Highly Efficient Thermoelectric Unicouples

Thermal-to-electric conversion efficiencies as large as 15 percent have been predicted.

NASA's Jet Propulsion Laboratory, Pasadena, California

Advanced unicouples that contain carefully tailored combinations of thermoelectric materials are being investigated in an effort to develop thermoelectric power generators that would operate with relatively high energy-conversion efficiencies between heat-source temperatures of about 1,000 K and heat-sink temperatures of about 300 K. [A thermoelectric unicouple is a part of a thermoelectric generator; a unicouple comprises one electron-conduction (n-type) leg and one hole-conduction (p-type) leg thermally connected in parallel and electrically connected in series.] Thermoelectric generators have no moving parts, are reliable, can operate unattended in hostile environments, and are environmentally benign; because of these characteristics, they are particularly attractive for extracting electric power from waste heat from such diverse sources as geothermal power plants, industrial heat-generating processes, and automobile exhausts.

To achieve high thermal-to-electric energy-conversion efficiencies, it is desirable to operate thermoelectric devices over large temperature gradients and to maximize the performances of the thermoelectric materials in the devices. However, no single thermoelectric material is suitable for maximizing the efficiency over the entire range between the contemplated source and sink temperatures. Therefore, the approach taken in designing the present advanced thermoelectric unicouples involves the use of different thermoelectric materials, each of which exhibits optimum performance in a different portion of the temperature range.

The figure schematically depicts a thermoelectric unicouple of the present type optimized for a source temperature of 973 K and a sink temperature of 300 K. This unicouple is made of high-performance thermoelectric materials, including (1) previously known state-of-the-art materials, (2) novel p-type alloys based on Zn₄Sb₃ and CeFe₄Sb₁₂, and (3) novel n-type alloys based on CoSb₃. Each of the p and n legs comprises segments of the various materials joined both thermally and electrically in series.

In general, the relative positions and lengths of the segments in each leg of a unicouple like this one must be chosen, taking account of the thermal conductivities of the materials, so that each segment is exposed to that portion of the source-to-sink temperature range for which the material of that segment exhibits its maximum thermoelectric performance. In addition, the ratio between the cross sections of the n and p legs must be chosen to optimize overall performance in the face of differences between the thermal and electrical conductivities in the two legs. To a first approximation (ignoring contact resistances and other relatively small effects), the energy-conversion efficiency of the unicouple depends on the ratios among the lengths of the segments but not on the overall length of the legs. However, the overall electrical and thermal resistances and the output electric power do depend on the overall length of the legs.

 

An Advanced Thermoelectric Unicouple is made of high-performance p- and n-type materials, each positioned and dimensioned so that it is exposed to a temperature interval in which its thermoelectric performance is maximized. The relative lengths of the segments shown here to scale are chosen to obtain a predicted overall thermal-to-electrical energy-conversion efficiency of about 15 percent. Fabrication and testing of the predicted prototype unicouple is in progress.

 

This work was done by Thierry Caillat, Alex Borshchevsky, Jeff Snyder, Jean-Pierre Fleurial, Andrew Zoltan, and Leslie Zoltan of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Electronic Components and Systems category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

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