Thermoelectric energy conversion: Conversion of heat into electrical energy or vice versa

Temperature gradient creates a potential difference.

Temperature gradient creates a potential difference.

Connecting the 2 ends via an electrical conductor results in a current flow.

Connecting the 2 ends via an electrical conductor results in a current flow.

Thermoelectrics are materials which are capable of converting heat into electrical energy and vice versa. This fascinating phenomenon is nowadays commercially used in power generators (e.g., in the telecommunication industry, or in spacecrafts), food refrigerators, air conditioning, cryotherapy, pacemakers, and sensors (e.g. thermocouples). The automobile industry is eager to use this technique, e.g. for environmentally harmless air conditioning or as a power source for the radio or headlights, driven by the exhaust heat. The applications are to date limited due to the somewhat low efficiency η (ca. 5% - 10%):


 

Efficiency of thermoelectrical power generation expressed as a function of temperature.

Typical values might be TH = 800°C, TC = 400°C, zT = 1, yielding a theoretical η = 7.6%.

Thermoelectrics are evaluated based on their thermoelectric figure-of-merit zT, which is close to 1 in the materials commercially used. The Seebeck coefficient α and the electrical conductivity σ can be measured using our ZEM-3 M-8 (ULVAC-RIKO), and the thermal conductivity κ can be determined wither our DLF-1 (TA Instruments).

DLF-1 (TA Instruments) in the lab.
DLF-1 (TA Instruments) in lab.

An increase of zT by a factor of two or more would be necessary to become competitive to Freon compressors as used in conventional refrigerators.

The best thermoelectrics exhibit an intermediate charge carrier concentration (e.g., small band-gap semiconductors), a high mobility of the charge carriers (achieved by using elements with similar electronegativities) and low thermal conductivity, which can be reached by using heavy elements, mixed occupancies, rattling of atoms, and low-symmetry structures.

To improve on these characteristics is the ultimate goal in our research group.

Figure of merit versus temperature for four materials.

Temperature dependence of the thermoelectric figure-of-merit of environmentally benign materials (own research).

Recommended references

  • Y. Shi, C. Sturm, H. Kleinke, J. Solid State Chem. 270, 273 (2019):
      a review covering various chalcogenides for power generation.
  • X. Cheng, N. Farahi, H. Kleinke, JOM 68, 2680 (2016):
      a review covering environmentally benign Mg2Si materials for power generation.
  • H. Kleinke, Chem. Mater. 22, 604 (2010):
      a review covering advanced thermoelectrics for power generation.
  • D. C. Ramirez, L. R. Macario, X. Cheng, M. Cino, D. Walsh, Y.-C. Tseng, H. Kleinke, ACS Appl. Energy Mater. 3, 2130 (2020):
      upscaling the synthesis of environmentally friendly thermoelectrics.
  • P. Jafarzadeh, M. Oudah, A. Assoud, N. Farahi, E. Müller, H. Kleinke, J. Mater. Chem. C 6, 13043 (2018):
      new copper sulfide-tellurides with high zT and good stability.
  • Y. Shi, A. Assoud, S. Ponou, S. Lidin, H. Kleinke, J. Am. Chem. Soc. 140, 8578 (2018):
      the first composite structure with zT above unity.
  • N. Farahi, S. Prabhudev, G. A. Botton, J. R. Salvador, H. Kleinke, ACS Appl. Mater. Interfaces 8, 34431 (2016):
      nano- and microstructure engineering enhances zT to 1.4 for Bi-doped Mg2(Si,Sn).
  • Q. Guo, A. Assoud, H. Kleinke, Adv. Energy Mater. 4, 1400348 (2014):
      Tl10-δSnδTe6 and Tl10-δPbδTe6 exceed zT = 1.
  • H. R. Freer, A. V. Powell, J. Mater. Chem. C 8, 441 (2020):
      future opportunities for thermoelectric materials.