What Is the Thermoelectric Effect?

The thermoelectric effect is a set of phenomena describing the direct interconversion of heat and electricity in conducting and semiconducting materials. It encompasses three related but distinct effects: the Seebeck effect, in which a temperature gradient across a material generates a voltage; the Peltier effect, in which an electric current creates a heat flux at the junction between two different conductors; and the Thomson effect, in which current flow through a material with a temperature gradient causes reversible heat exchange with the surroundings. Together, these phenomena constitute the thermodynamic basis for thermoelectric technology.

The effect arises from the transport behavior of charge carriers in materials. When a temperature gradient is established across a conductor or semiconductor, charge carriers on the hot side have more thermal energy and tend to diffuse toward the cold side, producing a net charge separation and an associated electrical potential. The magnitude of this potential per unit temperature difference is the Seebeck coefficient, also called thermopower, measured in microvolts per kelvin. Maximizing this coefficient while simultaneously maintaining high electrical conductivity and low thermal conductivity is the central materials challenge in thermoelectric engineering.

Seebeck Effect

The Seebeck effect was discovered by Thomas Johann Seebeck in 1821, when he observed that a circuit made of two dissimilar metals deflected a compass needle in the presence of a temperature difference between the two junctions. The effect is quantified by the Seebeck coefficient, which is positive for p-type materials (holes carry current) and negative for n-type materials (electrons carry current). Practical thermoelectric generators exploit this effect by placing p-type and n-type semiconductor legs between a hot surface and a cold surface, connecting them electrically in series so that the voltages from each leg add, while the thermal paths are in parallel to equalize heat flow. The ETH Zurich study on thermoelectric energy harvesting demonstrates power generation from gradients as small as 1 K, illustrating the sensitivity of the Seebeck effect in optimized materials.

Peltier Effect

Jean Charles Athanase Peltier observed in 1834 that passing an electrical current through a junction of two different conductors caused one junction to absorb heat and the other to release it. This reversible heat exchange at the junction is the Peltier effect, and its magnitude is described by the Peltier coefficient, which equals the Seebeck coefficient multiplied by the absolute temperature at the junction. The Peltier effect is the operating principle behind thermoelectric coolers, which pump heat from a cold reservoir to a hot one without refrigerants or moving parts. The direction of pumping reverses when current direction is reversed, giving these devices precise bidirectional temperature control.

Thomson Effect and Figure of Merit

William Thomson (Lord Kelvin) demonstrated in 1851 that a conducting material carrying current along a temperature gradient exchanges heat with its surroundings at a rate proportional to both the current and the temperature gradient, independent of the Joule heating effect. This Thomson effect, though small in most practical devices, completes the thermodynamic description of thermoelectricity. The overall performance of a thermoelectric material is characterized by the dimensionless figure of merit ZT, where Z equals the square of the Seebeck coefficient multiplied by electrical conductivity, divided by thermal conductivity, and T is the absolute temperature. High ZT values require high Seebeck coefficient, high electrical conductivity, and low thermal conductivity, three properties that are difficult to optimize simultaneously because they are physically coupled. Research published in PMC on bismuth telluride thermoelectric generation describes how ZT values have grown from around 0.5 to well above 1.0 through nanostructuring and alloying strategies. Improvements in ZT are also reviewed in PMC studies on thermoelectric figure of merit enhancement.

Applications

The thermoelectric effect has applications in a wide range of disciplines, including:

• Spacecraft power systems using radioisotope thermoelectric generators
• Solid-state refrigeration in medical diagnostics and photonic devices
• Industrial waste heat recovery converting exhaust heat to electricity
• Wearable energy harvesting from body heat to power health monitors
• Temperature sensing and measurement through thermocouple calibration