Pumped-storage hydroelectricity (PSH), or pumped hydroelectric energy
storage (PHES), is a type of hydroelectric energy storage used by
electric power systems for load balancing. The method stores energy in
the form of gravitational potential energy of water, pumped from a
lower elevation reservoir to a higher elevation. Low-cost surplus
off-peak electric power is typically used to run the pumps. During
periods of high electrical demand, the stored water is released
through turbines to produce electric power. Although the losses of the
pumping process makes the plant a net consumer of energy overall, the
system increases revenue by selling more electricity during periods of
peak demand, when electricity prices are highest.
Pumped-storage hydroelectricity allows energy from intermittent
sources (such as solar, wind) and other renewables, or excess
electricity from continuous base-load sources (such as coal or
nuclear) to be saved for periods of higher demand. The
reservoirs used with pumped storage are quite small when compared to
conventional hydroelectric dams of similar power capacity, and
generating periods are often less than half a day.
Pumped storage is the largest-capacity form of grid energy storage
available, and, as of 2017, the United States Department of Energy
Global Energy Storage Database reports that PSH accounts for over 96%
of all active tracked storage installations worldwide, with a total
installed nameplate capacity of over 168 GW. The round-trip energy
efficiency of PSH varies between 70%–80%, with some
sources claiming up to 87%. The main disadvantage of PSH is the
specialist nature of the site required, needing both geographical
height and water availability. Suitable sites are therefore likely to
be in hilly or mountainous regions, and potentially in areas of
outstanding natural beauty, and therefore there are also social and
ecological issues to overcome. Many recently proposed projects, at
least in the U.S., avoid highly sensitive or scenic areas, and some
propose to take advantage of "brownfield" locations such as disused
1.1 Basic principle
1.2 Types: natural or man-made reservoirs
1.3 Economic efficiency
1.3.1 Small-scale facilities
3 Worldwide use
4 Pump-back hydroelectric dams
5 Potential technologies
5.2 Underground reservoirs
5.3 Decentralised systems
5.4 Underwater reservoirs
6 See also
8 External links
Power distribution, over a day, of a pumped-storage hydroelectricity
facility. Green represents power consumed in pumping; red is power
At times of low electrical demand, excess generation capacity is used
to pump water into the upper reservoir. When there is higher demand,
water is released back into the lower reservoir through a turbine,
generating electricity. Reversible turbine/generator assemblies act as
a combined pump and turbine generator unit (usually a Francis turbine
Types: natural or man-made reservoirs
In open-loop systems, pure pumped-storage plants store water in an
upper reservoir with no natural inflows, while pump-back plants
utilize a combination of pumped storage and conventional hydroelectric
plants with an upper reservoir that is replenished in part by natural
inflows from a stream or river. Plants that do not use pumped-storage
are referred to as conventional hydroelectric plants; conventional
hydroelectric plants that have significant storage capacity may be
able to play a similar role in the electrical grid as pumped storage
by deferring output until needed.
Taking into account evaporation losses from the exposed water surface
and conversion losses, energy recovery of 70-80% or more can be
regained. This technique is currently the most cost-effective
means of storing large amounts of electrical energy, but capital costs
and the presence of appropriate geography are critical decision
factors in selecting pumped-storage plant sites.
The relatively low energy density of pumped storage systems requires
either large flows and/or large differences in height between
reservoirs. The only way to store a significant amount of energy is by
having a large body of water located relatively near, but as high
above as possible, a second body of water. In some places this occurs
naturally, in others one or both bodies of water were man-made.
Projects in which both reservoirs are artificial and in which no
natural inflows are involved with either reservoir are referred to as
"closed loop" systems.
These systems may be economical because they flatten out load
variations on the power grid, permitting thermal power stations such
as coal-fired plants and nuclear power plants that provide base-load
electricity to continue operating at peak efficiency, while reducing
the need for "peaking" power plants that use the same fuels as many
base-load thermal plants, gas and oil, but have been designed for
flexibility rather than maximal efficiency. Hence pumped storage
systems are crucial when coordinating large groups of heterogeneous
generators. Capital costs for pumped-storage plants are relatively
high, although this is somewhat mitigated by their long service life
of up to 75 years or more, which is three to five times longer than
The upper reservoir (Llyn Stwlan) and dam of the Ffestiniog Pumped
Storage Scheme in north Wales. The lower power station has four water
turbines which generate 360 MW of electricity within 60 seconds of the
Along with energy management, pumped storage systems help control
electrical network frequency and provide reserve generation. Thermal
plants are much less able to respond to sudden changes in electrical
demand, potentially causing frequency and voltage instability. Pumped
storage plants, like other hydroelectric plants, can respond to load
changes within seconds.
The most important use for pumped storage has traditionally been to
balance baseload powerplants, but may also be used to abate the
fluctuating output of intermittent energy sources. Pumped storage
provides a load at times of high electricity output and low
electricity demand, enabling additional system peak capacity. In
certain jurisdictions, electricity prices may be close to zero or
occasionally negative on occasions that there is more electrical
generation available than there is load available to absorb it;
although at present this is rarely due to wind or solar power alone,
increased wind and solar generation will increase the likelihood of
such occurrences. It is particularly likely that pumped storage will
become especially important as a balance for very large scale
photovoltaic generation. Increased long distance transmission
capacity combined with significant amounts of energy storage will be a
crucial part of regulating any large-scale deployment of intermittent
renewable power sources. The high non-firm renewable electricity
penetration in some regions supplies 40% of annual output, but 60% may
be reached before additional storage is necessary.
While smaller scale pumped storage experiences an economy of scale
penalty, there are small-scale installations of such technology,
including a recent 13 MW project in Germany. Shell Energy has proposed
a 5 MW project in the U.S. state of Washington. Some have proposed
small pumped storage plants in buildings, although these are
economically unfeasible given the economies of scale present.
Also, a large volume of water is required for a meaningful storage
capacity which is a difficult fit for an urban setting.
Nevertheless, some authors defend its technological simplicity and
secure provision of water as important externalities.
The first use of pumped storage was in the 1890s in Italy and
Switzerland. In the 1930s reversible hydroelectric turbines became
available. These turbines could operate as both turbine-generators and
in reverse as electric motor driven pumps. The latest in large-scale
engineering technology are variable speed machines for greater
efficiency. These machines operate in synchronization with the network
frequency when generating, but operate asynchronously (independent of
the network frequency) when pumping.
The first use of pumped-storage in the United States was in 1930 by
the Connecticut Electric and Power Company, using a large reservoir
located near New Milford, Connecticut, pumping water from the
Housatonic River to the storage reservoir 230 feet above.
See also: United States Department of Energy Global Energy Storage
In 2009, world pumped storage generating capacity was 104 GW,
while other sources claim 127 GW, which comprises the vast majority of
all types of utility grade electric storage. The EU had 38.3 GW
net capacity (36.8% of world capacity) out of a total of 140 GW of
hydropower and representing 5% of total net electrical capacity in the
Japan had 25.5 GW net capacity (24.5% of world capacity).
In 2010 the United States had 21.5 GW of pumped storage generating
capacity (20.6% of world capacity). PSH generated (net) -5.501 GWh
of energy in 2010 in the US because more energy is consumed in
pumping than is generated. Nameplate pumped storage capacity had grown
to 21.6 GW by 2014, with pumped storage comprising 97% of grid-scale
energy storage in the US. As of late 2014, there were 51 active
project proposals with a total of 39 GW of new nameplate capacity
across all stages of the FERC licensing process for new pumped storage
hydroelectric plants in the US, but no new plants were currently under
construction in the US at the time.
The five largest operational pumped-storage plants are listed below
(for a detailed list see List of pumped-storage hydroelectric power
Bath County Pumped Storage Station
38°12′32″N 79°48′00″W / 38.20889°N 79.80000°W /
38.20889; -79.80000 (Bath County Pumped-storage Station)
Guangdong Pumped Storage Power Station
23°45′52″N 113°57′12″E / 23.76444°N 113.95333°E /
23.76444; 113.95333 (Guangzhou Pumped Storage Power Station)
Huizhou Pumped Storage Power Station
23°16′07″N 114°18′50″E / 23.26861°N 114.31389°E /
23.26861; 114.31389 (Huizhou Pumped Storage Power Station)
Okutataragi Pumped Storage Power Station
35°14′13″N 134°49′55″E / 35.23694°N 134.83194°E /
35.23694; 134.83194 (Okutataragi Hydroelectric Power Station)
Ludington Pumped Storage Power Plant
43°53′37″N 86°26′43″W / 43.89361°N 86.44528°W /
43.89361; -86.44528 (Ludington Pumped Storage Power Plant)
Note: this table shows the power-generating capacity in megawatts as
is usual for power stations. However, the overall energy-storage
capacity in megawatt-hours (MWh) is a different intrinsic property and
can not be derived from the above given figures.
The five countries with largest power capacity by 2017 :
Pump-back hydroelectric dams
Conventional hydroelectric dams may also make use of pumped storage in
a hybrid system that both generates power from water naturally flowing
into the reservoir as well as storing water pumped back to the
reservoir from below the dam. The
Grand Coulee Dam
Grand Coulee Dam in the US was
expanded with a pump-back system in 1973. Existing dams may be
repowered with reversing turbines thereby extending the length of time
the plant can operate at capacity. Optionally a pump back powerhouse
such as the Russell
Dam (1992) may be added to a dam for increased
generating capacity. Making use of an existing dams upper reservoir
and transmission system can expedite projects and reduce costs.
Pumped storage plants can operate with seawater, although there are
additional challenges compared to using fresh water. In 1999, the
30 MW Yanbaru project in Okinawa was the first demonstration of
seawater pumped storage. It has since been decommissioned. A 300 MW
Lanai Pumped Storage Project was considered for Lanai,
Hawaii, and seawater-based projects have been proposed in Ireland.
A pair of proposed projects in the
Atacama Desert in northern Chile
would use 600 MW of photovoltaic solar (Skies of Tarapacá)
together with 300 MW of pumped storage (Mirror of Tarapacá)
raising seawater 600 metres (2,000 ft) up a coastal
The use of underground reservoirs has been investigated. Recent
examples include the proposed Summit project in Norton, Ohio, the
proposed Maysville project in
Kentucky (underground limestone mine),
and the Mount Hope project in New Jersey, which was to have used a
former iron mine as the lower reservoir. The proposed energy storage
Callio site in
Pyhäjärvi (Finland) would utilize the deepest
base metal mine in Europe, with 1,450 metres (4,760 ft) elevation
difference. Several new underground pumped storage projects have
been proposed. Cost-per-kilowatt estimates for these projects can be
lower than for surface projects if they use existing underground mine
space. There are limited opportunities involving suitable underground
space, but the number of underground pumped storage opportunities may
increase if abandoned coal mines prove suitable.
Small pumped-storage hydropower plants can be built on streams and
within infrastructures, such as drinking water networks and
artificial snow making infrastructures. Such plants provide
distributed energy storage and distributed flexible electricity
production and can contribute to the decentralized integration of
intermittent renewable energy technologies, such as wind power and
solar power. Reservoirs that can be used for small pumped-storage
hydropower plants could include natural or artificial lakes,
reservoirs within other structures such as irrigation, or unused
portions of mines or underground military installations. In
Switzerland one study suggested that the total installed capacity of
small pumped-storage hydropower plants in 2011 could be increased by 3
to 9 times by providing adequate policy instruments.
In March 2017 the research project StEnSea (Storing Energy at Sea)
announced their successful completion of a four-week test of a pumped
storage underwater reservoir. In this configuration a hollow sphere
submerged and anchored at great depth acts as the lower reservoir,
while the upper reservoir is the enclosing body of water. Electricity
is created when water is let in via a reversible turbine integrated
into the sphere. During off-peak hours the turbine changes direction
and pumps the water out again, using "surplus" electricity from the
grid. The quantity of power created when water is let in grows
proportionally to the height of the column of water above the sphere,
in other words: the deeper the sphere is located, the more potential
energy it can store, which can be transformed into electric power. On
the other hand, pumping the water back out at greater depths also uses
up more power, since the turbine-turned-pump must act on the same
entire column of water.
As such the energy storage capacity of the submerged reservoir is not
governed by the gravitational energy in the traditional sense, but
rather by the vertical pressure variation.
While StEnSea's test took place at a depth of 100 m in the fresh
water Lake Constance, the technology is foreseen to be used in salt
water at greater depths. Since the submerged reservoir needs only a
connecting electrical cable, the depth at which it can be employed is
limited only by the depth at which the turbine can function, currently
limited to 700 m. The challenge of designing salt water pumped
storage in this underwater configuration brings a range of advantages:
No land area is required,
No mechanical structure other than the electrical cable needs to span
the distance of the potential energy difference,
In the presence of sufficient seabed area multiple reservoirs can
scale the storage capacity without limits,
Should a reservoir collapse, the consequences would be limited apart
from the loss of the reservoir itself,
Evaporation from the upper reservoir has no effect on the energy
Transmission of electricity between the reservoir and the grid can be
established from a nearby offshore wind farm limiting transmission
loss and obviating the need for onshore cabling permits.
A current commercial design featuring a sphere with an inner diameter
of 30 m submerged to 700 m would correspond to a 20 MWh
capacity which with a 5 MW turbine would lead to a 4-hour
discharge time. An energy park with multiple such reservoirs would
bring the storage cost to around a few eurocents per kWh with
construction and equipment costs in the range €1,200-€1,400 per
kW. To avoid excessive transmission cost and loss, the reservoirs
should be placed off deep water coasts of densely populated areas,
such as Norway, Spain, USA and Japan. With this limitation the concept
would allow for worldwide electricity storage of close to
For comparison, a traditional, gravity-based pumped storage capable of
storing 20 MWh in a water reservoir the size of a 30 m
sphere would need a hydraulic head of 519 m with the elevation
spanned by a pressurized water pipe requiring typically a hill or
mountain for support.
Renewable energy portal
Compressed air energy storage
Grid energy storage
List of energy storage projects
List of pumped-storage hydroelectric power stations
United States Department of Energy Global Energy Storage Database
Seawater Pumped Storage Okinawa,
Japan - Yanbaru
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