Power Generation-Gas TURBINE

Applications of gas turbines:

Gas turbines, also known as combustion turbines, are common prime movers for many applications.Their historically high fuel consumption, especially for small units (less than 10,000 kW), as well as atpart load, and at high inlet air temperatures has made them less desirable than diesel engines for prime power plant applications. They have been utilized extensively in standby and peaking applications where their relatively low installed cost outweighs other factors. Open or simple cycle gas turbines are used in virtually all power plant applications and only this type will be addressed here. Combined cycle systems, where heat is recovered from the gas turbine exhaust and used to make steam which then drives
a steam turbine, have become much more common in recent years, but they will not be discussed here.

Gas turbine operating characteristics:

Gas turbines are based on the Brayton or Joule cycle which consists of four processes: compression with no heat transfer, heating at constant pressure, expansion with no heat transfer, and in a closed cycle system, cooling at constant pressure. In open cycle gas turbines, the fourth step does not exist since inlet air is taken from the atmosphere and the exhaust is dumped to atmosphere. Due to its higher temperature, there is more energy available from the expansion process than is expended in the compression. The net
work delivered to drive a generator is the difference between the two. The thermal efficiency of the gas turbine is a function of the pressure ratio of the compressor, the inlet temperature of the power turbine, and any parasitic losses (especially the efficiency of the compressor and power turbine). Practical limitations on thermal efficiency due to losses and materials technology yield a maximum of about 40 percent at pressure ratios of 30 to 40 and temperatures of approximately 2,500°F. These temperatures and pressure ratios are found only in recently developed, large gas turbines. Typically pressure ratios of 5 to
20 and turbine inlet temperatures from 1,400 to 2,000°F are common in gas turbines for this application, resulting in efficiencies from 20 to 33 percent. As improved materials and cooling technologies are introduced to smaller units, the efficiencies can be expected to improve if the cost is not prohibitive.