What are the four types of hydropower plant?
Hydropower comes in four forms: reservoir plants, where a dam impounds a river and the storage allows peaking and frequency regulation; run-of-river plants, which generate whatever the river delivers; pumped storage, which pumps water uphill and releases it later and is storage rather than generation; and micro hydro, typically under 100 kW for off-grid villages. Global hydro capacity was about 1,400 GW in 2023, roughly 15% of world electricity.
Reservoir hydropower is the archetype: a dam blocks the river to create a large reservoir, and water is routed down penstocks to drive the turbines. The reservoir buys enormous operational flexibility — release more at peak, store at night, and provide peaking and frequency regulation. Reservoir size sets the regulating horizon: a small one manages a daily cycle, a large one a seasonal cycle, and the largest can carry surplus from a wet year into a dry one. The costs are equally real: flooded land, displaced communities, blocked migratory fish routes and altered downstream sediment transport.
Run-of-river schemes are usually far gentler on river ecology, because they do not inundate large areas or greatly change the seasonal distribution of flow. But their output depends entirely on inflow and drops sharply in the dry season, which is why system planners treat them as an unreliable green resource that must be paired with flexible generation or storage. Low-head, high-flow reaches typically use Kaplan turbines or bulb turbines, the latter sealing the generator inside a streamlined casing placed directly in the flow path.
Pumped-storage hydropower (PSH) is technically not generation at all but storage: electricity pumps water from a lower to an upper reservoir to charge, and the water is released through turbines when the grid needs it. Round-trip efficiency of 70%–85% is slightly below lithium-ion, but the energy capacity reaches the gigawatt-hour scale and beyond, and PSH accounts for over 90% of all grid storage worldwide. Bath County in Virginia, at 3,006 MW, is the largest single storage facility on Earth; in Japan pumped storage is about 9%–10% of total generating capacity.
Micro hydropower generally means installations below 100 kW, well suited to mountain streams and off-grid supply for remote villages. Nepal has among the world's highest micro-hydro penetration, with something over ten thousand micro and small hydro plants built as of 2022, bringing electricity to countless mountain communities. Together the four forms make hydropower the largest renewable electricity source in the world: global output exceeded 4,300 TWh in 2023, equal to solar, wind and biomass combined.
| Form | Storage and regulation | Output behaviour | Example |
|---|---|---|---|
| Reservoir | Large reservoir; daily to multi-year regulation | Peaking and frequency regulation; highest flexibility | Three Gorges Dam, 22.5 GW, 32 x 700 MW Francis units |
| Run-of-river (RoR) | No impoundment or only a small regulating pond | Generates what the river delivers; falls sharply in dry season | Treated as an unreliable green resource; needs flexible backup |
| Pumped storage (PSH) | Two reservoirs separated by elevation | Round-trip efficiency 70%–85%; storage, not generation | Bath County, USA, 3,006 MW, world's largest single storage facility |
| Micro hydro | Typically under 100 kW, no significant reservoir | Follows stream flow; used for off-grid supply | Nepal, over ten thousand micro and small plants as of 2022 |
Reservoir size sets the regulating horizon — a small reservoir manages a daily cycle, a large one a season, and the largest can carry the surplus of a wet year through to a dry one.
Sources
- IEA, World Energy Outlook 2023 (about 1,400 GW of hydro capacity and over 4,300 TWh in 2023, roughly 15% of world electricity)
- The Full Spectrum: Every Energy Source Explained — A Singapore Perspective, Chapter 9.2
This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第九章 9.2