Combined Cycle Power Plants achieve high levels of efficiency by combining two thermodynamic cycles: the Joule cycle (gas turbine) and the Rankine cycle (steam turbine). In this setup, gas is burned in a gas turbine to generate mechanical energy, which is converted into electricity by a generator. The hot exhaust gases from the gas turbine are used to produce steam that powers a secondary steam turbine, also driving a generator to produce additional electricity.
To achieve maximum efficiency, the pressure in the steam turbine's condenser must be kept as low as possible. Lower pressure levels improve the thermal efficiency of the steam cycle, maximizing energy output and reducing fuel consumption. This principle is outlined in the Clausius-Rankine cycle.
The pressure in the condenser is influenced by cooling water temperature and is below atmospheric levels, effectively creating a vacuum. This vacuum can be compromised if air leaks into the system, typically at flange connections or other points between the turbine and condenser. An increase in air content raises the total pressure, reducing efficiency, power output, and increasing CO2 emissions.