7. How do thermoelectric generators (TEGs) work?
Thermoelectric generators are solid-state heat engines that operate according to the Seebeck Effect – a theory that claims a temperature difference across thermoelectric material can be converted directly into electrical power.
A thermoelectric generator is made of many pairs of p-type and n-type elements. The p-type elements are made of semiconductor materials doped such that the charge carriers are positive (holes) and Seebeck coefficient is positive. The n-type elements are made of semiconductor material doped such that the charge carriers are negative (electrons) and the Seebeck coefficient is negative.
Connecting a p-type element to an n-type element creates a voltage potential across the junction. This voltage potential is proportional to the differences in the Seebeck coefficient in each element and the temperature of the junction.
When one electrically connects a p-type element to the n-type element, the mobile holes in the p-type element “see” the mobile electrons in the n-type element and migrate just to the other side of the junction (See Figure 1).
When one electrically connects a p-type element to the n-type element, the mobile holes in the p-type element “see” the mobile electrons in the n-type element and migrate just to the other side of the junction. For every hole that migrates into the n-type element, an electron from the n-type element migrates into the p-type element. Soon, each hole and electron that “switch sides” will be in equilibrium and act like a barrier, preventing more electrons or holes from migrating. This is called the depletion zone (see figures 2 and 3).
Heating this depletion zone area and cooling other ends of the element can break down this depletion zone. The mobile holes in the p-type are excited by the heat and move further into the element with the extra kinetic energy. The same happens to the mobile electrons in the n-type material. The net effect: many of the holes pile up at the cold end of the p-type element and many of the electrons pile up at the cold end of the n-type element, thereby creating a voltage potential across the p-n junction when measured from cold end to cold end (see figure 4).
Add a voltage potential in picture from cold end of left to right.
By placing an electrical load or wire from the cold end of the p-type element to the n-type element, the electrons from the n-type element will “see” all of the holes piled up at the end of the p-type element and hitch a ride along the wire into the p-type material. In response, a hole from the p-type element will “see” a vacancy in the n-type element and migrate in that direction. The end effect is current flow across a voltage potential (from the p-n junction) (see figure 5), and electrical power is created. This power is a function of many things such as temperature difference, Seebeck coefficients, and the electrical load that connects the cold sides. And of course, this concept can be extrapolated for many p-n couples.
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