The Energy SuperGrid

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Superconducting Cables

Almost immediately after its discovery in 1911, superconductivity and superconducting wires, with their ability to carry direct current without loss, were proposed for electricity transmission and distribution cable application.  However, the early superconductors were primarily elemental metals whose superconducting properties disappeared for even moderate currents and magnetic fields.  Furthermore, the necessity to supply large amounts of liquid helium for their operation was a major, if not overwhelming, barrier.  Not until the discovery of “hard” superconducting alloys such as NiTi and Nb₃Sn capable of sustaining practical levels of current in the years following World War II, the ability to manufacture long wire lengths of these materials, and the increasing availability of efficient helium liquefaction equipment, could transmission of electricity via superconductivity be seriously considered. 

In 1967, Richard Garwin and Juri Matisoo at IBM published a paper proposing the construction of a 100 GW, 1000 km, dc superconducting transmission line based on the then newly discovered type II compound, Nb ₃Sn, refrigerated throughout its entire length by liquid helium at 4.2 K.  At the time it was thought remote nuclear power plant farms or hydroelectric facilities would provide a major portion of the then burgeoning national demand for electricity, and that the “high power bandwidth” transmission at near zero loss available from deployment of superconducting cables would become economical.  In principle, their idea presaged many aspects of the SuperGrid concept.  In the 1970s and early 1980s, more studies on the feasibility of both ac and dc superconducting cables appeared, and two watershed ac superconducting cables were built and successfully tested at Brookhaven, NY, and Graz, Austria, the latter actually undergoing live grid service for several years.  At least two reports published during this period explored the joint use of hydrogen with superconducting wires for electricity transmission.  Bartlit, Edeskuty and Hammel considered an energy transmission line employing low temperature superconductors cooling by liquid helium with liquid hydrogen serving as a heat shield, the hydrogen to be delivered eventually as rocket fuel for NASA.  In 1975, a report assembled by Stanford University and NIST examined the use of “slush hydrogen” at 14 K as cryogen for a cable using Nb₃Ge with a transition temperature near 20 K as the superconductor; however, no attention was given the use of hydrogen as an energy agent itself.

 Following on the discovery of high temperature superconductors in 1986 and the appearance of practical tape and wire in the early 1990s, Schoenung, Hassenzahl and Grant revisited the work of Garwin and Matisoo in light of these new events, and concluded that an HTSC dc “electricity pipeline” cooled by liquid nitrogen could compete economically with conventional high voltage dc transmission lines or gas pipelines for distances greater than 200 km.  Although today several prototype HTSC superconducting cable demonstrations are planned or actually undergoing test worldwide, all target ac applications at transmission and distribution voltage levels at 66 kV and greater, we must emphasize that the major advantage of superconductivity is the ability to transport very large dc currents at relatively low voltage.  Only under constant current conditions are superconductors perfect conductors, otherwise heat-producing hysteretic losses occur requiring additional cryogenic capacity above and beyond that to remove ambient thermal in-leak to the cable.  Moreover, the use of lower voltages will reduce dielectric stress and improve cable reliability and extend lifetime.

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On the Practical Construction of a SuperCable

Although the elements of a SuperCable, delivering both electrical and chemical power, are conceptually simple, for capacities on the scale of 5 GWe via superconducting wire and 10 GWth via chemical potential energy in the form of hydrogen or methane for a distance on the order 1000 km, it’s actual construction would be an immense undertaking, rivaling today’s natural gas pipeline expansion.  The provision of refrigeration and vacuum infrastructure along the right-of-way alone would require deployment of these technologies on an unprecedented level, surpassing even those servicing the largest hadron colliders in existence or under plan.

Some idea of these requirements can be culled from the seminal paper by Garwin and Matisoo ( GM) published in 1967 proposing a 1000 km, 100 GW superconducting dc cable, and the re-visitation of the concept in 1997 by Schoenung, Hassenzahl and Grant ( SHG) in the context of high temperature superconducting materials not available to the former.  Useful background is also provided by the late-90s study at Fermilab of an 233-km circumference low-field 100 TeV hadron collider energized by a low temperature superconducting carrying 100,000 A dc.

From a macroscopic standpoint, the construction of a SuperCable with these kinds of  specifications bears great similarity to that of several high capacity, long distance natural gas pipelines under consideration today, such as the 1300-km, 30-in, 18 GW _th Mackenzie Valley Project to run from gathering well fields near the Mackenzie River – Artic Ocean Delta southward to the northern border of Alberta.

In fact, this pipeline provides a useful exercise for examining the effort and resource the Mackenzie Project will require as a scoping scenario for a SuperCable.

• To cost $3 B USD

• Construction to begin 2006 after seven years of environmental studies, 20 years of negotiation with native population, five years pre-permitting.

• Commissioning and delivery of up to 1.2 B cu-ft/day in 2009 – 2010 to northern Alberta with fan-out to Eastern Canada and Mid-West US on existing pipeline network.

• Will employ up to 7000 workers housed in 40 “temporary villages.”

• Will consume 5 cubic football fields of sand and gravel (granular particulates).

• Principal transportation of material, heavy construction equipment and pump station machinery will be over specially built barges to perhaps ten landings to be constructed along the banks of the Mackenzie River.

• Construction of the pipeline proper to take place over three winter seasons with transportation over “artic roads” to be rebuilt every year.  Pipe will be laid in backfilled trenches.  Some helicopter delivery is anticipated.

• Four or five re-pressuring stations will be positioned along the route.

• The width of the right-of-way will be 40 - 50 meters, with pipe laid in surface trenches 2 - 3 meters deep, except for "bridges" across intervening rivers and streams.

• Some “manufacturing activity” (welding and lining of pipe) will be done on-site.

Similar “macroscopic” issues are to be expected with respect to the “laying” of a SuperCable, especially with respect to right-of-way if trenching and surface construction techniques are employed.

On the other hand, whole new issues arise should "deep tunneling" be considered a possible venue for the SuperCable.  What differentiates tunneling for the SuperCable from tunnels that supply water to New York or highways under the Alps is distance...a few kilometers vs. hundreds.  I know of only one carefully thought out study of truly long distance underground tunnels -- that mentioned above concerning a proposed future low-field hadron collider located at Fermilab to be housed in a 233-km, 4-m diameter, long circumferential tunnel, 50-m below the surface.  The tunnel would be excavated and mud removed by a series of novel, battery-powered tunnel boring machines.  A lot of work has been undertaken at Fermilab on the issues -- mechanical, geological and legal -- surrounding deep tunnel boring that could apply to the SuperCable.

Due to the special requirements to support superconductivity, there are a number of “microscopic” aspects present for the SuperCable that are absent in gas pipeline construction.  These “peculiarities” will heavily impact local construction methods to be employed in building the SuperGrid.

Drawing on the GM and SHG studies, and experience being gained at present designing, manufacturing and deploying prototype high temperature superconducting (HTS) cables, the following should be noted:

Present HTS wire technology is capable of manufacturing high performance tapes of 2-3 kilometers in length in production volumes exceeding 100,000 kilometers per year (there are only two such facilities worldwide, however).  There is no length limitation that does not also practically govern aluminum or copper wire.  On the other hand, the current “1G” wire is 65% silver by volume and thus expensive.

Although the HTS wires for cables operate quite satisfactorily at the relatively “hot” temperature of boiling liquid nitrogen, extensive refrigeration an vacuum pumping equipment is required in support.  The line-length heat sources are, in order of severity, for a dc cable:

• Radiative heat in-leak, 300 – 77 K

• Turbulent flow friction heat from flowing cryogens and/or gases

• Type II hysteretic losses due to current ripple superposed on the dc level

• Convection losses through the cryostat vacuum, 300 – 77 K, which must be maintained at 10^-5 torr or lower throughout the length of the cable.

• Thermal conductivity through internal support infrastructure (e.g., struts and spacers between concentric tubular structures.

The SHG report estimated for a 1650 km line, vacuum stations would be needed every kilometer and re-cooling refrigerators every 10.  Each of these units requires electric power.  How is this to be supplied?. An ancillary conventional power line in parallel with the SuperCable?  Tap offs from the superconducting cable with inverters?  Tap off of hydrogen to power a fuel cell supply?  Or some combination of all?

How should the immense energy stored in the magnetic field surrounding the 100,000 amp cable conductor in the event of a fault be “dumped?”  GM suggest a parallel resistive shunt that would dissipate that energy to the environment and help damp the resulting L (di/dt) high voltage transients.  Perhaps this could be the same line as that supplying power to vacuum pumps and refrigeration compressors. 

All these “micro-issues” complicate construction of the SuperCable significantly beyond that experienced with gas pipelines.  Having said that, there are several common challenges.  Among these are:

Size of the “structure quanta” to be transported to the construction site, e. g., length of each pipe (or SuperCable) segment to be transported to the construction site.  This is essentially limited by the length of a truck trailer or railroad car (crudely the distance between home plate and pitcher's mound, roughly 20 meters, but could be much longer by barge (how do you get the stock to the barge landing?).

1. Sidebar.  During the westward expansion of the railroad, rail stock and laborer quarters and supplies simply followed, as the track was extended…an option not available to us.

2. ‘Coptering is a possibility.  I have seen ski lift towers put in place in a matter of days once footings were laid and the structures pre-fabed in the area parking lot.

If the SuperCable “quanta” consisted of  a prefabricated flexible tubular structure that could be coiled and spooled, in principle transportation of larger sections could be accomplished.  Nexans has estimated the maximum length of a flexible superconducting cable of nominal overall diameter 20-cm that could be built and spoiled for “conventional” shipment by truck or barge is 600 meters.  Given the possibly more complex design of the SuperCable, and especially the “hybrid-SuperCable” involving LN2, pressurized supercritical H ₂ and a superconducting conductor (utility parlance has it that a “conductor” is something you wind with wires, and a cable is what you put around the “conductor.”  A “conductor” becomes a “line” if you’re talking about overhead transmission…confusing to the layman…nonetheless we will try to stick to this usage herein).

These considerations give rise to additional “on-site” construction challenges unique to the SuperCable.

Creation and maintenance of sufficient vacuum in the cryostat.

1. All present and planned demonstrations of HTS ac cables, save one, intend to continuously pump on the cryostat portion of the cable assembly during operation.  The sustaining of this vacuum proved to be the Achilles' Heel of the Pirelli-Detroit Edison Frisbie Substation demonstration.  Pirelli had employed welding and final inspection techniques appropriate to hermetically sealed conventional underground cables (this is my impression from what was actually fabricated, not directly substantiated by Pirelli).  This weld failed on installation.

2. On the other hand, the LIPA cable installation being undertaken by AMSC and Nexans will use permanently evacuated, to !0^-6 torr, 100 meter segments separated from each other.  Nexans has been quite successful providing cryogenic transfer lines of total length several hundred meters to the European Space Program, with vacuum integrity lifetime guaranteed.

On-site fabrication of the “conductor.”  First of all, the manufactured wire length is currently limited to about three kilometers.  Although this could be extended to perhaps five.  Depending on the “pitch angle” the tape is wound on the conductor, its total length could be a kilometer or more.  This is in far excess of the practical length that a rigid or flexible SuperCable “package” could be fabricated in the factory and subsequently shipped.  However, a much longer flexible conductor, perhaps 2 - 3 kilometers, of small diameter (2 - 3 cm) could be "plant fabricated" then coiled, shipped and threaded through a pre-laid rigid housing.  This would be straightforward for a monopole design, not so simple for coaxial.

Joining and jointing.  This is far more complicated task, for reasons listed above, for the SuperCable than for gas pipelines, which involves more than “simple welding and coating” they require.  The complication is exacerbated if the optimal design is shown to be coaxial instead of two independent monopoles.

1. The manufactured wire length is currently limited to about three kilometers.  Although this could be extended to perhaps five.  Depending on the “pitch angle” the tape is wound on the conductor, an electrical joint or splice will be required every kilometer.

2. A fair amount of experience on the design and construction of HTS superconducting joints exists already by virtue of past and present cable prototype efforts in the US, Europe and Japan.  However, these joints are not "perfectly conducting" as they are in MRI magnets.  A specification issue for the SuperCable would be the relative level of hysteretic losses due to ripple compared to resistance in the joints.  The joint locations may also provide an opportunity for the installation of fault current shunts of the type discussed earlier.

Taking all the above under consideration, we propose several rough monopolar SuperCable designs along the following lines:

Essentially, this is the configuration of a "gas pipe," except that it carries liquid hydrogen with a flexible superconducting "conductor," overall diameter, D _C, with superconducting tapes or wires of thickness t_S laid around a stranded aluminum cable, laying along the bottom of the hydrogen transport tube of diameter, D_H, and cooled by flowing hydrogen slightly sub-cooled to 20 K, held under 2 - 7 atmospheres of pressure.  The overall diameter, D _O, approximately 2 × D_H, is set by the layers of super-insulation and thickness of the high voltage insulation depicted by the outer black ring.  All surfaces out to D _O are "at potential," and reflect the "room temperature dielectric" nature of the monopole approach, allowing normal dielectric materials to be employed.

This design is actually quite simple.  One can envision concentric 20-m sections of the outer pipe, enclosing D _H and D_O, containing both the electrical and thermal insulation, factory prefabricated and shipped to the site and joined in 5-piece sections (100-m), and then evacuated to 10 ^-5 torr and "permanently" sealed with annular "walls" at each end.  Subsequently, five of these completed 100-m segments are connected to form a kilometer unit.  At this point, a 1000-m continuously long flexible superconducting "conductor" is unreeled from its spool and threaded and laid as shown in the above figure and jointed to the next conductor, threaded through another 1000-m segment...and so on...and so on.

To put some practical dimensions on this embodiment of the SuperCable, assume a total power flow of 15 GW...5 electrical ( 25,000 V bipolar at 100,000 A dc circuit) and 10 "thermal" in the form of flowing liquid hydrogen.  These values approximate the Mackenzie Valley Pipeline, assuming 1/2 of the methane potential energy is converted to electricity at wellheads in the Northwest Territory gathering fields. Taking well-established values for HTS wire critical current and reasonable levels of fluid flow velocities, we find the following numbers for D _C and D_H:

Next we need to calculate the thermal losses and pressure drops induced by radiative heat inleak, thermal conduction from ambient, and frictional forces arising from viscous fluid flow respectively.  But first, let's consider an alternative design, one in which we use liquid nitrogen as the cryogen and supercritical gaseous hydrogen at 77 K and 1850 psia as the chemical energy delivery agent (hydrogen gas at this temperature and pressure equals half the energy density of liquid hydrogen at 20 K).

...to be continued.

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