Table of Contents
Overview Of Hydropower Plant Types
Hydropower plants can be grouped in several ways, but the most useful classification for beginners looks at how water is used and how the plant interacts with the river and the electricity system. The main categories are run‑of‑river plants, reservoir or storage plants, pumped storage plants, and a few special configurations such as diversion schemes and small or micro hydropower. Each type has distinct operating characteristics, environmental footprints, and roles in the power system.
Run‑Of‑River Hydropower
Run‑of‑river plants use the natural flow of the river with little or no water storage. Water is diverted from the river into an intake, passes through turbines, and is then returned downstream. Some plants include a small weir or pond to smooth very short‑term variations in flow, but this storage is limited.
Because run‑of‑river output depends directly on river flow, electricity generation varies strongly with seasons and weather. During rainy or snowmelt periods, power output is high. During dry seasons, it can fall sharply. These plants normally provide what is called “run‑of‑river energy,” which is relatively inflexible and cannot easily be ramped up or down on demand.
From an infrastructure perspective, run‑of‑river projects often involve smaller dams than large storage plants and can be built as diversion schemes. In a diversion project, water is captured upstream, sometimes by a low dam, and channeled through a tunnel or canal to a powerhouse located at a lower elevation. After passing through the turbine, water returns to the river. This configuration can achieve a significant drop in height without creating a large reservoir.
Run‑of‑river projects are generally seen as having less extensive flooding of land compared with large storage dams, but they can still alter river flow patterns, affect aquatic habitats, and change local river uses. Their lower storage capacity also limits their role in providing grid flexibility and firm capacity.
Reservoir Or Storage Hydropower Plants
Reservoir or storage hydropower plants use a dam to create a large reservoir upstream. The water level behind the dam can be controlled, which allows operators to decide when to release water through the turbines to generate electricity.
The key feature of storage plants is their ability to store potential energy in the form of water at height. This stored water can be released when electricity demand is high or when other generation sources are not available. As a result, storage hydropower can provide base load energy as well as peak power, depending on how it is scheduled.
In a typical storage plant, the dam raises the water level and creates a substantial hydraulic head. The powerhouse is usually located at or near the base of the dam. Operators use spillways and gates to manage water levels and flows for both energy production and flood control.
Storage plants can smooth seasonal variations in river flow. During wet seasons, reservoirs are filled, while in dry seasons, stored water is drawn down to maintain generation. This makes them valuable for long‑term energy management and for supporting variable renewables such as wind and solar.
However, the creation of large reservoirs is associated with major changes to river systems and surrounding landscapes. Large storage projects flood valleys, alter sediment transport, and transform ecosystems upstream and downstream. These dimensions are explored in more detail in chapters that focus on environmental and social issues.
Pumped Storage Hydropower As A Special Type
Pumped storage hydropower is often considered a separate category, but it is useful to see it as a special type of hydropower plant that includes at least two reservoirs at different elevations. During periods of low electricity demand and low prices, excess electricity is used to pump water from the lower reservoir to the upper reservoir. During periods of high demand, water is released back down through turbines to generate electricity.
Pumped storage plants primarily act as energy storage systems rather than net energy sources. Their round‑trip efficiency, the ratio of energy generated to energy used for pumping, is typically less than 1. In simplified form, if $E_{pump}$ is the electrical energy used to pump water up and $E_{gen}$ is the electrical energy produced when the water flows back, then:
$$\text{Round‑trip efficiency} = \frac{E_{gen}}{E_{pump}}$$
Pumped storage hydropower is the dominant technology for large‑scale electricity storage worldwide and plays a critical role in balancing variable renewable energy such as wind and solar.
Some pumped storage schemes are “pure” storage with minimal natural inflow, while others are “mixed” schemes where natural river flow also contributes to the upper reservoir. Plant design can be either conventional, with surface reservoirs, or involve underground caverns and tunnels in mountainous terrain.
Run‑Of‑River Versus Storage Plants In Power Systems
The differences between run‑of‑river and storage hydropower are most visible in their operating roles. Run‑of‑river plants typically provide energy that follows hydrological conditions and have less ability to shift output in time. They may run at high capacity factors when water is abundant and at low capacity during dry periods. Storage plants, by contrast, can adjust output to match daily and seasonal demand patterns more closely.
In power systems, storage hydropower is often used as a flexible resource to provide peak capacity, fast ramping, and ancillary services such as frequency regulation. Run‑of‑river plants often operate as steady contributors when water is available, with limited dispatch flexibility. A system that combines both can benefit from predictable baseline energy and flexible peak support.
From a planning perspective, selecting between run‑of‑river and storage types depends on many factors such as river hydrology, topography, environmental and social impacts, and the needs of the electricity grid. These choices are typically evaluated in broader project development and planning processes.
Head‑Based Classification: High, Medium, And Low Head Plants
Another way to classify hydropower plants looks at the height difference between the water surface upstream and the water surface downstream. This height difference is called the hydraulic head $H$. It determines how much potential energy is available, along with the flow rate $Q$ of water.
The theoretical power $P$ available from flowing water can be approximated by:
$$P = \rho g Q H$$
where $\rho$ is the density of water and $g$ is the acceleration due to gravity.
Higher hydraulic head or higher flow rate both increase the potential power output of a hydropower plant, as captured by the relationship $P = \rho g Q H$.
High head plants use large vertical drops, sometimes several hundred meters, typically found in mountainous regions. They often involve diversion tunnels or penstocks carrying water from an intake high in the mountains down to a powerhouse much lower. Because the head is high, a relatively small flow can yield significant power.
Medium head plants operate with intermediate height differences that are common in foothill regions or along rivers with moderate gradients. Their civil structures, including dams and canals, are sized to the available topography and flow.
Low head plants use small drops, from a few meters to several tens of meters, as found in lowland rivers or canals. To generate useful power at low head, higher flow volumes are usually required. Low head projects are often associated with run‑of‑river configurations and can sometimes be retrofitted onto existing weirs or navigation dams.
Head classification influences the choice of turbines, the layout of penstocks and canals, and the overall design. It is one of the first technical parameters considered when assessing a hydropower site.
Scale‑Based Classification: Large, Small, Mini, Micro, And Pico
Hydropower plants are also categorized by their installed capacity. There is no single global standard, and thresholds vary by country, but the general idea is to distinguish large projects from smaller, more localized systems.
Large hydropower typically refers to plants above tens or hundreds of megawatts of capacity. These are often associated with major river basins, substantial reservoirs, and national or regional grids. They can supply large fractions of a country’s electricity and often have additional roles such as flood control, navigation, and irrigation support.
Small hydropower usually covers plants below a certain capacity threshold, often in the range from 1 megawatt to tens of megawatts. Below small hydro, the categories of mini, micro, and pico hydropower are used to describe even smaller systems that serve villages, clusters of homes, or individual households. A micro hydro system, for instance, can supply a rural community by tapping a nearby stream with a modest head.
These smaller systems may be run‑of‑river or diversion based and generally have less environmental and social impact compared with large reservoir projects, simply because the scale of intervention is smaller. They are often used in off‑grid or weak‑grid areas and can be combined with other renewables like solar in hybrid systems.
Scale influences not only technical design but also financing, ownership, and regulatory requirements. For example, community‑owned micro hydro projects are often developed under different frameworks than national large dam projects.
In‑River And Reservoir‑Based Configurations
Within the broad categories already discussed, it is useful to distinguish between in‑river plants and reservoir‑based plants. In‑river projects place most of the infrastructure within or immediately adjacent to the river channel, and they rely principally on the river’s flow at that location. Weir‑type run‑of‑river schemes are a common example.
Reservoir‑based plants create a significant upstream water body behind a dam. The plant may be located at the dam toe or some distance away. Diversion schemes can combine aspects of both, since they sometimes use a small intake structure on the river and route water through separate conduits to a powerhouse.
In‑river plants are often associated with lower storage capacity and more continuous flow regimes, while reservoir‑based plants add both storage and more extensive changes to the hydrological system. This distinction is important for understanding both technical operation and environmental consequences.
Choosing Among Hydropower Plant Types
The selection of a hydropower plant type for a particular site depends on natural conditions and human objectives. River flow variability, topography, geology, and environmental sensitivities shape what is technically feasible. At the same time, electricity system needs influence whether flexible storage or mainly run‑of‑river energy is more valuable.
Where there is a steep gradient and limited flow, a high head diversion scheme may be chosen. Where a broad river and relatively flat terrain exist, low head run‑of‑river or reservoir projects may be more appropriate. Pumped storage is typically sited in areas with suitable elevation differences and where grid conditions make storage profitable and useful.
Understanding these main types of hydropower plants, and their characteristic ways of using water and providing electricity, provides a foundation for exploring design details, environmental issues, and system integration in other chapters.