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==Basic principle==
[[File:Pumpspeicherkraftwerk engl.png|thumb|Power distribution, over a day, of a pumped-storage hydroelectricity facility. Green represents power consumed in pumping. Red is power generated.]]
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.
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 combined pump and turbine generator units (usually [[Francis turbine]] designs).<ref>{{Cite web |title=Pumped-Hydro Energy Storage |url=http://web.engr.oregonstate.edu/~webbky/ESE471_files/Section%203%20Pumped%20Hydro.pdf |url-status=live |archive-url=https://web.archive.org/web/20201031005617/http://web.engr.oregonstate.edu/~webbky/ESE471_files/Section%203%20Pumped%20Hydro.pdf |archive-date=31 October 2020 |access-date=28 August 2020}}</ref> Variable speed operation further optimizes the round trip efficiency in pumped hydro storage plants.<ref>{{Cite web |date=4 July 2018 |title=Variable Speed Is Key To World's Biggest Pumped Hydro Energy Storage Project, China's Fengning Plant |url=https://cleantechnica.com/2018/07/04/variable-speed-is-key-to-worlds-biggest-pumped-hydro-energy-storage-project-chinas-fengning-plant/ |url-status=live |archive-url=https://web.archive.org/web/20200807144356/https://cleantechnica.com/2018/07/04/variable-speed-is-key-to-worlds-biggest-pumped-hydro-energy-storage-project-chinas-fengning-plant/ |archive-date=7 August 2020 |access-date=28 August 2020}}</ref><ref>{{Cite journal |last1=Joseph |first1=Anto |title=Reliability of Variable Speed Pumped-Storage Plant |last2=Chelliah |first2=Thanga |last3=Lee |first3=Sze |last4=Lee |first4=Kyo-Beum |journal=Electronics |year=2018 |volume=7 |page=265 |doi=10.3390/electronics7100265 |doi-access=free |issue=10}}</ref>▼
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 (usually [[Francis turbine]] designs).<ref>{{Cite web |title=Pumped-Hydro Energy Storage |url=http://web.engr.oregonstate.edu/~webbky/ESE471_files/Section%203%20Pumped%20Hydro.pdf |url-status=live |archive-url=https://web.archive.org/web/20201031005617/http://web.engr.oregonstate.edu/~webbky/ESE471_files/Section%203%20Pumped%20Hydro.pdf |archive-date=31 October 2020 |access-date=28 August 2020}}</ref>
In micro-PSH applications, a group of pumps and [[Pump As Turbine]] (PAT) could be implemented respectively for pumping and generating phases.<ref name=":1a">{{Cite journal |last1=Morabito |first1=Alessandro |title=Pump as turbine applied to micro energy storage and smart water grids: A case study |last2=Hendrick |first2=Patrick |date=2019-10-07 |journal=Applied Energy |volume=241 |pages=567–579 |doi=10.1016/j.apenergy.2019.03.018 |s2cid=117172774 |doi-access=|bibcode=2019ApEn..241..567M }}</ref> The same pump could be used in both modes by changing rotational direction and speed:<ref name=":1a" /> the operation point in pumping usually differs from the operation point in PAT mode.▼
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▲In micro-PSH applications, a group of pumps and [[Pump As Turbine]] (PAT) could be implemented respectively for pumping and generating phases.<ref name=":1a">{{Cite journal |last1=Morabito |first1=Alessandro |title=Pump as turbine applied to micro energy storage and smart water grids: A case study |last2=Hendrick |first2=Patrick |date=2019-10-07 |journal=Applied Energy |volume=241 |pages=567–579 |doi=10.1016/j.apenergy.2019.03.018 |s2cid=117172774 |doi-access=|bibcode=2019ApEn..241..567M }}</ref>
The same pump could be used in both modes by changing rotational direction and speed:<ref name=":1a" /> the operation point in pumping usually differs from the operation point in PAT mode.
==Types==
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