Thermionic Converter Based Rectifier
Creation of a High-Current Low-Voltage Thermionic Rectifier with the Phase Control of the Output Voltage
Tech Area / Field
- CHE-IND/Industrial Chemistry and Chemical Process Engineering/Chemistry
- NNE-MEC/Miscellaneous Energy Conversion/Non-Nuclear Energy
3 Approved without Funding
State Enterprise Krasnaya Zvezda, Russia, Moscow
- Russian Academy of Sciences / Physical Technical Institute, Russia, St Petersburg\nFEI (IPPE), Russia, Kaluga reg., Obninsk
- [Individual specialist]
Project summaryProblem Topicality
The electro-chemistry is one of the most power-consuming fields of engineering. Thus, the electric power consumption by the galvanizing departments of only the USA motor-car industry is valued at $5 billons per year. An important feature of electrochemical plants is that they are the low-voltage (3-50 V) high-current (tens of kA) consumers of direct current, and hence there arises a complicated engineering problem of rectification of the mains frequency alternating current into the direct current. In the capacity of conventional valves they use mainly the powerful silicon thyristors, which have the high direct voltage drop (~2 V), but at the same time provide a possibility of the output voltage phase control by varying the sine wave conduction angle. Therefore their electrical efficiency is relatively low: for example, the rectifiers of one of the world leaders in this field – the “COSEL” company (Japan) – have the electrical efficiency of 65-85% for the output voltage between 5 and 24 V.
The problem of reduction or even elimination of the electrical energy losses when rectifying the low voltage can be solved cardinally by using the thermionic converters of thermal energy into electrical energy as the valves. At the anode temperature Ta <800 K the reverse current in them is negligible, and for the modes with the current-voltage characteristic in the operational quadrant they operate as ideal valves with a zero direct voltage drop. This makes it possible to completely exclude the rectified voltage losses in the valves and obtain the electrical efficiency of rectification of ~100%.
The practical implementation of this technology and the creation of a full-scale module of a thermionic rectifier (TIC-rectifier) with the gas heating of a cathode made up the task of the ISTC Project 2306. The result of the successful execution of this project was the demonstration of a real possibility to solve all the engineering problems of the gas heating and, correspondingly, a possibility to reduce the electric energy losses in galvanizing by 15 to 20% due to the use of thermionic conversion of the alternating current into the direct current. However, although a diode option of a TIC-rectifier provides such an essential electrical energy saving, it yields to conventional semiconductor rectifiers in its functional capabilities – its use requires the rectified voltage amplitude control system, while the thyristor rectifiers provide a simple phase control of an output voltage by varying the sine wave conduction angle (exactly for this reason almost all rectifiers for galvanizing are made on a basis of thyristors rather than diodes, in spite of the fact that the latter have twice as lower direct voltage drop). This was fully confirmed by the expertise of a commercialization potential of the results of the ISTC Project 2306, which also pointed out that it is desirable to expand the rectified voltage range up to tens of volts thus essentially expanding possible areas of the thermionic rectifiers’ use (in particular, in the electrolysis units).
The real way for the solution of this problem was found in the process of research work at the ISTC Project 2306: the engineering possibility of the creation of a fundamentally new device – a cesium thermionic converter-thyristor (TIC-thyristor) for rectifiers with the gas heated cathode and control grid ensuring the variation of a moment of initiation of the low-voltage arc discharge – was shown. This mode was realized in a three-electrode TIC with the electrical heating of a cathode being the thyristor analog. Combination of the grid-controlled discharge initiation with the gas heating allows the realization of a high-current low-voltage rectifier with the minimal (down to zero) direct drops and the phase control of an output voltage, thereby providing this rectifier with the functional capabilities identical to those of semiconductor thyristor rectifiers. At that, the essentially higher affordability of a new device, which provides saving of 15 to 20% of the electrical energy, gives to it the deciding competitive advantages over the existing low-voltage high-current rectifiers and makes the prospect for the transition of the electro-chemical industries (galvanizing, electrolysis) to the proposed project solutions commercially reasonable and real.
Thus, the successful execution of the ISTC Project 2306 by and large makes it possible to define the problem, the solution of which imparts to TIC-rectifiers functional capabilities identical to those of semiconductor rectifiers and gives to TIC-rectifiers the deciding competitive advantages in view of their essentially higher affordability.
Aim of this work is the creation of a prototype of a commercial thermionic high-current gas-heated rectifier with the phase controlled output voltage for the rectified voltage range between 3 and ~50 V and the demonstration of new design solutions.
Methodology Features, Content and Results of Work
Main directions of research work for the solution of the problems put by are supposed to be as follows:
- the use of the results of a fundamental research on the physics of the direct conversion of the thermal energy into the electrical energy for the optimization of the thermionic phase-controlled rectifier (TPC-rectifier) characteristics;
- the modification and use of the primary design solutions of a rectifier on a diode thermionic converter (TIC) basis for the creation of a TPC-rectifier;
- the research in the field of the design, material and engineering problems concerned with the creation of a TPC-rectifier with the output voltage phase control in the range of 3-6-12-24-50 V;
- the research aimed to reduce the heat-flux rate and the density of generated electrical energy (hereinafter – energy intensity), i.e. to lower the TPC-rectifier cathode temperature (Tc), anode temperature (Ta), grid temperature (Tgrid) and thermal power by increasing the thermionic efficiency (decreasing the VB barrier index value).
The work in these directions will allow further system improvement of the gas heated thermionic rectifier (TIC-rectifie)r design used in the ISTC Project 2306, which ensures the solution of the problems put by in the present project. The basic improvements are as follows:
- the introduction of a control grid into the TPC-rectifier interelectrode gap;
- the use in the TPC-rectifier prototype of the interelectrode gap sealing using the “cold seal” scheme;
- the use of the possible means to lower the TPC-rectifier cathode temperature (Tc), anode temperature (Ta), grid temperature (Tgrid) and thermal power by increasing the thermionic efficiency (decrease of VB barrier index value) by selecting of the cathode, anode and grid materials and of the current rectification process parameters.
The research performed in the ISTC Project 2306 showed that the use of a grid makes it possible to conveniently realize the phase control of the rectifier output voltage in a wide range of thermionic converter modes. Near optimal for this aim is a large-structure grid with a loop of ~1 mm, which leads to an increase of the direct voltage drop only by 0,1-0,3 V as compared to a diode, thus making it possible to keep the rectification electrical efficiency close to 100%.
The use of the “cold seal” scheme not only increases the device breakdown strength, but also essentially simplifies the construction, reduces its price, increases its reliability, makes the rectifying valve repairable.
It is supposed to increase the thermionic efficiency by both using the Cs/O systems and realizing the new idea of using the thin nano-dimension insulating and semiconductor layers on the anode as was suggested by collaborators.
The expected results of work on the project are as follows:
- The creation and test of the electrically heated TPC-rectifier simulator.
- The development, creation and test of a pilot production prototype of a thermionic gas heated TIC-rectifier module.
- The substantiation of a possibility to replace conventional rectifiers in the galvanizing and electrolytic industries by the more efficient TPC-rectifiers.
- Recommendations on the adaptation of the created technology of the TPC-rectifiers’ fabrication to a mass production.
- Recommendations on the application of the engineering solutions on the TPC-rectifier fabrication worked out in the course of the project execution for the creation of the electrogenerating TIC to be used as the high-temperature topping in the terrestrial heat generating power systems.
The scientific importance of the project is determined by the elaboration of fundamental issues and the development of the newest physical circuitries in the field of both the direct conversion of the thermal energy into the electrical energy and the direct current low-voltage power engineering.
The commercial efficiency of the project is conditioned by the fact that the created competitive engineering solutions and systems have prospects of their industrial application for the consumers of the low-voltage direct current electrical energy.
The institutions participating in the project were for years engaged in the study and development of thermionic converters (TIC) for various power systems. In particular, the FSUE “Red Star” in co-operation with the SSC RF-IPPE developed the “TOPAZ” thermionic reactors, which successfully operated as a component of the “Plasma-A” space vehicle of the “Cosmos” series (## 1818 and 1867). In the years 2004 through 2006 the specialists of these institutions together with the group of the A.F. Ioffe PTI successfully fulfilled the work at the ISTС Project #2306.
Participation in the project will enable the “weapon” personnel of the SSC RF-IPPE and FSUE “Red Star” to re-direct their efforts toward the civil activities for a long period of time, and the results of work will allow the creation of new jobs for the scientists of the former Soviet defense complex with the outlook for their assistance in the development of a large-scale production of the TIC-rectifiers and, possibly, also the electrogenerating TIC for the terrestrial applications.
The Project duration is 24 months. The work will be performed at the FSUE “Red Star”, the SSC RF-IPPE and the A.F. Ioffe PTI of RAS.
The tentative labor input into the Project is 625 man-months (13749 man-days). The volume of work supposed to be performed by the weapons personnel makes 325 man-months or 52%.