Electrical machine with superconducting coils

Abstract

An electromechanical machine includes at least one coil made from a material that becomes electrically superconducting when its temperature is below a critical temperature. A functional part is contained in an internal volume of a thermally insulating and fluid-tight enclosure of the machine. A wall of the insulating enclosure is traversed in a fluid-tight fashion by at least one shaft for transmitting mechanical power between the functional part located in the internal volume of the insulating enclosure and a space outside the insulating enclosure. The functional part can be used as a heat sink, pre-cooled to maintain the temperature conditions for maintaining superconductivity inside the insulating enclosure.

Claims

1. Electromechanical machine comprising at least one part composed of a coil made from a material that becomes electrically superconducting when its temperature is below a critical temperature; a functional part of the electromechanical machine contained in an internal volume delimited by a wall of a thermally insulating and fluid-tight enclosure; and wherein the internal volume has a total capacity for storing energy in the form of heat, in response to a temperature change in the internal volume from a temperature of a cryogenic fluid to a temperature at most equal to the critical temperature, equal to or greater than a quantity of heat Emax introduced into the internal volume after the electromechanical machine is used for a predetermined duration and under operating conditions corresponding to an uninterrupted mission previously established as a worse-case mission with respect to the quantity of heat Emax.

2. Electromechanical machine according to claim 1, wherein the total capacity for storing energy in the form of heat comprises: a capacity for storing thermal energy in static form via accumulation of a quantity of heat in elements of the functional part of the electromechanical machine between the critical temperature of the superconducting material and the temperature of the cryogenic fluid used; a capacity for storing energy via a latent heat of vaporization of a quantity of the cryogenic fluid filling a reservoir; and the capacity for storing thermal energy in static form plus the capacity for storing energy via latent heat of vaporization of the cryogenic fluid representing at least the quantity of heat Emax.

3. Electromechanical machine according to claim 1, wherein materials composing the functional part are selected from materials having a high heat capacity, greater than 400 J/kg C., to form a heat sink accumulating at least a substantial part of the quantity of heat Emax.

4. Electromechanical machine according to claim 1, wherein the materials composing the functional part are selected from materials having a high heat capacity, greater than 800 J/kg C., to form a heat sink accumulating at least a substantial part of the quantity of heat Emax.

5. Electromechanical machine according to claim 3, wherein the materials having the high heat capacity are arranged and geometrically configured to promote heat exchanges between the materials and the internal volume.

6. Electromechanical machine according to claim 1, wherein the internal volume comprises a reservoir to store a cryogenic fluid in a liquid state, for a temperature lower than the critical temperature, that is non-insulated from the internal volume in terms of heat conduction.

7. Electromechanical machine according to claim 6, wherein the reservoir is formed by an internal separator determining, between the internal separator and an external separator of the wall, a volume of the reservoir, and determining, on a side of an internal surface, a smaller volume in which the functional part is located.

8. Electromechanical machine according to claim 1, wherein the wall of the thermally insulating and fluid-tight enclosure comprises openings to connect the internal volume with outside of the thermally insulating and fluid-tight enclosure, the openings comprising gates or valves to control a circulation of fluids through the openings.

9. Electromechanical machine according to claim 1, wherein the wall of the thermally insulating and fluid-tight enclosure comprises openings traversed in a fluid-tight fashion by at least one shaft to transmit mechanical power between the functional part located in the internal volume and a space outside the thermally insulating and fluid-tight enclosure.

10. Electromechanical machine according to claim 9, wherein said at least one mechanical shaft is made from a material having a thermal conductivity less than 25 W/m C.

11. Electromechanical machine according to claim 1, wherein the wall of the thermally insulating and fluid-tight enclosure comprises openings traversed in a fluid-tight fashion by conductive electric cables.

12. Electromechanical machine according to claim 11, wherein the conductive electric cables running through the wall of the thermally insulating and fluid-tight enclosure are made from a material having a thermal conductivity less than 25 W/m C.

13. Electromechanical machine according to claim 1, further comprising a control device to control and monitor a temperature of at least one of the internal volume and the coils made of the superconducting material, the control device comprising at least one temperature sensor attached to the functional part.

14. Aircraft comprising the electromechanical machine of claim 1.

15. Vehicle comprising the electromechanical machine of claim 1, the electromechanical machine being used as a propulsion engine of the vehicle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is described in reference to the figures which, in a non-limiting way, schematically represent:

(2) FIG. 1: a schematic sectional view of an electromechanical machine according to the invention;

(3) FIG. 2: a block diagram of the method for designing the electromechanical machine of the invention; and

(4) FIG. 3: a block diagram of a method for cooling the machine of the invention for purposes of a mission.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) The various components and elements of the electromechanical machine are not shown to scale.

(6) In FIG. 1, the accessory elements, mounts, electrical cables, sensors, etc., are not shown.

(7) The electromechanical machine 100, schematically illustrated in FIG. 1, comprises a functional part 10 and comprises a thermal control system 20 for regulating a temperature of said functional part.

(8) The functional part 10 performs the expected functions of the electromechanical machine 100, typically the functions of an electric motor and/or of an electric generator, in this case comprising a rotor 11.

(9) In its general principals and its structure, the functional part 10 is similar to that of the known electromechanical machines comprising a moving part, in this case a rotor 11, and a stator 12. It also comprises, in a known way, magnetic parts, for example magnets and/or parts made of magnetic materials, and comprises at least one electrical conductor, for example a coil made with an electrically conductive material.

(10) In the example illustrated in FIG. 1, a person skilled in the art will recognize a rotary machine, electric motor, or electric generator comprising stator coils 120 and rotor coils 110.

(11) This example is non-limiting, since any electromechanical machine comprising coils for creating magnetic fields may be used in the context of the present invention.

(12) The electrically conductive materials, in the case of the electromechanical machine 100, are superconducting materials whose electrical resistance becomes zero at a temperature below a critical temperature Tc characteristic of the material used.

(13) The superconducting material is for example a high-temperature superconducting material whose critical temperature is greater than or equal to the cryogenic vaporization temperature of a gas (at ordinary temperature) such as liquid diatomic nitrogen, 77 Kelvin at ordinary ambient pressure, liquid diatomic hydrogen, 20 Kelvin at ordinary ambient pressure, or liquid helium, about 4 Kelvin at ordinary ambient pressure.

(14) Furthermore, non-electric parts of the functional part 10, for example a magnetic mass of the rotor 11 or a cage of the stator 12, are made to create an accumulating heat sink with a desired capacity, the functions of which will be described below.

(15) Accordingly, the materials used to produce said non-electric parts are chosen, within the limits required for their mechanical properties, so as to have the highest possible specific heat capacities Cp.

(16) For example, the non-electric parts are made by incorporating ferrous materials (Cp of iron=460 J/Kg C.), aluminum (Cp of aluminum=890 J/Kg C.), boron (Cp=1300 J/Kg C.) or beryllium (Cp=1800 J/Kg C.).

(17) Firstly, these materials, or other materials having high specific heat capacities, are preferable to the polymer materials often used in electric motors and generators, and secondly, a sufficient mass of these materials must be incorporated in order to obtain the desired heat accumulation capacity.

(18) Such a result, which in theory is easy to obtain in the case of electromechanical machines that are high-powered, and hence of high mass, can also be achieved or approximated by incorporating into the internal volume 22 of the insulating enclosure 21 accessories such as reducers or mechanical motion converters which, due to the amounts of power to be transmitted by these reducers or converters, generally represent a mass of materials capable of accumulating energy in the form of heat that is sizeable relative to the mass of the electromechanical machine 100.

(19) The result of these constraints is that the architectural and design criteria taken into consideration by the person skilled in the art of electromechanical machine design are different in this case from those considered in ordinary design rules.

(20) The thermal control system 20 primarily comprises an insulating enclosure 21 for thermally insulating the functional part 10, a device for cooling an internal volume 22 of the insulating enclosure 21 and a system for controlling and monitoring the temperature in said insulating enclosure.

(21) The insulating enclosure 21 is primarily formed by a wall 23 surrounding the internal volume 22.

(22) This wall 23 is made to limit the flow of heat between the internal volume 22 of the insulating enclosure, at low temperature, for example at a temperature below 100 Kelvin, and a space outside the insulating enclosure which may be at temperatures on the order 400 Kelvin, or even higher in certain environments.

(23) This type of insulating enclosure is known, particularly in the field of cryostats or Dewar vases.

(24) In a known way, the wall 23 most often comprises several separators 230, 231, 232 apart from each other and delimiting spaces between them. The spaces between the separators determine separation volumes 221, 222.

(25) The most external separation volume 221, in which a partial gas gap is produced and/or which contains a thermal insulator, for example a silica aerogel, provides a first insulation.

(26) Openings 24, 25, 26, 27 of the wall, which are necessarily present in the wall, are fluid-tight so as to limit, as much as possible, the fluid exchanges between the inside of the insulating enclosure 21 and the outside.

(27) Such openings are disposed so as to provide access to parts inside said insulating enclosure from outside said insulating enclosure.

(28) In the case of the exemplary embodiment illustrated in FIG. 1, at least one mechanical power transmission shaft 111, for example connected to rotating parts of the electromechanical machine 100, runs through the insulating wall along with a bundle of electrical cables.

(29) In one embodiment, not shown, the electromechanical machine does not comprise a shaft running through the wall, and a mechanical transmission shaft entirely outside the insulating enclosure is driven in motion by a magnetic coupling with internal moving parts of the functional part inside the insulating enclosure.

(30) Preferably, all of the elements running through the wall 23 of the insulating enclosure and the separators 230, 231, 232 are made of materials chosen for their poor heat-conducting properties.

(31) The concept of poor heat-conducting materials is relative in this case insofar as functional criteria, for example mechanical strength for a shaft 111 of the electromechanical machine or electrical conductivity for a power supply cable or measurement sensor, must necessarily be taken into consideration.

(32) For example, a mechanical shaft is made from a titanium alloy whose thermal conductivity of around 20 W/m C. is lower than that of ordinary steel, which has a thermal conductivity at least twice as high, while having good mechanical strength, or an electrical cable, at least in its part running through the wall of the insulating enclosure, is made from an iron-nickel alloy with 36% nickel (such as Invar), which is also a poor heat conductor, for a metal, with a thermal conductivity of 13 W/m C., and whose electrical resistivity, although nearly five times that of copper, is not disadvantageous in a short length of cable.

(33) Other materials can be used as long as they have similar or better characteristics in terms of poor thermal conductivity, such as polymer matrix composites.

(34) The thermal control system 20 also comprises a heat exchanger incorporated into the insulating enclosure.

(35) The concept of a heat exchanger in this case should be considered in a broad sense. The heat exchanger in this case incorporates a set of elements and features of embodiment distributed throughout the electromechanical machine 100 which promote the transfer of heat between the various elements in said electromechanical machine.

(36) The heat exchanger comprises, in particular, openings 25, 26 disposed in the wall 23 of the insulating enclosure so as to enable the circulation of a fluid between the inside of said insulating enclosure and the outside, both in the direction of a filling of the internal volumes of said insulating enclosure and in the direction of the drainage of said volumes. Said openings are provided with sealing devices 251, 261, either controlled gate-type devices, or automatic valve-type devices. The passages 25, 26 and the sealing devices are made to limit the heat exchanges between the inside of the insulating enclosure and the outside, as mentioned above, particularly by using poor heat-conducting materials to produce them.

(37) In one embodiment, the heat exchanger makes use of geometric characteristics of the non-electric parts, made of materials chosen for their heat-accumulation properties, of the functional part 10 that promote heat exchanges inside the insulating enclosure 21.

(38) Such geometric characteristics consist for example in bores 112, 122 running through the non-electric parts so that a surface area of said non-electric parts in contact with the surrounding fluid is increased so as to promote heat exchanges.

(39) In one embodiment, an internal separator 232 encloses the functional part 10 in fluid-tight fashion inside a smaller volume 223 of the insulating enclosure 21.

(40) The wall of the internal separator 232 determines, with a more external separator 231 of the wall 23 of the insulating enclosure, a reservoir 222 surrounding the smaller volume 223 in which the functional part 10 is located. In this case, the internal separator 232 does not have any particular thermal insulation properties, since as much thermal transparency as possible is sought. The internal separator 232 is for example made of aluminum alloy.

(41) In this case, using a reservoir 222, at least one filling and/or drainage opening 25 connects to said reservoir, and said at least one opening, or at least one other opening connecting to said reservoir, is provided with a device, not shown, for regulating the pressure inside the reservoir 222 so as to evacuate a fluid located inside it with a pressure higher than a set pressure.

(42) In this case, according to an embodiment illustrated in FIG. 1, at least one filling and/or drainage opening 26 runs in fluid-tight fashion through the volume of the reservoir 222 and the separators 230, 231, 232 of the insulating wall 23 so as to connect to the smaller volume 223.

(43) The thermal control system 20 also comprises the gates or valves, measurement sensors, and electrical cables, not shown, necessary or useful to the operation and monitoring of said thermal control system and of the temperature of the functional part 10 of the electromechanical machine 100. As indicated above, all of the passages in the walls are fluid-tight and if necessary, thermally insulated.

(44) In a non-illustrated embodiment, the opening that connects to the reservoir 222 and is provided with a pressure regulating device also connects to the smaller volume 223. The cryogenic fluid released by the reservoir 222 is thus injected into the smaller volume 223 that encloses the functional part 10, which is cooled before said fluid itself is evacuated through an opening 26 of the insulating wall 23 connecting to said smaller volume.

(45) The electromechanical machine 100 and its structure, particularly the way in which its structural elements must be designed and produced, will be more clearly understood in the description of the principles implemented in an exemplary design, FIG. 2, of such an electromechanical machine and in the description of the operational implementation, FIG. 3, of such an electromechanical machine, which will be described in the context of an electromechanical machine of the electric generator type onboard an aircraft.

(46) In addition to the performance customarily expected of an electromechanical machine intended for a specific use, a person skilled in the art in charge of designing an electromechanical machine implementing the principles of the invention will establish, in a first phase 210, in accordance with the various potential missions for the aircraft, the maximum duration of continuous operation of the electromechanical generator.

(47) This maximum duration of continuous operation is, in practice, the maximum possible duration of a mission of the aircraft, including reserves, which is known, for example 6 hours of mission, and takes into account a safety coefficient, for example 20%, or 7.2 hours of continuous operation.

(48) Based on the available technologies in the field of superconducting materials, the person skilled in the art will then, in a second phase 220, determine the maximum temperature Tmax that the electric generator must not exceed during the maximum duration of the mission including the margin, 7.2 hours in the example, in order to remain operational throughout the mission.

(49) This temperature is for example 75 Kelvin, for a high-temperature superconducting material having a critical temperature Tc at least slightly higher than that value.

(50) In a third phase 230, the quantity of energy in the form of heat that will be supplied to the electromechanical machine during the previously established duration is determined.

(51) This thermal balance takes into account a flow of heat from the outside which will reheat the functional part 10, which flow of heat is a function of the performance of the thermal insulation provided by the insulating enclosure 21, an exterior temperature, and the temperature actually maintained in said insulating enclosure.

(52) This thermal balance also takes into account the heat generated by the functional part 10 inside the insulating enclosure 21. In essence, even though the electrically conductive elements are superconducting under the temperature conditions maintained in said insulating enclosure, the operation of the electromechanical machine 100 dissipates internal energy in the form of hysteresis losses in the magnetic parts, creating a heat supply, which will be determined for a worst-case mission profile based on the criteria of said heat supply.

(53) The maximum amount of energy Emax (Joule) that the electromechanical machine will receive in the event of the worst-case mission is then deduced from the thermal balance.

(54) In a fourth phase 240, taking into account an initial temperature Tmin, for example the temperature of liquid nitrogen at ambient atmospheric pressure at ground level, inside the insulating enclosure 21, and the maximum admissible temperature Tmax, a total static heat capacity CCs (Joule/ C.) of the elements inside the insulating enclosure, i.e. apart from material phase changes, is determined.

(55) It will be noted that the total static heat capacity CCs in this case is primarily supplied by structures of the functional part 10.

(56) It is possible, by adapting the static heat capacity CCs via adjustments in the dimensions of the elements of the functional part 10, for the electromechanical machine 100 to be capable of performing the mission without having its internal temperature exceed the maximum temperature Tmax, if:
CCs(TmaxTmin)>=Emax

(57) It is then verified in step 250 whether or not this condition is fulfilled.

(58) If this condition is fulfilled, the electromechanical machine 100 will, in theory, have only one way to maintain cold, via a static accumulation of cold, and the essential features of the electromechanical machine, for the thermal control functions, are defined in this phase.

(59) If this condition is not fulfilled, the quantity of cryogenic liquid which must be eliminated in order to compensate for the difference between Emax and the term CCs(TmaxTmin), firstly by raising the temperature of said cryogenic liquid to a boiling point and secondly via the change of said cryogenic liquid from the liquid phase to the vapor phase, will be determined in a fifth phase 260. The cryogenic liquid in this case is chosen so as to have a boiling point lower than the critical temperature Tc.

(60) In the case of liquid nitrogen at the atmospheric pressure of 101325 pa, the latent heat of vaporization is around 200 kJ/kg.

(61) The quantity of cryogenic liquid required in this case will determine the volume of the reservoir 222.

(62) It is clear that the design cycle just described in simplified fashion will be conducted by the person skilled in the art in a series of iterations, given that the thermal design process is not analytical and requires that intermediate results be applied to the initial hypotheses in order to converge on a final result.

(63) Despite the complexity introduced by the need to maintain the functional part 10 at a low temperature, the electromechanical machine 100 is lighter and of smaller dimensions than a conventional electromechanical machine with the same electrical and/or mechanical performance, particularly due to the use of coils made of superconducting materials, which enables currents to pass through the wires of the coils without overheating.

(64) Furthermore, the thermal control system 20 that maintains the conductive elements at the cryogenic temperature is totally static.

(65) When applied to the functional part 10 as a whole, the thermal control system 20 is much simpler, lighter, and more reliable than in the known cryogenic systems for cooling the electrically conductive parts.

(66) This result is obtained by means of a specific implementation of the electromechanical machine 100.

(67) When the electromechanical machine 100 must be used, it is cooled prior to performing the mission, for example of the aircraft in which it is installed.

(68) In a first step 310, a source of cryogenic liquid, for example liquid nitrogen at the temperature of 77 Kelvin or less, for an external cooling system is connected to a filling opening 25, 26 of the insulating enclosure, and if necessary, a cryogenic liquid recovery unit is connected to a drainage opening. In the case where the insulating enclosure 21 comprises a smaller volume 223, the openings 26 connecting to said smaller volume are connected first 320.

(69) In a second step 330, cryogenic liquid is delivered through the filling opening into the smaller volume 223, or into the internal volume of the insulating enclosure 21 if it does not include a smaller volume, in order to fill said smaller volume, or said internal volume, and to immerse the functional part 10 located inside it. During this second step, the quantity of cryogenic liquid is continuously adjusted, as necessary, in order to compensate for an evaporation of said cryogenic liquid.

(70) It will be noted that in this second step, the geometric shapes chosen for the elements of the functional part 10 used as a heat sink will provide an increased surface area of contact between said elements and the cryogenic liquid, which has the effect of accelerating the temperature adjustment of said functional part.

(71) When the temperature of the functional part 10 is lowered and stabilized at the temperature of the cryogenic liquid, which is for example monitored by temperature sensors permanently installed in the intermediate enclosure, which sensors are connected during this step to the cooling system, the cryogenic liquid is drained 340 out of the intermediate enclosure which, if necessary, is emptied without having its temperature increased by this operation.

(72) In a third step 321 when the internal volume 22 of the insulating enclosure 21 includes a smaller volume 223, which can be performed simultaneously with the step 320, the source of cryogenic liquid, for example liquid nitrogen at the temperature of 77 Kelvin or less, the external cooling system is connected to a filling opening of the insulating enclosure, and if necessary, a cryogenic liquid recovery unit is connected to a drainage opening, for the openings 25 connecting to the reservoir 222.

(73) In a fourth step 331 the reservoir 222 is filled with cryogenic liquid.

(74) Preferably, the external cooling system is kept connected so as to maintain the cryogenic liquid at a desired level as long as possible, and is not disconnected until just prior to the start of the mission.

(75) It should be noted that the presence of the reservoir 222 surrounding the smaller volume 223, in which the previously or simultaneously cooled functional part 10 is located, ensures that said smaller volume and the functional part 10 are maintained at the temperature of the cryogenic liquid until the start of the mission.

(76) When the external cooling device is disconnected 350, the electromechanical machine 100 becomes autonomous in terms of controlling its temperature and capable of maintaining the internal temperature in the insulating enclosure 21 below the critical temperature Tc for a maximum duration corresponding to the capacity for storing cold defined during the design of said electromechanical machine.

(77) As the mission progresses, the heat produced by the functional part 10, minimized by the use of superconductors, and that derived from the flow of heat linked to the temperature difference between the inside and the outside of the insulating enclosure 21 cause the temperature inside the enclosure to increase to the boiling temperature of the cryogenic liquid, after which the temperature is held constant at said boiling temperature during an evaporation phase of the cryogenic liquid, and finally, the temperature gradually increases from the boiling temperature to the ambient temperature. The projected maximum operating temperature must not be reached during the mission. The vapors caused by the boiling of the cryogenic liquid are evacuated by the pressure regulating device.

(78) When the mission has ended, the electromechanical machine 100 is again cooled and/or the quantity of cryogenic liquid is topped off for a new mission.

(79) Advantageously, when a vehicle, for example an aircraft, comprises a plurality of electrical machines of the invention, said vehicle includes a cryogenic fluid distribution system to which the external cooling device is connected, and a centralized monitoring of the temperatures of the various electric machines connected to this system is also performed.

(80) It is clear from the provided examples of the production, design, and use of an electromechanical machine according to the invention that it is subject to variants while remaining within the general principles of the invention.

(81) In particular, the structure of the insulating enclosure, the shape and arrangement of a cryogenic reservoir incorporated into the insulating enclosure, the cooling and cryogenic liquid filling means, and the control and monitoring devices may take various forms while performing the same functions as those described.

(82) Likewise, there may be any number of cryogenic coils of the functional part, or any number of mechanical parts independent of the functional part, in the same insulating enclosure.

(83) The person skilled in the art will also be capable of selecting materials and parameters such as the type and temperature of the cryogenic fluid based on the specific requirements specific of the individual case. This, the cryogenic fluid may be nitrogen, hydrogen, or helium depending on the requirements linked to the critical temperature of the superconducting material used.

(84) This results in an electromechanical machine 100 that benefits from the advantages of superconducting materials without the disadvantage of complex cooling units, thanks to a cold-containment enclosure containing the entire functional part 10 of said electromechanical machine used as a heat sink, and to the transfer of the complex cold-production means to offboard systems.

(85) Such a machine is for example an electric generator whose shaft 111 for driving the moving parts is connected to an external mechanical power source of a propulsion engine or a gas generator of an auxiliary power unit.

(86) Such a machine is for example an electric motor of an actuator or an electric propulsion engine of a vehicle.