3.2. Gas turbines

3.2.1. Basic principles

1. Compression: A rotating compressor acts as a fan to drive the working fluid into the heating system. The fluid is pressurised adiabatically, thus its temperature increases. Compressors are of the dynamic type, in which each stage increases the fluid velocity, then lets it diffuse to gain pressure.

2. Combustion: The fluid is heated by internal combustion, in a continuous process taking place at constant pressure. A steady supply of fuel mixes with air at high velocity from the compressor and burns as it flows through a flame zone. Combustion occurs in a very small volume, partly because it takes place at high pressure. The flame does not touch the container, which is cooled by the inlet airflow pattern.

3. Expansion: The working fluid at high pressure is then released to the turbine, which converts the fluid's energy into useful work as the temperature of the working fluid decreases. Part of this work is returned to the compressor. The remainder is used for the application intended: Generation of electricity, pumping, turbojet propulsion... Axial flow turbines are able to lower the stagnation pressure by about 30 - 50 %. Temperature falls between 8 and 16 %. Several stages are combined for increased work output.

Figure 13: Gas turbine cycles

If the fluid employed is recycled the turbine works on closed cycle, whereas if the fluid is not re-used it is open cycle.

3.2.2. Efficiency in gas turbines

Ideal gas turbine cycles are based on the Joule or Brayton cycles, ie, compression and expansion at constant entropy, and heat addition and release at constant pressure (Harman (1981)) . In an ideal cycle, efficiency varies with the temperature ratio of the working fluid in the compression process, which is related to its pressure ratio. The inlet temperature in the turbine section is generally limited by turbine technology, materials strength, corrosion and other considerations. The increment of temperature also depends on the initial temperature of the working fluid.

Various process modifications can be applied to improve the thermal efficiency of gas turbines. For example, regeneration improves efficiency by transferring waste heat to the inlet air, thus reducing fuel consumption. Co-generation is a process where the exhaust gases are used to raise steam in a boiler. The steam can be used in a different application, be it an industrial process, district heating, etc. The quality of the fuel used must be higher as contaminants likely to produce corrosion must be avoided. In combined cycles, the waste heat generated in a gas turbine is re-directed to a boiler in order to drive a steam turbine. Both the gas turbine and the steam turbine generate electricity.

Some effects must be considered which diminish efficiency in real operating cycles, such as inefficiency in compression and expansion, loss of pressure during heat addition and rejection, variation of working fluid specific heat with temperature, incomplete combustion, etc.

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