Hydroelectric dams are renewable energy machines that use stored water to produce electricity on demand. A dam raises the water level upstream, creating a height difference called head that gives the water gravitational potential energy. When water flows through an intake and penstock, it spins a turbine connected to a generator.
The shape and materials of the dam determine how safely it holds back the reservoir.
Key Facts
- Hydropower uses gravitational potential energy: E = mgh.
- Ideal water power is P = ρgQh, where ρ is water density, Q is flow rate, and h is head.
- Real electric output is Pout = ηρgQh, where η is efficiency.
- Gravity dams resist water pressure mainly with their own weight and broad base.
- Arch dams curve upstream and transfer much of the water force into strong canyon walls.
- Embankment dams use compacted earth or rock and need a watertight core or facing to reduce seepage.
Vocabulary
- Gravity dam
- A heavy concrete or masonry dam that uses its weight to resist the horizontal force of reservoir water.
- Arch dam
- A curved dam that transfers water pressure sideways into the valley walls, allowing a thinner structure.
- Embankment dam
- A dam built from compacted soil, rock, or both, usually with a waterproof core to control leakage.
- Head
- The vertical height difference between the reservoir water surface and the turbine outlet.
- Penstock
- A large pipe or tunnel that carries high pressure water from the reservoir to the turbine.
Common Mistakes to Avoid
- Thinking all dams hold back water in the same way, which is wrong because gravity dams rely on weight, arch dams rely on shape and canyon support, and embankment dams rely on compacted fill and seepage control.
- Ignoring head in hydropower calculations, which is wrong because the same flow rate produces more power when the water falls through a greater vertical height.
- Using mass flow and volume flow interchangeably, which is wrong because P = ρgQh uses volume flow rate Q and the density ρ converts it to mass flow rate.
- Assuming a larger dam always produces more electricity, which is wrong because output depends on head, flow rate, turbine efficiency, and how much water is available.
Practice Questions
- 1 A hydroelectric plant has a head of 80 m, a flow rate of 25 m3/s, and an efficiency of 0.90. Using ρ = 1000 kg/m3 and g = 9.8 m/s2, calculate the electric power output.
- 2 A reservoir releases 12,000 kg of water through a height drop of 45 m. Calculate the gravitational potential energy available before turbine losses using E = mgh.
- 3 A narrow rocky canyon and a wide valley with thick soil are both being considered for dams. Explain which location is better suited for an arch dam and which is better suited for an embankment dam, and justify your choices.