Thermoelectric Advances to Capture Waste Heat in Automobiles he field of thermoelectrics has seen a renaissance of significant progresses in improving materials efficiency in the past 2 decades, largely driven by its promises of capturing waste heat and boosting energy efficiency. The solidstate and compact nature makes thermoelectric generators (TEGs) ideal for waste heat recovery in automobiles and other areas where space is limited. However, designing and implementing TEGs in automobiles is quite challenging and complex due to the requirement of efficient, low-cost, robust, and nontoxic thermoelectric material and devices, as well as the need to deal with the unavoidable consequences of adding extra exhaust pressure drop and weight to the automotive system. The figure of merit ZT of thermoelectric materials has undergone considerable increases from near 1 to above 2.4 in the past 2 decades.1−5 Among all of the reported thermoelectric materials, half-Heuslers and skutterudite materials are two most promising candidates for automotive applications due to their suitable working temperature, adequate thermal stability and mechanical strength, and reasonable cost.6−9 As shown in Figure 1, implementing thermoelectric materials in the car and extracting power from the exhaust waste heat requires three major leaps. First and foremost, the thermoelectric materials need to be made into an efficient, robust, and high-power-density TE module to enable reliable operation in the automotive exhaust environment for decades. The electrical contact and thermal contact resistances have been an outstanding issue that needs to be addressed in order to minimize parasitic losses and extract the maximum potential from the intrinsic properties of TE materials. Second, an efficient, compact, and antifouling heat exchanger is required to transfer the maximum amount of waste heat into the TE modules with minimum pressure drop. Third, sophisticated system-level design and engineering is required to realize thermal and electrical integrations of the TEG into the automobiles in order to deliver a meaningful net increase of the fuel efficiency with a competitive cost.10 Through investment from both the government and private industry, several research teams have successfully fabricated and implemented TEGs in automobiles. Figure 2 shows a highpower-density and high-temperature TEG designed and fabricated by Zhang et al. using high-efficiency nanostructured bulk half-Heusler materials.11 The TEG modules generate a power density above 4 W/cm2 with a temperature difference of 500 °C, and the TEG system delivere over 1000 W of electricity using the exhaust waste heat from a diesel engine, making it by far one of the largest and highest-performing TEGs fabricated using high-efficiency nanostructured materials.11 There are also a few other reports on bismuth-telluridebased TEGs for automotive waste heat recovery that can operate with a hot-side temperature below 300 °C and produce relatively low power density.12,13 Despite the above successful demonstrations of TEGs in automobiles, challenges still remain to make TEGs a cost-competitive
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and commercially viable technology for broad application. The conventional TEG manufacturing process is relatively expensive, inflexible, and inadaptable for different applications. Advanced manufacturing processes such as screen printing have recently been employed to fabricate low-cost and highly flexible thermoelectric devices for waste heat recovery applications.14,15 While the efficiency of thermoelectric devices is unlikely to compete with other dominant thermal energy conversion technologies in the foreseeable future, reducing the cost of thermoelectric devices and translating significant materials advances into marketable products should be the next goal to rejuvenate thermoelectric industry.
Yanliang Zhang*
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Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yanliang Zhang: 0000-0001-7423-8001 Notes
Views expressed in this Energy Focus are those of the author and not necessarily the views of the ACS. The author declares no competing financial interest.
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REFERENCES
(1) Tan, G.; Zhao, L. D.; Kanatzidis, M. G. Rationally Designing High-Performance Bulk Thermoelectric Materials. Chem. Rev. 2016, 116, 12123−12149. (2) Biswas, K.; He, J.; Blum, I. D.; Wu, C. I.; Hogan, T. P.; Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G. High-Performance Bulk Thermoelectrics with All-Scale Hierarchical Architectures. Nature 2012, 489, 414−418. (3) Zhao, L.-D.; Lo, S.-H.; Zhang, Y.; Sun, H.; Tan, G.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Ultralow Thermal Conductivity and High Thermoelectric Figure of Merit in SnSe Crystals. Nature 2014, 508, 373−377. (4) Minnich, A. J.; Dresselhaus, M. S.; Ren, Z. F.; Chen, G. Bulk Nanostructured Thermoelectric Materials: Current Research and Future Prospects. Energy Environ. Sci. 2009, 2, 466. (5) Mehta, R. J.; Zhang, Y.; Karthik, C.; Singh, B.; Siegel, R. W.; Borca-Tasciuc, T.; Ramanath, G. A New Class of Doped Nanobulk High-Figure-of-Merit Thermoelectrics by Scalable Bottom-up Assembly. Nat. Mater. 2012, 11, 233−240. (6) Joshi, G.; Yan, X.; Wang, H.; Liu, W.; Chen, G.; Ren, Z. Enhancement in Thermoelectric Figure-of-Merit of an N-Type
Received: May 8, 2018 Accepted: June 1, 2018
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DOI: 10.1021/acsenergylett.8b00749 ACS Energy Lett. 2018, 3, 1523−1524
Energy Focus
Cite This: ACS Energy Lett. 2018, 3, 1523−1524
Energy Focus
ACS Energy Letters
Figure 1. Schematic of a TEG system for turning the automotive engine exhaust waste heat into electricity.
Figure 2. High-power-density and high-temperature nanostructured bulk TEG for automotive waste heat recovery. Reprinted from ref 11. Half-Heusler Compound by the Nanocomposite Approach. Adv. Energy Mater. 2011, 1, 643−647. (7) Joshi, G.; He, R.; Engber, M.; Samsonidze, G.; Pantha, T.; Dahal, E.; Dahal, K.; Yang, J.; Lan, Y.; Kozinsky, B.; et al. NbFeSb-Based pType Half-Heuslers for Power Generation Applications. Energy Environ. Sci. 2014, 7, 4070−4076. (8) Tang, Y.; Gibbs, Z. M.; Agapito, L. A.; Li, G.; Kim, H. S.; Nardelli, M. B.; Curtarolo, S.; Snyder, G. J. Convergence of Multi-Valley Bands as the Electronic Origin of High Thermoelectric Performance in CoSb3 skutterudites. Nat. Mater. 2015, 14, 1223−1228. (9) Zong, P.; Hanus, R.; Dylla, M.; Tang, Y.; Liao, J.; Zhang, Q.; Snyder, G. J.; Chen, L. Skutterudite with Graphene-Modified GrainBoundary Complexion Enhances ZT Enabling High-Efficiency Thermoelectric Device. Energy Environ. Sci. 2017, 10, 183−191. (10) Kempf, N.; Zhang, Y. Design and Optimization of Automotive Thermoelectric Generators for Maximum Fuel Efficiency Improvement. Energy Convers. Manage. 2016, 121, 224−231. (11) Zhang, Y.; Cleary, M.; Wang, X.; Kempf, N.; Schoensee, L.; Yang, J.; Joshi, G.; Meda, L. High-Temperature and High-PowerDensity Nanostructured Thermoelectric Generator for Automotive Waste Heat Recovery. Energy Convers. Manage. 2015, 105, 946−950. (12) Hsu, C. T.; Huang, G. Y.; Chu, H. S.; Yu, B.; Yao, D. J. Experiments and Simulations on Low-Temperature Waste Heat Harvesting System by Thermoelectric Power Generators. Appl. Appl. Energy 2011, 88, 1291−1297.
(13) Liu, X.; Deng, Y. D.; Li, Z.; Su, C. Q. Performance Analysis of a Waste Heat Recovery Thermoelectric Generation System for Automotive Application. Energy Convers. Manage. 2015, 90, 121−127. (14) Varghese, T.; Hollar, C.; Richardson, J.; Kempf, N.; Han, C.; Gamarachchi, P.; Estrada, D.; Mehta, R. J.; Zhang, Y. HighPerformance and Flexible Thermoelectric Films by Screen Printing Solution-Processed Nanoplate Crystals. Sci. Rep. 2016, 6, 6−11. (15) Han, C.; Tan, G.; Varghese, T.; Kanatzidis, M. G.; Zhang, Y. High-Performance PbTe Thermoelectric Films by Scalable and LowCost Printing. ACS Energy Lett. 2018, 3, 818−822.
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