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Use pressure to increase temperature of solar oven?

by Sravoff
Tags: solar, thermocouple, thermodynamic
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Sravoff
#1
Jul6-12, 01:26 PM
P: 13
Hello, I want to build an electrical circuit powered by a temperature diffence between two unlike couple, such as copper and iron. I know a good thermocouple would be better, but I don't think I'll be able to get a hold of those materials as easily.

My current plan is to use a solar oven to heat up the iron/copper junction, and use evaporation too cool down another section of the circuit in order to maximize the energy difference.

I started thinking about a pressure cooker and thought if I sealed the junction in a metal container full of water, as the temperature increased, so would the pressure, which would also increase the temperature. In this way would the temperature increase more than just linearly? T=at^2 instead of T=at, where T is temp, a is some constant, t is time--time in focus of the paraboloid I am using to focus the sun.

My reasoning was also influenced by this thread, but it doesn't address my question specifically:
http://www.physicsforums.com/archive...p/t-39446.html

In order for my reasoning to be sound, in the Ideal Gas Law PV=nRT; P must be a function of T, and T a function of P, which seems very wonky.

However, back to the pressure cooker, they use pressure to increase heat, or at least retain more heat (as opposed to losing heat out the top of a pot with no lid) so there could be a benefit to pressurizing the junction.

What am I looking at here, and what kind of strange made-up alleys am I finding myself in?

Thank you
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CWatters
#2
Jul7-12, 08:16 AM
P: 3,264
as the temperature increased, so would the pressure, which would also increase the temperature
If that were possible you would have thermal run away and BOOM.

It doesn't work like that. Pressure cookers work by preventing water from boiling. That way you can raise the temperature of the water above 100C. In an ordinary pan the water turns to steam and dissapears leaving only 100C water behind. However there is no free lunch. You don't get the extra temperature without adding more energy.
mfb
#3
Jul7-12, 09:53 AM
Mentor
P: 12,081
As you use water for cooling, it would be a bad idea to increase its temperature. If you want to play with pressure, lower the pressure. However, this requires more energy than you will get by the better temperature difference in the metal.

Sravoff
#4
Jul7-12, 11:51 AM
P: 13
Use pressure to increase temperature of solar oven?

Thanks CWatters, I thought there was something strange going on with that reasoning and now I see exactly what is.

Mfb, I was going to use two reservoirs of water, one sealed for the pressure, and another for the evaporation, which would have to be refilled.

Thanks all!
mfb
#5
Jul8-12, 06:50 AM
Mentor
P: 12,081
Why do you plan to use water for the hot part? A metal surface, painted black, should work fine and you can get a higher temperature - especially as the colder part is quite hot with ~100C.
Sravoff
#6
Jul8-12, 01:04 PM
P: 13
I was thinking the pressure with the water could create a sort of controlled thermal run away, which is not the case. So I will be just using one piece of metal.
Lunar-Scooter
#7
Jul9-12, 01:30 AM
P: 4
Solar oven?

More temperature = more energy in a given area / volume.

Use a magnification system to heat the junction to very high temperatures.

A simple 120mm glass lense will be more than concentrated enough to "burn" (oxidise) a small thermocouple. Place it in argon in a long small corked test tube, that will stop it from oxidising, but only up to the melting temperature.

I thought the junction really only needed to be a few hundred *C anyway.
Sravoff
#8
Jul9-12, 10:53 AM
P: 13
I don't have much of a budget, no means to gather the best, possibly necessary, materials, soi am sticking to what I have. I would love to use a fresnel lens, but I don't have access to one, so parabolic solar oven it is.

Because the seebeck effect only produces tens of microvolts per degree of Kelvin difference, I want as many degrees as I can get.


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