Factsheet to accompany the report “Pathways for energy storage in the UK”
Supercapacitor (electrochemical
double-layer capacitor)
Brief description of technology
Electrochemical double-layer capacitors (EDLCs),
often called ‘Supercapacitors’ or ‘Ultracapacitors’,
are storage devices that can store or deliver
energy at a high rate, but have limited storage
capacity when compared with most batteries.
They have a high power density (power per unit
volume), but a low energy density (energy per unit
volume). Traditionally, EDLCs have been installed
complementary to a battery storage system to
increase the overall power density – the rate at
which the system can charge or discharge – of the
facility, handling the peak transients in supply or
demand while the battery storage system provides
the overall energy storage capacity. Figure 1
compares the energy density with the power
density for various energy storage devices,
including ‘Ultracapacitors’.
Technical/economic data
2008 figures for EDLCs put the average cycle life at
over half a million cycles, with an overall cycle
efficiency range from 75% to 95%. Costs are
estimated at $10-20/Wh and $25-50/kW, with a
specific energy of 5Wh/kg and specific power 510kW/kg [2] (See table 1).
Energy efficiency is current dependent as losses
occur in the devices proportional to current, and
self-discharge of the EDLC is high, at 6.5% loss
after 12 hours at room temperature [3].
Application/markets
Until higher energy densities are realised outside
of the academic environment, EDLCs can find
application in power quality and voltage
stabilisation on timescales of seconds, for system
operator ancillary services. However, due to their
high power handling capability, they can be
combined with low power density battery storage
systems to improve response times in an
otherwise high energy capacity system. This
complementary system can have benefits to the
battery technology too, where high current can
have a detrimental effect on the battery lifetime.
Advantages/disadvantages
Figure 1: Plot of energy density vs power density
for various storage devices [1]
EDLCs have been in development since the 1970’s
and use a different storage mechanism to
conventional capacitors. In most EDLCs, two metal
electrodes are coated with a very high surface area
type of ‘activated carbon’ and are separated by a
thin porous insulator. Depending on whether an
aqueous or non-aqueous electrolyte is used, a
breakdown voltage of around 1.3 V or 3.7 V
respectively can be achieved, with the higher
voltage attained at the expense of introducing
higher equivalent series resistance (ESR) which is
the limiting factor when it comes to the charging
or discharging rate of the device. Thus the EDLCs
must be connected in series and charge balancing
between the devices must be done to achieve high
terminal voltages.
EDLCs can handle very many charge-discharge
cycles and therefore typically have a low cost per
cycle when their long lives are accounted for.
However, the cost per kWh of energy storage
remains high, as they possess very low energy
density and require a large volume to store a large
amount of energy. This means they are typically
only useful for short endurance applications.
Lifetimes are measured in hundreds of thousands,
to millions, of cycles, as compared to batteries
which have lifetimes limited to hundreds or
thousands of cycles at deep discharge. However,
unlike practical batteries, the voltage across any
capacitor, including EDLCs, drops significantly as it
discharges. Effective storage and recovery of
energy therefore requires complex electronic
control and switching converters, with consequent
energy loss in the converter electronics, lowering
the overall system efficiency.
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Factsheet to accompany the report “Pathways for energy storage in the UK”
Energy Density
2.5-15 Wh/kg,
500-5000
Wkg[4]
Typical
Rated
Capacity
(MW)
0-10[5]
0-0.3[4]
Nominal
Duration
Milliseconds1 hour [5, 4]
Cycle
Efficiency [%]
90[6]
95[7]
84-98[5]
<75-95+[2]
Energy Cost
[$/kWh]
10000[7]
300-2000[4]
1000020000[2]
Power
Capacity
Capital Cost
[$/kW]
Typical
Life
90-510[6]
500[7]
100-300[4]
25-50[2]
8-20+ years[5]
20+ years[4]
1 million cycles[6]
25000 cycles [7]
100,000+
cycles[4]
500,000 cycles[2]
Table 1: Technical and economic data for EDLCs
Current status
In their 2008 paper for the Power Systems Energy
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Research Center , Smith and Sen reported that
research and application of EDLC in large power
systems was in its infancy and that the authors
were unaware of any large scale installations in the
transmission grid [8]. Until cost can be reduced
and energy density increased, the authors advise
that both simulation and experimental tests have
shown EDLCs to be of benefit in large scale battery
systems where they reduce battery current and
stress, thus extending the life of the battery by
reducing the number of cycles.
Large systems with energy densities over 20
3
kWh/m are still in the early development stage
[4], although there are a large number of potential
developers.
Time to commercialisation and R&D needs
Possible improvements in energy density are being
investigated and preliminary results from using
nanotubes to increase the surface area of the
electrodes are promising and indicate that 100 fold
improvements in energy density may be
achievable [8].
However, this is yet
tocommercialised outside of an academic
environment.
Three aspects of research have been identified
that would facilitate increasing the energy density:
•
•
•
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Design of carbons with a bespoke pore size to
increase capacitance
Combination of EDLC with a pseudo-capacitive
charge storage mechanism
The development of new electrolytes and
device architectures to allow increased cell
voltage [9].
www.PSerc.org
Safety, security, environmental and public
perception issues
Because of their capacity to deliver very high
transient currents, EDLCs are more prone to
sparking which can become a safety consideration
when used in conjunction with batteries which
may produce flammable gasses. However, in
general, most EDLCs have a low environmental
impact given they do not use heavy metals or toxic
electrolytes, although the use and disposal of
materials within them will be influenced by
legislation [10]. Also, the high cycle life tends
towards a storage device which will last the
lifetime of the given application.
References
[1] Supercapacitors chart based on data from
Maxwell Technologies. [Online]. Available: http://commons.wikimedia.org/wiki/File:Supercapacitors_chart.svg
[2] J. R. Miller and A. F. Burke, “Electrochemical
capacitors : Challenges and opportunities for realworld applications,” Electrochemical Society
Interface, vol. 17, no. 1, pp. 53–57, 2008. [Online].
Available:
http://scholar.google.com/scholar?hl=en&btnG=Search&q=intitle:Electroche
mical+Capacitors+:+Challenges+and+Opportunities
+for+Real-World+Applications#0
[3] A. Ruddell, “Energy storage in power
systems,” in SUPERGEN Wind Training Seminar,
March 2011.
[4] H. Chen, T. N. Cong, W. Yang, C. Tan, Y. Li,
and Y. Ding, “Progress in electrical energy storage
system: A critical review,” Progress in Natural
Science, vol. 19, no. 3, pp. 291 – 312, 2009.
[Online].
Available:
http://www.sciencedirect.com/science/article/pii/S100200710800381X.
[5] M. Beaudin,
H. Zareipour,
A. Schellenberglabe, and W. Rosehart, “Energy
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Factsheet to accompany the report “Pathways for energy storage in the UK”
storage for mitigating the variability of renewable
electricity sources: An updated review,” Energy for
Sustainable Development, vol. 14, no. 4, pp. 302 –
314,
2010.
[Online].
Available:
http://www.sciencedirect.com/science/article/pii/S0973082610000566
[6] S. Faias, P. Santos, J. Sousa, and R. Castro,
“An overview on short and long-term response
energy storage devices for power systems
applications,” in International conference on
renewable energies and power quality, 2008.
[7] S. Schoenung, “Energy storage systems cost
update: A study for the doe energy storage
systems program,” Sandia National Laboratories,
Technical Report, April 2011.
[8] S. Smith and P. Sen, “Ultracapacitors and
energy storage: Applications in electrical power
system,” in Power Symposium, 2008. NAPS
’08.40th North American, sept.2008, pp. 1 –6.
[9] P. Simon, “Electrochemical capacitors for
power grid storage technology: State of the art
and next challenges,” in Trans-Atlantic Workshop
on Storage Technologies for Power Grids,
Washington, 2010.
[10] C. Naish, I. McCubbin, O. Edberg, and
M. Harfoot, “Outlook of energy storage
technologies,” European Parliament’s committee
on Industry, Research and Energy (ITRE), Tech.
Rep., February 2008.
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