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Author: Daniel C. Riley Publisher: ISBN: Category : Languages : en Pages :
Book Description
Thermionic energy conversion (TEC) is a direct heat-to-electricity conversion technology with the potential to leapfrog state-of-the-art solid-state conversion in efficiency and power density. In a thermionic energy converter, electrons evaporate from a hot electrode, the cathode, into a vacuum gap and are collected by a cooler electrode, the anode, to generate electric current. In the 1960s-1970s numerous groups reported thermionic converters with power densities above 10 W/cm^2 and conversion efficiencies of ~15%. However most of this work was tied to the US space-nuclear program which ended in 1973, and thermionics research has never fully recovered. As a result two central challenges yet remain in thermionics: (1) High operating temperatures necessary to produce electric current result in difficult materials challenges, and (2) low operating voltages due to losses associated with space charge and high anode work functions. However, new opportunities to tackle these challenges are available as a result of the breathtaking rise of semiconductor fabrication technology. In this work I present a new physical mechanism called photon enhanced thermionic emission (PETE). This concept is an improvement on thermionic emission by using light to boost the average energy of carriers in a hot p-type semiconductor cathode. Additionally, unlike in a photovoltaic cell, the waste heat from recombination losses and sub-bandgap light absorption is utilized to heat the cathode. Thus a PETE cathode can produce efficient electron emission at lower temperatures than a thermionic cathode. I will describe theoretical calculations showing that a PETE device may exceed 40% solar power conversion efficiency, and the conversion efficiency may exceed 50% if a PETE device is used in tandem with a solar thermal backing cycle. I will also describe an experimental demonstration of the PETE effect in an ultra-high vacuum photoemission measurement. In the cathode of an energy converter based on photon-enhanced thermionic emission (PETE) photoexcited carriers may need to encounter the emissive surface numerous times before having sufficient thermal energy to escape into vacuum and therefore should be confined close to the surface. However, in a traditional planar geometry, a thin cathode results in incomplete light absorption. Nanostructuring has the potential to increase light capture and boost emission by decoupling the lengths associated with photon absorption and electron emission. Nanostructures may complicate the properties of the emissive surface; therefore, the effect of nanostructuring on emission efficiency needs to be studied. In this work I describe results from a suite of simulation tools we have developed to capture the full photoemission process: photon absorption, carrier transport within the active material, and electron ballistics following emission. I show that the theoretical efficiency of a negative electron affinity emitter may be increased with nanostructures if light absorption and electron escape ballistics are considered. I then describe measurements of the photoemission efficiency of fabricated nanostructures that were designed based on the results of the simulation suite. I will also present a fundamentally new method to increase the operating voltage of a TEC by lowering the anode work function using the surface photovoltage effect. When a semiconductor surface is illuminated, photo-excited carriers form an internal dipole, or surface photovoltage (SPV), in the band-bending region and begin to flatten the bands near the surface. This SPV is analogous to the photovoltage in a photovoltaic cell and can reduce the effective work function of the material. I will describe an experimental demonstration using the SPV effect to produce a low work function surface. I will also describe a proof-of-concept demonstration of the SPV effect applied to improve the I-V characteristics of thermionic device. This generic physical process extends across materials systems and forms a realistic path to ultra-low work functions in devices to enable efficient thermionic energy conversion.
Author: Daniel C. Riley Publisher: ISBN: Category : Languages : en Pages :
Book Description
Thermionic energy conversion (TEC) is a direct heat-to-electricity conversion technology with the potential to leapfrog state-of-the-art solid-state conversion in efficiency and power density. In a thermionic energy converter, electrons evaporate from a hot electrode, the cathode, into a vacuum gap and are collected by a cooler electrode, the anode, to generate electric current. In the 1960s-1970s numerous groups reported thermionic converters with power densities above 10 W/cm^2 and conversion efficiencies of ~15%. However most of this work was tied to the US space-nuclear program which ended in 1973, and thermionics research has never fully recovered. As a result two central challenges yet remain in thermionics: (1) High operating temperatures necessary to produce electric current result in difficult materials challenges, and (2) low operating voltages due to losses associated with space charge and high anode work functions. However, new opportunities to tackle these challenges are available as a result of the breathtaking rise of semiconductor fabrication technology. In this work I present a new physical mechanism called photon enhanced thermionic emission (PETE). This concept is an improvement on thermionic emission by using light to boost the average energy of carriers in a hot p-type semiconductor cathode. Additionally, unlike in a photovoltaic cell, the waste heat from recombination losses and sub-bandgap light absorption is utilized to heat the cathode. Thus a PETE cathode can produce efficient electron emission at lower temperatures than a thermionic cathode. I will describe theoretical calculations showing that a PETE device may exceed 40% solar power conversion efficiency, and the conversion efficiency may exceed 50% if a PETE device is used in tandem with a solar thermal backing cycle. I will also describe an experimental demonstration of the PETE effect in an ultra-high vacuum photoemission measurement. In the cathode of an energy converter based on photon-enhanced thermionic emission (PETE) photoexcited carriers may need to encounter the emissive surface numerous times before having sufficient thermal energy to escape into vacuum and therefore should be confined close to the surface. However, in a traditional planar geometry, a thin cathode results in incomplete light absorption. Nanostructuring has the potential to increase light capture and boost emission by decoupling the lengths associated with photon absorption and electron emission. Nanostructures may complicate the properties of the emissive surface; therefore, the effect of nanostructuring on emission efficiency needs to be studied. In this work I describe results from a suite of simulation tools we have developed to capture the full photoemission process: photon absorption, carrier transport within the active material, and electron ballistics following emission. I show that the theoretical efficiency of a negative electron affinity emitter may be increased with nanostructures if light absorption and electron escape ballistics are considered. I then describe measurements of the photoemission efficiency of fabricated nanostructures that were designed based on the results of the simulation suite. I will also present a fundamentally new method to increase the operating voltage of a TEC by lowering the anode work function using the surface photovoltage effect. When a semiconductor surface is illuminated, photo-excited carriers form an internal dipole, or surface photovoltage (SPV), in the band-bending region and begin to flatten the bands near the surface. This SPV is analogous to the photovoltage in a photovoltaic cell and can reduce the effective work function of the material. I will describe an experimental demonstration using the SPV effect to produce a low work function surface. I will also describe a proof-of-concept demonstration of the SPV effect applied to improve the I-V characteristics of thermionic device. This generic physical process extends across materials systems and forms a realistic path to ultra-low work functions in devices to enable efficient thermionic energy conversion.
Author: George N. Hatsopoulos Publisher: ISBN: 9780262080606 Category : Languages : en Pages : 683
Book Description
Prepared under the auspices of the U.S. Department of Energy Division of Power Systems. Volume I of Thermionic Energy Conversiondealt with processes and devices (MIT Press, 1974). Its four chapters summarized information basic to the field and not subject to substantial alteration by future developments. Volume II begins with chapter 5 and describes the scientific and engineering aspects of thermionic conversion and the experience with actual operational hardware. It summarizes the experience and analytic methods that should be useful in the development of new applications of thermionic conversion. Thermionic Energy Conversionis intended for scientists and engineers working in the field of energy conversion. With increased interest in maximizing the efficiency of present energy conversion systems, it is an important reference for scientific and engineering libraries as well. The book benefits especially from the authors' actual experience-Hatsopoulos and his firm, Thermo Electron Corporation, are leaders in this field. Gyftopoulos and his students have made basic contributions to the theory of thermionic conversion. Both authors have developed a unified quantum theory of mechanics and thermodynamics. The book covers Elements of Quantum Mechanics; Principles of Thermodynamics; Thermodynamics of Stable Equilibrium States; Thermodynamics of Steady States; Emission Phenomena; Collisionless Transport Phenomena; Analysis of Performance Characteristics of High-Pressure Diodes; Experimental Techniques; Experimental Studies and Correlations of Characteristics; Metallurgy of Electrode Materials; Design and Fabrication of Practical Convertors; and Thermionic Power System Engineering.
Author: Jae Hyung Lee Publisher: ISBN: Category : Languages : en Pages :
Book Description
Thermionic energy converters (TECs) are heat engines that convert heat directly to electricity at very high temperatures. This energy conversion process is based on thermionic emission--the evaporation of electrons from conductors at high temperatures. In its simplest form, the converter consists of two electrodes in the parallel plate capacitor geometry, and it uses the thermionically emitted current to drive an electrical load. This dissertation presents research on five key areas of microfabricated thermionic energy converters ([mu]-TECs). First, the numerical calculation of the emitter-collector gap that maximizes the power conversion efficiency of thermionic energy converters (TECs) is discussed. Thermionic energy converters require emitter and collector work-functions that are relatively low, to reach useful efficiencies at typical operating temperatures of 1000 - 1500 oC. 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 heat loss via near-field radiative heat transfer. The numerical calculation results show that, for typical TECs made with cesiated tungsten electrodes, the optimal gaps range from 900 nm to 3 [micrometers]. I then discuss several prototypes of mechanically and thermally robust [mu]-TECs, including the stress-relieved emitter design, emitter-collector structural design, as well as a recent approach for the stand-alone (encapsulated) [mu]-TECs. Thermionic emission from the SiC emitter was demonstrated for the first time. The stress-relieved design emitters were analyzed, and the work-function of the SiC emitter was estimated at temperatures of up to 2900K. Also described are both the planar and the U-shaped suspension for microfabricated TECs ([mu]-TECs). Our initial planar [mu]-TECs achieved emitter temperatures of over 2000 K with incident optical intensity of approximately 1 W/mm2 (equivalent to 1000 Suns), remained structurally stable under thermal cycling, and maintained a temperature difference between the emitter and the collector of over 1000 K. Conformal sidewall deposition of poly-SiC on a sacrificial mold is used to fabricate stiff suspension legs with U-shaped cross sections, which increases the out-of-plane rigidity and prevents contact with the substrate during the heating of the suspended emitter. By extending the conventional technique of cesium coatings to SiC, we reduce the work-function from 4 eV to 1.65 eV at room temperature. Subsequently, we tested [mu]-TECs with both barium and barium oxide coatings. The coatings reduced the work-function of the SiC emitter to as low as ~2.14 eV and increased the thermionic current by 5-6 orders of magnitude, which is a key step toward realizing a efficient thermionic energy converter. Encapsulation of [mu]-TEC was achieved by an anodic bond between pyrex and the silicon substrate with via feedthroughs. Last, I introduce the photon-enhanced thermionic emission (PETE) concept, and show why a single crystal photo-emitter is needed. I cover my recent fabrication development of smart-cut layer transfer using Spin-on-Glass (SoG). In addition, a novel layer transfer technology that can transfer any device materials onto the glass substrate, which I call "Anything on Glass, " is briefly described. I, then, describe how the first demonstration of the photon-enhanced thermionic emission (PETE) from the microfabricated emitter was achieved. The p-type SiC emitter was used to demonstrate PETE in an uncesiated and microfabricated sample, bringing this energy conversion approach closer to practical applications.
Author: Nathan Snyder Publisher: Elsevier ISBN: 0323142486 Category : Technology & Engineering Languages : en Pages : 798
Book Description
Progress in Astronautics and Rocketry, Volume 3: Energy Conversion for Space Power focuses on the use of dependable electric power sources on space vehicles. Composed of various literature, the book first discusses the physics of thermoelectricity, thermoelectric generator of materials, the use of semiconductors in thermoelectric conversion, and the use of high temperature thermoelectric materials for power generation. The text also presents experiments on the effect of irradiation on thermoelectric materials, thermoelectric elements in space power systems, and thermionics. The book then describes photovoltaic effect and conversion of solar energy; trends in silicon solar cell technology; the use of silicon solar cells in energy conversion; and how radiation affects solar cell power systems. The text notes the specifications of batteries if used in communications satellites; the use of positive-displacement engines and turbines on cryogenic power systems; and the characteristics of magnetohydrodynamic (MHD) generators in space power conversion. The book is a good source of information for readers and scientists wanting to explore the potential of energy conversion in space power technology.
Author: George N. Hatsopoulos Publisher: MIT Press (MA) ISBN: Category : Science Languages : en Pages : 288
Book Description
Good,No Highlights,No Markup,all pages are intact, Slight Shelfwear,may have the corners slightly dented, may have slight color changes/slightly damaged spine.
Author: Samuel James Rosenthal Publisher: ISBN: Category : Languages : en Pages :
Book Description
A new physical mechanism for direct conversion of solar energy to electricity, called photon-enhanced thermionic emission (PETE), has attracted significant attention as a technology with the promise of very high power conversion efficiencies. The PETE mechanism involves thermionic emission of photoexcited electrons from a high temperature semiconductor cathode, followed by collection at a lower temperature, low work function anode. Due to its combination of photovoltaic and thermal processes, PETE has the potential to achieve efficiencies far above the fundamental limits for single-junction photovoltaics. In this work, two approaches to engineering electron transport in PETE energy converters are presented. A triode device with an electron-transparent gate electrode can be used to overcome space charge limitations that may reduce PETE converter efficiency. Graphene is considered as a candidate gate material, and SEM-based experimental measurements of its transparency to low energy (down to ~5 eV) electrons are reported. Even with an effective gate, the PETE process requires emission of electrons into vacuum, posing significant challenges to realizing efficient converters. To overcome this challenge, a solid-state PETE device structure is proposed, in which the vacuum gap is replaced by semiconductor nanowires that bridge the electrodes. These nanowires provide a path for electron transport while minimizing thermal conduction between the electrodes. Theoretical efficiencies for this device architecture are reported, and the implications of these results for realizing solid-state PETE converters are discussed.
Author: Daniel C. Riley
Author: Daniel C. Riley
Author: V. A. Baum
Author: George N. Hatsopoulos
Author: Dr. Walter L. Knecht
Author: Jae Hyung Lee
Author: Nathan Snyder
Author: George N. Hatsopoulos
Author: 
Author: 
Author: Samuel James Rosenthal