Carnot Cycle Limitations

Heat Sources and Efficiency

• Heat sources used in power plants include nuclear fission and combustion of fossil fuels like coal, natural gas, and oil.
• The efficiency of the Rankine cycle is limited by the high heat of vaporization of the working fluid.
• Steam turbine entry temperatures are around 565°C, while steam condenser temperatures are around 30°C.
• Theoretical maximum Carnot efficiency for the steam turbine is 63%, while actual overall thermal efficiency for a modern coal-fired power station is around 42%.

Parts of a Rankine Cycle

Boiler

• A closed vessel or arrangement of vessels and tubes, along with a furnace or heat source, to generate steam or vapor.
• Types of boilers include:
  • Water-tube boiler: water circulates in tubes exposed to fire and enveloped by hot gases.
  • Fire-tube boiler: hot gases from a fire pass through one or more tubes running through a sealed container of water.

Turbine

• A rotary mechanical device that extracts energy from fluid flow and converts it into useful work.
• Has at least one moving part called a rotor assembly, with blades attached to a shaft or drum.

Condenser

• A device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it.
• Latent heat is given up by the substance, transferring to the condenser coolant.

Pump

• A device that moves fluids (liquids or gases) or slurries by mechanical action.
• Classified into three major groups: direct lift, displacement, and gravity pumps.

Rankine Cycle Process

• Isentropic compression: liquid condensate is compressed adiabatically from state 1 to state 2 by centrifugal pumps.
• Isobaric heat addition: constant-pressure heat transfer to the liquid condensate from an external source.
• Isentropic expansion: steam from the boiler expands adiabatically from state 3 to state 4 in a steam turbine to produce work.
• Isobaric heat rejection: constant-pressure process in which heat is rejected from the partially condensed steam.

Carnot Cycle vs. Rankine Cycle

• Carnot cycle is the most efficient cycle operating between two specified temperature limits.
• However, it is not suitable for steam power cycles due to turbine erosion, compressor design, and condensation control.
• Rankine cycle is a more realistic model for vapor power cycles, describing the process of steam-operated heat engines in thermal power generation plants.

Learn about the reasons why the Carnot cycle is not a suitable model for steam power cycles, including issues with turbine erosion, compressor design, and condensation control. Understand the limitations of this efficient cycle operating between two specified temperature limits.