Hydroelectric Plant

A hydroelectric plant converts the potential energy of water stored at a height into electricity. Power available is $P=\rho g Q H\eta$.

  Reservoir
  ~~~~~~~~~~~|  Dam            Surge tank
  ~~~~~~~~~~~|====+             |
  ~~~~~~~~~~~|    Intake  ______|_____
  ~~~~~~[Trash   rack]--/ Penstock    \
         [Gate]                        \
                                       [Turbine]=[Gen]
                                        |
                              Draft tube|  Tailrace -->
                                       ~~~~~~~~~~~~~~

Essential components

1. Catchment area and reservoir: collects and stores rainfall and river water. Its storage gives the control of flow through the year.
2. Dam: raises the water level to create head and storage. Types: gravity, arch, buttress, earth or rockfill. It has a spillway for passing flood water safely.
3. Intake structure with trash rack and gates: takes water into the waterway. The trash rack keeps out logs and debris; gates control and stop the flow. A silt-settling basin (desilter) is also used on rivers carrying silt.
4. Headrace (canal or tunnel): carries water from the intake to the forebay or surge tank.
5. Forebay or surge tank: the surge tank is a vertical open chamber near the penstock. It absorbs pressure rise (water hammer) when the turbine gates close suddenly and supplies water when load increases.
6. Penstock: a steel or reinforced concrete pipe taking water at high pressure from the forebay to the turbine.
7. Powerhouse: contains the turbines, generators, governors, control panel, and cranes.
8. Water turbine: converts water energy to mechanical energy (Pelton for high head, Francis for medium, Kaplan for low head).
9. Generator: coupled to the turbine to produce electricity.
10. Draft tube (reaction turbines): a diverging pipe below the runner which allows the turbine to be set above the tailwater level and recovers the kinetic energy of the exit water.
11. Tailrace: channel carrying the used water back to the river.
12. Switchyard and transformer: step up voltage for transmission.

Hydroelectric plants can be classified in several ways.

(The head and capacity limits differ slightly between textbooks.)

Run-of-river plant

Uses the natural flow of the river without significant storage; a small weir or barrage diverts water. Output varies with river flow, so it supplies base load in the wet season only and needs a standby in the dry season. Low cost and little submergence. Many hydro plants in the Himalayan rivers are of this type.

Storage (reservoir) plant

A dam stores water in the wet season for use in the dry season. Output can be controlled according to demand, so the plant can supply base load or peak load. Also gives flood control and irrigation benefits, but needs high cost and land submergence.

Pumped storage plant

Two reservoirs at different levels. During off-peak hours surplus power pumps water from the lower to the upper reservoir; at peak hours the water runs down through the turbines (often a reversible pump-turbine) to generate power.

   Upper reservoir ~~~~
                      \  penstock
                       \
         Pump/Turbine --- generator/motor
                       /
   Lower reservoir ~~~~

Merits: improves the load factor of the whole system, gives quick start-up for peak load, and uses surplus off-peak energy. Overall cycle efficiency is about 70 to 75 %.