Methods to improve gas turbine efficiency

A simple open-cycle gas turbine has a low thermal efficiency (about 20–30 %) because (i) the compressor consumes a large part (60–70 %) of turbine work and (ii) the exhaust leaves at 400–550 °C, carrying away much heat. The following methods improve efficiency and/or output.

1. Regeneration

A heat exchanger (regenerator) transfers heat from the hot turbine exhaust to the compressed air before it enters the combustion chamber. Fuel required for the same turbine inlet temperature falls, so efficiency rises while net work is unchanged. Effective only when exhaust temperature is higher than compressor delivery temperature (low–moderate pressure ratios). Effectiveness $\varepsilon = \dfrac{T_x - T_2}{T_4 - T_2}$, usually 0.6–0.8.

2. Intercooling

Compression in two or more stages with cooling between stages reduces compressor work ($W \propto T_{inlet}$). Net work and work ratio increase. Alone it slightly increases fuel needed, but with regeneration it gives a definite gain in efficiency.

3. Reheating

Expansion in two turbines with reheating of gas between them (in a second combustor) increases turbine work, so net output rises; efficiency rises only when combined with regeneration because exhaust is hotter.

4. Combination (intercooling + reheating + regeneration)

Approaches the Ericsson cycle; gives the highest efficiency and work ratio, but plant is complex and costly.

5. Higher turbine inlet temperature

Efficiency and specific work increase with $T_3$. Achieved with nickel-based superalloys, ceramic coatings and blade cooling (air or steam).

6. Optimum pressure ratio and better components

For given $T_1, T_3$ there is a pressure ratio giving maximum work and another giving maximum efficiency. Increasing isentropic efficiencies of compressor and turbine (better blade design) and reducing pressure losses in ducts, filters and combustor all improve performance.

7. Waste-heat recovery / combined cycle

Exhaust heat raises steam in a heat recovery steam generator (HRSG) for a steam turbine (combined cycle, overall 50–60 %), or is used for process heating (cogeneration).

8. Water/steam injection and inlet air cooling

Injecting water or steam increases mass flow and output; cooling the inlet air (evaporative or chiller) increases air density and reduces compressor work, especially in hot weather.