---
title: "Hydro and thermal power sources"
subject: "Power System Analysis I"
chapter: 1
key: "semester_4/power_system_analysis_i:hydro-thermal-sources"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/hydro-thermal-sources
site: BCT Engineering
---

# Hydro and thermal power sources

Power System Analysis I · Chapter 1: General Background

- Asked in: 2069 Bhadra, 6 marks
- Video: [Power Plant Explained | Working Principles](https://www.youtube.com/watch?v=HGVDu1z5YQ8) — RealPars
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/hydro-thermal-sources
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-1

## Question

Describe in brief hydro and thermal power sources with basic principles of electricity generation and major features.

## Short answer

Hydropower uses water head and flow to drive a turbine (P=ηρgQH), with free fuel, high efficiency (85-90%), low running cost, but high capital. Thermal power burns fuel to produce steam for a turbine (Rankine cycle), with lower efficiency (30-40%), high fuel cost, pollution, and slow start.

## Answer

#### Hydropower
**Principle:** Water stored at a height has potential energy. It flows through a penstock to a turbine, which turns an alternator. Power:
$$
P = \eta\rho gQH\ \text{W}
$$
where $Q$ = flow (m³/s), $H$ = net head (m), $\eta$ = overall efficiency.
```text
 Dam/Intake -> Headrace -> Surge tank -> Penstock
     -> Turbine --[Generator]--> Tx -> Grid
     -> Tailrace -> River
```
**Major features**
- Fuel is free; very low running cost; renewable and non-polluting.
- High efficiency (85–90 %) and long life (50+ years).
- Quick start and load change: useful for peak load and frequency control.
- High initial cost and long construction time; site depends on geography; often far from load centres.
- Output varies with season (run-of-river plants drop in dry season).
- Examples: Kali Gandaki A (144 MW), Upper Tamakoshi (456 MW).

#### Thermal (steam) power
**Principle:** Fuel (coal, oil, gas) is burned in a boiler; the heat turns water into high-pressure steam, which expands in a steam turbine coupled to an alternator, then is condensed and pumped back (Rankine cycle). Chemical energy → heat → mechanical → electrical energy.
```text
 Fuel -> [Boiler] -> steam -> [Turbine]--[Gen]-> Grid
            ^                    |
       [Feed pump] <- [Condenser] <- cooling water
```
**Major features**
- Lower initial cost and shorter construction; can be built near loads or fuel source.
- High running cost due to fuel; overall efficiency only 30–40 %.
- Pollution: CO₂, SO₂, NOx, ash; needs chimneys, precipitators and ash disposal.
- Needs large amounts of cooling water.
- Slow start; best for base load.

| Feature | Hydro | Thermal |
|---|---|---|
| Fuel cost | Nil | High |
| Efficiency | 85–90 % | 30–40 % |
| Capital cost | High | Lower |
| Pollution | Very low | High |
| Starting | Quick | Slow |
| Load type | Base and peak | Base |

## Check yourself

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

1. Compared with thermal, hydro has
   A. High fuel cost
   B. Low capital cost
   C. Free fuel and low running cost but high capital cost
   D. Slow start
   Answer: C. Water is free; civil works are costly. (Key point: Hydro running cost)

2. Thermal efficiency is about
   A. 30-40 %
   B. 85-90 %
   C. 60-70 %
   D. 100 %
   Answer: A. Heat lost in the condenser. (Key point: Thermal efficiency)

3. Which starts quickly?
   A. Thermal
   B. Both slow
   C. Neither
   D. Hydro
   Answer: D. Hydro suits peak load and frequency control. (Key point: Starting)

4. The thermal power plant works on the
   A. Otto cycle
   B. Diesel cycle
   C. Rankine cycle
   D. Carnot cycle
   Answer: C. Steam is generated, expanded, condensed and pumped back. (Key point: Thermal cycle)

## Related in Power System Analysis I

- [Major electricity sources and sustainability](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aelectricity-sources-sustainability) — Power System Analysis I, chapter 1
- [Conventional sources and features of a station](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aconventional-sources-features) — Power System Analysis I, chapter 1
- [Major components of a power station and functions](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Apower-station-components-functions) — Power System Analysis I, chapter 1
- [Major components of a power station](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Apower-station-major-components) — Power System Analysis I, chapter 1, asked 2×
- [Basic structure of power system](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Abasic-structure-power-system) — Power System Analysis I, chapter 1
- [Two sources via 0 - j5 Ω: power flow](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Atwo-sources-0-j5) — Power System Analysis I, chapter 3

## Related in other subjects

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- [Quarterly progress report: hydro-electricity supply centre (memo)](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Aquarterly-progress-report-hydro-supply-memo) — Communication English, chapter 7
- [Energy sources and solar power generation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aenergy-sources-solar-power) — Power Plant Design, chapter 1
- [Thermal, hydro and solar PV plants compared](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Athermal-hydro-solar-pv) — Power Plant Design, chapter 1, asked 8×