Download High temperature superconductors

High temperature
superconductors
Fig. 1
䊳 Second-generation conductors
are pending industrial implementation
䊳 Greater grid security thanks
to fault current limiters
䊳 Concepts for future electricity grids
with superconducting cables
䊳 Industrial heating processes use
up to 40% less energy while improving
the production workflow
Second-generation high-temperature superconductor (right) conducts the same
rated current of 200 A without losses as the illustrated copper cable (left).
A
pprox. 100 years ago, the physicist H.K. Onnes discovered
that mercury suddenly loses its electrical resistance below
a temperature of 4.2 Kelvin. Subsequently, superconducting
metal alloys with transition temperatures of up to 23 K were developed.
In the last 30 years, this facilitated pioneering developments, primarily
in medical technology, but also in the physical research. However,
considering its versatility, superconductivity long remained a niche
technology. The main reason is the difficulty of maintaining such low
temperatures. As only helium is liquid below 23 K, it is the only possible
coolant. However, it is expensive and difficult to handle. Also, a significant amount of energy is required to liquefy it.
The response in 1986 was appropriately positive when the physicists
J.G. Bednorz and K.A. Müller developed materials which became
superconductive at temperatures as high as 35 K. By contrast to the
already known metals and alloys, these materials were surprisingly
ceramic substances. Just one year later, the new materials achieved
transition temperatures above 77 K. This allowed liquid nitrogen, a
widely-available, inexpensive industrial product, to be used as a
coolant. This greatly expands the possible areas of application, and
it also makes energy technology applications, and in particular effi-
ciency technologies, possible. However, there is still need for further
development to turn the brittle ceramic materials into superconductive
wires or bands in the required quality and quantity at acceptable costs.
Pilot projects in electricity grids and for motors and generators and
applications in industrial processes show that significant energy
savings are implemented compared with conventional technology.
There is often a secondary benefit, which makes superconductivity
particularly attractive compared with conventional technology. For
example, superconductors are smaller, lighter and more efficient
than conventional conductors, and also allow improvements in
processes in some cases.
The potential of the technology has been identified around the world.
Countries such as the USA, Japan, Korea, China and India have
increased their research activity. Germany is among the front
runners, having offered significant research subsidies in the nineties.
Medium-sized companies in particular can offer highly-developed
products and system solutions. In order to maintain and expand this
good position, the German Federal Ministry of Economics and
Technology promotes the development of the technology and its
applications in numerous projects.
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HTS wires and bands
Fig. 4: Layer architecture of 2G conductors
Fig. 2: Manufacturing 1G conductors
Protective layer
HTS layer
Buffer layer
Extruding
Rolling
Currently, two compounds with the technical
and commercial potential for manufacturing HTS wires are known. Both are ceramic oxide materials with copper oxide
surfaces as a common basis: They are bismuth-(lead)-strontium-calcium-copper-oxide
((Bi,Pb)2Sr2Ca2Cu3O10+x or BSCCO) with a
transition temperature of Tc=110 K and yttrium-barium-copper-oxide (YBa2Cu3O7-x)
with Tc=92 K (YBCO). Producing flexible
and long HTS conductors using these two
materials presents researchers with a variety
of challenges. The materials are brittle and
fragile. They only achieve the required flexibility in sufficiently small dimensions, and
in combination with a metallic matrix or a
metal substrate. The orientation of the crystal structure (texture) also affects the current conductivity via multiple scales and
grain boundaries of the crystalline structure
limit the rated currents which can be
reached. Finally, the microstructure also determines the magnetic field strength until
which the HTS material remains superconductive. In energy technology applications in
particular, this critical magnetic field is often the deciding factor.
While industrial manufacturing processes
are available for the first generation, which
featured embedded long thin silver fibres as
metals, the development is focusing to an
increasing extent on different thin-film
technologies for the band conductors of the
second generation.
Annealing
The first generation – Wires
First generation HTS wires (1G) are made
with BSCCO using the powder-in-tube
method:
The initial material is poured into a malleable silver tube as a fine powder, and the
tube is then extruded to a thin wire. The
diameter reduces from approx. 35 to 2 mm.
The powder particles adapt to the thinner
wire cross-section. In turn, several of these
individual wires are combined in a silver
tube and extruded to a multi-filament wire.
In a subsequent multi-stage annealing
process in an oxygen atmosphere and at
temperatures of up to 900 °C, the superconductive phase is formed as finest crystallite
in the parallel filaments, which are each
separated by the metallic sleeve material.
Multiple rolling phases between the heat
treatments ensure that the crystallite and
conductive CuO surfaces are aligned in
parallel.
1G conductors based on BSCCO are reliable
and durable. The established manufacturing process currently guarantees sufficient
availability. However, the material costs are
high, as the wires have a silver content of
60% and higher. If all crystal axes are not
absolutely perfectly aligned, causing a reduction in conductivity, in particular in magnetic fields above a temperature of 50 K, this
reduces the areas of application.
Fig. 3: Current capacity in magnetic fields
1 bar
106
FCL
Cable
Generators, motors
Transformers
Jc [A/cm2]
Coating rate
Performance (current density)
5 MA/cm2
μm/s
0.01 bar
2 MA/cm2
104
103
2G HTS conductor
[77 K]
nm/s
10-7 bar
1G HTS conductor
[77 K]
102
2
The second generation – Bands
HTS band conductors of the second generation (2G) achieve higher current densities,
are more suitable for magnetic field applications due to the different material class
(YBCO instead of BSCCO), and make lowcost mass production possible. However,
only two American companies currently
offer significant quantities, but European
and in particular German companies plan
to establish production lines. The coated
conductors are based on a layer architecture: On a metallic base band, first ceramic
buffer layers and then the actual superconductor layer are precipitated.
The German Federal Ministry of Economics and Technology subsidises various
processes for industrial production in three
large-scale joint projects. Physical methods
like sputtering, laser ablation or electron
beam evaporation create layers with maximum performance. THEVA GmbH has
already succeeded in exceeding a current
density of 1000 A/mmÇ with electron beam
evaporation. However, as the physical
processes take place in a high vacuum,
lower coating rates are reached than with
chemical processes such as chemical solution deposition (CSD). Metal organic chemical vapour deposition (MOCVD), which
only requires a moderate vacuum, takes the
middle ground.
Fig. 5: Comparison of coating processes2
Pressure
105
Metal substrate
BINE projektinfo 06/10
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CSD
10
15
20
MOCVD
PVD
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Application research
A variety of research projects, demonstration projects and studies, some of which are
funded by the German Federal Ministry of
Economics and Technology, are investigating the broad range of uses of HTS superconductors. These are among others the
following:
■ A 4 MW ship drive motor with a
great torque at low rotational speeds is
currently being tested by Siemens.
■ When renovating a hydroelectric power
plant in Hirscheid, an HTS generator
with higher output capacity replaces a
conventional generator of the same size.
This saves conversion in the building,
which is listed for protection.
■ Converteam and Zenergy are developing
generators for wind power plants and
hydroelectric power plants, including
a gearless 8 MW wind turbine which
is 75% lighter than the conventional
technology.
■ Oswald develops motors in the medium
power range for vehicles, generators and
tool machines.
■ German companies (Nexans and NKT)
are major participants of cable projects
at an international level.
As an example, two developments will be
introduced which offer users other key
advantages in addition to higher energy
efficiency, represent an economic solution
and are interesting for a large number of
potential users.
Industrial heat treatment
In industrialised countries, 1 to 5% of the
overall electricity consumption is used for
material heating in extruding plants. A
newly developed superconductive magnetic
heater consumes approx. 40% less energy
than an induction furnace fired with
alternating current and around 60% less
energy than a gas furnace of comparable
power. The system investment for a magnetic
heater is recouped through increased productivity and reduced energy costs in less
than two years on average. The developers
of the system, Zenergy Power GmbH and
Bültmann GmbH, won the German 2010
Innovation Prize for Climate and the Environment.
The magnetic heater operates on the eddy
current brake principle. The system contains
a superconductive magnetic coil powered
with direct current. The extrusion billets
are rotated by two electromotors in the field
of the magnetic coil. The braking effect of
the magnetic field must be overcome, which
causes the rotating material to be heated.
Short-circuit resistance
of electricity grids
Superconductive fault current limiters increase
the short-circuit resistance and reliability of
electricity grids and can also reduce the
investment costs. During short circuits,
current levels occur in electricity grids
which can exceed the rated current many
times. The grid must be designed for such
loads, i.e. must be short-circuit resistant, to
prevent major damage. Relative to normal
operation, this results in oversizing, which
is primarily reflected in the investment
costs. In the past, grid failures were common, as a result of short circuits in the grid.
Fault current limiters which restrict the
level of short-circuit current can make a significant contribution to increasing the security, availability and reliability of grids.
Ideal FCLs have a low alternating current
resistance (impedance) in normal operation, rapid and effective current limiting in
the event of failures and automatic reusability readiness. Previous measures to restrict
short-circuit currents are used in medium
voltage and either result in permanent increases in impedance during normal operation, or they must be replaced every time
they are triggered (e.g. fuses or Is-limiters,
which disconnect the current path via an
explosive charge). To date, there were no
fault current limiters for the high-voltage
grid – even conventional solutions from
medium-voltage systems cannot be applied
here.
By contrast, superconductive FCLs fulfil all
requirements of an ideal fault current limiter. They take advantage of the fact that the
superconductive state transitions to a normal
conductive state above the maximum current
density. Thus, the superconductor instantly
establishes a high resistance, which restricts
the current efficiently to a designed value,
regardless of how high the expected shortcircuit current is.
Thus, the FCL functions without external
signals and is intrinsically safe. It reactivates
automatically after a brief cooling phase
without further maintenance. There is great
interest in the technology. In Germany, four
companies are currently developing superconductive fault current limiters, some of
which feature different principles.
Nexans SuperConductors devices are the
first in commercial use in public electricity
grids in England and Germany. One of these
has protected the electricity supply of coal
pulverisers and breakers against short circuits in the Saxon lignite power station in
Boxberg since the end of 2009. The operator
is of the opinion that the technology will
Fig. 6: Magnetic heater
Fig. 7: Fault current limiter in the Boxberg
power station
provide a significant gain in health and safety
and plant reliability. If the design proves itself, such fault current limiters could protect the entire internal electricity supply of
power station against short-circuit currents.
Superconductive FCLs are suitable for new
power stations, as well as for extensions
such as retrofitting CO2 separating systems.
The project won the Energy Masters Award
2010.
Superconductive fault current limiters for
high-voltage systems are currently in development, but not yet in practical use. They
should also facilitate the construction of
more efficient grid structures: For example,
110 kV sub-grids, each connected by a 400
kV transformer, can be coupled using FCLs.
This coupling means that one of the two
transformers can be omitted (n-l principle),
thus avoiding both the investment costs and
losses, which reduces operating costs.
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Prospects
PROJECT ORGANISATION
PROJECT ADDRESSES
• Adelwitz Technologiezentrum GmbH
(ATZ), Arzberg, Germany
▼
▼
A new generation of high temperature superconductors is in the starting blocks and
will open the technology for a wide range of applications. While the first generation
of HTS wires was 60% silver, which made it almost prohibitively expensive, the new
HTS band conductors require hardly any expensive raw materials. Various manufacturing processes are being improved or developed, and taken from the laboratory
phase to industrial pilot production with the support of the German Federal Ministry
of Economics and Technology. However, the production capacities remain low. A twokilometre pilot section for a medium-voltage superconductive cable, currently being
considered by power company RWE, would currently require the entire global annual
production of HTS band conductors.
Depending on the market penetration of HTS components, experts estimate the future
demand for HTS wire at between 10,000 km per annum in the next few years to
500,000 km in five to ten years. If the expected significant cost decrease as a result of mass
production is achieved, high temperature superconductors could play an increasingly
important role in power supply and efficiency technologies. For example, it facilitates
lossless power cables, more compact motors and generators with greater efficiency
and efficient heating processes and frictionless superconductive bearings for industrial
applications.
The technology is considered particularly important for the adjustment of the electricity
grid to the changing, increasingly decentralised generation structures. A current study
documents significant advantages, in conurbations in particular. More compact and
lighter transformers and cables can be implemented, while reduced impedance increases
grid stability, and they can be designed with active short-circuit current limiting.
Superconductive magnetic energy storage systems can compensate for power and voltage
fluctuations in the grid. The great current-bearing capacity facilitates new grid topologies,
in which individual grid levels are no longer necessary. Overall, the use of various HTS
components can help save energy in urban grids. Using Cologne as an example, the
study predicts an annual CO2 emission reduction of 60,000 tonnes due to lower grid
losses. Pilot large-scale grid applications are pending implementation. For example,
plans are underway in the USA to connect two large and one small electricity grids
totalling 5 GW via superconductive high-voltage direct current cables. The “Tres Amigas”
project is intended to facilitate the integration of renewable energy sources into the
power supply and also improve the grid stability in the USA.
The range of German institutions and commercial companies researching this area is
unique in Europe and compares well with leading technology nations such as the USA
and Japan. This means that Germany is well positioned to take on a leading role in
superconductive energy technology, similar to the success it has enjoyed with many
environmental technologies and renewable energy sources such as wind and solar
power.
ADDITIONAL INFORMATION
Internet
• www.ivsupra.de
• Bruker HTS GmbH, Hanau, Germany
• GTT Gesellschaft für Technische
Thermochemie und -physik mbH,
Herzogenrath, Germany
• Karlsruhe Institute of Technology
(KIT), Eggenstein-Leopoldshafen,
Germany
• Nexans SuperConductors GmbH,
Hürth, Germany
• Oswald Elektromotoren GmbH,
Miltenberg, Germany
• Theva Dünnschichttechnik GmbH,
Ismaning, Germany
• Zenergy Power GmbH,
Rheinbach, Germany
Picture credits
• Background photo, p. 1, Figs. 1-6:
Zenergy Power GmbH, Rheinbach, Germany
• Fig. 7: Nexans SuperConductors GmbH,
Hürth, Germany
• Background photo, p. 4: Theva
Dünnschichttechnik GmbH,
Ismaning, Germany
Service
• This Projektinfo brochure is also available
as an online document at www.bine.info
under Publikationen/Projektinfos.
Additional information in German,
such as other project addresses and links,
can be found under “Service”.
■ Project Funding
Federal Ministry of Economics
and Technology (BMWi)
11019 Berlin, Germany
Project Management Organisation Jülich
Research Centre Jülich
Dr. Claus Börner
52425 Jülich, Germany
■ Project Number
0327431H
0327429B, C, D, E
IMPRINT
■ ISSN
0937 – 8367
■ Publisher
FIZ Karlsruhe
76344 Eggenstein-Leopoldshafen
Germany
■ Copyright
Text and illustrations from this publication
can only be used if permission has been granted
by the BINE editorial team. We would be
delighted to hear from you.
■ Editor
Dr. Franz Meyer
BINE Information Service
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KERSTIN CONRADI · Mediengestaltung, Berlin, Germany
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