--- title: "Working and performance of combined plant" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-working-performance" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-working-performance site: BCT Engineering --- # Working and performance of combined plant Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2081 Baisakh, 6 marks - Video: [Combined Steam And Gas Turbine Power Plant](https://www.youtube.com/watch?v=uD0kH-0HME8) — 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-working-performance - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question Explain the working of combined power plant. How can we determine the performance of a combined power plant? ## Short answer Combined plant sends gas turbine exhaust to an HRSG that raises steam for a steam turbine. Performance is determined by overall thermal efficiency η_cc ≈ η_GT + η_ST − η_GT·η_ST (unfired) or η_cc = (W_GT+W_ST)/(Q_GT+Q_sf). ## Answer #### Working of 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. ```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 |<---+ +-----------+ ``` Working: 1. Air is compressed in the compressor (1-2), burnt with fuel in the combustion chamber (2-3) and expanded in the gas turbine (3-4), which drives the compressor and generator G1. 2. Exhaust gas at about 500-600 C enters the **HRSG**. Here it heats the feed water in the economiser, evaporates it in the evaporator and superheats it in the superheater (d-a). The cooled gas goes to the stack at about 100-150 C (4-5). 3. Steam expands in the **steam turbine** (a-b) and drives generator G2. 4. Exhaust steam is condensed (b-c) and pumped back to the HRSG (c-d). 5. Optionally, a duct burner before the HRSG burns extra fuel (supplementary firing) to raise steam output. Heat added at high temperature in the gas cycle and rejected at low temperature in the steam condenser gives a high overall efficiency. #### Determining performance 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What defines a combined power plant? A. Heat rejected by the topping cycle becomes the heat input of the bottoming cycle B. Two identical steam cycles in parallel C. One cycle with two boilers D. Two gas turbines sharing a fuel tank Answer: A. Two cycles in series, the first's rejected heat feeding the second. (Key point: Topping and bottoming) 2. What sequence of processes does the exhaust gas perform in the HRSG? A. Heats feed water in the economiser, evaporates in the evaporator, superheats in the superheater B. Compresses air, burns fuel, expands C. Condenses steam and pumps it D. Only superheats steam Answer: A. The HRSG contains economiser, evaporator and superheater in that order along the water-steam path. (Key point: HRSG sequence) 3. For an unfired HRSG with no losses, which formula gives overall efficiency? A. eta_GT + eta_ST - eta_GT x eta_ST B. eta_GT x eta_ST C. eta_GT + eta_ST D. eta_GT - eta_ST Answer: A. The steam cycle receives Q(1 - eta_GT), so total = eta_GT + (1 - eta_GT) eta_ST. (Key point: Efficiency formula) 4. With eta_GT = 0.35 and eta_ST = 0.32, the combined efficiency is A. 0.670 B. 0.558 C. 0.112 D. 0.452 Answer: B. 0.35 + 0.32 - 0.112 = 0.558. 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