Solar Power Technology

In a gas turbine plant where oil, natural gas, or synthesis gasmaybe used as a fuel, the hot combustion gases are directly used to drive a gas turbine, rather than transferring heat to steam and driving a steam turbine. This requires a different turbine, appropriate for the much higher temperature of the combustion gases and their different thermodynamic properties compared to steam. Gas turbines are easily brought on line and have flexible load match. But their cycle efficiencies are lower than those of steam plants, and the fuel is more expensive.

Therefore, gas turbines are mostly used for peak load production and for auxiliary power, such as during major plant outages. However, recently many natural gas-fueled gas turbine plants have been installed in the United States and other countries; but these usually employ the combined cycle mode, which has a higher efficiency than the single cycle mode.

Compressed air enters a combustion chamber, where liquid or gaseous fuel is injected. The combustion of the fuel increases the temperature of the combustion gas, producing a net work output of the turbine–compressor system. The temperature of the combustion gases is on the order of 1100–1200 ◦C, which is the maximum tolerable by present-day steel alloys used for gas turbine blades. Even at these temperatures, thermal stresses and corrosion problems are manifested, so that turbine blade cooling from the inside or outside of the blades by air or water is necessary.

Gas turbines are of the reaction type, where blades form a converging nozzle in which the combustion gases expand, thus converting enthalpy to kinetic energy. As in steam turbines, staged turbines are employed, consisting of several rows of moving and fixed blades.

The working fluid in gas turbines, composed of nitrogen, excess oxygen, water vapor, and carbon dioxide, is not recycled into the compressor and combustion chamber but is, instead, vented into the atmosphere. In some systems, a part of the energy still residing in the exhaust gas is recovered in heat exchangers to heat up the air entering the combustion chamber in order to enhance the overall thermal efficiency of the Brayton cycle, but eventually the exhaust gas is vented. This is in contrast to steam turbines where the working fluid, steam, is recycled into the boiler as condensed water.