In passenger cars about 15% of fuel burned is lost due to waste heat production. This of course varies with driving patterns and engine efficiency, but still constitutes a considerable share of fuel use, as well as of corresponding climate gas and air pollutant emissions. Finding ways to recvover parts of waste heat thus would significantly contribute to the economic and environmental sustainability of car travel.
The aim of the project was to clarify important issues regarding the technical and economic feasibility of the pilot scale of modules based on the concepts which were developed in the previous TEG (thermoelectric generator) project.
The project was divided into several main issues:
- Efficient design of the TEG unit
- Skutterudite synthesis under technological boundary conditions
- Leg production in pilot scale
- Thermo mechanical reliability aspects
According to the different topics, the following key results have been achieved:
Efficient design of the TEG unit: The simulation task has been completed at AVL List GmbH by a 1D model and the 3D model with an integration into existing simulation codes AVL BOOST® RT or AVL FIRE®. It is now possible to model the complete system and to analyze the influences and interactions. With this additional information, it was possible to perform a realistic view of the overall system.
Skutterudite synthesis under technological boundary conditions: The powder quantities of both skutterudite types have been reproducibly scaled from a few kilograms to currently 1t in pilot scale. Great emphasis was placed on technical grade materials in order to build an economically viable production route. This made it possible to achieve thermoelectric properties corresponding to approximately 95% of the values obtained in the laboratory. It can be stated that the material quality and production capacity is already unique in the world; nevertheless further improvements are already under investigation.
Leg production in pilot scale: A close-to-series pilot production was achieved with a facility which was installed during the project period. The achieved reproducible mechanical and thermoelectric properties of skutterudite legs are comparable to those of well-known international research groups. The generally high demands of the geometric tolerances and surface qualities of the legs still require intense development in terms of production technologies for the implementation of a large-scale processing route. The thermoelectric performance of skutterudites could also be confirmed on the basis of the legs. The study of long-term stability has shown high stability of the thermoelectric power factor. The corrosion and oxidation protection for high temperature application however – under technical and economic considerations – remains a high barrier for bringing the product into the market. The chances of success for the intended individual leg protection shall be assessed as low.
Thermo mechanical reliability aspects: For a detailed analysis of the mechanical stress situation, the extensive mechanical characterization was complemented by accompanying simulations. These studies on the skutterudite material behavior permitted an assessment of the very challenging, thermo mechanical behavior in the environment of use for the first time. The present results indicate that currently a TEG can only be implemented in the car with high effort (especially at operating temperatures above 450 °C and short-term peak temperatures of 600 °C at the leg). The required limitation of the continuous operating temperature to about 450 °C and the associated reduction in efficiency leads in addition with necessary technical measures to high cost per watt of electrical power. It still will take a certain time to solve the technical challenges in order to be economically successful. Other research groups are facing these technological challenges too, as can be concluded from an increasing number of publications on the topic "optimization". For stationary "off grid" solutions, the application is still interesting – e.g. recent progress in the field of solid oxide fuel cells indicates promising advancements.