Efficiency of Thermoelectric Devices

Nowadays there are many alternative power sources and for thermoelectric generators and coolers to be competitive , their efficiency has to increase from the value which is attainable now (about 10%) .

Efficiency of thermoelectric devices is mainly determined by the properties of the semiconductor materials used to manufacture the modules called in abbreviation-TE- modules. One module is used to produce low power devices while many connected in series is needed to produce high power devices. The properties of these semiconductor materials determine how effective they are to produce poor or good efficiency, the effectiveness is called figure of merits (Z) which depend on Seebeck coefficient (s), electrical conductivity (σ), and thermal conductivity (k) according to the formula

                   Z = σS2
                           
  k

The formula indicates that a good thermoelectric module should have a high electrical conductivity and Seebeck coefficient as (electric crystal) and low thermal conductivity as a (phonon glass).

Figure of merit Z has only increased from 0.6 to about 1.0 over the last five decades and research indicate that for thermoelectric devices to be competitive for high power applications, the figure of merit need to be about 3.0.

The variables in the above formula which are mainly the electrical and thermal conductivity interact with each other in reality and so it is very challenging to produce a module that has a high electrical conductivity and in the same time low thermal conductivity .

Latest laboratory research is concentrating on experimenting with many different materials to reduce the thermal conductivity and reports indicate that it is possible now to get figure of merit of about 2.0 .

Some manufacturers specialize in production of thermoelectric devices claim that they have produced new generation of these devices using newly developed materials with comparatively high Z.

In conclusion it is quite clear that efficiency of thermoelectric devices is not expected to be any where near that of mechanical compressors or pumps (40% to 50%) in the near future but continuous improvement is expected.

It is worth noting that efficiency is not the only important factor in determining the use of thermoelectric generators and coolers but one needs to take into consideration the many advantages of thermoelectric devices in such location as for example using coolers by trucks and automobile users where portable devices are necessary and also while in a picnic or in  remote locations that require only low power devices and when reliability is a priority.

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