--- title: "Methods to improve gas turbine efficiency" subject: "Power Plant Equipment" chapter: 6 key: "semester_7/power_plant_equipment:gt-efficiency-methods" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-efficiency-methods site: BCT Engineering --- # Methods to improve gas turbine efficiency Power Plant Equipment · Chapter 6: Gas Turbine Power Plant - Asked in: 2078 Bhadra, 6 marks; 2074 Chaitra, 10 marks (asked 2 times) - Video: [VIDEO 3: METHODS TO IMPROVE THE THERMAL EFFICIENCY OF OPEN CYCLE GAS TURBINE](https://www.youtube.com/watch?v=KoIvNrLMXHY) — SNIT MECH S4 - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-efficiency-methods - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-6 ## Question Explain the methods of efficiency improvement in gas turbine power plant. ## Short answer Efficiency is improved by regeneration, intercooling, reheating, higher turbine inlet temperature, better component efficiencies, and combined cycles. Regeneration reduces fuel use, intercooling reduces compressor work, reheating increases turbine work, and combined cycles recover exhaust heat. Optimum pressure ratio and inlet air cooling also help. ## Answer A simple open-cycle gas turbine has a low thermal efficiency (about 20–30 %) because (i) the compressor consumes a large part (60–70 %) of turbine work and (ii) the exhaust leaves at 400–550 °C, carrying away much heat. The following methods improve efficiency and/or output. #### 1. Regeneration A heat exchanger (regenerator) transfers heat from the hot turbine exhaust to the compressed air before it enters the combustion chamber. Fuel required for the same turbine inlet temperature falls, so **efficiency rises** while net work is unchanged. Effective only when exhaust temperature is higher than compressor delivery temperature (low–moderate pressure ratios). Effectiveness $\varepsilon = \dfrac{T_x - T_2}{T_4 - T_2}$, usually 0.6–0.8. #### 2. Intercooling Compression in two or more stages with cooling between stages reduces compressor work ($W \propto T_{inlet}$). **Net work and work ratio increase.** Alone it slightly increases fuel needed, but with regeneration it gives a definite gain in efficiency. #### 3. Reheating Expansion in two turbines with reheating of gas between them (in a second combustor) increases turbine work, so **net output rises**; efficiency rises only when combined with regeneration because exhaust is hotter. #### 4. Combination (intercooling + reheating + regeneration) Approaches the Ericsson cycle; gives the highest efficiency and work ratio, but plant is complex and costly. #### 5. Higher turbine inlet temperature Efficiency and specific work increase with $T_3$. Achieved with nickel-based superalloys, ceramic coatings and **blade cooling** (air or steam). #### 6. Optimum pressure ratio and better components For given $T_1, T_3$ there is a pressure ratio giving maximum work and another giving maximum efficiency. Increasing isentropic efficiencies of compressor and turbine (better blade design) and reducing pressure losses in ducts, filters and combustor all improve performance. #### 7. Waste-heat recovery / combined cycle Exhaust heat raises steam in a heat recovery steam generator (HRSG) for a steam turbine (combined cycle, overall 50–60 %), or is used for process heating (cogeneration). #### 8. Water/steam injection and inlet air cooling Injecting water or steam increases mass flow and output; cooling the inlet air (evaporative or chiller) increases air density and reduces compressor work, especially in hot weather. | Method | Main effect | |---|---| | Regeneration | Less fuel, higher efficiency | | Intercooling | Less compressor work, more net work | | Reheating | More turbine work, more net work | | Higher $T_3$ | Higher efficiency and output | | Combined cycle | Uses exhaust heat, highest efficiency | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is simple open-cycle GT efficiency low? A. Fuel is poor B. The turbine is too large C. Pressure ratio is zero D. Compressor takes 60-70% of turbine work and exhaust is hot Answer: D. These two losses dominate. (Key point: Why low efficiency) 2. When is regeneration effective? A. When turbine exhaust is hotter than compressor delivery (low-moderate pressure ratio) B. At very high pressure ratios C. Whenever the air is intercooled only D. Never with a turbine Answer: A. If T4 < T2 the regenerator would cool the air. (Key point: Regenerator condition) 3. What does reheating do? A. Lowers compressor work B. Raises turbine work and net output C. Raises compressor outlet temperature D. Cuts fuel use alone Answer: B. Efficiency gains need regeneration too. (Key point: Reheating) 4. Combined cycle improves efficiency by A. Cooling the inlet air B. Adding more compressor stages C. Using exhaust heat to raise steam for a steam turbine D. Lowering turbine inlet temperature Answer: C. HRSG recovery gives overall 50-60%. 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