EI-FACT SHEET 2014 - 02
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EI Fact sheet: Storage technologies for the power system
Introduction
Electricity is a real-time product. In this perspective, electricity
is generated to match demand expectations, with the help of
stockpiling at the fuel side of generation. Power plants may have
substantial fuel reserves; from days to weeks for coal-fired ones
up to years’ worth in a nuclear power plant. Remaining mismatch
in supply and demand can (in principle) be bridged with a number
of solutions, one of which is electricity storage, be it directly or
indirectly. In some countries, indirect electricity storage -via hydropump stations- is well established, but in most countries, that type
of electricity storage is only available to a limited extent. In
Belgium, the available storage capacity in current electric power
system (~5.8 GWh) represents less than an hour’s worth of
typical demand (~10 GW).
The storage of electricity represents a combination of three
services: consuming electric power, accumulating the energy in
some form, and finally producing electric power again. As energy
is never destroyed, the storage of electricity employs some kind
of conversion process, but with efficiencies less than 100%. The
energy stored at a particular instant can be zero (when the
storage device has been completely discharged), thereby limiting
the ability to ‘produce’ power. Conversely, when the storage
facilities are completely filled, it is impossible to for them consume
more electric power.
Drivers
Renewable energy sources (RES) are being deployed rapidly
in the European power system. As technologies such as wind
turbines and solar photovoltaics now make up a substantial
amount of the electricity generation, their inherent intermittency
presents a challenge for the operation of the power system.
Moreover, good sources of renewable energy are often found
far away from population centers. To increase the flexibility, the
generation patterns by such RES sources are forecasted;
however prediction errors remain. As wind speed and solar
irradiation cannot be controlled, typical generation technologies
based on these sources provide instantaneous power, offering
little opportunity to control the output except for curtailment.
Often RES targets (e.g., the EU’s 20-20-20 targets) are expressed
in terms of final-energy consumption, meaning that about ~ 34 %
of the electric-energy consumption must be of renewable origin,
in turn requiring very large installed power capacities (especially
for PV). Overproduction must be “absorbed” somewhere, which
is then “released” when RES produce little electricity. Storing such
RES-generated electricity —otherwise squandered— is just one
of the ways in which the integration of RES in the power system
can be managed. Other techniques to take on the challenges
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related to integrating RES include active demand response,
distribution and transmission grid upgrades or extension (perhaps
with HVDC) and flexible operation of conventional power plants.
A variety of storage possibilities exist, each with their particular
characteristics (size in terms of stored energy, max power uptake
and delivery, efficiency, cost, etc.). In addition, depending on the
technology, response time, space requirements and landscape
impact may also limit the amount of value that can be provided. A
storage facility can be deployed where it is enabled by a matching
combination of technological parameters, geological features and
local grid capabilities.
Large-scale storage systems may provide a number of services
in the wholesale-market context. An arbitrage strategy can be
performed to obtain positive cash flows by exploiting price
differences in the electricity market. Furthermore, as part of a
production portfolio, storage can be used for balancing and cost
minimization. Moreover, storage systems may provide reserves
and can have black-start capability. Finally, storage can be used to
manage congestion issues in the grid.
For example, consider a remote onshore wind farm connected
to a relatively weak grid. Local conditions permit harvesting more
wind energy by enlarging the farm, but the grid connection may
lack the capability to handle more power. Upgrading the grid can
take a long time. If a storage technology is found which can be
deployed in the close vicinity of the wind farm, the farm can be
expanded while avoiding excessive curtailment and having to wait
for grid upgrades to come in place.
Storage in distribution grids is currently rare, however this is
expected to increase. The value of electric energy increases
as it is transported from the transmission grid through the
distribution grid down to the end user. Therefore, the cost of
curtailment and losses is higher as well. Trying to avoid grid
problems, “clever” grid tariffs or renewable energy support
schemes may be designed with the aim of increasing selfconsumption of local generation such as PV. Storage can be
used to increase self-consumption and as storage becomes
more distributed, congestion issues, which are inherently local,
could perhaps be better managed. Moreover, at the low
voltage level, storage could be used to improve security of
supply through islanding. Having said the above, it is important
to remain vigilant for overall system effects: promoting
storage at the local level might resolve local “problems” but
may introduce side effects on a higher level. In any case,
the ancillary services for the grids (on the distribution and
transmission level) will have to be paid for by all users of the
grid.
EI Fact sheet: Storage technologies for the power system
EI-FACT SHEET 2014 - 02
Scope
This factsheet discusses a selected number of mechanical,
chemical and electrochemical storage technologies. This
document does not aim to provide a complete overview, nor
does it choose a ‘best’ technology. It aims to introduce the
operating principles, to discuss characteristics and to provide
insights in how to match current and near-term storage
technologies to applications in the power system. The market
value of storage facilities on the wholesale and the local level is
not considered in this factsheet.
Technology overview
Pumped Hydro Storage
Pumped Hydro Storage (PHS) systems store electricity
through a mechanical conversion. An electric motor pumps
water from a reservoir to another reservoir at higher
elevation to store energy. The energy is released by
reversing the process, thereby operating like a classic hydro
plant. PHS represents over 99 % of the worldwide installed
capacity (> 127 GW) of electricity storage. This technology is
mature and losses are moderate; the cycle efficiency is of the
order of 65 % - 80 % depending on the case.
The energy-storage capacity of PHS is a function of height
difference between and volumetric capacity of the
reservoirs. Typically, substantial civil engineering work is
required to obtain sizable energy-storage capacities.
Furthermore, in the western world, a lot of the ‘easy’
locations may already have been captured. Except for
converting current hydro plants to PHS, the potential is
severely limited by the requirement of specific geological
features.
Consequently, hydro plants and PHS typically are not found
close to population centers. Therefore, it is difficult for
this technology to help avoid congestion-related issues at
times of peak use of the transmission grid. System-level
services can however be provided at reasonable cost. For
example, a price-arbitrage strategy can be performed, and
black-start capabilities as well as reserves can be offered. PHS
systems offer power ratings of up to several GWs for a few
tens of hours (typically). Usually, PHS is limited to large-scale
applications.
Improved PHS technologies are being developed, to
increase the flexibility in terms of services and land use.
Classically, reverting from generation to pumping mode
in PHS costs time because dewatering the turbine room is
required. Furthermore, in pumping mode the operating
power of a pump-turbine is fixed. Variable-speed
technologies enable more flexibility and improve the
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dynamic behavior of PHS. On the other hand, using the sea as
a reservoir can minimize land use. Furthermore, energy-island
concepts have been developed on paper, where the second
reservoir is an artificial atoll. Surface area may represent a
smaller issue as on land, but large height differences are more
difficult to obtain.
Battery Energy Storage Systems (BESS)
Battery Energy Storage Systems (BESS) consist of batteries to
store electricity and a converter to exchange energy with the
grid. The batteries consist of reversible electrochemical cells
fixed in packs and modules, mechanically designed to allow
for efficient cooling as well as to protect against (accidental)
damage. The charging and discharging characteristics of the
chemistry under consideration determine the power rating.
Charging and discharging power ratings are often of different
value. Using conventional battery technology, BESS can be
considered solid-state systems, as they have no moving parts1.
Efficiencies and dynamics are high, but the required investment
can be substantial and the technology is considered to be of
lower maturity than PHS.
A wide range of battery chemistries is considered for grid
application. Three categories are discussed: conventional
low-temperature batteries, high-temperature batteries, and
batteries with circulation of the chemically active components
(flow batteries).
Pb-Acid and Li-ion
Common low-temperature battery chemistries are Pb-acid
and Li-ion. The Pb-acid chemistry is well known and relatively
inexpensive; however, its energy and power densities are low.
Li-ion encompasses a wide range of chemistries, offering
different trade-offs of energy density, power density and
cost. In general though, Li-ion is more efficient but also more
expensive than Pb-acid. However, prices have decreased from
more than 1000 $/kWh a few years ago to below 500 $/kWh
because of economies of scale being explored with the rollout
of electric vehicles. Within the Li-ion chemistry family, Lititanate and LiFePO4 chemistries are most often considered for
distribution grid applications, because of their improved power,
efficiency, cycle-life and thermal stability characteristics. BESS
systems with Li-ion or Pb-acid have been demonstrated up to a
few tens of MWs with 15 min to about 10 h of storage capacity.
High temperature batteries
Currently, the most commonly used battery technology in
the distribution grid2 is sodium-sulfur (NaS), a kind of hightemperature chemistry. This storage technology is deployed
globally at more than 200 locations, with a total of more than
300 MW, 1.9 GWh. It consists of molten salts of sodium and
sulfur, materials that are available at low cost. This chemistry
The heating, ventilation and air conditioning system may contain moving
parts.
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Also in general in the electric power system
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EI Fact sheet: Storage technologies for the power system
EI-FACT SHEET 2014 - 02
offers long cycle-life, high efficiency and a good energy
density. However, the heating system represents a cost
and causes losses. NaS BESS are operated in e.g. Japan, US,
Germany and France. NaS is used to provide frequency
regulation services on islands or to decrease variability in the
output of wind-power plants. Typical installed commercial systems have ratings from 1 MW up to 34 MW, with about 6 h
of storage capacity. Other high temperature battery chemistries such as with NaNiCl2 are commercially available, but the
maturity in grid applications is lower.
Flow batteries
Flow batteries (also known as redox-flow batteries) use
chemical components dissolved in a liquid electrolyte. These
components are then pumped through an electrochemical cell
stack. The electrolytes are stored in tanks, away from the cell
stack. This separation of the active materials minimizes selfdischarge losses. Therefore, this technology offers the potential
of much longer-term storage than conventional batteries. The
tank size determines the energy content of the system, thereby
decoupling the energy content and power rating of the system.
The cycle-life and scalability are high, but the maturity is low,
the energy density is low and the complexity is high because
of the pumps and control systems. Limited commercialization
has taken place with zinc-bromide (ZnBr) flow battery systems.
Another technology (VRB) is based on vanadium-vanadium
redox reactions. Flow battery system ratings typically are of a
few (tens) of MW for a few hours.
Interaction between batteries for mobile and
stationary applications
The batteries of electric vehicles could be used as gridconnected storage through concepts often referred to as
Vehicle-to-Grid (V2G). The reader is referred to the
KU Leuven EI fact sheet on electric vehicles for more
information.
Compressed Air Energy Storage (CAES)
Compressed Air Energy Storage (CAES) stores electricity
by compressing air and storing it in a high-pressure
containment. It is mainly applicable in large-scale bulk applications.
To generate electricity again, the compressed air is expanded
over a turbine coupled to a generator. When air is compressed,
its temperature increases. This heat, next to the increased
pressure, represents part of the energy stored. If the
compressed air cools down after pressurizing, the thermal is
energy is lost and the system efficiency decreases. To make up for
the heat loss, a fuel, typically natural gas, is used to reheat before
and during expansion taking place.
Therefore, such CAES technology is considered similar to a
gas turbine, where the compression and expansion stages are
decoupled in time. The volume of the storage reservoir
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determines the storage capacity. Furthermore, the pressure in
the reservoir may vary substantially as a function of the energy
content, which limits the turbines in the recovery of the stored
energy. Natural storage reservoirs are used, such as salt caverns,
former mines and former natural gas sites. Consequently, the
application of this technology is limited by geography. Only a few
commercial systems exist. For example the 321 MW CAES in
Huntorf, Germany is operating since 1978 and uses salt caverns.
CAES system ratings are in the range of hundreds of MWs for
a few hours to a day. CAES is competitive with PHS in terms of
scale and storage duration.
Newer CAES technologies try to improve the efficiency, negate
the need of additional fuel, and negate the need of underground
geological features3. The efficiency can be improved by storing heat,
for example using adiabatic CAES technology (mostly referred
to as AACAES —Advanced Adiabatic Compressed Air Energy
Storage). CAES with aboveground storage reservoirs are
developed using manufactured pressure reservoirs.
Flywheels
Flywheels store electricity by converting it to mechanical kinetic
energy. When the flywheel is turning, it continually loses energy,
largely because of friction losses.To minimize such losses, magnetic
bearings are used and the flywheel is operated in a partial vacuum.
Nevertheless, flywheels are designed for seconds to a few tens
of minutes worth of capacity. Consequently, they are used in
applications were high power exchange is more valuable than high
energy-storage capacity. This technology is mostly for limited-size
applications and short-duration purpose. For example, a 20 MW
flywheel power plant is in operation in the USA (NY state) and is
used for primary frequency regulation.
Power to gas (P2G)
Power-to-gas (P2G) technology stores energy in a chemical form,
by converting electricity to methane. Technologically, methane can
be fed into the gas grid, were ample storage capacity is available.
However, the synthetic methane will have to compete with the
available natural gas streams in the market. P2G builds upon
electrolysis technology, which uses electricity and water to
generate hydrogen gas.The hydrogen formed, in combination with
a source of CO2 and electricity is then chemically converted to
CH4 in a process called “methanation” (via the so-called Sabatier
reaction). This technology has a high potential, but maturity of the
process is low and costs are uncertain. The efficiency of P2G is
relatively low at about 60 %, but it is the sole electricity storage
technology that offers seasonal storage capabilities. A 2 MW P2G
unit recently started operation in Falkenhagen, Germany. In some
schemes, the methanation step (i.e. the combination with CO2) is
skipped and hydrogen is fed into the natural-gas grids. Mixing H2,
however, is only possible in (very) limited quantities.
See
e.g.
http://www.me.umn.edu/~lixxx099/papers/Mohsen_Li_
ACC12_modelingCAES.pdf
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EI Fact sheet: Storage technologies for the power system
EI-FACT SHEET 2014 - 02
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Conclusion
Fig. 1 illustrates techno-economic storage technology
parameters. PHS technology is mature and dependable and due
to its relatively low cost it will remain the most common storage
technology for a long time. BESS with NaS batteries operate
commercially, and numerous demonstration projects with other
chemistries are developed to prove market readiness despite
higher investment required. P2G is the only technological path
that offers long-term storage capability, but currently the maturity
is low.
Operators may use these storage technologies to offer a range
of services in the liberalized electricity market. The market may
provide a number income streams that offer opportunities to
upgrade current storage assets or to build new ones. Depending
on the availability of such income streams, stakeholders develop
business cases and make investments. Potential investors can
choose from a range of storage technologies with varying degrees
of maturity. Investment studies must consider multiple services
and markets and take an application-focused approach.
Legend
PHS
CAES
ZnBr
VRB
Li-ion
Pb-acid
NaS
Flywheel
Fuel cell
P2G
Cycle
life
(-)
1000
10 000
100 000
Calendar
life
(a)
10
20
30
Roundtrip
efficiency
(%)
60
70
80
90
100
Power
rating
kW
MW
GW
Cost
(USD/kW)
0
1000
2000
3000
4000
5000
0
1000
2000
3000
4000
5000
Cost
(USD/kWh)
Charge/discharge
duration
minute
Figure 1: Comparison of techno-economic storage technology
parameters. The symbols indicate the typical value; the lines indicate
variation in cited values.
hour
day
week
season
Maturity
low
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high
Energy
density
(Wh/l)
1
10
100
1000
EI Fact sheet: Storage technologies for the power system