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. A PSH system 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.
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.[1][2]The reservoirs used with pumped storage can be quite small, when contrasted with the lakes of conventional hydroelectric plants of similar power capacity, and generating periods are often less than half a day.
The round-trip efficiency of PSH varies between 70% and 80%. Although the losses of the pumping process make 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. If the upper lake collects significant rainfall, or is fed by a river, then the plant may be a net energy producer in the manner of a traditional hydroelectric plant.
Pumped storage is by far the largest-capacity form of grid energy storage available, and, as of 2020[update], accounts for around 95% of all active storage installations worldwide, with a total installed throughput capacity of over 181 GW and as of 2020 a total installed storage capacity of over 1.6 TWh.[3]
In closed-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 if appropriately equipped.
Taking into account conversion losses and evaporation losses from the exposed water surface, energy recovery of 70–80% or more can be achieved.[8][9][10][11][12] This technique is currently the most cost-effective means of storing large amounts of electrical energy, but capital costs and the necessity 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 as possible above, 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.[13]
Along with energy management, pumped storage systems help stabilize electrical network frequency and provide reserve generation. Thermal plants are much less able to respond to sudden changes in electrical demand that potentially cause 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 they 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 use of such generation will increase the likelihood of those occurrences.[citation needed]
It is particularly likely that pumped storage will become especially important as a balance for very large-scale photovoltaic and wind generation.[16] 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.[17] 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.[18][19][20]
Smaller pumped storage plants cannot achieve the same economies of scale as larger ones, but some do exist, including a recent 13 MW project in Germany. Shell Energy has proposed a 5 MW project in Washington State. Some have proposed small pumped storage plants in buildings, although these are not yet economical.[21] Also, it is difficult to fit large reservoirs into the urban landscape (and the fluctuating water level may make them unsuitable for recreational use).[21] Nevertheless, some authors defend the technological simplicity and security of water supply as important externalities.[21]
Pumped storage plants can operate with seawater, although there are additional challenges compared to using fresh water, such as saltwater corrosion and barnacle growth.[28] Inaugurated in 1966, the 240 MW Rance tidal power station in France can partially work as a pumped-storage station. When high tides occur at off-peak hours, the turbines can be used to pump more seawater into the reservoir than the high tide would have naturally brought in. It is the only large-scale power plant of its kind.
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 seawater-based Lanai Pumped Storage Project was considered for Lanai, Hawaii, and seawater-based projects have been proposed in Ireland.[29] 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) lifting seawater 600 metres (2,000 ft) up a coastal cliff.[30][31]
Freshwater from the river floods is stored in the sea area replacing seawater by constructing coastal reservoirs. The stored river water is pumped to uplands by constructing a series of embankment canals and pumped storage hydroelectric stations for the purpose of energy storage, irrigation, industrial, municipal, rejuvenation of over exploited rivers, etc. These multipurpose coastal reservoir projects offer massive pumped-storage hydroelectric potential to utilize variable and intermittent solar and wind power that are carbon-neutral, clean, and renewable energy sources.[32]
In Bendigo, Victoria, Australia, the Bendigo Sustainability Group has proposed the use of the old gold mines under Bendigo for Pumped Hydro Energy Storage.[36] Bendigo has the greatest concentration of deep shaft hard rock mines anywhere in the world with over 5,000 shafts sunk under Bendigo in the second half of the 19th Century. The deepest shaft extends 1,406 metres vertically underground. A recent pre-feasibility study has shown the concept to be viable with a generation capacity of 30 MW and a run time of 6 hours using a water head of over 750 metres.
US-based start-up Quidnet Energy is exploring using abandoned oil and gas wells for pumped storage. If successful they hope to scale up, utilizing some of the 3 million abandoned wells in the US.[37][38]
Using hydraulic fracturing pressure can be stored underground in impermeable strata such as shale.[39] The shale used contains no hydrocarbons.[40]
Using a pumped-storage system of cisterns and small generators, pico hydro may also be effective for "closed loop" home energy generation systems.[43][44]
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 densely it can store energy.As such, the energy storage capacity of the submerged reservoir is not governed by the gravitational energy in the traditional sense, but by the vertical pressure variation.
RheEnergise[45] aim to improve the efficiency of pumped storage by using fluid 2.5x denser than water ("a fine-milled suspended solid in water"[46]), such that "projects can be 2.5x smaller for the same power."[47]
The first use of pumped storage was in 1907 in Switzerland, at the Engeweiher pumped storage facility near Schaffhausen, Switzerland.[48][49] In the 1930s reversible hydroelectric turbines became available. This apparatus could operate both as turbine generators and in reverse as electric motor-driven pumps. The latest in large-scale engineering technology is 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 70 metres (230 ft) above.[50]
In 2009, world pumped storage generating capacity was 104 GW,[51] while other sources claim 127 GW, which comprises the vast majority of all types of utility grade electric storage.[52] The European Union 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 EU. Japan had 25.5 GW net capacity (24.5% of world capacity).[51]
The six largest operational pumped-storage plants are listed below (for a detailed list see List of pumped-storage hydroelectric power stations):
Australia has 15GW of pumped storage under construction or in development. Examples include:
In June 2018 the Australian federal government announced that 14 sites had been identified in Tasmania for pumped storage hydro, with the potential of adding 4.8GW to the national grid if a second interconnector beneath Bass Strait was constructed.
The Snowy 2.0 project will link two existing dams in the New South Wales'' Snowy Mountains to provide 2,000 MW of capacity and 350,000 MWh of storage.[67]
In September 2022, a pumped hydroelectric storage (PHES) scheme was announced at Pioneer-Burdekin in central Queensland that has the potential to be the largest PHES in the world at 5 GW.
China has the largest capacity of pumped-storage hydroelectricity in the world.
In January 2019, the State Grid Corporation of China announced plans to invest US$5.7 billion in five pumped hydro storage plants with a total 6 GW capacity, to be located in Hebei, Jilin, Zhejiang, Shandong provinces, and in Xinjiang Autonomous Region. China is seeking to build 40 GW of pumped hydro capacity installed by 2020.[68]
There are 9 power stations capable of pumping with a total installed capacity of 1344 MW and an average annual production of 2247 GWh. The pumped storage hydropower in Norway is built a bit differently from the rest of the world. They are designed for seasonal pumping. Most of them can also not cycle the water endlessly, but only pump and reuse once. The reason for this is the design of the tunnels and the elevation of lower and upper reservoirs. Some, like Nygard power station, pump water from several river intakes up to a reservoir.
The largest one, Saurdal, which is part of the Ulla-Førre complex, has four 160 MW Francis turbines, but only two are reversible. The lower reservoir is at a higher elevation than the station itself, and thus the water pumped up can only be used once before it has to flow to the next station, Kvilldal, further down the tunnel system. And in addition to the lower reservoir, it will receive water that can be pumped up from 23 river/stream and small reservoir intakes. Some of which will have already gone through a smaller power station on its way.
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 in the United States was expanded with a pump-back system in 1973.[73] 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 dam''s upper reservoir and transmission system can expedite projects and reduce costs.
Emerging as a big player in renewable energy, pumped storage hydropower has many advantages and disadvantages. By using water from reservoirs and harnessing the power of gravity, pumped storage hydropower offers a dynamic solution to energy management. Think of it like a giant battery but with water. It''s smart, but not without its headaches.
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