Seebeck Coefficient and Electrical Conductivity

In recent years, the conversion of waste heat into electrical power has become increasingly important. So-called thermoelectric generators (TEGs) have been developed, which can be applied anywhere a useable temperature difference is available. TEGs are used, for example, in vehicle exhaust systems in order to reduce fuel consumption. Currently, they deliver up to 600 W of electrical power. However, their use is also feasible for large-scale applications in the future, such as waste heat recovery in power plants. Tech-nologies are currently being tested which will allow residual heat in cooling towers to be converted into electrical power in the future. This means, for example, the entire interior area of a cooling tower could be coated with flexible TEG material to make technical use of the enormous amounts of energy which are converted during the evaporation of approximately 1500 liters of water per second. For this purpose, special, electrically conductive and ecologically harmless thermoelectric polymers could be employed; these would be structured into thin TEG films by means of special 3-D printing techniques. Along with the positive environmental aspect, such technologies could contribute significantly toward increasing the overall efficiency and profitability of industrial plants.

To realize such applications, thermoelectric materials with high working temperatures and optimized efficiency must be developed. The thermoelectric figure of merit (ZT) is used to assess performance.

To achieve as high a figure of merit as possible requires high electrical conductivity (σ), a high Seebeck coefficient S and low thermal conductivityt λ:

Seebeck-Effect

Seebeck-Effect is a thermoelectric effect.
Phenomenon: A temperature difference causes a electrical potential.

Efficiency of Thermoelectric Materials
Figure of Merit (ZT)

In the field of thermoelectrics, it is important to determine the figure of merit. This figure of merit is the efficiency of a thermoelectric material. This means, one can determine the quality of a material to be used in a thermoelectric generator. The figure of merit can be determined with the Seebeck coefficient S, the electrical conductivity σ and the thermal conductivity λ multiplied with the temperature.

A good thermoelectric material should posses

  • Large Seebeck-Coefficient
  • High electrical conductivity
  • Low thermal conductivity

A high electrical conductivity is necessary to minimize Joule heating, whilst a low thermal conductivity helps to retain heat at the junctions and maintain a large temperature gradient.

Since Z varies with temperature, a useful dimensionless figure of merit can be defined as ZT.
The figure of merit of a thermoelectric material is defined as:

ZT: Figure of Merit [--]
S: Seebeck-Coeffizient or thermo power [μV/K]
T: Temperature [K]
σ: Electrical Conductivity [S/cm]
λ: Thermal Conductivity [W/mK]

The thermal conductivity λ is calculated by the thermal diffusivity a, the specific heat capacity cp and the density ρ.

The thermal diffusivity can be measured with the LFA.

The specific heat capacity can be measured with the DSC or LFA; the density with a dilatometer.

Therefore, by measuring the Seebeck coefficient and the electrical conductivity, we can now measure all values necessary for determination of the figure of merit.

Recommended Literature

Application Literature

Thermoelectric Materials