The thermodynamic process used in gas turbines is the Brayton cycle. Two significant performance parameters are the pressure ratio and the firing temperature. The fuel-to-power efficiency of the engine is optimized by increasing the difference (or ratio) between the compressor discharge pressure and inlet air pressure. This compression ratio is dependent on the design. Gas turbines for power generation can be either industrial (heavy frame) or aeroderivative designs. Industrial gas turbines are designed for stationary applications and have lower pressure ratios – typically up to 18:1. Aeroderivative gas turbines are lighter weight compact engines adapted from aircraft jet engine design which operate at higher compression ratios – up to 30:1. They offer higher fuel efficiency and lower emissions, but are smaller and have higher initial (capital) costs. Aeroderivative gas turbines are more sensitive to the compressor inlet temperature.
The temperature at which the turbine operates (firing temperature) also impacts efficiency, with higher temperatures leading to higher efficiency. However, turbine inlet temperature is limited by the thermal conditions that can be tolerated by the turbine blade metal alloy. Gas temperatures at the turbine inlet can be 1200ºC to 1400ºC, but some manufacturers have boosted inlet temperatures as high as 1600ºC by engineering blade coatings and cooling systems to protect metallurgical components from thermal damage.
Because of the power required to drive the compressor, energy conversion efficiency for a simple cycle gas turbine power plant is typically about 30 percent, with even the most efficient designs around 40 percent. A large amount of heat remains in the exhaust gas, which is around 600ºC as it leaves the turbine. By recovering that waste heat to produce more useful work in a combined cycle configuration, gas turbine power plant efficiency can reach 55 to 60 percent. However, there are operational limitations associated with operating gas turbines in combined cycle mode, including longer startup time, purge requirements to prevent fires or explosions, and ramp rate to full load.
| Typical performance values for new gas turbines
|Gas turbine type
Simple cycle (%), LHV
Combined cycle (%), LHV
| Small scale heavy duty
| Large scale heavy duty