Calculating Power at the Focal Point with Temperature Measurements

[Z0rb]

Hello All,

I am new to this forum, and right up front I will say I know very little about physics in general. That does not however limit my ability to question and ask for help from people in the know.

Here is my dilemma. I have a small Fresnel lens approx. 6 cm X 15 cm. Or 90 square centimeters. I would like to know how much power in Watts this is really focusing.

I did a little math under the assumption of each square meter receiving 1000 Watts of solar energy on a sunny day at the equator and came up with about 9 watts of calculated power at the focal point.

This of course is a figure that assumes there is no loss in the system. So to get a better idea of how much power is REALLY at the focal point, I want to make some measurements.

I have a K-Type thermocouple hooked into a meter that can read up to 750 degrees Celsius with moderate accuracy. I’ll place the thermocouple at the focal point. The focal point appears to be about 1 cm in diameter. The diameter of the thermocouple is about half of that.

Is it possible (and if so can I have it explained to me), to calculate how much wattage is at the focal point, by taking a temperature reading over time?

Any help or explanation would be great.

Thanks

 

[Andy Resnick]

Your initial assumption is reasonable- 9 watts. For more accuracy, you need the right tools, and a thermocouple is not it.

Even so, you have the beginnings of an interesting experiment- the power will depend on the atmospheric conditions (air absorption), and since you have some tools, why not do the experiment and see what happens?

However, I would instead focus the light into a (known) volume of water into which the thermocouple is inserted, rather than directly onto the thermocouple. That will give you better stability and accuracy.