---
title: "Advantages of combined power plant"
subject: "Power Plant Equipment"
chapter: 8
key: "semester_7/power_plant_equipment:combined-advantages"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-advantages
site: BCT Engineering
---

# Advantages of combined power plant

Power Plant Equipment · Chapter 8: Combined Power Plant

- Asked in: 2073 Chaitra, 6 marks
- Video: [Advantages Of Combined Cycle Power Plant](https://www.youtube.com/watch?v=TIc08Hz1geQ) — BEST MECHANICAL ENGINEERING
- 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
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8

## Question

Briefly explain about the advantages of combined power plant.

## Short answer

Advantages are high overall efficiency (50-60%) versus 35-40% steam and 30-38% gas, lower fuel cost per kWh, lower capital cost, short construction, quick start, less cooling water, lower emissions, less space, and flexibility. Combined efficiency equals eta_GT plus eta_ST minus eta_GT times eta_ST.

## Answer

A combined power plant uses the waste heat of a gas turbine exhaust to raise steam for a steam turbine. Its main advantages:

- **High overall efficiency** (50-60%) against 35-40% for steam and 30-38% for gas turbine plants, so **lower fuel cost per kWh**.
- **Lower capital cost per kW** than coal or nuclear plants; the gas turbine part is compact and factory built.
- **Short construction time** (2-3 years); can be built in phases (gas turbine runs first in open cycle).
- **Quick start-up and good load-following**, so useful for intermediate and peak loads as well as base load.
- **Less cooling water** needed, since only the steam part (about one-third of output) has a condenser.
- **Lower emissions** of CO2, SO2 and particulates per kWh (usually natural gas fuel); no ash handling.
- **Less space** and smaller staff; high reliability and availability.
- **Flexible:** supplementary firing, cogeneration (process steam), repowering of old steam plants.

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

For example, $\eta_{GT} = 0.35$ and $\eta_{ST} = 0.30$ give $\eta_{cc} = 0.545$ (54.5%), far higher than either plant alone.

## Check yourself

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

1. Which is listed as a flexibility of the combined plant?
   A. It cannot be supplementary fired
   B. It cannot supply process steam
   C. Supplementary firing, cogeneration and repowering of old steam plants
   D. It works only at base load
   Answer: C. The plant can use supplementary firing, supply process steam and repower old steam plants. (Key point: Cogeneration and repowering flexibility)

2. The combined efficiency is given by
   A. eta_GT * eta_ST
   B. eta_GT + eta_ST - eta_GT*eta_ST
   C. eta_GT + eta_ST
   D. (eta_GT + eta_ST)/2
   Answer: B. The steam cycle uses the heat rejected by the gas cycle, so the product is subtracted: 0.35 + 0.30 - 0.105 = 0.545. (Key point: Combined efficiency formula)

3. Why is a combined plant useful for intermediate and peak load as well as base load?
   A. It cannot be shut down
   B. It has quick start-up and good load-following
   C. It stores energy in batteries
   D. It has no gas turbine
   Answer: B. Quick start-up and good load-following let it handle varying demand. (Key point: Load-following)

## Related in Power Plant Equipment

- [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×
- [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×
- [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 introduction and popular designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-intro-designs) — Power Plant Equipment, chapter 8
- [Combined plant layouts, performance and economics](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-layout-economics) — Power Plant Equipment, chapter 8
- [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

## 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
- [Types of power supply system for industrial plant](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aindustrial-supply-system-types) — Industrial Power Distribution and Illumination, chapter 1, asked 2×
- [Thermal versus nuclear power plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Athermal-vs-nuclear) — Power Plant Design, chapter 1, asked 2×
- [Simple Rankine plant 2 MPa / 20 kPa: efficiency, power](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Arankine-2mpa-20kpa-50kgs) — Fundamental of Thermodynamics & Heat Transfer, chapter 6