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

# Combined cycle advantages and working principle

Power Plant Equipment · Chapter 8: Combined Power Plant

- Asked in: 2073 Shrawan, 6 marks; 2080 Bhadra, 2+4 marks (asked 2 times)
- Video: [Combined Cycle Plants -  Power Plant Engineering](https://www.youtube.com/watch?v=6xNwZKOtg_o) — Lead GATE -  ME,XE,PI
- 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-advantages-working
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8

## Question

Enumerate the advantages of a combined cycle plant. With the help of a neat diagram, explain the principle of working of a combined cycle plant to enhance the efficiency of electricity generation.

## Short answer

A combined cycle plant uses a gas turbine’s hot exhaust (500–600°C) in an HRSG to raise steam for a steam turbine. This yields 50–60% efficiency, quick start-up, lower fuel cost, less cooling water, and reduced emissions.

## Answer

A **combined cycle plant** uses the hot exhaust of a gas turbine to generate steam for a steam turbine, so the same fuel produces power in two cycles.

#### Advantages

- **High overall efficiency**, 50-60%, against about 35-40% for steam and 30-35% for simple gas turbine plants.
- **Lower fuel cost and lower CO2 per kWh.**
- **Short construction time**; gas turbine part can run first (phased construction).
- **Low capital cost per kW** and less space than a coal plant.
- **Quick start-up**; good for intermediate and peak loads.
- **Less cooling water** (only the steam part, about one-third of output, needs a condenser).
- Lower emissions (natural gas fuel).

#### Working principle

```text
      air 1                fuel
      |                     |
      v                     v
   +------+     2     +-----------+   3   +------+
   | Comp |---------->| Combustion|------>|  GT  |===> G1
   +------+           |  chamber  |       +------+
      ||              +-----------+        ||  |
      ++=============== shaft =============++  | 4 hot exhaust
                                               v   (~500-600 C)
   stack <-- 5 --+-------------------------------+
                 |  HRSG (economiser,            |
                 |  evaporator, superheater)     |
                 +-------------------------------+
                     ^ d                  | a steam
                     |                    v
                 +------+             +------+
                 | Pump |             |  ST  |===> G2
                 +------+             +------+
                     ^ c                  | b
                     |   +-----------+    |
                     +---| Condenser |<---+
                         +-----------+
```

1. Air is compressed in the compressor (1-2) and fuel is burnt in the combustion chamber (2-3).
2. Hot gas expands in the gas turbine (3-4) and drives generator G1.
3. The exhaust gas (about 500-600 C) passes through the **HRSG** where it heats feed water, evaporates it and superheats the steam; gas leaves through the stack at about 100-150 C (4-5).
4. Steam expands in the steam turbine (a-b) and drives generator G2; it is condensed (b-c) and pumped back to the HRSG (c-d).

The gas cycle adds heat at a very high temperature (about 1200-1400 C) and the steam cycle rejects heat at a low temperature (about 35-45 C). Using the gas cycle's waste heat as the steam cycle's input gives

$$
\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST}
$$

For example, $\eta_{GT} = 0.35$ and $\eta_{ST} = 0.30$ give $\eta_{cc} = 0.35 + 0.30 - 0.105 = 0.545$, i.e. about 54.5%.

## Check yourself

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

1. Roughly how does combined cycle efficiency compare with other plants?
   A. 50-60 % versus about 35-40 % for steam and 30-35 % for simple gas plants
   B. 30-35 % versus 50-60 % for steam plants
   C. The same as a simple gas turbine plant
   D. About 80 % versus 40 % for steam
   Answer: A. Using the gas turbine exhaust to run a steam cycle lifts overall efficiency to 50-60 %. (Key point: Efficiency advantage)

2. Why does a combined cycle plant need less cooling water than a steam plant of equal output?
   A. It has no condenser at all
   B. Only the steam part, about one-third of output, needs a condenser
   C. The gas turbine uses the cooling water instead
   D. Steam is recycled without condensing
   Answer: B. Only the steam turbine portion rejects heat to a condenser, which is about one-third of plant output. (Key point: Cooling water)

3. Why is the combined cycle efficiency high?
   A. Heat is added at a very high temperature in the gas cycle and rejected at a low temperature in the steam condenser
   B. Both cycles reject heat at the same high temperature
   C. Fuel is burnt twice at the same temperature
   D. The pump work is eliminated
   Answer: A. Gas cycle heat addition at about 1200-1400 C and steam cycle rejection at about 35-45 C give a wide effective temperature range. (Key point: Why high efficiency)

4. For an unfired HRSG with no losses, eta_GT = 0.35 and eta_ST = 0.30, what is the combined efficiency?
   A. 0.650
   B. 0.545
   C. 0.105
   D. 0.325
   Answer: B. eta_cc = 0.35 + 0.30 - 0.35 x 0.30 = 0.545. (Key point: Combined efficiency formula)

## Related in Power Plant Equipment

- [Working of combined gas-steam plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-gas-steam-working) — Power Plant Equipment, chapter 8
- [Working and performance of combined plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-working-performance) — Power Plant Equipment, chapter 8
- [Combined plant components and T-s diagram](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-components-ts) — Power Plant Equipment, chapter 8, asked 4×
- [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 plant heating feed water with exhaust](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-layout) — 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×

## Related in other subjects

- [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
- [Rankine cycle working principle](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Arankine-cycle-working-principle) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Brayton cycle working principle with P-v and T-s](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Abrayton-cycle-working-principle) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Ideal diesel cycle working principle](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Adiesel-cycle-working-principle) — Fundamental of Thermodynamics & Heat Transfer, chapter 6