The invention relates to a device for transporting current through a superconducting power cable, wherein the device comprises an inner tube and an outer tube which are arranged concentrically relative to each other, wherein the power cable is disposed within the inner tube, which inner tube also houses a cryogenic cooling medium for cooling the power cable, wherein a space having an annular cross-section is present between the inner tube and the outer tube, and wherein the outer tube is thermally isolated from the inner tube . It is noted that the term "superconducting" as used within the framework of the invention is understood to refer to the phenomenon that below a particular (generally very low) temperature the electrical resistance of an electrical conductor is much lower than at room temperature. At such low temperatures, more current can be transported through the conductor, given an identical potential difference between ends of the electrical conductor. This can be explained as follows. The electrical resistance of an electrical conductor is a measure of the heat produced by the conductor as a result of current being passed

therethrough, and that on the basis of the following

formula :

P = 12 . R

In the above formula, P is the heat produced in the

conductor per unit time, expressed in Watt (W) , I is the current intensity, expressed in Ampere (A) , and R is the electrical resistance, expressed in Ohm (Ω) . In superconducting conditions, significantly more current I can be transported, with the same heat production P, since the resistance R is much lower. The heat production P can be maintained at the same level in superconducting conditions if the conductor is cooled to below a temperature at which the material of the conductor becomes superconducting, and is maintained at that temperature during use. A device as described above is generally known. To cool the sc power cable to a temperature of, for example, 77K, 20K or even 4K, use is made of a cryogenic medium, for example liquid nitrogen (N2), liquid hydrogen (H2) or liquid helium (He) , which is pumped through the inner tube, with the cryogenic medium flowing round the power cable. As already said before, the outer tube is thermally isolated from the inner cable by creating a vacuum in the annular space between the inner tube and the outer tube. As a result, the cooling of the power cable is optimised, since outside heat has hardly if any influence on said cooling. It is noted that cooling the power cable along its entire length may present problems in practice. The fact is that if said cooling should temporarily fail in whole or in part, the temperature of the power cable may soon rise locally above the temperature at which the material of the conductor becomes superconducting, causing the resistance of the power cable to increase. As a result, also the amount of heat produced in the power cable will increase, in turn causing the temperature of the power cable to increase. Thus, transporting the power in the power cable under

superconducting conditions will soon be no longer possible. This may lead to major economic and financial losses, in particular in case the power supply may not decrease at all or only for a (very) short period of time. Such an

undesirable situation is also referred to as a "quench" in practice. Accordingly this is the reason why a cryogenic medium, such as helium, hydrogen (H2) or nitrogen in a liquid or supercritical state is used for cooling the power cable, since said substances are capable of absorbing relatively much heat (from the environment and, to a lesser degree, from the power cable) . A drawback of the known device is the following. As

explained above, the cryogenic medium must absorb and discharge the heat present in the device. Said heat is (i) heat produced by the power cable, (ii) ambient heat which, in spite of the vacuum created in the annular space, penetrates the device to a certain extent, and (iii) frictional heat produced as a result of the cryogenic medium flowing through the inner tube. The flow rate of the

cryogenic mediate in the inner tube must to that end be sufficiently high, because the cryogenic medium may

evaporate locally if the flow rate of the cryogenic medium is too low. As a result, vapour bubbles will form in the inner tube, which may adversely affect the flow rate of the cryogenic medium in the inner tube and thus also the cooling of the power cable. To help maintain the flow rate of the cryogenic medium in the inner tube, a relatively large pressure difference is created in the prior art between the inlet of the cryogenic medium at one end of the inner tube and the outlet of the cryogenic medium at the other end of the inner tube. However, said relatively large difference in pressure of the cryogenic medium between the two ends of the inner tube also means a relatively large difference locally in the temperature of the cryogenic medium. After all, the cryogenic medium, which will at some point boil in the inner tube as a result of the aforesaid heat absorption, has a lower boiling temperature at a lower pressure and a higher boiling temperature at a higher pressure. The aforesaid difference in temperature of the cryogenic medium will also result in a difference in the temperature of the power cable at the two ends of the inner tube (i.e. the extent to which the power cable is cooled by the cryogenic medium) , so that the prior art has a limitation as regards the maximum length of the inner tube and the outer tube, and thus of the power cable. Another drawback of the known device is the fact that a complex transport system of pipes and valves for the cryogenic medium is needed for cooling the power cable, whilst also a complex control system is needed for

monitoring and maintaining the temperature and flow rate of the cryogenic medium. Furthermore, the evaporated cryogenic medium must constantly be converted into liquid form in the prior art.

The object of the invention is to improve the prior art, in particular to provide a device for transporting current through a superconducting power cable, wherein in principle no limitation applies as regards the maximum length of the power cable, and wherein a complex transport system of pipes and valves for the cryogenic medium is not needed.

In order to accomplish that object, a device of the kind described in the introduction is according to the invention characterised in that a vapour return line is disposed within the inner tube, wherein an outer wall of the vapour return line is provided with several spaced holes, and wherein an underpressure relative to the cryogenic medium prevails in the vapour return line for causing a portion of the cryogenic cooling medium that has been forced through the holes in the vapour return line to evaporate. By

creating an underpressure in the vapour return line relative to the pressure of the cryogenic medium in the inner tube, a (small) portion of the cryogenic medium will be forced through the holes in the outer wall of the vapour return line. Said portion of the cryogenic medium will evaporate in the vapour return line (after all, on account of the

pressure difference the boiling temperature of the cryogenic medium in the vapour return line is lower than the boiling temperature of the cryogenic medium in the inner tube) , with the energy required for said evaporation being extracted from the cryogenic medium in the inner tube. The cryogenic medium in the inner tube is thus cooled along the entire length of the vapour return line, as a result of which not only a major energy saving is realised in comparison with the prior art, but in addition the formation of the

aforesaid vapour bubbles in the inner tube is prevented. Based on the invention, the cryogenic medium no longer needs to flow around the power cable in the inner tube, so that the above-described problems regarding the pressure

differences and the temperature differences of the cryogenic medium near the ends of the inner tube belong to the past. It stands to reason that a return line extending outside the inner tube and the outer tube for circulating the cryogenic medium will no longer be needed in that case. The invention thus makes it possible in principle to select any length for the device, i.e. the inner tube and the outer tube and the power cable. The outer tube is preferably thermally isolated from the outer tube by creating a vacuum in the annular space. The diameter of the holes preferably ranges between 0.01 and 0.25 mm, preferably between 0.03 and 0.1 mm.

In a preferred embodiment of a device according to the invention, the vapour return line is disposed in the

cryogenic cooling medium, with the cryogenic cooling medium surrounding the power cable. In another preferred variant, the power cable is disposed within the vapour return line, whilst the cryogenic cooling medium surrounds the vapour return line. In yet another preferred variant, the power cable is disposed in the cryogenic cooling medium, with the vapour return line surrounding the cryogenic cooling medium.

In another preferred embodiment of a device according to the invention, the vapour return line is in open communication with the environment for discharging the evaporated portion of the cryogenic cooling medium into the environment (i.e. the atmosphere) . Preferably, the pressure in the vapour return line is at least substantially the same as the ambient pressure in that case.

In another preferred embodiment of a device according to the invention, the vapour return line is connected to a pump for discharging the evaporated portion of the cryogenic cooling medium. Said pump is in particular a vacuum pump.

In another preferred embodiment of the device according to the invention, the vapour return line is connected to an overpressure system for creating an overpressure in the vapour return line. Any clogged holes in the outer wall of the vapour return line can thus be (periodically) opened by the overpressure, to which end use is made of the cryogenic medium in gaseous form or any other substance which is suitable for that purpose.

In another preferred embodiment of the device according to the invention, the vapour return line is divided into coupled sections having a length of between 2 and 20 m, preferably between 4 and 15 m, more preferably about 6 m. The flow rate of the evaporated cryogenic medium in the vapour return line must not run up too high, because too large a pressure difference across the ends of the vapour return line might make it difficult for the cryogenic medium to enter through the holes in the outer wall of the vapour return line on account of the pressure difference being too small, whilst in addition the frictional heat in the vapour return line must remain within bounds. In those cases where the length of the power cable exceeds the maximum length of the vapour return line, segments of the vapour return line are according to the invention coupled together, thus making it possible to cool the entire power cable yet.

The invention also relates to a method for transporting current through a superconducting power cable, wherein use is made of an inner tube and an outer tube which are

arranged concentrically relative to each other, wherein the power cable is placed in the inner tube and wherein a cryogenic medium is introduced into the inner tube as well for cooling the power cable, wherein a space having an annular cross-section is formed between the inner tube and the outer tube, and wherein the outer tube is thermally isolated from the inner tube by the creation of a vacuum in the annular space, characterised in that a vapour return line is placed in the inner tube, in an outer wall of which vapour return line several spaced holes are formed, wherein an underpressure relative to the cryogenic medium is created in the vapour return line for causing a portion of the cryogenic cooling medium that is forced through the holes in the vapour return line to evaporate.

The invention will now be explained in more detail below with reference to figures illustrated in a drawing, which show various preferred embodiments of the device according to the invention.

With reference to figures 1, 2 and 3, which are cross- sectional views, the device 1 for transporting current through a superconducting power cable 2 comprises an inner tube 3 and an outer tube 4, which are arranged

concentrically relative to each other. As shown, the power cable 2 is disposed within the inner tube 3, whilst the inner tube 3 also contains a cryogenic cooling medium 5 for cooling the power cable 2. Present between the inner tube 3 and the outer tube 4 is a space 6 having an annular cross- section. In said space 6 a vacuum prevails, which functions to thermally isolate the outer tube 4 from the inner tube 3.

In figure 1 a vapour return line 7 extends through the cryogenic cooling medium 5, whilst the cryogenic cooling medium 5 surrounds the power cable 2. In another preferred variant, which is shown in figure 2, the power cable 2 is disposed within the vapour return line 7, whilst the

cryogenic cooling medium 5 surrounds the vapour return line 7. In yet another preferred variant, which is shown in figure 3, the power cable 2 extends through the cryogenic cooling medium 5, whilst the vapour return line 7 surrounds the cryogenic cooling medium 5. The cryogenic cooling medium is preferably liquid nitrogen (N ₂ ) .

The vapour return line 7 locally passes through a wall of the inner tube 3 and a wall of the outer tube 4 to the outside and blows off into the atmosphere or is connected to a vacuum pump. A wall 8 of the vapour return line 7 is provided with small holes, for example having a diameter of 0.05 mm, along its entire length, preferably at fixed intervals. Said holes do not necessarily have to be circular in shape, they may also be elliptical or angular, for example. The operation of the device is as follows. The inner tube 3 functions as a process conduit and is filled with a liquid cooling medium, for example liquid nitrogen (N ₂ ) at a pressure of, for example, 1 bar. The vacuum pump realises a lower pressure, for example 0.9 bar, in the vapour return line 7. As a result, liquid nitrogen flows through the small holes into the vapour return line 7, which is configured as a thin pipe. The liquid that enters the vapour return line 7, where a pressure of 0.9 bar prevails, has a lower boiling temperature than the liquid in the adjacent inner tube 3, where a pressure of 1 bar prevails. The consequence is that the liquid being under a pressure of 0.9 will evaporate. The heat that is required for said evaporation is extracted from the environment, and the vacuum pump discharges the vapour through the vapour return line 7. The liquid in the inner tube 3 surrounding the superconductor 2 gives off heat in this way and will not evaporate, thus preventing the formation of bubbles around the superconductor 2. Because only a minimal amount of liquid nitrogen passes through the small holes, the flow rate in the inner tube for the superconducting power cable 2 is minimal. This is not objectionable at all, as vapour bubbles no longer form there, which vapour bubbles would have to be discharged, as is the case in prior art

installations. As a result, there is no longer any heat production resulting from liquid friction in the inner tube 3. In addition, the return line for liquid containing vapour bubbles as described for prior art installations is no longer needed, since all the liquid nitrogen now exits the installation in vapour form via the vapour return line 7. As already noted before, the vapour return line 7 preferably consists of coupled sections 9 having a length of about 6 m. Figure 4 shows such a segment 9 in top plan view (figure 4a) and in a longitudinal sectional view (figure 4b) , whilst a detail (figure 4c) is likewise shown in longitudinal

sectional view. Parts corresponding to parts shown in figures 1, 2 and 3 are indicated by the same numerals. Each segment 9 has an insertion part at one end and an insertion opening 11 at the other end. As shown, adjacent sections 9 can be coupled together by inserting an insertion part 10 of one section 9 into an insertion opening 11 of the other section. The outer tube 4 has an outlet 4 for each section 9, through which the vapour return line 7 blows off into the atmosphere . It is noted that the invention is not limited to the

illustrated embodiments, but that it also extends to other preferred variants that fall within the scope of the

appended claims.