Thermionic energy conversion plasmas

Rasor, N. S.

doi:10.1109/27.125041

Cited by 73 publications

(42 citation statements)

References 13 publications

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“…The first thermionic conversion device was proposed by Schlichter in 1915(Schlichter, 1915, but the first demonstrations of practical levels of power generation were not made until the 1950s when both the United States and the former Soviet Union (USSR), as well as Western European nations, began exploring TEC for space applications in earnest (Rasor, 1991). Early United States developments aimed for solar energy and radioisotopes as thermal sources, and the Jet Propulsion Laboratory Solar Energy Technology program developed thermionic convertors that operated at ~1,900 K and ~150 W with 7-11% efficiency for up to ~11,000 h. However, the state-of-the-art at the time proved uncompetitive with emerging photovoltaic and thermoelectric energy conversion strategies, and the program was discontinued in the 1970s.…”

Section: History Of Tecmentioning

confidence: 99%

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

Haase

George

et al. 2017

Front. Mech. Eng.

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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 perspective article provides an assessment of the potential of TEC systems for space and terrestrial applications in the twenty-first century, overviewing recent advances in the field and identifying key research challenges. Recent developments as well as persisting research needs in materials, device design, fundamental understanding, and testing and validation are discussed.Keywords: thermal energy conversion, thermionic energy conversion, thermionic emission, heat engine, electron emission iNTRODUCTiON The direct conversion of heat to electricity without any intermediate steps or moving parts remains one of the most promising, yet challenging, methods of power production. The promise of high conversion efficiency, device simplicity, and robust operation continues to push research and technology development at the cutting edge. Furthermore, because of the wide variety of possible heat sources, ranging from combustion of fossil or other fuels, nuclear reactors, solar heat, or even waste heat from the human body, the applications for thermal-to-electric energy convertors are vast, spanning many orders of magnitude of possible temperature and power ranges.Thermionic energy conversion (TEC) for direct conversion of heat to electrical energy occurs when electrons thermally emit from a hot surface, traverse a gap, and are collected by another surface. This process, starting with thermionic emission, produces a current of electrons that can subsequently drive an electrical load to produce work. Particularly well suited for high-temperature applications, since it was first proposed by Schlichter (1915), thermionic emission has been pursued as a power generation method for over a century Gyftopoulos, 1973, 1979), yet has seldom been realized in either space or terrestrial applications. However, recent advances in material science and nanotechnology as well as our evolving understanding of the underlying physical processes afford new opportunities to develop practical thermionic convertors, reinvigorating the field. Thermionic energy conversion has a long and storied history, particularly in the space programs of the United States and the former Soviet Union. However, while the field was vibrant and much progress was made during the 1960s through 1980s, including space flight demonstrations, TEC has largely been supplanted by alternative energy conversion technologies, specifically thermoelectrics and photovoltaics, in both the public and research community consciences. Much of thermionic-based research tapered off after a 2001 report by the National Research Council titled Thermionics Quo Vadis? An Assessment of the DTRA's Advanced Thermionics Research and Development Program (Na...

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Section: History Of Tecmentioning

confidence: 99%

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

Haase

George

et al. 2017

Front. Mech. Eng.

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“…With a work function greater than 4 eV, bare tungsten cathodes would require exceptionally high temperatures, greater than 2,000°C, to emit sufficient current for a TEC device. Past TEC implementations reduced this high-temperature requirement by "cesiating" the surface of the tungsten cathodes (Wilson, 1959;Rasor, 1963Rasor, , 1991. Because cesium is electropositive with respect to tungsten, it creates a polarized surface layer on the cathode, resulting in a surface with a lower work function than bare tungsten (Jenkins, 1969).…”

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confidence: 99%

“…Past implementations of such devices utilized tungsten cathodes with cesium gas fed into the cathode-anode gap (Wilson, 1959;Hatsopoulos, 1963;Hernqvist, 1963;Rasor, 1963Rasor, , 1991Witting and Gyftopoulos, 1965). With a work function greater than 4 eV, bare tungsten cathodes would require exceptionally high temperatures, greater than 2,000°C, to emit sufficient current for a TEC device.…”

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confidence: 99%

Thermionic Emission from Diamond Films in Molecular Hydrogen Environments

Paxton

Ravipati

Brooks³

et al. 2017

Front. Mech. Eng.

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Diamond-based low-work function thermionic electron emitters are in high demand for applications ranging from electron guns and space thrusters to electrical energy converters. A key requirement of such diamond-based electron sources is hydrogen termination of the surfaces which can significantly reduce the emission barrier. However, at high temperatures (≤600°C), terminated hydrogen begins to desorb causing degradation in thermionic emission performance. The purpose of this study is to examine low-pressure hydrogen operating environments as a means to overcome this high-temperature performance limitation by enabling increased thermionic emission currents with improved stability at temperatures ≤600°C. A series of isothermal and isobaric experiments were performed in both nitrogen and hydrogen gas environments to determine the performance enhancement. Diamond electron emitters in both the as-grown and hydrogenated states were characterized at temperatures of 600, 625, and 650°C. An increase in thermionic emission current over vacuum operation was observed following the introduction of hydrogen. Upon evacuation of hydrogen to vacuum, the emission current decreased back to baseline levels. Further experiments in gas environments at a constant pressure (~5.5 × 10 −6 Torr) were conducted at temperatures ranging from 700 to 900°C. It was observed that the hydrogen environment promoted increased emission current while also enabling the diamond electron emitters to stably emit at increased temperatures compared with vacuum operation. Analogous experiments using nitrogen environments did not show any measurable performance enhancements, thus verifying that hydrogen is responsible for the observed effect. These results suggest diamond-based electron emitters can have improved thermionic emission performance at temperatures ≤600°C when operating in hydrogen gas environments.

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“…(Paramonov and El-Genk, 1994) to TOPAZ-I (Voss, 1994 Figure 3-1 shows a simplified flat-plate thermionic converter (Rasor, 1991).…”

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confidence: 99%

“…Electrons are "boiled" off the hot emitter and travel across the gap and are "condensed" on (Rasor, 1991) The most common gas that is used in this type of diode is cesium which has the lowest ionization potential of any chemical element (Angelo and Buden, 1985).…”

mentioning

confidence: 99%