Thermionic Energy Conversion Devices Research Articles

Prediction of Thermionic Energy Conversion Performance and Parametric Effects Using Genetic Algorithms to Fit Physics-Inspired Model Equations to Prototype Test Data

Abstract Thermionic converters have potential as an energy conversion technology for high-temperature space and terrestrial applications using concentrated solar, nuclear reaction, and combustion processes as the heat source. Recent studies have generated experimental performance data for narrow-gap thermionic energy conversion devices. This investigation explores the use of genetic algorithms to fit existing data with physics-inspired model equations. The resulting model equations can be used for performance prediction for system design optimization or to explore parametric effects on performance. The model equations incorporate Richardson’s law for current density, including both the saturated and Boltzmann regimes, with appropriate relations for power delivered to the external load. The transition regime is characterized using two separate models, each accounting for nonuniformity in emission surfaces and irregularities in the manufacturing process. The trained models enable performance prediction of small-gap thermionic energy conversion devices. In this study, data were fitted for two different prototype designs. The prototype test data and postulated values for the work functions and a transition regime parameter are substituted into physics-inspired model equations, yielding performance models with three adjustable constants. Optimized values of these constants are determined using a genetic algorithm to best fit the experimentally determined performance data for prototype thermionic conversion devices tested in earlier studies. This approach is demonstrated to fit the performance data to within 9%. This methodology also allows the user to back-infer the effective work function values, which were found in this study to be consistent with independent measurement.

Influence of Thermionic Emission on He Ionization and Plasma Enhancement in Thermionic Energy Conversion

In this paper, the influence of thermionic emission on He ionization and plasma enhancement in thermionic energy conversion (TEC) are studied by experiment and numerical simulation. A 1D unsteady plasma TEC model, which includes a He ionization model, plasma conservation equations, and a thermionic emission formula for the wall, is developed. A He plasma thermionic energy conversion device composed of a barium–tungsten cathode and a tungsten anode is established. The volt–ampere curves of the He plasma TEC device are measured at 1050 K, 1150 K, 1250 K, 1300 K, and 1350 K temperatures. Both important cathode parameters, work function and emission area, are estimated. Based on the modelling simulation and the experiment, the He ionization mechanism in plasma TEC is discovered. The effects of cathode temperature on several distributions of plasma reaction rates, particle number density, and potential in He plasma TEC are described. Some important parameters, including electron mobility, resistivity, and plasma equilibrium are analyzed. The relationship of thermionic emission on plasma enhancement to the output power of plasma TEC is presented. The output powers of plasma TEC and vacuum TEC are compared at various cathode temperatures. A dimensionless analyzing method concerning thermionic emission intensity and plasma enhancement power is proposed. A brief dimensionless relationship is deduced regarding thermionic emission intensity and the plasma enhancement contribution of TEC. The principles and methods for quantitative calculations concerning the output power of plasma TEC under the action of thermionic emission are established. It is possible to do quantitative research on the effects of thermionic emission on plasma-enhanced TEC.

Performance characteristics of a novel high-efficientgraphene thermionic power device

According to the theories of the solid physics and irreversible thermodynamics, the performance characteristics of a novel high-efficient graphene thermionic power device (TPD) are studied. The temperature of the cathode plate and anode plate are determined by solving the energy balance equation of hot and cold sides of the TPD. The effects of the output voltage and the work function of the cathode on the volt-ampere characteristics of the TPD and the temperature of the two electrodes are analyzed to determine the parametric characteristics of the TPD at the maximum power density and efficiency. The power density and efficiency are compromised, and the parametric optimal designs are given. The influence of the temperature of heat source at high temperature on optimization performance is analyzed. The results obtained here can provide theoretical guidance for developing the thermionic energy conversion devices.

三类微型能量转换系统有限时间热力学性能优化的研究进展

With the development of molecular biology technology, nanotechnology and micro electronic technology, more and more attentions have been paid to the microscopic energy conversion systems. The research on energy conversion mechanism and efficiency for micro-energy conversion system is a new subject concerning the interdisciplinary integration of thermodynamics, statistical mechanics, physics theory, etc. Performance optimization is one of the key science problems in revealing the mechanism of energy conversion and improving efficiency of energy utilization for micro-energy conversion system. On the basis of introducing the origin and development of the finite time thermodynamics theory, this paper reviews the recent advances of thermodynamic optimization for three kinds of typical microscopic energy conversion systems, i.e., the thermal Brownian motors, energy selective electron engines and the thermionic energy conversion devices, and proposes the future research prospects.

Bulk-Like Laminated Nitride Metal/Semiconductor Superlattices for Thermoelectric Devices

Bulk-like thermionic energy conversion devices have been fabricated from nanostructured nitride metal/semiconductor superlattices using a novel lamination process. 5- $\mu{\rm m}$ thick $({\rm Hf}_{0.5}{\rm Zr}_{0.5}){\rm N}$ (6-nm)/ScN (6-nm) metal/semiconductor superlattices with a 12 nm period were deposited on 100-silicon substrates by reactive magnetron sputtering followed by a selective tetra methyl ammonium hydroxide substrate etching and a gold-gold lamination process to yield 300 $\mu{\rm m}\times\,$ 300 $\mu{\rm m}\times\,$ 290 $\mu{\rm m}$ microscale thermionic energy conversion elements with 16,640 superlattice periods. The thermionic element had a Seebeck coefficient of ${-}{\rm 120}~\mu{\rm V}/{\rm K}$ at 800 K, an electrical conductivity of ${\sim}{2500}~\Omega^{-1}{\rm m}^{-1}$ at 800 K, and a thermal conductivity of 2.9 and 4.3 W/m-K at 300 and 625 K, respectively. The temperature dependence of the Seebeck coefficient from 300 to 800 K suggests a parallel parasitic conduction path that is dominant at low temperature, and the temperature independent electrical conductivity indicates that the $({\rm Hf}_{0.5}{\rm Zr}_{0.5}){\rm N}/{\rm gold}$ interface contact resistivity currently dominates the device. The thermal conductivity of the laminate was significantly lower than the thermal conductivity of the individual metal or semiconductor layers, indicating the beneficial effect of the metal/semiconductor interfaces toward lowering the thermal conductivity. The described lamination process effectively bridges the gap between the nanoscale requirements needed to enhance the thermoelectric figure of merit ZT and the microscale requirements of real-world devices. $\hfill[2013\hbox{--}0158]$

Increasing the efficiency of a thermionic engine using a negative electron affinity collector

Most attention to improving vacuum thermionic energy conversion device (TEC) technology has been on improving electron emission with little attention to collector optimization. A model was developed to characterize the output characteristics of a TEC where the collector features negative electron affinity. According to the model, there are certain conditions for which the space charge limitation can be reduced or eliminated. The model is applied to devices comprised materials reported in the literature, and predictions of output power and efficiency are made, targeting the sub-1000 K hot-side regime. By slightly lowering the collector barrier height, an output power of around 1 kW, at ≥20% efficiency for a reasonably sized device (∼0.1 m2 emission area) can be achieved.

Spatial correlation of photo-induced and thermionic electron emission from low work function diamond films

Hydrogen terminated, nitrogen doped diamond thin films have been the focus of recent research for application in thermionic energy conversion devices and possibly in solar cells. Nitrogen doped diamond films can attain negative electron affinity (NEA) through treatment with hydrogen plasma, which also produces a very low work function surface. Photoemission and thermionic emission spectroscopy measurements confirm a work function of approximately 2eV for such films. The research presented here includes results from imaging these thin films with photo-electron emission microscopy (PEEM) and thermionic electron emission microscopy (ThEEM), in addition to spectroscopic studies using ultraviolet photoelectron spectroscopy (UPS). From the images it can be concluded that the photo- and thermionic emission are spatially uniform and do not originate from different isolated emission sites. This observation holds true up to the highest resolution and for all temperatures investigated (300–800K). While relatively uniform, the emission is found to be influenced by the surface morphology and film microstructure. The spatial intensity distributions of the PEEM and ThEEM images are very similar, as reflected by the structure present in both of these images. This observation indicates that both emission processes are enabled by the low work function of the film.

Thermoelectric properties of HfN/ScN metal/semiconductor superlattices: a first-principles study

Nitride-based metal/semiconductor superlattices are promising candidates for high-temperature thermoelectric applications. Motivated by recent experimental studies, we perform first-principles density functional theory based analysis of electronic structure, vibrational spectra and transport properties of HfN/ScN metal/semiconductor superlattices for their potential applications in thermoelectric and thermionic energy conversion devices. Our results suggest (a) an asymmetric linearly increasing density of states and (b) flattening of conduction bands along the cross-plane Γ–Z direction near the Fermi energy of these superlattices, as is desirable for a large power factor. The n-type Schottky barrier height of 0.13 eV at the metal/semiconductor interface is estimated by the microscopic averaging technique of the electrostatic potential. Vibrational spectra of these superlattices show softening of transverse acoustic phonon modes and localization of ScN phonons in the vibrational energy gap between the HfN (metal) and ScN (semiconductor) states. Our estimates of lattice thermal conductivity within the Boltzmann transport theory suggests up to two orders of magnitude reduction in the cross-plane lattice thermal conductivity of these superlattices compared to their individual bulk components.

Theory of space charge limited regime of thermionic energy converter with negative electron affinity emitter

A theory of the space charge limited output current regime of a vacuum thermionic energy conversion device (TEC) employing a negative electron affinity (NEA) diamond emitter electrode is derived. The theory is developed by assuming that the electrons behave as a collisionless gas and self-consistently solving Vlaslov’s equation and Poisson’s equation. Special cases of the theory are addressed. Calculations are performed to model a TEC with a nitrogen doped diamond emitter material under various conditions. It is shown that the NEA material outperforms a similar positive electron affinity material in terms of output power and efficiency because the NEA lowers the electrostatic boundary condition at the emitter and therefore mitigates the negative space charge effect.

Considerations for a high-performance thermionic energy conversion device based on a negative electron affinity emitter

A theory is developed to model the effect a negative electron affinity (NEA) emitter electrode has on the negative space charge effect of a vacuum thermionic energy conversion device (TEC). The theory is derived by treating the electrons in the interelectrode space as a collisionless gas and self-consistently solving the Boltzmann transport equation and Poisson equation. The theory determines the point on the voltage-current characteristic such that the maximum motive due to space charge is at the same level as the conduction band minimum. It is shown that emitter electrodes with an NEA significantly mitigate the negative space charge effect; therefore a TEC employing such an electrode will outperfrom a similar TEC with conventional electrodes in terms of output power. Additionally, it is shown that a TEC with an NEA emitter electrode can have a greater interelectrode spacing than a TEC with conventional electrodes operating under similar conditions where the outputs of both TEC's are comparable.

Using negative electron affinity diamond emitters to mitigate space charge in vacuum thermionic energy conversion devices

A negative electron affinity (NEA) diamond surface is employed as an emitter electrode in a vacuum thermionic energy conversion device in order to mitigate the negative space charge effect. The motive diagram of an NEA device operating at the virtual saturation point is compared to a similar device with a conventional emitter material operating in the space charge limited regime in order to understand how NEA mitigates space charge. Output current characteristics are calculated for various NEA values, and the results are compared to an ideal (no space charge) model. Increasing the value of the NEA causes the output current characteristic to approach that of the ideal model. Motive diagrams for various values of NEA are calculated and used to explain this phenomenon. It is shown that an NEA device can achieve a maximum output power density equal to the maximum output power density of a similar ideal device.

Enhanced solid-state thermionic emission in nonplanar heterostructures

Conservation of transverse momentum during thermionic emission from planar structures is a key factor limiting the number of hot electrons emitted and the efficiency of solid-state thermionic energy conversion devices. In this letter, electron emission from nonplanar potential barrier structures is analyzed using a Monte Carlo transport model. Compared to the planar structures, nonplanar tall barriers can achieve much larger emission currents. Although the average energy of the transmitted electrons drops a little, the thermoelectric figure of merit can be increased with a nonplanar barrier structure. The improvement of the thermoelectric properties is attributed to the combined effects of increased effective interface area and reduced probability of total internal reflection at heterostructure interfaces.

A study of the compatibility of thermionic converter materials with caesium

The reasons for the current interest in thermionic energy conversion devices in the field of nuclear energy are briefly outlined. In a caesium thermionic device the problem of corrosion of the constructional materials by caesium is of prime importance. A method has been devised for carrying out compatibility tests in caesium vapour and investigations at temperatures up to 600° C are described. The materials studied include metals and ceramics likely to be employed in a thermionic converter device, and it is shown that in many cases severe corrosion can occur at temperatures as low as 300° C. Particular attention is paid to the behaviour of metal-ceramic seal assemblies where the choice of materials, particularly the metallising and brazing components, is of great importance. Most conventional commercially available seals are not entirely satisfactory and are unlikely to be suitable for prolonged use at temperatures in the region of 600° C. It is concluded that it may prove necessary to develop new methods of sealing high purity alumina to metals before the full potential of the thermionic converter can be realised.