Thermal electric generators, Part 1: Principles and implementation

TEGs – thermal electric generators – are used for energy harvesting and enhanced efficiency, as well as stand-alone primary power sources for both mundane and highly advanced situations.

Every application needs a source of energy which can be “drawn down” to provide power for the system. Obviously, the easier and cheaper it is to get this power, the better and more effective the result. That’s why thermoelectric generators (TEGs) can be attractive add-ons or even primary power sources, as they capture and transform some of the heat energy which is around us and transforms it into electrical energy. TEGs are also seeing increased use as a harvested energy source for battery-free IoT nodes where both power needs and data rates are low.

This FAQ looks at the physics principles of TEGs and the component parts of a complete TEG system. It then looks at some applications of the TEG principles.

Q: What is a TEG?

A: A TEG is a source of electrical power which is derived from the temperature difference between a cold side and a hot side. The larger this differential, the greater the amount of power that can be extracted.

Q: What’s the physics principle behind the TEG?

A: It’s the Seebeck effect, which is known to many engineers because it is also the source of the well-known thermoelectric effect in thermocouples, where a small but well-defined voltage is created by the temperature difference between the dissimilar metal leads and the ambient environment.

Q: TEG seems like getting something for nothing – is it?

A: Yes and no. While the heat-side source may be available for “free” in some cases, such as the heat given off by a source which is already creating or dissipating heat, there are still many issues associated with developing a complete and effective installation. Of course, if the heated side is not free and requires some sort of specially generated heat source, it is not free even if everything else is – and the “everything else” has cost in size, weight, complexity, and dollars.

Q: What are the basic categories if TEG application?

A: TEGs fall into two primary classes of application:

1) where The TEG is the primary or sole source of power such as for energy harvesting, to power a small IoT temperature sensor node, or providing power in a spacecraft (more on this in Part 2), or in-between loads. Fire-sourced TEGs are sometimes used for supplemental power in off-grid areas where sunlight is limited or sporadic, to provide power when the sun is inadequate.

2) for extracting and recovering heat energy that would otherwise go to waste, such as the heat given off by an engine, and using this co-generation recovered energy to provide additional power to the system.

Q: How much power can a TEG generate?

A: The answer depends on many factors such as the size and temperature differential. As a primary power source, a basic TEG might provide anywhere from 10 W to several hundred watts. Of course, a tiny IoT node may need less than a watt and would be sized accordingly.

Q: How long can a TEG last? Is there a wear-out mechanism?

A: There are no moving parts, and so there is no mechanical wear. A TEG will last as long as any well-designed electronic product. Some vendors claim lifetimes of 100,000 hours, and Part 2 will have an example which has lasted far beyond that period. The main cause of failure is a heat-induced failure as with other electronics, especially if there are heating/ cooling cycles and thermal stress.

Q: Is the Seebeck effect which underlies the TEG the same as the Peltier effect, where a current through a special material pair can be used to produce local heating or cooling depending on the direction of current flow?

A: No, the Peltier effect and the Seebeck effect are tangible outcomes of two different principles although they are related, as are most principles of thermal physics. The Peltier effect relates to creating heating and cooling as a temperature differential from an electrical power source, while the Seebeck effect creates a voltage and electrical power from a temperature differential. Thus, the Peltier Effect is the complement of the Seebeck effect. Instead of applying heat differential to create current, in the Peltier effect the electrical current results in the production of heating or cooling.

Q: Of what does the power-producing core element of a TEG consist?

A: The core, called a thermoelectric module (TEM) can be a thermocouple, or a semiconductor “sandwich,” usually comprised of a specialized p-type and n-type ceramic, such as bismuth telluride, which as a high thermoelectric coefficient, Figure 1. The material must have good thermal conductivity to allow the energy from the heated side to pass to the colder side.

Fig 1: The basic physics principle of the Seebeck effect is that a voltage is generated across a temperature differential as heated, higher-energy electrons flow towards the cooler, lower-energy ones (Image source: TEGmart)

Q: What does the final arrangement look like?

A: In practice, the TEG core is built of many small blocks known as couples, and is called a thermopile, Figure 2. The couples can be connected in series to increase the output voltage or parallel to increase the current.

Fig 2: In most designs, the TEG core is fabricated from a series/parallel array of many smaller couples as a single thermopile. (Image source: TEGmart)

Q: What are the additional challenges in developing a viable TEG?

A: The cold side must be kept cold, to maintain the critical temperature differential. In some applications such as outer space, there’s plenty of cold due to the vacuum of space; in others such as a TEG installed in a wood stove, there’s a need for cooling (passive convection, active convection, or even water) to maintain the cool side. Some complete TEG modules come with a fan or even a water-cooled radiator assembly, Figure 3.

Fig 3: The TECTEG TEG12VDC -24AIR with 30-W peak rating is designed to be inserted into a heating-systems exhaust vent (a separate pipe vent is used to allow cooler air to enter the intake fan to maintain the differential temperature); the electrical output is regulated and provides a constant voltage/constant current adjustable output for trickle charging of batteries. (Image source: TECTEG MFR)

Q: Is that all there is?

A: The hot side can be fairly hot, as the effectiveness of the TEG s a function of the temperature difference, and a hotter hot side leads to a more electrical output. Therefore, the physical elements such as solder, wires, and other components must be able to withstand temperatures which are often in hundreds of degrees.

Q: What else does it take to make a viable TEG?

A: The small voltage from the TEM core must be captured and the converted to a useful, regulated voltage source. In many but not all cases, this will require a storage battery, power-harvesting and conversion circuits, and associated electronics.

Q: A TEG sounds like “free” energy, so why isn’t it used more often?

A: The biggest issue is efficiency versus cost. Although the number varies with the specific installation, a TEG can convert only about 5-10% of the incident heat energy into electrical power. In some cases, this is simply too low to justify the cost; in others, a TEG is the only option, and the low efficiency is acceptable.

Q: Can I build my own TEG?

A: Yes, absolutely. You can easily buy complete basic TEMs (thermocouple or bismuth telluride modules) as well as all other components, and custom-build a TEG system.

Part 2 of this FAQ looks at a range of actual TEG applications, spanning from low-cost, mass-market situations where a TEG approach makes sense as a reliable, cost-effective solution, to unique, highly sophisticated ones where there is no other alternative.

References

Thermocouples, Seebeck Effect, and Gas Heaters