Catch a Few Rays ... to Wash the Dishes

IF you've ever used a magnifying glass to focus sunlight on a leaf or piece of paper, you know that if you hold the glass long enough at the right height, the sunlight can burn the material.

That little experiment, which uses a lens to focus the sun's rays on a tiny spot, hints at the huge amount of energy the sun sends our way every day. It also gives a strong clue as to why many people want to directly tap that energy for everyday use.

If you had used a match to light the leaf, the burning wood or paper in the match would have given off small amounts of a gas known as carbon dioxide (abbreviated as CO2), which in large quantities is becoming harmful to our atmosphere.

If you had used a match, you also would have smelled a slight odor of rotten eggs: burning sulfur from the match tip. Sulfur is found in coal and other plant-based fuels and combines with oxygen when burned to form another gas, sulfur dioxide (SO2). When SO2 in the atmosphere mixes with rainwater, it forms acid rain, which can weaken or kill trees in forests and fish in streams, ponds, and lakes.

But what if you could get the heat you need for some jobs from the sun, rather than from coal, oil, or natural gas? Unlike the match, which burns out, or the other fuels, which eventually will run out, the sun represents a virtually limitless energy source. And it doesn't pollute. Even if the sun is blocked by cloudy weather, you still can use it often enough to cut the amount of other fuels you need.

One use for solar energy is to heat water for showers or washing dishes. Using a cardboard box and other supplies from around the house, you can build a model hot-water heater. The instructions appear on this page.

Once you build it, the model will show you not only how quickly the water can heat, but what kind of weather conditions you need for the heater to work best. To help you see this, record the water temperature at different times, say once every hour.

If you know how to draw graphs, you can make a graph for each type of weather condition, plotting the temperature from bottom (lowest temperature) to top (highest temperature) on the left side while plotting the time in one-hour segments from left to right across the bottom.

Use it during days with different kinds of weather. How long does the water take to heat up on a sunny day? On a partly cloudy day? Does it heat up at all on a cloudy day?

Have you ever heard of a solar oven? It uses the same approach as the hot-water heater, only instead of heating water, you can bake bread or cook roasts. Depending on how they are made, these ovens can reach temperatures of up to 400 degrees.

Once you have built and tested the model it in its ''household'' mode, you can take the experiment one step further to see how some people use solar energy to make electricity.

In the Mojave Desert in California, one electric company has built a tower with a water tank on top. On the ground sit dozens of special mirrors designed to do what that magnifying glass did to the leaf - in this case, reflect and focus the sun's energy on the tank to boil the water. The water turns to steam, which is used to generate electricity.

To see how this works, you'll have to find or buy one or two inexpensive magnifying makeup mirrors. Then, set up the solar hot-water heater so that you can reflect sunlight into it from the mirrors. Using the thermometers, once again plot on a graph the time it takes for the water to heat. Hint: To keep the sunlight shining into the heater, you'll have to keep moving the mirrors to follow the sun as it moves across the sky.

You won't make steam, but you will see how using the mirrors helps heat the water faster and to a higher temperature.

* Pete Spotts can be reached by e-mail at spotts@csps.com.

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