Editing Pumped-storage hydroelectricity (section)

==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.<ref name="EconomistPSH">{{Cite news |date=2011-03-03 |title=Energy storage - Packing some power |newspaper=[[The Economist]] |url=http://www.economist.com/node/21548495?frsc=dg%7Ca |url-status=live |access-date=2012-03-11 |archive-url=https://web.archive.org/web/20200306081719/https://www.economist.com/technology-quarterly/2012/03/03/packing-some-power |archive-date=6 March 2020}}</ref><ref name="thier">{{Cite web |last=Jacob |first=Thierry |date=2011-07-07 |title=Pumped storage in Switzerland - an outlook beyond 2000 |url=http://www.stucky.ch/en/contenu/pdf/Pumped_storage_in_Switzerland_Dr_Jacob.pdf |access-date=2012-02-13 |website=Stucky|archive-url=https://web.archive.org/web/20110707003324/http://www.stucky.ch/en/contenu/pdf/Pumped_storage_in_Switzerland_Dr_Jacob.pdf |archive-date=7 July 2011 }}</ref><ref name="Levine">{{Cite web |last=Levine |first=Jonah G. |date=December 2007 |title=Pumped Hydroelectric Energy Storage and Spatial Diversity of Wind Resources as Methods of Improving Utilization of Renewable Energy Sources |url=http://www.colorado.edu/engineering/energystorage/files/MSThesis_JGLevine_final.pdf |publisher=University of Colorado |page=6|archive-url=https://web.archive.org/web/20140801113053/http://www.colorado.edu/engineering/energystorage/files/MSThesis_JGLevine_final.pdf |archive-date=1 August 2014 }}</ref><ref name="yang">{{Cite book |last=Yang |first=Chi-Jen |url=https://books.google.com/books?id=TPReBwAAQBAJ&dq=info:1fSw0yVikpMJ:scholar.google.com&pg=PA25 |title=Pumped Hydroelectric Storage |date=11 April 2016 |publisher=Duke University|isbn=9780128034491 }}</ref><ref name="ESA">{{Cite web |title=Pumped Hydroelectric Storage {{!}} Energy Storage Association |url=http://energystorage.org/energy-storage/technologies/pumped-hydroelectric-storage/ |archive-url=https://web.archive.org/web/20190119150459/http://energystorage.org/energy-storage/technologies/pumped-hydroelectric-storage/ |archive-date=19 January 2019 |access-date=15 January 2017 |website=energystorage.org}}</ref> 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.<ref name="ferc-psp-def">{{Cite web |title=FERC: Hydropower - Pumped Storage Projects |url=https://www.ferc.gov/industries/hydropower/gen-info/licensing/pump-storage.asp |archive-url=https://web.archive.org/web/20170720223612/https://www.ferc.gov/industries/hydropower/gen-info/licensing/pump-storage.asp |archive-date=20 July 2017 |access-date=15 January 2017 |website=www.ferc.gov}}</ref>

These systems may be economical because they flatten out load variations on the power grid, permitting [[thermal power station]]s such as [[Fossil fuel power plant|coal-fired plant]]s and [[nuclear power plant]]s 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 [[unit commitment problem in electrical power production|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,<ref>{{Cite web |date=30 December 2020 |title=Pumping power: Pumped storage stations around the world |url=https://www.drax.com/power-generation/pumping-power-pumped-storage-stations-around-the-world/ |url-status=live |archive-url=https://web.archive.org/web/20211119125837/https://www.drax.com/power-generation/pumping-power-pumped-storage-stations-around-the-world/ |archive-date=19 November 2021 |access-date=19 November 2021}}</ref><ref>{{Cite web |title=Erneuter Abschreiber beim Pumpspeicher Engeweiher |date=28 June 2017 |url=https://www.shn.ch/region/stadt/2017-06-28/erneuter-abschreiber-beim-pumpspeicher-engeweiher |url-status=live |archive-url=https://web.archive.org/web/20210420045611/https://www.shn.ch/region/stadt/2017-06-28/erneuter-abschreiber-beim-pumpspeicher-engeweiher |archive-date=20 April 2021 |access-date=9 March 2020}}</ref> which is three to five times longer than utility-scale batteries. When [[Negative pricing#Electricity|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 price]]s 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 Power Station|Ffestiniog Pumped Storage Scheme]] in North [[Wales]]. The lower power station has four water turbines which generate 360&nbsp;MW of electricity within 60 seconds of the need arising.]]

Along with energy management, pumped storage systems help stabilize [[utility frequency|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,<!--diurnal--> but they may also be used to [[Load following power plant|abate]] the fluctuating output of [[intermittent energy source]]s.<!--week/month--> Pumped storage provides a load at times of high electricity output and low electricity demand, enabling additional system peak capacity. In certain jurisdictions, [[Electricity market|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 power|wind]] or [[Solar power|solar]] power alone, increased use of such generation will increase the likelihood of those occurrences.{{citation needed|date=November 2021}}

It is particularly likely that pumped storage will become especially important as a balance for very large-scale [[photovoltaic]] and wind generation.<ref>{{Cite book |url=http://www.iea-pvps.org/products/rep8_02s.htm |title=Summary Energy from the Desert - Practical Proposals for Very Large Scale Photovoltaic Power Generation (VLS-PV) Systems |publisher=Earthscan |archive-url=https://web.archive.org/web/20070613185611/http://www.iea-pvps.org/products/rep8_02s.htm |archive-date=13 June 2007 |editor=Kurokawa, K. |editor2=Komoto, K. |editor3=van der Vleuten, P. |editor4=Faiman, D. |via=IEA Photovoltaic Power Systems Programme}}</ref> 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.<ref>{{Cite web |title=Reducing Wind Curtailment through Transmission Expansion in a Wind Vision Future |url=http://www.nrel.gov/docs/fy17osti/67240.pdf |url-status=live |archive-url=https://web.archive.org/web/20170116163807/http://www.nrel.gov/docs/fy17osti/67240.pdf |archive-date=16 January 2017 |access-date=14 January 2017}}</ref>  The high [[Intermittent energy source#Penetration|non-firm renewable electricity penetration]] in some regions <!--parts of Germany, Australia and Denmark--> supplies 40% of annual output, but 60% may be reached before additional storage is necessary.<ref>{{Cite web |date=7 December 2015 |title=German grid operator sees 70% wind + solar before storage needed |url=http://reneweconomy.com.au/german-grid-operator-sees-70-wind-solar-storage-needed-35731/ |url-status=live |archive-url=https://web.archive.org/web/20170202011740/http://reneweconomy.com.au/german-grid-operator-sees-70-wind-solar-storage-needed-35731/ |archive-date=2 February 2017 |access-date=20 January 2017 |website=Renew Economy |quote=Schucht says, in the region he is operating in, 42 percent of the power supply (in output, not capacity), came from wind and solar – about the same as South Australia. Schucht believes that integration of 60 to 70 percent variable renewable energy – just wind and solar – could be accommodated within the German market without the need for additional storage. Beyond that, storage will be needed.}}</ref><ref>{{Cite web |last=Dehmer |first=Dagmar |date=8 June 2016 |title=German electricity transmission CEO: '80% renewables is no problem' |url=https://www.euractiv.com/section/energy/interview/german-electricity-transmission-ceo-80-renewables-is-no-problem/ |url-status=live |archive-url=https://web.archive.org/web/20161018130552/http://www.euractiv.com/section/energy/interview/german-electricity-transmission-ceo-80-renewables-is-no-problem/ |archive-date=18 October 2016 |access-date=1 February 2017 |website=[[Der Tagesspiegel]] / [[EurActiv]].com |quote=There are a certain number of myths in the energy industry. One of them is that we need more flexibility in the system to integrate renewables, like energy storage, interruptible loads or backup power plants. That's a myth. We are well on track to having a system that can accommodate between 70-80% renewable energy without the need for more flexibility options.}}</ref><ref name="enet42">{{Cite web |date=15 January 2016 |title=New record-breaking year for Danish wind power |url=http://energinet.dk/EN/El/Nyheder/Sider/Dansk-vindstroem-slaar-igen-rekord-42-procent.aspx |archive-url=https://web.archive.org/web/20160125083857/http://energinet.dk/EN/El/Nyheder/Sider/Dansk-vindstroem-slaar-igen-rekord-42-procent.aspx |archive-date=25 January 2016 |publisher=[[Energinet.dk]]}}</ref>

=== 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&nbsp;MW project in Germany. Shell Energy has proposed a 5&nbsp;MW project in Washington State. Some have proposed small pumped storage plants in buildings, although these are not yet economical.<ref name=":0">{{Cite journal |last1=de Oliveira e Silva |first1=Guilherme |title=Pumped hydro energy storage in buildings |last2=Hendrick |first2=Patrick |date=2016-10-01 |journal=Applied Energy |volume=179 |pages=1242–1250 |doi=10.1016/j.apenergy.2016.07.046|bibcode=2016ApEn..179.1242D }}</ref> Also, it is difficult to fit large reservoirs into the urban landscape (and the fluctuating water level may make them unsuitable for recreational use).<ref name=":0" /> Nevertheless, some authors defend the technological simplicity and security of water supply as important [[Externality|externalities]].<ref name=":0" />