--- title: "Operational characteristics of combined cycle plant" subject: "Power Plant Design" chapter: 1 key: "semester_8/power_plant_design:ccgt-operational-characteristics" source: https://www.bctengineer.com/q/semester_8/power_plant_design/ccgt-operational-characteristics site: BCT Engineering --- # Operational characteristics of combined cycle plant Power Plant Design · Chapter 1: Energy Sources and Electric Power Generation - Asked in: 2071 Bhadra, 4 marks - Video: [Gas Turbines and Combined Cycle Power Plants Explained (Combustion Turbines)](https://www.youtube.com/watch?v=N-V_uGeCFNg) — saVRee - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/ccgt-operational-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-1 ## Question Discuss about operational characteristic of combined cycle gas turbine plant for power generation. ## Short answer CCGT combines gas turbine Brayton cycle with steam Rankine cycle via HRSG. Efficiency 50-60%, quick start 10-30 min for gas turbine, good load following, part-load efficiency falls, low emissions, ambient sensitive, low capital and short construction. Operates on natural gas or light oil, suitable for intermediate/peak load. ## Answer A **combined cycle gas turbine (CCGT)** plant combines a gas turbine (Brayton cycle) with a steam turbine (Rankine cycle). The hot exhaust of the gas turbine (about 450–600 °C) produces steam in a **heat recovery steam generator (HRSG)**, which drives a steam turbine. ```text Air -> Compressor -> Combustor -> Gas turbine -> G1 ^ fuel | exhaust | Steam turbine <-- HRSG --> stack | | G2 Condenser ``` #### Operational characteristics 1. **High efficiency** – about 50–60% (gas turbine alone 30–40%), the highest of all thermal plants. 2. **Quick start** – gas turbine starts in 10–30 minutes; full combined cycle in about 1–3 hours. Suitable for intermediate and peak load. 3. **Good load following** – can ramp fast; supports variable renewables. 4. **Part-load behaviour** – efficiency falls at part load; usually operated above about 40–50% load; several smaller units improve flexibility. 5. **Fuel** – natural gas (cleanest) or light oil; fuel cost is the main running cost. 6. **Ambient sensitivity** – output and efficiency drop at high air temperature and high altitude (lower air density). 7. **Low emissions** – less CO₂ per kWh than coal; low SO₂ and particulates. 8. **Low capital cost and short construction time** (2–3 years); compact; little cooling water (only steam part). 9. **Phased operation** – gas turbine can run in simple cycle while the steam part is built or maintained. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a CCGT plant produce its steam? A. Burning coal in a boiler B. Gas turbine exhaust heat in an HRSG C. Nuclear reaction D. Solar collectors Answer: B. The hot gas turbine exhaust (450-600 C) raises steam in the heat recovery steam generator. (Key point: Combined cycle principle) 2. CCGT efficiency is about A. 50-60% B. 30-40% C. 85-92% D. 10-20% Answer: A. The combined cycle reaches 50-60%, higher than the gas turbine alone (30-40%). (Key point: Efficiency) 3. How does high ambient air temperature or altitude affect CCGT? A. Output and efficiency rise B. No effect C. Output and efficiency drop because of lower air density D. Plant stops permanently Answer: C. Less dense air reduces mass flow through the gas turbine. (Key point: Ambient sensitivity) 4. Which is true of operation? A. Takes days to start B. Efficiency falls at part load, but it ramps fast and suits intermediate load C. Efficiency rises at part load D. Cannot follow load Answer: B. Quick start and good load following, but part-load efficiency drops. 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