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Electricity Storage
A Briefing provided by the Institution of Engineering and Technology
www.theiet.org/factfiles
About This Briefing
Contents
The Institution of Engineering and Technology acts as a voice
for the engineering and technology professions by providing
independent, reliable and factual information to the public
and policy makers. This Briefing aims to provide an accessible
guide to current technologies and scientific facts of interest to
the public.
Introduction������������������������������������������������������������������������������������� 3
Does the electricity supply system need storage?������������������������������ 3
Does wind generation strengthen the case for storage?��������������������� 3
What storage technologies are available or being developed?������������ 4
Are some storage technologies better than others?���������������������������� 6
How can storage help electricity networks?��������������������������������������� 6
Are there any structural barriers to the deployment of storage?���������� 8
What other benefits can storage plants deliver? ���������������������������� 8
What role can heat storage play in the operation of the system?�������� 8
Conclusion��������������������������������������������������������������������������������������� 9
Additional reading���������������������������������������������������������������������������� 9
End notes���������������������������������������������������������������������������������������� 9
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Cover images (clockwise from top left)
„„ 20 MW Flywheel energy storage plant, Stephentown, USA
courtesy of Beacon Power, LLC
„„ 600 kW Vandium Redox flow battery, Oxnard, USA
courtesy of PD Energy
„„ Compressed air energy storage plant, Huntorf, Germany
courtesy of E.ON Kraftwerke GmbH
„„ Pumped hydro plant, Dinorwig, UK
courtesy of First Hydro Company
Enquiries
[email protected]
Electricity Storage
A Briefing provided by The Institution of Engineering and Technology
© The IET 2012
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1. Introduction
There is a growing interest in the use of energy storage to
enable electricity supply systems to operate more efficiently
and securely. The main drivers for this interest are the need
to integrate a significant capacity of wind power, and other
intermittent renewable generation, into the system and to
make more efficient use of the transmission and distribution
networks.
Storage capacity can be deployed in the form of electricity
storage devices connected at strategic points of the system
and/or the use of thermal storage devices that are directly
linked to the system.
For the purposes of this briefing , the electricity supply system
includes the generating stations and the transmission and
distribution system right down to consumer metering points.
For simplicity it is simply referred to as “the system”.
This briefing answers some key questions about the role of
storage in the system.
2. Does the electricity supply system need storage?
To answer this question, it is necessary first to understand the
particular characteristics of the system.
The system combines a process industry with a commodity
supply chain like many others. It takes a raw material (the
primary energy in, for example, coal, gas or wind), converts it
to a product (electricity) and distributes it to customers over
the transmission and distribution networks. This is no different
in principle from the supply chain for petrol, paper, food and
so on.
However, there is one very significant difference in practice.
Most products can be stored as they move from production
to consumption. Whether it’s liquid fuels in tanks or food
in warehouses and freezers, storage is used to decouple
production from consumption. This is very beneficial as it
allows the production processes to be made very efficient potentially operating on a continuous basis rather than flexing
with short-term changes in demand. In contrast, because it
has always proved very difficult to store electricity, (pumpedhydro is the only widely used technology), the system has
been developed so that production and consumption are
balanced, second-by-second, on a continuous basis. The
electricity supply chain is unique in this respect.
Whilst, in engineering terms, operating the system in this
way is a remarkable achievement, it means that the assets
employed are not used as effectively as in other process
industries and their related supply chains. This can be quite
easily demonstrated.
The annual average electricity demand is currently about
60% of the peak demand. As a margin of generating capacity
is needed above the peak demand to allow for planned and
unplanned outages, say 20%, we need generating capacity of
120% of peak demand. It follows therefore that the installed
generating capacity is twice the average demand so that, on
average, it is only utilized for 50% of the time. In other process
industries asset utilisation would typically exceed 90%. This
low utilisation does not only affect generation. It has even
more of an effect on transmission and distribution assets.
This is primarily because network capacity margins have to
be provided ‘locally’ as they can’t be pooled nationally as
generation can. The cost of this low utilization of the system
has to be reflected in the price of electricity.
So, in answer to the question, “Does the electricity supply
system need storage?”, the answer is as follows. Storage is not
an absolutely essential part of the electricity supply system as
the services it provides can be delivered in alternative ways, in
particular by controlling generation and/or demand. However,
as demonstrated by the use of pumped hydro, storage can
form part of the overall portfolio necessary to securely meet
system demand. In the future, if storage could be deployed
at a lower overall cost (capital and operating costs) than the
competing alternatives then it would be a beneficial addition to
the system.
It should also be noted that, depending on the particular
technology employed and its location on the system, storage
can offer services to the system over and above capacity
replacement. These services can strengthen the economic
case for its deployment.
One example of these services is back-up generating capacity
either for very fast response (i.e. within seconds) or with some
notice period. Given the projected increase in wind generation
and the prospect of larger nuclear power stations, these
‘ancillary services’ are expected to become increasingly sought
after by power system operators.
3. Does wind generation strengthen the case for
storage?
The growth of wind generation and other intermittent,
essentially uncontrollable, sources of electricity, results in
a less predictable world for the system operator who has to
balance supply and demand. There are a number of ways
of dealing with this. The ‘conventional’ way is to provide
higher levels of reserve or back-up generation so that any
shortfalls can be quickly met and any excesses are managed
by constraining generation output or exporting electricity to
another country’s system using interconnectors. An alternative
way is to encourage consumer demand to be either responsive
to changes in generation output or directly controllable by the
system operator. A more radical way is to do what most supply
chains do, as already described. Intermittent generation can
be effectively decoupled from the system using storage as a
buffer. Importantly, the store does not have to be co-located
with the wind farm. It can provide this buffer wherever it is
connected provided there are no network constraints.
It is generally accepted that it will be necessary to use a
combination of these solutions to address the challenge of
having large amounts of variable output wind generators on
Electricity Storage
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3
the system. However, economics and the practicalities of
deploying these solutions will determine how successful each
one is.
The ‘conventional’ solution of reserve generation is within
the control of parties directly involved in the supply chain.
Typically, gas-fired generation could be built to fulfill this
role. Use of the demand side will ultimately depend on the
willingness of consumers to change their patterns of behavior,
or the evolution of sources of demand that are relatively
insensitive to some degree of interruption, such as the
charging of electric vehicles, or electrically supplied heating.
The success of storage will depend on a number of factors
including cost, the ability to deploy it where it is needed, and
its environmental impact.
So, the deployment of wind generation does strengthen the
case for storage but storage will still have to compete with
other sources of flexibility.
4. What storage technologies are available or being
developed?
Superconducting magnetic energy storage (SMES) - the
electrical resistance of certain materials reduces to zero
when cooled to very low temperatures. This allows very high
currents to be passed through them without loss. This property
is applied in a SMES plant. A coil of superconducting wire is
cryogenically cooled. By passing a DC current through the coil
a magnetic field is produced which acts as the energy store.
The superconducting device is connected to an AC system
using an inverter/rectifier in a similar way to a conventional
battery plant. The import and export of energy by the device
can be controlled in a number of ways. This is primarily a
device to enhance power quality. Though its response time is
close to zero it cannot store very much energy. These devices
are commercially available and have been used in the US to
support grid systems.
Power quality is the term given to the quality of the power
supply from the grid system. It is described in terms of its
continuity (lack of supply interruptions), voltage stability (the
voltage should always remain within upper and lower limits)
and harmonic content (a more complex characteristic involving
distortion of the voltage waveform).
The US-based Electricity Storage Association maintains a
very good web site with details of most of the leading storage
technologies. This can be found at its websitei. The following
notes provide a summary of this information. Cost and
performance data for most of the developing technologies is
available from a number of sources. However, it is difficult to
ensure the accuracy of this data and so it is not included in
this briefing.
It is helpful to understand the difference between energy and
power when considering storage. The energy in a storage
device is equivalent to the petrol in a car’s petrol tank - so the
bigger the tank the more energy can be stored. The power
that a storage device can deliver is equivalent to the size of
the car’s engine. With some storage technologies, such as
conventional lead-acid batteries, the relationship between
energy and power is fixed so if you want more storage you also
get more power. With others you can specify the power output
and the storage capacity independently. Examples of this
include pumped-hydro and flow batteries.
Energy storage technologies can be divided into three main
categories:
„„ Primary - superconducting and capacitor technologies;
„„ Mechanical - pumped-hydro, compressed air, flywheels;
„„ Electrochemical - conventional and flow batteries.
The key performance characteristics of storage technologies
are summarised in Table 1.
Primary
In primary storage devices the electrical energy is stored
without conversion to another form (as is the case in a battery
or pumped hydro plant).
Superconducting magnetic energy storage (SMES)
courtesy of Bruker Advanced Supercon
Capacitors - there have been significant developments in
capacitor technology in recent years. These are referred to
as “ultra” or “super” capacitors and use new techniques to
dramatically increase the energy density of these devices.
Capacitors have the ability to discharge very rapidly and in
this respect are superior to some competing technologies.
However, they do not yet offer solutions in the utility-scale
storage market (i.e. of a scale suitable for the grid system) and
so are not discussed further here.
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Mechanical
Pumped hydro - this is the most widely used storage technology
at present. There is an excellent example of it in North Wales
- the Dinorwig plant which can provide power within seconds
and generate up to 1700MW for some 5 hours. A pumped
hydro plant consists of two reservoirs, one higher than the
other, and a pumping/generating plant. Energy is stored by
pumping water from the lower to the upper reservoir giving
it potential energy. The energy can be stored in the upper
reservoir for considerable periods of time subject to losses
through leakage and evaporation. When energy is required by
the grid, water is allowed to flow from the upper to the lower
reservoir through a turbine which then drives a generator to
produce electricity. This is a tried and trusted technology.
There are four such plants in the UK, however, further
deployment has been constrained by the lack of suitable sites
and the high capital cost.
Huntorf compressed air energy storage plant
courtesy of E.ON Kraftwerke GmbH
Flywheels - a flywheel device stores energy (kinetic) in a
rotating mass. A motor/generator is able to either drive the
flywheel, inputting energy, or be driven by it, extracting energy.
Losses are reduced by using very sophisticated bearing
technology (e.g. magnetic levitation) and a vacuum enclosure
to reduce drag and corrosion. Like SMES, these devices are
essentially low energy and are primarily used in power quality
applications rather than bulk storage.
Dinorwig power station, Wales
courtesy of First Hydro Company
Cryogen-based energy storage - electricity is used to liquefy
air or nitrogen which can be stored at cryogenic temperatures
(i.e. extremely low) in large volumes at atmospheric pressure.
A low grade heat source (e.g. ambient air) can then be used to
heat the cryogen that boils and produces a high pressure gas
to drive a turbine which then drives a generator.
Compressed air energy storage (CAES) - this is in many
ways similar to a pumped hydro plant. However, rather than
pumping water ‘up a hill’, air is pumped into a cavern or
vessel and compressed. This compressed air can then be
used directly in a gas turbine (GT), effectively replacing the
compression stage of the GT. This is really a hybrid between a
storage plant and a conventional prime mover plant as primary
fuel is required. A CAES plant can be built on a similar scale to
pumped hydro. Commercial plants are in operation in the US
and Germany.
Flywheel energy storage plant, Stephentown, USA
courtesy of Beacon Power, LLC
Electrochemical
Batteries - there are a number of battery technologies,
some commercially available and others in various stages of
development. The best known technology is of course leadacid. This is certainly a proven technology but has never
gained acceptance for utility-scale applications. Newer battery
technologies include:
„„ Nickel-based batteries - nickel cadmium and nickel metal
hydride (NiMH) batteries are commercially available; the
latter being used particularly in portable electronic devices
and power tools. They are also used in electric vehicle (EV)
applications.
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„„ Lithium-ion (Li-ion) - currently used in small portable
electronic devices because of their very high energy
density. One company is pursuing this technology for large
scale energy storage systems.
„„ Metal-air - in this system metal is actually ‘consumed’
to produce electricity. A number of companies are
developing variations of this technology but again, utility
scale application is not a near-term prospect.
„„ Sodium sulphur - a high temperature device (300 degrees
Celsius) employing liquid sodium and sulphur separated
by a ceramic electrolyte. This technology is commercially
available from NGK, Japan and many installations are
now in service. Multi-MW storage plants have been built,
mainly in Japan, but a fire at an installation in 2011 has
raised concerns about its safety.
then be used as a primary fuel if required. Alternatively, it can
be converted back to electricity by combining it with oxygen in
a conventional fuel cell.
Flow Batteries - flow batteries offer a development of
conventional batteries. Whereas in a conventional battery the
electrolyte is contained within the battery, in a flow battery
the electrolytes flow through the battery and can be stored in
separate vessels. The advantages of this system include:
„„ the ability to specify the energy storage capacity
independently from the power output of the device;
„„ significantly reduced self-discharge, as most of the
electrolyte is stored away from the ‘active’ part of the
device.
The basic arrangement of a flow battery is shown in Figure 1.
Electrolyte
tank
Ion-selective
membrane
Electrode
-
Electrolyte
tank
+
Regenerative
fuel cell
Electrolyte
Electrolyte
Pump
Vandium-Redux Flow battery, USA
courtesy of Prudent Energy
5. Are some storage technologies better than others?
As discussed above, there are a number of storage
technologies competing to be deployed in electricity supply
systems. They all have different characteristics and could
potentially meet different needs in the operation of the system.
These different needs are essentially time-related. So, for
example, storage could be used to help balance the system on
a second-by-second basis, over a daily cycle or even between
seasons. These are very different applications and would
require very different technologies.
As a general rule, short term applications (i.e. second-bysecond balancing) require high power outputs and longer term
applications require high energy capacities. In Table 1, the key
performance characteristics of a number of technologies are
provided. They include the expected power rating, storage time
and efficiency. It is important to understand that no storage
technology is 100% efficient, so you will always get less energy
out than you have put in.
Pump
Power source/load
Figure 1: Schematic overview of a Redox-Flow-Battery
courtesy of ISEA, RWTH Aachen University (original image)
There are several competing flow battery technologies
currently under development. They all employ the same basic
arrangement but use different chemical ‘couples’.
Regenerative hydrogen fuel cells
This is a fundamentally more complex approach to storage
compared with the other systems described here. Electricity
is used to electrolyse water into hydrogen and oxygen. The
hydrogen is stored in pressurised tanks. The hydrogen can
In order to fully address the asset utilisation issue explained
earlier it would be necessary to store energy between
seasons. This is not expected to be commercially viable in the
foreseeable future, though long term storage of heat is possible
under some circumstances - see section 9.
6. How can storage help electricity networks?
Storage can also help address the challenge of using
networks more efficiently. In fact, the closer storage is located
to consumers the greater its value. This is because it can
potentially increase the utilisation of all the assets ‘above’ it in
the supply chain (i.e. network and generation assets). It could
be argued that ideally all consumers should own a storage
device so that the demand presented to the system is less
uneven and thus requires less system capacity. It is possible
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Table 1
Comparison of Storage Technologies
(All data sourced from “Pathways for energy storage in the UK” - Centre for Low Carbon Futures)
Technology
Indicative rated
capacity (MW)
Nominal duration1
Cycle efficiency
(%)2
Maturity
Likely scale of
application
Pumped hydro
100 - 5000
1 - 24+ hrs
70 - 87
Mature &
Commercial
Large Grid
Compressed air
energy storage
50 - 300
1 - 24+ hrs
70 - 89
Commercial
Large Grid
Cryogen-based
energy storage
10 - 200
1 - 24+ hrs
40 - 90+
Early Commercial
Grid/EV3/
Commercial UPS4
Flywheel
0.4 - 20
1 - 15 mins
80 - 95
Demo/Early
Commercial
Small Grid/House/
EV
Hydrogen storage
and fuel cell
0 - 50
seconds - 24+ hrs
20 - 85
Demo
Grid/House/EV/
Commercial UPS
Battery
(Flow)
0.03 - 3
seconds - 10 hrs
65 - 85
Research/Early
Demo
Grid/House/EV/
Commercial UPS
Battery
(Lithium)
1 - 100
0.15 - 1 hr
75 - 90
Demo
Grid/House/EV/
Commercial UPS
Battery
(Metal-air)
0.01 - 50
seconds - 5 hrs
~75
Research/Early
Demo
Grid/House/EV/
Commercial UPS
Battery
(Sodium sulphur)
0.05 - 34
seconds - 8 hrs
75 - 90
Commercial
Grid/House/EV/
Commercial UPS
Battery
(Nickel)
up to 40
seconds - hrs
60 - 90
Early Commercial
Grid/House/EV/
Commercial UPS
Battery
(Lead-acid)
up to 40
seconds - 10 hrs
63 - 90
Mature &
Commercial
Grid/House/EV/
Commercial UPS
Superconducting
Magnetic Energy
Storage
0.1 - 10
milliseconds seconds
90 - 97+
Early Commercial
Small Grid/
Commercial UPS
Supercapacitor
0 - 10
milliseconds - 1 hr
<75 - 98
Early Demo
Small Grid/House/
EV
Mechanical
Electro-mechanical
Primary
Table Notes:
1
The typical period that the technology can maintain its rated output from a fully charged state.
2
The proportion of the energy used to charge the device that can will be returned to the system.
3
EV = Electric vehicle
4
UPS = Uninterruptible Power Supply
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that the storage capacity of electric vehicles at domestic
premises could be used in this way.
There is expected to be a tension here between small scale
distributed storage, which offers the greatest system benefit,
and larger scale storage plants which could benefit from
economies of scale and simpler, more predictable control.
While it is theoretically possible to co-ordinate the operation of
thousands or millions of distributed resources, this has never
been done in a power system context (with the exception of
some load control schemes).
As discussed in section 8, storage plants can also provide
additional ancillary services, such as voltage control, that
further assist the operation of networks. They can also
potentially enhance the resilience of the system either locally
or, if there is sufficient capacity, more widely.
7. Are there any structural barriers to the deployment
of storage?
This question is much discussed at present and has to be
considered on a market-specific basis. In the UK, there are a
number of small scale distribution network-connected storage
projects, most of which have been undertaken using funds
from the Innovation Funding Initiative (IFI) or Low Carbon
Network Funds. However, it is argued that the commercial
and regulatory structure of the electricity supply system (i.e.
under the Third Energy Packageii) does make it more difficult
to promote storage projects. In particular, the disaggregation of
the supply chain (i.e. compared to a fully vertically integrated
structure) makes it more difficult for the owner of a storage
plant to be properly rewarded for all the services that the plant
provides. DECC and Ofgem are now actively considering the
commercial and regulatory issues impacting on storage.
It is also notable that, at present, storage developers do not
have access to the subsidies that renewable generators do.
It is important to understand that the most effective storage
deployment strategies need to consider the operation of the
system as a whole. It is valid to argue that this is more difficult
to achieve in a disaggregated market. However, if this is
recognised, market mechanisms can be developed to address
this.
8. What other benefits can storage plants deliver?
This does depend on the specific characteristics of the
technology involved. However, a number of storage
technologies are connected to the system with equipment
that can also help control the voltage at different parts of the
system. In theory these devices can connect to the system
at any voltage and so a domestic solar panel can have one
in the same way that a large store connected to the high
voltage transmission system could. Voltage control can be very
beneficial and can attract ancillary service payments from the
system operator.
Storage could also help improve continuity of supply, for
example in microgrids, when generation shortfalls occur. (The
term “microgrid” is used here to mean a local grid that has
sufficient generation to meet its own needs that may or may
not be connected to the main grid system.)
9. What role can heat storage play in the operation of
the system?
Thermal storage is much less costly than electricity storage.
It can also operate over long timescales and is very efficient.
However, unlike electricity or chemical storage which can
easily be used to produce heat, the process cannot be
reversed without high costs and inefficiencies. Hence the
primary benefit of thermal storage comes from time shifting
electricity demand for heat to avoid peaks and higher prices
by consuming at periods of lower demand and lower electricity
prices.
The primary role that heat storage can play in the operation
of the electricity system is to provide demand that can be
supplied at times that benefit the operation of the system, such
as when surplus renewable electricity is available. Thermal
storage can be considered at a local (or distributed) level and
at a network level.
Local Heat Storage
Essentially there are two categories
„„ hot water;
„„ storage heating.
Hot water storage in the UK is substantial, circa 14 m
households each with 100 litre hot water cylinders. However,
this is declining due to the increase in combination boilers
being installed. Electric storage heating is mainly restricted to
flats (due to safety concerns with gas in such buildings) and
represents circa 7% of the building stock. Both hot water and
storage heating present a significant opportunity for demand
side management.
Network Heat Storage
Large scale thermal storage can be used to support heat
networks to meet peak demands and as back-up or for
planned and unplanned plant outages. Generally they are
designed for short term storage (hours/days) although longer
term or seasonal storage using underground reservoirs can
also be used. Very substantial thermal storage systems can be
constructed but they are constrained by the size of the heat
load connected to the heat network. Hence the scope for such
systems in the UK is very limited without a large increase in
heat loads supplied by heat networks.
In Denmark where 60% of space and water heating is supplied
by heat networks, storage capacities of 70,000m3 have been
constructed (comparable to an average gasometer) which
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can store nearly 4 GWhth of heat energy with an output of
600MWth. Such systems are heated by conventional CHP
plants or conventional gas boilers. However, Denmark plans
to increase electric heating using very large heat pumps
thereby developing a substantial demand side management
opportunity.
End notes
10.Conclusion
i
http://www.electricitystorage.org/about/welcome
ii
http://ec.europa.eu/energy/node/50
iii
http://www.ofgem.gov.uk/Networks/ElecDist/lcnf/Pages/lcnf.aspx
So, in conclusion, the growth of storage technologies could be
very significant provided that they:
„„ become economically competitive with alternative
solutions;
„„ can be located without undue constraints;
„„ can offer a wide range of power and energy ratings;
„„ are environmentally acceptable.
It is recognised that achieving the first of these ‘success
criteria’ may only be achieved with support of some kind to
acknowledge that new technologies typically follow a ‘learning
curve’ that leads to lower unit prices as volumes increase.
A number of demonstration projects are being pursued
internationally. In the UK, Ofgem’s Low Carbon Networks
Fundiii has resulted in a number of network storage trials
being funded. It is also likely that the development of storage
for transport applications could result in technologies that
are deployable on power systems. There will be international
sales potential for cost-effective storage technologies and their
associated control and management systems.
11.Additional reading
Four recent reports are recommended as additional reading:
„„ ‘Energy Storage Systems in the UK Low Carbon Energy
Future: Strategic Assessment’, Carbon Trust
http://www.carbontrust.com/resources/reports/technology/
energy-storage-systems-strategic-assessment-role-andvalue
„„ ‘The Future Role of Energy Storage in the UK’, The Energy
Research Partnership
http://www.energyresearchpartnership.org.uk/tiki-index.
php?page=page12
„„ ‘Pathways for Energy Storage in the UK’, Leeds University
Centre for Low Carbon Futures
http://www.electricitystorage.co.uk/
„„ Electricity System: assessment of future challenges, DECC,
August 2012
https://www.gov.uk/government/publications/electricitysystem-assessment-of-future-challenges
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