Gas turbine improvement methods and intercooling

The thermal efficiency of a simple open-cycle gas turbine is low (about 20–30 %) because the compressor absorbs a large part of the turbine work (60–70 %) and the exhaust leaves at a high temperature. Its efficiency and output are improved by the following methods.

Methods of efficiency improvement

1. Regeneration (heat exchanger): hot turbine exhaust preheats the compressed air before the combustion chamber, so less fuel is needed for the same turbine inlet temperature.
2. Intercooling: compression is done in stages with cooling of air between stages; compressor work falls.
3. Reheating: expansion is done in stages; gas is reheated between turbines, raising turbine work.
4. Higher turbine inlet temperature using better blade materials and blade cooling.
5. Higher (optimum) pressure ratio with high component (isentropic) efficiencies.
6. Combined cycle / waste-heat recovery: exhaust heat raises steam for a steam turbine.
7. Water or steam injection, and reducing pressure losses in ducts and combustor.

Intercooling

In intercooling the air is compressed in a low-pressure (LP) compressor, cooled in an intercooler at nearly constant pressure back to (ideally) the inlet temperature, and then compressed in a high-pressure (HP) compressor.

 Air  +-----+   +-------+   +-----+   +----+
 -1-->| LPC |-2>| Inter |-3>| HPC |-4>| CC |
      +-----+   | cooler|   +-----+   +----+
         |      +-------+      |        |5
         +========= shaft =====+====+---+--+
                                    |  T   |-6-> exhaust
                         Gen <======+------+

 T
 |                   5
 |                  /|
 |                 / |
 |         4     /   |
 |         |---'     |
 |         |   2     6
 |         |  /|    /
 |         | / |   /
 |         3   1--'
 +----------------------- s
 1-2 LP comp, 2-3 intercool,
 3-4 HP comp, 4-5 heating,
 5-6 turbine, 6-1 exhaust

How it improves the plant:

• Work of compression is proportional to the absolute temperature of the air ($W_c = c_p T_{in}[r_p^{(\gamma-1)/\gamma} - 1]$). Cooling the air to $T_3 \approx T_1$ before the HP stage reduces total compressor work. With perfect intercooling and equal stage pressure ratios ($r_{p1} = r_{p2} = \sqrt{r_p}$), compressor work is minimum.
• Turbine work is unchanged, so net work output and work ratio increase considerably.
• However, the air leaves the HP compressor colder ($T_4 < T_2'$ of single-stage), so more fuel must be burnt from 4 to 5. Alone, intercooling may give only a small increase (or even a decrease) in efficiency.
• When intercooling is used with a regenerator, the lower compressor delivery temperature allows more exhaust heat to be recovered, and thermal efficiency increases appreciably.

Thus intercooling mainly raises specific output and, combined with regeneration, raises thermal efficiency.