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Thermionic Energy Conversion in the Twenty-First Century: Advances and Opportunities for Space and Terrestrial Applications

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Thermionic Energy Conversion in the Twenty-First Century: Advances and Opportunities for Space and Terrestrial Applications https://ntrs.nasa.gov/search.jsp?R=20170011125 2019-08-31T13:52:57+00:00Z REVIEW published: 08 November 2017 doi: 10.3389/fmech.2017.00013 Thermionic Energy Conversion in the Twenty-first Century: Advances and Opportunities for Space and Terrestrial Applications David B. Go1,2*, John R. Haase1,2, Jeffrey George 3, Jochen Mannhart 4, Robin Wanke 4, Alireza Nojeh 5 and Robert Nemanich6 1 Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN, United States, 2 Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, United States, 3 Johnson Space Center, National Aeronautics and Space Administration (NASA), Houston, TX, United States, 4 Max Planck Institute for Solid State Research, Stuttgart, Germany, 5 Department of Electrical and Computer Engineering, Quantum Matter Institute, University of British Columbia, Vancouver, BC, Canada, 6 Department of Physics, Arizona State University, Tempe, AZ, United States Thermionic energy conversion (TEC) is the direct conversion of heat into electricity by the mechanism of thermionic emission, the spontaneous ejection of hot electrons from a surface. Although the physical mechanism has been known for over a century, it has yet to be consistently realized in a manner practical for large-scale deployment. This Edited by: perspective article provides an assessment of the potential of TEC systems for space Guillermo Rein, Imperial College London, and terrestrial applications in the twenty-first century, overviewing recent advances in the United Kingdom field and identifying key research challenges. Recent developments as well as persisting Reviewed by: research needs in materials, device design, fundamental understanding, and testing and Dong Liu, University of Houston, validation are discussed. United States Keywords: thermal energy conversion, thermionic energy conversion, thermionic emission, heat engine, electron Liang Chen, emission Xi’an Jiaotong University, China *Correspondence: David B. Go [email protected] INTRODUCTION The direct conversion of heat to electricity without any intermediate steps or moving parts remains Specialty section: This article was submitted to one of the most promising, yet challenging, methods of power production. The promise of high Thermal and Mass Transport, conversion efficiency, device simplicity, and robust operation continues to push research and a section of the journal technology development at the cutting edge. Furthermore, because of the wide variety of possible Frontiers in Mechanical Engineering heat sources, ranging from combustion of fossil or other fuels, nuclear reactors, solar heat, or even Received: 19 July 2017 waste heat from the human body, the applications for thermal-to-electric energy convertors are Accepted: 17 October 2017 vast, spanning many orders of magnitude of possible temperature and power ranges. Published: 08 November 2017 Thermionic energy conversion (TEC) for direct conversion of heat to electrical energy occurs Citation: when electrons thermally emit from a hot surface, traverse a gap, and are collected by another Go DB, Haase JR, George J, surface. This process, starting with thermionic emission, produces a current of electrons that can Mannhart J, Wanke R, Nojeh A and subsequently drive an electrical load to produce work. Particularly well suited for high-temperature Nemanich R (2017) Thermionic applications, since it was first proposed bySchlichter (1915), thermionic emission has been pursued Energy Conversion in the Twenty- as a power generation method for over a century (Hatsopoulos and Gyftopoulos, 1973, 1979), yet first Century: Advances and Opportunities for Space and has seldom been realized in either space or terrestrial applications. However, recent advances in Terrestrial Applications. material science and nanotechnology as well as our evolving understanding of the underlying physi- Front. Mech. Eng. 3:13. cal processes afford new opportunities to develop practical thermionic convertors, reinvigorating doi: 10.3389/fmech.2017.00013 the field. Frontiers in Mechanical Engineering | www.frontiersin.org 1 November 2017 | Volume 3 | Article 13 Go et al. TEC Twenty-first Century Thermionic energy conversion has a long and storied history, Forward” discusses future research needs, focusing primarily on particularly in the space programs of the United States and the for- the need to establish a set of guidelines for testing and evaluating mer Soviet Union. However, while the field was vibrant and much TEC devices. We conclude with “Summary and Final Thoughts,” progress was made during the 1960s through 1980s, including which offers some perspectives and suggestions for a path space flight demonstrations, TEC has largely been supplanted by forward. alternative energy conversion technologies, specifically thermo- electrics and photovoltaics, in both the public and research com- BACKGROUND AND FUNDAMENTALS munity consciences. Much of thermionic-based research tapered off after a 2001 report by the National Research Council titled Fundamental Physics of TEC Thermionics Quo Vadis? An Assessment of the DTRA’s Advanced The basic concept of TEC is outlined in Figure 1A. As heat is Thermionics Research and Development Program (National added to the emitter (cathode) and the emitter temperature rises, Research Council, 2001) provided a fairly negative perspective on electrons have sufficient energy to escape the solid and move the viability of TEC. However, since that time the TEC research freely in the vacuum, a process likened to evaporation or boiling community has responded by discovering and inventing new and off of electrons. These electrons then move across an electrode novel approaches to devices, materials, and operating strategies gap to a collector (anode), and completing the circuit with a that highlight the potential of TEC to achieve the high conversion load produces electrical power. Figure 1B shows this process of efficiency and performance predicted by theory. thermionic emission from the perspective of the energy levels As both space and terrestrial energy needs continue to grow, it is of the electrons. To be emitted, electrons in the emitter must be not only compelling but necessary to develop new technologies for energized to be above the vacuum potential barrier of the emit- power generation. For space applications, new missions, includ- ter, where the difference between the vacuum potential and the ing to Mars, and new technologies, such as miniature satellites Fermi energy is called the work function (ϕ). If no collisional (e.g., CubeSats), will require power sources that deliver high or space-charge effects limit the electron transport through the power in reliable and sustained ways over long periods of time. interelectrode gap, the electrons traverse the gap and enter the From the terrestrial perspective, there is an ongoing need to find collector, where the ideal output voltage (Vout) is approximately alternative energy sources as well as improve the efficiency of the contact potential difference. existing fossil fuel-driven power sources, especially with grow- Thermionic emission is described by the Richardson– ing concerns about the environmental and climate impact of Dushman equation (Richardson, 1921), which relates thermi- traditional coal and oil-fired power plants, environmental and onic current density (j) to the temperature of the emitter (Te) as economic challenges facing fossil fuel extraction, and the ongoing follows: challenge of fossil fuel depletion. 2 φ 2 In October, 2014, a Workshop on Thermionic Energy jA=−AT0 e exp/()Acm , (1) ()kTBe Conversion for Space and Earth was held in Houston, TX (United States) to bring together researchers, thought leaders, and stake holders to discuss the current state of TEC and opportunities and needs for future research and development to address the challenges. Perhaps more importantly, the Workshop served to centralize and collect the TEC community, and bring together somewhat independent and isolated researchers to a common table to discuss and promote this important energy technol- ogy. This perspective article is a result of this workshop and ensuing discussions among the participants. The purpose of this article is not only to discuss recent advances but also to offer some perspectives on the field in terms of possibilities as well as challenges, from the collective wisdom of the workshop participants. We do note that it is not the intent of the article to provide a detailed review of recent research, and for that we point the reader to other recent review articles (Khoshaman et al., 2014; McCarthy et al., 2014; Khalid et al., 2016; Trucchi and Melosh, 2017). This perspective article is organized into several sections as follows. “Background and Fundamentals” gives background on thermionic emission and TEC, including establishing the funda- mentals of TEC and the terminology that will be used throughout this article as well as opportunities for TEC implementation. “Challenges and Recent Advances” outlines recent advances in FIGURE 1 | (A) Schematic of the thermionic energy conversion (TEC) TEC research, focusing on the primary components of a TEC process. (B) Electromotive diagram showing electron energy levels during TEC. device—the emitter, the electrode gap, and the collector. “Going Frontiers in Mechanical Engineering | www.frontiersin.org 2 November 2017 | Volume 3 | Article 13 Go et al. TEC Twenty-first Century 2 2 where A0 = 120 A/cm K is the Richardson constant and
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