---
title: "How a regenerator improves gas turbine output"
subject: "Power Plant Equipment"
chapter: 6
key: "semester_7/power_plant_equipment:gt-regenerator-explain"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-regenerator-explain
site: BCT Engineering
---

# How a regenerator improves gas turbine output

Power Plant Equipment · Chapter 6: Gas Turbine Power Plant

- Asked in: 2079 Bhadra, 6+2 marks
- Video: [Regeneration in Gas turbine](https://www.youtube.com/watch?v=ToIg4g-0BNQ) — Gear Institute Mechanical Engineering Videos and Physics by Gear Institute
- 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-regenerator-explain
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-6

## Question

Explain how regenerator increases the output of the gas turbine power plant along with the neat sketch and show the cycle in p-v and T-s diagram.

## Short answer

A regenerator transfers heat from turbine exhaust to compressed air before combustion, so fuel heat input becomes cp(T3 - Tx) while net work is unchanged, increasing efficiency. Effectiveness is (Tx - T2)/(T4 - T2). It is useful when exhaust temperature exceeds compressor delivery temperature, typically at low to moderate pressure ratios.

## Answer

A **regenerator** is a counter-flow heat exchanger that uses the hot exhaust gas leaving the turbine to preheat the compressed air before it enters the combustion chamber. Heat that would be thrown away is returned to the cycle, so less fuel is burnt for the same turbine inlet temperature.

#### Sketch
```text
          +---------- REGENERATOR ----------+
          |  air 2 -> x       y <- gas 4    |
          +---|-------^-------|-------^-----+
              |       |       v       |
 Air 1 ->[ C ]+       |    exhaust    |
           ||   x---->[ CC ]--3-->[ T ]+
           ||           ^          ||
           ++====shaft==|==========++==> Generator
                      fuel
```
Processes: 1–2 compression, 2–x air heated in regenerator, x–3 combustion, 3–4 expansion, 4–y gas cooled in regenerator, y–1 exhaust to atmosphere.

#### p–v and T–s diagrams
```text
 p
 |  2    x    3
 |  *----*----*
 |   \          \
 |    \            \
 |     \              \
 |      *------*--------*
 |      1      y        4
 +--------------------------- v
```

```text
 T
 |             3
 |            *
 |           /|
 |      x   / |
 |      *--'  |
 |     /      * 4
 |  2 *    _-'
 |    |  *'
 |    | / y
 |    *
 |    1
 +-------------------- s
```
Heat from 4–y (gas side) supplies 2–x (air side). Ideal: $T_x = T_4$, $T_y = T_2$.

#### How the regenerator increases performance
- Heat supplied from fuel becomes $Q_s = c_p(T_3 - T_x)$ instead of $c_p(T_3 - T_2)$; since $T_x > T_2$, **fuel consumption falls**.
- Compressor and turbine work are unchanged, so **net work is the same** while heat input is less, therefore
$$
\eta = \frac{W_t - W_c}{c_p (T_3 - T_x)}
$$
increases (often by 5–10 percentage points).
- **Effectiveness** $\varepsilon = \dfrac{T_x - T_2}{T_4 - T_2}$ (practical 0.6–0.8). Higher effectiveness needs a larger, costlier heat exchanger and gives more pressure loss.
- Regeneration is useful only when $T_4 > T_2$, i.e. at low and moderate pressure ratios. At high pressure ratios the compressor delivery is hotter than the exhaust and regeneration would reduce efficiency.
- For a fixed fuel supply the turbine inlet temperature can be raised, which increases the **output**; also exhaust is cooler, reducing waste heat.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the regenerator do?
   A. Cools air before compression
   B. Heats fuel
   C. Cools turbine blades
   D. Uses hot exhaust to preheat compressed air before combustion
   Answer: D. It reduces fuel for the same turbine inlet temperature. (Key point: Regenerator purpose)

2. With regeneration heat supplied is
   A. cp(T3 - Tx)
   B. cp(T3 - T2)
   C. cp(T4 - T2)
   D. cp(Tx - T2)
   Answer: A. Tx > T2 so fuel falls while net work stays the same. (Key point: Heat supplied)

3. Effectiveness is defined as
   A. (T4 - Tx)/T4
   B. (Tx - T2)/(T4 - T2)
   C. (T3 - Tx)/(T3 - T2)
   D. T2/T4
   Answer: B. Actual air-side rise over ideal rise. (Key point: Effectiveness)

4. When does regeneration not help?
   A. At low pressure ratio
   B. With high effectiveness
   C. At high pressure ratios when T2 exceeds T4
   D. With intercooling
   Answer: C. Hot compressor delivery would be cooled by the exhaust. (Key point: Limitation)

## Related in Power Plant Equipment

- [Gas turbine 800 kW with regenerator](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-800kw-regenerator) — Power Plant Equipment, chapter 6, asked 2×
- [Gas turbine with 78% effective regenerator](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-regenerator-78) — Power Plant Equipment, chapter 6
- [Gas turbine 6:1: power output and efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-6-1-power-efficiency) — Power Plant Equipment, chapter 6, asked 2×
- [Gas turbine 6:1: generator power output](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-6-1-power-output) — Power Plant Equipment, chapter 6, asked 2×
- [Ideal gas turbine with regenerator at 1400 K](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-ideal-regenerator-1400k) — Power Plant Equipment, chapter 6
- [Reheat gas turbine power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-reheat-plant) — Power Plant Equipment, chapter 6

## Related in other subjects

- [Combined cycle gas turbine versus hydro](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Accgt-vs-hydro) — Power Plant Design, chapter 1
- [Actual gas turbine cycle with component efficiencies](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aactual-brayton-cycle-numerical) — Turbo Machines, chapter 3
- [Closed gas turbine cycle components; thermal efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aclosed-gas-turbine-cycle-efficiency) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Modifications of the basic gas turbine cycle](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Avariations-basic-gas-turbine-cycle) — Turbo Machines, chapter 6