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Small hydro is the generation of hydroelectric power on a smaller scale as compared to traditional large-scale hydro. Exact definitions vary by country, but small hydro power (SHP) projects are typically less than 50 Megawatt (MW) and can be further subdivided by scale into "mini" (<500kW), "Micro hydro" (<100 kW), and "Pico hydro" (<10 kW). Maximum power generation capacity is the primary factor of SHP classification. Factors like dam height, weir height, reservoir area, outlet structures and operating procedures are not standardized under this metric.
SHP projects have grown rapidly in the past two decades. Quicker permitting processes can make them easier to develop and contribute to distributed generation in a regional electricity grid. Small hydro projects may be built in isolated areas that would be uneconomic to serve from a national electricity grid, or in areas where a national grid does not exist. They produce power on a scale suitable for local community use, promoting energy independence. Rural areas face challenges in SHP integration due to an absence of political focus, accurate data, and sustainable funding.
The exact socio-environmental effects of smaller scale hydro are not yet fully understood. Many countries do not require environmental impact assessments for smaller installations.
In a number of communities which lack essential electricity access, small hydro offers a reliable source of decentralized electricity. Small hydro projects do not always require significant government assistance and gaps in governance allow them to be built fairly easily. Given these policy gaps, small hydropower as a renewable, climate mitigation strategy can also negatively affect local livelihoods in the absence of community-minded policy.
The environmental impacts of small hydropower projects are understudied. Within run-of-river design projects, the greatest harm for water systems are flow regime alteration, loss of river cohesion and connectivity, and habitat degradation effecting fish and macroinvertebrates.
pg12 Over 50% of the world's potential small hydro power was found in Asia; however, a report noted that "It is possible in the future that more small hydropower potential might be identified both on the African and American continents".
In the mountains and rain forests of British Columbia, Canada there are a great many sites suitable for hydro development. However environmental concerns towards large reservoirs after the 1980s halted new dam construction. The solution to coping with increased demand was to offer contracts to independent power producers, who have built 100 run of the river projects under 50 MW. Power production without reservoirs varies dramatically, but older conventional dams retain or release water to average out production though the year. In 2014 these independent producers generated 18,000 GWh from 4,500 MW of capacity.
As of 2022, the global capacity (for projects ≤10 MW) is approximately 79.0 GW, with China holding over 53% of the world's SHP installed capacity. Under this definition (≤10 MW), installed SHP capacity increased by 11% in the Americas from 2019 to 2022. Because of local differences in SHP definitions, it is likely that the installed capacity of SHPs across the globe is higher than these totals. Countries such as China, India, and Brazil, are significantly expanding their small hydro capacity in the 21st century. The continents of Asia, Africa and the Americas hold the most potential for small hydro power growth..]]
Post 20th century environmental doctrine is moving away from large-scale hydropower construction due to increased awareness of ecological problems associated with dams. Examples of previous dam deconstruction projects include the Restoration of the Elwha River and Un-Dam the Klamath river movement in the United States. Both of these projects deconstructed dams with generation capacities less than 30 MW.
Small hydro projects may be created from the re-development of existing dams whose primary purpose is flood control or irrigation. Old hydro sites may also be re-developed with water rights re-used, salvaging substantial investment in installation technology such as penstock pipe and turbines. Either of these cost saving advantages can make the return on investment for a small hydro site well worth the use of existing sites.
Brazil is another country which is investing heavily in small hydro. Brazil itself is a leader in hydroelectric generation, the world's third most hydropower installed capacity country at 79 GW, behind the United States at 100 GW, and China in first place with 171 GW. 51 new small hydro projects are, as of 2024, being constructed in Brazil.
Small "run of the river" projects do not have a conventional dam with a reservoir, only a weir to form a headpond for diversion of inlet water to the turbine. Unused water simply flows over the weir and the headpond may only be capable of a single day's storage, not enough for dry summers or frozen winters when generation may come to a halt. A preferred scenario is to have the inlet in an existing lake.
Modular "micro hydrokinetic" systems have been developed for Acequia. "Irrigation districts across the U.S. have installed power plants at diversion points and in-canal drops, which are traditionally used for flow measurement, to stabilize upstream heads and to dissipate energy where there is significant elevation change throughout the canal system."
Countries like India and China have policies in favor of small hydro, and the regulatory process allows for building dams and reservoirs. In North America and Europe the regulatory process is too long and expensive to consider having a dam and a reservoir for a small project.
Small hydro projects usually have faster environmental and licensing procedures, and since the equipment is usually in serial production, standardized and simplified, and the civil works construction is also reduced, the projects may be developed very rapidly. The physically smaller size of equipment makes it easier to transport to remote areas without good road or rail access.
One measure of decreased environmental impact with lakes and reservoirs depends on the balance between stream flow and power production. Reducing water diversions helps the river's ecosystem, but reduces the hydro system's return on Investment (ROI). The hydro system design must strike a balance to maintain both the health of the stream and the economics.
Part of the balance between a small hydro project's return on investment and environmental concern is the proximity of the project to the national power grid. The more isolated a small hydro project is the more cost effective its construction will be.
In India, SHPs under 25 MW do not require an Environmental Impact Assessment and instead have a Detailed Project Report (DPR). These reports provide basic information to policy and decision makers, speeding up timelines for small hydro developments. Planning and approval processes under India's DPRs do not require community engagement nor consultation.
In Matsuguma Village, Japan, a 30 kW small hydropower plant was completed in 2020 under a private-public partnership. The local government approached the community explaining their goal to supply power to the regional electric grid. Community members collaborated with a business model based "by the local community and for the local community." By selling the electricity produced, community members obtained energy democracy, improved the infrastructure of the village and improved their welfare and quality of life.
Multiple small hydropower projects are occasionally placed segmentally on rivers, but little research has been done on the effects of multiple installations in a collective area. In India, cumulative effects of multiple SHP projects are not considered prior to granting hydropower developments. In China, researchers found the effects of river connectivity on a river with 31 small hydro projects to significantly outweigh the impacts associated with a river containing 4 larger-scale projects.
Previous studies on the sustainability of hydropower focus on greenhouse gas emission estimations, which are generally lesser than other conventional or renewable energy systems. Assessments of sustainability often do not consider additional parameters, such as environmental impacts, land use, social costs and cultural effects. With climate change threatening the reliability of seasonal river flows, the efficiency and sustainability of small hydro for energy generation is unknown.
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