APPENDIX 12: ELECTRICITY STORAGE
Introduction
Electricity storage devices can be used to balance
fluctuations in the supply and demand of electricity. Depending
on the timescales over which these fluctuations occur, applications
fall into three categories:
- Power quality: over short timescales
(e.g. a fraction of a second) electricity storage devices can
improve the quality and reliability of power supplies;
- Bridging power: on intermediate timescales (e.g.
minutes) they can be used in transmission and distribution networks,
to ensure grid stability and continuity of supply when switching
between energy sources;
- Energy management: over longer timescales (e.g.
several hours) they can improve the efficiency of electricity
generation.
This note relates to energy management applications.
It focuses on their use in large scale electricity generation
(that is, connected to transmission systems) although they also
have uses in local distribution networks.
Characteristics of electricity storage devices
The energy delivered by renewables can fluctuate
on a hourly, daily or even seasonal basis. Electricity storage
devices therefore offer one possible means of ensuring continuity
of supply from intermittent sources such as wind or solar power.
The suitability of a particular storage device for use in conjunction
with renewables depends on a number of technical parameters of
which the most important are:
- Capacity: the amount of electricity
that a system can deliver at a given moment. Devices with capacities
of less than ~100 kW are not suitable for use in energy management;
- Discharge time: the timescale over which energy
is delivered by the device. Discharge times need to be of the
order of hours;
- Alternating/direct current: suitability would
depend on whether the device generated a direct current (e.g.
a battery) or an alternating current (e.g. a rotating device such
as a flywheel). Direct current is suitable for transmission over
long distances but most current networks use alternating current,
so the electricity would either need to be converted to work on
existing networks, or networks would have to be adapted.
Many other factors need to be taken into account
such as reaction time (i.e. the time the device would take to
come online), specific locational requirements, energy density
(which will determine the size of the device), efficiency and
lifetime (expressed in terms of the number of charge/discharge
cycles). Although these factors will influence capital costs as
well as operational and maintenance costs, it is difficult to
speculate on the economic feasibility of different technological
options as this will be influenced heavily by market factors (see
below).
How they work
The graph over the page illustrates how specific
storage techniques are suited to different applications based
on their capacity and discharge time. Techniques suitable for
use in energy management applications are described below, with
more information provided in the table over the page.
Pumped hydroelectric
This is currently the most widely used storage technology
(e.g. the United Kindom's Dinorwig plant in Wales). These systems
consist of two vertically separated water reservoirs. Water can
be pumped from the low to the high reservoir at off peak times,
and then used to generate electricity when required.
Graph illustrating capacity (horizontal
axis) and discharge time
(vertical axis) of electricity storage technologies.
Source: Electricity Storage Association[101]
Batteries
Batteries work by using a chemical reaction to produce
a voltage between their output terminals (electrodes). In a rechargeable
battery, the reaction is reversible and the battery can be recharged
at off-peak times. Lead acid batteries are the most widely used
but various advanced battery designs are also being developed
including:
- Sodium sulphide batteries: These
consist of a positive electrode (molten sulphur) separated from
negative electrode (molten sodium) by a solid ceramic electrolyte
through which only the positive sodium ions can flow. During discharge
sodium and sulphur ions combine to form sodium polysulphides.
During the charge cycle the sodium polysulphides can be converted
back into sulphur ions and sodium ions. The sodium ions flow back
through the membrane and recharge the battery;
- Flow batteries (also referred to as regenerative
fuel cells). There are various different types of flow battery
at different stages of development (note that Regenesys, the United
Kingdom's only project in this area has been discontinued—see
table). Energy is stored in two separate charged electrolytic
solutions held in separate tanks (i.e. not within the battery
cell itself) and pumped into the battery cell. They are easier
to recharge than other battery types, and also have the advantage
that the total amount of energy that can be delivered (which can
be increased simply by increasing the amount of electrolyte in
the tanks) is independent of their power (i.e. the rate at which
that energy can be delivered).
Compressed air electricity storage
Off peak electricity is used to pre-compress air
(which can be stored in underground mines or caverns) which can
then be used to generate electricity as required in a gas-turbine
power plant. They can produce two to three times as much energy
as a conventional gas plant for the same amount of fuel.
Note that with the exception of pumped hydro, there
is limited current use of, and limited investment in, development
of other energy storage technologies suitable for energy management.
Future prospects
Looking to the future of the transmission network,
National Grid Company (NGC) who operates the network in England
and Wales, reports that it has no great concerns over the need
for electricity storage over the next twenty years. Indeed, NGC
claims that it is able to handle intermittency from renewables
within the network in many different ways and storage is not a
particularly critical component of its strategy. Similarly, Ofgem,
the energy regulator, reports that although they followed developments
of the Regenesys project with interest, they have not studied
the potential for electricity storage over the coming decades
in great detail. Overall, there is a sense amongst key stakeholders
that the evolution of the electricity transmission network over
the next twenty years will not be influenced significantly by
the absence of large scale electricity storage.
Within smaller scale networks, there may be potential
for storage[102] and
indeed Ofgem is considering a pricing structure to incentivise
network operators to encourage embedded generation, in which storage
may play a part. However, there is no major investment in this
area.
101 The Electricity Storage Association is a trade
association established to foster development and commercialisation
of energy storage technologies. More information can be found
at: http://www.electricitystorage.org Back
102
For example, fuel cells such as that used in the demonstration
combined heat and power system installed at Woking could in principle
be configured for storage. Back
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