Because the National Science Foundation (NSF) and Department of Energy (DOE) have long invested in research and development of thermoelectric materials and devices, NSF and DOE developed the jointly funded partnership. The awards associated with this solicitation will potentially enable the broad application of thermoelectric waste heat recovery devices at a scale commensurate with the global vehicle manufacturing enterprise.

Automotive thermoelectric waste heat recovery. Providing a means to economically convert the otherwise wasted heat that is contained in a vehicle’s exhaust into electrical power is a key opportunity to (i) decrease fuel consumption and (ii) reduce emissions. A promising approach to take advantage of this energy harvesting opportunity is through incorporation of thermoelectric devices.

Such devices might be installed within the exhaust system of a vehicle to convert the energy within the hot combustion products into high-grade electrical power. In this way thermoelectric devices could reduce the mechanical generation of electricity in any vehicle, thus allowing for smaller alternators and reduced engine load that would thereby increase fuel economy.

Thermoelectric devices are based upon materials that exhibit the so-called Seebeck effect, which is the development of electrical voltage potentials within the material that are proportional to spatial temperature differences, also within the material. Extensive fundamental research is currently underway to develop materials that are effective in this process. For the most part, current research is focused on manipulating the nanostructure of solid state thermoelectric materials in order to:

1. increase the material’s Seebeck coefficient, while simultaneously
2. reducing the thermal conductivity and
3. increasing the electrical conductivity of the material.

In doing so, the efficiency by which the thermoelectric material can convert heat into electrical power is increased.

A typical thermoelectric device consists of numerous materials, including not only the thermoelectric material but also electrical connections necessary to extract the electricity from the thermoelectric material. The multiple interfaces existing between materials and across system levels can lead to unwanted thermal and electrical contact resistances internal and external to the thermoelectric device which degrade performance. Mismatches in the thermal expansion coefficients of the various materials can also significantly reduce the durability and lifetime of thermoelectric devices, threatening the potential for broad application.

A critical issue is the fact that the amount of electrical power ultimately produced by the thermoelectric device is directly related to the temperature distribution within the thermoelectric material. This distribution cannot be determined or controlled without, for example, understanding and predicting the very complex convective and radiative heat transfer processes external to the thermoelectric device. Hence, the ultimate efficiency, durability, manufacturability, and cost of thermoelectric devices hinge upon a highly-linked set of interdisciplinary challenges.

Six key elements—as indicated in the figure above—form an interdependent network which governs performance of a thermoelectric module or device. The agencies expect that successful proposals will address at least three of them.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

The program. Proposals will be judged based upon the potential success of the engineering approach in achieving the goals of cost-effectiveness and large scale deployment of thermoelectric devices for exhaust waste heat recovery. Proposals that target (i) incremental improvements in energy conversion efficiency, (ii) a single discipline or traditional line, or (iii) concepts that cannot be potentially implemented at a large scale (e.g., that require chemical elements that do not exist in sufficient quantity on Earth with little possibility of integration on a large scale) will not be competitive. The synthesis of diverse disciplinary knowledge, concepts, methodologies, and technologies must be clearly described.

The Solid State Energy Conversion Activity (SSECA) within the Vehicle Technologies Program (VTP) of DOE and the Thermal Transport Processes Program (TTP) within the Chemical, Bioengineering, Environmental and Transport Systems (CBET) Division of the Directorate for Engineering at NSF will manage the partnership. Each of these programs includes strong components of thermoelectrics within the portfolio of projects they support.

The SSECA has led the effort to realize the potential of solid state energy conversion to recover a significant fraction of the waste heat from automotive exhaust systems through the industry collaborations it supports. The TTP is a leader in supporting fundamental research and development activities in thermoelectric materials, principally at universities.

The agencies anticipate that six or more continuing grants will be made in FY 2010. Each project team may receive support of up to a total of $500,000 per year for up to three years on a continuing basis, pending availability of funds and research progress made. The agencies do not expect that all awards will receive the maximum amount; the size of awards will depend on the type of research program that is proposed.

Resources

• NSF/DOE Partnership on Thermoelectric Devices for Vehicle Applications Grant