---
title: "Classification of hydroelectric plants"
subject: "Fluid Machines"
chapter: 2
key: "semester_5/fluid_machines:hydroelectric-plant-classification"
source: https://www.bctengineer.com/q/semester_5/fluid_machines/hydroelectric-plant-classification
site: BCT Engineering
---

# Classification of hydroelectric plants

Fluid Machines · Chapter 2: Hydroelectric Plant

- Asked in: year not given, 5 marks
- Video: [Types of hydro power plants, Classification of Hydro power plants, #HydroPlant](https://www.youtube.com/watch?v=2Ya1i7oQphg) — Mechanical Engineering Management
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/fluid_machines/hydroelectric-plant-classification
- Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-2

## Question

Classify hydroelectric power plants. Explain run-of-river, storage and pumped storage plants with their merits.

## Short answer

Run-of-river plants use natural river flow with little storage, low cost, base load only in wet season. Storage plants store water behind a dam for controlled power generation year‑round. Pumped storage pumps water to an upper reservoir with off‑peak surplus, then generates peak power, improving system load factor with about 70–75% round‑trip efficiency.

## Answer

Hydroelectric plants can be classified in several ways.

| Basis | Classes |
|---|---|
| Available head | Low head (below 15 m), medium head (15 to 70 m), high head (above 70 m) |
| Quantity of water | Run-of-river, storage (reservoir), pumped storage |
| Load served | Base load, peak load plants |
| Capacity | Micro (below 100 kW), mini, small, medium, large |
| Layout | Dam-toe, canal or diversion, tunnel type |

(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 on 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. It also gives flood control and irrigation benefits, but the cost is high and a large area of land is submerged.

#### 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.

```text
   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 %.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A run-of-river plant:
   A. Uses two reservoirs
   B. Has a large reservoir
   C. Uses natural river flow with little storage, so output varies with flow
   D. Always supplies peak load
   Answer: C. It diverts flow with a small weir; output varies with river flow and needs standby in the dry season. (Key point: Run-of-river)

2. A storage plant can supply base or peak load because:
   A. It pumps water
   B. Water is stored in the wet season and output controlled to demand
   C. It has higher head
   D. It has no dam
   Answer: B. Stored water lets output follow demand; it also gives flood control and irrigation benefits. (Key point: Storage plant)

3. In a pumped storage plant, pumping is done:
   A. Never
   B. At peak hours
   C. Only in the wet season
   D. During off-peak hours using surplus power
   Answer: D. Surplus off-peak power pumps water up; at peak hours it runs down through turbines. (Key point: Pumped storage)

4. Overall cycle efficiency of pumped storage is about:
   A. 95 to 99%
   B. 40 to 50%
   C. 20 to 30%
   D. 70 to 75%
   Answer: D. The answer gives about 70 to 75%. (Key point: Pumped storage efficiency)

## Related in Fluid Machines

- [Components of a hydroelectric power plant](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Ahydroelectric-plant-components) — Fluid Machines, chapter 2
- [Turbomachines: definition and classification](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aturbomachine-definition-classification) — Fluid Machines, chapter 1

## Related in other subjects

- [Pumped storage plant: power, turbine, setting, surge](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apumped-storage-turbine-surge-numerical) — Hydropower Engineering, chapter 7
- [Importance of storage plants; pumped storage layout](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astorage-importance-pump-storage-layout) — Hydropower Engineering, chapter 2
- [When pumped storage is economically feasible](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Apumped-storage-economic-feasibility) — Power Plant Equipment, chapter 1
- [Francis turbine in pumped storage plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Afrancis-turbine-pumped-storage) — Power Plant Equipment, chapter 1