Thermoelectrics are a part of everyday life; they can be applied to a wide range of fields such as electricity generation, refrigeration, air conditioning, heating or cooling, and more. But even if you don’t know what thermoelectrics are, you don’t have to be an expert at science to understand how they work. We used thermoelectris to develop our batteryless PowerPot generator as well as to inspire a range of other products.
Let’s take a look at some of the basic science behind thermoelectrics and learn more!
Thermoelectric generators use a difference in temperature to generate electrical power. On the flip side, they can also use a power source to create a temperature difference — how cool is that? Take mini-fridges as an everyday example: some of these appliances use thermoelectric generators to cool whatever food or beverages you store in them.
So, how do thermoelectrics generate electricity from a temperature difference? In order to understand this process, we need to know how electrons move in metals. You may have heard before that metals are good conductors; this is because electrons can freely move within them like fluids.
To illustrate how electron movement works, let’s say you have a pipe full of water and raise one end of the pipe. The water will flow down the pipe from the high end to the low end because you’ve increased the potential energy when you raised one end of the pipe. As a result, the water flows in a downward direction. In thermoelectric material, electrons move in the same way when they’re heated.
Adding heat to one end of thermodynamic material causes electrons to flow from the hot end toward the cold end; this hot-to-cold movement of the electrons creates an electrical current. In order to generate more electrical current and power, a larger temperature difference is needed.
However, heating thermoelectric generators can get a little tricky because as you heat the hot side, the cold side also heats up. And as we’ve mentioned before, you need to maintain a difference in temperature across thermoelectric material in order to generate electricity and power.
Now that you have a brief overview of how thermoelectrics work, let’s dive a little deeper into the details! As we’ve explained before, thermoelectric power is basically the conversion of a difference in temperature directly into electrical power. It mainly results from two physical effects: theSeebeck effect andPeltier effect.
The Seebeck effect was first discovered in 1821 by Thomas J. Seebeck — hence the phenomenon was named after him. Using a compass, Seebeck observed the creation of an electromagnetic field when a loop consisting of two different materials was heated on one side. He also noticed that a voltage difference occurs due to the electromagnetic field and voltage’s strength being proportional to the temperature difference between the hot and cold sides of the material. The Seebeck coefficient (S) can be found below, where it varies with material and temperature.
Let’s break this equation down into smaller parts:
Materials that have a negative Seebeck coefficient (-S) have electrons as the dominant charge carriers (n-type). However, materials that have a positive Seebeck coefficient (S) haveholes (absences of electrons) as the dominant charge carriers (p-type). The majority charge carriers — the charge carriers that are largest in quantity — move from the heated side of a material to the cooler side.
With minority charge carriers, they move in a cold-to-hot direction. However, due tophonon drag and the fact that minority charge carriers shift to the opposite direction, they move at a slower rate. So, in order to have current flow in a device, both n-type and p-type materials are needed.
To summarize the Seebeck effect shortly, here are a few things to remember about the phenomenon:
The Peltier Effect is named after Jean C.A. Peltier, who first discovered the phenomenon in 1834. Peltier noticed that when a circuit composed of two different materials passes current, heat is absorbed on one end and released at the other. Unlike Joule heating (which is an irreversible process and quadratic in nature), the process that Peltier observed is linearly dependent and thermodynamically reversible. The latter forms the basis for thermoelectric cooling and temperature control, which are the most common applications of thermoelectric devices today.
In a reverse process, however, power can be generated by applying a temperature difference to create a current flow. The two figures below show a thermoelectric power (TEP) device for cooling and power generation purposes. Note that the current flow is labeled according to the direction of the electrons.
If we want to measure the efficiency of a material in terms of its capability of generating power, we can use what’s known as the figure of merit (Z). In the equation below, the figure of merit is defined in terms of three components: the Seebeck coefficient (S), the electrical conductivity (𝝈), and the thermal conductivity (𝜿). Out of the three, the figure of merit depends mostly on the Seebeck coefficient of the material.
To maximize power generation, the thermal conductivity must be minimized while both the Seebeck coefficient and electrical conductivity must be maximized.
Now that you have some background knowledge about thermoelectrics and how it works, we’ll leave you with a simple example to help solidify your understanding: the Power Practical PowerPot. Although the PowerPot is no longer available, the way it works is pretty neat. And here’s a fun fact: it made an appearance on Shark Tank!
The PowerPot is a batteryless thermoelectric generator that uses heat to generate electricity and charge USB devices. To create a temperature differential, water (or even snow) can be placed in the pot and heated over a variety of heat sources (such as a stove or a fire). The simplified rendering below shows the PowerPot’s temperature distribution when operating.
Within seconds, the PowerPot will start to create an electrical current due to electrons moving from the hot side to the cold side of the pot. This generated electricity can be used along with the high temperature cable to safely charge any USB device!
As you can see from this example, thermoelectrics can be applied to our everyday lives in many different ways. So the next time you think about grabbing a cold beverage from your mini-fridge, just know that thermoelectric processes may be happening behind the scenes!
Contributing Writer: Rebecca Lee