How a regenerator improves gas turbine output

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

          +---------- 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

 p
 |  2    x    3
 |  *----*----*
 |   \          \
 |    \            \
 |     \              \
 |      *------*--------*
 |      1      y        4
 +--------------------------- v

 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.