Mirrored Parabola

Temperatures rise above 100 F during Sacramento summers. Inevitably, I'll get caught standing somewhere with the direct sun pounding my skin. This year, the heat of the sun cooked up a new idea: A big, reflecting parabola, designed to focus a bunch of sunlight onto a tea kettle.

My first task was to evaluate various reflective surfaces. The obvious solution was mirrors. The cheapest one foot squares I found were mirror tiles, priced at $1.66 each. In the photo on the right, you can compare the aluminum foil square with the glass mirror.

I could make a huge cardboard and aluminum foil mirror for just pennies. A similar amount of energy would be reflected, but it wouldn't be nearly so precisely directed.


Before I started building, I tried an experiment out on the sidewalk.

I bought 12 mirror tiles and propped them up on bricks in the afternoon sun. A green utility box made a great target for my very small array (VSA) of mirror tiles. I aimed each brilliant reflection onto the same spot.

It was cool! That one very bright spot was being hit by sunlight from 12 square feet of sun-power. 

Melanie and I dared each other to stand in it.

Her eyeballs began to melt and her hair caught fire. Ok, not really. She reported that it felt really, really hot. My camera tried to compensate for the very bright light, rendering the rest of the scene quite dark.

When I tried, steam came out of my ears, I developed a hypercritical case of skin cancer and my teeth exploded.

Ok, not really. It was amazingly hot. Too hot to endure for more than a few seconds.


I learned that plenty of effort has been exerted in an attempt to calculate an accurate Solar Constant, the light and heat energy that strikes earth's surface.

The energy that hits a particular square of land depends upon the sun's angle, the weather, the density of the atmosphere above that spot, etc. The figure scientists use most is 1.37 kW / m, or 433 btu per ft per hour, or 127 watts per square foot.

By reflecting 16 square feet of this energy onto one square foot, I could harness 2,032 watts of free solar energy!

It would have been best to build a big, reflective dish.

Unfortunately, I didn't. I was intimidated by the complexity, precision of the structure and the price of the mirrors. Besides, large dishes of this shape are everywhere, what fun would that be?

My goal was to have the mirror array ready to boil water on the 5th of July, during our annual block party.

I ended up building a simple line of mirrors in a parabolic curve.

I used new mirror tiles, and cut down some large scrap mirror to match.

Cutting mirror is just like cutting glass. The first step is to etch a straight line into the surface, and the next step is to snap it in half like a breadstick. It is surprisingly easy and fun.

Please continue reading part 2 of the mirrored parabola.

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October 17th, 2003.  

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