--- title: "Working principle of RoR, PRoR and ST plants" subject: "Hydropower Engineering" chapter: 2 key: "semester_7/hydropower_engineering:ror-pror-storage-working-principle" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/ror-pror-storage-working-principle site: BCT Engineering --- # Working principle of RoR, PRoR and ST plants Hydropower Engineering · Chapter 2: Planning of Hydropower Projects - Asked in: 2076 Ashwin, 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/ror-pror-storage-working-principle - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-2 ## Question Explain the working principle of RoR, PRoR and ST plants with the help of figures. Also comment on the suitability of those plants in the context of Nepal. ## Short answer RoR diverts river flow via weir and waterway without storage, so output varies seasonally. PRoR adds a small pond to store water for peak hours. Storage plants impound monsoon flow behind a dam for year-round firm energy. In Nepal, RoR is cheap but creates winter deficit; PRoR meets peak demand; storage is ideal long-term but costly. ## Answer #### 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. ```text 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. ```text 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does RoR output change through the year? A. No storage, output follows river flow B. Pondage for days C. Large reservoir D. Turbine fault Answer: A. High in monsoon, low in winter. (Key point: RoR output) 2. How does a PRoR plant give larger installed capacity? A. Has no pond B. Stores water in off-peak and uses it 4-6 hours in peak C. Has seasonal reservoir D. Uses pumps Answer: B. Daily pondage allows peaking. (Key point: PRoR) 3. Which is best for the long term in Nepal? A. Tidal B. Thermal C. RoR only D. Storage, since it gives dry-season energy Answer: D. Storage gives dry-season energy but is costly. (Key point: Nepal suitability) 4. What happens if Nepal builds too many RoR plants? A. Cheaper winter power B. Spill energy in monsoon and winter deficit C. No problem D. Excess dry-season energy Answer: B. Surplus in monsoon, deficit in winter. (Key point: Too many RoR) ## Related in Hydropower Engineering - [RoR and PRoR from hydrograph equation (Q40)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-pror-hydrograph-equation-q40) — Hydropower Engineering, chapter 3 - [Development stages and PRoR working principle](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astages-pror-working-arrangement) — Hydropower Engineering, chapter 2 - [Top producers and early plants of Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Ahydropower-history-nepal-world) — Hydropower Engineering, chapter 1 - [Common RoR intakes in Nepal; site selection](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-common-intakes-nepal-location) — Hydropower Engineering, chapter 5, asked 2× - [Three alternative RoR layouts](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-three-alternative-layouts) — Hydropower Engineering, chapter 2 - [Remedial measures for sediment deposition in RoR](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-sediment-deposition-remedial-measures) — Hydropower Engineering, chapter 5 ## Related in other subjects - [ROR versus PROR hydropower plants](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aror-vs-pror) — Power Plant Design, chapter 3 - [Kaplan turbine suitability in Nepal](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Akaplan-suitability-nepal) — Power Plant Design, chapter 3 - [Working principle of PCM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apcm-working-principle) — Communication Systems, chapter 4 - [Combined cycle advantages and working principle](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages-working) — Power Plant Equipment, chapter 8, asked 2×