--- title: "Combined plant layouts, performance and economics" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-layout-economics" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-layout-economics site: BCT Engineering --- # Combined plant layouts, performance and economics Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2078 Bhadra, 2+4+2 marks - 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-layout-economics - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question What is combined Power plant? Sketch the layout with necessary labels, for various types of combined power plant. Discuss the performance and economics of combined power plants. ## Short answer A combined plant runs a topping and bottoming cycle in series, typically gas turbine (Brayton) exhausting at 450-600 C into a steam Rankine cycle, giving 50-60% efficiency. Layouts are HRSG, exhaust as combustion air in a fired boiler, and exhaust feed water heating. Economics: low capital cost per kW, short construction, quick start, good load following, less cooling water and space. ## Answer #### What is a combined power plant A **combined power plant** (combined cycle plant) uses two thermodynamic cycles in series, so that the heat rejected by the first (**topping**) cycle becomes the heat input of the second (**bottoming**) cycle. The most common is the **gas turbine - steam turbine combined cycle**: a gas turbine (Brayton cycle, heat added at 1100-1500 C) exhausts at 450-600 C, and this exhaust raises steam for a steam turbine (Rankine cycle, heat rejected at 30-45 C). Overall efficiency of 50-60% is achieved, compared with 30-40% for each plant alone. #### Layouts of various types #### Design 1: Unfired (or supplementary fired) heat recovery boiler (HRSG) ```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 |<---+ +-----------+ ``` The GT exhaust raises all the steam in a heat recovery boiler. A duct burner may add supplementary fuel. This is the most common modern design (efficiency 50-60%). #### Design 2: Exhaust used as combustion air in a fired boiler ```text air fuel |1 | v v [C]--2-->[CC]--3-->[GT]==> G1 | 4 hot exhaust (~15% O2) v used as combustion air fuel --> +-----------------+ a +----+ | Fired steam |---->| ST |==> G2 stack <----| boiler | +----+ +-----------------+ | b ^ d v | +------+ c +-----------+ +--| Pump |<----| Condenser | +------+ +-----------+ ``` The hot, oxygen-rich exhaust replaces the air preheater and forced draft air of a conventional fired boiler. Steam turbine output is large compared to gas turbine output. #### Design 3: Exhaust heats the feed water ```text air fuel | 1 | v v [C]--2-->[ CC ]--3-->[GT]==> G1 || | 4 exhaust gas ++=====shaft=========+ v +--------------------------+ stack <-5| Feed water heater (gas | | to water heat exchanger) | +--------------------------+ ^ d | e hot feed water | v +------+ +----------+ a +----+ | Pump | | Steam |---->| ST |==> G2 +------+ | boiler | +----+ ^ c +----------+ | b | ^ fuel v | +-----------------+ +-----------| Condenser | +-----------------+ ``` The exhaust heats the condensate/feed water of a conventional steam plant, replacing bled-steam feed heaters; the boiler supplies the rest of the heat. #### Performance and economics Performance of a combined plant is judged by its **overall thermal efficiency** (or heat rate): $$ \eta_{cc} = \frac{W_{GT} + W_{ST}}{Q_{GT} + Q_{sf}} $$ where $W_{GT}$, $W_{ST}$ are net outputs of gas and steam units, $Q_{GT}$ is fuel heat to the gas turbine and $Q_{sf}$ is any supplementary fuel heat. For an unfired HRSG with no losses, all heat rejected by the gas turbine, $Q_{GT}(1-\eta_{GT})$, goes to the steam cycle, so $$ \eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\,\eta_{ST} $$ Example: $\eta_{GT} = 0.35$, $\eta_{ST} = 0.32$ gives $\eta_{cc} = 0.35 + 0.32 - 0.112 = 0.558$ (55.8%). If the HRSG (heat recovery) effectiveness is $\eta_{HR}$ (part of exhaust heat lost to stack), $\eta_{cc} = \eta_{GT} + \eta_{HR}\,\eta_{ST}(1 - \eta_{GT})$. Other performance figures: **heat rate** = 3600 / $\eta_{cc}$ kJ/kWh (about 6000-7000 kJ/kWh), **power ratio** $W_{ST}/W_{GT}$ (about 0.5), part-load efficiency and stack temperature. Economics: - **Capital cost** per kW is lower than for coal or nuclear plants (the gas turbine is factory made, the HRSG is simple, and there is no coal or ash handling); it is often about half that of a coal plant, or less. - **Construction time** 2-3 years; phased construction earns revenue early. - **Fuel cost** is the largest cost item; high efficiency reduces it, but gas/oil price decides the cost of energy. - **Operating and maintenance cost** is moderate; fewer staff, but GT hot parts need regular overhaul. - **Cost of electricity** is lowest for base and intermediate load where cheap natural gas is available; supplementary firing gives cheap peaking capacity. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is overall thermal efficiency of a combined plant defined? A. (W_GT + W_ST) / (Q_GT + Q_sf) B. W_ST / W_GT C. Q_GT / W_GT D. W_GT / W_ST Answer: A. Total net output divided by the total fuel heat, including any supplementary firing. (Key point: Performance measure) 2. How is heat rate related to efficiency? A. Heat rate = 3600 / eta_cc kJ/kWh B. Heat rate = eta_cc x 3600 C. Heat rate = 1 / 3600 eta D. Heat rate = eta_cc Answer: A. One kWh is 3600 kJ, so fuel energy per kWh is 3600/eta. (Key point: Heat rate) 3. How does the capital cost per kW of a combined plant compare with a coal plant? A. About 60-70 % of coal plant, since no coal or ash handling is needed B. Double the coal plant C. Same as nuclear D. Ten times greater Answer: A. Gas turbines are factory made, HRSG is simple and there is no coal/ash handling. (Key point: Capital cost) 4. What is the largest cost item of a combined plant and what decides cost of energy? A. Fuel; gas/oil price B. Labour; wage rate C. Cooling water; river flow D. Land; plot price Answer: A. Fuel is the largest cost; high efficiency reduces it, but fuel price determines electricity cost. (Key point: Largest cost item) ## Related in Power Plant Equipment - [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 - [Advantages of combined power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages) — Power Plant Equipment, chapter 8 - [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 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× - [Layout of diesel power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-plant-layout) — Power Plant Equipment, chapter 5, asked 3× ## Related in other subjects - [RoR plant layouts; storage over RoR](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-layouts-storage-importance) — Hydropower Engineering, chapter 2 - [Importance of storage plants; pumped storage layout](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astorage-importance-pump-storage-layout) — Hydropower Engineering, chapter 2 - [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 - [Development cycle; storage plant layout](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adev-cycle-storage-layout) — Hydropower Engineering, chapter 2