The basic idea
Hydropower captures the energy of moving water. Water that sits at a height has stored energy. When it flows downhill, that energy can be used to turn a turbine, and the turbine drives a generator. Rivers are refilled by rain and snowmelt, which is why hydropower is usually classed as renewable, though the supply depends on climate and weather and is not constant.
Two things determine how much power a site can produce:
- Head, the vertical drop the water falls through.
- Flow, the volume of water passing through per second.
More of either means more potential power. A tall dam on a modest river and a low weir on a large river can end up with comparable output for different reasons.
From water to electricity
In a typical plant, water enters through an intake and travels down a pipe called a penstock, gaining speed as it drops. It strikes or passes through a turbine, causing it to spin. The turbine shaft turns a generator, producing electricity that is sent through a substation to the grid. The water then returns to the river downstream.
Different turbine designs suit different conditions. Some are built for high head and lower flow, others for low head and high flow. The choice is an engineering decision made for each site.
A well-designed hydropower plant can convert a large share of the water’s energy into electricity, and plants often operate for many decades with maintenance and equipment refurbishment. That long life is one of hydro’s recognized strengths.
The main types of hydropower plants
Run-of-river
These plants divert some of a river’s natural flow through a channel or pipe past a turbine, with little or no storage. Their impact on the landscape is usually smaller than a large dam, but their output follows the river, so it rises and falls with seasons and rainfall.
Reservoir (storage) plants
A dam blocks a river and creates a reservoir. Operators can release water when electricity is needed and hold it back when it is not. This makes reservoir hydropower one of the more controllable renewable sources. It can help balance an electricity grid, for instance by responding quickly when demand rises or when other sources fall short. Reservoirs may also serve other purposes, such as flood control, irrigation and drinking water supply.
Pumped storage
Pumped storage plants move water between two reservoirs at different heights. When electricity is plentiful or cheap, water is pumped uphill. When power is needed, it is released back down through turbines. Strictly speaking, this stores energy rather than generating new energy, and it is a major form of large-scale energy storage today. The technique is covered in more detail under pumped hydro.
Small and micro hydro
Small installations can power a village, a farm or a single building. They can be valuable in remote areas, though suitable sites and maintenance capacity are needed.
Environmental and social trade-offs
Hydropower produces no direct combustion emissions, and over its life its emissions are generally low compared with fossil fuels. That does not make it impact-free, and its effects vary greatly from site to site.
- Rivers and wildlife. Dams can block fish migration, change water temperature and flow patterns, and trap sediment that would otherwise nourish downstream land and deltas. Fish passages and managed releases can reduce but not always eliminate these effects.
- Flooded land. Creating a reservoir can submerge forests, farmland, homes and cultural sites. Some projects have involved moving communities, which raises questions of fairness, compensation and consent.
- Reservoir emissions. Decaying organic matter in some reservoirs, especially in warm regions, can release methane and carbon dioxide. The size of this effect varies by site and is an active area of study, so a life cycle assessment is useful for comparing projects.
- Dependence on climate. Droughts, shrinking snowpack and changing rainfall patterns can reduce output. Planning increasingly has to consider how water availability may shift in the coming decades, and estimates vary.
- Dam safety. A failure can be catastrophic downstream, which is why design standards, inspections and emergency planning are regulated closely.
Where hydropower fits
Hydropower has been one of the largest sources of renewable electricity worldwide for a long time. Countries with mountainous terrain and plentiful rivers tend to rely on it more. Many of the best sites in some regions are already developed, so future growth often comes from upgrading existing plants, adding turbines to existing dams, or building smaller or pumped schemes rather than new large dams.
Its flexibility is valuable alongside wind power and solar power, because those depend on weather and hydropower can often be adjusted. Whether hydropower is the right choice for a location depends on local geography, ecology, cost, community views and the results of formal project permitting and environmental review.
Working around dams and plants
Hydropower stations involve strong water flows, heavy equipment and high-voltage systems. Work on them is done by trained, authorized staff following official lockout procedures and site rules. This article does not describe operating methods, and the public should heed warning signs and sirens near dams and tailraces because water levels can change quickly.
This is general information, not engineering or safety advice. Details differ by country and by plant.