Thermoelectric

Thermoelectric energy conversion



Voyager 1
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Hi-Z, San Diego
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Hi-Z, San Diego
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D.T.S., Halle (Saale)
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PV-TE Hybridsystem
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Initially, thermoelectric energy conversion, which means direct conversion of thermal into electric energy by means of a solid state effect, was developed as power supply of deep space missions. Their significance for wider application is growing for terrestrial utilization with respect to energy savings, reduction in CO2 and pollutants emission in economically and ecologically relevant dimensions.

Current developments focus on

  • mobile auxiliary current sources, e.g. for the generation of additional electric power in the motorcar, or to supply telecommunication repeater stations remote from mains connection; there, thermoelectrics are integrated in hybrid systems together with photovoltaic generators;
  • the recovery of waste heat in heavy industries  and power stations using converter blocks up to the megawatt range have already been drafted: 
  • integrated miniaturised generators with output power in the microwatt range for self-powered sensors and micro-devices.

In international cooperation (involving RU, CZ) prototypes of functionally graded converter modules for hot gas waste heat to be used in mobile thermoelectric auxiliary current sources are under development.

Extensive work is dedicated to iron disilicide as an inexpensive, technologically simple, long-term stable high-temperature material and qualified for the application from room temperature up to about 800 °C. Besides the opportunity to control temperature dependent sensor properties, this functional material paves the way to raise the output power and the efficiency of thermoelectric energy conversion by the principle of functionally grading, taking advantage of the control of microstructure by rapid solidification, thermal spraying, and additional thermal treatment. In extensive developments the influence of multiple doping and non-conductive additives is examined to maximise the material’s thermoelectric quality such that a variation of material’s properties (Seebeck coefficient, electrical and thermal conductivity) is achieved to adjust to a certain application situation.

To simulate functionally graded energy converters and to determine the optimal concentration function of the material along the thermoelectric element, a one-dimensional finite element algorithm was implemented (Galerkin method). Methods of analytic- numeric modelling are involved for the formulation of practically relevant analytical approximations as well as accurate calculations for the estimation of performance parameters of thermoelectric components (efficiency and the electric output power of thermogenerators, temperature differences and the cooling power of Peltier-modules). An exact one-dimensional continuous theoretical description of a homogeneous Peltier leg was developed under correct provision for real temperature dependence of thermoelectric properties. In the case of an inhomogeneous (segmented or continuously graded) pellet it is adjusted and was transferred to a similar treatment of thermogenerator elements.

Technologies for hot side contacting are being developed for thermoelectric generators based on iron disilicide, bismuth telluride, as well as cobaltate ceramics. Besides good mechanical stability, low electrical and thermal contact resistance, the contact at generator’s hot side must be stable at operating temperatures and must not react neither to the silicide or oxide thermoelectric nor be oxidized/degraded by surrounding atmosphere. The characterisation of contacts includes yield mechanical strength, determination of contact resistance via travelling potential micro-probe and long-term studies for stability.


Contact
Univ.-Prof. Dr. Wolf Eckhard Müller
German Aerospace Center

Institute of Materials Research
, Thermoelectric Functional Materials
Tel: +49 2203 601-3556

Fax: +49 2203 696480

E-Mail: Eckhard.Mueller@dlr.de
URL for this article
http://www.dlr.de/wf/en/desktopdefault.aspx/tabid-1696/3089_read-4912/