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.

 

[astrokreng]

for your question. Calculating the power at the focal point of your Fresnel lens using temperature measurements is possible, but it will require some additional information.

First, you will need to know the specific heat capacity of the material that your lens is made of. This is a measure of how much energy is needed to raise the temperature of a certain amount of the material by one degree.

Next, you will need to measure the change in temperature over a certain amount of time at the focal point. This will give you the change in energy (in joules) at the focal point.

Using the specific heat capacity, you can then calculate the amount of energy in joules that is needed to raise the temperature of the thermocouple by the measured amount. This will give you the power (in watts) at the focal point.

However, keep in mind that this calculation will only give you an approximation of the actual power at the focal point. Factors such as losses in the system, efficiency of the lens, and other variables may affect the accuracy of your calculation. It would be best to repeat the measurement multiple times and take an average to get a more accurate result.

I hope this helps and good luck with your experiment!