Field of the invention

The present invention relates to a superconducting cable, in particular to a

superconducting cable arrangement comprising at least one superconducting cable extending through a primary cooling duct for pumping a refrigerant or cryogenic medium there through. In a further aspect the present invention relates to a method of cooling a superconducting cable from a non-superconducting state to a superconducting state. Background art

US patent application US 2014/0051582 discloses an arrangement with at least one superconductive cable and a first cryostat surrounding the cable. A second cryostat is provided around the first cryostat coaxially therewith and at a distance therefrom. The second cryostat is composed of two coaxially arranged pipes at a distance from each other, wherein thermal insulation is enclosed between the pipes, and wherein during operation of the arrangement a liquefied gas is forced through the second cryostat.

Chinese patent application CN 101699570 discloses a liquid nitrogen superconducting circuit, comprising a concentric arrangement of a wire extending through a tube transporting liquid nitrogen, which, in turn, extends through a composite outer tube. A continuous liquid nitrogen chamber is formed between the wire and the liquid nitrogen tube. The wire is suspended by a wire holder in the middle of the nitrogen tube and the liquid nitrogen tube is suspended in the centre of the composite outer tube by a liquid nitrogen tube holder. A continuous vacuum chamber is formed between the outer tube and liquid nitrogen tube. The composite outer tube may be composed of a high-strength ductile material forming a support inner layer, an insulating layer and a protective outer layer.

Summary of the invention

The present invention seeks to provide a superconducting cable arrangement that allows for faster cooling of a superconducting cable toward a superconducting state whilst keeping thermally induced stresses within the superconducting cable to a minimum for avoiding damaging the cable as a result of, e.g., intense phase changes of cooling mediums used during a cool-down process. The superconducting cable arrangement further provides reduced design complexity and allows for smaller dimensions (e.g. diameters) of the cable arrangement and ensures that fast cool-down times can be achieved consistently and safely.

According to the present invention, a superconducting cable arrangement of the type defined in the preamble is provided comprising a superconducting cable extending through a primary channel of a primary tubular member configured to transport a first refrigerant, and a secondary tubular member comprising a secondary channel configured to transport a second refrigerant, wherein the secondary tubular member is eccentrically, i.e. non-concentrically or asymmetrically, arranged with respect to a central axis of the primary tubular member. For cooling down the superconducting cable from a non-superconducting state to a superconducting state, a first refrigerant may flow through the primary channel as a result of which a temperature gradient or "cold front" develops within the superconducting cable moving in a flow direction of the first refrigerant from a start point of the superconducting cable to an end point thereof. This temperature gradient or "cold front" develops due to the relatively high thermal mass of the superconducting cable providing "inertia" against temperature differences.

As the first refrigerant flows through the primary channel, the second refrigerant flows through the secondary channel of the secondary tubular member, wherein the first refrigerant may be the same as the second refrigerant, but this is not required. According to the present invention the eccentrically arranged secondary tubular member allows the primary tubular member to attain and maintain a lower temperature much faster during the cool-down process, i.e. the pre-cooling stage, along its length. Because of the additional cooling provided by the secondary tubular member, the warmer end of the superconducting cable toward the end point cools in accelerated and even fashion, thereby reducing the temperature gradient within the superconducting cable and in turn the thermally induced stresses due to cable shrinkage. Therefore, damage to the superconducting cable is avoided by the even yet fast pre-cooling stage.

In an embodiment, the secondary tubular member is thermally connected to an inner surface or an outer surface of the primary tubular member along a length thereof, e.g. its entire length. Through this embodiment in conjunction with the eccentric arrangement of the secondary tubular member, an eccentric/asymmetric double tubular arrangement of the primary and secondary tubular member is obtained and formed as a single piece or unitary component. This eccentric double tubular arrangement does not require spacers for providing the secondary channel per se, but, if needed, a spacer may be used for preventing direct contact between the secondary tubular member and the superconducting cable such that excessive localised cooling of the cable is avoided. This embodiment further allows for a continuous thermal exchange surface between the primary and secondary tubular member, i.e. between the primary and secondary channels thereof, in lengthwise fashion such that the secondary channel exhibits no constrictions providing a reduced flow resistance to the second refrigerant.

In a further aspect the present invention relates to a method for cooling a superconducting cable to a superconducting state, wherein the superconducting cable extends through a primary channel of a primary tubular member configured to transport a first refrigerant, and wherein a secondary tubular member is provided comprising a secondary channel configured to transport a second refrigerant, wherein the secondary tubular member extends through or encloses at least in part the primary tubular member, the method comprising the steps of:

providing the superconducting cable in a non-superconducting state;

selecting the second refrigerant as a liquid refrigerant and pumping the second refrigerant through the secondary channel;

waiting until the superconducting cable reaches a temperature of 200 Kelvin or lower, and then selecting the first refrigerant as a liquid first refrigerant and pumping the liquid first refrigerant through the primary channel, and allowing the superconducting cable to reach a superconducting state.

The method of the present invention provides a pre-cooling phase/stage by utilizing the secondary tubular member and pumping/injecting a liquid second refrigerant, such as liquid nitrogen, into the secondary channel without causing an intense phase change (e.g. liquid to gas expansion, boiling etc.) of a liquid refrigerant directly along an outer surface of the

superconducting cable during the pre-cooling phase. This avoids excessive (local) shrinkage of the cable and resulting tension forces therein. Because the second refrigerant is a liquid refrigerant an accelerated yet equable and even cool-down of the superconducting cable along its length is possible, particularly the warmer end thereof. As the liquid second refrigerant facilitates pre-cooling of the superconducting cable, it is possible to introduce a liquid first refrigerant (e.g. liquid nitrogen) into the primary channel sooner during the pre-cooling phase, wherein the liquid first refrigerant can be injected into the primary channel well before a steady superconducting state of the cable has been reached.

Note that the secondary tubular member may but need not be eccentrically arranged, i.e. non-concentrically or asymmetrically, with respect to a central axis of the primary tubular member as mentioned above in light of the superconducting cable arrangement.

Depending on a sufficiently low temperature of the superconducting cable, i.e. 200 Kelvin or lower, it is possible to introduce a liquid first refrigerant into the primary channel and prevent excessive boiling of the liquid first refrigerant along an outer surface of the superconducting cable. The temperature of 200 K or lower can be seen as a (switching) temperature being indicative of the possibility to safely switch to a liquid first refrigerant. When such a switch to the liquid first refrigerant can be made at a particular temperature of 200 K or lower depends on, e.g., materials used for the superconducting cable. So given a particular superconducting cable it is possible to determine at which pre-cooling temperature a liquid first refrigerant can be pumped into the primary tubular member for bringing the cable to a steady superconducting state without damaging it.

Therefore, according to the method of the present invention the secondary tubular member and the second refrigerant flowing there through yields a significant reduction of the cool- down time yet prevent damaging the superconducting cable. Consequently, a surprising but effective solution is provided for avoiding excessive long cool-down times associated with presently known cool-down methods for superconducting cables, where cool-down times of days instead of weeks are now possible.

Short description of drawings

The present invention is discussed in more detail below, with reference to the attached drawings, in which:

Figure 1 shows an embodiment of a superconducting cable arrangement providing improved convective cooling of a superconducting cable according to the present invention; Figure 2 shows an exemplary embodiment of an eccentrically arranged secondary tubular member for a superconducting cable arrangement according to the present invention; and

Figure 3 shows another exemplary embodiment of a secondary tubular member for a superconducting cable arrangement according to the present invention; and

Figure 4 shows an embodiment of a primary tubular member as a double tubular arrangement for a superconducting cable arrangement according to the present invention.

Description of embodiments

In the present state of the art cooling down a superconducting cable is based on utilizing a refrigerant in heat exchange with the cable. In a typical scenario the refrigerant is a cold gas (e.g. nitrogen gas) during an early stage of the cooling process, where at some point in time the refrigerant is changed to a cold liquid (e.g. liquid nitrogen). This process may be carefully and gradually executed, so that this cooling technique provides equable and controlled cooling of the superconducting cable that does not cause damage to the cable by thermally induced shrinkage nor by forces on the cable as a result of high refrigerant flow rates there along. However, cooling down the superconducting cable through such conventional means is rather slow and may take weeks.

For demonstration purposes and technical feasibility studies of superconducting cables, such a slow cool-down process need not be an immediate problem, but for most intended users of superconducting cables the availability of the system is severely hindered by such a slow process and as such the cool-down process should be significantly shorter, e.g. days instead of weeks. For example, commercial and permanent deployment of superconducting cables in transmission and/or distribution networks require much shorter cool-down times to minimize network interruption or network unavailability due to maintenance or repair schedules, particularly when there is no or insufficient redundancy in the network.

From the above considerations there is a need for a superconducting cable arrangement that provides much shorter cool-down times than cool-down times that are possible with conventional superconducting cable arrangements.

In general, according to the present invention a faster and accelerated cool-down process for superconducting cables is achieved by a primary tube through which the superconducting cable extends, and whereby a secondary tube is provided and eccentrically arranged with respect to a central (longitudinal) axis of the primary tube. The primary and secondary tube each comprise or define a primary and secondary channel, respectively, configured to transport a suitable first and second refrigerant respectively, e.g. a cryogenic medium. The secondary tube typically exhibits a low thermal mass and as such exhibits a cool-down time which is considerably shorter than the cool-down time of a superconducting cable. The main cooling principles made possible by the superconducting cable arrangement of the present invention are based on either improved convective cooling by the secondary tube of the superconducting cable, and/or wherein the superconducting cable arrangement further improves radiation assisted cooling, e.g. radiative cooling, by cooling down the superconducting cable by absorbing heat radiation emitted by the superconducting cable through the secondary tube (i.e. second refrigerant). In exemplary embodiments the secondary tube extends through the primary tube or encloses at least in part the primary tube.

For further details on the superconducting cable arrangement reference is made to Figure 1 showing a first embodiment of a superconducting cable arrangement 1 providing improved cooling of a superconducting cable 2 according to the present invention. In the embodiment shown, the superconducting cable arrangement 1 , hereinafter "cable arrangement", comprises a superconducting cable 2, hereinafter "cable", extending through a primary channel 3 of a primary tubular member 4 configured to transport a first refrigerant, e.g. a first cryogenic medium (gas, liquid). In a group of embodiments, the cable 2 may be seen as flexible wire-like superconductor or an elongated rigid bar/rod shaped superconductor (e.g. "busbar") Hence, without loss of generality, the superconducting cable 2 as used in the present invention may be considered to be a flexible and/or rigid elongated superconducting element.

The cable arrangement 1 further comprises a secondary tubular member 6 comprising a secondary channel 5 which is configured to transport a second refrigerant, e.g. a second cryogenic medium (e.g. gas, liquid). The secondary tubular member 6 is eccentrically arranged with respect to a central axis "C" of the primary tubular member 4, allowing the secondary tubular member 6 to be arranged in relative close proximity to the superconducting cable 2 for improved convective and/or radiative heat exchange therewith.

In the exemplary embodiment shown in Figure 1 , the secondary tubular member 6 extends through the primary tubular member 4 in eccentric or non-concentric fashion as mentioned above. This allows for further improved pre-cooling of the superconducting cable 2.

When cooling the cable 2 from a non-superconducting state to a superconducting state, the second refrigerant is pumped through the secondary channel 5 that cools down the secondary tubular member 6. In case a first refrigerant is not pumped through the primary channel 3, then radiation from the cable 2 is absorbed by the secondary tubular member 6 and heat is drained off by the second refrigerant. If a first refrigerant is in fact pumped, e.g. in gaseous form, through the primary channel 3 while the secondary tubular member 6 is already cooled down, then as a result convection will occur in the primary channel 3 where heat from the cable 2 is transferred to the secondary tubular member 6 and subsequently removed through the secondary channel 5 and second refrigerant. Therefore, the cable 2 is cooled down in a fast and far more homogeneous fashion along the cable 2 than with a conventional cold front moving along the cable 2. That is, after the cable 2 is sufficiently pre-cooled by the second refrigerant, a colder (normally liquid) first refrigerant can be safely pumped into the primary channel 3 to replace the gas and further cool the pre-cooled cable 2. From this point on a temperature gradient or "cold front" develops within the cable 2, but this temperature gradient is reduced and causes less stress along and within the cable 2, thereby preventing e.g. intense boiling of a liquid first refrigerant as it absorbs heat from the cable 2.

According to the present invention, the secondary tubular member 6 allows the primary tubular member 4 to attain a lower (cryogenic) temperature much faster during a cool-down process along its length. That is, the secondary tubular member 6 enables effective and efficient pre-cooling of the cable 2, so that a warmer end of the cable 2 cools in accelerated but even manner. Here, the warmer end of the cable 2 may be envisaged as a portion of the cable 2 toward the end point not yet reached by the "cold front". The pre-cooling of the cable 2 yields a reduced temperature gradient within the cable 2 and, as a result, thermally induced stresses due to cable shrinkage are avoided as well as damage to the superconducting cable 2.

With reference to Figure 2, in the depicted embodiment the secondary tubular member 6 is thermally connected to an inner surface 4a or an outer surface 4b of the primary tubular member 4 along a length thereof. In this embodiment the secondary tubular member 6 remains, indeed, eccentrically arranged with respect to the central axis "C" of the primary tubular member 4, but where the secondary tubular member 6 can be disposed on the inner surface 4a or the outer surface 4b. Furthermore, the primary and secondary tubular member 4, 6 form an eccentric/non-concentric double tubular arrangement as a single piece or unitary component which does not require spacers to provide the secondary channel 5. However, spacers may be used, for example, to avoid direct contact between the cable 2 and the secondary tubular member 6. In any case, the secondary tubular member 6 provides a continuous, non-constricted secondary channel 5 in lengthwise fashion reducing flow resistance to the second refrigerant along the primary and secondary tubular members 4, 6. The reduced flow resistance also holds for the secondary tubular member 6 extending though the primary tubular member 4 as shown in the embodiment of Figure 1 .

In an exemplary embodiment, the secondary tubular member 6 is mechanically connected to the inner surface 4a or the outer surface 4b, providing the unitary double tubular arrangement of the primary and secondary tubular members 4, 6. In a further exemplary embodiment the secondary tubular member 6 may be welded to the primary tubular member 4.

It is noted that the secondary tubular member 6 need not be thermally connected to the primary tubular member 4 in straight fashion there along. In particular, there may be an embodiment wherein the secondary tubular member 6 is arranged in helical fashion along the inner surface 4a or the outer surface 4b of the primary tubular member 4. In this way a more circumferentially distributed cooling of the primary tubular member 4 is achieved. In a further embodiment, a plurality of secondary tubular members 6 may be provided and circumferentially arranged, e.g. spaced apart, along the inner surface 4a or the outer surface 4b of the primary tubular member 4. This embodiment also provides circumferentially distributed (convective) cooling of the primary tubular member 4 during a cool-down process.

In an embodiment, as shown in Figure 2, the secondary tubular member 6 comprises a curved cross section 7 having a first end 7a and a second end 7b, the first end 7a and the second end 7b being both thermally connected (e.g. welded) to the primary tubular member 4. In this embodiment the curved cross section 7 encloses at least in part the secondary channel 5 for transporting the second refrigerant and allows for a thermal bridge along the curved cross section 7 through the first end 7a and the second end 7b toward the primary tubular member 4, e.g. to the inner surface 4a or outer surface 4b thereof. This thermal bridge along the curved cross section 7 may further contribute to an accelerated cooling of the primary tubular member 4 when the second refrigerant flows through the secondary channel 5.

In the exemplary embodiment depicted in Figure 2, the curved cross section 7 circumferentially encloses only a part of the outer surface 4b, so wherein the first end 7a and the second end 7b are spaced apart along a circumferential portion of the primary tubular member 4. For example, this may be implemented as having the curved cross section circumferentially enclosing only a part of the outer surface 4b of the primary tubular member 4. This embodiment provides an effective way of providing a secondary tubular member 6 eccentrically with respect to the central axis C of the primary tubular member 4. In this embodiment the secondary channel 5 provides a cold inner or outer surface 4a, 4b to absorb heat from the cable 2 by radiation and/or convection. Moreover, this embodiment provides an enlarged thermal exchange surface between the primary and secondary tubular member 4, 6, wherein this thermal exchange surface is represented by the enclosed portion of the outer surface 4b (or inner surface 4a) of the primary tubular member 4. This thermal exchange provides improved cooling of the inner and outer surface 4a and 4b making the cooling of the cable 2 through radiation and/or convection more effective. Note that it is not required that the curved cross section 7 as depicted in Figure 2 is a smooth arc. For example, the curved cross section 7 may be curved in that it has a more triangular or rectangular shape having one or more bent corners.

It is important to note that the first end 7a and the second end 7b may meet in a single point at the inner surface 4a or the outer surface 4b of the primary tubular member 4, thus yielding a secondary tubular member 6 which is in "line thermal contact" with the primary tubular member 4 along a length thereof. For example, both the primary tubular member 4 and the secondary tubular member 6 may be round tubes so that these tubes are thermally connected (e.g.

mechanically affixed, welded etc.) to one another along a line in order to have the secondary channel 5 provide a cold inner or outer surface 4a, 4b that cools the cable 2 by absorbing the heat radiation from the cable 2 and/or heat transferred by convection.

Figure 3 depicts such an embodiment wherein both the primary tubular member 4 and the secondary tubular 6 may be in line thermal contact with each other. The primary tubular member 4 is cooled directly by the second refrigerant in the secondary channel 5 and provides a required cold surface to the primary tubular member 4 for cooling the cable 2 by absorbing the heat radiated therefrom and/or transferred by convection from the cable 2. This embodiment is a special case wherein the first end 7a and the second end 7b of the curved cross section 7 are spaced apart along a circumferential portion of the primary tubular member 4 but to such an extreme that the first and the second end 7a, 7b meet at a single point on the other side of the primary tubular member 4. As a result, the secondary tubular member 6 effectively encloses the primary tubular member 4 such that the secondary channel 5 surrounds the outer surface 4b of the primary tubular member 4. This in turn provides a large heat exchange surface with the primary tubular member 4 to accelerate the cool-down process without increasing thermally induced stresses within the cable 2. The cool-down process of the cable 2 may be further facilitated by allowing improved thermal contact of the cable 2 with the primary tubular member 4. To that end an embodiment is provided wherein the cable arrangement 1 further comprises a cable support member 8 thermally connected to the superconducting cable 2 and the primary tubular member 4, e.g. to the inner surface 4a, allowing heat from the cable 2 to be removed by the primary tubular member 4 as it is in thermal contact with the first refrigerant during the cool-down process. This embodiment can be envisaged as enlarging a thermal contact surface of the cable 2 coming into contact with the first refrigerant. In an embodiment the cable support member 8 is a metallic cable support member, e.g. aluminium, for providing a thermal bridge between the cable 2 and the primary tubular member 4. In a further embodiment the cable support member 8 takes the form of a support block or connecting strip thermally connecting the cable 2 to the primary tubular member 4. In yet a further embodiment the cable support member 8 comprises a granular material acting as a thermally conducting bed on which the cable support member 8 rests to exchange heat between the primary and secondary tubular member 4.

According to the present invention, the secondary tubular member 6 improves pre-cooling of the cable 2 such that the warmer end thereof is cooled faster without increasing stresses within the cable 2 during further pre-cooling. This is achieved through physical separation of the second refrigerant flowing through the secondary tubular member 6 from an outer surface of the cable 2, so that the second refrigerant, e.g. a liquid refrigerant, is not in direct contact with the cable 2, thereby avoiding boiling phenomena directly on the cable 2. Furthermore, the second refrigerant cools the primary tubular member 4 so that the cable 2 attains a lower temperature much faster over its entire length during the cool-down phase.

The heat exchange between the primary and secondary tubular member 4, 6 and the cable 2 as outlined above can be further optimized by allowing heat exchange between the second refrigerant and the primary tubular member 4 to occur in distributed fashion around the primary tubular member 4. That is, in an embodiment a plurality of the secondary tubular members 6 are circumferentially arranged, e.g. circumferentially spaced apart, along the inner surface 4a or the outer surface 4b of the primary tubular member 4. This allows the primary tubular member 4 to be evenly cooled in circumferential fashion so that it attains a lower temperate much faster during cool-down. In an alternative embodiment the secondary tubular member 6 may even be arranged in helical fashion around the primary tubular member 4 to also achieve improved circumferential convective cooling thereof.

In addition to convective cooling of the cable 2 through the secondary tubular member 6, the cool-down process of the cable 2 may be further assisted through enhanced radiative cooling. For example, Figure 4 shows an embodiment of a superconducting cable arrangement 1 providing improved radiation-assisted cooling of the superconducting cable 2. In the depicted embodiment, the primary tubular member 4 is a double-walled tubular member having an inner tubular part 10 comprising the inner surface 4a of the primary tubular member 4 as described for the embodiments described above with reference to Figure 1 . Furthermore, the double-walled tubular arrangement further comprises an outer tubular part 1 1 comprising the outer surface 4b of the primary tubular member 4. An annular channel 12 is provided between the inner tubular part 10 and the outer tubular part 1 1 configured to hold a low absolute pressure (e.g. 1 Pa), such as a vacuum. This double-walled tubular arrangement as an embodiment of the primary tubular member 4 allows the outer tubular part 1 1 to be cooled by the second refrigerant flowing through the secondary tubular member 6. This cooling of the outer tubular part 1 1 in turn allows the inner tubular part 10 to be cooled through radiation assisted cooling, wherein convective thermal exchange between the outer tubular part 1 1 and the inner tubular part 10 is minimized by the low pressurised/vacuum annular channel 12.

In an advantageous embodiment the superconducting cable arrangement 1 may even be configured to provide an adjustable gas pressure within the annular channel 12, so that an optimum can be achieved between convective versus radiative cooling dependent on the cool- down process and/or operational conditions of the cable 2. It may even be conceivable that the pressure within the annular space 12 is controllable along a limited length portions of the cable arrangement 1 , e.g. near special points of interest such as curves or bends.

In an embodiment, the outer tubular part 1 1 has a much lower thermal mass than the cable 2 for allowing the outer tubular part 1 1 to be cooled down rapidly, e.g. preferably at least 10 times faster than the cable 2.

A spacer 9 may be provided within the annular channel 12 to keep the inner and outer tubular part 10, 1 1 at a distance and to fix the channel geometry. In an advantageous embodiment the spacer member 9 is arranged between the inner tubular part 10 and the outer tubular part 1 1 .

Radiative cooling provided by the cable arrangement 1 as depicted in Figure 4 can be enhanced by considering emissivity's of surfaces. The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation and is expressed as a ratio value between 0 and 1 . For example, in an embodiment the outer tubular part 1 1 comprises an inner surface 1 1 a having an emissivity of 0.5 or higher, so that heat being emitted by the cable 2 can be absorbed by the outer tubular member 1 1 , i.e. the second refrigerant flowing though the secondary tubular member 6. In an advantageous embodiment the emissivity of the inner surface 1 1 a of the outer tubular part 1 1 need not be constant and may have an emissivity which is adjustable/controllable between 0 and 1 . This embodiment allows the emissivity of the inner surface 1 1 a to be controlled toward a high value, e.g. 0.5 or higher, during the cool-down process, and subsequently be controlled toward a lower value, e.g. lower than 0.5, when operational superconducting temperatures of the cable 2 have been reached. In another embodiment, the outer tubular part 1 1 comprises an outer surface 1 1 b having an emissivity below 0.5, thereby (partially) blocking external heat load onto the cable arrangement 1 .

In addition to the emissivity of the outer tubular part 1 1 , choosing particular emissivity's of the inner tubular part 10 may also be advantageous. For example, in an embodiment the inner tubular part 10 comprises an outer surface 10a having an emissivity of 0.5 or higher, thereby facilitating absorption of heat by the outer tubular part 1 1 emitted by or coming from the cable 2. In particular applications it may be advantageous to locally adapt the cooling process of the cable 2 for specific length portions thereof. For example, in an embodiment the emissivity of the outer surface 10a of the inner tubular part 10 may vary along a length portion of the superconducting cable 2. In this embodiment the emissivity of the outer surface 10a of the inner tubular part 10 need not be a constant value along the entire length of the inner tubular part 10. Instead, the emissivity may be chosen differently at locations of interest along the

superconducting cable arrangement 1 , such as at corners or bends.

According to the present invention, the cool-down process of a superconducting cable 2 is accelerated through the cable arrangement 1 without causing damage to the cable 2. However, the cable arrangement 1 need not only reduce the time it takes to bring the superconducting cable 2 from a non-superconducting state toward a superconducting state, but the cable arrangement 1 can be adapted to maintain the superconducting state more efficiently. To that end there is an embodiment wherein the primary channel 3 of the primary tubular member 4 is connectable to the secondary channel 5 of the secondary tubular member 6. In this embodiment it is possible to allow the first refrigerant to return through the secondary tubular member 6 when, for example, superconducting operating conditions have been reached. In this way the secondary tubular member 6 becomes a return line for the first refrigerant, where, at operating conditions, the primary channel 3 is connected to the secondary channel 5 by means of some thermal switch or valve. For example, in an embodiment the superconducting cable arrangement 1 further comprises an adjustable gas-gap switch thermally connecting the primary and secondary tubular members 4, 6. In particular, the gas-gap switch allows the intensity of thermal conductance between the primary and secondary tubular member 4, 6 to be changed from a high thermal conductance during cool-down whilst lowering the thermal exchange at operating conditions. This ensures that the thermal load imposed by the return flow through the secondary channel 5 on the supply flow through the primary channel 3 is minimized.

As depicted in Figures 1 to 4, to reduce heat load on the superconducting cable 2 from outside of the superconducting cable arrangement 1 , an embodiment is provided wherein the cable arrangement 1 comprises a tubular jacket member 14 arranged around the primary and the secondary tubular members 4, 6. Furthermore, a further spacer member 16 is arranged between the tubular jacket member 14 and the primary and/or secondary tubular member 4, 6 providing a further annular channel 18 configured to hold a low absolute pressure (e.g. 1 Pa or lower), such as a vacuum. In this embodiment the tubular jacket member 14 and further annular channel 18 limit external heat load on the primary and secondary tubular members 4, 6 such that the cool- down time can be further reduced and allow the cable arrangement 1 to maintain the

superconducting cable 2 in a superconducting state more efficiently. Radiative heat exchange between the tubular jacket member 14 and the primary and/or secondary tubular member 4,6 may be reduced by an embodiment wherein the primary and/or the secondary tubular members 4, 6, are enclosed/wrapped by a plurality of thermally insulating layers. In an advantageous embodiment, the further spacer member 16 exhibits a low thermal conductance to limit heat load on the primary and/or secondary tubular members 4, 6. In a further aspect the present invention relates to a method for cooling a superconducting cable 2 to a superconducting state, wherein the superconducting cable 2 extends through a primary channel 3 of a primary tubular member 4 configured to transport a first refrigerant, and wherein a secondary tubular member 6 is provided and comprises a secondary channel 5 configured to transport a second refrigerant for further cooling. That is, the secondary tubular member 6 may extend through the primary tubular member 4 or the secondary tubular member 6 may encloses at least in part the primary tubular member 4, see e.g. the Figures 1 to 4.

In an embodiment and in view of the superconducting cable arrangement 1 as described above, the secondary tubular member 6 may be eccentrically arranged with respect to the central axis "C" of the primary tubular member 4.

The method commences by providing the superconducting cable 2 in a non- superconducting state, e.g. at room temperature, and selecting the second refrigerant as a liquid refrigerant, such as liquid nitrogen. Once the second refrigerant has been selected, the method continues by pumping the liquid second refrigerant through the secondary channel 5. In this step it is understood that the secondary tubular member 6 comprising the secondary channel 5 avoids direct physical contact between the liquid second refrigerant and an outer surface of the superconducting cable 2. So the physical separation of the liquid second refrigerant from the cable 2 prevents any intense and excessive phase changes (e.g. boiling), refrigerant speeds and pressures to occur directly at the outer surface of the cable 2, thereby preventing damaging said cable.

As the liquid second refrigerant is pumped through the secondary channel 5, the method continuous by waiting until the superconducting cable 2 reaches a temperature of 200 Kelvin or lower. Within this temperature range it can be assumed that the temperature gradient along the cable 2 is sufficiently reduced and that a liquid first refrigerant (e.g. liquid nitrogen) can be introduced into the primary tubular member 4 without causing intense boiling phenomena of the liquid first refrigerant somewhere along the outer surface of the superconducting cable 2.

Therefore, having determined a safe switching temperature of 200 K or lower, the method then comprises the step of selecting the first refrigerant as a liquid first refrigerant and pumping the liquid first refrigerant through the primary channel 3 of the primary tubular member 4. As the liquid first refrigerant flows through the primary channel 3, the method continues by allowing the superconducting cable 2 to reach a (steady) superconducting state, i.e. to reach operating conditions.

The method of the present invention provides a pre-cooling phase by utilizing the secondary tubular member 6 and pumping/injecting a liquid second refrigerant, such as liquid nitrogen, into the secondary tubular member 6 without causing an intense phase change (e.g. liquid to gas expansion, boiling etc.) of a liquid refrigerant directly along an outer surface of the superconducting cable 2 during the pre-cooling phase. This avoids excessive (local) shrinkage of the cable 2 and excessive tension forces therein. Because the second refrigerant is a liquid refrigerant an accelerated yet equable and even cool-down of the superconducting cable 2 along its length is possible, particularly the warmer end thereof. As the liquid second refrigerant facilitates pre-cooling of the superconducting cable 2, it is possible to introduce a liquid first refrigerant (e.g. liquid nitrogen) into the primary channel 3 sooner during the pre-cooling phase, wherein the liquid first refrigerant can be injected into the primary channel 3 well before a steady superconducting state of the cable 2 has been reached.

Depending on a sufficiently low temperature of the superconducting cable, i.e. 200 Kelvin or lower, it is possible to introduce a liquid first refrigerant into the primary channel 3 and prevent excessive boiling of the liquid first refrigerant along an outer surface of the superconducting cable 2. The temperature of 200 K can be seen as an upper limit temperature being indicative of the possibility to safely switch to a liquid first refrigerant. When such a switch to the liquid first refrigerant can be made at a particular temperature (200 K or lower) depends on materials used for the superconducting cable 2. So given a particular superconducting cable 2 it is possible to determine at which pre-cooling temperature and time point a liquid first refrigerant can be pumped into the primary tubular member 4 for bringing the cable 2 to a steady superconducting state without damaging it.

So based on specific properties of the superconducting cable 2, it may well be the case that in particular embodiments the step of waiting until the superconducting cable 2 reaches a temperature of 200 Kelvin or lower, may comprise waiting until the superconducting cable 2 reaches a temperature of 190 K or lower, 180 K or lower, 170K or lower, 160 K or lower, 150 K or lower, 140 K or lower, 130 K or lower etc.

So to summarize, according to the method of the present invention the secondary tubular member 6 and the second refrigerant flowing there through yields a significant reduction of the cool-down time yet prevent damaging the superconducting cable 2. Consequently, a surprising but effective solution is provided for avoiding excessively long cool-down times associated with presently known cool-down methods for superconducting cables.

According to the method of the present invention, waiting until the superconducting cable

2 reaches a temperature of 200 Kelvin or lower may be understood to mean an embodiment wherein each point along the superconducting cable 2 exhibits a temperature of 200 K or lower. In a further embodiment, the step of waiting until the superconducting cable 2 reaches a temperature of 200 Kelvin or lower may mean waiting until a start point and/or an end point of the

superconducting cable 2 reaches a temperature of 200 Kelvin or lower. For example, it possible to wait until a start point of the cable 2, e.g. the point at which the second refrigerant is injected into the secondary channel 5, to reach a temperature of 200 Kelvin or lower. In another embodiment it is of course possible to wait until an end point of the cable 2 reaches a temperature of 200 Kelvin or lower. This embodiment may be advantageous because when the end point, i.e. the warmer end of the cable 2, reaches a temperature of 200 Kelvin or lower, then one could infer that the rest of the cable 2 toward the start point may have reached a temperature of 200 Kelvin or lower as well. In yet another embodiment it is possible to wait until a mid-point of the cable 2 reaches a temperature of 200 Kelvin or lower. Any combinations of these embodiments are possible depending on the actual application and feasibility to measure a cable temperature at one or more points along the superconducting cable 2. Lastly, as mentioned above, for these embodiments it is also the case that depending on specific properties of the cable 2, the upper limit temperature for switching to a liquid first refrigerant may be 190 K or lower, 180 K or lower, 170K or lower, 160 K or lower, 150 K or lower, 140 K or lower, 130 K or lower etc.

In an embodiment, prior to waiting until the superconducting cable 2 reaches a temperature of 200 Kelvin or lower, the method may comprise providing a vacuum in the primary channel 3 of the primary tubular member 4. In this embodiment it is understood that as the liquid second refrigerant flows through the secondary channel 5, the primary tubular member 4 and the primary channel 3 thereof hold a vacuum. Heat coming from the superconducting cable 2 is then absorbed by the liquid second refrigerant through radiation and allows for an accelerated and gradual pre-cooling of the cable 2.

In an alternative embodiment, prior to waiting until the superconducting cable 2 reaches a temperature of 200 Kelvin or lower, the method may comprise the step of selecting the first refrigerant as a gaseous refrigerant (e.g. nitrogen gas) and pumping the gaseous first refrigerant through the primary channel 3. In this embodiment it is understood that as the liquid second refrigerant flows through the secondary channel 5, the gaseous first refrigerant flows through the primary channel 3. Heat coming from the superconducting cable 2 is then absorbed by the gaseous second refrigerant and subsequently absorbed by the liquid second refrigerant, also allowing for an accelerated and gradual pre-cooling of the cable 2.

In a further embodiment, the method may comprise the step of pumping the liquid first refrigerant through the secondary channel 5 in a flow direction opposite to a flow direction in the primary channel 3 during the superconducting state of the superconducting cable 2. So in this embodiment, during the superconducting steady operating conditions of the superconducting cable 2, the liquid first refrigerant flows through the primary channel 3 from a start point toward an end point of the cable 2 and subsequently flows through the secondary channel 5 back toward the start point. The secondary channel 5 thus provides a return line for the liquid first refrigerant so that a pumping system may only be needed at a single end (e.g. start point) of the

superconducting cable 2.

The present invention embodiments have been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the claims.