Hydropower source and plant types

The source of hydropower is the water cycle: the sun evaporates water, which falls as rain or snow at a height and runs back to the sea. Water stored or flowing at a height (head) has potential energy, which is converted into kinetic energy and then into mechanical and electrical energy by a turbine and generator. Power: P = ρ g Q H η.

Classification of hydropower plants

1. By capacity (Nepal): pico (< 1 kW), micro (1–100 kW), mini (100 kW–1 MW), small (1–10 MW), medium (10–100 MW), large (> 100 MW). (Limits differ by country.)
2. By head: low (< 15 m), medium (15–70 m), high (> 70 m).
3. By storage / water use: run-of-river (ROR), peaking run-of-river (PROR), storage (reservoir), pumped storage.
4. By load: base load, peak load.

Types

• Run-of-river (ROR): uses the natural flow of the river, with a small weir to divert water; no or very little storage. Output varies with river flow. Most Nepali plants are of this kind, for example Khimti and Upper Bhotekoshi.
• Peaking run-of-river (PROR): has a small daily pondage so it can generate more in peak hours, for example Kali Gandaki A, Middle Marsyangdi and Upper Tamakoshi.
• Storage (reservoir) plant: a dam stores water in the monsoon and releases it when needed, so output can be controlled; for example Kulekhani I (60 MW), Nepal's main storage plant, whose water is reused by Kulekhani II (32 MW) downstream.
• Pumped storage plant: water is pumped to an upper reservoir using surplus electricity and released through turbines at peak time. It works as a large battery.
• Micro/pico hydro: small plants of a few kW for villages.

 Intake -> Canal -> Forebay -> Penstock -> Turbine -> Tailrace
 (weir)                (head H)           + Generator