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Name: Ben
Status: Student
Grade: 6-8
Location: CA
Country: United States
Date: January 2009


Question:
I am building a solar furnace by focusing the reflection of 144 mirrors (1 inch each) onto a single point. I want to measure the light intensity at that point. My intent is to setup the focal point at various distances and measure the differences in efficiency. All the light meters I have found would burn up.



Replies:
You have a couple of problems here. First, as apparently you have found, optical light meters will not withstand the amount of energy you are focusing on them. However the focus of the optical radiation may not be the same as the focal point of the infrared (heat) focal point. Each one may be off by a little, but if you are using 144 "mirrors" the effect could add up quickly.

A more direct measure would be to use a thermocouple, or similar thermal device, in place of an optical detector. The advantages are that the thermocouple is measuring "directly" what you want to know -- where is the "focus" of the thermal energy. Also thermocouples (again, other thermal heat detectors could be used) are much more rugged to exposure to high temperature, and they are at least as small as any optical detector for the same conditions. You should be able to "focus" the heat from each mirror onto the target.

Finally, be aware that the optical properties of materials to infrared (heat) radiation may be different than the optical properties of "visible" light -- that is, some optical element may be highly reflective to visible light but absorbent to infrared (heat) radiation -- or the other way around.

Your's is an interesting project, but you need to be careful to make sure that what you "see" is what you "get".

Vince Calder


Typically light meters are for measuring the intensity of light that you can see with your eye. They are very sensitive, and they are not used to measure heat.

The purpose of your solar furnace is heat something. You are using sunlight, which is bright, but a lot of the heat is also coming from invisible infrared wavelengths that the sun also emits.

What you want is called a "calorimeter." That is a fancy name for a heat meter. There are many kinds of calorimeters.

One kind of calorimeter looks like a small hockey puck, and you put in the path of a laser beam to measure the laser beam power. You do not want to keep it there too long or it will get too hot.

Another type of calorimeter looks like a cooking pot, to which you add chemical liquids, and the calorimeter measures heat produced when the chemicals react.

What you want is something that you can put at the center of your furnace to measure heating. You probably do not want to buy a professional model.

Instead, I suggest getting a block of something, like an iron or aluminum. This will be your calorimeter. You will want to paint the block black so most of the sunlight is absorbed and have a way to measure its temperature; a thermocouple would be best. You can determine the "heat capacity" of your block by multiplying the specific heat of the metal (from handbooks) by the mass of the block.

If you put the block in your furnace for a while, say 10 seconds or 100 seconds, it will get warm. If you measure the temperature change you can determine the amount of heat absorbed during that time. You heat intensity will be the temperature rise multiplied by the heat capacity divided by the number of seconds in the furnace. Your intensity should have the units of "watts."

An alternative method would be to get your block warm and then remove it from the furnace and place it in a small container of water. The block will cool and the water will heat up. Water has a heat capacity of 4.18 joules/gram. You can write a mathematical expression that sets the amount of heat lost by the block equal to the amount of heat gained by the water, after both come to the same temperature.

Robert Erck



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