--- title: "Gas turbine improvement methods and regeneration" subject: "Power Plant Equipment" chapter: 6 key: "semester_7/power_plant_equipment:gt-methods-regeneration" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-methods-regeneration site: BCT Engineering --- # Gas turbine improvement methods and regeneration Power Plant Equipment · Chapter 6: Gas Turbine Power Plant - Asked in: 2071 Shrawan, 8 marks - Video: [Regeneration, Intercooling, and Reheating in 13 Minutes!](https://www.youtube.com/watch?v=EBQJv8pry2k) — Less Boring Lectures - 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-methods-regeneration - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-6 ## Question List the common methods used for the performance improvement of the gas turbine power plants. Explain how regeneration increases efficiency of the plant. ## Short answer Common methods: regeneration, intercooling, reheating, higher turbine inlet temperature, combined cycles. Regeneration preheats compressed air with exhaust, reducing fuel input for the same net work, thus raising thermal efficiency; works best when turbine exhaust is hotter than compressor outlet. ## Answer #### Common methods to improve gas turbine performance 1. **Regeneration** – exhaust heat preheats compressed air. 2. **Intercooling** – multi-stage compression with cooling between stages. 3. **Reheating** – multi-stage expansion with reheating between turbines. 4. Combination of regeneration, intercooling and reheating. 5. **Higher turbine inlet temperature** (better materials, blade cooling). 6. Optimum pressure ratio and higher compressor/turbine efficiencies; low pressure losses. 7. **Combined cycle / cogeneration** using exhaust heat. 8. Water/steam injection and inlet air cooling. #### How regeneration increases efficiency In a simple open cycle the exhaust leaves the turbine at 400–550 °C, often hotter than the air leaving the compressor. A **regenerator** (counter-flow heat exchanger) passes this hot exhaust over tubes carrying the compressed air. ```text +------- REGENERATOR -------+ | air 2 --> x y <-- gas 4 | +--^--------|---|--------^--+ | v v | Air 1 -->[ C ] [ CC ]-3->[ T ]-+ || ^ fuel || ++=====shaft=====++==> Gen exhaust from y ``` 1. Air leaves the compressor at $T_2$ and is heated in the regenerator to $T_x$ by the exhaust gas. 2. The combustion chamber only has to raise the air from $T_x$ to $T_3$, so heat supplied falls from $c_p(T_3 - T_2)$ to $c_p(T_3 - T_x)$. 3. Compressor and turbine works are not changed, so **net work stays the same while fuel input decreases**: $$ \eta = \frac{W_t - W_c}{c_p(T_3 - T_x)} > \frac{W_t - W_c}{c_p(T_3 - T_2)} $$ 4. The exhaust leaves at the lower temperature $T_y$, so less heat is wasted. 5. **Effectiveness** $\varepsilon = \dfrac{T_x - T_2}{T_4 - T_2}$, practically 0.6–0.8. With an ideal regenerator ($\varepsilon = 1$), $\eta = 1 - \dfrac{T_1}{T_3}r_p^{(\gamma-1)/\gamma}$, which increases as pressure ratio decreases. **Limitation:** regeneration works only when $T_4 > T_2$ (low/moderate pressure ratios). It adds cost, weight and pressure losses, so it is mainly used in stationary base-load plants. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is NOT a common method to improve gas turbine performance? A. Intercooling B. Reheating C. Regeneration D. Lowering turbine inlet temperature Answer: D. Higher, not lower, turbine inlet temperature improves performance, along with intercooling, reheat and regeneration. (Key point: Methods of improvement) 2. What does the regenerator do? A. Cools air before compression B. Uses hot exhaust to heat compressed air before the combustion chamber C. Adds fuel to the exhaust D. Raises pressure ratio Answer: B. Exhaust passes over tubes carrying compressed air, raising air temperature from T2 to Tx. (Key point: How regeneration works) 3. Why does efficiency increase with regeneration? A. Net work increases while fuel stays the same B. Net work stays the same while heat supplied falls C. Compressor work is halved D. Turbine work rises Answer: B. Compressor and turbine work are unchanged, but the combustor needs cp(T3 - Tx) instead of cp(T3 - T2). (Key point: Why efficiency rises) 4. Regeneration only helps when: A. T4 > T2, i.e. low or moderate pressure ratios B. T2 > T4, i.e. very high pressure ratios C. T3 equals T1 D. Pressure ratio is infinite Answer: A. Heat can only flow from exhaust to compressed air when the exhaust is hotter than compressor exit air. 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