applied superconductivity_02

7/30/2019 applied superconductivity_02

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2 Overview

A.M. Wolsky, E.J. Daniels, and RF. GieseArgonne National Laboratory

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4 Applied Superconductivity

The recent and sudden discovery of a family of materials that become superconducting attemperatures above 77 K raises the likelihood that further advances are at hand and thatthese advances will lead to commercial applications that conserve energy.

Materials in their superconducting state offer a means to circulate directelectric currents (DC) with no resistive loss. Materials in their superconducting statealso offer a means to convey low-frequency alternating currents (i.e., AC at 60 Hz) withunusually small losses. The absence or significant reduction of losses prompts universalinterest in superconductors as energy savers.

Materials become superconducting only in certain circumstances, which differfor each material. These circumstances (e.g., low temperature) are unusual and havebeen expensive to arrange and maintain. In the past, that expense has been too great topermit widespread commercial applications of superconductivity, although commercialapplications have been made in high-energy physics, medical magnetic resonance imaging(MRI), and -- most recently -- industrial materials separation. Now, there is hope forfurther advances that will lower the cost of applications and enable adoption of thetechnology by utilities and industry.

The most well-known characteristic affecting superconductivity is thetemperature of the material. Niobium-tin, Nb$n, becomes superconducting when itstemperature is less than 16.05 K; the corresponding transition temperature for niobium-titanium, NbTi, is 9.8 K. (On this scale, the Kelvin scale, room temperature is generallyconsidered as 298 K.) The total cost of refrigeration to cool these materials to 1.8-4 Kand maintain their operating temperatures is formidable. This cost includes capital andoperating components. Capital is required to purchase thermal insulation, which slowsthe rate at which ambient heat reaches the superconductor and, in some cases, topurchase equipment to refrigerate the coolant. Operating costs pay for the coolant (i.e.,helium) makeup and, in some cases, for the energy required to remove the heat thatpenetrates the thermal insulation.

As noted above, the new materials (e.g., YBa2Cu307_x) become superconductingat temperatures in the range 77-100 K. This range of temperatures is above a significantthreshold -- it provides the opportunity to use liquid nitrogen instead of liquid helium asthe superconductor coolant. Furthermore, operating in this temperature regime wouldreduce the total cost of refrigeration for two reasons: (1) for the same insulation, therate of heat transfer from ambient temperature to cold superconductor declines as thecold temperature increases (alternatively, the same heat-transfer rate may be obtainedwith less costly thermal insulation) and (2) the cost of removing the heat that penetratesthe thermal insulation declines as the cold temperature increases.

The cost savings for heat removal depend on the type of refrigeration andinsulation system used in a particular superconductor application. Under idealizedconditions, the energy required to remove one unit of heat at 77 K is less than 5% of theenergy required to remove the same amount at 4 K, and the amount of heat that can beremoved by the vaporization of 1 L of liquid nitrogen is 60 times that of 1 L of liquidhelium. Because the cost of liquid nitrogen (per liter) is less than 10% of the cost ofliquid helium, this represents a significant potential for cost reduction in therefrigeration of superconductors. As a practical example, a typical MRI solenoid,


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Overview 5

maintained below 4.2 K, provides a magnetic field of 1.5-2.0 T in a l-m bore. Thecapital cost of the thermal insulation (also known as a cryostat) is about $100,000, andthe annual cost of liquid helium makeup is about $30,000. Were the solenoid maintainedat 77 K, the capital cost of the needed thermal insulation would be $50,000, and theannual cost of liquid nitrogen makeup would be $3,000 - a very substantial reduction inthe total cost of refrigeration.

The second circumstance affecting superconductivity is the strength of themagnetic field around the material. if this field strength is too great, superconductivitycannot be achieved. The new materials are expected to maintain superconductivity atfield strengths greater than those that would prevent superconductivity in thecommercial materials. This property could enable the production of lighter-weightmagnets with strong fields induced by currents circulating within the superconductoritself. Present practice is to insert iron, with a density of 7.9 g/cm, within the core ofan electromagnet, where the field it contributes is at most 2.2 T. However, magneticfield strength is also limited by the ability to accept the mechanical stress that themagnetic field exerts on the currents that produce it. For example, the outward stressor pressure on the interior walls of a long, air-filled solenoid producing the magneticfield B is given by 3.9 atm x (B/l T)2 - thus, a 5-T field exerts a stress of 97.5 atm --and the concomitant tension (tangent to the solenoids wall and perpendicular to itsradius) is given by the product of that pressure and the solenoids radius.

The third circumstance affecting superconductivity is the electric currentdensity, usually described in amperes per square centimeter (A/cm2), within thematerial. The maximum or critical current density depends on the material, itstemperature, and the magnetic field around it. Although the popular press has givenmuch more attention to the critical temperature than to the critical current density, thelatter is now equally important, or more so, for the following reasons:

1. Weight. The weight of material (the density of YBa2Cu307_x isabout 6.3 g/cm3) required to convey a given total current for agiven distance is inversely proportional to the current density withinthe material. Reduced weight means reduced cost for supportingstructures or increased payload for levitation (cranes or trains).This is a reason for avoiding the use of iron.

2. Size. The volume of material required to convey a given totalcurrent for a given distance is inversely proportional to the currentdensity within the material. Reduced size means increasedopportunity to replace equipment for which floor space has alreadybeen allotted.

3. Flexibility. Over equal lengths, material with a large cross sectionis less flexible than material with a small cross section, and thusless easily wound in the form of wire or tape. The needed crosssection is inversely proportional to the current density within thesuperconducting filaments embodied in the wire. Increased flexi-bility means increased ease of handling and increased reliability inthe face of mechanical perturbations.


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4. Cost of Raw Materials. The cost of raw materials is likely to beproportional to the weight or volume of the final product super-conductor, which (as noted above) is inversely proportional to thecurrent density within it.

In addition to current density, three other classes of engineering propertiesdeserve attention. The first is the ability of new superconductors to join with or becoated by other materials. Present practice often requires that superconductors formcomposites with other materials. For example, the tape used in Brookhaven NationalLaboratorys transmission line is a sandwich of stainless steel (for strength), Nb3Sn, andcopper (to shunt current during a fault). The usefulness of new superconductors willalmost certainly increase when they, too, can be part of such composites.

The second class of properties involves chemical stability. The new materialsshow a propensity to lose oxygen and, with it, their superconducting properties. It maybe important to know if the composites required for electrical systems also act topreserve the chemical stability of the superconductor.

The third class of properties affects the AC losses in the new superconductors.As already noted, superconductors circulate only direct currents without loss. However,many applications in the electric power system require superconductors to experiencetime-dependent magnetic fields, or AC currents. Hysteresis loss deserves attention, asdoes the effect on losses of the condition of the superconductors surface.

When superconductors with favorable properties ;Ipe fabricated, they are likely tofind profitable applications on both sides of the meter. Below, we describe our essentialfindings, including the essential findings of the topical sections that follow. Some ofthese findings are also presented in Table 2.1.

1. The critical current densities that have been observed in bulksamples of the new superconductors are too small to permit theirterrestrial commercial application. Research should be devotedto increasing these critical current densities.

2. Because the chemical stability of the new superconductors in thepresence of oxygen (e.g., air) and water is unknown, theirpotential for terrestrial application cannot be evaluated withoutspeculation. Research should be devoted to measuring thechemical stability of these materials and, if needed, to developingsuitable protective coatings. These coatings might also serve toadd mechanical strength (e.g., stainless steel) or provide a heatsink and electrical shunt (e.g., copper and aluminum).

3. The AC properties of the new superconductors are unknown.Thus, their potential for application in generators, transformers,AC power lines, and motors cannot be evaluated withoutspeculation. Research should be devoted to measuring the ACproperties of new superconductors.


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Overview 7

TABLE 2.1 Design Goala and Economic Benefits for Selected &@catiOns

Application

Life-Cycle Dollar SavingsDesign of High-T System (%IOperating DesignCurrent Operating Compared with Compared withField Liquid Helium Conventionaltnsity

(10 A/cm2) (T) Systema Systemb

Geerators,c 300 MU 3d 2 27e 63eTransform rs,P1,000 MVA

10 0.30g 36 60

Transmission lines,113,000 HVA, 230 kV

23h no.1 23 43i

SMES systems,5,000 MWh

60i 1.6-5 5-a Note k

Motors 0.1-0.251 2-3 llrn 21mHaglev systems 1 3 NA NAMagnetic separators 3p 2-5 15 20

aSavings = [(LHe - High T,)/LHe] x 100.bSavings = [(Conventional - High T,)/Conventional] x 100.Generators, which account for l-22 of the capital cost of conventional power

plants, convert shaft power to electrical power. The rest of the plantproduces shaft power and is unaffected by superconductivity. Superconduc-tivity may substantially affect future power plants using MHD or fusion.

dDesired bulk critical current density = 4.5 x IO4 A/cm2; operatingcurrent density in wire (including copper cladding) = lo4 A/cm2.

eBased on materials and operating costs, with refrigeration costs propor-tional to refrigeration power.

fl MVA = 1 MU, if there is no phase difference between current and voltage.go.30 T maximum in the coil windings and 1.75 T in the transformer core.hBulk critical current density = 230 x lo4 A/cm2;,sity = 23 x lo4 A/cm2,

bulk operating current den-or equivalent operating surface current = 500 A/cm.

tconventional underground transmission.J Desired bulk critical current density = 70 x lo4 A/cm2.kDepends on utility characteristics (e.g., load shape and capacity mix).Based on copper windings with a iron core.mAssuming a 20% capital cost reduction for coolant refrigeration.Based on both U.S. and J apanese research during the 1970s.Not available.pBased on a small prototype.


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4. Although the foregoing research and the specific advances calledfor below may increase the efficiency of electricity productionand transmission from all sources, the impact on each may bedifferent. In particular, the choice between renewable andselected conventional sources may be affected. We offer thefollowing examples:- Peaking power is now supplied by units fueled by natural gas.

In the future, such units may compete with superconductingmagnetic energy storage (SMES) for the peak market. Thus,SMES may provide the means for solar energy (e.g., wind poweror photovoltaic cells) to displace natural gas. Solar energy willcontinue to compete with coal and nuclear fuel, burned inotherwise idle capacity (if any), for the SMES chargingmarket.

- Because of their ability to charge and discharge rapidly, smallSMES units may also play a role in absorbing transient powerand discharging level power. This conversion of transient inputto level output may ease the burden of incorporating windgeneration into the grid.

- The cost of electricity delivered to the shoreline from offshoreocean thermal energy conversion facilities might be reduced byusing superconducting, rather than conventional, transmissionlines under water.

5. If current densities of about IO4 A/cm2 can be achieved in wire(including copper cladding) at about 77 K and 2 T, and if thesuperconductor otherwise behaves as Nb3Sn or NbTi, then large(300-MWe) generators using the new superconductors will be moreeconomical than either conventional generators or low-T generators. In particular, a high-T, 300-MWe generator mig&have an efficiency of 99.7% (compared with efficiencies of 99.5%for a low-T, generator and 98.6% for a conventional generator).Increased efficiency would reduce the quantity of air pollutionfrom combustion or reduce the cost of air-pollution control.Engineering research and economic evaluation should be devotedto smaller generators (e.g., 60 MWe), for which there is now agreater demand than for 300-MWe generators.

6. If current densities of about 10 x lo4 A/cm2 can be achieved at77 K, and if the material otherwise behaves as Nb3Sn or NbTi,then the cost of service of a l,OOO-MWe, high-T, superconductingtransformer would be 64% of the cost of service of a low-T,transformer and 40% of the cost of service of a conventionaltransformer. These cost comparisons reflect the higherefficiency of the high-T, transformer (99.92%) compared with the


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Overview 9

lower efficiencies of the low-T, (99.85%) and conventional(99.7%) transformers.

7. I f current densities of about 100 x lo* A/cm2 can be achieved inwire at 77 K and 1.8-5.0 T, and if the material otherwise behavesas NbTi, then the capital cost of large (l,OOO-MWe, 5,000-MWh)SMES facilities might be reduced by 3-8%. The low end of thisrange accounts for savings in thermal insulation and refrigeration,whereas the high end includes savings from inexpensive (2.2-e/g)superconductor materials. Under reasonable assumptions, thesesavings might make SMES competitive with gas-fired peakingplants. Lower current densities (e.g., 60 x lo* A/cm21 might besufficient to make SMES economical. Research should be devotedto determining the effect of increased specific heat, concomitantwith the increase in operating temperature from 1.8 to 77 K , onSMES reliability.

8. I f operating current densities of about 23 x IO* A/cm2, withcritical current densities of about 230 x lo* A/cm2, can beachieved in tapes at 77 K and less than 1 T, and if the materialotherwise behaves as Nb3Sn, then the cost of service for a 66-mi,lO,OOO-MWe, AC superconducting transmission line appears to beroughly 60% of the cost of service of conventional underground,oil-filled-pipe transmission. This cost advantage reflects lowertransmission loss (0.73%) in the superconducting line than in theconventional underground line (3.60%). Both lines are moreexpensive than a conventional aerial transmission line. However,concern about the health and environmental effects from aerialtransmission and the ability to obtain aerial rights of way mayresult in future mandates to construct underground lines.Research and economic evaluation should be devoted to lower-capacity (e.g., 300-1,000 MWe) transmission lines, for which thereis a greater demand than for lO,OOO-MWe lines.

9. I f current densities of about 0.1-0.25 x lo* A/cm2 could beachieved in wire at 77 K in the range of 2-3 T, and if the materialotherwise behaves as Nb3Sn or NbTi, then a conservativeestimate indicates that a large (e.g., 1,500-hp) high-T,superconducting motor, with an iron alloy core, might provideshaft power for 90% of the cost of service of a conventionalmotor. This saving reflects the assumed high efficiency (97%) ofthe high-T, superconducting AC motor and the lower efficiency(95%) of a conventional AC motor. I f the capital cost of thesystem were reduced by about 20% by redesign of therefrigeration system, the high-T, superconducting motors cost ofservice would be about 80% that of a conventional motor.


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10. Most recently, low-T, superconductors have been commerciallyapplied to high-gradient magnetic separation (HGMS) of magneticcontaminants in kaolin processing. Superconductors offer anumber of advantages in industrial processing (e.g., reducedweight, increased throughput, and reduced floor space), inaddition to their 80% reduction in power consumption (includingrefrigeration power) compared with conventional HGMS. Theprimary advantage of high-T, superconductors for industrialapplications, compared with low-T, systems, would be a capitalcost reduction of lo-1596 due to elimination of the heliumrefrigeration/reliquefaction system. Thus, compared with low-T,or conventional HGMS, the cost savings of a high-T, super-conducting HGMS system would be about 15 or 2096, respectively.In addition to competing with conventional HGMS systems inindustry, high-T, superconducting magnets may be applicable toother industrial processes, including (1) gas/gas separation,(2) materials handling, and (3) materials fabrication (e.g., pressfitting of components).

11. High-speed rail is being actively considered for at least a dozencorridors in the United States. Like other systems, magneticlevitation (maglev) is unlikely to be economical without indirectbenefits being added. Advances in superconductivity are unlikelyto change this situation, because present designs allocate onlyabout 1% of the system capital cost to the levitating magnets onthe train. However, if the new superconductors can operate at77 K as well as NbTi operates at 4.2 K, these superconductorsmay offer an ease of operation and promise an increase in systemreliability that will make high-T, maglev systems the preferredchoice among high-speed rail technologies.

Many of the superconductor applications discussed above and illustrated in Table2.1 exhibit large economic savings, even for the liquid-helium-cooled versions..Moreover, several have been developed through the prototype stage. Why have none ofthem been commercialized? First, most technologies employing superconductivity havelarge economies of scale that require large capital investments and the associatedfinancial risks. Second, many of the technologies (generators, transformers, transmissionlines, and SMES) are in the electric utility sector. This sector has curtailed investmentsin recent years because of (1) recent completion of a large capacity-expansion program,(2) slow growth in electricity demand, (3) an existing capacity that consists of equipmentwith long lifetimes, and (4) an uncertain regulatory environment. Also, the electricutility industry places a very high premium on system reliability. These factors havecombined to delay the adoption of any new, superconducting technologies.

The overall impact of successful development of the new high-temperaturesuperconductors cannot be gauged precisely. Not only is there a large uncertaintyconcerning the emerging technologies, but both conventional and low-temperature-superconductor technologies continue to improve. However, if even a fraction of thepotential improvement in energy efficiency is realized, the associated economic benefits


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Overview 11

may be important. In 1983, about 7% of the electricity generated in the United Stateswas lost before it reached the customers meters. Superconductivity may enableincreased profitability and improved electrical system efficiency, with concomitantreductions in environmental impact. For example, a 3.6% loss is expected from a 66-miconventional, underground AC transmission line, while the loss from the competingsuperconducting, underground AC transmission line is expected to be only 0.7%.Improvements are also likely to extend to the customers side of the meter. About 64%of the electricity sold is transformed to shaft power by motors with efficiencies that nowrange from 72% for small motors to 95% for large industrial motors (e.g., 1,500 hp or1,119 kWe). The efficiency of large motors might well be raised to 97%. Further,materials-separation processing in industry is now energy-intensive (about 3 quads peryear*). As familiarity with new superconductors increases, magnetic separation mayreplace present practice in several applications (e.g., cleaning boiler feedwater).

At present, no one knows if or when needed advances wil l be made. Enthusiasmamong researchers is very high, and progress is reported each week. I f confirmed, arecent announcement that critical current densities of about lo3 A/cm2, at 1 T, havebeen observed in a bulk superconductor marks an important step toward commerciallyuseful material.

*One quad = 1Ol5 Btu.

applied superconductivity_02

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