1. How does this technology work?
The basic concept behind thermoelectric (TE) technology is the Peltier effect —a phenomenon first discovered in the early 19th century. The Peltier effect occurs whenever electrical current flows through two dissimilar conductors; depending on the direction of current flow, the junction of the two conductors will either absorb or release heat. Explaining the Peltier effect and its operation in thermoelectric devices, is a very challenging proposition because it ultimately keys on some very complex physics at the sub-atomic level. Here we will attempt to approach it from a conceptual perspective with the goal of giving readers an intuitive grasp of this technology (i.e., without getting too bogged down in the minutia).

In the world of thermoelectric technology, semiconductors (usually Bismuth Telluride) are the material of choice for producing the Peltier effect—in part because they can be more easily optimized for pumping heat, but also because designers can control the type of charge carrier employed within the conductor (the importance of this will be explained later). Using this type of material, a Peltier device (i.e., thermoelectric module) can be constructed—in its simplest form—around a single semiconductor 'pellet' which is soldered to electrically-conductive material on each end (usually plated copper). In this 'stripped-down' configuration, the second disimilar material required for the Peltier effect, is actually the copper connection paths to the power supply.

figure 1

It is important to note that the heat will be moved (or 'pumped') in the direction of charge carrier movement throughout the circuit (actually, it is the charge carriers that transfer the heat). In this example, 'N-type' semiconductor material is used to fabricate the pellet so that electrons (with a negative charge) will be the charge carrier employed to create the bulk of the Peltier effect. With a DC voltage source connected as shown, electrons will be repelled by the negative pole and attracted by the positive pole of the supply; this forces electron flow in a clockwise direction (as shown in the drawing). With the electrons flowing through the N-type material from bottom to top, heat is absorbed at the bottom junction and actively transferred to the top junction—it is effectively pumped by the charge carriers through the semiconductor pellet.

figure 2

In the thermoelectric industry, 'P-type' semiconductor pellets are also employed. P-type pellets are manufactured so that the charge carriers in the material are positive (known in electronics as 'holes'). These 'holes' enhance the electrical conductivity of the P-type crystaline structure, allowing electrons to flow more freely through the material when a voltage is applied. Positive charge carriers are repelled by the positive pole of the DC supply and attracted to the negative pole; thus 'hole' current flows in a direction opposite to that of electron flow. Because it is the charge carriers inherent in the material which convey the heat through the conductor, use of the P-type material results in heat being drawn toward the negative pole of the power supply and away from the positive pole. This contrasting heat-pumping action of P and N-type materials is very important in the design of practical TE devices (as will be explained in the next FAQ). While the illustration here—for simplicity's sake—shows 'hole' flow through the connections to the power supply, in reality, electrons are the charge carriers through the copper pathways.

2. Why are two types of material (P and N) required?
Unfortunately, while you can make a simple thermoelectric device with a single semiconductor pellet, you can't pump an appreciable amount of heat through it. In order to give a TE device greater heat-pumping capacity, multiple pellets are used together. Of course, the initial temptation would be to simply connect them in parallel—both electrically and thermally—as shown in the accompanying drawing. While this is possible , it does not make for a very practical device. The 'fly in the ointment' here, is that the typical TE semiconductor pellet is rated for only a very small voltage—as little as tens of millivolts—while it can draw a substantial amount of current. For example, a single pellet in an ordinary TE device might draw five amps or more with only 60 mV applied; if wired in parallel in a typical 254-pellet configuration, the device would draw over 1000 amps with the application of that 60 mV (assuming that the power supply could deliver that sort of current).

figure 3

The only realistic solution is to wire the semiconductors in series, and doing so in a way that keeps them thermally in parallel (i.e., pumping together in the same direction). Here, we might be tempted to simply zig zag the electrical connections from pellet to pellet (see illustration) to achieve a series circuit. This is theoretically workable, however, the interconnections between pellets introduce thermal shorting that significantly compromises the performance of the device. Fortunately, there is another option which gives us the desired electrical and thermal configuration while better optimizing the thermoelectric effect.

figure 4

figure 5

By arranging N and P-type pellets in a 'couple' and forming a junction between them with a plated copper tab, it is possible to configure a series circuit which can keep all of the heat moving in the same direction. As shown in the illustration, with the free (bottom) end of the P-type pellet connected to the positive voltage potential and the free (bottom) end of the N-type pellet similarly connected to the negative side of the voltage, an interesting phenomenon takes place. The positive charge carriers (i.e, 'holes') in the P material are repelled by the positive voltage potential and attracted by the negative pole; the negative charge carriers (electrons) in the N material are likewise repelled by the negative potential and attracted by the positive pole of the voltage supply. In the copper tabs and wiring, electrons are the charge carriers; when these electrons reach the P material, they simply flow through the 'holes' within the crystalline structure of the P-type pellet (remember, it is the charge carriers inherent in the material structure which dictate the direction of heat flow). Thus the electrons flow continuously from the negative pole of the voltage supply, through the N pellet, through the copper tab junction, through the P pellet, and back to the positive pole of the supply—yet because we are using the two different types of semiconductor material, the charge carriers and heat are all flowing in the same direction through the pellets (bottom to top in the drawing). Using these special properties of the TE 'couple', it is possible to team many pellets together in rectangular arrays to create practical thermoelectric modules. These devices can not only pump appreciable amounts of heat, but with their series electrical connection, are suitable for commonly-available DC power supplies. Thus the most common TE devices now in use—connecting 254 alternating P and N-type pellets—can run from a 12 to 16 VDC supply and draw only 4 to 5 amps (rather than 1000 amps at 60 mV).

figure 6

Of course, in fabricating devices with multi-pellet arrays, you must have a means to mechanically hold everything together. A solution is to mount the conductive tabs to thin ceramic substrates (as shown in the illustration); the outer faces of the ceramics are then used as the thermal interface between the Peltier device and the 'outside world'. Note that ceramic materials have become the industry standard for this purpose because they represent the best compromise between mechanical strength, electrical resistivity, and thermal conductivity.

3. Do these P and N couples function like diodes?
No. It is easy to see why many people expect couples to work like diodes, given the pairing of P and N materials, but there is a crucial difference. In the manufacturing of diodes, a depletion region is created between the P and N layers. When the diode is forward-biased, charge carriers are drawn into the depletion region and the diode becomes conductive; when reverse-biased, charge carriers are drawn away from the depletion region and the diode acts like an open circuit. Without a depletion region, a TE couple cannot act like a diode; the couple will conduct in both electrical polarities and there is no fixed voltage drop across the couple (unlike the nominal 0.6 to 0.7 VDC typically dropped across a forward-biased silicon diode).

figure 7

4. How is a typical thermoelectric (TE) system configured?
Let's look conceptually at a typical thermoelectric system designed to cool air in an enclosure (e.g., picnic box, equipment enclosure, etc.); this is probably the most common type of TE application. Here the challenge is to 'gather' heat from the inside of the box, pump it to a heat exchanger on the outside of the box, and release the collected heat into the ambient air. Usually, this is done by employing two heat sink/fan combinations in conjunction with one or more Peltier devices. The smaller of the heat sinks is used on the inside of the enclosure; cooled to a temperature below that of the air in the box, the sink picks up heat as the air circulates between the fins. In the simplest case, the Peltier device is mounted between this 'cold side' sink and a larger sink on the 'hot side' of the system. As direct current passes through the thermoelectric device, it actively pumps heat from the cold side sink to the one on the hot side. The fan on the hot side then circulates ambient air between the sink's fins to absorb some of the collected heat. Note that the heat dissipated on the hot side not only includes what is pumped from the box, but also the heat produced within the Peltier device itself (V x I).

figure 8

Let's look at this in terms of real numbers. Imagine that we have to pump 25 watts from a box to bring its temperature to 3°C (37.4°F) from a 20°C (68°F) ambient. To accomplish this, we might well have to take the temperature of the cold side sink down to 0° C (32°F). Using a Peltier device which draws 4.1 amps at 10.4 V, the hot side of the system will have to dissipate the 25 watts from the thermal load plus the 42.6 watts it takes to power the TE module (for a total of 67.6 watts). Employing a hot side sink and fan with an effective thermal resistance of 0.148 C°/W (0.266F°/W), the temperature of the hot side sink will rise approximately 10°C (18°F) above ambient. It should be noted that, to achieve the 17° C drop (30.6°F) between the box temperature and ambient, we had to create a 30° C (54°F) temperature difference across the Peltier device.

5. Can Thermoelectric systems be used for heating as well?
Yes. One of the benefits of TE technology is that you can switch the direction of heat pumping by simply reversing the polarity of the applied voltage—you get heating with one polarity, cooling with the other. Thermoelectric modules make very efficient heaters—in fact, because of the unique properties of Peltier devices, any given TE system will have a greater capacity for heating a load than cooling it.

6. Are TE systems used only for heating or cooling air?
No. Systems are often designed for pumping heat from both liquids and solids. In the case of solids, they are usually mounted right on the TE device; liquids typically circulate through a heat exchanger (usually fabricated from an aluminum or copper block) which is attached to the Peltier unit. Occasionally, circulating liquids are also used on the hot side of TE cooling systems to effectively dissipate all of the heat (i.e., a liquid-to-liquid system). Note that liquid cooling is never achieved by immersing the Peltier device in the fluid—thermoelectric modules are not the equivalent of 'electric ice cubes'.

7. Do I have to use a heat sink in my design?
Whether heating or cooling a thermal load, you must employ some form of heat sink to either collect heat (in heating mode) or dissipate collected heat into another medium (e.g., air, water, etc.). Without such provisions, the TE device will be vulnerable to overheating; once it reaches the reflow temperature of the solder employed, the unit will be destroyed. When the heat sink is exchanging heat with air, a fan is usually required, as well.

8. Can these devices be immersed?
Only for cleaning purposes and never while under power. TE devices should always be dry when under use to prevent thermal and electrical shorting.

9. What type of products currently use this technology?
There are an increasing number and variety of products which use thermoelectric technology—from picnic boxes to water coolers, laser applications, and highly-specialized instrumentation and testing equipment. The compatibility of many TE's with automotive voltages, makes them especially suitable for small cooling jobs in that industry. With each new year, the imaginations of design engineers widen with the immense possibilities of thermoelectric heating and cooling.

10. Why would I want to use a thermoelectric system instead of compressor-based technology?
Both technologies have their advantages and disadvantages, but where thermoelectric technology really shines, is in making it feasible to do very small cooling jobs—ones which would be wholly impractical with a compressor-based system. Can you imagine cooling an individual integrated circuit with compressed gasses? What about thermally cycling a test tube or cooling a very small enclosure? TE's are also strong in products which demand both heating and cooling in the face of a changing operating environment; here a simple switching of TE current polarity allows the system to shift to the mode required. In addition, unlike compressor technology, TE system components can be mounted in any physical orientation and still function properly. Of course, one other advantage of TE systems, is that they do not require evaporative chemicals which may be harmful to the environment. Thermoelectric devices open up a whole new world to cooling and heating possibilities.

11. Are there situations where compressor-based systems make more sense?
Yes. Generally, whenever a small compressor-based system would clearly be 'overkill' in providing a cooling solution, TE systems become the most viable choice. You find a 'gray area' amidst the medium-sized cooling jobs; here decisions ultimately come down to critical cost/benefit or design engineering considerations which are unique to each application. Given the present state of technology—unless there are unique overriding concerns—the compressor-based approach has distinct advantages in larger cooling systems such as standard-sized refrigerators and air-conditioning systems for buildings & vehicles. However, ongoing research into materials may one day make thermoelectrics practical for many of these larger applications.

12. For heat-only applications, do thermoelectric devices have advantages over resistive heaters?
Yes. Resistive devices create heat solely by virtue of the power dissipated within them. TE devices, on the other hand, not only provide this I 2R heating, but also actively pump heat into the thermal load; this, potentially, makes them much more efficient than resistive heaters. Unfortunately, the need for a DC power source and the generally higher cost of TE systems compared to resistive heaters, precludes their use in most heat-only applications. Furthermore, Peltier devices have a far more limited temperature range than most resistive heaters. Generally, TE devices are only used for heating in systems that also require cooling