Gate currents need to be suppressed, which can be done by applying magnetic fields or using electron-transparent gate materials Wanke et al.
This version of thermionic power conversion is called thermoelectronic power generation, as it is independent of the emission mechanism and furthermore uses only electrons, no ions, as active particles Meir et al. Whereas gate losses and, possibly, the generation of magnetic fields by permanent magnets or high- T C superconducting coils deserve consideration, the gate electrode offers tunability of the devices, for example, for an electronic adjustment of the output power or emitter temperature.
Thermoelectronic convertors are promising candidates as they do not require a Cs plasma, yet work with large emitter—collector spacings such that they feature durability, relative robustness, and operation also at very high emitter temperatures.
Figure 5. Using gates to solve the space-charge problem. In thermoelectronic generators as shown in panel A , a gate voltage allows efficient operation at large emitter—collector spacings, d ec. B Measured data data points together with the results of numerical simulations lines for various hole diameters w of the gate.
Adapted from Meir et al. These recent advances in device design that take advantage of new microfabrication capabilities have opened up the possibility of overcoming the space-charge challenge that has historically plagued TEC devices. Yet, additional work is required to bring these and other new ideas to fruition in practical operation.
We summarize a few opportunities in the future that are needed to make this happen. The availability of microfabrication and semiconductor-fabrication technologies as well as the advancements of materials science afford great opportunities for the fabrication of integrated thermionic convertors with microscale electrode gaps.
First, test devices with microscale gaps have been built and explored via semiconductor technologies Lee et al. The next big steps will be the demonstration of efficiency and performance under real operating conditions.
The use of vacuum-encapsulated wafer technologies affords new opportunities for modular, lightweight designs, which in principle might also be fabricated in a cost-effective manner. As the devices are built with little mass, they can be robust and cost-effective, as is characteristic for MEMS-type devices. Further, emitter and collector assemblies can be fabricated by standard thin-film processes, and the possibilities for the design of devices with microscale gaps are tremendous.
These need to be pursued as possible routes to devices with small thermal losses and the desired long-term stability. Model calculations show high efficiencies at these large emitter—collector spacings, which still must be demonstrated experimentally. One key issue is the design of the gates such that they allow high electron transparency and thereby feature small gate currents to minimize gate losses. Another key issue is establishing techniques to fabricate integrated devices with the desired emitter—collector spacings and small thermal losses.
The use of optimized materials for the gates, microscale structuring of the gates, and possibly of magnetic fields offer opportunities to realize such devices in flip-chip-based semiconductor technology. In such devices, nanostructuring of the emitter or collector electrodes may be used to match the gate microstructure for optimal device performance, for example, by channeling electron emission through the gate holes.
This radically new concept demands exploration of its advantages, in particular its possible synergy with microplasmas or low-temperature plasmas Go and Venkattraman, Fundamental questions concerning energy transfer between the electrodes, losses, and the role of space charges are yet to be investigated.
Like with microfabrication technology, great advances have been made in recent years in the science of microplasmas Becker et al. New and promising ideas to reduce these losses by optimized plasma generation have been presented Moyzhes and Geballe, , but to the best of our knowledge have not yet been explored experimentally. Owing to the small emitter—collector distances of thermionic convertors, the handling and optimization of plasmas confined to the emitter—collector gap is challenging.
The principles of electron motion in space, of heat transport, and of plasma discharges have long been known. In recent years, however, three-dimensional simulation software has started to offer tremendous advantages in the design of thermionic convertors, as the behavior of space charges in inhomogeneous electromagnetic fields is difficult to assess otherwise.
Although existing code has already proven helpful, further advances are nevertheless required. It would be beneficial if codes were available that are able to simulate the performance of convertors on the system level. This has to be based on the calculation of the electron motion in the space charge arising from large numbers of electrons emitted through a process characterized by their thermal energy distribution and moving in inhomogeneous electromagnetic fields.
To assess device performance on a system level, it is mandatory that such codes integrate all physical processes, the electron transport together with the generation, distribution, and flow of the thermal energy. Such algorithms need to be iterative, and the demand on their numerical precision is high, as the velocity spread of the electrons can be considerable, as can be the spread of length scales, in particular if emission and absorption processes on nanostructured surfaces are to be taken into account.
For thermionic convertors that utilize a plasma or a gas, precise calculation of the device performance is even more demanding. Note that it is not unknown aspects of physics that challenge the simulation, nor is it that algorithms are in principle unknown. Rather, it is the large number of particles and the multitude of length and time scales involved that prohibit a good simulation of a high-performance power convertor with a macroscopic side length. The collector in a TEC device is often the most overlooked component, yet it still directly impacts the overall performance of the thermionic convertor.
For typical TEC configurations, the maximum power is obtained when operating at a voltage equal to the difference of the emitter and collector work function as shown in Eq. Consequently, there is a strong advantage to having a low work function collector. For typical operation, electrons are collected in states above the collector work function, and thermalization to the collector thermal distribution is a primary mechanism for emitter-to-collector heat transfer.
Tunneling into states below the vacuum level may prove to be an advantage in reducing this loss mechanism. Absorption of thermal radiation from the emitter is the second major source of heat transfer, and a high-infrared reflectivity of the collector may enhance the system efficiency.
In some operating conditions, back emission of electrons from the collector will contribute to the space-charge barrier. Evidence suggests that an NEA surface on the collector may reduce the space-charge barrier. A high Richardson constant is presumed to indicate a low value of the electron reflectivity at the collector surface; consequently, a high value is desired. However, if the Richardson coefficient is too high, then back emission from the collector can reduce efficiency; so there needs to be an optimal intermediate value.
Table 2 summarizes some of these requirements necessary for a high-performance collector. Table 2. Collector requirements and advantages, and device-related requirements. Much like the emitter, many of the challenges with collectors can be narrowed down to materials challenges. However, the collector has generally received far less attention than the emitter. Still, the following overviews both historical and more recent approaches in collector design. The scandate and phosphorus-doped, n-type diamond surfaces exhibit the lowest work functions for non-cesiated surfaces reported to date Koeck et al.
In fact, they may be the lowest overall, and these surfaces could provide significant advantages for specific TEC configurations. Scandium oxide films integrated with a porous metal could provide both the low work function and low electrical resistance required for effective collector operation in a TEC system Gibson et al. Reports have indicated a thermionic work function of 0. The 0. However, the results were deduced from thermionic emission rather than collection. Recently, it has been proposed that a grid structure near the collector surface could enable electron tunneling into states below the vacuum level Figure 6 Pan et al.
This effect could lower the effective work function and increase the efficiency through reducing thermalization heat transfer. Simulations have also indicated that an NEA collector would result in a substantial reduction of the space-charge barrier Smith, Back emission is more complicated for semiconductor surfaces, and whether it becomes important still needs to be addressed. Figure 6. The red region depicts the potential energy landscape in the vacuum gap, shaped by the grid voltage.
Cathode-emitted electrons with energies above-but-close-to the collector Fermi level green line can tunnel through the thin barrier setup by the electric field outside the collector, circumventing the collector work function purple line. Reprinted, with permission, from Pan et al. Copyright IEEE. There have been few studies of materials properties for collector applications, and they are usually coupled to a specific emitter in a TEC configuration.
While many aspects of the collector surface can be deduced by analogy from emitter characteristics, it would appear that focusing on collector performance may lead to much more well-defined optimal characteristics. It will be essential to provide a fundamental understanding of how the Richardson constant impacts electron collection and, moreover, whether the Richardson constant can be optimized with different surface terminations. The development of a database of materials and surfaces with theoretical underpinning would be crucial for guiding the development of new systems.
At a system level, simulating thermal properties will be required. The research on emitter materials has uncovered several candidate collectors with some of the lowest reported work functions.
New research should focus on enhancing the electron absorption i. A theoretical study has suggested that an NEA collector surface could reduce the space-charge barrier Smith, There are opportunities to expand the study to determine the relation to specific materials configurations and to provide experimental verifications.
Similarly, there is an opportunity to understand the role of quantum tunneling at the collector surface. Particular focus for integrated collector design would be on grid or nanostructures to enhance tunneling effects and to integrate efficient cooling to control the system temperature. Quantum tunneling and NEA surfaces are two new concepts that merit further study. The previous sections have outlined specific areas of research need at the individual component level. However, beyond improving and optimizing individual components, there remains system-level challenges that the TEC research community must overcome.
Given that TEC technology has not yet reached a state of widespread adoption, a complete set of standards has not been fully established for testing all aspects of fabricated devices or characterizing the materials used in various device components, although worthwhile progress has been made in defining a figure of merit for convertor performance Shefsiek, In this section, we will first review the main requirements for characterization and testing in thermionics, then describe the current state-of-the-art, and finally discuss the future needs and some of the opportunities available for further research and development.
A thorough understanding of the properties of the materials used for the emitter, collector, and other parts of the device is crucial to be able to optimize device design and performance. A critical parameter for both the emitter and the collector is the work function and, generally speaking, materials with very low work function or even NEA are required.
The work function, however, is a rather difficult property to deal with. It not only depends on the atomic composition and structure of the bulk but is also heavily influenced by the surface morphology and any type of coating, both intentional and unintentional.
Moreover, work function depends on temperature, for instance due to thermal expansion Olawole and De, , and this could be particularly important for thermionic convertors, given the elevated temperatures in use. The thermal conductivity and electrical conductivity are other important considerations. The thermal conductivity plays a direct role in determining the operating temperature and also partially determines the undesirable heat transfer from the emitter to the rest of the structure.
One may argue that, unless high-frequency pulsed operation is needed, a low thermal conductivity is desired for the emitter to minimize unwanted heat transfer to the surroundings, especially in applications where it is possible to directly heat the emitter surface, such as in solar thermionics. For the collector, the situation might be the opposite since the heat load deposited by the incoming electrons needs to be removed.
For both the emitter and the collector, a high electrical conductivity is needed and, naturally, this usually comes at the price of a high thermal conductivity, which could be detrimental to the operation of the emitter.
Therefore, it is important to have accurate data for these conductivity values and their interdependencies as a function of temperature. Such data are not always available in the literature for the temperature ranges in question. Perhaps even more crucially, the physics of emission may be different in novel materials with reduced dimensionality and modifications to the Richardson—Dushman law may be needed Liang and Ang, However, almost all of these are essentially limited to ex situ materials characterization scenarios and do not necessarily allow the direct measurement or extraction of properties under actual device operation or even emulated operating conditions such as at very high temperatures.
Given the importance of in situ materials characterization as discussed before, this can represent a significant limitation.
There are two aspects to device testing and characterization: one involves the measurement of the external or performance parameters.
Depending on the particular application at hand, the important parameters could include maximum input power, operating temperature, output power, power per unit volume or mass, output current, current density, output voltage, output impedance, power conversion efficiency, robustness for example, to ionizing radiation , stability, and lifetime. The other aspect of device characterization is related to measuring and monitoring the internal device parameters such as the surface temperatures of the emitter and the collector, the electron and ion if applicable current and their spatial distribution in the gap region, the voltage drop across different components and contact points, and the mechanical stress build-up in various regions of the device.
Understanding these is crucial in the evaluation of a particular design or prototype and its subsequent improvements. It is, therefore, necessary to have the ability to characterize these properties in situ during device operation or at least under representative conditions.
As far as device operation itself is concerned, standards for performing and reporting measurements, which would allow for meaningful comparisons across a wide variety of devices, need more attention, such as the concept of barrier index as a figure of merit Shefsiek, Such concepts will allow the reemerging thermionics community to communicate advances more succinctly and effectively.
Similarly, comparing devices that operate using different types of input power—waste heat, nuclear, solar, etc. In practice, a thermionic convertor will inevitably be used in a larger system, at a minimum including a source of input power and a load, if not being part of a more elaborate power grid with dynamic load properties and time-varying requirements, and including sophisticated power electronic circuitry. Examples include solar simulators and the National Renewable Energy Laboratory test facility, or the figure-of-merit ZT parameter.
If progress in thermionic convertors is to be accelerated, which will inevitably require the participation of many researchers from various backgrounds and geographical locations, effective communication of results and comparison of obtained performance data are crucial. This necessitates the development of not only standard test equipment and central facilities but also well-defined performance reporting criteria.
For instance, which one is a more meaningful parameter to be compared: power density or total power? And, if it is the former, how is it exactly defined? Similarly, should device efficiency be reported as a function of emitter temperature, or that of the temperature difference between emitter and collector, or is it simply the maximum efficiency that matters? The list goes on. The answer to many such questions is far from trivial or even unique, and requires careful thinking, deliberation, and consensus building.
Existing models and theoretical studies are typically confined to a sub-section of a thermionic convertor, such as electron emission from the emitter or transport through the gap.
For example, the different regions of the current—voltage characteristics of a thermionic device—the retarding mode, the space-charge mode and the saturation mode—are sensitive to various device and material parameters in different ways. Therefore, using an accurate model for the current—voltage characteristics, one could extract multiple parameters such as the emission spot area, temperature, and work function by fitting the model predictions to current—voltage sweeps Khoshaman et al.
Similar approaches may be used for extracting other internal device parameters, too. It is not a coincidence that research in TEC has recently been attracting increasing levels of interest. With the advent of such technologies, two other developments have taken place, which could have a major impact on TEC: the ability to structure materials and engineer material properties previously unthinkable when traditional thermionic designs were being implemented half a century ago, and new physical phenomena that could open up opportunities for radically new device concepts.
The creation of nanostructured emitters that take advantage of the promising properties of nanotubes and nanowires, the development of novel grid structures to minimize space-charge effects Meir et al. With such developments, however, some of the challenges and opportunities discussed above become more pronounced, while new ones also arise.
For instance, in situ materials property measurements in functional devices becomes increasingly important as large temperature gradients could lead to significant variations in work function, electrical and thermal conductivities, etc. As such, not only is it now crucial to develop new experimental and in situ characterization techniques and standards but it is also imperative to create theories, models, and simulation tools that allow for the accurate analysis of the entire device in a comprehensive manner, such that solving the reverse problem, that is extracting the material properties and internal device parameters with high levels of detail and precision from measured external device characteristics, also becomes a practical reality.
Thermionic emission is over a century old—perhaps older than vacuum electronics itself. While thermionic devices have also largely given way to their solid-state counterparts, i. This lag highlights the fundamental challenge of energy conversion compared with information processing using electronics, where the flow of two quantities—charge and heat—as opposed to only one—charge—is of primary concern.
Vacuum, by its very nature, provides a great barrier to heat transfer—a fact that gives TEC a fundamental advantage over thermoelectrics. On the other hand, electrons are naturally found in matter, and taking them out into vacuum is a difficult and often harsh process. In addition, once they are in vacuum, making them behave the way we desire and go where needed is far from trivial, especially for low-kinetic-energy electrons with a wide energy distribution and high current density, as is often the case in thermionics.
As we have seen in the previous chapter, much progress has been made on all these fronts. Emitters and collectors with low work function and good stability have been demonstrated. It thus appears that many of the fundamental pieces of the puzzle are now in place for us to be able to finally produce practical devices with useful performance levels. What has been missing from the recent history of TEC research and development is a coherent effort and a strong and tightly knit community of researchers with enough resources to drive this much-needed technological development and innovation forward.
Time is now ripe for such a major technological push and a concerted effort to continue to advance the fundamental science that underpins TEC. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors would like to acknowledge all participants at the Workshop on Thermionic Energy Conversion for Space and Terrestrial Applications, and especially William F.
Schmitt Society through the Arthur J. Schmitt Presidential Fellowship. AN also acknowledges CMC Microsystems for the provision of products and services that facilitated this research.
Aamodt, R. Thermionic emission from molybdenum in vapors of cesium and cesium fluoride. Adams, S. Solar thermionic space power technology testing: a historical perspective. Energy Convers. Baksht, F. Thermionic convertors and low-temperature plasma.
N 80, Google Scholar. Barmina, E. Nano-textured W shows improvement of thermionic emission properties. A , 1—4. Becker, K. Microplasmas and applications. D Appl. Belbachir, R. Thermal investigation of a micro-gap thermionic power generator. Benke, S. Child, C. Discharge from hot CaO. Series I 32, Diederich, L. Electron affinity and work function of differently oriented and doped diamond surfaces determined by photoelectron spectroscopy. Fitzpatrick, G. Fomenko, V. Gibson, J.
Thermionic conversion uses high temperature thermal energy to drive electrons across a gap and thus creating an electrical current. A thermionic converter is a solid state device with no moving parts, with the main challenge being the high temperatures.
The thermionic emitter also serves as an absorber of concentrated solar radiation to provide heating. In this project we are developing the basic materials, coatings and processes necessary to create an efficient thermionic converter, constructing a demonstration device with both thermionic and thermo-electric stages, and testing the device in a solar furnace to validate its operation.
Energy Research Center. Search form Search. In contrast to conventional thermionic converters, the two electrodes are at the same temperature, and the cathode is side-illuminated rather than front-illuminated. This configuration leads to several advantages: close coupling of multiple units in a series connection high-voltage configuration; higher area for electron emission; and recovery of waste heat at a higher temperature compared to heat recovery from the anode of a conventional thermionic converter.
High performance photo-thermionic solar converters. N2 - A new class of solar energy converters is described, based on photon-enhanced thermionic emission of electrons into a vacuum region between two electrodes.
0コメント