Plenary Lecture

Heat Transfer in Thermoelectricity: Modelling,
Optimization and Design

Professor Myriam Lazard
Institut Superieur d’Ingenierie de la Conception
27 rue d’Hellieule, 88100 Saint Die
FRANCE
E-mail : mlazard@insic.fr

Abstract: When a temperature difference exists, a potential for power production ensues: it is the principle of thermoelectricity, it could provide an unconventional energy source for a wide range of applications even if the efficiency of the thermoelements is rather low. As a consequence, there is an increasing use of thermoelectric devices in many fields such as aerospace, automotive and building applications. On the other hand, the coupled effects involved in such systems usually leads to complex modelling. In order to predict the performances of the device, several methods could be used: experimental, numerical and semi-analytical. For the experimental ones, the device must already exist whereas numerical and semi-analytical methods could provide more or less realistic predictions.
In a first part, a semi-analytical method has been chosen in order to better understand the underlying physical phenomena and the contribution of the different effects. A thermal modelling of a thermoelectric leg is presented. The aim is to determine the expressions of the temperature within the thermoelement and also the heat fluxes. Indeed these two quantities are needed to determine the performance of the device by calculating the efficiency of the element or for instance by evaluating the COP. The steady-state and the transient cases are considered. The Joule contribution is taken into account (introducing a source term in the heat transfer equation) and the effect due to the Thomson coefficient is investigated.
In the second part, the design of a thermoelement for instance applied to Radioisotope Thermoelectric Generators (RTGs) is investigated. As no single thermoelectric material presents high figure of merit over a wide temperature range, it is therefore necessary to use different materials and to segment them together in order to have a sandwiched structure: in this way, materials are operating in their most efficient temperature range. Even if the thermoelectric figure of merit is an intensive material property of prime importance, it is not the only one: indeed the expression of the reduced efficiency involves another parameter called the compatibility factor, which must be considered and controlled to determine the relevance of segmentation. Not only the reduced efficiency but also the compatibility factor are then plotted for different n-type and p-type elements such as skutterudite as function of the temperature. Thanks to these considerations, the design of the segmented thermoelectric device is investigated in order to optimize the efficiency and once the materials chosen, to determine the best operating conditions and especially the relative current density which is the ratio of the electric current density to the heat flux by conduction.

Brief Biography of the Speaker:
- Associate Professor, Mechanical Engineering, Institute in Engineering and Design (InSIC), Ecole des Mines, (since 2002).
- Post Doctoral Position, Thermoelectricity, Lab. Physics of Materials (LPM), Ecole des Mines de Nancy (2001).
- PhD, “Modelling of the combined conductive-radiative heat transfer in a semi-transparent medium. Parameters estimation”, Lab. Energetics & Mechanics Theoretical and Applied, (LEMTA), Institut National Polytechnique de Lorraine (2000).
- MSME and BSME, Institut National Polytechnique de Lorraine (1995).
- MSMaths and BSMaths, Elie Cartan, Universtite Henri Poincare (1995,1994).
Research interests include:
- Heat Transfer in Manufacturing Processes (turning, injection...)
- Radiative Transfer in Semi-Transparent Media
- Inverse Problems, Parameters Estimation
- Thermoelectricity : modelling and simulations
Teaching :
- Undergraduate and graduate levels, Engineering schools
- Heat transfer, thermodynamics,numerical methods, mathematics
Member of the Editorial Board of the journal CESES