insignificant on a clear, sunny day. Compare this though with some other designs, and you will see that this could be an important factor, and if the local weather is hazy, or foggy, it may reduce the efficiency of a parabolic dish significantly.
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