Six key elements of a thermoelectric waste heat recovery module for vehicle applications.

The six key elements indicated in the figure are an interdependent network which governs performance of a thermoelectric module or device. It is expected that successful proposals will address at least three of the key elements summarized below.

• Key Element 1: Materials. In addition to seeking improvements in a thermoelectric material's energy conversion efficiency or figure of merit (ZT), the cost and availability of the material itself must be considered. Materials that are rare, or are being used extensively in other alternative energy technologies that would limit their supply and availability for thermoelectric devices, show little promise for potential large-scale deployment for vehicle applications. The development of materials which are comparatively easy to manufacture, and with the potential for large scale production volumes (on the order of several thousand tons per year for automotive use), have greater promise to be integrated into thermoelectric packages.
• Key Element 2: Thermal management. The manner in which the temperature distribution within a thermoelectric device is established, and its evolution, is directly related to thermal management, specifically the process by which the hot and cold sides of the thermoelectric module are convectively and radiatively heated or cooled. System level thermal management will require bridging scales of nanometers to meters. Opportunities exist for incorporating novel thermal management techniques, including but not limited to jet impingement, effective interface materials and adhesives, mini- or microchannel cooling, and single and multiphase concepts. Efficient simulation tools and supporting experimental data for model validation are needed for an effective design.
• Key Element 3: Durability. Thermoelectric devices for automotive applications will be subjected to temperature variations and mechanical stresses (for example, vibrations) that will challenge their ability to remain operable over automotive life cycles (approximately 15 years). Robust designs are necessary to ensure long life under operational conditions.
• Key Element 4: Interfaces. Interfaces between various materials represent vital thermal and electrical links in any thermoelectric device. Furthermore, the temperature swings associated with exhaust waste heat harvesting can potentially lead to de-lamination of interfaces due to mismatches in material coefficients of thermal expansion. Research is needed to develop durable and inexpensive bonding techniques, specific to thermoelectric harvesting of vehicle waste heat.
• Key Element 5: Heat sink design. The electrical power produced by a thermoelectric device will hinge upon minimizing the thermal resistance between the device and the surroundings. Design of efficient heat sinks are critical to this process, as is reducing the thermal resistance between the thermoelectric device and heat sink. New approaches are needed to develop novel heat sink designs, specific to thermoelectric harvesting of waste heat in vehicle applications. Concepts based on multiphase fluids, finned structures, microchannels and heat pipes to name a few are envisioned, though designs which are perceived to be too difficult to manufacture or too expensive will not be competitive.
• Key Element 6: Metrology. Metrology to characterize materials and the thermal performance of thermoelectric devices is essential to establish the efficacy of any design. Use of testing and measurement concepts which are standardized (for example, traceable to NIST standards) is important to evaluate the efficiency of proposed new thermoelectric materials (measuring ZT) at relevant temperatures. At the device or system level, it is anticipated that successful proposals will include a plan for experimental calibration and measurement of relevant performance parameters and the ability to assess accuracy, repeatability and the effect of measurement intrusiveness.

This figure appears in NSF 10-549