Thermoelectricity

We know that electricity can be generated employing various methods. Electricity can be generated from wind, with the help of windmills, from water at hydroelectric power plants. Similarly, electricity can be generated from heat energy as well. So thermoelectricity as the name itself suggests is the electric current generated from heat energy. The conversion of heat energy into electrical energy with the help of a thermocouple generates thermoelectricity. 

The thermocouple combines two dissimilar metals (conductors) to develop a potential difference at the junction of the conductors. At the junction of two dissimilar metals (i.e., at the junction of a thermocouple) or within a single conductor, when the temperature difference exists an emf will be generated, resulting in the flow of electricity in a given circuit. The emf so generated is called thermo emf and the electricity is known as thermoelectricity or thermoelectric current. This effect is known as the thermoelectric effect.

Peltier Effect

The ability to cool air or exchange heat is important in many situations. From computer chips that require to stay from overheating to spacecraft that require to face up to temperature extremes the planning of cooling systems may be a business.

The Peltier effect is one of the most interesting phenomena. A potential difference applied across a thermocouple generates a temperature difference between the junctions of the various materials within the thermocouple. This in turn results in the generation of thermoelectricity. Peltier module consists of two dissimilar materials fused together and maintained at room temperature.

The Peltier effect is the opposite of the Seebeck effect (it is named after the scientist who discovered it in 1821). Seebeck Effect suggests that, if two dissimilar metals (conductors) are connected in two separate places, and the intersections are kept at two different temperatures (usually one at zero and another at room temperature), then a potential difference (emf) between the junctions (the intersections) will be generated. This generation of emf when a thermocouple maintained at two different temperatures is known as the Seebeck effect. 

Later, in the year 1834, Jean Peltier found that even the opposite (reverse) of the Seebeck effect is also true, which says that, a potential difference (or emf) and thus a current can also result in a temperature difference, regardless of what the desired amount of temperature is. The Peltier module is also the same as the Seebeck construction or the Seebeck module, and the only difference is the result. Hence it was concluded and was in good agreement with the result is that the Peltier effect is the opposite of the Seebeck effect.

The Peltier effect is the reverse phenomenon of the Seebeck effect. The electric current flowing through the junction of two conductors will emit or absorb heat per unit time at the junction to balance the temperature difference in the chemical potential of the two conductors. 

One of the important applications of the Peltier effect is an electronic refrigerator that can be constructed with the help of a Peltier module known as the Peltier cooler. The Peltier cooler has been applied to many types of electrical equipment such as infrared detectors, CPU coolers, wine cellars, etc. because the cooling power of the Peltier cooler is lower than that of compressor-based refrigerators. 

Peltier Cooler

Theory

Peltier found there was an opposite phenomenon to the Seebeck Effect, whereby thermal energy might be absorbed in one dissimilar metal junction and discharged at the opposite junction when an electric current flows within the closed circuit.

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In the above figure, the circuit is modified to obtain a different configuration that illustrates the Peltier Effect. If a voltage (V) is applied to terminals T\[_{1}\] and T\[_{2}\], an electrical current (I) will flow in the given circuit. As a result of the flow of electric charges or the electric current, a slight cooling effect (Q\[_{cool}\]) will occur at thermocouple junction A (where heat is being absorbed), and a heating effect (Q\[_{heating}\]) will occur at junction B (where heat is dissipated). We have to note that this effect may be reversed whereby a change in the direction of the flow of electric current will reverse the direction of heat flow. 

Joule heating, having a magnitude of I\[^{2}\]R (where R is the electrical resistance used), also occurs in the conductors as a result of the flow of electric current. This Joule heating effect acts in opposition or contradiction to the Peltier Effect and hence it results in a net reduction of the available cooling. The Peltier effect can be expressed mathematically as:

⇒ Q\[_{cool}\] = β x I

⇒ Q\[_{cool}\] = αT x I

Where,

β - The differential coefficient between the two materials

I - The electric current flowing in the given circuit

Working of Peltier Cooler

In the world of thermoelectric technology and thermoelectric generation, semiconductors (usually the materials such as Bismuth Telluride) are the material of choice for producing the Peltier effect because they are efficient enough and can be more easily optimized for pumping heat. Using this type of semiconducting material, a Peltier module (i.e., thermo electric module) can be constructed in its simplest form around a single semiconductor pellet which is soldered to the electrically conductive material on each end (usually plated copper). In this configuration, the second dissimilar material required for the Peltier effect is really the copper connection paths to the facility supply.

It is important to note that heat will be transformed in the direction of charge carrier motion throughout the given circuit (in fact, it is the charge carriers that transfer the heat). While performing thermoelectric experiments, Peltier found there was an opposite phenomenon to the Seebeck Effect, in which thermal energy can also get absorbed in one dissimilar metal junction and dissipated at the other junction when an electric current flowed within the closed circuit.

By arranging n-type and p-type pellets in the form of a single pair or a couple (as demonstrated in the given figure below) and forming a junction between them with the help of plated copper tab, it is possible to construct a series circuit that can keep all of the heat moving in the same direction. As represented in the given illustration, the free or in other words bottom end of the p-type pellet connected to the positive terminal of the source, and the free (bottom) end of the n-type pellet similarly connected to the negative terminal of the source. 

At the same time, we know that for n-type of semiconductor, heat is absorbed from the junction near to the negative terminal of the source and heat is released at the junction near the positive terminal of the source. For the p-type semiconductor, heat is absorbed from the junction near to positive terminal of the source and released at the junction near the negative terminal of the source.

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By arranging the given circuit as shown in the above figure, it is possible to expel heat to one part and absorb it from another end. Using these salient features and special properties of the TE couple, it is possible to team many such pellets all together in rectangular arrays to create practical thermoelectric modules and many thermoelectric devices.

Did You Know?

All the Peltier modules and the Peltier thermoelectric coolers are very useful cooling devices, but they are far from perfect. One of the biggest issue or drawback with a Peltier module is its inefficiency. A Peltier cooler is found to be comparatively less efficient than a conventional coolant-based device. Even though they can be used to create minute air conditioning units it would be impractical to use them to cool down an entire building.

Another drawback of the Peltier cooler is its lifespan. The Peltier module or the Peltier cooler will not last forever, all thermoelectric coolers or just thermo cooler will experience declined efficiency as they age. technically, even some conventional air conditioning systems also suffer the same drawback.