Pumped-storage hydroelectricity


Pumped-storage hydroelectricity, or pumped hydroelectric energy storage, 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 or excess electricity from continuous base-load sources to be saved for periods of higher demand.
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, accounted 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.
As of 2025, according to International Hydropower Association, worldwide PSH provides 200 GW power and 9000 GWh energy storage, while the Battery energy storage system market is catching up very fast in terms of power generation capacity. As of May 2025, China’s cumulative BESS installations are reported at 106.9 GW and 240.3 GWh.

Basic principle

A pumped-storage hydroelectricity generally consists of two water reservoirs at different heights, connected with each other.
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. Pumped storage plants usually use reversible turbine/generator assemblies, which can act both as a pump and as a turbine generator.
Variable speed operation further optimizes the round trip efficiency in pumped hydro storage plants.
In micro-PSH applications, a group of pumps and Pump As Turbine could be implemented respectively for pumping and generating phases.
The same pump could be used in both modes by changing rotational direction and speed: the operation point in pumping usually differs from the operation point in PAT mode.

Types

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.

Economic efficiency

Taking into account conversion losses and evaporation losses from the exposed water surface, energy recovery of 70–80% or more can be achieved. 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.
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 proven long service life of decades - and in some cases over a century, which is three to five times longer than utility-scale batteries. When electricity prices become negative, pumped hydro operators may earn twice - when "buying" the electricity to pump the water to the upper reservoir at negative spot prices and again when selling the electricity at a later time when prices are high.
File:Stwlan.dam.jpg|thumb|right|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 need arising.
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.
It is particularly likely that pumped storage will become especially important as a balance for very large-scale photovoltaic and wind 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.

Small-scale facilities

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. In 2016, proposals were made for small pumped storage plants in buildings, although these are not economical. Also, it is difficult to fit large reservoirs into the urban landscape. Nevertheless, some authors defend the technological simplicity and security of water supply as important externalities.

Location requirements

The main requirement for PSH is hilly country. The global greenfield pumped hydro atlas lists more than 800,000 potential sites around the world with combined storage of 86 million GWh, which is about 100 times more than needed to support 100% renewable electricity. Most are closed-loop systems away from rivers. Areas of natural beauty and new dams on rivers can be avoided because of the very large number of potential sites. Some projects utilise existing reservoirs such as the 350 Gigawatt-hour Snowy 2.0 scheme under construction in Australia. Some recently proposed projects propose to take advantage of "brownfield" locations such as disused mines such as the Kidston project under construction in Australia.

Environmental impact

Water requirements for PSH are small: about 1 gigalitre of initial fill water per gigawatt-hour of storage. This water is recycled uphill and back downhill between the two reservoirs for many decades, but evaporation losses must be replaced. Land requirements are also small: about 10 hectares per gigawatt-hour of storage, which is much smaller than the land occupied by the solar and windfarms that the storage might support. Closed loop pumped hydro storage has the smallest carbon emissions per unit of storage of all candidates for large-scale energy storage.

Potential technologies

Seawater

Pumped storage plants can operate with seawater, although there are additional challenges compared to using fresh water, such as saltwater corrosion and barnacle growth. 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. A pair of proposed projects in the Atacama Desert in northern Chile would use 600 MW of photovoltaic solar together with 300 MW of pumped storage lifting seawater up a coastal cliff.