1. How does this technology work?
This gets complicated—but then if it was easy, everybody would be explaining it. We’ll just take it one small step at a time.

First, we need to understand the relationship between the flow of electricity and the existence of charge carriers. According to electron theory, electricity is the movement of electrons in a circuit. It occurs whenever there is a continuous conductive path across an applied voltage. The voltage provides an electromotive force which sets the electrons into motion. The resulting electrical current is measured in terms of the number of electrons moving past a given point in one second, where one ampere (or ‘amp’ for short) equals the movement of 6.25 x 1018 ( ten to the 18th power) electrons per second.

figure 1
Charge carriers are the physical components of a material which allow it to conduct electricity. The precise nature of these carriers, is a function of the material’s atomic structure. In the simplest examples, like copper, the material is a pure element which has only a single valence electron in its outer shell (see Figure 1). The fewer the number of electrons in an element’s outer shell, the more loosely bound it is to the atom’s nucleus, and the easier it is to make it flow with the application of a voltage. Other elements with a single outer electron include silver and gold and they are excellent conductors.

figure 2

Conductivity takes a somewhat different form when it comes to semiconductor material. For electronic applications, semiconductor materials are ‘grown’ into crystalline structures which are given conductive properties by virtue of the impurities (or dopants) which are added. In their purest form (i.e., without dopants), the base semiconductor materials form crystalline lattices which become very stable by sharing electrons among the constituent atoms. Figure 2 shows such a configuration for a silicon crystal. In looking at the shell mapping, be aware that the electrons (shown in red), are actually in constant motion as they orbit the nuclei in the lattice. The shared electrons, however, are continually pulled into the orbits of adjacent nuclei to maintain the structural stability of the lattice. In this pure state, the material is not very conductive.

figure 3

Once the impurities are added to the mix, however, the conductive properties are radically affected. For example, if we have a crystal formed primarily of silicon (which has four valence electrons), but with arsenic impurities (having five valence electrons) added, we wind up with ‘free’ electrons which do not fit into the crystalline structure (see Figure 3). These electrons are thus ‘loosely bound’ and when a voltage is applied, they can be easily set in motion to allow electrical current to pass. The loosely bound electrons are considered the charge carriers in this ‘negatively doped’ material (which is referred to as ‘N’ material).

figure 4

It is also possible to form a more conductive crystal by adding impurities which have one less valence electron. For example, if Indium impurities (which have three valence electrons) are used in combination with silicon, this creates a crystalline structure which has ‘holes’ in it—that is, places within the crystal where an electron would normally be found if the material was pure. These ‘holes’ make it much easier to convey electrons through the material upon the application of a voltage. In this case, ‘holes’ are considered to be the charge carriers in this ‘positively doped’ conductor (which is referred to as ‘P’ material).

It is critical here to understand that the existence of charge carriers is entirely a property of a given material. The vast, vast majority of conductors—including those employed to make electrical connections—use electrons as the charge carriers and would be considered ‘N’ material. ‘P’ material can only be fabricated within crystalline structures.

Okay, now that we have a basic understanding of electricity and the nature of charge carriers, we need to come to grips with an important concept in power generation. Sometimes it is possible to set charge carriers in motion through interaction with other energy sources. For instance, if a magnetic field is moved along a conductor, the effect of that field upon the electrons (assuming that there is a complete path), will cause electrical current to flow. In essence, if you can force charge carriers to move, you can create voltage and current flow. This is not only true when there is an interaction between charge carriers and magnetic fields, but when those carriers are set in motion by the flow of heat.

Thus we come to the nitty gritty of Seebeck technology. Whenever an electrical conductor is strung between two different temperatures, the conductor is capable of transferring thermal energy from the warmer side to the colder one. Furthermore, the physical process of transferring that heat, also tends to move electrical charge carriers within the conductor in the same direction as the heat. Conceivably then, this charge carrier movement can be used to generate electrical current—if we can find a way to effectively complete the circuit.

Here, however, we run up against a major issue. If the conductor which completes the circuit is identical to the first conductor, the flow of thermal energy will create a potential for equal charge carrier movement in both conductors. Furthermore, the potential for current flow in one conductor is in complete opposition to that in the other conductor. The result is no net current flow.

If we employ two dissimilar conductors, on the other hand, we get quite a different result. With differing capacities for moving charge carriers in response to thermal flow, the current level in one conductor will overcome (or in some cases, complement) the potential for thermally-generated current flow in the other conductor. The net effect is a continuous current level which is equal to the generated current capacity of the primary conductor (for the given temperature difference) minus the generated current capacity of the second conductor. The existence of this net current flow, indicates that a voltage is created through the movement of heat and we can get a direct measurement of this voltage level by breaking the circuit and measuring across the opened terminals with a voltmeter. Note that the ability of two dissimilar conductors to produce a voltage when a temperature difference is applied, is called the Seebeck effect. The voltage which results is referred to as Seebeck voltage.

Probably the most well-known example of this phenomenon, is the common thermocouple. For example, with a K-type thermocouple made of two wires—one composed of a nickel-chromium alloy and the other from nickel-aluminum, if one junction is at 100° C and the other junction (the so-called ‘reference junction’) is at 0° C, a voltage of approximately 4.096 millivolts is produced. In general, the voltage generated by a thermocouple is a function of two things: 1) the temperature difference (DT) between the two thermocouple junctions, and 2) the nature of the conductors employed (including their temperature dependencies).

Of course, thermocouples are used primarily for temperature measurement—not power generation. Thermoelectric power generation (TEG) devices typically use special semiconductor materials which are optimized for the Seebeck effect. The circuit shown in Figure 10 demonstrates the simplest possible example. It shows a single ‘N’-type semiconductor pellet connected across a voltmeter.

figure 10

figure 11

As the heat moves from the hot to the cold side of the pellet, the charge carriers (i.e., electrons from the dopants) are carried with the heat. Heat also effects charge carrier movement in the return path (typically copper wire). Because the heat movement can carry far more charge carriers in the semiconductor material than in the circuit’s return path, however, a significant potential difference (i.e., Seebeck voltage) is generated. In this example, the Seebeck voltage would be about 20 mV.

In thermoelectric power generation, ‘P’ pellets are also employed. Figure 11 shows a basic configuration. Note how the flow of electrons goes in a direction opposite to that of hole flow.
It is through the use of both N and P type materials in a single power generation device, that we can truly optimize the Seebeck effect. As shown in Figure 12, the N and P pellets are configured thermally in parallel, but electrically in a series circuit. Because electrical current (i.e., moving electrons) flows in a direction opposite to that of hole flow, the current generating potentials in the pellets do not oppose one another, but are series-aiding. Thus, if each pellet developed a Seebeck voltage of 20 mV, this combination of an N and P pellet would generate approximately 40 mV rather than zero volts.

figure 12

Of course, in truly practical TEG’s, many such P & N couples are employed to bring the Seebeck voltage up to useful levels. The illustration in Figure 13 shows a three-couple device (more typically, a Seebeck module would have 127 couples or more). Note the direction of electrical current flow in the N/P series configuration (assuming a load is connected across the Seebeck device).

figure 13

2. Do TEG’s employ silicon-based semiconductor material?
They can. Tellurex, however, uses bismuth/telluride structures to optimize performance. While similar dopants are employed in both semiconductor technologies, the crystalline latices which form from Bismuth/Telluride, are far more complex. The same principles of ‘N’ and ‘P’ material apply, though.

3. How is a typical TEG system configured?
Fundamentally, there are four basic components: a heat source, a TEG module (i.e., a thermoelectric generator—also known as a Seebeck device), a ‘cold-side’ heat sink, and the electrical load. The system may also include a voltage regulation circuit, or a fan for the heat sink. The illustration in Figure 14 shows one example.
In this case we have a burner box with a propane fuel source. It is shown with the burner box open on one end, but in reality, it would be enclosed. The TE module is then sandwiched between the heat source and the cold-side sink. While this example shows only a single TEG module, in reality, several modules might be deployed in whatever series/parallel electrical arrangement best served the load.

figure 14

4. Do I have to use a heat sink in my design?
It would be virtually impossible to get an adequate DT without some type of heat sink. However, you can sometimes reduce the size requirement for the sink (i.e., fin surface area) if you can find a way to insure good air flow.

5. Are any special precautions required for the hot side of the system?
Yes. First and foremost, you want to prevent the hot-side temperature of the TE device from exceeding the melting temperature of the solder employed to secure the semiconductor pellets to the copper tabs. It is recommended that the temperature be kept below 200° C. Toward this end, it is a good idea to use some type of ‘heat spreader’ to prevent hot spots at the hot-side module interface. Usually this means employing a relatively thick casting or extrusion between the heat source and the module.

On the mechanical side—especially when using multiple devices—you need to find a means of applying compression between the hot and cold sides, which will apply even pressure across the modules and, most importantly, prevent the hot-side interface from bowing. If there is too great an expanse between compression points, the hot side interface can distort to the point where some modules are crushed or the thermal interface is compromise.

6. What does the specification, THot, mean?
This is the temperature at the mounting surface of the module, which comes in contact with the heat source (i.e., the hot side of the system).

7. What does the specification, TCold, mean?
This is the temperature at the mounting surface of the module, which comes in contact with the cold-side heat sink.

8. What does ‘no-load voltage’ (VNL) mean?
This is the voltage output of the TEG system when no electrical load is connected.

9. What does ‘load voltage’ (VL) mean?
This is the voltage output of the TEG system when an electrical load is connected.

10. What does internal resistance (RInt) mean?
This is the electrical resistance of the TEG module (or module array).

11. What does ‘power conversion efficiency’ mean?
It is the ratio of power output to power input, expressed as a percentage. In this case, power output would be the wattage dissipated in the electrical load and power input would be the rate of energy use (e.g., watts, BTU’s/hr) to create the necessary DT.

12. What does ‘electrical efficiency’ mean?
It is the ratio of electrical power dissipated in the load to the total amount of power generated (including the dissipation in the internal resistance).