Small hydro. Small hydro is the development of hydroelectric power on a scale suitable for local community and industry, or to contribute to distributed generation in a regional electricity grid. Exact definitions vary, but a "small hydro" project is less than 50 megawatts (MW), and can be further subdivide by scale into "mini" (<1MW), "micro" (<100 kW), "pico" (<10 kW). In contrast many hydroelectric projects are of enormous size, such as the generating plant at the Three Gorges Dam at 22,500 megawatts or the vast multiple projects of the Tennessee Valley Authority.

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. By not requiring large dams or large water storage capabilities small hydro projects are relatively benign on the environment.  This makes small hydro projects an attractive compromise for renewable energy activists, environmentalists, and investors.

Description
The use of the term "small hydro" varies considerably around the world, the maximum limit is usually somewhere between 10 and 30 MW. While a minimum limit is not usually set, the US National Hydropower Association specifies a minimum limit of 5 MW.  In California, hydroelectric generating stations with a maximum capacity of less than 30 MW are classified as small, and are eligible for inclusion in the state's renewable portfolio standard, while hydroelectric generating stations with a higher capacity are classified as large and are not considered renewable. The "small hydro" description may be stretched up to  in the United States, Canada and China. In India, hydro projects up to 25 MW station capacities have been categorized as Small Hydro Power (SHP) projects.

Small hydro can be further subdivided into mini hydro, usually defined as 100 to 1,000 kilowatts (kW), and micro hydro which is 5 to 100 kW. Micro hydro is usually the application of hydroelectric power sized for smaller communities, single families or small enterprise. The smallest installations are pico hydro, below 5 kW.

Since small hydro projects usually have correspondingly small civil construction work and little or no reservoir, they are seen as having a relatively low environmental impact compared to large hydro.

An additional defining feature of small hydro is known as run-of-river, or that the physical impact of the project is relatively minuscule compared to major hydroelectric dams which require a water storage lake. Little water is stored behind the project, if at all, and the river is usually able to continue flowing.

Growth
According to a report by REN21, during 2008 small hydro installations grew by 28% over year 2005 to raise the total world small hydro capacity to 85 gigawatts (GW). Over 70% of this was in China (with 65 GW), followed by Japan (3.5 GW), the United States (3 GW) and India (2 GW). Global growth is expected to be 2.8% annually until the mid-2020s when capacity will be about 150 gigawatts.

China planned to electrify a further 10,000 villages between 2005 and 2010 under their China Village Electrification Program, including further investments in small hydro and photovoltaics. By 2010, China had 45,000 small hydro installations, especially in rural areas, producing 160 Twh annually. Over 50% of the world's potential small hydro power was found in Asia; however, the report noted, "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.

History 
Wood water wheels along riversides may be considered the first examples of "small hydro". Up to the 17th century the efficiency of water wheels neared 70%. However, as the need for power generation increased small hydropower projects were phased out in favor of the large scale dams using newly designed turbines. 

Post 20th century environmental doctrine is moving away from large-scale hydropower construction due to the increased awareness of ecological problems associated with dams. For instance, the large artificial lakes often flood habitats and communities and disrupt species reliant on consistent river flow.  Examples of previous projects which sought to remove dams include the Restoration of the Elwha River and Un-Dam the Klamath river movement.

Since the 1960's the number of new small hydro projects have declined despite the capacity far exceeding current usage. Despite this trend, some countries, including China, India, and Brazil, are significantly expanding their small hydro capacity in the 21st century.

Generation

Hydroelectric power is the generation of electric power from the movement of water. A  hydroelectric facility requires a dependable flow of water and a reasonable height for the water to fall, called the head. In a typical installation, water is fed from a reservoir through a pipe into a turbine. The water flowing through the turbine causes an electrical generator to rotate, converting the motion into electrical energy.

Small hydro may be developed by constructing new facilities or through re-development of existing dams whose primary purpose is flood control, or irrigation. Old hydro sites may be  re-developed, sometimes salvaging substantial investment in the installation such as penstock pipe and turbines, or just re-using the water rights associated with an abandoned site. 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.

Project design
Many companies offer standardized turbine generator packages in the approximate size range of 200 kW to 10 MW. These "water to wire" packages simplify the planning and development of the site since one vendor looks after most of the equipment supply. Because non-recurring engineering costs are minimized and development cost is spread over multiple units, the cost of such package systems is reduced.  While synchronous generators capable of isolated plant operation are often used, small hydro plants connected to an electrical grid system can use economical induction generators to further reduce installation cost and simplify control and operation.

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 irrigation canals. "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.

Advantages and disadvantages 
The primary advantages of small hydro development include low cost to build, environmental justice, and ability to remain disconnected from centralized power grids.

Rural or isolated areas that are expensive to connect to national power grids are where most small hydro developments are made. For instance, rural areas in India or other countries that have flowing water regimes utilize small hydro to provide a renewable source of energy without connection to the national grid.  Additionally, in communities which are geographically isolated from national power grids small hydro projects provide the greatest reduction in green house gas emissions. 

For investors, environmentalists, and policy makers small hydro projects are considered most viable when there is little ecological impact and projected profit after construction.  It is shown to be relatively easy for stakeholders to greenlight small hydro developments if these conditions are met, even to a slight degree. 

A final noted advantage of small hydro over larger hydropower developments or fossil fuel plants is an element of environmental justice. In a number of communities which lack essential electricity access small hydro offers a reliable and clean source of electricity.  Small hydro projects do not normally require significant government assistance. Gaps in governance allow small hydro projects to be built and which provide local power to local communities. 

Disadvantages of small hydro do exist primarily in habitat alteration and potential cost increase.

Some of the disadvantages of small hydro come in the form of how the running water system is disturbed. 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 primarily. 

Although the cost of small hydro projects are generally far lower than large-scale hydroelectric systems, there is a cost to construction. Considering this, if a small hydro project proves to be uneconomical it will have an outsized budget expenditure relative to large projects which run over-budget.

Sample list of small installations worldwide
Africa
Zengamina, a 700 kW plant in Kalene Hill, Mwinilunga District in northwestern Zambia. 2008
Asia
Meenvallam Small Hydroelectric Project, Palakkad district, Kerala, India.
Bario Asal & Arur Layun Micro-Hydro Community Project, Kelabit Highlands, Sarawak, Malaysia. 2009
Ambangal Mini Hydro Electric Power Plant, Kiangan, Ifugao, Philippines (2010)
Asiga River Mini Hydro Electric Power Plant, Santiago, Agusan del Norte, Philippines (2019)
Balongbong Mini Hydro Electric Power Plant, Bato, Catanduanes, Philippines (1983)
Cantingas Mini Hydro Electric Power Plant, San Fernando, Romblon, Philippines (2009)
Likud Mini Hydro Electric Power Plant, Asipulo and Kiangan, Ifugao, Philippines (2015)
New Bataan Hydro Electric Power Plant, New Bataan, Davao de Oro, Philippines (2018)
San Luis Mini Hydro Electric Power Plant, San Luis, Aurora, Philippines (2011)
Villasiga Mini Hydro Electric Power Plant, Bugasong, Antique, Philippines (2016)
Pakpattan Hydro Power Project, a 2.82 MW plant on Pakpattan Canal in Pakpattan District, Pakistan 2015
Europe
Green Valleys Project, Brecon Beacons National Park, Wales.  Joint winner of £300K in Big Green Challenge 2009.
St Catherine's, a National Trust site near Windermere, Westmorland, United Kingdom.
North America
Ames Hydroelectric Generating Plant, Colorado, United States. On the List of IEEE Milestones
Childs-Irving Hydroelectric Facilities, Arizona, United States. 1916 Now decommissioned 
Snoqualmie Falls, Washington, United States. 1957
Malibu Hydro, British Columbia, Canada. 2005
Menihek Hydroelectric Generating Station, Newfoundland and Labrador, Canada. 1954
Cloudworks Energy is one of many Independent power producers that have built projects in British Columbia, Canada

See also

Electricity generation
Hydro power
Low head hydro power
Micro hydro up to 100 kW
Pico hydro up to 5 kW
Conduit hydroelectricity
International Center on Small Hydro Power

References

Hydroelectricity
Power station technology
Renewable energy
Distributed generation