Gas turbine improvement methods and regeneration

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.

          +------- 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.