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Name: Kelly
Status: student
Grade: 9-12
Location: Outside U.S.
Country: USA
Date: N/A 


Question:
Hi, I am working on a Passive solar water distillation pump for my science fair project. I want to know what you might suggest to use as a metal to transfer heat gathered from a Fresnel lens at the top of the unit down to the bottom of the unit which would go deep in the ground to vaporize water. I am looking at the standard silver copper gold and aluminum but want to know which would be best to use in a potable water system with the least amount of corrosion. Maybe copper with a silicone insulator? or silver coated with a silicone insulation? What would be your suggestion?



Replies:
Hi Kelly,

If I understand your question correctly, what you want is a good thermal conductor to transfer heat from one place to another. The best thermal conductor by at least an order of magnitude or two, is a heat pipe. However this would clearly be impractical from a cost standpoint.

If you are using a length of metal to transfer the heat (as you seem to describe), then as you can easily see from easily available thermal conductivity data of various metals, silver would clearly be the best choice, but also the most impractical from a cost standpoint. The second best metal, based on thermal conductivity, is (as you probably know already) copper. Aluminum is the third best.

An important concept to know is that of "thermal resistance". This is the resistance of your completed metal "bar" to the flow of heat. You want the metal bar to have the lowest thermal resistance possible (in other words, the highest thermal conductivity). Several factors influence this. Too achieve the lowest possible thermal resistance, you need to have....

- A metal bar made from a metal with the highest practical thermal conductivity. Copper is the most practical metal in this case.

- A metal bar that has the largest possible cross sectional area. Just as more water will flow in a wide stream than a narrow one, more heat will flow (and the thermal resistance will be less) through a conductor with a larger cross sectional area.

- A metal bar that is as short as possible. The longer the thermal path (i.e. bar length) is, the higher the thermal resistance, and the less easily heat will flow.

This should help you arrive at the most optimal design. Please note that if you look up the thermal conductivity of the metal you use, and knowing the dimensions of the metal bar, you can fairly easily calculate a good approximation of the temperature drop as heat flows along the bad from the heat source, to the water being heated. Your teacher should be able to explain how to do this.

As for the use of an insulator, it is likely unnecessary. The amount of heat lost from the bar, will be very small as compared to that being conducted through the bar to the water, assuming that the length of the bar is not excessive. Silicone is not a particularly good thermal insulator, but it is able to withstand high heat. A better insulator would be something like ordinary fiberglass insulation used in insulating houses. But my guess is that insulation is not worth the trouble in this case.

Hope this was helpful.

Bob Wilson
Canisius College


Kelly

Yours is an impressive project.

Here is a URL that lists the heat conductivity of materials. The higher the number the higher the rate of thermal transfer.

http://physics.info/conduction/

From this article:

Heat conduction (as opposed to electrical conduction) is the flow of internal energy from a region of higher temperature to one of lower temperature by the interaction of the adjacent particles (atoms, molecules, ions, electrons, etc.) in the intervening space.

The best ordinary metallic conductors are (in decreasing order) silver, copper, gold, aluminum, beryllium, and tungsten. Diamond beats them all, and graphite beats diamond only if the heat can be forced to conduct in a direction parallel to the crystal layers.

This article:

http://answers.yahoo.com/question/index?qid=20080317125910AAh1EyX

says that "The only reason gold is used in so many applications that require both high electrical and thermal conductivity, is that gold is very hard to oxidize and corrode, whereas silver and copper oxidize easily.

Copper is widely used as an electrical conductor because it is cheap compared with silver."

You can find more information by searching for "best metallic heat conductor" at http://www.google.com

I don't think you will be able to generate and transfer enough heat to vaporize water, but good luck trying. That's why it is called an experiment.

Sincere regards,
Mike Stewart


Kelly,

I see some basic problems with your design that I believe need attention.

First, is that any water that is vaporized at the bottom, will likely condense on the relatively cold rock that fills the tube, and simply run back before reaching the top.

The long thin copper rod will offer very significant thermal resistance to the heat flow needed, and this will (I believe) significantly reduce the rate of vaporization of ground water.

You will need to blacken the copper ball's surface (a black copper oxide layer should do the trick) in order increase its emissivity to close to 1.0, that is, to make it appear "black" (and thus to be as perfect an absorber) to both infrared AND visible wavelengths.

I am not too clear on the function of the radiant fins, since they will mainly not be in contact with water. Better, might be a circular disk buried a millimeter or two in the wet soil.

Vaporizing (boiling) water requires a lot of heat energy. Further, in order to get any water vapor to rise through the rocks to the condenser, the rocks will ALL have to be heated to at least the boiling point of water (otherwise they will just condense the steam and the water will run back). I am not convinced that the relatively thin copper rod can conduct that much heat to the water below, and to heat the rocks as well.

Also, be aware that in condensing the water vapor, all the heat you put into boiling it, must be REMOVED to get it to condense! This means that a simple metal ring will not suffice.... it will simply be heated by the (hopefully) rising steam, and once it gets hot, it will no longer condense any water.

I suggest a simple experiment... Get a copper rod (without the ball on top) that is the same diameter and length that you are proposing. Put one end in a cup of water about an inch deep. Heat the upper end with a blowtorch (to simulate your Fresnel lense). Now observe how much water appears to be boiling in the cup. This will quickly give you an idea of the rate of water vaporization you can expect. Even with a heavily insulated rod, I believe you will find that it is much less than you expected.

I'd be pleased to discuss this further, as well as to suggest other possible approaches to this problem. You can email me direct at R.F.Wilson@telus.net

Best regards,

Bob Wilson



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