Thermoelectric materials
show the
thermoelectric effect
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
in a strong or convenient form.
The ''thermoelectric effect'' refers to phenomena by which either a
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
difference creates an
electric potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
or an electric current creates a temperature difference. These phenomena are known more specifically as the
Seebeck effect
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
(creating a voltage from temperature difference),
Peltier effect
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
(driving heat flow with an electric current), and
Thomson effect
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
(reversible heating or cooling within a conductor when there is both an electric current and a temperature gradient). While all materials have a nonzero thermoelectric effect, in most materials it is too small to be useful. However, low-cost materials that have a sufficiently strong thermoelectric effect (and other required properties) are also considered for applications including
power generation
Electricity generation is the process of generating electric power from sources of primary energy. For utilities in the electric power industry, it is the stage prior to its delivery ( transmission, distribution, etc.) to end users or its stor ...
and
refrigeration
The term refrigeration refers to the process of removing heat from an enclosed space or substance for the purpose of lowering the temperature.International Dictionary of Refrigeration, http://dictionary.iifiir.org/search.phpASHRAE Terminology, ht ...
. The most commonly used thermoelectric material is based on
bismuth telluride
Bismuth telluride (Bi2Te3) is a gray powder that is a compound of bismuth and tellurium also known as bismuth(III) telluride. It is a semiconductor, which, when alloyed with antimony or selenium, is an efficient thermoelectric material for refriger ...
().
Thermoelectric materials are used in thermoelectric systems for
cooling or heating in niche applications, and are being studied as a way to
regenerate electricity from waste heat.
Thermoelectric figure of merit
The usefulness of a material in thermoelectric systems is determined by the
device efficiency. This is determined by the material's
electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
(''σ''),
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
(''κ''), and
Seebeck coefficient
The Seebeck coefficient (also known as thermopower, thermoelectric power, and thermoelectric sensitivity) of a material is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material ...
(S), which change with
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
(''T''). The maximum efficiency of the energy conversion process (for both power generation and cooling) at a given temperature point in the material is determined by the thermoelectric materials
figure of merit
A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. Examples
*Clock rate of a CPU
* Calories per serving
*Contrast ratio of an LCD
*Frequency response of a speaker
* ...
, given by
Device efficiency
The efficiency of a thermoelectric device for electricity generation is given by
, defined as
The maximum efficiency of a thermoelectric device is typically described in terms of its device
figure of merit
A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. Examples
*Clock rate of a CPU
* Calories per serving
*Contrast ratio of an LCD
*Frequency response of a speaker
* ...
where the maximum device efficiency is approximately given by
where
is the fixed temperature at the hot junction,
is the fixed temperature at the surface being cooled, and
is the mean of
and
. This maximum efficiency equation is exact when thermoelectric properties are temperature-independent.
For a single thermoelectric leg the device efficiency can be calculated from the temperature dependent properties ''S'', ''κ'' and ''σ'' and the heat and electrical current flow through the material.
In an actual thermoelectric device, two materials are used (typically one n-type and one p-type) with metal interconnects. The maximum efficiency
is then calculated from the efficiency of both legs and the electrical and thermal losses from the interconnects and surroundings.
Ignoring these losses and temperature dependencies in ''S'', ''κ'' and ''σ'', an inexact estimate for
is given by
[Ioffe, A.F. (1960) ''Physics of semiconductors'', Academic Press Inc., New York] where
is the electrical resistivity, and the properties are averaged over the temperature range; the subscripts n and p denote properties related to the n- and p-type semiconducting thermoelectric materials, respectively. Only when n and p elements have the same and temperature independent properties (
) does
.
Since thermoelectric devices are heat engines, their efficiency is limited by the
Carnot efficiency , the first factor in
, while
and
determines the maximum reversibility of the thermodynamic process globally and locally, respectively. Regardless, the
coefficient of performance
The coefficient of performance or COP (sometimes CP or CoP) of a heat pump, refrigerator or air conditioning system is a ratio of useful heating or cooling provided to work (energy) required. Higher COPs equate to higher efficiency, lower energy ( ...
of current commercial thermoelectric refrigerators ranges from 0.3 to 0.6, one-sixth the value of traditional vapor-compression refrigerators.
Power factor
Often the thermoelectric power factor is reported for a thermoelectric material, given by