Working principle of RoR, PRoR and ST plants

Run-of-river (RoR) plant

A small weir or barrage raises the river level and diverts the flow into the intake. The plant uses the flow as it comes; no significant storage exists. Hence the output changes with river flow; it is high in the monsoon and low in winter. Pondage (storage for less than a day) is not provided.

 River --> [Weir] --> Intake --> Desander
                                    |
                     Headrace tunnel/canal
                                    |
                    Forebay/surge --> Penstock
                                    |
                      Powerhouse --> Tailrace --> River

Peaking run-of-river (PRoR) plant

It has a small reservoir or pond near the headworks that stores water for a few hours (daily regulation). In the lean period, water is stored in off-peak hours and used for 4-6 hours in the peak hours, so the installed capacity is larger than that of the RoR plant. Example: Kaligandaki A, Upper Tamakoshi.

 Off-peak: Q stored in pond (small turbine use)
 Peak:     Q_river + Q_stored --> turbine (full power)

Storage (ST) plant

A high dam creates a reservoir to hold monsoon flow for seasonal or annual use. It gives firm energy through the year and permits the largest installed capacity and best regulation. Example: Kulekhani I.

      Reservoir
  ____/~~~~~~~\____
 |    Dam        |  --> Intake --> Tunnel/Penstock --> PH

Suitability in Nepal

• RoR: Cheap and quick but gives little dry-season energy; Nepal has surplus in monsoon and deficit in winter, so too many RoR plants lead to spill energy and an imbalance.
• PRoR: Best for the near term; it meets the evening peak. Many projects are of this type.
• Storage: Best for the long term because it gives dry-season energy, flood control and irrigation; but it needs high cost, long time, land and resettlement. Prefer a mix with priority to storage and PRoR projects.