Book Description

We conducted a vehicle analysis to assess the feasibility of thermoelectric technology for waste heat recovery and conversion to useful electrical power and found that eliminating the 500 W of electrical power generated by the alternator corresponded to about a 7% increase in fuel economy (FE) for a small car and about 6% for a full size truck. Electric power targets of 300 W were established for city and highway driving cycles for this project. We obtained critical vehicle level information for these driving cycles that enabled a high-level design and performance analysis of radiator and exhaust gas thermoelectric subsystems for several potential vehicle platforms, and we identified the location and geometric envelopes of the radiator and exhaust gas thermoelectric subsystems. Based on this analysis, we selected the Chevrolet Suburban as the most suitable demonstration vehicle for this project. Our modeling and thermal analysis assessment of a radiator-based thermoelectric generator (TEG), however, revealed severe practical limitations. Specifically the small temperature difference of 100°C or less between the engine coolant and ambient air results in a low Carnot conversion efficiency, and thermal resistance associated with air convection would reduce this conversion efficiency even further. We therefore decided not to pursue a radiator-based waste heat recovery system and focused only on the exhaust gas. Our overall approach was to combine science and engineering: (1) existing and newly developed TE materials were carefully selected and characterized by the material researcher members of our team, and most of the material property results were validated by our research partners, and (2) system engineers worked closely with vehicle engineers to ensure that accurate vehicle-level information was used for developing subsystem models and designs, and the subsystem output was analyzed for potential fuel economy gains. We incorporated material, module, subsystem, and integration costs into the material selection criteria in order to balance various materials, module and subsystem design, and vehicle integration options. Our work on advanced TE materials development and on TEG system design, assembly, vehicle integration, and testing proceeded in parallel efforts. Results from our two preliminary prototype TEGs using only Bi-Te TE modules allowed us to solve various mechanical challenges and to finalize and fine tune aspects of the design and implementation. Our materials research effort led us to quickly abandon work on PbTe and focus on the skutterudite materials due to their superior mechanical performance and suitability at automotive exhaust gas operating temperatures. We synthesized a sufficiently large quantity of skutterudite material for module fabrication for our third and final prototype. Our TEG#3 is the first of its kind to contain state-of-the-art skutterudite-based TE modules to be installed and tested on a production vehicle. The design, which consisted of 24 skutterudite modules and 18 Bi-Te modules, attempted to optimize electrical power generation by using these two kinds of TE modules that have their peak performance temperatures matched to the actual temperature profile of the TEG during operation. The performance of TEG#3 was limited by the maximum temperature allowable for the Bi-Te TE modules located in the colder end of the TEG, resulting in the operating temperature for the skutterudite modules to be considerably below optimum. We measured the power output for (1) the complete TEG (25 Watts) and (2) an individual TE module series string (1/3 of the TEG) operated at a 60°C higher temperature (19 Watts). We estimate that under optimum operating temperature conditions, TEG#3 will generate about 235 Watts. With additional improvements in thermal and electrical interfaces, temperature homogeneity, and power conditioning, we estimate TEG#3 could deliver a power output of about 425 Watts.