Electricity Made from Thin Air (and Heat) 😱

An overview of thermoelectricity, materials, devices and the challenges ahead

It’s Saturday morning. And as a new baker, you’re delighted when you see an order for 100 themed cookies, even though it has the label SAME-DAY ORDER.

You know it’ll be a task but you love the job, so you accept the challenge. Hours upon hours are spent mixing the ingredients, forming a dough, making circles, transferring them to the tray, baking them, and then meticulously decorating them, one by one.

Each step, you lose a little more of your sanity — you get tired and fed up and all you want to do is finish the job.

That’s why when you do, you feel great.

But as soon as you send them over to your customer, they take nearly all the beautifully-iced cookies from your box and dump them in the trash.

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This is the current state of our energy system. Rather than the cause being the conversion of ingredients to cookies (and the unexpected actions of your friend), 72% of the global primary energy consumption is lost thanks to the conversion of heat energy to mechanical energy to electrical energy.

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Although the root cause of energy loss can be difficult to address, we’ve been aware of the problem for a while, so we have various solutions to harness the high-temperature heat spewing from sources like industrial plants and power stations.

But what we’ve left untouched for the most part, is low-grade waste heat (less than 230 °C), as recovering it is far more challenging, infeasible and often less economically viable. The higher the temperature of the waste heat is, the more efficient can this energy be exploited further.

This is because, as we’ll see below, all energy conversion processes associated with heat boil down to some sort of temperature difference thanks to the second law of thermodynamics — even in an engine, for example. This means that, in the case of low-grade waste heat, where the temperature difference is too small, the efficiency becomes very low, and the energy profit becomes negligible.

However, with the sheer amount of waste heat being lost to the environment, it would be a shame to not make an effort to use it. This effort has been spearheaded by thermoelectric devices.

Thermoelectric-wha?!

Looking at the word, it can be understood that thermoelectricity has to do with heat and, you guessed it, electricity.

But more specifically, thermoelectricity is as a phenomena by which a temperature difference results in an electric voltage, thereby directly converting thermal energy into electrical energy with no moving parts or fluids.

Under the umbrella term thermoelectric effect, there are three phenomena.

Note: definitions for all unfamiliar terms that follow have been provided below. The * symbol will indicate whether a definition is provided.

The Big 3

Seebeck effect 🌟

The Seebeck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances. If the pair is connected through an electrical circuit, direct current (DC) flows through that circuit.

Peltier effect

Opposite to the Seebeck effect, the Peltier effect states that the electrical current flowing through a junction* connecting two materials will emit or absorb heat per unit time at the junction. Heat is given out or absorbed depending on the pairs of metals and the direction of the current.

Thomson effect

William Thomson (later well known as Lord Kelvin) discovered a third thermoelectric effect which provides a link between the Seebeck effect and Peltier effect. He found that when a current is passed through a wire of a single homogeneous material along which a temperature gradient exists, heat must be exchanged with the surrounding in order that the original temperature gradient may be maintained along the wire. (The exchange of heat is required at all places of the circuit where a temperature gradient exists.)

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For the purpose of this article, all you’ll need to know is the Seebeck effect. And to make sure you know it well, let’s go a little deeper into the how.

The inner workings of the Seebeck effect

Why exactly does temperature gradient → voltage gradient → electric current?

To know that, we need to get down to the atomic, and then sub-atomic level.

Essentially, the Seebeck effect occurs due to the movement of charge carriers within the thermoelectric material. When heat is applied to a thermoelectric material, the charge carriers on the hot side become more energetic and consequently diffuse away to the cool side of the material, leading to a buildup of charge carriers at one end.

Temperature gradient and charge buildup. (source)

This buildup of charge creates a more negative charge at the colder part than the warmer part, which leads to the generation of voltage potential that is directly proportional to the temperature difference across the semiconductor..

Charge carriers are simply particles that carry an electric charge, namely electrons and holes*. In semiconductors, which we’ll talk more about below, charge carriers are the ones responsible for all the fun stuff.

Thermoelectric materials

After understanding the basic concepts of the thermoelectric effect, you may be wondering “what makes a material thermoelectric”?

Interestingly, all materials have a nonzero thermoelectric effect, but in most, it is far too minute. This means that a thermoelectric (here on referred to as TE) material simply shows the thermoelectric effect in a strong or convenient form.

These materials are most commonly extrinsic semiconductors because of their band structure.

If you’re new to semiconductors, head on over to my 2-minute overview to learn the basics.

As a review:

• Semiconductors have an electrical conductivity in between that of an insulator and that of a conductor
• This is due to their band structure, which allows charge carriers to travel to the conduction band (although not too easily) and participate in electric current

To understand why semiconductors make good candidates for TE materials, there’s one last piece to the puzzle 🧩

The Fermi level.

The Fermi level is the highest filled energy level at 0 Kelvin. In other words, the full energy level* with the highest energy is the Fermi level.

(Source)

As indicated in the above diagram, the Fermi level typically lies in the centre of the valence band (VB) and conduction band (CB). What’s interesting is that if you can bring the Fermi level higher, electrons will have an easier go at making a transition to the conduction band, thus increasing the conductivity.

For thermoelectric materials, this exactly is desirable; a Fermi level that lies close to the edge of the conduction band (but not in it).

As mentioned, the Fermi level of pure semiconductors (also called intrinsic semiconductors) lies in between the VB and CB. So how could we change that?

The answer is adding impurities to the lattice of the semiconductor — a method called doping. This results in an extrinsic semiconductor, which are most frequently used in thermoelectric devices.

Extrinsic semiconductors are characterized by an excess of electrons or holes. To think about how this happens, imagine a crystal silicon atom.

Here, every silicon atom has 14 electrons, 4 of which are valence. These are the important ones, as they take part in chemical reactions.

Since two electrons are needed to form a covalent bond, the two electrons on either side of the atoms form a bond, resulting in 4 bonds for each silicon atom.

When you dope the silicon atom with phosphorous, whose atoms have 5 valence electrons, for example, what happens is that the phosphorous uses 4 of its electrons to form bonds with the neighbouring silicon atoms. The remaining free electron still tries to form a bond, but is much less secure in place, making it free to move to the conduction band.

The result of this is a n-type semiconductor — where the main charge carrier is electrons.

If the opposite were to happens, where an element with fewer valence electrons than the silicon is added to the mix, the would be an electron vacancy, or hole, in place of where the bond previously was. This creates a p-type semiconductor, wherein the main charge carrier is holes.

Now that we understand why semiconductors make good candidates for TE materials, I think I should break the news…

The typical efficiency of a thermoelectric device is merely 5–8%.

Measuring and optimizing thermoelectricity

With the efficiency being so low, you may wonder why and how it can be increased. This is in fact the central challenge with leveraging thermoelectricity for real-world applications, and what most researchers have been focusing on for several years.

The performance of a thermoelectric material is measured by a figure of merit called ZT. While there are many different equations that are tied to this numerical value, what’s needed to maximize the thermoelectric ZT value of a material is:

• A large Seebeck coefficient (the magnitude of the induced thermoelectric voltage in response to a temperature gradient across the material)
• High electrical conductivity
• Low thermal conductivity

This is important if electrical energy is to be generated permanently from a temperature difference — because if temperature differences could equilibrate very quickly and the entire material would soon have the same temperature everywhere, the thermoelectric effect would come to a standstill.

Other important criteria in TE devices can be broken down into mechanical and chemical properties:

Mechanical

• Fairly high compression strength so it will not break into pieces when put under pressure.
• The materials coefficient of thermal expansion (CTE) — there should be minimal thermal stress mismatch between n-type, p-type, and contact materials which can lead to failure of the TE-module
• Also important to match the elastic modulus so that stresses are uniformly distributed throughout the material layers

Chemical

• With large temperature variations over time, materials are vulnerable to instability and degradation. Must be aware of effects such as grain growth and diffusion and accumulation of dopants, or forming unstable oxidation layers.
• So, they need to be stable in respect to oxidation and sublimation. They shouldn’t demonstrate grain growth* or accumulate dopants.

How do we optimize the ZT?

Looking at each of these points in the criteria as levers, the question becomes “How do we pull them in a manner so as to increase the efficiency? ”

The reason that this question (which has plenty of answers) is so difficult to answer is because each of the levers are in some way, conflicting with the other.

All properties in ZT rely on the carrier concentration, however there are prominent trade-offs between each lever which makes them difficult to optimize.

The conflicting nature of favourable properties make them hard to optimize.

For example, electrical conductivity is directly related to thermal conductivity, making it very hard to have one high and one low.

For these reasons, we’ve basically been relying on the same three materials for thermoelectric devices since the 1950s and 60s, when the effect was first discovered. These materials are bismuth (Bi2Te3) telluride, lead telluride (PbTe) and Silicon germanium (SiGe).

While effective, these materials are toxic to the environment, giving them a pretty big downside. The reason that they are still in use today is that even with continuous advancements in the materials side of things, the cost and feasibility of commercializing high-performing, novel materials is usually a difficult barrier to overcome.

Key challenges

Other challenges with leveraging the thermoelectric effect to produce electricity can be summarized as follows:

• Improving ZT value while maintaining low cost
• TE material supply and toxicity
• Material properties are highly temperature-dependent, but few applications have heat sources at one single temperature
• The several components of TEGs currently need to be assembled by hand
• High manufacturing costs — not only material synthesis, but also system component costs like substrates and heat exchangers, for example

Bottom line

While the technicalities may not be so simple to figure out, there’s no doubt that thermoelectric devices will play a role in our energy systems’ future — the fact that the technology is so commonly stated in the top 10 energy innovations that deserve attention serves as assurance.

In this article, I hope to have gotten across the core principles of the thermoelectric effect, why semiconductors are ideal thermoelectric materials, material properties needed for use in TE devices and some key challenges.

In my next article, I’ll be going over these materials integrate with devices and an exciting application, so stay tuned by clicking the follow button!

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Dictionary 📙

Junction = A point or area where multiple conductors or semiconductors make physical contact. In this case, the junction is where the positive and negative legs of the thermocouple wire come together.

Holes = An electron hole is one of the two types of charge carriers that are responsible for creating electric current in semiconducting materials. A hole can be seen as the “opposite” of an electron — while not a physical particle like electrons, holes are the absence of an electron in an atom. They have a positive charge.

Energy level (also called electron shell) = Fixed distances from the nucleus of an atom where electrons may be found.

Grain growth = An increase in the size of crystallites (grains) in a material at high temperatures.

FF, or fill factor = The ratio of the actual maximum obtainable power

Thick-film = A thick, engineered layer of material deposited on a substrate with a thickness typically measured in micrometers, used in electronic and optical applications