What’s the difference between an open cycle and a combined cycle gas plant?

Open cycle gas turbines draw air from outside of the plant and compress the air in a centrifugal or an axial flow compressor. The compressor heats the air and feeds it into a combustion chamber where the gas is burnt at a specified temperature. The pressure from the burning gas causes rotation in the plant turbines. The rotational movement in the turbines generates electricity as well as driving the compressor, while the burnt gas fumes are released back into the atmosphere. 

Combined cycle gas plants use both gas and steam turbines and can generate significantly more electricity than open cycle plants. These plants are capable of generating 50% more electricity because the ”waste heat” generated by the gas turbine is fed through a waste heat boiler to generate steam which is then fed to the steam turbine to generate additional electricity.