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Superconducting transmission lines –Sustainable electric energy
transfer with higher public acceptance?
Heiko Thomas
a,
n
, Adela Marian
a
, Alexander Chervyakov
a
, Stefan Stückrad
a
,
Delia Salmieri
b
, Carlo Rubbia
a,b
a
Institute for Advanced Sustainability Studies e.V. (IASS), Germany
b
European Organization for Nuclear Research (CERN), Geneva, Switzerland
article info
Article history:
Received 27 November 2014
Received in revised form
18 June 2015
Accepted 21 October 2015
Keywords:
Environmental impact
Superconducting transmission
Public acceptance
HVDC transmission
Future grid
abstract
Despite the extensive research and development investments into superconducting science and tech-
nology, both at the fundamental and at the applied levels, many benefits of superconducting transmis-
sion lines (SCTL) remain unknown to the public and decision makers at large. This paper aims at
informing about the progress in this important research field. Superconducting transmission lines have a
tremendous size advantage and lower total electrical losses for high capacity transmissionplus a number
of technological advantages compared to solutions based on standard conductors. This leads to a
minimized environmental impact and enables an overall more sustainable transmission of electric
energy. One of the direct benefits may be an increased public acceptance due to the low visual impact
with a subsequent reduction of approval time. The access of remote renewable energy (RE) sources with
high-capacity transmission is rendered possible with superior efficiency. That not only translates into
further reducing CO
2
emissions in a global energy mix that is still primarily based on fossils, but can also
facilitate the development of RE sources given for instance the strong local opposition against the
construction of new transmission lines. The socio-economic aspects of superconducting transmission
lines based on the novel magnesium diboride (MgB
2
) superconductor and on high-temperature super-
conductors (HTS) are compared to state-of-the-art HVDC overhead lines and underground cables based
on resistive conductors.
&2015 Published by Elsevier Ltd.
Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
2. Superconducting transmission lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
2.1. Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
2.2. State of the art –industrial development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
2.3. Main obstacles for widespread utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
2.4. Future potential and path forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3. Advantages of superconducting transmission lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4. Methods............................................................................................................65
5. Visual impact of high-capacity transmission lines –a chance for superconducting power lines? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.1. Overhead line HVDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.2. Standard HVDC underground cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.3. Superconducting transmission line (cable/underground) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6. Technical aspects of superconducting transmission lines relevant for local communities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6.1. Electro-magnetic (EM) fields...................................................................................... 67
6.2. No heat dissipation into the soil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.3. Potential health hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
7. Sustainability relevance for the environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
http://dx.doi.org/10.1016/j.rser.2015.10.041
1364-0321/&2015 Published by Elsevier Ltd.
n
Correspondence to: IASS e.V., Berliner Strasse 130, 14467 Potsdam, Germany. Tel.: þ49 331 28822428.
E-mail addresses: heiko.thomas@iass-potsdam.de,heiko.thomas.ut@gmail.com (H. Thomas).
Renewable and Sustainable Energy Reviews 55 (2016) 59–72
7.1. CO
2
emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
7.2. Efficiency of SCTL with respect to renewable energy transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
8. Cost ............................... ..................................................... ........................... 69
8.1. Capital cost of SCTL in comparison to standard technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
8.2. Right-of-way associated cost. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
9. Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
References.............................................................. .............................................. .. 71
1. Introduction
The objective of this paper is to outline the advantages of
superconducting transmission lines with respect to public and
social acceptance [1–3] and to show the status of existing and
planned superconducting transmission lines, highlighting the
positive sustainability characteristics of this technological option
[4]. The path forward from industrial development to an utility
application with obstacles and the future potential are also given.
Thus the paper aims at giving interested stakeholders a more
complete picture of superconducting transmission lines going
beyond the technical aspects that constitute the focus of existing
literature [5]. Only few of the recent reviews provide an insight
into the social acceptance of superconducting transmission lines,
for instance through detailed cost analyses [6]. This paper evalu-
ates the benefits of superconducting transmission lines and sub-
sequently gives a comparison to established standard transmission
technologies, which increasingly cause public concern as a result
of the electric grid expansion plans [7,8]. Experience shows that
one of the main hindrances to awider use of this technology is not
the research and development (R&D) part, but simply economic
reasoning and missing technology awareness.
Theneedtoupgradeandexpandtheelectricgridtomeetthe
requirements imposed by the access and utilization of renewable
energy sources, intermixed with a global growing energy demand, is
increasingly challenged by environmental questions and the public
community affected by these grid extension plans. This intensifies the
urge to develop new sustainable technologies that can alleviate the
multiple intricate problems arising from ecological, social and eco-
nomic boundary conditions, in order to find suitable solutions for all
involved stakeholders. Apart from the affected local communities, key
stakeholders are the transmission and distribution system operators,
transmission line manufacturers and potential investors.
Superconducting transmission lines are an innovative option to
transfer electric energy and are now being tested and accepted by a
growing number of operators and utilities as part of the electric
distribution grid (for example, the AmpaCity project in Germany
and the LIPA
1
project in the US will be discussed in detail in Section
2.2). This paper highlights the potential of SCTL to minimize the
environmental and visual impact and thereby to increase the public
acceptance of transmission lines compared to the case of standard
HVDC overhead and underground lines. From a technical point of
view, the higher transmission efficiency and the ability to use lower
operating voltages while still preserving the total capacity are the
dominant advantages. However, from the perspective of local resi-
dents and communities affected by new transmission line projects,
the low visual impact due to the incredibly small size seems to be
the main advantage. Moreover, estimated costs will be presented in
Section 8.1 and hint that SCTL can be competitive with standard
cables and even with overhead lines.
One of the showpiece examples for the dilemma explained
above is the electric grid expansion as a consequence of the Energy
Transition (Energiewende)in Germany. The grid expansion has
been challenged for years by a strong opposition against the
construction of new transmission lines, especially of new overhead
lines. As an example, around 21,000 objections were recorded in
the case of the Wahle-Mecklar overhead power line (380 kV,
190 km long) in Lower Saxony and Hesse [7], which amounts to
about one complaint per 9 m of transmission line. Protesters and
affected residents demand the use of underground cables despite
the significantly higher costs. Furthermore, a new bill is now (Nov
'15) debated by the ministry that states the preference of under-
ground cables for the HVDC corridors in Germany. This is appar-
ently triggered by the fact that the approval process of corridor C
(Südlink) especially is hindered by public opposition. Another
example in Europe is the France–Spain interconnection project
that originated from an agreement signed in 1984 between France
and Spain to transfer electricity to Spain. All projects considered
until 2003 were abandoned due to multiple reasons including
public opposition (for instance against the alteration of nature
reserves and touristic areas). The project proposed in 2003 (now
called INELFE
2
) still faced a lot of public opposition and concern
[9]. Several NGOs were formed by local communities and gov-
ernments (“Non à la THT”/“No to the extra high voltage”,“Defensa
de la Terra”). Finally, Spain and France had to ask for a European
coordinator in 2006 to facilitate the implementation of the project.
Of all non-commissioned transmission line projects in the ten-
year-network-development-plan (TYNDP) 2010 at the time of
publishing the TYNDP 2012, 34% was delayed and 3% was canceled
[10,11]. The most frequent reason was public opposition by the
local residents that led to the substitution of some OHL
3
sections
with underground cables. The resulting average delay amounted
to more than 2 years [11]
The concerns related to (overhead) transmission lines are con-
nected to one or several of the following issues [12,13,1]:
Visual impact
Destruction or alteration of the natural landscape
Possible impact on health
Environmental impact
Lower property value
This is mirrored in the obstacles identified for specificprojectsin
the 2007 Priority Interconnection Plan (PIP) by the European Com-
mission [14]. As pointed out by Buijs [15] most projects in the PIP
encountered obstacles related to the NIMBY
4
phenomenon, i.e. people
oppose projects that affect them directly and potentially have a
negative impact on their lifestyle, despite actually having a positive
attitude towards the general idea of for instance the “Energiewende”
[16]. Out of 32 projects, 11 encountered obstacles related to electro-
magnetic fields (EMF), 9 related to environmental issues and 7 related
to visual impact. Only the number of obstacles encountered due to the
1
Long Island Power Authority.
2
Interconnexion électrique France Espagne.
3
Overhead line.
4
Not in my backyard.
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–7260
authorization procedure and legal framework (12) were higher. The
explanation of public opposition purely by the NIMBY phenomenon
may be outdated as the actual reasoning is overall more complex and
multi-layered [17].
With respect to these contention points, superconducting
direct current (DC) transmission lines are likely to generate a very
different reaction due to their ability to carry more energy at a
much smaller size, while at the same time being buried under-
ground, and thus out of sight. This would substantially reduce the
approval times. Also, the cost for right-of-way would be lowered
due to the small size of SCTL.
2. Superconducting transmission lines
2.1. Technology
Superconductors (SC) are materials that can conduct electric
energy without losses below a certain critical temperature TC, i.e.
they are non-resistive below TC. That distinguishes them from
standard conductors like copper that are resistive and have power
losses dissipated as heat. A cryogenic envelope is needed to keep
the superconductor cooled below its critical temperature (see
Fig. 1). State-of-the-art cryogenic envelopes allow less than 1 W of
heat per meter length to enter the cryogenic system as heat influx
from the environment. Since the second law of thermodynamics
states that in a heat engine not all supplied heat can be used to do
work, the mechanical power that is needed at room temperature
in order to have the desired cooling power at the cryogenic tem-
perature is much higher. The theoretically most efficient thermo-
dynamic cycle is the Carnot process characterized by the Carnot
factor, which defines the efficiency of the process and depends on
both the cryogenic temperature and the higher temperature of the
environment (T¼300 K). The Carnot factor is 3 for liquid nitrogen
(T¼77 K) and 14 for liquid hydrogen (T¼20 K), meaning that the
cooling efficiency is 4–5 times higher if using liquid nitrogen
compared to liquid hydrogen. However, in a superconducting
transmission line the electric losses due to cooling can be kept
small for all considered coolants, as compared to the transferred
power and to the losses of standard conductors.
The critical temperature of a SC varies in a wide range and
there are basically two types of superconductors, low-temperature
superconductors (LTS) like niobium titanium (NbTi, T
C
¼9.2 K) and
high-temperature superconductors (HTS) like yttrium-barium-
copper-oxide (YBCO, T
C
¼93 K). Most LTS need to be cooled by
liquid helium (T¼4.2 K), while HTS can be cooled by liquid
nitrogen (T¼77 K) allowing for a simpler design of the cryogenic
envelope and opening the door for electric grid applications. With
the discovery of superconductivity below T¼39 K in magnesium
diboride (MgB
2
)in2001[18], a promising new superconductor has
come on the scene, that can be cooled by either gaseous helium or
liquid hydrogen, is based on raw materials that are very abundant
in nature and is therefore cheaper than any other competing
superconductor.
In the more than 100 years since its discovery [19], super-
conductivity has been successfully applied to a significant number
of large-scale particle-physics experiments, for instance
Acronyms and nomenclature
AC alternating current
BImSchV Federal Emission Control Act Concerning Electroma-
gnetic Fields
DC direct current
EMF electro-magnetic fields
GHe gaseous helium
GIL gas insulated line
HTS high temperature superconductors
HV high voltage
HVDC high voltage direct current
LH2 liquid hydrogen
LN2 liquid nitrogen
LNG liquefied natural gas
LTS low temperature superconductors
MgB
2
magnesium-di-boride
MRI magnet resonance imaging devices
NbTi niobium titanium
NIMBY not in my backyard
OHL overhead line
E polyethylene
RES renewable energy share
ROW right-of-way
SC superconductors
SCTL superconducting transmission line
SF
6
sulfur hexafluoride
TL transmission line
TSO transmission system operators
TYNDP ten-year-network-development-plan
VSC voltage source converter
XLPE cross linked polyethylene
YBCO yttrium–barium–copper-oxide
cm [39] centimeter
g gram
GW gigawatt
GW h gigawatt hours
Hz hertz
K kelvin
kA kiloampere
kA m kiloampere meter
km kilometer
kV kilovolt
kW h kilowatt hours
m meter
mm millimeter
MW megawatt
mT microtesla
Fig. 1. Design of a high temperature superconducting (HTS) cable for AC operation
with 3 phases cooled by liquid nitrogen (copyright Nexans).
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–72 61
superconducting magnets, superconducting accelerator cavities
and detectors used in accelerators at CERN, DESY, Brookhaven and
Fermilab, as well as the fusion machine ITER. Additionally, super-
conductivity is today widely used in a number of commercial
applications, for instance in NMR magnets, generators (wind tur-
bines, hydro power plants, ship engines), transformers, wireless
receivers in communication technology, inductive (metal) heating
systems, magnetic levitation train (Maglev), fault current limiters,
and superconducting magnetic energy storage (SMES).
One of the first proposed practical applications of super-
conductivity, envisaged for it already in 1915 by its discoverer
Heike Kamerlingh-Onnes, is the transmission of electric power
without losses. Apart from the lack of resistive losses, the very
high current densities associated with superconductors allow for
much smaller dimensions of the conductor and cable compared to
the case of standard conductors.
The overall design of SCTL shares many similarities with natural
gas pipelines, as far as carrying a highly pressurized medium and
the need for refrigeration/compressor stations along the line.
However, the dimensions are smaller (a few 10 cm compared to
140 cm diameter) and the maximum pressure is much lower
(20 bar or less compared to 85 bar). There is no availability data for
large-scale SCTL with cooling stations several tens of km apart
because they have not been implemented so far. To give an
impression of the reliability and availability of a large cryogenic
system one can refer to the Large Hadron Collider (LHC) of CERN
which has the longest and most complex cryogenic system in the
world with a length of 27 km. The magnets operate at a tempera-
ture of 1.9 K, which is much more challenging than the cooling
temperatures of 15 K or 70 K necessary for MgB
2
and HTS SCTL.
10,080 ton of liquid nitrogen and 136 ton of liquid helium are
necessary to keep 36,000 ton of cold mass (magnets, equipment) at
its nominal operating temperature. The system consists of about
60.000 inlets and outlets and has been running continuously from
2007 to 2013. It achieved a global availability of 94.8% for the year
2012 and an availability of 99.3% for each of the eight 3.3 km long
cryogenic segments [20]. The non-availability time was caused by
the cryogenic system (3.3%), by scientists conducting experiments/
users (0.4%) and by other events (1.2%) triggered by single experi-
ment events, IT or electricity supply by utilities. Thus, the cryogenic
system of SCTL considered in this paper can have a much higher
availability. Not only would the setup be much simpler for cooling
only a bi-polar conductor, but the operating temperatures would be
much higher and operation less demanding.
2.2. State of the art –industrial development
The idea of employing superconducting transmission lines to
transfer GW of power over long distances has been around for
decades [21] and is now making its way into real world grid
applications [22–25] because SCTL offer benefits to TSOs that cannot
be provided by standard solutions. At the moment we see the
technology stage in between innovation (demonstration projects)
and niche application (field projects) with high learning rates [26].
As for every commercially available product, a crucial point is the
economic advantage for the operator and for the end user. The
superconducting tapes and wires, i.e. the superconducting con-
ductors itself, are more advanced in terms of technological devel-
opment because they are increasingly used in a wide range of
applications like magnet resonance imaging devices (MRI), electric
generators or current leads for electric energy intensive industries
like metal refining. Here, an accelerated cost reduction due to
economy-of-scale reasons is likely. It is worth noting that the flex-
ible cryogenic envelopes are already commercially available, they
are used to transfer for instance liquefied natural gas (LNG).
As of now, many demonstrators and proof-of-principle super-
conducting cables have been commissioned by utility companies
worldwide or are already in operation and fully integrated in the
electric grid, as listed in Table 1. The average length of these
superconducting cables is a few hundred meters and capacities are
fairly low, but nonetheless these SCTL offer intrinsic advantages
like the ability to tailor the voltage level, especially to lower it. The
AmpaCity HTS cable [27,22] connecting two power substations in
the city center of Essen holds the record with 1 km length (Spring
2014), but will be soon surpassed by the St. Petersburg cable with
2.5 km length [24,28]. In the case of AmpaCity, the responsible
utility company RWE was convinced by an economic study that
showed that a SC cable is one of the two cheapest options to
upgrade the existing grid. In particular, by employing a SC cable,
one can take advantage of its high current density to operate
at a lower voltage (10 kV) and one can thus eliminate the aging
110-10 kV AC transformers.
Table 1
Global superconducting cable projects that are planned to operate in the electric grid.
Project Location Length [m] Capacity [MVA] Schedule Operator
LIPA Long Island/USA 600 574 (138 kV AC, 2.4 kA) In operation since 2008 LIPA
AmpaCity Essen/Germany 1000 40 (10 kV AC, 2.3 kA) Start of operation 01/2014 RWE
Amsterdam/NL 6000 250 (50 kV AC) Proposed Alliander
St. Petersburg Project St. Petersburg/Russia 2500 50 (20 kV DC, 2.5 kA) Start of operation 2015 FGC UES
a
Ishikari Ishikari/Japan 2000 100 (710 kV DC, 5 kA) Start of construction spring 2014 City of Ishikari
Icheon/Korea 100 154 (154 kV AC, 3.75 kA) Operating since 11/2013 KEPCO
b
Jeju Island/Korea 1000 154 (154 kV AC, 3.75kA) Operation 2015 KEPCO
Jeju Island/Korea 500 500 (80kV DC) Operation 2014 KEPCO
HYDRA Westchester county/USA 170 96 (13.8 kV AC/4 kA) Start of construction early 2014 ConEdison
Yokohama/Japan 250 200 (66 kV AC, 5kA) Operation stopped December
2013, continuation planned with
new high-performance refrig-
erator 2015.
TEPCO
c
China 360 13 (1.3 kV DC, 10 kA) Operation since 2011 IEE CAS
d
REG
f
Chicago/US 5 km to be specified Planning since 2014 ComEd
e
Tres Amigas New Mexico/US 750/5000 Postponed Tres Amigas LLC
a
Federal Grid Company United Energy System.
b
Korea Electric Power Corporation.
c
Tokyo Electric Power Company.
d
Institute of Electrical Engineering, Chinese Academy of Sciences.
e
Commonwealth Edison.
f
Resilient Electric Grid
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–7262
Another prominent example is the LIPA project [29],a600m
superconducting power cable operating in the grid at 138 kV and
2400 Ampere since 2008 and based on HTS material. It was
commissioned by the Long Island Power Authority (LIPA) that was
established in 1998 as the primary electric service provider for
Long Island. Expecting a significant increase in energy demand
until 2020, LIPA made substantial investments in system upgrades
and improvements, thereby acknowledging the promise of the
superconducting technology.
LIPA recognized superconducting power lines as a possible
solution to various needs and related problems [23]:
a. Right-of-way (ROW) congestion: superconducting cables pro-
vide increased power transfer capability within existing ROWs
b. Public acceptance: permission problems for overhead lines
c. Potential cost savings: cheaper than upgrading to 345 kV over-
head transmission systems
Despite the increasing number of demonstrator projects, the
awareness or acceptance as a mature technology among decision
and policymakers is small. According to the TYNDP 2012 (page
206) superconducting transmission lines are still seen as a tech-
nology that is in the research stage, i.e. the lowest development
stage, with no practical application yet [11]. However, as described
earlier, the LIPA cable has been operating in the grid since 2008
with a nominal capacity of 574 MW (reduced to 150 MW by
bottlenecks created by the standard technology grid) and the
AmpaCity in Essen has shown reliable operation since early 2014.
If successful in the longer term, it can lead to retrofitting 30 km of
standard technology transmission lines in Essen. These projects
can be rated as being in large-scale testing phase (stage 2 follow-
ing the TYNDP 2012). Stage 1 technologies are mature and have
already proven their general reliable applicability within the
existing meshed grid.
As a last example of reliable operation, Fig. 2 shows the HTS
test facility of TEPCO in Yokohama [30]. During two years of
operation within the Asahi substation of an HTS cable built by
Sumitomo Electric Industries (SEI), no faults were reported. The
installation, including the refrigeration system, was remotely
monitored from TEPCO in Tokyo with no service man at the sta-
tion. TEPCO has shown continued interest in superconducting
transmission lines because the coastal area of Japan especially
around Tokyo is very densely populated. Due to the small size of
SCTL, existing ROW could be used, rendering new transmission
corridors unnecessary and/or making system upgrades possible.
2.3. Main obstacles for widespread utilization
From a technological point of view, SCTL have a higher com-
plexity than standard transmission lines. The fact that during
operation they rely on a fluid at cryogenic temperatures can be
seen as a disadvantage. The cryogen cannot be allowed to transi-
tion into the gas phase and because the cooling system is powered
by electric energy, it needs an absolutely reliable power source.
The electric energy could be tapped from the TL itself and backed
up by on-site RE sources in combination with energy storage
devices in remote areas. Here it is worth pointing out that the
natural gas pipeline system has a very similar setup and has a
proven record of operating reliably over many decades. The
complex large-scale cryogenic system of the LHC
5
at CERN, that
achieved availabilities above 99% per year, can also be taken for
comparison [31]. In terms of maintenance, no significant degra-
dation of the superconducting cable is expected compared to
standard cables.
However, for longer-distance field installation, the cryogenic
envelope and cooling system and the joints connecting the various
cable segments represent the main technical and engineering chal-
lenges. This stems from the need for a good high-voltage electric
insulation combined with the need of perfect thermal insulation
when creating a temperature bridge from room to cryogenic tem-
peratures. The design of the superconducting cable itself also
requires substantial engineering for optimum performance (espe-
cially for AC operation due to the fast switching magnetic field). But
these challenges have been already addressed and solutions only
need to be adapted to the specific transmission line project. Few
official technical guidelines and specification codes for operation
exist, although recently there have been increased efforts in this
direction.
From an economic point of view, the projected capital cost of
superconducting transmission lines and the necessity of economic
competitiveness play a vital role in the utilization and further
application of SCTL in the electric grid. No grid operator will install
an SCTL if the benefits do not outweigh the disadvantages when it
comes down to projected costs. The high cost of the HTS tapes very
surely hindered the utilization of SCTL on a larger scale up to now.
With an increased factory output and new cost-saving production
technologies these costs can be reduced, likely to 50$/kA m for
HTS tapes in the near future. Until HTS reaches economic com-
petitiveness, MgB
2
based SCTL will see increased interest,
Fig. 2. TEPCO/Sumitomo 66 kV AC HTS test station at Asahi substation in Yokohama/Japan: left image shows the cable with a joint, right image shows the 66 kV AC end
stations responsible for the transition from standard conductor to superconductor and from room to cryogenic temperature (2014).
5
Large Hadron Collider.
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–72 63
development and application. From the perspective of an indus-
trial company, the development of superconducting power line
technology involves substantial financial investments and can
therefore be considered rather risky, given the small niche appli-
cation market at the moment. Also, the investments that were
already made in R&D of “standard”transmission line options have
to be first amortized.
Also, the awareness of the regulatory bodies with regard to
SCTL is quite low, hence a lot of dissemination work is needed in
the future, as outlined in the next section.
2.4. Future potential and path forward
As already mentioned, SCTL have a much higher capacity per
size/width ratio than any other transmission line option. That
makes them the first choice if limited space meets high-capacity
transmission needs, for instance in ROW-impacted areas, like
urban areas or densely populated areas in general. If SCTL are
economically competitive compared to standard transmission
lines (TL), they can potentially replace vast fractions of the existing
medium- and high-voltage grid (as in the AmpaCity project).
Theoretically, the complete high-voltage grid could be changed
into a superconducting low- to medium-voltage grid, making
high-voltage up and down transformers unnecessary with a direct
power plant to city connection at the turbine output voltage
(10–30 kV).
SCTL are inherently predestined to transfer large amounts of
electric energy due to the absence of losses except for the cooling
losses. The higher the capacity, the more attractive is a SCTL with
respect to energy efficiency (please see chapter 9) [32]. The
underlying reason is that the design and size of a SCTL do not
change much when increasing the capacity, due to the high cur-
rent density of superconductors. The cost/capacity ratio is smaller
for higher capacities, especially for cheaper superconductors like
MgB
2
because the costs for the cryogenic envelope and trenching
are practically fixed and only the additional SC material has to be
paid for. However, low-capacity SCTL can still be economically
competitive and be used to overcome disadvantages of existing
grids. For instance low-voltage SCTL can be used to remove high-
voltage lines and transformers. A technical advantage of the SCTL
is that the capacity is not reduced in hot climates compared to
standard TL. For the transport of tens of GW over distances of
several 1000 km, standard solutions are not suitable yet because
their electric losses and voltage drops will be too high [33]. Here,
SCTL may be the only viable option.
In the next years, it will be very important to continue to show
economic competitiveness with standard options and demonstrate
reliability under real grid operating conditions. To have a sig-
nificant impact on the energy efficiency and sustainability of the
electric grid, projects with longer length of superconducting lines
need to be pursued. Moreover, it is mandatory to have codes and
standards for operation and safety issued by official international
bodies.
Beyond these steps, the visibility of this technology to key
stakeholders (TSOs, DSOs, regulatory bodies) needs to be increased
by strong information and dissemination activities.
Most of these points are addressed by a newly started colla-
borative project on novel energy transmission applications within
the European Commission's 7th research framework, which was
funded with EUR 63 million. The project acronym BEST PATHS
stands for “BEyond State-of-the-art Technologies for rePowering
AC corridors and multi-Terminal HVDC Systems”and was chosen
to reflect the variety of the five demonstrators involved. Thus, one
of the five constituent demonstrators is a superconducting high-
power transmission line based on the novel MgB
2
technology
pioneered in an experimental collaboration between CERN and the
IASS. Headed by the leading cable manufacturer Nexans and
bringing together transmission operators, industrial manu-
facturers and research organizations, this project envisages the
development of a monopole cable system operating in helium gas
in the range 5–10 kA/200–320 kV, which corresponds to a trans-
mitted power of 1–3.2 GW. The current international practices will
be taken into consideration by using the recommendations issued
by the International Council on Large Electric Systems (CIGRÉ).
The research and demonstration activities will be accompanied by
a comprehensive dissemination package.
3. Advantages of superconducting transmission lines
SCTL share the advantages of underground cables compared to
overhead lines:
1. Very low visual impact on the landscape due to their under-
ground location.
2. Generation of lower electromagnetic fields that could affect the
surrounding area.
3. Smaller environmental footprint than overhead lines (except for
wetlands).
4. Minimization of land use and property acquisition, leaving the
value of local real estate unchanged.
5. No affection by most natural weather phenomena such as wind,
fog, snow and ice.
6. No emission of noise.
In addition to these advantages buried cables have in general,
SC cables comprise several other advantages compared to standard
HVDC underground cables. The following points highlight the
advantages of superconducting power lines compared to the most
modern underground standard HVDC cables (7320 kV XLPE
HVDC):
1. A size advantage (a few 10 cm width of only one needed SC
cable compared to a 17 m wide trench consisting of 24 cables
for 10 GW capacity for a standard HVDC 7320 kV cable
installation - not including 2.5 m safety area on both sides).
2. Much smaller land use potentially as low as 10% of standard
HVDC cable installations depending on the capacity, area
(urban or land) and regulations.
3. Appealing option for long-distance and high-capacity electric
energy transport if underground cables are required because
standard conductor cables have high losses ( 46%/1000 km at
100% load for 7320 kV XLPE HVDC).
4. Adjusting the nominal current to meet the desired or existing
operating voltage, especially that of medium and low voltage
grids. Thus eliminating transformers results in less occupied
space and less components in the grid chain that are prone to
technical failures.
5. Much better option for hot climates because of the vacuum-
isolated cryogenic envelope that prevents heat from entering
the system and therefore stabilizes the temperature of the SC
conductor. The capacity of standard HVDC cables is reduced by
higher soil temperatures (the resistivity of Cu and Aluminum
increases with higher temperatures and so do the power
losses).
6. Do not heat the surrounding soil (does not alter soil humidity).
7. Option for a hybrid transmission line, transferring not only
electrical energy but also hydrogen, the fuel with the highest
energy density per weight (Please note that the efficiency of
the hydrogen liquefaction process is rather low and that it
takes 40% of the chemical energy of hydrogen to liquefy it from
300 K to 20 K).
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–7264
8. Much easier use of existing right-of-ways (ROW) to transfer
GWs of power
9. Can potentially be operated in AC with much smaller losses
than standard HVDC cables. No cost-intensive AC–DC converter
would be needed.
10. The cryogenic system can store energy by cooling to lower oper-
ating temperatures at times of high renewable energy input.
It is apparent, that the advantages of SCTL address many public
concerns. Especially the size advantage can potentially decrease the
public opposition against new transmission lines. The technological
advantages of SCTL are evident and TSOs can profitfrominstalling
and operating SCTLs (potentially reduced delays, technological
advantages) creating a win-win situation with affected
communities.
4. Methods
The characteristics of superconducting transmission lines used
for comparison with standard technologies in this paper stem
from a long-distance SCTL design that was developed at the
Institute for Advanced Sustainability Studies e.V. (IASS) in Potsdam
[34]. The important numbers highlighted in this paper which are
relevant to public and social acceptance, i.e. size, cost, EM-fields
and efficiency are based upon this design and were derived
according to standard thermo- and fluid dynamic and electrostatic
theories resulting in a design that fulfills the requirements for
efficient long-distant electric energy transfer. The cryogen pres-
sure dictates the stainless steel wall thickness (cost factor) and the
distance between cooling stations plus the necessary mass flow (to
remove the heat) determines the hydraulic and finally the outer
diameter (size, cost factor) which subsequently determines the
heat influx and the transmission efficiency. Please be referred to
the existing literature for a complete technical description [6,34]
as this is beyond the scope of this paper.
Based on the superconducting material MgB
2
, it can either be
cooled by liquid hydrogen or gaseous helium plus liquid nitrogen
with cooling stations located every 300 km in the first design
phase. The design can easily be adapted to other separations. In a
cooperation between CERN and the IASS Potsdam a super-
conducting prototype cable based on MgB
2
was successfully tested
in 2014 with a direct current rating of 20 kA [35]. The test con-
figuration consisted of 2 20 m long MgB
2
cables that were
Fig. 3. Possible layouts to fulfill HVDC 5 GW power transmission requirements with ROW in forested areas and associated costs. The size proportions are meant to be
realistic. A capacity of 5 GW was chosen with respect to the planned North–South HVDC transmission lines in Germany. The ROWs for 10 GWand 3000 km length are 245 m
(7800 kV), 22 m (7320 kV underground cable) and 5.5 m (superconducting line).
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–72 65
immersed in helium gas to maintain the required temperature
(20 K). The total diameter of the cable setup and the cryogenic
envelope was only 16 cm. This success caused various interests on
the industrial and transmission system operator (TSO) side and led
to the formation of a European consortia of industry, research
centers and TSOs with the goal to design and test a high-voltage
(7320 kV) prototype MgB
2
cable to validate its operation in a real
grid (BEST PATHS project as part of the 7th European framework
programme).
For completeness, a 7100 kV DC long-distance SCTL design
developed by EPRI based on high temperature superconductors
(HTS) is used for comparison [6]. The standard technologies taken
for comparison are the 7320 kV HVDC XLPE cable as a direct
competitor –as it is also buried underground –and 7500/800 kV
HVDC overhead lines (OHL) as it is the preferred technology by
TSOs for high capacity long distance transfer of electric energy up
to now.
Please be referred to the existing literature for a complete
technical description of superconducting transmission lines [6,34]
as this is beyond the scope of this paper.
5. Visual impact of high-capacity transmission lines –a chance
for superconducting power lines?
The main objection point of communities who oppose the
construction of new transmission lines is the visual impact.
Whereas smaller electric powers could be transferred by using
standard underground cables (either AC or DC) and that way
reduce the visual impact, the transmission of electric power in the
range of several GW will leave a substantial corridor width if
established transmission technologies are employed. In urban
areas it is practically impossible to install new multi-GW trans-
mission lines using standard technologies. Examples for such
urban areas can be the Ruhr district in Western Germany as a
consequence of new North–South HVDC corridors to be build and
the coastal lowlands of Japan where existing ROWs have to be
used to increase the transmission capacity.
5.1. Overhead line HVDC
A single pylon of a 7800 kV 6.4 GW HVDC power line has a
height of 50–90 m and the width of the corridor was estimated to
be 125 m adapting similar calculations from 380 kV AC corridors
and regulations in Germany. Two HVDC lines capable of transfer-
ring 10 GW (max. 12.8 W) have a width of 245 m. Towers for
7500 kV HVDC TLs have similar dimensions and require much
broader ROWs due to the lower capacity which scales with the
square of the voltage. The visual impact is humongous: A 50 m
high construction can be seen from 25 km if standing at sea level
(using the earth curvature). That means that a HVDC overhead TL
construction is potentially compromising the landscape of 50 km
2
per every km length. However, state-of-the-art HVDC power lines
did show quite a technological development during the last years
and are practically the first choice for long-distance electrical
energy transfer at the moment until new advantageous technol-
ogies –as SCTL show reliable operation. The right-of way of
7800 kV HVDC overhead lines (OHL) is compared to standard
7320 kV HVDC underground cables and superconducting cables
in Fig. 3.
Despite SCTL have much smaller dimensions than standard cables
they still require a ROW of several meter width because of additional
unforested zones added as protection measurements against tree
roots.ROWforSCTLcanbemuchsmallerinurbansettings.
5.2. Standard HVDC underground cable
A single XLPE cable has a current rating of 1.760 A and is thus
capable of transferring 563 MW at 320 kV. The conductor diameter
with an area of 2500 mm
2
is an industrial limit and a substantial
increase of ampacity for a single cable in the near future is unli-
kely. These types of cables were used for the France–Spain inter-
connector (INELFE project). Other cables with higher voltages
using oil-impregnated paper for electric insulation are not con-
sidered in this discussion. Recently, the development of a 525 kV
XLPE HVDC cable with a capacity rating of 2.6 GW per bi-polar
system was presented by ABB.
To transfer 5 GW of electric power overa distance of 700 km six
bi-polar systems (12 single cables) are required assuming a loca-
tion in northern France. The trench width for 6 bi-poles is 8 m. To
transfer 5 GW in northern Africa 8 bi-poles are necessary and the
separation between cables has to be much higher resulting in a
trench width of 19 m [36]. To transfer 10 GW over 3000 km the
total trench width would be 22 m and much wider in northern
Africa (430 m).
Cables heat up the surrounding soil and thus reduce the
capacity of other cables in a system because the soil temperature is
increased.
Fig. 4. Possible layouts to fulfill HVDC 10 GW power transmission requirements with ROW in forested areas and associated costs using underground cables. The size
proportions are meant to be realistic. The ROW for a 7800 kV OHL with 10 GW capacity and 30 00 km length is 245 m (not shown).
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–7266
5.3. Superconducting transmission line (cable/underground)
The ROW corridors of superconducting transmission lines are
determined not only by the size of the cable system but also by the
existing regulations for medium- and high-voltage cable installa-
tions. That includes an operation of the cable according to effective
regulations concerning limits of the electro-magnetic stray field
(Federal Emission Control Act Concerning Electromagnetic Fields/
26. BImSchV in Germany). The ROW in forested areas is wider than
in un-forested and urban areas because a protective zone (2.5 m
on each side in Germany) is mandatory to prevent tree roots from
damaging the cable unless the cable is installed in another pro-
tective tube (Fig. 4).
The exact ROW of a superconducting multi-GW transmission
line in urban settings can only be estimated at the moment
because respective regulations and norms for such high capacity
TL cables may not be based on existing regulations. Therefore only
the pure size and width of the cable system in urban settings is
compared (Fig. 5).
The magnetic field of a DC bi-polar SC TL with a coaxial design
is zero as long as the opposing currents (bi-polar TL) are equal. It
may not be possible to ensure this for all times and a magnetic
field can remain in that case for short periods. This depends on the
cable design, how the cable is operated as part of the grid and the
safety handling of cable failures. However, the ROW of a multi-GW
SCTL has potentially the width of less than 50 cm in un-forested
urban areas.
6. Technical aspects of superconducting transmission lines
relevant for local communities
6.1. Electro-magnetic (EM) fields
EM fields are a major concern for local residents affected by
transmission lines and hence a major objection point in grid
planning. DC power lines have favorable magnetic field char-
acteristics compared to AC power lines because inherently they do
not have an oscillating magnetic field but a static field. The mag-
netic field is proportional to the current transferred in the
conductor and inversely proportional to the distance from the
conductor. It can be minimized by choosing a proper layout for the
cable system design. For instance, a coaxial design of a bi-polar
cable leads to zero magnetic fields assuming both (oppositely
directed) currents are equally high. Such a design is not available
for high capacity standard conductor HVDC cables at the moment
because the dissipated heat cannot be removed properly from the
inner conductor of such (coaxial) design. Contrary to that, super-
conductors do not experience any resistivity and therefore do not
dissipate heat. There are no electrical fields outside a DC cable due
to the shielding of the conductor.
Because the current density of SC is much higher than that of
standard conductors, the current carried by a SC cable can be
much higher. This would result in equally stronger magnetic fields
of a single line (mono-pole) because the magnetic field is pro-
portional to the transferred current (compare 1.7 kA for a standard
HVDC cable with potentially 40 kA or more for SC). However, if bi-
polar designs are used, what will be the case also due to con-
tingency reasons, the single magnetic fields of both poles partially
or totally cancel each other out. Fig. 6 displays the magnetic field
of a bi-polar SCTL with a nominal current rating of 40 kA per pole
as it was developed at IASS for long-distance electric energy
transport. This calculation shows that high currents of super-
conducting transmission lines do not pose a threat. The pole
centers are separated by 6 cm, and the cable is buried 1.5 m
underground. The resulting magnetic field is 210 mT at surface
level and 54 mT 1 m above surface right above the cable. This
calculation does not include the superposition with the Earth
magnetic field (50 mT) –the total magnetic field depends on the
orientation of the cable. That is lower than the limit for static
magnetic fields of 500 mT of the revised Federal Emission Control
Act in Germany (26. BImSchV –signed into law in 8/2013) that for
the first time includes limits for DC lines. It is worth mentioning
that the new amendment of the 26th BImschV bans new AC low
frequency (50 Hz) overhead power lines in new corridors spanning
residential housing for voltages of 220 kV and more. For AC low-
frequency magnetic fields, the maximum magnetic flux allowed in
Germany is 200 mT for 50 Hz.
Fig. 5. ROW in urban areas for 5 GW underground transmission systems.
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–72 67
6.2. No heat dissipation into the soil
Besides a lower visual impact due to their extremely small size
and potentially no electro-magnetic fields due to their coaxial
cable design that is not applicable for standard HVDC cables
superconducting transmission lines have more advantages. Dis-
sipated heat from standard HVDC underground cables into the
surrounding soil results in a temperature rise and moisture
migration. Less moisture of the soil results in a higher heat resis-
tivity of the soil and leads to higher temperatures of the conductor
itself that in turn dissipates even more heat. This circle can ulti-
mately lead to a thermal runaway and a cable breakdown, the
temperature limit of soil next to a standard HVDC XLPE cable is
55 °C in summer [36]. The consequences of heated soil have not
been investigated in detail so far but an impact on local fauna is to
expect. A maximum temperature increase of 5 K at 50 cm below
soil surface has been recommended by the German Ministry for
Environment. The most relevant negative impact of cables is on
wetlands. The use of overhead lines is recommended here and for
the case of drinking water resources. Irreversible damage of local
hydrology can often be prevented by proper soil management
during the construction. Superconducting cables only exchange a
minimum amount of heat with the environment due to their
almost perfect vacuum insulation and therefore do not alter the
soil except for trenching. Only one trench is necessary for 10 GW
capacity transmission for SCTL. An example is the installation of
the AmpaCity superconducting cable in the city center of Essen/
Germany buried underneath a road.
6.3. Potential health hazards
To get an idea how vulnerable an underground SCTL potentially
is leakage and accident statistics of gas pipelines can be used for
comparison. The 5-year moving average failure frequency in 2010,
which represents the 5-year incident frequency from 2006–2010,
equals 0.16 per 1000 km per year [37,38,10]. That would mean that
for a 1000 km long North-South HVDC transmission line in Ger-
many 1 leakage appears over a period of 6 years. Most leakages are
caused by external interference (48.4%) like excavation or ram-
ming damage, 16.7% by construction defects/material failure, 16.1%
by corrosion, 7.4% by soil movements like landslides, 4.8% by
accidently connecting high pressure to low pressure or even water
pipes and 6.7% by maintenance, lightning, design errors and others
(2006–2010). The released gas ignited in 4.5% of these accidents
from 1970–2010. It is worth mentioning that the failure frequency
due to corrosion is only 0.01 per 1000 km per year for poly-
ethylene (PE) coated gas pipelines. Flexible cryogenic transfer lines
for liquid gas (for instance Cryoflex from Nexans) have a PE-
coating. With respect to superconducting lines gas pipelines have
only one cylinder, i.e. only one wall between the gas and the
environment and provide therefore less safety in case of an acci-
dent. It is very unlikely that coolant can enter the environment
due to an internal failure of the SCTL during a lifetime of 30–
40 years.
The coolants helium, hydrogen and nitrogen are not toxic and
have no direct health impact except that they can potentially
replace oxygen in air and lead to suffocation or cryogenic com-
bustions caused by splashing liquid coolants in the case of a major
leak. This is however very unlikely because the outer coolant
cylinder is embraced by another cylinder that holds the outer
vacuum space between environment and cryogenic envelope. Any
leaks will immediately lead to higher vacuum pressures and
should give enough time to take measures. Contrary to that gas-
insulated lines (GIL) employ sulfur hexafluoride (SF
6
) to elec-
trically insulate the conductor of standard underground HV lines.
SF
6
is extremely toxic and potential gas leaks are a danger to
health and life.
Hydrogen pipes are routinely operated in industry and hydro-
gen liquefaction is state of the art. For instance is the German
industrial gas supplier Linde operating a 80 km long hydrogen gas
pipeline network since 1994 and producing 33,000 l of liquid
hydrogen (LH2) per hour in Leuna/Germany [39].
Leaks in the cryogenic system of a superconducting transmis-
sion line using hydrogen as the coolant can potentially lead to
explosions if hydrogen is released into the atmosphere and mixes
with oxygen. However, natural gas pipelines share similar con-
cerns and are widely accepted. Hypothetically, released hydrogen,
nitrogen or helium will not contaminate the environment as oil
and gas does. If hydrogen mixes with air, only mixtures that
contain 4–75.6% hydrogen are explosive when lightened. For
comparison, explosive gasoline mixtures are formed if they do
contain 1–11% gasoline vapor and explosive propane mixtures if
they do contain 2.1–9.5% propane. These gases create explosive
mixtures with air at much lower concentrations than hydrogen
Fig. 6. Stray magnetic field of a 40 kA bi-polar transmission line as proposed from the IASS. The poles are separated by 6 cm. The overall magnetic field generated by both
poles is much lower than the magnetic field of a single pole due to opposite direction of current flow and hence partial cancellation of magnetic fields. A coaxial design of the
conductor would theoretically result in a non-existing magnetic field as long as both opposing currents are equal. The calculation does not include any soil with high iron
content or magnetic shielding (SC can act as a magnetic shield for static fields).
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–7268
but leaking air into a hydrogen reservoir creates an explosive gas
mixture at 24.4% air content already. Also, air is solid at liquid
hydrogen temperatures and special care has to be taken to prevent
solid air–liquid hydrogen mixtures, which are highly explosive.
The minimum energy required to ignite hydrogen gas mixtures is
0.019 mJ, that is 1/10 of the energy required to ignite propane [40].
However, hydrogen gas mixtures cannot ignite without an energy
influx, i.e. they cannot self-ignite.
7. Sustainability relevance for the environment
7.1. CO
2
emissions
Superconducting transmission lines can potentially have much
lower power losses than standard options, especially than stan-
dard cables. These power losses of transmission lines can be linked
to CO
2
emissions because losses in transferred electrical energy
need to be compensated by an increase of the generated electric
energy. It would have therefore not a local but a national impact
and also be relevant for policy makers. In the case of Germany
between 5% and 6% are lost during transmission from generation
to consumption due to losses in power lines and transformers.
Though a big portion of losses (about 50% in Germany) are accu-
mulated in the low-voltage level grid primarily due to its long
length, losses in the transmission and regional distribution grids
cannot be neglected in aspects of energy efficiency. The renewable
energy share (RES) of the electricity mix in Germany was 33% in
the first half of 2015. The average emission of CO
2
per generated
kW h was 562 g in 2012 and a similar number was forecasted for
2014. The CO
2
emissions of superconducting transmission lines are
compared to standard overhead and underground transmission
lines in Tables 2and 3.
The CO
2
emissions are proportional to the power losses and can
therefore be lower for superconducting transmission lines com-
pared to standard transmission lines depending on the cryogenic
system used, the capacity and the load factor. Especially if liquid
nitrogen is used to separate the cryogenic system from the
environment, losses can be significantly lower for SCTL than for
standard lines. However, reduced load factors as mentioned above
will alter this advantage to a certain degree. A MgB
2
based SCTL of
4 GW capacity cooled with liquid hydrogen experiences half the
losses as 7320 kV XLPE standard cables at 50% load. The annual
CO
2
emissions due to electrical losses of both underground TL
variants with 810 km length and 4 GW net capacity (North-South
HVDC corridors in Germany without converter losses) would be
equal to about 3.4% and 6.5% of the CO
2
emissions of a standard
coal power plant with an electric energy generation of 4300 GW h/
year and 1000 g of CO
2
emitted per kW h generated. This further
changes in favor of the SCTLs with increasing capacity and load
factor due to the fixed amount of energy needed for cooling. As an
example, the CO
2
emissions for 3000 km long, TLs with 10 GW
net-capacity are listed in Table 3. The emissions associated with
the electric losses for standard cables would be equivalent to the
emissions of almost 3 coal power plants!
7.2. Efficiency of SCTL with respect to renewable energy transfer
It is evident that SCTL have the potential to reduce the electric
transmission losses compared to standard transmission options,
especially underground cables what is also mirrored in potentially
lower CO
2
emissions associated with the electric losses. However,
this has to be evaluated for the specific project individually and
depends not only on the actual design of the cryogenic envelope
but on the actual load because the electric losses for super-
conducting transmission lines are constant whereas the electric
losses of standard conductors in DC mode are proportional to the
load squared (load
2
).
Two facts make it hard to achieve 100% load:
1. The load varies over the year, over the day.
2. The fluctuating nature of renewable energy generation –with
an RES of the energy mix in Germany of already 33% (2015) and
80% by 2050.
Very sophisticated energy management systems in combina-
tion with massive energy storage capabilities can overcome the
differences of supply and demand and achieve high load factors.
Intelligent grids can increase the load factor of superconducting
transmission lines and reduce at the same time the load of stan-
dard transmission lines by redirecting the current flow. Excess
energy could also be stored by cooling the cryogen to lower
temperatures and a warm-up to regular operating temperatures at
times of low load with no use of electric energy. Neglecting the
fact that SCTL are not ready to be utilized, i.e. actually constructed,
for long-distance transmission projects probably for the next 10
years, calculations show that SCTL can have favorable efficiencies
for the planned North–South HVDC corridors in Germany assum-
ing realistic load factors of 30–80% depending on the corridor [41]
and the officially specified capacities of 2–12 GW per corridor in
the grid development plan scenario B for 2023 and 2032 [32].
8. Cost
8.1. Capital cost of SCTL in comparison to standard technologies
As no large-scale installations of several 10 or 100 km length
have been built yet (2015) and costs depend on the specific design
and terrain, costs can only be estimated for superconducting
Table 2
CO
2
emissions associated with power losses of transmission lines (4 GW, 810 km
length) assuming the RES of 2012 in Germany. Electricity mix 2012 (562 g/kW h).
Load: 50%. Coal power plant: 4300 GW h/year with 1000 g of CO
2
emitted per kW h
generated.
CO
2
equivalent
emission of
losses
MgB
2
LH2 MgB
2
GHe þ
LN2
HTS cable 7500 kV
HVDC OHL
7320 kV
HVDC
cable
Electricity mix 2012 (562 g/kW h)
Per year [t] 146,717 46,683 36,012 115,836 279,571
For 40 years [t] 5,868,696 1,867,312 1,440,498 4,633,439 11,182,826
Coal power
plant CO
2
equivalent
emission [%]
3.4 1.1 0.8 2.7 6.5
Table 3
CO
2
emissions associated with power losses of transmission lines (10 GW, 3000 km
length) –load: 100%, otherwise same assumptions as in Table 2.
CO
2
equiva-
lent emission
of losses
MgB
2
LH2 MgB
2
GHe þ
LN2
HTS cable 7500 kV
HVDC OHL
7320 kV
HVDC cable
Electricity mix 2012 (562 g/kW h)
Per year [t] 543,398 172,899 133,379 4,640,140 12,203,002
For 40 years
[t]
21,735,912 6,915,972 5,335,178 185,605,613 488,120,066
Coal power
plant CO2
equivalent
emission
[%]
12.6 4.0 3.1 107.9 283.8
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–72 69
transmission lines. The cost for operation and maintenance were
assumed to be 1% per year of the initial capital cost following
standard economic calculations. There may be no or less degra-
dation of the (super)-conductor itself for SCTL contrary to standard
conductors. Furthermore, HVDC installations need AC–DC con-
verter (and DC–AC) and the costs for converter can make up for a
substantial part of the costs, especially for short high-capacity
power lines. For instance, two 65 km long HVDC superconducting
TLs each with 2 GW capacity, as needed in Northwestern Germany,
would have estimated total capital costs of 1980M€(HTS –today
costs) and the costs for the converter alone would be 1620M€
(82%). That means that the technology used to transmit electric
energy may not have a substantial financial impact for HVDC
applications depending on length and capacity and therefore the
type of transmission line can be chosen that meets other than
financial requirements best, i.e. has the smallest ecologic footprint
etc. In any case, cost of superconducting transmission lines can be
cost competitive with standard technologies, for instance with
7500 and 7800 kV HVDC overhead lines and 7320 kV HVDC
XLPE cables. The cost per capacity and length of HVDC transmis-
sion options are displayed in Fig. 7.
The cost calculation was made for two designs developed at
IASS using MgB
2
as the superconducting material and cooled by
liquid hydrogen and liquid nitrogen/gaseous helium. The cost for
trenching and installation are included in the total cost but not the
converter cost for easier comparison. The fulfillment of the (n1)
principle was not investigated but the cost of two SCTL systems
each with 50% capacity is shown for comparison. The cost for the
single MgB
2
transmission line is given for a one tube, bi-polar
design with two separate cables inside the cryogenic tube.
Two 7500 kV HVDC transmission lines systems are assumed
to be needed to transfer 4 GW, this is in accordance with the
current planning status of Südlink –the corridor C of the German
Grid Development Plan 2013 (Netzentwicklungsplan) with 4 GW
capacity and 810 km length [46]. Südlink is needed to mainly
transfer wind energy from the North to the South of Germany and
operation is projected to start until 2023. At the moment Südlink
is planned to be build with 7500 kV, not with 7800 kV. Reasons
can be of technical nature –voltage limitation of the voltage
source converter (VSC) technology or voltage mismatches of OHL
and standard cables –or simply the effort to prevent even stronger
public opposition due to the higher voltage.
MgB
2
based SCTL become cheaper compared to standard
solutions with increasing capacity because the conductor itself
accounts only for a small fraction of the total cost which are
dominated by the cost of the cryogenic system that is practically
cost-independent of the capacity for longer length. For capacities
of more than 2 GW even a redundant design with 2 separate lines
is competitive with standard cables. Competitiveness with OHL is
reached between 3 and 6 GW depending on the actual electric
design.
8.2. Right-of-way associated cost
When transmission line installations are constructed on or
cross private property and public land usually a one-time com-
pensation fee has to be paid by the transmission system operator
(TSO). These cost can make up for a substantial fraction of the total
cost. In northern Germany, compensation payments to munici-
palities and land owners for limited future planning options and
lowered property value due to new 380 kV AC OHL are being
discussed and negotiated right now and add up to about 75k€/km
[47,48]. That includes 5000€per tower erected and 20% of market
property value payments of spanned area to land owners besides
40,000€/km payments to municipalities. This value is much higher
in urban areas where OHL may cause the loss of building land for
owners because regulations prohibit the construction of housing
structures underneath OHL. The dimensions of 380 kV AC towers
are comparable with towers for 7800 kV HVDC lines and ROW
costs should be comparable. Two 7800 kV HVDC lines are
necessary to transfer 10 GW and have a combined span width of
80 m and thus ROW costs of 150k€. That would represent 5% of the
construction cost (4M$/km) for the transmission line itself
excluding costs for AC–DC converter. Including the converter, the
fraction of total costs for ROW payments can be much lower.
SCTL would have less impact on property value and would
result in lower ROW associated cost. However, the cost advantage
Fig. 7. Indicative capital cost per capacity and length for HVDC options. The cost of two redundant SCTL systems is shown with respect to the (n1) criterion and possible
redundancy requirements. The step like appearance of standard transmission lines stems from fixed costs like towers, trenching, installation or cables systems ( 7320 kV
XLPE) needed to accommodate increased capacity. For SCTL increased capacity is accommodated for by adding more superconducting material without changing the design
and thus only small further additional costs in case of MgB
2
appear. Refs.:[42,43,6,44,45].
H. Thomas et al. / Renewable and Sustainable Energy Reviews 55 (2016) 59–7270
due to lower ROW payments seems to play a bigger role only in
urban areas.
Delayed approval and operation can cause indirect costs induced
by ROW issues. In a study initiated by the German Federal Ministry for
the Environment the cost for a 1-year delay of operating two 65 km
long transmission lines with approx. 2 GW maximum capacity each in
2020 in Northwestern Germany to transfer wind energy southwards
was calculated to be 120M€[42]. The BMU study is comparing a pure
OHL and partly (10%) cabled transmission lines, all AC operated. As a
result, costs are comparable if costs due to a 1-year delay are included
assuming shorter approval times for partly cabled TLs. The amount of
energy which could not be transferred due to the delay in the ZIP code
2 area (NW Germany) is 7300 GW h, the total installed wind power in
that area will be 20 GW in 2020 [49].Potentially,thedeploymentof
new technologies can alleviate the opposition against new transmis-
sion lines to such an extent that the approval time will be reduced by
severalyears.Inthatcaseeventechnologieswithamultiplecost
factor compared to the standard solution (OHL) can achieve benefits
in this way that make them more cost effective [15].
9. Summary and outlook
Superconducting transmission lines are an innovative and
promising transmission option that can be one of the many
components needed to achieve a more sustainable transmission
and distribution of electric energy. The manifold advantages of
SCTL like a potentially much higher efficiency and small size
requirements can have a direct positive impact on the environ-
ment and would likely increase the public acceptance. The access
and utilization of renewable energy sources can indirectly be
facilitated by faster approval procedures. SCTL have a much lower
visual impact on landscape than standard OHL and also require
much less space than standard cables for high capacity transmis-
sion. They can alleviate the ROW problematic and lead to an
increased public acceptance. Underground transmission lines with
several GW of power could be realized using existing right-of-ways
like highways, train tracks or standard cable conducts in cities. For
small capacities the size advantage of SCTL is less evident but a
technological advantage can still exist, especially for transmission
system operators. For instance can the operating voltage be tai-
lored by adjusting the operating current with still keeping the
same small size what allows to reduce the amount of transformer
equipment and free space. Superconducting transmission lines
based on MgB
2
can potentially be cost competitive without taking
savings from reduced ROW requirements into account. Cost of
SCTL will also decrease in time due to cheaper production pro-
cesses of the superconductor itself and if the demand and output
is higher (both, HTS and MgB
2
). Standard transmission lines have
gone through this process already long time ago and cost reduc-
tions are not expected. The losses and related costs depend on the
actual load factor and the capacity. In general, high capacities and
high load factors work in favor of SCTL. The size of super-
conductors makes it possible to minimize the outside magnetic
field by choosing a proper layout for the cable system design. A
coaxial design of a bi-polar cable leads to zero magnetic fields if
both opposing current are equal. Heat dissipation of resistive
conductors deny this design option for high capacity standard
HVDC cables. For long distance and high capacity transmission
SCTLs can be the best choice of all transmission options. However,
the global output of superconducting wires and tapes would not
be enough to supply one multi-GW and 1000 km long transmis-
sion line at the moment but can once SCTLs are accepted as a
mature and cost competitive technology and output is increased to
meet demand. Reduced CO
2
emissions are another sustainability
asset and can further increase the acceptance of SCTL. It is very
important to inform about the progress in the area of super-
conducting power transmission. This is all the more necessary
since on-going events in Europe as well as other regions show that
it is crucial to engage the local communities in the early stages of
the planning process. It is crucial to further invest in and foster the
development of superconducting transmission lines in order that
they become a commercially available option for the future electric
grid as an alternative to standard technologies with a number of
advantages.
Acknowledgment
This work was funded by the German Federal Ministry of
Education and Research (BMBF) and the State of Brandenburg/
Germany.
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