practical applications of electrical conductors

Stefan FassbinderDeutsches Kupfer-Institut

January 2010

Electrical co

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ucto

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Electrical conductors

Practical applications of electrical conductors

With a conductivity that is about 60% of that of copper, aluminium just

fails to make it onto the podium of the best three metallic conductors.

Silver takes gold, so to speak, with the silver medal going to copper, and

gold coming in third to take bronze. Aluminium follows a little behind

gold to take fourth place, but well ahead of the rest of the fi eld of metal

conductors. Non-metallic conductors (carbon, electrolyte solutions,

conducting polymers, superconductors and nanotubes) have their role to

play – often in new applications - and rarely compete with metals.

2



1 Metals: familiar and versatile materials

1.1 Metallic conductors: the choice is limited

With a conductivity that is about 60% of that of copper, aluminium just fails to make it onto the podium of the best

three metallic conductors. Silver takes gold, so to speak, with the silver medal going to copper, and gold coming

in third to take bronze. Aluminium follows a little behind gold to take fourth place, but well ahead of the rest of the fi eld

(see Table 1). The high prices of gold and silver makes their use in cables, wires, conductors and electrical machines

uneconomical, though they do fi nd application as bond wires in integrated circuits where they are used in minute

quantities. All other known elements and compounds trail the top four metals in terms of electrical conductivity

by some way, with many materials not electrically conducting at all. Alloys, which are mixtures of diff erent metals,

have much lower electrical conductivity than pure metals. The only two metals therefore off ering high electrical

conductivity at economically viable prices are aluminium and copper, with the latter setting the benchmark for

all other materials. According to documents published by the German Copper Institute (DKI), the conductivity

of copper used for the conduction of electricity (Cu-ETP-1, Cu-OF-1 or Cu-OFE) is 58.58 MS/m1. The IEC standard

60028 was already quoting a value of 58.51 MS/m in 1925. This corresponds to 101 % of the value in the International

Annealed Copper Standard (IACS), which in 1913 set the standard electrical conductivity of engineering copper to be

58.00 MS/m2, 3 – the benchmark against which other electrically con ducting materials must be measured.

Ω*m

(at 20°C)

Ω*m

from to

Silver

Copper (99.95%)

Gold

Aluminium

CuCrZr alloy

Tungsten

Brass (CuZn37)

Iron

Stainless steel

Lead

Resistance wire (CuNi44Mn1)

Coal, graphite

0.0160 * 10-6

0.0172 * 10-6

0.0220 * 10-6

0.0283 * 10-6

0.0375 * 10-6

0.0550 * 10-6

0.0645 * 10-6

0.1000 * 10-6

1.0000 * 10-6

0.2080 * 10-6

0.4900 * 10-6

40.0000 * 10-6

Sea water

Tap water

Distilled / demineralized water

Ice

Garten soil, top soil, peatland soils

Porous limestone

Wet concrete

Dry concrete

Sand

Gravel, crushed stone

Quartzite, weathered limestone

Rock

0,1

5

2000

10000

5

30

30

2000

200

2000

300

1000

1

100

100000

100000

50

100

100

10000

2500

3000

1000

10000

Table 1: Resistivity values of selected metallic materials compared to the resistivities of various types of water,

soils and rocks, which are often are treated as ‘conducting’ when discussing earthing systems

Aluminium is a light metal with a density of only about 30% that of copper. Furthermore, the day-to-day trading

price of aluminium, which is always quoted per unit weight (strictly, per unit mass), is usually slightly, and sometimes

signifi cantly, lower than that of copper. However, the crucial quantity determining the amount of conducting material

required in a particular application is the conductor cross-section. What counts is therefore the volume and not

the mass (or weight) of material. Although the better conductivity of copper means that two litres of copper can

replace more than three litres of aluminium, copper as conductor material requires twice the mass of aluminium.

So why is it then that in Western Europe, for instance, aluminium is hardly ever used in the manufacture of electrical

machines? Or why are electrical machines using copper lighter and more compact that aluminium designs (for the

same effi ciency)?

3


1.2 Electrical machines

Consider an electric motor in which aluminium rather than copper is used for the motor windings. If this motor

is to be technically equivalent to one wound with copper (particularly with respect to effi ciency), the current densities

have to be reduced by 40%, that is the cross-sectional area of the conductor will have to be increased by 64%, thus

increasing the size of the laminated core and all other mechanical components. However, the electrical sheet steel

used for the laminated core also has its price on the markets, which essentially cancels out the savings made by using

aluminium rather than copper in the windings. As a result, work has been underway for a number of years aimed

at casting the rotor cage from copper.4 A number of these new rotors are now commercially available and have already

been used in the fi rst practical applications (Figure 1). The problem of casting the rotor cages from copper was the

much higher melting point of copper (1083 °C) compared to a much more con venient 660 °C for aluminium. This led

to a signifi cantly higher rate of wear of the casting mould. Fortunately, these problems have now been solved and

moulds with economically feasible life times are now available.5

1.3 Cables and wiring

Space is really a critical criterion when discussing electrical cables and wires. In a low-voltage (LV) plastic-sheathed

cable with conductor cross-sections of up to 10 mm2 per conductor (Figure 5) or in high-voltage (HV) cables (Figure

2), the lion’s share of the cross-sectional area is occupied by the insulating material. If aluminium rather than copper is

used as the conductor material, the additional cross-sectional area required is more or less negligible in comparison.

Figure 1: Squirrel-cage rotors cast from copper were

exhibited to the public for the fi rst time at the Hanover

Trade Fair in 2003

Figure 2: In high-voltage cables the insulating material

makes up a greater fraction of the total cross-sectional

area than the conductor material

Comparison of Dimensions

Mineral-insulated cable2 * 1.5mm2 with copper sheath as protective earthCable comprises copper sheath and mineral insulant, optional outer sheath of LSF plastic

Cable diametre7.2 mm

Typical fire resistant cable3 * 1.5mm2 with protective earth

Cable comprises sheath and core insulation of organic insulant with glass fibre or mica filler andflame protection foil

Cable diametre13.2 mm

Figure 3: Mineral-insulated cables Figure 4: The structure of ‘fi reproof’ plastic-coated cable

and mineral-insulated cable

4

At least that is the situation for conventional plastic-coated cables. Mineral-insulated cables and wires (Figure 3) are

not only absolutely fi reproof6, they also take up much less space (Figure 4) than conventional plastic-sheathed cables.

For a time, these mineral-insulated cables were even equipped with an aluminium sheath, but this never became

established and copper sheathing remains the norm.

And in most European countries, copper is still used predominantly, if not exclusively, for electrical installation work

in buildings. So why is it that most European standards do not permit the use of aluminium conductors with cross-

sections up to 16 mm2 (or in some cases) up to 10 mm2?

There are three main reasons:

• Although aluminium is quite ductile, it is not as ductile as copper. The ends of stiff wires laid in walls

e.g. as connections to fl ush-mounted sockets or wall outlets tend to break after being repeatedly bent back and

forth. This can be problematic if the imminent fracture point is located inside the insulating sheath and if the wire

continues to be used. In such cases the fault can remain undetected until the wire has to carry a sizeable current

(that is one close to its rated maximum current) and although it could be years before this situation arises, when

it does, the conductor material will melt at the fracture point and sustained arcing can occur. Aluminium also

tends to form these local constrictions more readily than copper and as it has a lower melting point and a lower

coeffi cient of thermal conductivity than copper, this sort of local melting will occur more readily in wires and

cables with aluminium conductors. In the worst case, this can cause the aluminium to catch fi re and burn like

a fuse wire.

• When exposed to air, the surface of aluminium rapidly becomes covered by a hard, durable oxide layer that does

not conduct electricity, thus making it harder to ensure electrical con tact. The build up of oxide at points where

aluminium wires are terminated or connected, can increase the local electrical resistance of the conductor. The

increased resistance can cause elevated temperatures with the risk of heat damage to the insulating materials

and possibly fi re. Copper also undergoes oxidation when exposed to air, but perhaps surprisingly, the oxide

layer does not inhibit electrical contact, even though the copper oxides (CuO and Cu2O) have conductivities

some 13 orders of magnitude less than elementary copper and can therefore hardly be described as electrical

conductors.

• Aluminium has a propensity to undergo slow material creep. When subjected to high pressures, the material will

yield over time. One result of this is that originally tight con nections may gradually become loose. Connection

technology is available that can deal with this problem and it is worth investing the extra cost and eff ort involved

for installations in volving relatively few connection points (e.g. HV overhead transmission lines), but not for more

complex branched networks such as those found inside buildings.

Because of the second of the three problems listed above, connections involving the ends of aluminium conductors

should always be made as tight screw-fastened contacts. Unfortunately, the third problem discussed above means

that these joints are often not permanent. Spring con tacts can be helpful, but they tend to suff er from the problems

associated with the insulating aluminium oxide layers. In both cases, the result is a slow rise in the contact resistance

Figure 5: Even in building installation and service cables,

the conductor material still makes a smaller contribution to

the total cross-sectional area than the insulating materials

Figure 6: Only in low-voltage high-current cables does the

conductor material make up most of the cable’s total cross-

sectional area


5

at the connection point and thus to an increased risk of fi re. Grandfathering regulations continue to protect older

aluminium installations in Eastern Germany and in most countries in Eastern Europe, but the only real protection

being provided by this sort of regulation is protection from the threat of improvement! Fortunately, methods are

now available for ensuring proper electrical contact between these older ‘protected’ installations and newer electrical

systems. These con nectors combine spring-loaded contacts with a special contact paste made from grease and sharp

metal particles. When the connection is made the particles penetrate the existing aluminium oxide layer while the

grease protects the contact area from renewed corrosion.7

Copper is also the preferred conductor material in high-voltage cables. Although the use of aluminium would result

in only a slight increase in the overall conductor cross-section, the insulating materials and the exterior shielding

required for HV cables are expensive and the greater total cross-sectional area of the cable would cancel out the

savings made by using the cheaper conductor material – in contrast to the situation with low-voltage power cables

(Figure 6). It is also worth remembering that the cable shielding is always made from copper, because it is the only

material suitable for the job. If aluminium is chosen as the conductor material, then processing the scrap cable at the

end of its (admittedly long) service life will involve the ad ditional step of separating the two materials.

As a material, pure copper has a practically infi nite lifetime. It can be reprocessed an indefi nite number of times

without suff ering any loss of quality. About 45 % of the copper required today is generated from scrap, and the

products for which it is used (cables, transformers, water pipes or roofi ng) will remain in use for a long time, on

average around forty years. However, forty years ago, the demand for copper was only about half of what it is today.

It follows that about 90 % of the copper used at that time is still in use today. This applies equally to aluminium and

other metals. Metals are not consumed, they are used.

1.4 Which metal for which job?

Apart from their electrical conductivity, the other technologically important properties of copper and aluminium

diff er so signifi cantly (density is an obvious example) that their areas of ap plication are and have always been clearly

distinct (Figure 8). And not a lot has changed or is likely to change in that respect. The only really novel development

in recent years has been the introduction of cast copper rotor cages (Figure 1). There are really only three, now four,

areas in electrical engineering in which aluminium and copper are competing in the same market seg ments:

Copper Aluminium

Motors

Transformers

Building wire

All telecom wiring

Low and medium voltageunderground cables

All high andmedium voltageoverhead wiring

Busbars

HV andEHV

undergroundcables

Cast squirrel cage rotors forthree-phase

induction motors

New!

Figure 8: Practical uses of copper and aluminium in the electrical

engineering sector: areas in which both metals can be used are rare

Figure 7: The underground cable in use at

Dietlikon power station in Switzerland: a

compromise solution that combines the

technological properties of copper and the

price of aluminium


6

• Low- and medium-voltage cables: The decision here is which is the lesser of two evils: a greater cable cross-

section or a higher cable weight? Generally speaking, aluminium cable will be substantially cheaper. However,

it is still worth recalling that copper cable is more ductile and less susceptible to electrical contact problems and

thus off ers a greater margin of safety than a corresponding aluminium cable. Due to its smaller cross-section,

the copper cable will also be easier to install as the stiff ness of the cable depends on the square of the cross-

sectional area and thus on the fourth power of the diameter! It is also possible to get very small stranded copper

cable; stranded aluminium cable is only available at nominal cross-sectional areas of at least 10 mm2 and the

individual strands are still very thick com pared to those in the equivalently sized copper cable. For technical

reasons, so-called ‘fi nely stranded’ and ‘extra fi nely stranded’ conductors are only available in copper.8 As a result,

the fi nest aluminium conductors available are signifi cantly stiff er than the fi nest copper con ductors and this

diff erence has on occasion led to some rather costly surprises. On paper, the aluminium conductor may well be

cheaper to buy, but that fails to take into account the extra cost and eff ort involved in installing the less pliable

aluminium cables.9 Recently, a combination Cu-Al cable has appeared as a compromise solution and is being

used at the Dietlikon power utility in Switzerland as an underground cable in low-voltage distribution networks

(Figure 7). A representative from the Swiss Dietlikon plant gave a presentation on the product and the underlying

concept after being invited to attend meetings of DKE Com mittee 712 ‘Safety of Information Technology

Installations including Equipotential Bonding and Earthing’ (DKE: German Commission for Electrical, Electronic

and Information Technologies). The Dietlikon electricity utility is the fi rst known distribution network operator

that is systematically converting its distribution network to a fi ve-wire TN-S system – work that it of course only

carries out during repairs, network expansions and new installations. In this new cable, the phase conductors

have the same cross-section as the neutral conductor, which helps to achieve a symmetrical cable structure. The

phase conductors are made of aluminium, while the same-diameter neutral conductor is of copper, enabling it to

carry a greater current and thus making the cable better suited to coping with the harmonic pollution problems

that are so commonly discussed today. The protective earth conductor is confi gured in this case as a surrounding

copper-wire shield, which off ers far higher symmetry and EMC than a con ventional fi fth conductor.

• Transformers: The problem of winding space is not as acute in transformers as it is in electric motors, which

is why the use of aluminium can at least be taken into consideration. In fact the main leakage channel, i.e. the

gap between the HV and LV windings, must have a certain size for the following three reasons: insulation, limiting

the short-circuit current, and cooling.10 However, a transformer with aluminium windings will be larger if power

losses and all other important operational data, such as the short-circuit voltage, are to be kept at the same level

as an equivalent transformer with copper windings (after all, this is what we mean when we say two transformers

are equivalent). However, the total weight of the marginally larger transformer with aluminium windings will

be slightly lower. Diff erences in manu facturing costs pretty much cancel each other out and in the opinion

of a number of well-respected manufacturing companies, the choice of conductor material is primarily a question

of company philosophy.

• Busbars: In this application, spatial requirements weigh even less heavily in the decision-making process, but still

remain a factor. Secondly, busbar applications are characterized by a large amount of conducting material and

a small quantity of insulating material in a small space. This highlights the diff erences in material prices. Thirdly, the

large number of electrical connections within this small volume mean that the connectivity problems as sociated

with aluminium are more pronounced in such applications. When all these aspects are taken into consideration,

we are left with a stalemate and the question of which material to select becomes almost philosophical. However,

it is important to ensure that prices and costs are not being confused. If price is taken as the main criterion for

selection, aluminium generally tends to be preferred. But if all the costs (including operational costs) are taken

into account, it usually turns out that aluminium can learn a thing or two from copper.11 Copper it seems also has

the better appearance, because some of the aluminium busbars available are copper-coated – not to improve

electrical contact (because drilling, punching and screwing will anyway damage the copper coat), but simply for

aesthetic reasons (Figure 9, Figure 10).


7

• One new area of application is copper rotor cages (Figure 1): In this application, the crucial factor is the greater

electrical conductivity per unit volume of copper. This factor alone made it worthwhile tackling all the technical

problems associated with the development of these devices. For more information, the reader is referred

to descriptions available elsewhere.4, 5

Aluminium’s undisputed domain is that of overhead high-voltage cables,12 where space re quire ments are of no

signifi cance but where weight plays a critical role. The lower strength of alu minium means that the conductor cables

need to be reinforced with a steel core but this does not change the fact that the cables can be produced at low cost

and that the two materials can be readily separated from one another magnetically when scrapped.

2 Non-metallic conductors – a real alternative?

The so-called semiconductors like germanium and silicon, which in the periodic table of the elements are located

between the metals and the nonmetals and which are at the heart of the electronic systems that we know today, are

a subject in their own right and would go well beyond the scope of the present article. But semiconductors aside, we

can ask whether metals are the only other materials capable of carrying an electric current. Or are there other sub-

stances that could be usefully deployed as conductors?

2.1 A material for special purposes: Carbon

We are all familiar with the graphite electrodes in electric arc furnaces, and graphite electrodes were also used

in discharge lamps. In fact, the very fi rst incandescent lamps were produced not with tungsten fi laments but with

fi laments made of carbon. Carbon brushes are still used today to establish electrical contact with the commutator

segments in DC machines. They are called brushes because their predecessors were in fact made from braided copper

and looked like tiny brushes. But graphite has better lubricating properties than copper. In applications in which the

signifi cantly lower electrical conductivity of carbon is insuffi cient, sintered graphite-copper com posites are available

(Figure 11). As sintered materials are not alloys they do not suff er from the re duction in conductivity that typically

accompanies the alloying process.

2.2 Important in electrochemistry: liquid conductors

Electrolyte solutions are typically made up of ionic salts dissolved in water in which the charge-carrying ions are

free to move. The dissociation of ionic salts in water to yield conducting fl uids underlies such important processes

as electrolysis or the generation of electric power in a battery, and it gives soil its electrical conductivity, albeit one

that is very low and strongly weather dependent. In order to show compliance with some (often seemingly arbitrary)

soil re sistance limit value, those in the know will carry out the requisite earth resistance measure ments after a heavy

downpour of rain. It is worth emphasizing that the resistivity values shown on the left in Table 1 all have the factor

10-6 attached. The resistivity values of metals and those of what we commonly refer to as earth therefore diff er by

between 6 and 12 orders of magni tude!


Figure 9: There are copper busbars, aluminium busbars … Figure 10: …and copper busbars made of aluminium

8

2.3 Electrically conducting polymers: the material for a new generation of cables?

Plastic materials that are themselves able to conduct electricity (i.e. organic polymers that are ‘intrinsically conducting’)

are rare. Most electrically conducting polymers (so-called ‘conductive polymers’) are plastics that have been induced

to carry electrical current by adding fi llers such as stainless steel fl akes, steel fi bres, silver-coated glass beads, graphite

or carbon black. The fraction of these additives is usually limited to a few percent by volume so as to be able to con-

tinue to exploit the properties of the organic polymer itself. As a result, the electrical con duc tivities of these materials

are at least four orders of magnitude lower than in metal conductors, in some cases the conductivity is reduced

by as much as 14 orders of magnitude. These re lative ly low conductivities are adequate or even desirable as these

materials are predominantly used to discharge or prevent static charging and to shield high-frequency electrical

or electro magnetic fi elds and waves. When it comes to potential applications for conductive polymers, power and

data transmission cables are hardly top of the list. Although one speaker at a dis cussion meeting13 on the subject

did present a demonstration model in which a torch bulb (estimated current: 50 mA) was connected to a battery via

an electrically conducting polymer rod (estimated cross-section: 10 mm2). The current density in the conductor was,

however, still about three orders of magnitude lower than would be found in copper or aluminium. If these sorts

of materials are going to replace metals in certain applications, they are likely to be used in the form of very thin

foils or extremely thin layers laid down by vapour deposition that are designed to provide shielding from electrical

fi elds. In fact, such applications are already well-established. If the plastic casing of a device that has to be protected

from emitting radiation or protected against the eff ects of incoming radiation is already (slightly) conducting, then

this type of antistatic coating can be dispensed with. It is of course perfectly justifi ed to ask why one would want to

replace a metal casing with a plastic casing, when the metal was anyway better at providing the required screening

properties and when the plastic has fi rst to be made con ducting by incorporating metallic additives. The answer lies

in the extrudability of the plastic polymers and the greater scope they off er in terms of coloration and design. But who

knows? Perhaps metals will fi nd a way to close the gap.

Intrinsically conducting polymers were discovered about 25 years ago, with one such polymer having an electrical

conductivity similar to that of a metallic conductor. The problem with these materials, however, is that they are all

infusible, non-formable and insoluble – making them practically impossible to process. They are also susceptible

to attack by oxygen and when ex posed to air they fairly rapidly lose their conductivity, which it turns out is not

only directionally dependent but also varies strongly depending on the manufacturing process used. It is obvious

that materials with these properties will never be selected in favour of metal conductors, which exhibit far superior

processability and stability. In a number of instances it has proved possible to improve the properties of intrinsically

conductive polymers, but this has always resulted in a reduction in the material’s electrical conductivity by several

orders of magnitude. These materials are used in the same limited areas as the conductive fi lled polymers, namely in

the prevention of static charge build-up. One such material is polyethylene dioxythiophene (PEDT), which is used to

provide antistatic coatings for photographic fi lm. Without the PEDT coating, the fi lm would accumulate static charge

during the photographic development process. If allowed to build up, the charge can discharge as a fl ash of light

that would re-expose the fi lm and ruin the original image. The fi nal image would then look like it had been taken in

a thunderstorm.

It should be mentioned that electrically conducting polymers have been used for some time in power transmission

systems, specifi cally in HV cables, where so-called ‘semiconducting layers’ are introduced once around the conductor

and once between the inner insulation and the outer cable coat, the latter serving to provide ‘fi eld-strength control’.

This enables the electric fi eld to be kept as homogeneous as possible and prevents local spikes in the electric fi eld that

would cause partial discharging and the gradual destruction of the cable’s insulation.

Despite the currently rather limited applications of conductive polymers, one visionary at Kabel werk Brugg was

not deterred from publishing an article in an IEC information leafl et in which he describes a scenario for ‘Power

Networks 2050’ where each network is composed exclusively of cables that are made from conductive polymers

and that can therefore be manufactured in a single extrusion process. The exceptionally high insulating capacity

of the insulating material (around 100 kV/mm) would apparently also enable high-voltages to be used in living

areas. Working electricians will no doubt shudder at the thought. The high capacitance of such cables certainly

helps to reduce EMC problems, but the calculations on which this vision of the future is based completely ignore

the tripping conditions in these cables and fail to take a number of other important factors into account. The

company’s website14 makes no mention of such fl ights of fancy and an enquiry as to what had become of the idea

yielded the information that no one knows anything about it and the author of the original article left the company

some time ago.


9

2.4 Superconductors

Superconductivity is a physical phenomenon exhibited by certain materials in which at tem peratures below a material-

specifi c critical temperature the materials lose their ohmic re sistance making them in principle able to conduct electric

current without loss. The discovery of high-temperature superconductors a little more than 20 years ago resulted in

an astonishing increase in the critical temperature from around 4 K before the discovery to around 100 K afterwards.

In other words, the distance from the critical temperature to absolute zero increased by a factor of 25. Roughly put,

one could say that the use of superconductors in applications suddenly be came about 25 times easier. For instance,

for so-called high-temperature superconductors, the refrigerant medium is liquid nitrogen, which is far cheaper to

produce than the liquid helium pre viously required. But 100 K is still -173 °C and the eff ort required to maintain this

temperature is large. But this eff ort may well be worthwhile, particularly in applications that exploit another bene-

fi cial property of superconductors – their ability to carry current densities approximately one hundred times greater

than those in metals, where current densities are limited by thermal eff ects. Semiconductors are used to generate

extremely powerful magnetic fi elds for research in nuclear physics and for medical diagnostics. They are also used in

the construction of lighter machines for applications in which volume or weight are of crucial importance. For a long

time many of these highly specialized applications delivered behind-the-scenes benefi ts that re mained generally

unknown to the wider public. An industry association15 has now been established in Germany that is working to

promote superconducting applications and improve public recognition of these technical developments. Applications

include a drive system for a naval vessel and an 8 MW wind turbine. Superconducting short-circuit current limiters

also look set to revolutionize power network engineering. Until recently the demands for a vanishingly small network

impedance during normal operations and for a suffi ciently large impedance in the event of a short-circuit appeared

incompatible and a compromise solution was needed. It now seems that it is possible in principle to meet both

demands and a number of systems are currently undergoing practical testing. In addition to the critical temperature

another important parameter of any superconductor is its saturation current density, called quench, that is the

current density at which superconductivity suddenly collapses just as suddenly in fact as it appears. The re markably

simple solution to this problem involves a conventional metallic con ductor (usually made of copper) that surrounds

the superconductor and that carries the current for the very short period until the short-circuit has ceased with the

current limited by the ohmic resistance of the metallic conductor.


Figure 11: Brushes made from a sintered graphite-copper

composite are an alternative to pure carbon brushes

Figure 12: The structure of a superconductor: Copper is an

essential component of superconducting cables

10

The numerous reports in recent years of the potential of superconductors to save energy should, however, be viewed

with a healthy degree of scepticism. The power network com ponents that we have been discussing such as extra-

high-voltage underground cables and large transformers already have effi ciencies signifi cantly above 99 %, in fact

a high-power transformer (≈800 MVA) exhibits an effi ciency of 99.75 % at full load and 99.8 % at half load. In grids

such as those in Germany, Austria and Switzerland no more than 5 % of the electrical energy is lost along the path

between the power generating station and the domestic outlet socket – and most of that 5 % is lost in the heavily

branched low-voltage distribution network. Dis tribution trans formers have effi ciencies of ‘only’ 98.5 % at full load

and 99.0 % when operating at half load.16 Even if copper losses at half load are a quarter of their value under full load

conditions, the energy needed to cool the transformer down to the cryogenic temperatures of a superconductor

remains unchanged. A (relatively large) distribution transformer with a rated output of, say, 1 MVA and losses of 15 kW

(or signifi cantly less than 5 kW when operating at half-load) would have to be maintained at a temperature of 100 K in

order for any sort of energy savings to be made. And even then, only the copper losses would be eliminated, not the

iron losses that actually contribute substantially to the transformer’s life-cycle costs.

Calculations have shown that for an extra-high-voltage underground cable a positive energy balance would be achieved

at transmission powers of 5 GW and above. That corresponds to the total power output from four nuclear power plant

blocks. But a cable of this type does not exist as there is simply no demand for it at present and there is unlikely to be any

demand in the future. The model calculation is thus purely academic and of no real practical utility.

There have also been reports of energy savings of ‘up to 50 %’ if the wind turbine mentioned above is fi tted with

a superconducting generator. First of all, the expression ‘up to’ is usually of no practical worth as it only ever specifi es one

extremum, while the other extremum in the op posite direction and the average value are never mentioned. Secondly, what

is meant here is, of course, a reduction in the losses, which translates to an energy saving of about 1 % of the energy generated.

Wind turbines typically operate at full load for only a relatively few number of hours per year. It is all the more important

then to recall that the copper losses increase with the square of the load, but that the cooling for the superconducting

material is a permanent re quire ment and has to be maintained even during windless periods as the duration of such

periods is unpredictable. It is also worth noting that one could also save about 90 % of the power losses using conventional

copper conductors were these conductors cooled from the usual operating temperature to cryogenic temperatures. The

temperature dependence of the ohmic resistance of copper would eff ectively allow us to create a ‘90 % superconductor’

– but nobody would ever do this, because it is simply not worth it. Finally, we note that superconductivity functions only

fully with direct electric current, and is only partially present with alternating currents. Attempts to use superconductors

directly to avoid ohmic losses and thus save energy are well suited to newspaper reports or political sound bites, but they

tend to be compromised by practical re alities. Superconductors do though off er extremely interesting applications in areas

where copper and silver conductors cannot be used. Returning to the wind turbine discussed above, the generator can be

made smaller and lighter by using superconducting materials and this opens up new performance categories that would

unattainable with a conventional electric generator, as a conventional generator would be so heavy that no crane is currently

available that could lift it into place. A fact that is generally not mentioned too prominently in the relevant press releases.

2.5 Carbon again: Nanotubes

Some years ago the national papers started to report on something called ‘nanotubes’. As the name suggests,

nanotubes are tiny tubes of rolled-up graphite with diameters of around 1 nm. According to these reports, these novel

tubules have all sorts of benefi cial properties among them ‘high electrical conductivity’. But what’s ‘high’? The lowest

resistivity value measured so far is 0.34 Ω·mm2/m – exactly 20 times higher than that for copper.

Physicists have also apparently measured extremely high current carrying capacities for these nanotubes, with some

measurements claiming ampacities of 1011 A/mm2. How is that possible? The answer lies in the minute size of these

tubules, whose diameters are six orders of magni tudes smaller than the wires in a typical electrical installation cable,

meaning that their cross-sectional areas are twelve orders of magnitude smaller. Relative to the cross-sectional area,

a nanotube therefore has 106 times more surface area available than a conventional copper wire over which it can

dissipate heat – a similar ratio to that found between small and large trans formers.17 However, if the nanotubes are

bundled together to produce a conductor with a cross-section of 1 mm2, the bundle will not have much more surface

area available than a con ventional wire, as the following calculation shows: A cube of ‘nanotube material’ with an

edge length of 1 m has a resistance of 0.34·10-6 Ω. If it could be made, a ‘nanowire’ 1 m long and with a cross-sectional

area of 1 mm2 would have a resistance of 0.34 Ω. At the current density of 1011 A/mm2 mentioned above, the ‘nanowire’

would have to carry a current of 1011 A. The power loss in this one-metre-long ‘nanowire’ would therefore be:

P I R AV

AW= = ⋅ = ⋅2 22 2 2110 0 34 3 4 10. . .


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It goes without saying that the nanotubes would be destroyed within a nanosecond. But so far this question has no

real practical relevance because fi rstly, no one is seriously thinking about using these materials as electrical conductors

(information on possible future uses of nanotubes can be found on a dedicated website18) and secondly, the longest

nanotube created thus far is only 1 mm. While that may sound pretty modest on its own, relatively speaking it

corresponds to a length of 1 km in a conventional wire with a diameter of 1 mm. And we all know how important it is

for physicists to see things relatively.

(Endnotes)

1 Information leafl et i4 ‘Kupfer / Vorkommen, Gewinnung, Eigenschaften, Verarbeitung, Verwendung’ [‘Copper: Deposits, extraction, properties, processing,

use’, available from the German Copper Institute (DKI), Düsseldorf, Germany or at: www.kupferinstitut.de

2 www.burde-metall.at/iacs.htm

3 www.copper.org/applications/electrical/building/wire_systems.html

4 www.copper.motor.rotor.org

5 Stefan Fassbinder: ‘Eine runde Sache: Kupferrotoren’ [‘Turning to effi ciency: Copper rotors’] in de, 20/2004, p. 68

6 Stefan Fassbinder: ‘Brandsichere Kabel und Leitungen’ [‘Fireproof cables’] in etz, 1-2/1997, p. 48

7 Fritz Hengelhaupt: ‘Kontaktverbessernde Wirkung von Kontaktpasten für die Elektro-Installation’ [‘The use of contact pastes in electrical installation work’]

in de, vol. 15-16/2001, p. 38

8 EN 60228 (VDE 0295):2005-09

9 Stefan Fassbinder: ‘Rationalisierungsmaßnahmen in kommunalen Stromnetzen’ [‘Rationalization strategies in local electric power networks’] in de, 5/2001, p. 40

10 Stefan Fassbinder: ‘Verteiltransformatoren – Teil 3: Betriebsverhalten’ [‘Distribution transformers – Part 3: Operational behaviour’] in Schweizer Zeitschrift

für angewandte Elektrotechnik, 4/2005

11 alumno: Spanish for student or pupil

12 Stefan Fassbinder: ‘Erdkabel kontra Freileitung?’ [‘Underground cable vs. overhead cable?’], in de, 9/2001, appears in DKI reprint ‘Drehstrom, Gleichstrom,

Supraleitung – Energie-Übertragung heute und morgen’ [‘Three-phase AC, DC and superconducting systems – Power transmission now and in the future’]

from the German Copper Institute (DKI), Düsseldorf

13 www.otti.de

14 www.brugg.ch

15 www.ivsupra.de

16 Stefan Fassbinder: ‘Verteiltransformatoren – Teil 5: Wirkungsgrad von Verteiltransformatoren’ [‘Distribution transformers – Part 5: Effi ciencies of distribution

transformers’] in Schweizer Zeitschrift für angewandte Elektrotechnik, 6/2005

17 Stefan Fassbinder: ‘Verteiltransformatoren – Teil 1: Warum überhaupt Transformatoren in Versorgungsnetzen?’ [‘Distribution transformers – Part 1: Why

have transformers in distribution networks?’] in Schweizer Zeitschrift für angewandte Elektrotechnik, 1/2005, p. 79

18 http://www.pa.msu.edu/cmp/csc/nanotube.html


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practical applications of electrical conductors

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