Output Characteristics of Solar-Power-Driven Thermionic Energy Converter

Oginö, Kazuhíde; Muramatsu, T.; Kando, Masashi

doi:10.1143/jjap.43.309

Cited by 14 publications

(6 citation statements)

References 17 publications

(20 reference statements)

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“…Microfabricated vacuum TECs therefore show great promise for achieving efficient conversion of heat directly to electricity and have potential application in topping cycles for concentrated solar thermal power plants. 9,18,19 This work was supported in part by DARPA seed grant no. W91CRB-10-1-0001, DARPA and Robert Bosch North America through the Center on Interfacial Engineering for MEMS (CIEMS), and BASF through the Stanford Energy and Environment Affiliates Program.…”

Section: -3mentioning

confidence: 99%

Optimal emitter-collector gap for thermionic energy converters

Lee

Bargatin

Melosh

et al. 2012

Applied Physics Letters

134

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In this letter, we calculate numerically the emitter-collector gaps that maximize the power conversion efficiency of vacuum thermionic energy converters (TECs). The optimum arises because efficiency drops both at very large gaps, due to space-charge limitations on the TEC current, and at very small gaps, due to the increased parasitic heat loss via near-field radiative heat transfer. For typical TECs made with cesiated tungsten electrodes, the optimal gaps range from 900 nm to 3 lm and are approximately equal to the characteristic wavelength of the emitter thermal radiation, as given by Wien's displacement law. V

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Section: -3mentioning

confidence: 99%

Optimal emitter-collector gap for thermionic energy converters

Lee

Bargatin

Melosh

et al. 2012

Applied Physics Letters

134

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“…All-sapphire cells [24] and nickel heat-pipe ovens [25] could be fabricated to stand temperatures even above 1000 °C. These high temperature cells are of great importance for the thermionic solar converters using cesium vapor in order to diminish the negative effect of the space charge [26][27][28]. For the modeling of those devices it will be of considerable importance to take into account the self-emission of high temperature cesium vapor in which the amount of stable cesium dimers is almost negligible.…”

Section: Discussionmentioning

confidence: 99%

Cs₂‘diffuse bands’ emission from superheated cesium vapor

Pichler

Makdisi

Kokaj

et al. 2016

J. Phys. B: At. Mol. Opt. Phys.

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Thermal emission from superheated cesium vapor was studied to very high temperatures from 700 °C to 1000 °C. This was performed in the vapor condition only and with no liquid cesium present in the all-sapphire cell. We observed a number of atomic and molecular spectral features simultaneously in emission and absorption, especially peculiar thermal emission of cesium dimer diffuse bands (2 3Πg → a 3∑u+ transitions) around 710 nm coexisting with absorption bands around first resonance lines at 852 and 894 nm. We performed appropriate calculations of the diffuse band emission profiles and compared them with measured profiles. We also performed absorption measurements and compared observed diffuse band profiles with calculated ones. Possible applications of the observed phenomena will be discussed in terms of the solar energy conversion using dense cesium vapor.

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“…It was early discovered that electron emission from a partially cesium covered tungsten filament resulted in an increase in electron emission (Ito and Cappelli, 2012). This phenomenon of work function reduction due to adsorbed alkali-metal atoms is still exploited in thermionic conversion research and applications with more recent interest in solar or photo-enhanced thermionic conversion (Langmuir and Kingdon, 1925;El-Genk and Momozaki, 2002;Zhang et al, 2003;Ogino et al, 2004;Lee et al, 2009;Makdisi et al, 2014). An extensive experimental investigation of various metal work functions under cesium coverage is presented in Table 2 ( Reck et al, 2014).…”

Section: Novel Materials For Thermionic Converter Electrodes Work Funmentioning

confidence: 99%

Advances in Thermionic Energy Conversion through Single-Crystal n-Type Diamond

Koeck

Nemanich

2017

Front. Mech. Eng.

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Thermionic energy conversion, a process that allows direct transformation of thermal to electrical energy, presents a means of efficient electrical power generation as the hot and cold side of the corresponding heat engine are separated by a vacuum gap. Conversion efficiencies approaching those of the Carnot cycle are possible if material parameters of the active elements at the converter, i.e., electron emitter or cathode and collector or anode, are optimized for operation in the desired temperature range. These parameters can be defined through the law of Richardson-Dushman that quantifies the ability of a material to release an electron current at a certain temperature as a function of the emission barrier or work function and the emission or Richardson constant. Engineering materials to defined parameter values presents the key challenge in constructing practical thermionic converters. The elevated temperature regime of operation presents a constraint that eliminates most semiconductors and identifies diamond, a wide band-gap semiconductor, as a suitable thermionic material through its unique material properties. For its surface, a configuration can be established, the negative electron affinity, that shifts the vacuum level below the conduction band minimum eliminating the surface barrier for electron emission. In addition, its ability to accept impurities as donor states allows materials engineering to control the work function and the emission constant. Singlecrystal diamond electrodes with nitrogen levels at 1.7 eV and phosphorus levels at 0.6 eV were prepared by plasma-enhanced chemical vapor deposition where the work function was controlled from 2.88 to 0.67 eV, one of the lowest thermionic work functions reported. This work function range was achieved through control of the doping concentration where a relation to the amount of band bending emerged. Upward band bending that contributed to the work function was attributed to surface states where lower doped homoepitaxial films exhibited a surface state density of ∼3 × 10 11 cm −2 . With these optimized doped diamond electrodes, highly efficient thermionic converters are feasible with a Schottky barrier at the diamond collector contact mitigated through operation at elevated temperatures.

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Output Characteristics of Solar-Power-Driven Thermionic Energy Converter

Cited by 14 publications

References 17 publications

Optimal emitter-collector gap for thermionic energy converters

Optimal emitter-collector gap for thermionic energy converters

Cs₂‘diffuse bands’ emission from superheated cesium vapor

Advances in Thermionic Energy Conversion through Single-Crystal n-Type Diamond

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Output Characteristics of Solar-Power-Driven Thermionic Energy Converter

Cited by 14 publications

References 17 publications

Optimal emitter-collector gap for thermionic energy converters

Optimal emitter-collector gap for thermionic energy converters

Cs2‘diffuse bands’ emission from superheated cesium vapor

Advances in Thermionic Energy Conversion through Single-Crystal n-Type Diamond

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Product

Resources

About

Cs₂‘diffuse bands’ emission from superheated cesium vapor