Segmented Thermoelectric Unicouples Research Articles

Segmented thermoelectric unicouple for an operating temperature range of 30–320°C

The possibility of increasing the efficiency of a thermocouple in the temperature range of 30–320°C is studied using an approach associated with the development of segmented thermoelectric unicouples. n- and p-type thermoelectric unicouples are constructed from low-temperature thermoelectric materials based on bismuth telluride and the addition of an intermediate-temperature material based on PbTe and GeTe, respectively. The thermoelectric unicouples are fabricated by spark plasma sintering (SPS). This method provides a contact resistance of ≤10 μΩ cm. The properties of segmented and conventional unicouples are compared. The efficiency of segmented unicouples in comparison with conventional ones increases by almost 70% and attains 5.3% in the operating range of 30–320°C.

Towards high efficiency segmented thermoelectric unicouples

Segmentation of thermoelectric (TE) materials is a widely used solution to improve the efficiency of thermoelectric generators over a wide working temperature range. However, the improvement can only be obtained with appropriate material selections. In this work, we provide an overview of the theoretical efficiency of the best performing unicouples designed from segmenting the state‐of‐the‐art TE materials. The efficiencies are evaluated using a 1D numerical model which includes all thermoelectric effects, heat conduction, Joule effects and temperature dependent material properties, but neglects contact resistance and heat losses. The calculations are performed for a fixed cold side temperature of 300 K and different hot side temperatures of 700, 900, and 1100 K. We confirm that without taking into account the compatibility of TE materials, segmentation can even decrease the total efficiency. Choosing the TE materials carefully, one is, however, rewarded by a significant improvement in the total efficiency.

Efficient segmented thermoelectric unicouples for space power applications

This paper compares the performance of SiGe (Si 0.8Ge 0.2) and skutterudite segmented thermoelectric unicouples (STUs) at a hot side temperature of 973 K and cold side temperatures of 300, 573 and 673 K and for the same total length and cross sectional dimensions of the p-leg. The area of the n-leg and lengths of and the interfacial temperatures between segments of various materials in the STU legs are determined using a global optimization methodology. Results indicate that the STUs could potentially achieve peak efficiencies of 7.8% and 14.7% when operated at a cold side temperature of 573 K, typical of that in current radioisotope thermoelectric generators (RTGs), and 300 K, respectively. These efficiencies are 55% and 99% higher, respectively, than for SiGe at the same temperatures. However, due to the higher density of skutterudite, the electrical power densities corresponding to the peak efficiencies of the STUs are 39 and 109 W e/kg versus 92 and 232 W e/kg for SiGe at cold side temperatures of 573 K and 300 K, respectively. On the other hand, the heat inputs and heat rejection powers for the STUs are 42–55% and 39–53%, respectively, of those for SiGe at the peak efficiency and 70–75% and 67–77% of those of SiGe, respectively, at the peak electric power density. Therefore, when used in RTGs, STUs could halve the 238PuO 2 fuel mass and the radiator area, while operating at >45% higher electrical power density (>7 W e/kg) than SiGe in current RTGs (∼5.5 W e/kg).

High efficiency segmented thermoelectric unicouple for operation between 973 and 300 K

High efficiency segmented thermoelectric unicouples (STUs) made of n-type Bi 2Te 3 and CoSb 3 based alloys and p-type Bi 2Te 3 and CeFe 4Sb 12 based alloys have recently been developed at the Jet Propulsion Laboratory (JPL). A 1-D analytical model of STUs, which assumes zero side heat losses, is developed and its predictions are compared with some results of experiments performed at JPL to measure the performance parameters of two STUs, uni8 and uni12. Calculations of the effects of contact resistance and side heat losses on the performance of these STUs are presented and discussed. The results indicate that had uni8 and uni12 been well insulated and the contact resistance per leg ∼50 μΩ cm 2, they could have achieved a conversion efficiency of ∼12%, in the tests performed at T c=305–316 K and T h=872–885 K. However, the actual contact resistances of ∼146 and 690 μΩ cm 2 per leg and the side heat losses of 3.7 and 1.83 W, for uni8 and uni12, respectively, limited the peak conversion efficiency in the tests to ∼5.5% and 4.6%, respectively. Calculations indicated that STUs having a total contact resistance ∼50 μΩ cm 2 per leg and being well insulated on the sides could potentially achieve a peak conversion efficiency of 15% when operated between 973 and 300 K.

Synthesis and Thermoelectric Properties of CoP3

ABSTRACTIn an effort to expand the range of operation for highly efficient, segmented thermoelectric unicouples currently being developed at the Jet Propulsion Laboratory (JPL), skutterudite phosphides are being investigated as potential high temperature segments to supplement antimonide segments that limit the use of these unicouples at a hot-side temperature of about 873-973 K. We report here on the synthesis and transport properties of one of these phosphide skutterudite materials, CoP3. Large amounts of cobalt triphosphide have been prepared by direct reaction from stochiometric powders at synthesis temperatures ranging from 873 to 1223 K. The synthesized powders were analyzed by x-ray diffractometry and hot pressed at 1273 K. The samples were analyzed by microprobe analysis and electrical conductivity, Seebeck voltage and thermal conductivity measurements were performed. The thermoelectric properties are presented and discussed as a function of temperature up to 1000 K. Initial thermal stability results are presented to assess the potential of this material for high temperature operation.

Progress in the development of segmented thermoelectric unicouples at the Jet Propulsion Laboratory

ABSTRACTA new version of a segmented thermoelectric unicouple incorporating advanced thermoelectric materials with superior thermoelectric figures of merit has been recently proposed and is currently under development at the Jet Propulsion Laboratory (JPL). This advanced segmented thermoelectric unicouple includes a combination of state-of-the-art thermoelectric materials based on Bi2Te3 and novel materials developed at JPL. The segmented unicouple currently being developed is expected to operate between 300 and about 975K with a projected thermal to electrical efficiency of up to 15%. The segmentation can be adjusted to accommodate various hot-side temperatures depending on the specific application envisioned. Techniques and materials have been developed to bond the different thermoelectric segments together for the nand p-legs and low contact resistance bonds have been achieved. In order to experimentally determine the thermal to electrical efficiency of the unicouple, metallic interconnects must be developed for the hot side of the thermocouple to connect the n- and p-legs electrically. The latest results in the development of these interconnects are described in this paper. Efforts are also focusing on the fabrication of a unicouple specifically designed for thermal and electrical testing.