|Fig. 1: T-H phase diagram for various materials. Solid lines and dashed lines show irreversibility field and upper critical field. Data collected from [10,14].|
Superconductivity has found many exciting applications. Storing and transferring power are constituents of several of these applications. [1,2] This document talks about some such applications and some limitations.
Since these materials have "zero resistance", they can carry a "lot" of current with "no" loss and in principle they can store energy in the form of a current loop "forever"! Say this principle is true; the only costs would be to keep the material below the critical temperature and to convert the energy to a desired form. High power applications of superconductors were thought of since the advent of superconductivity but high field/current capability was shown in the early 60's.  Discovery of High TC superconducting materials (HTS) in the late 80's (LaBa2CuO4-x at 30°K, YBa2Cu3Ox at 92°K) and early 90's (Hg2Ba2Ca2Cu3Ox at 130°K) sparked off several government funded programs in Europe, Japan and the United States. [4-7]
The perpetual current loop to store energy, mentioned in the previous paragraph, is known as the superconducting magnetic energy storage (SMES). Similarly, a superconducting power transmission line would reduce resistive losses.  Let's note down a few numbers - transmission lines are quite efficient - they might lose about 7-10% of the power over more than a thousand kilometers; Motors run at nearly 90% efficiency on full load.  So we are not going to get orders of improvement in these. There are interesting numbers to talk about the advantages of using HTS in place of copper and other systems. Table 1 summarizes some of these for power cables. [9,10] What Table 1 does not tell us is the cost of cryogen needed to keep the superconductor up and running; neither does it tell us the cost for going from AC transmission to DC transmission - we still use AC transmission since its easier and cheaper than DC transmission!  Generators run at >90% efficiency on full load, although people who sell superconductivity claim to reduce the (already low) loss by 50%.  Superconducting fault current limiters are called upon because of their ability to respond in a very small time (a fraction of a millisecond).  But what about the numerous capacitor technologies that are being developed? SMES is a promising candidate and has lots of investments in it, since it is a far more generic way of storing energy and lets us efficiently retrieve it, with some practical limitations.  The improvements we get, big or small, have been sufficient fuel for some of these systems to go into production. Major components of the generation, transmission (power cables and devices for superconducting magnetic energy storage), distribution (transformers and fault current limiters) and end-use (motor) devices have been built, primarily using the (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) (a.k.a. BSCCO or "bisko") conductor and some are commercialized. [6,7] Apart from these numbers, its common to hear generic keywords like reliability, efficiency, low environmental impact, compact systems, high capacity, low loss, low carbon footprint, etc.
|Table 1: Comparison of some numbers for power transmission using conventional and HTS cables. Data taken from .|
Before we think of the materials in hand, let us find out the performance requirements of a conductor for different applications. Table 2 presents some numbers that have been agreed upon by researchers and the applied scientists as performance requirements of conductors for different applications.  To choose an HTS, we would like a high magnetic field performance, high (and optimal) current density, feasible critical temperature (great if it is above liquid nitrogen temperature of 77°K) and low cost. It is actually a trade-off and balance among these. Some of the competing superconductors' T-H phase diagram is shown in Fig. 1, and one can interpret many material properties from this diagram. [10,14] Among several superconductors, some prominent ones are the cuprates YBCO and BSCCO, relatively recently discovered Fe based compounds - FeSexTe1-x , NdFeAsO0.7F0.3 and Ba0.55K0.45Fe2As2 single crystals; conventional ones like NbTi, Nb3Sn and MgB2. [4,6,7,14]
|Table 2: Wire performance requirements for some devices. (Jc = 105 A cm-2) |
A good choice of a material from Fig. 1 would be that which allows for a high critical field at a sufficiently high temperature. Being above liquid nitrogen temperature of 77K would make implementation easier. One major hurdle in HTS like cuprates is that current is limited by issues with flux pinning, grain boundaries, misalignment of crystals and fabrication, which are fundamental material issues.  This reduces the current carrying "capacity" of the superconductor, so we end up using up only a fraction of the area of cross section of a conductor to carry current. For example, BSCCO has a high critical field (more than 20 T at 77°K) but suffers from the above mentioned materials issues so its practical field capability (a.k.a. irreversibility field) is at most 0.3 T at 77°K (as seen in Fig. 1), which is good for power applications, as read out from Table 2. A group that worked on the materials issues says that the best current density they could get in YBCO was 10-20% of the capacity of the conductor.  So we see that its not really practical to cover all the applications in Table 2 at 77°K and pushing on the materials issues to get there would give a quantum leap to the industry.
SMES is fundamentally a perfect conductor - not a perfect storage. The energy it can store is just the electricity and it is, by first principles, less than what can be packed in gasoline. So it becomes relevant when we are out of cheap gasoline and when other storage methods like hydro, thermal, etc. are comparable in economics and politics of implementation.
There are visionary ideas for the future applications of superconductivity. One of them was to generate solar power in areas where it is abundant and transport it in an inter continental grid to areas where it is required. Since the sun shines somewhere on the earth at any given time, this grid would obviate the need for storing energy!  The authors of this work estimate that this system would supply 70% of the electrical power needs of the United States in 2050 at a mere $400 billion. Considering the inflation of the world economy and the not so stable US Dollar, this might be a good investment! Another interesting work - a pilot system using superconducting coils on the train to generate the magnetic levitation has been built in Japan, J.R. Maglev.  This can go at 600km/h and they plan to connect major cities in Japan with this soon. A combination of superconducting solar power grid and this Maglev would be a great thing for a low-carbon- emission future.
While many of the performance requirements talked about here are not specific to superconductivity, the fact is that people (including the DOE) are trying to sell it. [6,7] Its also a fact that there are serious engineering limitations to get the materials to perform at their best field and current limits, which might improve with time.
© Suhas Kumar. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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