---
title: "Hydropower site power and turbine"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:hydro-site-power-numerical"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/hydro-site-power-numerical
site: BCT Engineering
---

# Hydropower site power and turbine

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: 2072 Kartik, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/hydro-site-power-numerical
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

A potential site has the net head of 100 m with 200 lit/sec of flow. What will be the power delivered from such site if the constructed power house overall efficiency is 50%? Which types of turbines would be suitable for such plants / site and also write its features.

## Short answer

Use P = ηρgQH with η=0.5, Q=0.2 m³/s, H=100 m. Hydraulic power is 196.2 kW, so delivered power is 98.1 kW (about 0.1 MW). Pelton or cross-flow turbines suit this medium-to-high head, low-flow site; Pelton is impulse type with nozzle and buckets, efficient at high head.

## Answer

#### Given
Net head H = 100 m, flow Q = 200 L/s = 0.2 m³/s, overall efficiency η = 50% = 0.5, ρ = 1000 kg/m³, g = 9.81 m/s².

#### Formula
Power delivered P = η ρ g Q H

#### Calculation
```text
Hydraulic power (input) = 1000 x 9.81 x 0.2 x 100
                        = 196,200 W = 196.2 kW
Power delivered = 0.5 x 196.2
                = 98.1 kW
```
**Answer: P ≈ 98 kW (about 0.1 MW)** of electrical power is delivered.

#### Suitable turbines
Head 100 m with a small flow (0.2 m³/s) is a medium-to-high head, low-flow site. Suitable options:
1. **Pelton turbine** (impulse), commonly used for micro-hydro in the hills of Nepal.
2. **Cross-flow (Banki) turbine** (impulse): cheap, locally made, good for 5–200 m heads.
3. A **Francis** turbine is possible (head 60–250 m), but at this small size Pelton or cross-flow is more practical and cheaper.

#### Features of a Pelton turbine
- Impulse type: high-velocity jets from nozzles strike double-hemispherical buckets on the runner periphery.
- Works at atmospheric pressure; a casing only prevents splash.
- Suited for heads above about 50 m (up to 1800 m) and small flow; low specific speed.
- Flow is regulated by a spear (needle) valve; a deflector protects against overspeed.
- Efficiency 85–90% (at full load), flat efficiency curve at part load; no draft tube; easy to maintain.
- Not suitable for low heads or large flows, and is vulnerable to sand erosion.

#### Features of a cross-flow turbine
- Drum-shaped runner with blades; water crosses the runner twice.
- Simple, low cost, easily made in local workshops; efficiency about 65–80% but flat over a wide flow range.

## Check yourself

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

1. Which formula gives power delivered from the site?
   A. P = ρ g H / Q
   B. P = η ρ g Q H
   C. P = ½ ρ Q H
   D. P = η ρ Q / H
   Answer: B. Power = efficiency x density x g x flow x head. (Key point: Power formula)

2. For H = 100 m, Q = 0.2 m³/s and η = 0.5, what is the delivered power?
   A. 196.2 kW
   B. 9.81 kW
   C. 98.1 kW
   D. 19.6 kW
   Answer: C. Gross power = 1000 x 9.81 x 0.2 x 100 = 196.2 kW; times 0.5 gives 98.1 kW. (Key point: Result of calculation)

3. Why are Pelton or cross-flow turbines suitable for this site?
   A. It is a low-head, high-flow site
   B. It is a medium-to-high head, low-flow site, and these are cheap and practical at small size
   C. It has no head
   D. Kaplan turbines are required
   Answer: B. 100 m head with 0.2 m³/s flow suits impulse turbines, which are practical and cheaper at micro-hydro scale. (Key point: Turbine selection)

4. Which statement about a Pelton turbine is correct?
   A. It runs at atmospheric pressure with jets striking double-hemispherical buckets, regulated by a spear valve
   B. It is a reaction turbine with a draft tube
   C. It is best for very low heads and large flows
   D. Its efficiency falls sharply at part load
   Answer: A. Pelton is an impulse turbine with nozzle jets, spear valve control, no draft tube and a flat part-load efficiency curve. (Key point: Pelton features)

## Related in Energy Environment and Society

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- [Francis overall efficiency with draft tube](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-draft-tube-efficiency-32mw) — Hydropower Engineering, chapter 7