High-Performance Silicon-Germanium-Based Thermoelectric Modules for Gas Exhaust Energy Scavenging

Some of the energy used in transportation and industry is lost as heat, often at high-temperatures, during conversion processes. Thermoelectricity enables direct conversion of heat into electricity, and is an alternative to the waste-heat-recovery technology currently used, for example turbines and other types of thermodynamic cycling. The performance of thermoelectric (TE) materials and modules has improved continuously in recent decades. In the high-temperature range (T (hot side) > 500A degrees C), silicon-germanium (SiGe) alloys are among the best TE materials reported in the literature. These materials are based on non-toxic elements. The Thermoelectrics Laboratory at CEA (Commissariat A l'Energie Atomique et aux Energies Alternatives) has synthesized n and p-type SiGe pellets, manufactured TE modules, and integrated these into thermoelectric generators (TEG) which were tested on a dedicated bench with hot air as the source of heat. SiGe TE samples of diameter 60 mm were created by spark-plasma sintering. For n-type SiGe doped with phosphorus the peak thermoelectric figure of merit reached ZT = 1.0 at 700A degrees C whereas for p-type SiGe doped with boron the peak was ZT = 0.75 at 700A degrees C. Thus, state-of-the-art conversion efficiency was obtained while also achieving higher production throughput capacity than for competing processes. A standard deviation < 4% in the electrical resistance of batches of ten pellets of both types was indicative of high reproducibility. A silver-paste-based brazing technique was used to assemble the TE elements into modules. This assembly technique afforded low and repeatable electrical contact resistance (< 3 n Omega m(2)). A test bench was developed for measuring the performance of TE modules at high temperatures (up to 600A degrees C), and thirty 20 mm x 20 mm TE modules were produced and tested. The results revealed the performance was reproducible, with power output reaching 1.9 +/- A 0.2 W for a 370 degree temperature difference. When the temperature difference was increased to 500A degrees C, electrical power output increased to > 3.6 W. An air-water heat exchanger was developed and 30 TE modules were clamped and connected electrically. The TEG was tested under vacuum on a hot-air test bench. The measured output power was 45 W for an air flow of 16 g/s at 750A degrees C. The hot surface of the TE module reached 550A degrees C under these conditions. Silicon-germanium TE modules can survive such temperatures, in contrast with commercial modules based on bismuth telluride, which are limited to 400A degrees C.