The Thermal Power Plant of the Future

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 5, Issue 10, October 2015)

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The Thermal Power Plant of the Future

Dr. Alexander Rubinraut

Freischützstr.110, 81927 Munich, Germany

Abstract - The project of a thermal power plant of the future, based on the new concept, providing for maximal usage of thermal energy, is considered. The gaseous working substance moves along a closed circuit, in which the working substance after heating in a gas heater comes into MHD AC generator, then goes through the blades of a gas turbine with electrogenerator and enters in a steam generator of waste heat, in the second circuit of which the steam turbine with condenser is installed. In order to ensure the long-term resource of the power plant operation , a new design of MHD AC generator, which has a superconducting winding, being cooled by liquid nitrogen, is developed. The project of power generating unit for thermal power plant of the future by capacity of 600 MW, which efficiency reaches 79%, is performed.

Keywords: Thermal power plant, combustion chamber, gas heater, MHD generator, steam generator of waste heat, steam turbine, gas turbine, turbogenerator, condenser, frequency converter.

I. INTRODUCTION

The development of power engineering goes the way of continuous improvement of the main electric power producer – the thermal power plant. And the main purpose here remains the maximal rise in the effectiveness of fuel utilization.

Currently the level of the thermal power plants (TPP) has considerably grown. In 2014 year, the coal-burning power plant in Wilhelmshaven (Kraftwerk Wilhelmshaven) has reached the cherished efficiency factor – 50% at fuel consumption 288 g/kWh. and carbon dioxide emission in atmosphere has decreased to 669 g/CO2 kWh [1].

It had taken place owing to introduction of up-to-date products of firm Siemens, which made it possible to raise the temperature on the blades of steam turbine up to 700°C.

While moving forward along the way of efficiency increase of fuel thermal power plant, the firm Siemens had developed the most powerful gas turbine in world. This turbine has capacity of 340 MW. It works in pair with a steam turbine on a turbogenerator and such a tandem had provided the record result. The efficiency of G and D installation at the thermal power plant in Irshing had exceeded 60% at capacity 340 MW [2].

Schematic diagram the thermal power plant GuD is shown at the Fig.1.

Fig. 1

As opposed to the traditional schema of a thermal power plant, at which the fuel combustion chamber is located in front of steam boiler outside of turbine hall, in schema (Fig. 1) the combustion chamber is located inside of gas turbine 3. After usage in gas turbine, the thermal flow from turbine exit is supplied to a steam heater, in which water steam is being arisen.

The additional energy is being produced with the help of the steam turbine 7, which is fixed on the common shaft together with turbogenerator 5 and gas turbine 3.

Yet it shall be stated, that in spite of the latest achievements, even at the most modern thermal power plant 40% of heat, which is being generated at electric power production, stays unused and is being spent as unwanted heating of atmosphere. The reason: the existence of restriction in relation to maximum allowed heating of turbine blades because of insufficient heat resistance of the blade material.

At the same time the modern designs of boiler furnaces allow heating up to 2000 °С.

Is it possible to overcome this thermal restriction? In this connection one must recall the experience of the numerous researches, performed in the twentieth century , concerning the creation of the so called magnetohydrodynamic generators (MHDG).

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In Moscow the special institute - the Institute of High Temperatures ) was founded (under The Academy of Sciences) , where the author worked as the leading researcher during five years.

It is necessary to remind, in what way it was supposed to carry out the working process by the electric power production with the help of a MHD generator .

The thermal energy, being obtained at fuel burning convert the working substance, for instance, air, into plasma state. At high temperature the gas is being ionized and becomes electrically conductive. At expansion process , the thermal energy turns into mechanical energy of the working substance movement inside the working chamber. Inside the working chamber two electrodes are installed. On outside of the working chamber, the superconducting DC winding is installed, which generates inside of the working chamber a magnetic field which is directed perpendicularly to movement direction of the working substance. At the movement of electrically conductive working substance in magnetic field between electrodes an emf and electric current are arisen. Numerous variants of MHD DC generator designs were invented in

60ths - 80th years of the last century. At that time the prototypes and experimental models of MHD generators were built and the corresponding researches were carried out in many countries [1] . It was supposed, that the installation of MHD generators at thermal power plants makes it possible to increase the effectiveness of the thermal cycle by 20%.

However these optimistic expectations have not come true , as all the existing designs of MHD generators could not to provide the sufficiently long-term operation in the capacity of energy source, that is to create the necessary resource. It is caused by the fact, that the design of DC MHD generator has an essential shortcoming. The electric current in working substance flows between metallic electrodes and the cathode is bombarded by ions of very high energy. The result of such a bombardment is rather fast cathode destruction. All the attempts to solve the problem of electrodes destruction problem had no success. The most successful designs of MHD generators had resource o hundreds of hours, when for operation in an industrial power supply system it is necessary to provide the resource of tens thousands of hours.

The MHD generator constructive decision was grounded on application of superconductors, which are able to create only constant magnetic field.

At present, as a result of advance in high-temperature superconductivity, the superconductors which operate in alternating current, are created.

The windings, which are manufactured out of the superconductors are able to create alternating magnetic field with high magnetic induction and with minimal energy losses at mains frequency 50-60 Hz.

On the basis of the high-temperature superconductors , the experimental (prototype) models of cables and transformers for industrial power supply systems are being produced. The winding of such transformers are being cooled with the help of liquid nitrogen. This positive experience can be used at creation of MHD AC generators.

This article treats the thermal power plant, which design excludes application of electrodes.

The energy transformation takes place by means of braking of the working substance in traveling magnetic field. The working substance in plasma state heads for a conic circular channel, on which external surface the three-phase AC winding is installed. The AC winding forms magnetic field , which moves along the conic circular channel. The speed of traveling magnetic field motion is being made less than the speed of working substance motion in the channel. Therefore the traveling magnetic field causes in electroconducting working substance the emf and eddy currents just as it takes place in rotor of a linear asynchronous electric motor

At interaction of eddy currents with traveling magnetic field, a force, which brakes the working substance flow, is appeared, and in AC three-phase winding the emf and active current, which is being handed over to the power supply system , are being generated.

The speed of the traveling magnetic field is defined by frequency of the current in the three-phase AC winding. Hereupon the electrical circuit of connections provides for the three-phase AC winding connection to the power supply system through the frequency converter.

II. FORMATION CONCEPTION OF THERMAL POWER PLANT

WITH ACMHDGENERATOR

The transformation process in thermal power plant with MHD generators begins with fuel combustion inside the combustion chamber, which results in transition of working substance( namely, of the air) in plasma state. The plasma plays role of conductor in electric generator, but while being gas, it is used also as the working substance of the thermal engine.

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The major parameter, on which depends the efficiency of energy transformation in MHD generator, is electrical conduction of gas.

The dependence of plasma electrical conduction was determined in course of researches which were carried out in the USSR and USA still 50 years ago [3].

At Fig.2 the temperature dependence of electrical conduction of different gases is given.

These data had been used for choice of thermal schema for thermal power plant.

The studies of plasma electrical conduction has shown the usage efficiency of adding into working substance of small amounts of materials having low ionization energy, for instance, of potassium or cesium. The addition of 1% of cesium or potassium increases electrons concentration and raises electrical conduction on two orders.

The maximal value of electrical conductance at the range of 2000-3000°K one can obtain at usage of argon with additive of 0,1% cesium (curve 1) or at usage helium with additive of 0,2% cesium (curve 2).

The usage of air with fuel combustion products as working substance also allows significantly to increase electrical conductance , if there are additives.

The temperature dependence of electrical conductance at presence of additive of 4% cesium is shown by curve 3 and temperature dependence of electrical conductance for air with fuel combustion products at presence of additive of 2% potassium is shown by curve 4

On the first stage of creation of thermal power plant with AC MHD generator the possibility of usage so called “the open thermal scheme” , which is shown at Fig.3, has been considered.

Fig. 3

This schema attracts by its simplicity. The fuel combustion products, which get warm to temperature of 2800°K in the combustion chamber 1, are used in MHD generator 2 as working substance. In order to increase plasma electrical conductance, 2% potassium additive is being introduced

The maximal temperature at blades of steam turbine 5 reaches 970°K. In the steam circuit a condenser 6 and a feed pump of traditional design are installed.

The calculations show that the efficiency of thermal power plant which is implemented in accordance with schema of Fig.3 does not exceed 0,7. Such a limitation is conditioned of insufficiently high temperature of plasma on MHD generator exit. In order to get the maximal value of plasma electrical conductance, which is necessary for electrical energy generation in MHD generator, the temperature of plasma on MHD generator exit must be not less 2000°K.

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The other shortcoming of the open schema is low electrical conductance of combustion products with additives of potassium at temperatures 2800-1800°K as compared with electrical conductance of inert gases with additive of cesium.

For the project of a thermal power station of the future, the new heat schema shown at Fig. 4, was developed.

Fig. 4

1-condensator 2-steam power turbine 3-input pipeline 4-gas-turbine engine 5-heat recovery tank 6-the section of steam superheater 7-the boiling section 8-the section of economizer 9-steam separator 10-the discharge of the saturated steam for consumer 11-the circulating pump 12-turbogenerator 13,15-respectively feed and condensing pumps 14-tank 16-the pipeline of water being added into feed tank 17,18- gas compressors of low and high pressure 19-combustion chamber 20-22- gas turbines of high, middle and low pressure 23-output gas pipeline; 24 – gas heater 25 – MHD generator 26 – gas pipeline

In this schema the shortcomings of the open schema shown at Fig.3. are overcome.

The fuel, which is being fed into combustion chamber 19, can be solid, liquid or gaseous. It burns in air gas jet which is being supplied under pressure. On the exit of the hot gas from the combustion chamber 19 the gas heater 24, which is a part of the closed gas circuit, is located.

The gas, heated in the gas heater 24, comes on the input of MHD generator 25 at temperature of 2600°K. As a working substance , argon with additive of 0,1% cesium (by mass), is used.

After usage in MHD generator 25, the working substance is being conveyed along the pipeline 26 at temperature 1600 °K on input of a gas turbine, which has wheels of high pressure, middle pressure and low pressure. After usage in the gas turbine , the working substance passes through the compressor 17, 18, which compensates a pressure drop in the closed circuit.

After exit out of gas turbine (at temperature 1000°K) the working substance gets into the steam boiler5. The steam boiler 5 has a hermetic gas chamber, in which the heat exchangers of a boiler 7, of steam heater 6 and of economizer 8 are located.

The heat exchangers are part of the traditional schema of the water-steam cycle of the thermal power plant.

The water steam, which is produced in the boiler 5 at temperature 950°K, is being conveyed on the input of the steam turbine 2. After usage the steam passes through condenser 1.

The circulation of water and steam is carried out with the help of the pump11. The working substance exits out of the gas chamber of the steam boiler through the pipeline 23 and is being conveyed onto the input of the gas heater 24. In such a way the primary heat circuit , in which argon circulates, is being closed. The turbo unit of thermal power plant consists of a turbogenerator 12 on one side of which (on shaft) a gas turbine and on the other side a steam turbine 2 are located, in much the same way how it was carried out by firm „Siemens” and is shown at Fig. 1. It shall be noted that the heat schema of power plant consists out of components, which are being widely used nowadays in industrial energetics. The new components are MHD AC generator 25 and the gas heater 24.

The gas heater is an unit , which transfers the heat of the heated air and of combustion products into argon flow inside of the closed circuit.

In this project of the thermal power plant the design, which is carried out on the basis of the latest developments of MAN company, is accepted. With the help of spray-type nozzles the hot air is being conveyed onto the plates of the heater, inside of which the channels for gas pass are located. The gas heater is assembled out of separate elements which are being connected to each other. Each element is made by means of a special casting out of material withstanding the temperature up to 3000°K. For instance, the ceramics on basis of aluminum dioxide Al2 O3 99,99%. can be applied.

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The proposed thermal schema (Fig.4) makes it possible to provide the energy production over the all range of temperatures change.

The steam and gas cycle of the useful heat usage is 0,58. The installation an AC MHD generator makes it possible to additionally use 0,27 of thermal energy.

If to take in consideration, that the energy losses in heat form in the gas heater are equal to 0,06, the total thermal efficiency of the power plant will reach 79% .

It means, that the proposed thermal schema of thermal power plant make it possible practically in twice to increase electrical energy production as compared with existing thermal power plants.

III. MHDGENERATOR OF ALTERNATING CURRENT.

[image:5.612.56.279.371.533.2]

The MHD generator of alternating current was proposed by author of this project in 2010 [4]. The design arrangement of alternating current generator is shown at Fig. 5.

Fig. 5

The working chamber of MHD generator has the form of conic ring channel, which is being formed with the help of two cones. The external cone is made out of heat-resistant material, for instance, out of ceramics.

It is jointed with input pipe 3, along which the gas being ionized at fuel combustion is being conveyed into working channel 1. On the exit, the working channel is jointed with the output tube. After passing through the working channel 1, gas is being directed further for usage in the turbine complex of the thermal power plant. The internal cone 5 also is made out of heat-resistant material. At the side of gas flow input, the cone 5 is jointed with a hydraulic mouthpiece 6 and from the side of exit- with hydraulic mouthpiece 7.

The end mouthpieces 6 and 7 provide the necessary strength of the form-generating cone 5.

The installation and fixation of the internal cone 5 at entrance of the external cone 2 are being carried out with the help of holder 8, which fastens the end mouthpiece 6 in the pipe 3. On the exit side of gas , the holder 9, which is installed in the pipe 4, holds the end mouthpiece 7. The end mouthpieces 6 and 7 as well the holders 8 and 9 are being made out of heat-resistant material, for instance out of the ceramics. For the travelling magnetic field creation, the design, which is shown at Fig. 5, provides for the installation of three-phase AC winding 10 on the external side of the conic ring channel 1. The three-phase AC winding 10 is being formed by means of connection of the separate coils of cylindrical form, each of which has the different diameter.

The efficiency of MHD generator depends upon the magnitude of the travelling magnetic field induction. Therefore the coils of the three-phase winding 10 are being made out of superconducting wire, for instance, out of wire made on the basis of magnesium - boron and are being put in liquid nitrogen.

For the ensuring of the long-term reliable operation at the temperature of liquid nitrogen, the winding 10 is being located in cryostat 11.

The other way to increase the magnetic induction in the working channel 1 is magnetic screening. For this purpose, a ferromagnetic screen is being installed outside of the superconducting AC winding 10. The external ferromagnetic screen 12 has conic form.

For the decreasing of energy losses because of influence of the alternating magnetic field, the screen 12 is being made laminated i.e. just like the transformer core.

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The cryostat 11, in which the superconducting AC winding is installed, is being made in form of Dewar vessel with vacuum screening isolation of the external surface 12, which provides minimal heat gain in the zone of the winding location.

The internal cavity of the cryostat 11 has form of cone and its external cavity has form of cylinder. The cryostat 11 is performed as a dividable assembly. In its metal structures, the alternating magnetic field induces eddy currents. In order to avoid the energy losses and screening of the magnetic field, the internal conic surface of the cryostat 20 is being made out of nonmetallic material, for instance, out of ceramics. The liquid nitrogen, which serves as a cooling agent, is being poured into the end chamber of the cryostat 22 into the axial cylindrical chamber 22. The chambers 21 and 22 are the cryostat screens against external heat gains. The feed of liquid nitrogen out of chamber 22 into the superconducting winding 10 is performed with the help of vacuum pipes 23, which pass through inside the cylindrical holder of the external ferromagnetic screen 24.The liquid nitrogen enters in the chamber 22, which is formed up by two cylindrical gaskets 16. In the chamber the liquid nitrogen cools the coil surfaces of the three-phase winding and the ferromagnetic screens 12 and 16.The gaseous nitrogen, which arises at boiling enters into the chamber of gas collection, which is located in the end part of the cryostat 26.

At operation of the generator , the energy losses because of the alternating magnetic field arises also in the internal ferromagnetic screen 13. Therefore, the design arrangement provides for water cooling of the screen 13. The cooling water is being supplied through the holder 9 into the conic chamber 27, which is located on the internal surface of the ferromagnetic screen 13.

[image:6.612.58.280.570.691.2]

The structural electric schema of MHD generator connection into energy system is shown at Fig. 6.

Fig. 6

The MHD generator, which is shown at Fig. 6 has too three-phase AC windings 2 and 3. The first winding 2 is being connected to buses of the energy system 8 via static frequency converter 4 and transformer 5. The second winding 3 is connected via static frequency converter 6 and transformer 7. The first winding 2 is fed by the alternating current of frequency f1 and the second winding 2 is fed by current of frequency f2. The frequencies f1 and f2 are being chosen so, that to provide for minimal sliding in each of separate winding 2 and 3.

The separation of the MHD generator winding makes it possible to significantly decrease the losses because of eddy currents in plasma and to increase the efficiency of the generator.

At starting of the generator the liquid nitrogen is supplied into the cryostat chamber 21,22 (Fig.

5).

After passage through the channel 23, the liquid nitrogen fills the chambers 25 and the coils 10 of the superconducting winding are cooled down. Simultaneously, the external ferromagnetic screen 12 also is being cooled down. At achievement the temperature of liquid nitrogen, the AC windings 2 and 3 (Fig.6) are beingconnected to theenergy system 8 with the help of frequency converters 4 and 6.

The voltage is being applied to the windings 2 and 3 and therefore the electrical current emerges, which is necessary for creation of traveling magnetic field along the working channel.

The MHD generator is being switched over to operating mode with the consumption of the reactive current, similarly to idling mode of asynchronous motor.

Through the input tube 3 (Fig.1), the hot gas - the electroconductive working substance in plasma state is being supplied into the working channel 1. At the movement of plasma in the traveling magnetic field , an emf and the active current arise. At braking of plasma, the mechanical energy is being transformed into electric energy and is being passed into energetic system 8. The gas, which had passed through the working channel of the MHD generator, enters into turbine cycle for the subsequent usage at an electric power plant.

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The working channel was divided (over the length) in separate parts and on the each part the energy parameters were determined. The total magnitudes of voltage and power on the winding ends are were determined by way of summation. At the calculation, the calculation techniques of conduction MHD generator and plasmatron, were also used[6], [3].

The magnetic field calculation in windings and in magnetic circuits of MHD generator is carried out by the finite-elements method. The energy transformation process in a MHD generator can be as a first approximation described with the help of asynchronous generator equations [7]. In this case the electromagnetic power being transferred from the plasma stream to the winding can be presented as

= p S l τ Δ /ρ

Where p - the number of pole pairs of the winding

S – the sliding of plasma stream in relation to the traveling magnetic field

- the linear speed of plasma stream

- the linear speed of traveling magnetic field, which

is related with geometrical sizes and frequency f by the relationship.:

= 2 τ p f

- maximal induction of the traveling magnetic field

l - length of the conic plasma channel τ – pole pitch of winding

Δ - radial height of the conic plasma channel ρ – resistivity of the plasma stream

Below an example of the MHDG possible implementation is given , where the MHDG is a part of power generating unit of a regional thermal power plant with 400 MW gas and steam turbines.

An AC MHD generator is installed at the TPS (thermal-power station) in accordance with schema, which is shown at Fig. 4. Its design arrangement is shown at Fig.5.

Connection to power supply system is carried out in accordance with schema, which is shown at Fig. 6.

The main data of the MHD generator with two windings, which is implemented in accordance with schema shown at Fig 6:

Geometrical sizes:

Input diameter of cone = 2,2 m

Output diameter of cone = 3,3 m

Length of the working channel l = 6 m Height of the working channel Δ = 0,1 m Pole pitch of the winding τ = 1,5 m

Initial data

The speed of the working substance on input

V1 = 300 m/s

The speed of the working substance on exit V2 = 100 m/s

The current frequency in the first winding

f 1 = 75 Hz

The current frequency in the second winding f 2 = 40 Hz

Magnetic induction of the traveling magnetic field Bm = 2,0 T

The calculated electric parameters of MHD generator The active power of the first winding

P1 = 130 MW

The active power of the second winding

P2 = 60 MW

Total power delivered by MHG generator

P1 + P2 = 190 MW

Voltage at the output of the windings

U1f = 6,3 kV U2f = 3,0 kV

The current in windings: I1 = 13 kA I2 = 13 kA

Power factor cos 1 = 0,45 cos 2 = 0,35

Reactive power consumed from the power supply system S = 225MVAR

The calculation of the superconducting AC winding

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Fig. 7

The end 6 of the first coil 1 is connected with the end 7 of second coil 2 by means of jumper 3.

As a result , at current flow the polar system with the beginning 4 and the end 5 is formed.

At Fig. 4 also the internal section of the coil is shown. The winding is manufactured out of the high high-temperature superconductor, produced by «American Semiconductor», which characteristics are given in [8].

Elementary conductors 8 are being gathered in parallel wires 9 by means of transposition „Rebel”, using the technology developed by KIT( Karlsruhe Institute of Technology) [9]

After forming and heat treatment in a press mold, the coils are coated by insulation layer of 3 mm thickness. In this calculation the size of the elementary conductor is 4,0 х 0,2 mm. The transposed wire 9 has sizes 10,0 х 10,0 mm. (2 conductors by width and 33 conductors by height ). In all in the coil there are 12 transposed parallel wires. The coil size: B= 26 cm, Н=66 mm. Current density j = 11 A/mm². After calculation of magnetic field, being created by the current I = 13 KA, the distribution pattern of the magnetic field in the winding and in the magnetic circuit was constructed. It was found, that the maximum value of magnetic induction in the magnetic circuit is 1,5 T.

In the working channel of the MHDG the maximal induction reaches the calculated value Bm = 2,0 T

In the winding: the longitudinal component of the magnetic induction parallel to the elementary conductor - = 0,4 T and the transverse component

perpendicular to the elementary conductor - = 0,2 T

As it can seen from the wire characteristics [8] at temperature 77 K° and at mean value of

current in the elementary conductor 16А (13 % of the rated value),the wire keeps the state of superconductivity.

The calculation of electric losses in the superconducting winding of the MHDG was carried out according to technique being stated in [10] for each winding separately, taking in account the different frequencies of the traveling magnetic field.

Primarily the losses in the winding at the length of 1 m were determined , which afterwards are totalized .

For hysteresis losses estimation the factor (B), calculated on basis of Been‟s model, was used. The cooperative losses, windage losses and the losses because of penetration were determined by means of Wilson formula [11].

AC losses on the length 1 meter of the winding were  Р1= 60 W

The total losses in the superconducting winding have made up  Р = 110 kW

In conclusion let us consider one more example of the possible future application of the MHD generator under consideration. The matter is the application of MHD generator at energy production at thermonuclear power plant.

At Fig. 8 the schema of the thermonuclear power plant, at which the reactor “Tokomak “ is installed, is shown.

Fig. 8

The thermal energy, emitted in circular channel, is being transferred to blanket, cooled by gaseous helium. The gaseous helium, which is in plasma state, is sent to a heat exchanger .

The water vapor being obtained in the heat exchanger is processed in a turbine-generator set in accordance with the traditional condenser schema.

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REFERENCES

[1] B. Müller. Ultraheiße Energiefabriken Pictures of the Future.

Siemens, Frühjahr 2008

[2] Der Koloss von Irsching. Pictures of the Future. Siemens, Herbst 2007 S.54

[3] Урусов И.Д. МГД генераторы. Наука. Москва 1966

[4] Rubinraut A. Magnetohydrodynamischer

Wechselstromgenerator Gebrauchtmusterschrift DE 20 2010 011 194 U1.

[5] Том Р„ Таррл. Магнитные системы МГД генераторов.

Энергоатомиздат 1985.

[6] Коротеев А.С., Миронов В.М., Свирчук Ю.С.

Плазмотроны . Машиностроение 1983.

[7] Иванов-Смоленский А.В. Электрические машины. Энергия

1980.

[8] American Superconductor HTS Wire 2004

[9] KTI. Roebel Assembled coated Conductors (RACC) Innovation

2004

[10] Сверхпроводниковые электрические машины и магнитные

системы. МАИ, 1993