Re: Electrical device that can heat and cool depending on the current

Date: Sun May 17 04:18:39 1998
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Physics
ID: 894291187.Ph

Message:

Hi Bert!

These devices are called TE modules (TE = thermoelectric) or Peltier coolers, and are based on the "Peltier effect" from physics. They are commonly sold as part of CPU fans for cooling your PC's processor, and as 6-pack coolers which are powered by the 12V jack in your car. They can be used in reverse: if one side is heated and the other is cooled, they generate electrical energy (a couple volts DC.) In this form they are used in the hot-plutonium thermoelectric generators used in deep-space probes. You can buy them for about $20 from various surplus electronics sources (see my SUPPLIERS page, try ALL and H&R.) Some science education suppliers also have them, try Arbor Scientific. (Note to experimenters: the device is held together by low-temp solder, so be careful when experimenting with flames, your TE module might melt!)

Quickie explanation:

To form a "thermocouple thermometer," twist the ends of a copper and an iron wire together, connect the free ends to an electric current meter, then stick the twisted ends into icewater or into a flame. The meter will indicate that charge is flowing. This is called the Seebeck Effect. This can be reversed: if an electric current is passed through the junction between dissimilar conductors, the junction will either become hot or cold depending on the direction of the current. This is called the Peltier Effect. Search WWW for info on these (there's some at a University site here) These effects have been known for a century, but only recently were used for heating and cooling. In metals they are very small effects, but they are considerably larger for N and P semiconductor junctions, and so the modern TE module was recently made possible by the availablity of inexpensive semiconductor materials.

More detailed explanation:

Thermocouples and "peltier-junction coolers" are very similar to solar cells and light emitting diodes. All of these use junctions between dissimilar conductors, and their operation relies upon electrons which jump between differing energy levels of the atoms in the different conductors. (Beware, this is the tip of the iceberg of quantum mechanics solid state physics!) I hope the following isn't getting too much into the complex stuff.

Solar cells use light to create voltage and current, while Light Emitting Diodes (LEDs) use voltage and current to create light. TE modules do something very similar, but they use heat vibrations rather than electromagnetic vibrations of visible light. They convert electric voltage and current directly into a temperature difference or vice versa, while LEDs and solar cells convert back and forth between light and electric voltage and current.

Note: a thermocouple junction becomes warm when electrical energy is applied, much like an LED emits light. But an LED does not absorb light when the electrical connections are reversed! (if it did, then we'd have an example of the infamous "Dark Emitting Diode" described by those who spread really bad electronics jokes.) Obviously the analogy is not perfect.

Conductors already contain charge

Matter is made of atoms, and atoms are made of positive and negative electric charge (protons and electrons,) therefor solid matter is actually a mass of positive and negative charge. In a conductive material, the positive charges are locked into a solid matrix, while the negative charges are not. The negative electrons wander all through the material and form a sort of "electric liquid." Overall, the material is electrically neutral, even though it behaves as a positively-charged sponge which is soaked with negatively-charged water.

Charge in conductors has a particular energy

The negative electrons of a conductor are still orbiting their protons. Sort of. They orbit in amongst all the protons of the material with wandering paths. They take a particular energy level, in somewhat the same way that the electrons of an individual atom can only orbit at a certain energy level. A block of conductive material is something like a gigantic atom, it has many atomic nucleii all spread out, but it has just one electron cloud with a particular energy level. Different materials have different energy levels. Junctions between different conductors is the key to the useful effects displayed by LEDs, solar cells, and thermocouple devices.

Charges going over the waterfall make "noise"

If two pieces of different conductor materials are touched together, there is a "step" at the spot where they join. This "step" is a difference in the energy levels of their charge fluids. If this composite conductor is used as a wire in a circuit, then their charges can be forced to flow over this energy "step". It's like a waterfall. If the electrons are forced to flow from the higher energy to the lower energy, each electron gives off a pulse of vibration as it falls down the step. (It's a lot like the electron of an atom jumping down in orbital level and emitting light.) In metals, the jump in energy is small, and the vibration is a heat vibration rather than a photon of light. So, whenever an electric current goes down an energy step as it travels from one type of material to another, it gives off energy. If the step is small, this energy will take the form of a rise in temperature.

A backwards waterfall is a vibration-sucker

If the direction of current is reversed, how can the charges get up the "step"? Backwards waterfalls don't REALLY work in real life. But in conductors, the electrons always have quite a bit of motion. They are jangling around in the materials. If something pushes them towards the energy step, many can easily jump up it. But when they do, they lose a lot of speed. Imagine a bouncing ball which bounces repeatedly off the floor, then by chance bounds up onto a tabletop. This can greatly slow the ball, even stop it. Same with the charges being forced across the junction between differing materials. The ones which don't bounce "high enough" cannot proceed across the junction, but those that do will be slowed. But why would this cool down the metals? Because the moving electrons ARE THE HEAT. The motion of the jangling electrons are actually the heat vibrations in the material. Yes, the protons wiggle too (as do the entire atoms.) But electrons cause protons to vibrate, and vice versa, and entire atoms share their vibrations with protons and electrons. These vibrations are heat vibrations, and if something slows down the electrons (or the protons, or the atoms), the material gets cooled.

Thermo-modules have TWO junctions

Solid state TE modules have hundreds of junctions. But we can imagine that they only have two. On one side they have a high-to-low energy step. On the other side they have a low-to-high energy step. (In reality, they have hundreds of up and down steps, with their conductor path woven back and forth in a zig-zag so all the upwards steps are on one side and all the downwards steps are on the other.) When a large electric current is created in the TE module, charges are pushed up the step on one side. That side becomes cold. The charges fall down the step on the other side, and that step becomes hot. Therefor, a TE module is a "heat pump." It's like a freon refrigerator, with cold evaporator coils inside and a hot condensor coil on the back.

Cool the refrigerator, and does the motor run backwards?

If we blow some exhaust into the tailpipe of a car while forcing the engine to turn, will it create gasoline and oxygen? No, because that type of system isn't reversible. However, heat pumps are reversible. If we could cool the coils in a refrigerator while heating the coils on it's back, if friction was low enough its pump would start turning. If we heat one side of the TE module and cool the other, a voltage will appear across its wires. The wires can be connected to a small motor, and the motor will run. There is even a physics-toy based on this effect. Inside the TE module, the hot junction is making electrons jump up the energy step while the cold junction is keeping them from bouncing back up once they have fallen down that energy step. If there is a complete circuit, then this pumps electrons around the circle in much the same way that a solar cell or a chemical battery does.

Primative TE modules once ran radios

The energy step between n-type semiconductor and p-type semiconductor is much larger than that in differing metal alloys. An n-p semiconductor junction becomes very cold when charges pass, while in metals the temperature difference is barely detectable. And any voltage which is developed from a temperature difference is fairly large in semiconductors (around 1/2 volt), while it's tiny in differing metals. Nevertheless, metals were once used as a "thermal battery". A large groups of wire segments was assembled in a zig-zag shape with their tips twisted together. Each "zig" was made of one metal alloy and each "zag" made of another. The wires were bent into a sunburst-shaped circle, with all of one group of twisted tips pointing into the center. An alcohol burner was placed in the center in order to heat the tips red hot. The tiny voltages from all the junctions added up in series. The two free ends of this zig-zag assembly supplied a small voltage, too small for most purposes but large enough to run an earphones-style radio in an emergency. This device was called a "thermocouple pile" or "thermopile."

Thermopiles were once used in research, for remote temperature measurement via infrared radiation. Put the junctions of your thermopile at the focus of a parabolic mirror, aim it at a distant object, and you can measure its temperature. This device is called a "bolomoter". From memory (I might have seen this in the book A STRESS ANALYSIS OF THE STRAPLESS EVENING GOWN:

A fascinating device, the Bolometer
it's a wonderful kind of thermometer
which can measure the temp.
of a polar bear's rump
at a distance of half a kilometer.

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.