Editing IPCC:AR6/WGIII/Chapter-6 (section)

==== 6.4.2.3 Hydroelectric Power ====

<div id="h3-3-siblings" class="h3-siblings"></div>

Hydropower is technically mature, proved worldwide as a primary source of renewable electricity, and may be used to balance electricity supply by providing flexibility and storage. The LCOE of hydropower is lower than the cheapest new fossil fuel-fired option. However, the future mitigation potential of hydropower depends on minimising environmental and social impacts during the planning stages, reducing the risks of dam failures, and modernising the ageing hydropower fleet to increase generation capacity and flexibility ( ''high confidence'' ).

Estimates of global gross theoretical available hydropower potential varies from 31–128 PWh yr –1 (112–460 EJ yr –1 ), exceeding total electricity production in 2018 ( [[#Banerjee--2017|Banerjee et al. 2017]] ; [[#BP--2020|BP 2020]] ; [[#IEA--2021d|IEA 2021d]] ). This potential is distributed over 11.8 million locations (Figure 6.12), but many of the locations cannot be developed for (current) technical, economic, or political reasons. The estimated technical potential of hydropower is 8–30 PWh yr –1 (29–108 EJ yr –1 ), and its estimated economic potential is 8–15 PWh yr –1 (29–54 EJ yr –1 ) ( [[#Zhou--2015|Zhou et al. 2015]] ; [[#van%20Vliet--2016c|van Vliet et al. 2016c]] ). Actual hydropower generation in 2019 was 4.2 PWh (15.3 EJ), providing about 16% of global electricity and 43% of global electricity from renewables ( [[#BP--2020|BP 2020]] ; [[#IEA--2020f|IEA 2020f]] ; [[#Killingtveit--2020|Killingtveit 2020]] ). Asia holds the largest hydropower potential (48%), followed by South America (19%) ( [[#Hoes--2017|Hoes et al. 2017]] ).

<div id="_idContainer036" class="Basic-Text-Frame"></div>

[[File:ecb735027f9e0293917943f24c6585a3 IPCC_AR6_WGIII_Figure_6_12.png]]

'''Figure 6.12 | Global map of gross hydropower potential distribution [GWh yr''' –1 '''].''' Source: data from Hoeset al. (2017).

Hydropower is a mature technology with locally adapted solutions ( ''high confidence'' ) ( [[#Zhou--2015|Zhou et al. 2015]] ; [[#Killingtveit--2020|Killingtveit 2020]] ). The peak efficiency of hydroelectric plants is greater than 85%. Hydropower plants without storage or with small storage typically produce a few kWs to 10 MWs (examples of plants producing higher amounts do exist), and are useful for providing electricity at a scale from households to small communities ( [[#El%20Bassam--2013|El Bassam et al. 2013]] ; [[#Towler--2014|Towler 2014]] ). However, hydropower plants without or with small storage may be susceptible to climate variability, especially droughts, when the amount of water may not be sufficient to generate electricity ( [[#Premalatha--2014|Premalatha et al. 2014]] ) ( [[#6.5|Section 6.5]] ).

Hydropower plants with storage may produce 10 GW, reaching over 100 TWh yr –1 (0.36 EJ yr –1 ), but generally require large areas. Pumped storage hydropower stores energy by pumping water to higher reservoirs during low-demand periods ( [[#Killingtveit--2020|Killingtveit 2020]] ). The storage in hydropower systems provides flexibility to compensate for rapid variations in electricity loads and supplies. The regulating characteristics of the storage play an important role in assuring continuity of energy supply from renewable sources ( [[#Yang--2018b|Yang et al. 2018b]] ).

Hydropower is one of the lowest-cost electricity technologies ( [[#Mukheibir--2013|Mukheibir 2013]] ; [[#IRENA--2021b|IRENA 2021b]] ). Its operation and maintenance costs are typically 2–2.5% of the investment costs per kW yr –1 for a lifetime of 40–80 years ( [[#Killingtveit--2020|Killingtveit 2020]] ). Construction costs are site-specific. The total cost for an installed large hydropower project varies from USD10,600–804,500 kW –1 if the site is located far away from transmission lines, roads, and infrastructure. Investment costs increase for small hydropower plants and may be as high as USD100,000 kW –1 or more for the installation of plants of less than 1 MW – 20% to 80% more than for large hydropower plants ( [[#IRENA--2015|IRENA 2015]] ). During the past 100 years, total installed costs and LCOE have risen by a few percent, but the LCOE of hydropower remains lower than the cheapest new fossil fuel-fired option ( [[#IRENA--2019b|IRENA 2019b]] , 2021).

Hydroelectric power plants may pose serious environmental and societal impacts ( ''high confidence'' ) ( [[#McCartney--2009|McCartney 2009]] ). Dams may lead to fragmentation of ecological habitats because they act as barriers for migration of fish and other land and water-borne fauna, sediments, and water flow. These barriers can be mitigated by sediment passes and fish migration aids, and with provision of environmental flows. Below dams, there can be considerable alterations to vegetation, natural river flows, retention of sediments and nutrients, and water quality and temperature. Construction of large reservoirs leads to loss of land, which may result in social and environmental consequences. Minimising societal and environmental impacts requires taking into account local physical, environmental, climatological, social, economic, and political aspects during the planning stage ( [[#Killingtveit--2020|Killingtveit 2020]] ). Moreover, when large areas of land are flooded by dam construction, they generate GHGs ( [[#Prairie--2018|Prairie et al. 2018]] ; [[#Phyoe--2019|Phyoe and Wang 2019]] ; [[#Maavara--2020|Maavara et al. 2020]] ). On the other hand, hydropower provides flexible, competitive low-emission electricity, local economic benefits (e.g., by increasing irrigation and electricity production in developing countries), and ancillary services such as municipal water supply, irrigation and drought management, navigation and recreation, and flood control ( [[#IRENA--2021b|IRENA 2021b]] ). However, the long-term economic benefits to communities affected by reservoirs are a subject of debate ( [[#de%20Faria--2017|de Faria et al. 2017]] ; [[#Catolico--2021|Catolico et al. 2021]] ).

Public support for hydroelectric energy is generally high ( [[#Steg--2018|Steg 2018]] ), and higher than support for coal, gas, and nuclear. Yet, public support for hydro seems to differ for existing and new projects ( ''high confidence'' ). Public support is generally high for small- and medium-scale hydropower in regions where hydropower was historically used ( [[#Gormally--2014|Gormally et al. 2014]] ). Additionally, there is high support for existing large hydropower projects in Switzerland ( [[#Rudolf--2014|Rudolf et al. 2014]] ; [[#Plum--2019|Plum et al. 2019]] ), Canada ( [[#Boyd--2019|Boyd et al. 2019]] ), and Norway ( [[#Karlstrøm--2014|Karlstrøm and Ryghaug 2014]] ), where it is a trusted and common energy source. Public support seems lower for new hydropower projects ( [[#Hazboun--2020|Hazboun and Boudet 2020]] ), and the construction of new large hydropower plants has been met with strong resistance in some areas ( [[#Vince--2010|Vince 2010]] ; [[#Bronfman--2015|Bronfman et al. 2015]] ). People generally perceive hydroelectric energy as clean and a non-contributor to climate change and environmental pollution ( [[#Kaldellis--2013|Kaldellis et al. 2013]] ). For example, in Sweden, people believed that existing hydropower projects have as few negative environmental impacts as solar, and even less than wind ( [[#Ek--2005|Ek 2005]] ). However, in areas where the construction of new large-scale hydroelectric energy is met with resistance, people believe that electricity generation from hydro can cause environmental, social, and personal risks ( [[#Bronfman--2012|Bronfman et al. 2012]] ; [[#Kaldellis--2013|Kaldellis et al. 2013]] ).

The construction time of hydroelectric power plants is longer than many other renewable technologies, and that construction time may be extended by the additional time it takes to fill the reservoir. This extended timeline can create uncertainty in the completion of the project. The uncertainty is due to insecurity in year-to-year variations in precipitation and the water inflows required to fill reservoirs. This is especially critical in the case of trans-boundary hydroelectric power plants, where filling up the reservoirs can have large implications on downstream users in other nations. As a result of social and environmental constraints, only a small fraction of potential economic hydropower projects can be developed, especially in developed countries. Many developing countries have major undeveloped hydropower potential, and there are opportunities to develop hydropower combined with other economic activities such as irrigation ( [[#Lacombe--2014|Lacombe et al. 2014]] ). Competition for hydropower across country borders can lead to conflict, which could be exacerbated if climate alters rainfall and streamflow ( [[#Ito--2016|Ito et al. 2016]] ).

<div id="6.4.2.4" class="h3-container"></div>

<span id="nuclear-energy"></span>