Thermionic converter

Last updated

A thermionic converter consists of a hot electrode which thermionically emits electrons over a potential energy barrier to a cooler electrode, producing a useful electric power output. Caesium vapor is used to optimize the electrode work functions and provide an ion supply (by surface ionization or electron impact ionization in a plasma) to neutralize the electron space charge.

Contents

Definition

From a physical electronic viewpoint, thermionic energy conversion is the direct production of electric power from heat by thermionic electron emission. From a thermodynamic viewpoint, [1] it is the use of electron vapor as the working fluid in a power-producing cycle. A thermionic converter consists of a hot emitter electrode from which electrons are vaporized by thermionic emission and a colder collector electrode into which they are condensed after conduction through the inter-electrode plasma. The resulting current, typically several amperes per square centimeter of emitter surface, delivers electrical power to a load at a typical potential difference of 0.5–1 volt and thermal efficiency of 5–20%, depending on the emitter temperature (1500–2000 K) and mode of operation. [2] [3]

History

After the first demonstration of the practical arc-mode caesium vapor thermionic converter by V. Wilson in 1957, several applications of it were demonstrated in the following decade, including its use with solar, combustion, radioisotope, and nuclear reactor heat sources. The application most seriously pursued, however, was the integration of thermionic nuclear fuel elements directly into the core of nuclear reactors for production of electrical power in space. [4] [5] The exceptionally high operating temperature of thermionic converters, which makes their practical use difficult in other applications, gives the thermionic converter decisive advantages over competing energy conversion technologies in the space power application where radiant heat rejection is required. Substantial thermionic space reactor development programs were conducted in the U.S., France, and Germany in the period 1963–1973, and the US resumed a significant thermionic nuclear fuel element development program in the period 1983–1993.

Thermionic power systems were used in combination with various nuclear reactors (BES-5, TOPAZ) as electrical power supply on a number of Soviet military surveillance satellites between 1967 and 1988. [6] [7] See Kosmos 954 for more details.

Although the priority for thermionic reactor use diminished as the US and Russian space programs were curtailed, research and technology development in thermionic energy conversion have continued. In recent years technology development programs for solar-heated thermionic space power systems were conducted. Prototype combustion-heated thermionic systems for domestic heat and electric power cogeneration, and for rectification, have been developed. [8]

Description

The scientific aspects of thermionic energy conversion primarily concern the fields of surface physics and plasma physics. The electrode surface properties determine the magnitude of electron emission current and electric potential at the electrode surfaces, and the plasma properties determine the transport of electron current from the emitter to the collector. All practical thermionic converters to date employ caesium vapor between the electrodes, which determines both the surface and plasma properties. Caesium is employed because it is the most easily ionized of all stable elements.

A thermionic generator is like a cyclic heat engine and its maximum efficiency is limited by Carnot's law. It is a low-Voltage high current device where current densities of 25-50 (A/squarecm) have been achieved at voltage from 1-2V. The energy of high temperature gases can be partly converted into electricity if the riser tubes of the boiler are provided cathode and anode of a thermionic generator with the interspace filled with ionized caesium vapor.

The surface property of primary interest is the work function, which is the barrier that limits electron emission current from the surface and essentially is the heat of vaporization of electrons from the surface. The work function is determined primarily by a layer of caesium atoms adsorbed on the electrode surfaces. [9] The properties of the interelectrode plasma are determined by the mode of operation of the thermionic converter. [10] In the ignited (or "arc") mode the plasma is maintained via ionization internally by hot plasma electrons (~ 3300 K); in the unignited mode the plasma is maintained via injection of externally produced positive ions into a cold plasma; in the hybrid mode the plasma is maintained by ions from a hot-plasma interelectrode region transferred into a cold-plasma interelectrode region.

Recent work

All the applications cited above have employed technology in which the basic physical understanding and performance of the thermionic converter were essentially the same as those achieved before 1970. During the period from 1973 to 1983, however, significant research on advanced low-temperature thermionic converter technology for fossil-fueled industrial and commercial electric power production was conducted in the US, and continued until 1995 for possible space reactor and naval reactor applications. That research has shown that substantial improvements in converter performance can be obtained now at lower operating temperatures by addition of oxygen to the caesium vapor, [11] by suppression of electron reflection at the electrode surfaces, [12] and by hybrid mode operation. Similarly, improvements via use of oxygen-containing electrodes have been demonstrated in Russia along with design studies of systems employing the advanced thermionic converter performance. [13] Recent studies [14] have shown that excited Cs-atoms in thermionic converters form clusters of Cs-Rydberg matter which yield a decrease of collector emitting work function from 1.5 eV to 1.0 – 0.7 eV. Due to long-lived nature of Rydberg matter this low work function remains low for a long time which essentially increases the low-temperature converter’s efficiency.

See also

Related Research Articles

Cathode An electrode where reduction take place

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit.

A radionuclide is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferred to one of its electrons to release it as a conversion electron; or used to create and emit a new particle from the nucleus. During those processes, the radionuclide is said to undergo radioactive decay. These emissions are considered ionizing radiation because they are powerful enough to liberate an electron from another atom. The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single nuclide the decay rate, and thus the half-life (t1/2) for that collection, can be calculated from their measured decay constants. The range of the half-lives of radioactive atoms has no known limits and spans a time range of over 55 orders of magnitude.

A nuclear electric rocket is a type of spacecraft propulsion system where thermal energy from a nuclear reactor is converted to electrical energy, which is used to drive an ion thruster or other electrical spacecraft propulsion technology. The nuclear electric rocket terminology is slightly inconsistent, as technically the "rocket" part of the propulsion system is non-nuclear and could also be driven by solar panels. This is in contrast with a nuclear thermal rocket, which directly uses reactor heat to add energy to a working fluid, which is then expelled out of a rocket nozzle.

Fusor An apparatus to create nuclear fusion

A fusor is a device that uses an electric field to heat ions to nuclear fusion conditions. The machine induces a voltage between two metal cages, inside a vacuum. Positive ions fall down this voltage drop, building up speed. If they collide in the center, they can fuse. This is one kind of an inertial electrostatic confinement device – a branch of fusion research.

Cold cathode Type of electrode and part of cold cathode fluorescent lamp.

A cold cathode is a cathode that is not electrically heated by a filament. A cathode may be considered "cold" if it emits more electrons than can be supplied by thermionic emission alone. It is used in gas-discharge lamps, such as neon lamps, discharge tubes, and some types of vacuum tube. The other type of cathode is a hot cathode, which is heated by electric current passing through a filament. A cold cathode does not necessarily operate at a low temperature: it is often heated to its operating temperature by other methods, such as the current passing from the cathode into the gas.

Neutron source Device that emits neutrons

A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear power.

Radioisotope thermoelectric generator Type of electric generator

A radioisotope thermoelectric generator is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioactive material into electricity by the Seebeck effect. This type of generator has no moving parts.

Thermionic emission Thermally induced flow of charge carriers from a surface

Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature. This occurs because the thermal energy given to the charge carrier overcomes the work function of the material. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as thermions. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission.

A magnetohydrodynamic generator is a magnetohydrodynamic converter that utilizes a Brayton cycle to transform thermal energy and kinetic energy directly into electricity. MHD generators are different from traditional electric generators in that they operate without moving parts to limit the upper temperature. They therefore have the highest known theoretical thermodynamic efficiency of any electrical generation method. MHD has been extensively developed as a topping cycle to increase the efficiency of electric generation, especially when burning coal or natural gas. The hot exhaust gas from an MHD generator can heat the boilers of a steam power plant, increasing overall efficiency.

An atomic battery, nuclear battery, radioisotope battery or radioisotope generator is a device which uses energy from the decay of a radioactive isotope to generate electricity. Like nuclear reactors, they generate electricity from nuclear energy, but differ in that they do not use a chain reaction. Although commonly called batteries, they are technically not electrochemical and cannot be charged or recharged. They are very costly, but have an extremely long life and high energy density, and so they are typically used as power sources for equipment that must operate unattended for long periods of time, such as spacecraft, pacemakers, underwater systems and automated scientific stations in remote parts of the world.

Electrodynamic tether Long conducting wires which can act as electrical motors or generators

Electrodynamic tethers (EDTs) are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy. Electric potential is generated across a conductive tether by its motion through a planet's magnetic field.

Hot cathode Type of electrode

In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating element. The heating element is usually an electrical filament heated by a separate electric current passing through it. Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface area. Cold cathodes rely on field electron emission or secondary electron emission from positive ion bombardment, and do not require heating. There are two types of hot cathode. In a directly heated cathode, the filament is the cathode and emits the electrons. In an indirectly heated cathode, the filament or heater heats a separate metal cathode electrode which emits the electrons.

Thermophotovoltaic (TPV) energy conversion is a direct conversion process from heat to electricity via photons. A basic thermophotovoltaic system consists of a hot object emitting thermal radiation and a photovoltaic cell similar to a solar cell but tuned to the spectrum being admitted from the hot object.

An optoelectric nuclear battery is a type of nuclear battery in which nuclear energy is converted into light, which is then used to generate electrical energy. This is accomplished by letting the ionizing radiation emitted by the radioactive isotopes hit a luminescent material, which in turn emits photons that generate electricity upon striking a photovoltaic cell.

The alkali-metal thermal-to-electric converter (AMTEC), originally called the sodium heat engine (SHE) was invented by Joseph T. Kummer and Neill Weber at Ford in 1966, and is described in US Patents 3404036, 3458356, 3535163 and 4049877. It is a thermally regenerative electrochemical device for the direct conversion of heat to electrical energy. It is characterized by high potential efficiencies and no moving parts except for the working fluid, which make it a candidate for space power applications.

Plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases. The plasma is generally created by radio frequency (RF) frequency or direct current (DC) discharge between two electrodes, the space between which is filled with the reacting gases.

Ionized-air glow Optical phenomenon

Ionized-air glow is the fluorescent emission of characteristic blue–purple–violet light, often of a color called electric blue, by air subjected to an energy flux either directly or indirectly from solar radiation.

A Johnson thermoelectric energy converter or JTEC is a type of solid-state heat engine that uses the electrochemical oxidation and reduction of hydrogen in a two-cell, thermal cycle that approximates the Ericsson cycle. It is under investigation as a viable alternative to conventional thermoelectric conversion. Lonnie Johnson invented it and claims the converter exhibits an energy conversion efficiency of as much as 60%, however, this claim is at a theoretical level based on comparison with a Carnot cycle and assumes a temperature gradient of 600 °C. It was originally proposed for funding to the Office of Naval Research but was refused. Johnson obtained later funding by framing the engine as a hydrogen fuel cell. Johnson had been collaborating with PARC on development of the engine.

Direct energy conversion (DEC) or simply direct conversion converts a charged particle's kinetic energy into a voltage. It is a scheme for power extraction from nuclear fusion.

Piezoelectric direct discharge (PDD) plasma is a type of cold non-equilibrium plasma, generated by a direct gas discharge of a high voltage piezoelectric transformer. It can be ignited in air or other gases in a wide range of pressures, including atmospheric. Due to the compactness and the efficiency of the piezoelectric transformer, this method of plasma generation is particularly compact, efficient and cheap. It enables a wide spectrum of industrial, medical and consumer applications.

References

  1. ↑ Rasor, N. S. (1983). "Thermionic Energy Converter". In Chang, Sheldon S. L. (ed.). Fundamentals Handbook of Electrical and Computer Engineering . Vol. II. New York: Wiley. p. 668. ISBN   0-471-86213-4.
  2. ↑ Hatsopoulos, G. N.; Gyftopoulos, E. P. (1974). Thermionic Energy Conversion . Vol. I. Cambridge, MA: MIT Press. ISBN   0-262-08059-1.
  3. ↑ Baksht, F. G.; G. A. Dyvzhev; A. M. Martsinovskiy; B. Y. Moyzhes; G. Y. Dikus; E. B. Sonin; V. G. Yuryev (1973). "Thermionic converters and low-temperature plasma (trans. from Termoemissionnye prebrazovateli i nizkotemperaturnaia plazma)": 490.{{cite journal}}: Cite journal requires |journal= (help)
  4. ↑ Mills, Joseph C.; Dahlberg, Richard C. (January 10, 1991). "Thermionic Systems for DOD Missions". AIP Conference Proceedings. 217 (3): 1088–92. Bibcode:1991AIPC..217.1088M. doi:10.1063/1.40069. Archived from the original on July 10, 2012.
  5. ↑ Gryaznov, G. M.; E. E. Zhabotinskii; A. V. Zrodnikov; Yu. V. Nikolaev; N. N. Ponomarev-Stepnoi; V. Ya. Pupko; V. I. Serbin; V. A. Usov (June 1989). "Thermoemission reactor-converters for nuclear power units in outer space". Soviet Atomic Energy. Plenus Pub. Co. 66 (6): 374–377. doi:10.1007/BF01123508. ISSN   1573-8205. S2CID   95666931.
  6. ↑ Bulletin of the Atomic Scientists. July 1993. pp. 12–.
  7. ↑ Proceedings of a Symposium Advanced Compact Reactor Systems: National Academy of Sciences, Washington, D.C., November 15-17, 1982. National Academies. 1983. pp. 65–. NAP:15535.
  8. ↑ van Kemenade, E.; Veltkamp, W. B. (August 7, 1994). "Design of a Thermionic Converter for a Domestic Heating System" (PDF). Proceedings of the 29th Intersociety Energy Conversion Engineering Conference. II.
  9. ↑ Rasor, Ned S.; Charles Warner (September 1964). "Correlation of Emission Processes for Adsorbed Alkali Films on Metal Surfaces". Journal of Applied Physics. The American Institute of Physics. 35 (9): 2589. Bibcode:1964JAP....35.2589R. doi:10.1063/1.1713806. ISSN   0021-8979.
  10. ↑ Rasor, Ned S. (December 1991). "Thermionic Energy Conversion Plasmas". IEEE Transactions on Plasma Science. 19 (6): 1191–1208. Bibcode:1991ITPS...19.1191R. doi:10.1109/27.125041.
  11. ↑ J-L. Desplat, L.K. Hansen, G.L. Hatch, J.B. McVey and N.S. Rasor, "HET IV Final Report", Volumes 1 & 2, Rasor Associates Report #NSR-71/95/0842, (Nov. 1995); performed for Westinghouse Bettis Laboratory under Contract # 73-864733; 344 pages. Also available in total as C.B. Geller, C.S. Murray, D.R. Riley, J-L. Desplat, L.K. Hansen, G.L. Hatch, J.B. McVey and N.S. Rasor, "High-Efficiency Thermionics (HET-IV) and Converter Advancement (CAP) programs. Final Reports", DOE DE96010173; 386 pages (1996).
  12. ↑ N.S. Rasor, "The Important Effect of Electron Reflection on Thermionic Converter Performance", Proc. 33rd Intersoc. Energy Conv. Engr. Conf., Colorado Springs, CO, Aug., 1998, paper 98-211.
  13. ↑ Yarygin, Valery I.; Viktor N. Sidelnikov; Vitaliy S. Mironov. "Energy Conversion Options For NASA's Space Nuclear Power Systems Initiative – Underestimated Capability of Thermionics". Proceedings of the 2nd International Energy Conversion Engineering Conference.
  14. ↑ Svensson, Robert; Leif Holmlid (May 15, 1992). "Very low work function surfaces from condensed excited states: Rydberg matter of cesium". Surface Science. 269–270: 695–699. Bibcode:1992SurSc.269..695S. doi:10.1016/0039-6028(92)91335-9. ISSN   0039-6028.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.