All copper or what?

Practical applications of electrical conductors

Electricity is applied for two fundamentally different areas of application: For the transmission of information and for the transfer of energy. Either of the two cases requires the use of electrical conductors, but which differ a lot in properties and appearance. So does their handling.

Metals: familiar and versatile materials

Aluminium is a good electrical conductor. With a conductivity that is about 65% 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 field (see Table 1). The high prices of gold and silver makes their use in cables, wires, conductors and electrical machines uneconomical, though they do find application as bond wires in integrated circuits where they are used in milligram 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 different metals, have much lower electrical conductivity than pure metals. The only two metals therefore offering 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/m. 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/m – the benchmark against which other electrically conducting materials must be measured.

The choice is limited

The only two metals therefore offering 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/m. 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/m – the benchmark against which other electrically conducting materials must be measured.

Table 1
Table 1: Resistivity values of selected metallic materials compared to the resistivities of various types of water, soils and rocks, which are often treated as ‘conducting’ when discussing earthing systems

Please note that the specific electrical resistance values of the metals and of carbon in Table 1 are given in micro-ohms times metres, those of the other materials, however, in ohms times metres! This already represents the mathematically simplified way of writing »ohms times square metre [of conductor cross section] per metre [of conductor length]«. So this must be imagined as the electrical resistance of a cube with an edge length of 1 m made of the respective material and contacted evenly on two opposite surfaces, with a homogeneously distributed current flowing through it. This means that, with the metallic materials, a voltage in the range of 1 millivolt suffices to drive a current in the range of 1 kiloamp through the cube! With the other materials, however, even a voltage of some kilovolts may be required for a current of just some milliamps!

Logically, the unit »ohms times square millimetre per metre« is preferred for conductor materials. While you cannot any longer shorten out one metre then, and you are kept busy to do more writing, but it still comes at some more ease to imagine a piece of wire with a length of 1 m and a cross sectional area of 1 mm² as a somewhat more practical reference.

Aluminium is a light metal with a density of only about 35% that of the heavy metal 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 significantly, lower than that of copper. Currently, a kilogram of aluminium costs less than half as much as a kilogram 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. At present, a litre of aluminium costs between five and six euros, while a litre of copper is easily ten times as expensive. Although the better conductivity of copper means that two litres of copper can replace three litres of aluminium, the decision to construct a conductor out of copper still makes a conductor some six to seven times as expensive as the equivalent aluminium device. So why is it then that in Western Europe, for instance, aluminium is hardly ever used in the manufacture of electrical machines? One notable exception being its use for cast rotor cages in AC induction motors.

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Electrical machines

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Figure 1: Squirrel-cage rotors cast from copper were exhibited to the public for the first time at the Hanover Trade Fair in 2003

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 efficiency), the current densities have to be reduced by a factor of 2/3, that is the cross-sectional area of the conductor will have to be increased by a factor of 3/2, thus increasing the size of the laminated core and all other mechanical components. However, the electrical sheet steel used for the laminated core is currently as scarce as copper on the world markets – a fact reflected in its price. This higher price 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. A number of these new rotors are now commercially available and have already been used in the first 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 convenient 660°C for aluminium. This led to a significantly higher rate of wear of the casting mould. Fortunately, these problems have now been solved and moulds with economically feasible lifetimes are now available.

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Electric cables

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Figure 2: In high-voltage cables the insulating material makes up a greater fraction of the total cross-sectional area than the conductor material

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.

At least that is the situation for conventional plastic-coated cables. Mineral-insulated cables and wires (Figure 3) are not only absolutely fireproof [6], 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.

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Figure 3: Mineral-insulated cables

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 flush-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 coefficient 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 fire 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 contact. 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 fire. 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 connections may gradually become loose. Connection technology is available that can deal with this problem and it is worth investing the extra cost and effort involved for installations involving relatively few connection points (e.g. HV overhead transmission lines), but not for more complex branched networks such as those found inside buildings.
Figure 4
Figure 4: The structure of ‘fireproof’ plastic-coated cable and mineral-insulated cable

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 contacts can be helpful, but they tend to suffer from the problems associated with the insulating aluminium oxide layers. In both cases, the result is a slow rise in the contact resistance at the connection point and thus to an increased risk of fire. 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 connectors 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.

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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

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 additional step of separating the two materials.

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Figure 6: Only in low-voltage high-current cables does the conductor material make up most of the cable’s total cross-sectional area

As a material, pure copper has a practically infinite lifetime. It can be reprocessed an indefinite number of times without suffering 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 roofing) 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.

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Which metal for which job?

Apart from their electrical conductivity, the other technologically important properties of copper and aluminium differ so significantly (density is an obvious example) that their areas of application 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 segments:

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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
  • 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 offers 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 stiffness 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 compared to those in the equivalently sized copper cable. For technical reasons, so-called ‘finely stranded’ and ‘extra finely stranded’ conductors are only available in copper.8 As a result, the finest aluminium conductors available are significantly stiffer than the finest copper conductors and this difference 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 effort 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 Committee 712 ‘Safety of Information Technology Installations including Equipotential Bonding and Earthing’ (DKE: German Commission for Electrical, Electronic and Information Technologies), meetings at which German experts are becoming increasingly scarce. The Dietlikon electricity utility is the first known distribution network operator that is systematically converting its distribution network to a five-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 configured in this case as a surrounding copper-wire shield, which offers far higher symmetry and EMC than a conventional fifth conductor.
Figure 8
Figure 8: Practical uses of copper and aluminium in the electrical engineering sector: areas in which both metals can be used are rare
  • 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. Differences in manufacturing 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.

 

 

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Figure 9: There are copper busbars, aluminium busbars …
  • 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 differences in material prices. Thirdly, the large number of electrical connections within this small volume mean that the connectivity problems associated 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 (note that in Spanish alumno stands for student or pupil). 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).

 

 

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Figure 10: …and copper busbars made of aluminium
  • 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.

 

 

Aluminium’s undisputed domain is that of overhead high-voltage cables, where space requirements are of no significance but where weight plays a critical role. The lower strength of aluminium 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.

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 substances that could be usefully deployed as conductors?

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Electrical conductivity as a second job: Steel and the like

At times conductors show up that are there anyhow, i. e. metal components of devices, structural metalwork and systems which, once they are in place, make their given electrical conductivity available to fulfil another useful purpose. Two classical examples shall be mentioned here:

Foundation earth electrodes

This designates the simultaneous usage of the given steel armour in armoured steel buildings as equipotential bonding and earthing mesh. However, this requires that this armouring is made up and assembled in a way as to match these requirements, i. e. all crossings and connection points need to provide safe and permanent electrical contacts. The foundation is »earthed« by being located underground anyway. However, if it is isolated against the earth against heat loss and the ingress of moisture, as is common practice nowadays, then it is also electrically insulated against earth, and an additional earthing rod or ring earth outside the building perimeter is required – and just handling such stuff anyway, it is usually made of steel. This is not a good idea at all, since these earthing electrodes – be they zinc galvanized or not – will be rusted away after just a few years. Only an assumed lightning protection remains left over, which is realized only when it is too late. All experts recommend to erect an earthing electrode made out of stainless steel from the »V4A« family of materials (even »V2A« steels are not sufficiently corrosion resistant) or of copper. However, stainless steel is even more expensive than copper and more difficult to process. Beyond, its conductivity is only about 1/50 that of copper, and hence it may occur that even larger cross sections are required, even though the conductor cross section is normally not a critical dimension here.

Train rails

Probably the most classical one of all such applications is found in the electrical operation of all sorts of railway vehicles: Electro-mobility may be – or supposed to become – hip and fashionable on the road; on track it is nearly as old as the railway system itself. This difference is based merely and solely on the fact it runs on an iron track, which can be used as a return conductor for the current at the same time. Stupidly enough, electric current always needs a return path, otherwise it will never ever flow anywhere. To suspend one wire overhead any sort of easement is not an art; with two wires the difficulties start – at the next parting of the ways at the latest.

Sure, mild steels or even high-strength steels are not electrical conductors. You may expect no more than 1/7 the conductivity of copper, but the provided cross section is many times larger than that of the copper wire overhead, and hence it suffices by far.

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Figure 12: Why is it that one piece of rail is connected to the other piece of rail with an electric cable? – Because a train rail is not only supposed to bring a train safely to its destination but also the »used« current safely back to its source

A material for special purposes: Carbon

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Figure 14: Brushes made of “copper graphite” are an alternative to pure carbon brushes

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 first incandescent lamps were produced not with tungsten filaments but with filaments 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 significantly lower electrical conductivity of carbon is insufficient, graphite-copper composites are available (Figure 14). These are not alloys, hence they do not suffer from the reduction in conductivity that typically accompanies the alloying process. Rather, a conductivity somewhere in the middle between those of the two components is reached.

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 fluids 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 resistance limit value, those in the know will carry out the requisite earth resistance measurements 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 differ by between 6 and 12 orders of magnitude!

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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 fillers such as stainless steel flakes, steel fibres, 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 continue to exploit the properties of the organic polymer itself. As a result, the electrical conductivities 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 relatively 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 electromagnetic fields 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 discussion meeting 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 fields. 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 effects of incoming radiation is already (slightly) conducting, then this type of antistatic coating can be dispensed with. It is of course perfectly justified 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 first to be made conducting by incorporating metallic additives. The answer lies in the extrudability of the plastic polymers and the greater scope they offer in terms of coloration and design. But who knows? Perhaps metals will find 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 exposed 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 filled 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 film. Without the PEDT coating, the film would accumulate static charge during the photographic development process. If allowed to build up, the charge can discharge as a flash of light that would re-expose the film and ruin the original image. The final 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, specifically 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 ‘field-strength control’. This enables the electric field to be kept as homogeneous as possible and prevents local spikes in the electric field 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 Kabelwerk Brugg was not deterred from publishing an article in an IEC information leaflet 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 flights 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.

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Superconductors

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Figure 15: The structure of a superconductor: Copper is an essential component of superconducting cables

Superconductivity is a physical phenomenon exhibited by certain materials in which at temperatures below a material-specific critical temperature the materials lose their ohmic resistance making them in principle able to conduct electric current without loss. The discovery of high-temperature superconductors in 1987 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 became 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 previously required. But 100 K is still -173 °C and the effort required to maintain this temperature is large. But this effort may well be worthwhile, particularly in applications that exploit another beneficial 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 effects. Semiconductors are used to generate extremely powerful magnetic fields 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 benefits that remained generally unknown to the wider public. An industry association 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 sufficiently 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 remarkably simple solution to this problem involves a conventional metallic conductor (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.

Meanwhile the idea has finally popped up to merge this component with a superconducting transformer. Hereby this might become a reasonable approach, while the transformer alone is not, since it exhibits too low losses to save more on these than the cooling consumes.

After all, the numerous reports in recent years of the potential of superconductors to save energy should, be viewed with a healthy degree of scepticism. The power network components that we have been discussing such as extra-high-voltage underground cables and large transformers already have efficiencies significantly above 99%, in fact a high-power transformer (≈800 MVA) exhibits an efficiency 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. Distribution transformers have efficiencies 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 significantly 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.

Whereas with underground and undersea cables, an »efficiency« always needs to be referenced to the respective length, since a metre of cable will always have the same amount of losses at equal current and voltage. According to latest information, the latest product in this area is characterized as having a loss level < 5% – even so with a transmission power magnitude of 2.6 GW and a length of 1500 km!

There have also been reports of energy savings of ‘up to 50%’ if the wind turbine mentioned above is fitted with a superconducting generator. First of all, the expression ‘up to’ is usually of no practical worth as it only ever specifies one extremum, while the other extremum in the opposite 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 requirement 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 effectively 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 realities. Superconductors do though offer 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.

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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 beneficial properties among them ‘high electrical conductivity’. But what’s ‘high’? The lowest resistivity value measured so far is 0.34 Ωmm²/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 magnitudes 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 transformers. 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 conventional 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:

It goes without saying that the nanotubes would be destroyed within a nanosecond. But so far this question has no real practical relevance because firstly, 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 website) 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.

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