---
title: "Combined cycle combinations and advantages"
subject: "Power Plant Equipment"
chapter: 8
key: "semester_7/power_plant_equipment:combined-combinations-advantages"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-combinations-advantages
site: BCT Engineering
---

# Combined cycle combinations and advantages

Power Plant Equipment · Chapter 8: Combined Power Plant

- Asked in: 2079 Baisakh, 2+2 marks
- Video: [Combined Steam and Gas Turbine power plant](https://www.youtube.com/watch?v=MCyaVx0829U) — Unique Series - Civil Mantraa
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-combinations-advantages
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8

## Question

What are the possible combinations of combined cycle power plants? Point out the advantages of combined cycles.

## Short answer

Possible combinations include gas turbine-steam turbine, diesel/gas engine-steam, MHD-steam, thermionic/thermoelectric-steam, nuclear gas-cooled reactor-steam, binary vapour (mercury-steam), and steam-organic Rankine. Advantages are 50-60% efficiency, lower fuel cost and CO2, low capital cost, quick start, less cooling water and space, and lower emissions.

## Answer

#### Possible combinations

A combined cycle couples a high-temperature topping cycle with a low-temperature bottoming cycle. Possible combinations:

1. **Gas turbine - steam turbine** (Brayton-Rankine), the most common.
2. **Diesel/gas engine - steam turbine** (engine exhaust raises steam).
3. **MHD (magnetohydrodynamic) generator - steam plant.**
4. **Thermionic - steam** and **thermoelectric - steam** plants.
5. **Nuclear (gas-cooled reactor) - steam / gas turbine plant.**
6. **Binary vapour cycle**, e.g. mercury - steam.
7. **Steam - organic Rankine cycle** (low-temperature bottoming).

#### Advantages of combined cycles

- High overall efficiency (50-60%), so lower fuel cost and lower CO2 per kWh.
- Low capital cost per kW and short construction time.
- Quick start and good load-following.
- Less cooling water and less space than a steam plant of the same rating.
- Lower emissions (clean fuels) and flexibility for cogeneration.

## Check yourself

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

1. Which is the most common combination of combined cycle?
   A. Gas turbine - steam turbine (Brayton-Rankine)
   B. Mercury - steam
   C. MHD - steam
   D. Thermionic - steam
   Answer: A. Brayton-Rankine is the most widely used. (Key point: Possible combinations)

2. Which pair is an example of a binary vapour cycle?
   A. Mercury - steam
   B. Gas turbine - steam
   C. Diesel - steam
   D. Steam - air
   Answer: A. Binary vapour cycles use two working vapours, e.g. mercury and steam. (Key point: Binary vapour)

3. Which is an advantage of combined cycles?
   A. Quick start and short construction time
   B. Very long construction time
   C. Higher CO2 per kWh
   D. Requires more cooling water than steam plants
   Answer: A. Combined cycles have quick start, short construction, lower CO2 and less cooling water. (Key point: Advantages)

## Related in Power Plant Equipment

- [Methods to utilize waste heat in combined plants](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-methods) — Power Plant Equipment, chapter 8, asked 2×
- [Combined cycle merits, demerits and designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-merits-designs) — Power Plant Equipment, chapter 8, asked 2×
- [Combined cycle advantages and three designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages-designs) — Power Plant Equipment, chapter 8, asked 2×
- [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×
- [Three popular combined cycle designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-three-designs) — Power Plant Equipment, chapter 8
- [Combined cycle components and waste heat use](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-utilization) — Power Plant Equipment, chapter 8

## Related in other subjects

- [Combined cycle gas turbine versus hydro](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Accgt-vs-hydro) — Power Plant Design, chapter 1
- [Gas vs vapor cycles; Otto cycle efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Agas-vs-vapor-cycles-otto-efficiency) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Operational characteristics of combined cycle plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Accgt-operational-characteristics) — Power Plant Design, chapter 1
- [Brayton cycle; Atkinson and Stirling cycles](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Abrayton-cycle-atkinson-stirling-notes) — Internal Combustion Engines, chapter 1