WO2014007903A2

WO2014007903A2 - Superconducting cables and method of cooling

Info

Publication number: WO2014007903A2
Application number: PCT/US2013/035193
Authority: WO
Inventors: William V. Hassenzahl; Jonathan DEMKO
Original assignee: Hassenzahl William V; Demko Jonathan
Priority date: 2012-04-04
Filing date: 2013-04-04
Publication date: 2014-01-09
Also published as: WO2014007903A3

Abstract

A superconducting cable and method of cooling said superconducting cable is disclosed. The superconducting cable includes a conductor for transmitting electricity, a cryogenic enclosure surrounding the conductor, a coolant return line adapted to return spent coolant for recharging, and an outer insulating sheath encasing the conductor, cryogenic enclosure, and coolant return line to form the superconducting cable. A gap is created between the enclosure and the conductor to allow a coolant to flow therebetween.

Description

SUPERCONDUCTING CABLES AND METHOD OF COOLING

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to superconducting cables and a method of cooling said superconducting cables and, more particularly, to the use of liquid air as a coolant for superconducting cables.

[0002] Superconducting power cables require a temperature that is sufficiently low to allow adequate performance of the superconducting materials. Depending on a variety of design details, the operating temperature for high-temperature superconductors (HTS) is typically below 77 Kelvin (K), which is the equilibrium boiling temperature of liquid nitrogen at atmospheric pressure, 1.013 kilo-Pascal (kPa). Because of the convenience of using liquid nitrogen, this is the operating temperature for several technologies based on HTS. The current carrying capacity of superconductors increases as the temperature decreases. As a result, many of the HTS applications use liquid nitrogen at temperatures slightly below 77K. However, techniques to reduce liquid nitrogen temperatures require a more complicated cryogenic system.

[0003] The lowest temperature available for cooling with nitrogen is its freezing point, 63.15K. However, the freezing point of liquid air is lower than that of pure nitrogen. At the typical atmospheric mixture of 79 percent (%) nitrogen and 20% oxygen, air remains a liquid down to at least 60.5K. At lower temperatures, part of the mix begins to solidify and separate, though some liquid remains down to about 57K.

[0004] Since the current carrying capacity of superconductors increases as the temperature decreases, lowering the operating temperature of, for example, a transmission cable by 2K through the use of liquid air will reduce the amount of superconductor needed in an application. Recent data suggest that the increase in critical current as the temperature decreases is significant. It is estimated that the reduction in superconductor requirements associated with the reduction of the maximum temperature in a system form 65K to 63K to be about 14% if the peak field is 0.4 Tesla (T), and about 13% if the peak field is between 0.6T and 0.8T. These magnetic fields are typical of what might exist in a superconducting DC cable.

[0005] With the advent of super grids and long distance superconducting DC cables, cooling of the cables is of utmost importance. As discussed, liquid nitrogen has been used as a coolant. However, other fluids such as gaseous helium, gaseous hydrogen, gaseous neon, and liquid argon have been discussed. Helium and hydrogen gas have been explored for DC cables, but they introduce complications for long distance use. Both argon and neon are too expensive to be used in large systems. Additionally, these coolants can create hazards in confined spaces.

[0006] Accordingly, there is a need for a method of cooling superconducting cables that reduces complications, that provides adequate cooling, that is safe in confined spaces, and that is inexpensive and readily available.

BRIEF SUMMARY OF THE INVENTION

[0007] These and other shortcomings of the prior art are addressed by the present invention, which provides a superconducting cable and method for cooling said superconducting cable.

[0008] According to one aspect of the invention, a superconducting cable includes a conductor for transmitting electricity, a cryogenic enclosure surrounding the conductor, a coolant return line adapted to return spent coolant for recharging, and an outer insulating sheath encasing the conductor, cryogenic enclosure, and coolant return line to form the superconducting cable. A gap is created between the enclosure and the conductor to allow a coolant to flow therebetween.

[0009] According to one aspect of the present invention, a method of cooling superconducting cables includes the steps of providing a coolant, and circulating the coolant through the superconducting cable. [00010] According to another aspect of the present invention, a method of cooling a superconducting cable includes the steps of providing a superconducting cable, having a conductor, an input line, and a return line. The method further includes the steps of providing liquid air to the input line and circulating the liquid air around the conductor to keep the conductor cooled, and circulating spent liquid air from the input line through the return line, wherein the return line returns the spent liquid air to a refrigerator for recharging.

BRIEF DESCRIPTION OF THE DRAWINGS

[001 1] The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

[0012] Figure 1 shows a liquid air facility according to an embodiment of the invention; and

[0013] Figure 2 shows a DC superconducting cable according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Referring to the drawings, an exemplary method of cooling superconducting cables according to an embodiment of the invention is illustrated in Figures 1 -2.

[0015] The present invention uses liquid air as a coolant for cooling superconducting cable systems. Air is typically liquefied to produce industrial gases such as oxygen, nitrogen and argon, which together form over 99% of the air in the atmosphere. The separation of air into its constituents is accomplished by a process sometimes referred to as fractional distillation. The industrial gas business is well established in industrialized countries and very large plants are used for this process so that the production of liquid air is measured in hundreds of tons. Also, liquid air is transported in pipes over considerable distances.

[0016] Apart from its role as a source of important industrial gases, liquid air itself is used for only a few applications. There are several reasons for this. For example, the constituent liquids can separate in stagnant vessels and, most importantly, the amount of oxygen in a tank increases because nitrogen boils off more readily. Thus, liquid air is not generally stored for long periods of time. Another reason liquid air is not often used is because the value of oxygen and argon makes air separation economical. Liquid nitrogen, essentially a byproduct of producing oxygen and argon, is available widely and is used extensively. Argon and oxygen are often supplied to users as a gas in high-pressure tanks, while nitrogen is delivered as a liquid in trucks with large cryogenic tanks.

[0017] Since air is available anywhere, it can be produced anywhere that it is needed as long as a power source, either mechanical or electrical, is available to operate a cryogenic refrigerator. In some remote locations, small liquefiers are used for the local production of liquid air, which can then be separated to produce oxygen and argon and liquid nitrogen.

[0018] Liquid air is an ideal material for cooling a long distance cable because it can be operated at a lower temperature than liquid nitrogen, thereby providing for more effective use of the superconducting material. In addition, since air is available everywhere, the use of liquid air eliminates the logistic issues of transporting other coolants to the site. Further, cryogenic systems are prone to leaks, which may or may not affect the operation of the system. However, there are many places, for example, in a vault for cable splices, where the need for adequate oxygen is critical, particularly during human maintenance activities. Leaks of any other coolant into the vault could displace the oxygen. Liquid air, however, contains the same mixture of oxygen as the atmosphere and thus it would provide a safe environment even if there were leaks. All other cryogenic coolants that might be used to cool a superconducting cable require special safety requirements associated with breathing if they are used in a confined area. These rules do not apply to liquid air systems. (Note that to be effective as a coolant the liquid air will have gone through a cleaning and filtration process that removes water, all particulates, and many other possible contaminates.)

[0019] Heat is generated internal to the cable and some heat flows into the cable from the exterior environment. Special refrigerators remove the heat by forcing the flow of a coolant along the length of a cable. Superconducting DC cables require refrigeration stations every 10 to 40 km to remove this heat. Superconducting AC cables may be most effective with other refrigerator separation distances. Specific separation distances and refrigerator power requirements will depend on cable design factors including heat input, cable power, AC or DC operation, type of coolant, maximum allowable pressure, pipe diameter, and altitude changes between refrigerators.

[0020] A potential negative for the use of a lower temperature is that the refrigerators that maintain the operating temperatures require more electrical power at the lower operating temperature. The penalty for operating at 62K instead of 64K is about 3%. This effect is somewhat compensated for by the fact that the pumping losses associated with pushing liquid air through a long cryogenic tube are less than the equivalent losses associated with liquid nitrogen.

[0021] Referring now to Figure 1 , liquid air plants 10 are placed at intervals along the route of a long length DC superconducting cable system, utilizing locally available electricity and ambient air. As shown, air is first drawn into an air purification system 1 1 that supplies clean air to a commercial refrigerator 12. The liquid air output of the refrigerator 12 is circulated through a superconducting cable 14, Figure 2, through "go" 16 and "return" 17 lines. A storage tank 13 provides backup capacity in case the refrigerator fails. The placement of liquid air plants along the superconducting cable line at 10km to 40km intervals avoids the need to provide for overland shipment of alternate coolants (e.g., liquid nitrogen) to the refrigeration stations. [0022] As shown in Figure 2, the superconducting cable 14 includes a cable 18 for transmitting electricity, the "go" line 16 formed by a cryogenic enclosure 20 that wraps around the cable 18 to create a gap 21 therebetween and allow the liquid air to be pushed/pulled through the gap 21 to provide cooling, and the "return" line 17 for returning the liquid air for cooling, and an outer insulating sheath, pipe, or enclosure 22 for encasing the cable 18, the "go" line 16, and the "return" line 17. As illustrated, a vacuum is maintained in the superconducting cable 14, by vacuum pump 23, Figure 1.

[0023] The foregoing has described a method for cooling superconducting cables. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.

Claims

We Claim:

1. A superconducting cable, comprising:

(a) a conductor for transmitting electricity;

(b) a cryogenic enclosure surrounding the conductor, wherein a gap is created between the enclosure and the conductor to allow a coolant to flow therebetween;

(c) a coolant return line adapted to return spent coolant for recharging; and

(d) an outer insulating sheath encasing the conductor, cryogenic enclosure, and coolant return line to form the superconducting cable.

2. The superconducting cable according to claim 1 , wherein the coolant return line is adjacent to the cryogenic enclosure.

3. The superconducting cable according to claim 1 , wherein coolant flowing through the gap is supplied from a refrigerator.

4. A method of cooling superconducting cables, comprising the steps of:

(a) providing a coolant; and

(b) circulating the coolant through the superconducting cable.

5. The method according to claim 4, wherein the coolant is liquid air.

6. The method according to claim 5, further including the step of positioning liquid air plants along a length of the superconducting cable, wherein each liquid air plant is positioned at a predetermined interval along the length of the superconducting cable.

7. The method according to claim 6, wherein the liquid air plant includes:

(a) an air purification system to purify incoming air;

(b) a refrigerator adapted to receive the purified air from the air purification system and refrigerate the air; and

(c) a liquid air storage tank to provide backup liquid air.

8. The method according to claim 5, wherein the liquid air is circulated through input and return lines of the superconducting cable.

9. A method of cooling a superconducting cable, comprising the steps of:

(a) providing a superconducting cable, having:

(i) a conductor;

(ii) an input line; and

(iii) a return line;

(b) providing liquid air to the input line and circulating the liquid air around the conductor to keep the conductor cooled; and

(c) circulating spent liquid air from the input line through the return line, wherein the return line returns the spent liquid air to a refrigerator for recharging.

10. The method according to claim 9, further including the step of positioning liquid air plants along a length of the superconducting cable at predetermined intervals to provide liquid air to the input line and receive spent liquid air from the return line.

1 1. The method according to claim 10, wherein the liquid air plants include:

(a) an air purification system to purify incoming air;

(c) a liquid air storage tank to provide backup liquid air.