HOW DO THERMOELECTRICS WORK?
Strictly speaking, thermoelectric generators take a temperature difference and turn it into electrical power. Amazingly, these materials can also be run in reverse! If you put power into a thermoelectric generator you will create a temperature difference. Small mini-fridges, for just a few sodas, use thermoelectric generators to efficiently cool a few drinks.
To understand how thermoelectrics generate the electricity from a temperature difference we have to know a bit about how electrons move in a metal. Metals are good conductors because electrons can move freely within them, similar to a fluid in a pipe. Imagine you have a pipe full of water and you raise one end, what happens? The water will flow down the pipe from the high end to the low end. This is because when you raised the pipe you increased the potential energy and the water wants to flow downhill. In a thermoelectric material the same thing happens to the fluid-like electrons when you heat it.
Heating one end of a thermoelectric material causes the electrons to move away from the hot end toward the cold end. When the electrons go from the hot side to the cold side this causes an electrical current, which the PowerPot harnesses to charge USB devices. The larger the temperature difference the more electrical current is produced and therefore more power generated.
The tricky part about thermoelectric generators is that as you heat the hot side, the cold side of the generator heats up too. In order to generate power with the a thermoelectric generator you need both a heat source and a way of dissipating heat in order to maintain a temperature difference across the thermoelectric materials. This is done with no moving parts by heating water in the PowerPot. Water holds several times more heat than aluminum per pound, so it makes a wonderful heatsink. Also, water never gets hotter than 212 F (100 C) at a boil, effectively limiting the maximum temperature of the “cold” side of the thermoelectric generator. This is why you always need to have something watery in the PowerPot or else it is possible to overheat the thermoelectric generator.
This rendering shows temperature distribution in the PowerPot during operation with some parts removed for clarity.
Background of Thermoelectrics
Thermoelectric power is the conversion of a temperature differential directly into electrical power. Thermoelectric power results primarily from two physical effects: the Seebeck effect, and Peltier effect.
The Seebeck effect is named after Thomas J. Seebeck, who first discovered the phenomenon in 1821. Seebeck noticed that when a loop comprised of two dissimilar materials was heated on one side, an electromagnetic field was created. He actually discovered the EM field directly with a compass! He noted that the strength of the electromagnetic field, and therefore the voltage, is proportional to the temperature difference between the hot and cold sides of the material. The magnitude of the Seebeck coefficient (S) varies with material and temperature of operation. The Seebeck coefficient is thus defined as:
In this equation ΔV is the voltage difference between the hot and cold sides, ΔT is the temperature difference between the hot and cold sides. The negative sign comes from the negative charge of the electron, and the conventions of current flow. A negative Seebeck coefficient results in electrons being the dominant charge carriers (n-type), whereas holes are the dominant carrier (p-type) in materials with a positive Seebeck coefficient. The majority charge carriers are said to move away from the heated side toward the cooler side. Minority charge carriers move in the opposite direction, but at a slower rate due to phonon drag and charge carrier diffusion rates. Thus, both n-type and p-type materials are required to realize current flow in a device.
Things to remember about the Seebeck effect:
- Solids have charge carriers that facilitate the flow of electrical power
- The charge carriers come in two flavors negative electrons "n-type" and positive "holes" that we use to keep track of mobile positive charge in "p-type" solids
- Heating one end of a conducting solid pushes on the charge carriers concentration and the distribution of charge creates voltage that can be measured this is called the Seebeck effect
The Peltier effect was first discovered in 1834 by Jean C.A. Peltier, for whom it was named. Peltier discovered that whenever a circuit of two dissimilar materials passes current, heat is absorbed at one end of the junction and released at the other. This is a linearly dependent and thermodynamically reversible process, unlike Joule heating which is irreversible and quadratic in nature mean. This process forms the basis for thermoelectric cooling and temperature control, these are currently the widest applications of thermoelectric devices.
However, applying a temperature differential the reverse process occurs, and current is caused to flow, thereby generating power. The figure below shows a TEP device in both cooling and power generation configurations.
A thermoelectric cooler (left), and power generator (right). Current flow is labeled in the direction of the electrons.
The efficiency by which a material is capable of generating power is governed by the figure of merit (Z). As seen in the equation below, the figure of merit is most dependent on the Seebeck coefficient of the material.
In the above equation, the figure of merit is defined in terms of the Seebeck coefficient, the electrical conductivity, and the thermal conductivity. Maximum power generation requires the minimization of the thermal conductivity, while maximizing the Seebeck coefficient and electrical conductivity.
The PowerPot is a thermoelectric generator that uses heat to generate electricity. The PowerPot has no moving parts or batteries, and since the thermoelectric technology is built into the bottom of the pot it can produce electricity from a wide variety of heat sources. Simply add water and place the PowerPot on a fire (e.g. wood, propane, butane, alcohol, gas) and it will start generating electricity within seconds. Just plug in the high temperature cable to the back of the pot and watch your USB devices safely charge from a fire.
The larger the temperature difference between the water in the pot and the bottom of the pot, the more electricity the PowerPot will produce. For example, melting snow in the PowerPot is a great way to generate electricity, because snow is so much colder than a flame. However, you don’t have to worry about overpowering your device, because the PowerPot has a built in regulator which insures that you safely charge your USB devices. The regulator outputs 5 volts (USB standard) and up to 1000 milliAmps of current, which is the most any smartphone/MP3 player on the market can handle. This means when you’re charging your USB device with the PowerPot, you will get the same charging time as you would from your wall outlet at home. Learn More