AU2012222562B2

AU2012222562B2 - Cooling device for cooling a superconductor, in particular in a magnetic resonance device or a rotor

Info

Publication number: AU2012222562B2
Application number: AU2012222562A
Authority: AU
Inventors: Michael Frank; Peter Kummeth; Heinz Schmidt; Peter Van Hasselt
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2011-03-02
Filing date: 2012-02-22
Publication date: 2016-02-25
Anticipated expiration: 2032-02-22
Also published as: ES2526542T3; EP2681575B1; AU2012222562A1; WO2012116911A1; DE102011004952A1; KR20140010391A; DE102011004952B4; EP2681575A1

Abstract

The invention relates to a cooling device (8, 25) for cooling a superconductor, in particular in a magnetic resonance device and/or a rotor (2) of a superconducting machine (1), wherein the superconductor and/or a component carrying the superconductor can be brought into contact with a liquid coolant (10, 27), wherein, in order to deliver the coolant (10, 27) to the superconductor and/or to circulate the coolant (10, 27) in a bath for the superconductor, use is made of a bubble pump (20, 36) arranged in a delivery line (18, 38).

Description

PCT/EP2012/052968 / 2010P20732WO 1 Description COOLING DEVICE FOR COOLING A SUPERCONDUCTOR, IN PARTICULAR IN A MAGNETIC RESONANCE DEVICE OR A ROTOR The invention relates to a cooling device for cooling a superconductor, in particular in a magnetic resonance device and/or a rotor of a superconducting machine, wherein the superconductor and/or a component supporting the superconductor can be brought into contact with a liquid coolant, to a magnetic resonance device and to a superconducting machine having such a cooling device, as well as to a method for delivering and/or circulating coolant for cooling a superconductor, in particular a superconducting winding. Superconductors, in particular the so-called high-temperature superconductors having a transition temperature above 77 K, are being increasingly used in a wide variety of applications. It is known for example to install superconducting windings, mainly of low-temperature superconductors, in magnetic resonance magnets, and, in particular high-temperature superconductors, in machines, i.e. generators and/or motors. It is necessary here to adhere strictly to the maximum permissible operating temperature of the superconductor in the respective case. For this purpose different cooling devices are known which use, as a coolant, cryogenic fluids such as helium, hydrogen, neon or nitrogen which provide the required cooling capacity when they evaporate. The evaporated coolant is returned either periodically by topping up or by PCT/EP2012/052968 / 2010P20732WO 2 condensation in a condenser chamber on one or more cold heads in an in particular closed coolant circuit. The problem may arise here that in some cases it is not practicable to feed the coolant directly to the superconductor over the shortest route. Thus, for example, magnetic resonance devices are known in which the superconducting magnetic resonance winding is kept in a bath of liquid coolant, in particular liquid helium, (so-called bath cooling of the superconductor). The level of said liquid coolant falls significantly over time due to evaporation, so that, prior to topping up, the upper part of the superconducting winding is exposed, i.e. is only surrounded by gas. This exposed part of the superconducting winding is then only cooled by thermal conduction from the cooled part of the winding located in the liquid coolant and may consequently have a higher temperature which limits the effective utilization of the superconductor, e.g. in respect of the permitted operating current. To cool superconducting machines in which, for example, a superconducting rotor winding may be fixed to a winding support inside the rotor, closed cooling circuits are mainly used in which, for example, neon gas or nitrogen as coolants are liquefied at a cold head with condenser in a closed system. From there the coolant flows into a hollow interior of the rotor, where it comes into contact with the thermally conductive winding support and evaporates there in order to achieve its cooling effect. This means that the interior of the rotor acts as an evaporator. The evaporated coolant is fed back to the condenser, where it is re-liquefied. In this process the so-called thermosiphon effect is utilized. The liquid coolant evaporates on the winding support and flows in PCT/EP2012/052968 / 2010P20732WO 3 gaseous form back to the condenser as a result of the pressure difference due to evaporation in the evaporator and condensation in the condenser chamber of the condenser. The term "heat pipe" is often also used to describe such coolant flows. With the known cooling devices, the liquid coolant is transported to the superconductor by gravity, which means that the condenser is disposed geodetically higher than the evaporator. This approach always proves difficult whenever tilting of the cooling device or rather of the superconducting synchronous machine can occur, as is possible, for example, with superconducting synchronous machines for marine applications, i.e. HTS motors or HTS generators, for example. In the case of seagoing vessels such as ships, tilting or inclination of the ship can easily occur. In addition to static tilting, known as "trim", dynamic tilting is also possible. The coolant may not then be able to reach the evaporator and produce its cooling effect. This results primarily in design limitations, particularly in fields of application where space is at a premium and does not allow a geodetically higher condenser with a cold head, which is usually connected to a compressor, to be provided. This applies not only to seagoing vessels or offshore platforms but also to other equipment subject to a height restriction, such as a railcar or the like. It is basically conceivable for cryogenic circulating pumps or feed pumps to be used. However, such pumps have a significant number of disadvantages. They have a short service life and are very prone to malfunction, and it is also extremely 4 difficult to maintain/repair them without interrupting operation. Moreover, it is extremely costly to implement complex moving components reliably at cryogenic temperatures. In addition, such a pump involves introducing heat and reduces the efficiency of a superconducting machine, so that a solution of this kind is to be avoided in practice. It is the object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages. A preferred embodiment provides a cooling device which allows coolant to be delivered and/or circulated against gravity without the disadvantages of an electrically operated pump. There is disclosed herein a method for cooling a superconductor, wherein the superconductor and/or a component supporting the superconductor is brought into contact with a liquid coolant, wherein the coolant is delivered to the superconductor by means of a bubble pump comprising a feed line and/or the coolant is circulated in a bath for the superconductor by means of the bubble pump, wherein the bubble pump includes a heater, wherein bubbles of gaseous coolant are produced in a defined manner by means of the heater, which bubbles convey the liquefied coolant in the feed line against gravity. It is therefore proposed according to the disclosure to use a bubble pump, the operating principle of which is known from coffee machines, for example, in a cryogenic environment in order to allow liquid coolant to be delivered by selectively evaporating a small part of the coolant which, as it rises, conveys liquid coolant periodically or quasi-continuously upward. The aim is therefore to apply the bubble pump principle to superconducting applications at cryogenic operating temperatures. This obviates the need for highly complex circulating pumps or feed pumps, since a bubble pump can be implemented at minimal cost and with very little effort, in particular with a very low power input. This provides an elegant means of delivering the liquid coolant against gravity, so that, in particular, it can also enable a condenser or rather the condenser chamber of the condenser to be disposed geodetically lower than the superconductor that is to be cooled, ultimately therefore lower than the evaporator. In a preferred embodiment of the invention it can be provided that the bubble pump comprises a heater for the defined production of bubbles of gaseous coolant which convey the liquefied coolant against gravity. A heater is therefore coupled to the feed line and is operated precisely in such a manner that a bubble filling up the cross-section of the feed line and buoying 5 up liquid coolant is produced which then rises against gravity in the feed line, entraining liquid coolant with it in the process. The heater can be operated periodically, for example, in order to produce a defined bubble size matched to the feed line by providing a defined thermal output. Even extremely low thermal outputs are completely adequate, so that the bubble pump can be implemented with little overhead and with scarcely no reduction in the efficiency of the cooling device or more specifically the application associated therewith. In particular, mechanical delivery as in the case of circulating pumps or feed pumps is unnecessary, so that feed sections in contact with the liquid coolant can be completely avoided. This therefore also provides mechanical simplicity. The heater, if it is located in the bath for the superconductor, can preferably include an insulating device that is embodied to thermally insulate the heater from the liquid coolant provided in the bath. For example, a vacuum flask or the like can be placed around the heater. An electrical heating means, for example, can be used as a heater. Preferably, the feed line runs substantially vertically at least in the region of bubble pump delivery. With a vertically running feed line, the maximum buoyant force of the generated bubbles is utilized, so that ideally a perpendicularly upward oriented feed section is provided. Preferably, a nonreturn valve is installed upstream of the bubble pump in the delivery direction. It has been shown that using an, in particular control-less, nonreturn valve of this kind again helps significantly to increase efficiency, since coolant is effectively prevented from flowing back into the bath or into a condenser chamber. Preferably, it can be provided that the superconductor that is to be cooled is a superconductor of a superconducting magnetic resonance winding and the liquid coolant not accommodating the entire winding in an evaporation chamber can be delivered by the bubble pump to the exposed part of the winding, in particular to an upper surface of the winding. In particular, an arrangement can be provided here in which a feed device is used which extends into the liquid coolant of the bath, in particular close to the bottom of the evaporation chamber, in particular the heater of the bubble pump also being disposed inside the liquid coolant bath. From there, liquid coolant can be conveyed upward through the feed line, i.e. out of the bath, to a feed device outlet opening extending outside the superconducting winding, where it reaches, e.g. drips onto, the exposed part of the superconducting winding. It should be noted that embodiments are conceivable here which only permit intermittent operation of the feed device for circulating the liquid coolant, e.g. if at least a predetermined part of the superconducting 6 winding is exposed, e.g. between top-ups. For this purpose a control device can be provided which either controls the bubble pump heater purely on a time basis or operates on the basis of data from a sensor, e.g. a fill level sensor. This means that, as long as the superconducting winding is completely submerged in the bath, no feeding with the aid of the feed device is necessary, but only when a section is exposed. On the other hand, however, the embodiment of the cooling device can also be selected such that, for example, the total amount of coolant used in the cooling device is reduced and essentially a part is exposed which is then nevertheless adequately cooled by the liquid coolant delivered by the bubble pump. It should finally be noted that it is self-evidently also possible to provide a plurality of such feed devices with bubble pumps in a cooling device or more specifically a magnetic resonance device of this kind. Alternatively, the cooling device can also be used as part of a superconducting machine, wherein it can be provided that the superconductor that is to be cooled is a superconductor of a superconducting rotor winding disposed in a rotor of a superconducting machine, wherein the coolant can be delivered from an in particular geodetically lower condenser chamber through the feed line into an interior space of the rotor acting as an evaporation chamber. Thus, in the context of superconducting machines, the present disclosure therefore enables coolant also to be fed "uphill", i.e. against gravity, at low cost and with little overhead, thereby overcoming the space and arrangement problems cited in the introduction, since the condenser's condenser chamber that is thermally coupled to a cold head can now be disposed geodetically lower than the superconductor that is actually to be cooled. In another preferred embodiment it can be provided that the coolant is conveyed in a closed cooling circuit, wherein the coolant which has been liquefied in a condenser chamber coupled to a cold head is delivered to the superconductor that is to be cooled in the interior of the rotor and conveyed back into the condenser chamber in gaseous form. A fundamentally known closed cooling circuit can therefore be used in which the inventively provided bubble pump can be beneficially integrated. In a preferred development of the present disclosure it can be provided that the feed line opens into a larger cross-section line segment leading into the interior of the rotor through a rotary feedthrough, in which line segment liquefied coolant can flow into the interior of the rotor, wherein gaseous coolant can simultaneously flow out of the interior of the rotor through another line segment. The advantage of such a line segment is that the bubbles of gaseous 7 coolant used for transportation also have sufficient space therein to flow into the interior of the rotor while the liquid coolant is flowing into the interior of the rotor. In particular, another line section for returning the gaseous coolant to the condenser can also be connected directly to the end of the line segment for the gaseous coolant at which the liquid coolant is supplied by the bubble pump. Using two line segments prevents liquid coolant from being sucked into the condenser chamber in the event of tilting. In addition to the cooling device, the present disclosure also relates to a magnetic resonance device, comprising a superconducting magnetic resonance winding and a cooling device, and to a superconducting machine, comprising a superconducting rotor winding disposed on a winding support in a rotor and a cooling device according to the invention. All the statements relating to the cooling device may be applied analogously to the magnetic resonance device and the superconducting machine, so that self-evidently the advantages achieved by the disclosure can be realized here also. A preferred embodiment of the present disclosure lastly also relates to a method for delivering and/or circulating coolant for cooling a superconductor, in particular a superconducting winding, wherein a bubble pump is used for delivering the coolant. As already explained in detail with reference to the cooling device according to the disclosure, a bubble pump is particularly suitable for delivering the liquid coolant against gravity in an easily implemented and efficient manner. The statements relating to the cooling device according to the disclosure ultimately also apply to the method according to the disclosure. In this method it can specifically be provided that bubbles of gaseous coolant are produced in a defined manner by a heater in the liquid coolant, said bubbles transporting the liquid coolant in an in particular substantially vertically running feed line, it being particularly advantageous if a nonreturn valve installed upstream of the heater is also used. It is conceivable, for example, that the heater, which can be implemented as an electrical heating means, is operated cyclically in order to produce, at regular/irregular intervals, bubbles of a particular buoyancy occupying the feed line which then transport liquid coolant in an upward direction through the feed line. In an alternative preferred embodiment, it can be provided that in the case of a superconducting winding located in a bath and containing the superconductor, coolant is 8 delivered by the bubble pump to an exposed part of the winding. However, if the superconductor is therefore disposed in a bath of liquid coolant, parts of the winding protrude from the liquid coolant, then the liquid coolant can be delivered to an exposed part of the winding by means of the bubble pump, e.g. by means of the feed device already described with reference to the cooling device. Here it is possible, particularly if coolant is only topped up periodically, to make the operation of the bubble pump or rather of the heating means dependent on whether a predetermined part of the winding is exposed, wherein, for example, time control can take place via a control device or the fill level of the bath can be measured, the bubble pump being activated by a control device from a particular threshold value onward. In a second alternative preferred embosiment of the method, it can be provided that in the case of a superconducting rotor winding disposed on a winding support in a rotor of a superconducting machine and comprising the superconductor, the coolant is delivered by the bubble pump against gravity to an interior space of the rotor acting as an evaporation chamber. Delivery against gravity is conceivable in this case, with the statements relating to the cooling device according to the disclosure being once again applicable to the method according to the invention. Further advantages and details of the invention will emerge from the preferred exemplary embodiments described below and with reference to the drawing, in which: Fig. 1 shows a superconducting machine having a cooling device according to the disclosure, and Fig. 2 shows a cooling device according to the disclosure for cooling a magnetic resonance winding. Fig. 1 shows a superconducting synchronous machine 1 according to the invention. It comprises a rotor 2 which is rotatable in a generally known manner inside a stator 3 having stator windings 4. The rotor 2, which is rotatable about an axis of rotation 5, comprises a superconducting winding 7 disposed on a thermally conductive winding support 6, said winding being made of a high-temperature superconductor. In order to keep the superconducting rotor winding 7 at its operating temperature, an inventive cooling device 8 is also provided which in this case conducts a coolant in a closed 9 cooling circuit. Neon or nitrogen, for example, can be used as the coolant. A hollow interior 9 of the rotor 2 is in this case used as an evaporation chamber into which liquid coolant 10 is introduced. In the evaporation chamber 9, the liquid coolant 10 is evaporated into gaseous coolant 11 and is [The next page is page 12] PCT/EP2012/052968 / 2010P20732WO 12 conveyed via a line segment 12 through which the gaseous coolant 11 is discharged from the interior 9 and which is disposed in parallel with a line segment 12' through which the liquid coolant 10 is conveyed into the interior 9, and also through another line section 13 to a condenser 14, a cold head 16 being thermally coupled to the condenser chamber 15 thereof. The cold head 16 can be operated via a compressor, for example. In the condenser chamber 15, the gaseous coolant 11 re condenses to liquid coolant 10 which is returned to the line segment 12 and from there to the interior 9 via a line section 17 and a feed line 18. The cooling device 8 is fixed except for the interior 9; the line segment 12 is coupled to the interior 9 by a rotary feedthrough 19 in a manner known from the prior art. In the superconducting synchronous machine 1, which can be operated as a generator or as a motor, the liquid coolant 10 is not transported by the force of gravity but via a bubble pump indicated in a general manner at 20. The feed line 18 is oriented vertically in this arrangement in order to utilize the maximum buoyant force of bubbles 21 (working gas bubbles) of gaseous coolant 11. The bubbles 21 transport liquid coolant 10 against gravity to the line segment 12. The bubbles 21 are produced by a heater 22, here an electric heating means which is coupled to the feed line 18 and produces bubbles 21 suitable for transporting the liquid coolant 10 by defined heat input to the liquid coolant 10. Here, for example, cyclical operation of the heater 22 can be provided.

PCT/EP2012/052968 / 2010P20732WO 13 Installed upstream of the heater 22 is a nonreturn valve 23 which increases the overall efficiency of the transportation by means of the bubbles 21. In this way it is possible, in a low-cost and easily implementable manner, to convey the liquid coolant 10 even against gravity, so that it presents no problem for the condenser chamber 15 to be disposed lower than the interior space 9. According to the invention, a bubble pump can also be particularly advantageously used in a magnetic resonance device, as Fig. 2 explains in greater detail. For the sake of simplicity, the magnetic resonance device is not shown in its entirety here, but only the superconducting magnetic resonance winding 24, which is made of a low-temperature superconductor, as well as the exemplary embodiment of an inventive cooling device 25 used there. It can be seen that the winding 24 is disposed in an evaporation chamber 26 which is typically located in a vacuum flask. For cooling, a liquid coolant 27, helium in this case, is used which evaporates on the winding 24, thereby providing its cooling effect. At particular time intervals, e.g. periodically or as a function of the measured values of a fill level sensor (not shown in greater detail here), liquid coolant 27 is topped up through a feed line 28. At this point, i.e. immediately after topping up, it can also be provided that the winding 24 is completely surrounded by the liquid coolant 27. However, the level of the liquid coolant 27 falls over time, so that the winding 24 is only PCT/EP2012/052968 / 2010P20732WO 14 partly surrounded by it. Higher temperatures can then occur on the upper, exposed part 29 of the winding 24. In order to counteract this, the cooling device 25 comprises a feed device 37 which is in turn fitted with a bubble pump 36. For this purpose a feed line 38 is provided via which liquid coolant 27 can be delivered through an orifice 29 onto an upper surface 30 of the winding 24. To deliver the coolant, bubbles 31 of gaseous coolant 32 are also used here which are produced by a heater 33 again embodied as an electric heating means. The heater 33 is positioned such that it is essentially located inside the liquid coolant 27, for which reason it is also surrounded by an insulating device 34 in the form of a thermal insulation, e.g. a vacuum chamber. In this case the heater 33 is again operated such that it produces bubbles 31 suitable for transporting the liquid coolant 27 through the vertical feed line 38. For example, cyclical operation of the heating means can be provided 33 for this purpose. It should be noted at this point that the feed device 37 need not be in continuous operation, but rather it can be provided that it is only activated if the level of the liquid coolant 27 falls below a particular value and/or a predetermined time has elapsed since the last top-up with liquid coolant 27. For this purpose a control device (not shown) can be provided which actuates the heater 33 accordingly. In addition, in the exemplary embodiment shown, a nonreturn valve 35 is also again provided upstream of the heater 33 in the delivery direction in order to improve the efficiency of delivery of the liquid coolant 27.

PCT/EP2012/052968 / 2010P20732WO 15 Thus, the inventively proposed use of a bubble pump 20, 36 also allows the liquid coolant 27 to be circulated and the winding 24 therefore to be more evenly cooled in a low-cost manner without using a circulating pump to mechanically deliver the coolant 27.

Claims

1. A method for cooling a superconductor, wherein the superconductor and/or a component supporting the superconductor is brought into contact with a liquid coolant, wherein the coolant is delivered to the superconductor by means of a bubble pump comprising a feed line and/or the coolant is circulated in a bath for the superconductor by means of the bubble pump, wherein the bubble pump includes a heater, wherein bubbles of gaseous coolant are produced in a defined manner by means of the heater, which bubbles convey the liquefied coolant in the feed line against gravity.

2. The method as claimed in claim 1, wherein the feed line runs substantially vertically at least in the region of delivery by means of the bubble pump.

3. The method as claimed in one of the preceding claims, wherein a nonreturn valve is installed upstream of the bubble pump in the delivery direction.

4. The method as claimed in any one of the preceding claims, wherein the superconductor that is to be cooled is a superconductor of a superconducting magnetic resonance winding and in that the bubble pump delivers a liquid coolant not accommodating the entire winding in an evaporation chamber to the exposed part of the winding, in particular to an upper surface of the winding.

5. The method as claimed in any one of claims 1 to 3, wherein the superconductor that is to be cooled is a superconductor of a superconducting rotor winding disposed in a rotor of a superconducting machine, wherein the coolant is delivered from an in particular geodetically lower condenser chamber through the feed line into an interior space of the rotor acting as an evaporation chamber.

6. The method as claimed in claim 5, wherein the coolant is conveyed in a closed cooling circuit, wherein the coolant is liquefied in the condenser chamber coupled to a cold head, then conveyed to the superconductor that is to be cooled in the interior of the rotor, and subsequently is conducted back to the condenser chamber in gaseous form.

7. The method as claimed in claim 5 or 6, wherein the feed line opens into a line segment of larger cross-section leading into the interior of the rotor through a rotary feedthrough, in which 17 line segment liquefied coolant flows into the interior of the rotor, wherein gaseous coolant flows in parallel out of the interior of the rotor through another line segment.

8. A magnetic resonance device, comprising a superconducting magnetic resonance winding as well as a cooling device which is embodied for cooling a superconductor of the magnetic resonance winding by means of a method as claimed in any one of claims 1 to 4.

9. A superconducting machine, comprising a superconducting rotor winding disposed on a winding support in a rotor as well as a cooling device which is embodied for cooling the superconductor by means of a method as claimed in any one of claims 1 to 3 or 5 to 7. Siemens Aktiengesellschaft Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON