Combined cycle gas plants are a type of natural gas power plant used to generate electricity, consisting of a simple cycle gas plant in combination with a second steam engine that uses the Rankine cycle. The hot exhaust gases from the initial gas turbine are sent to the steam engine, and the heat from them is used to generate steam. This steam can then expand through another turbine, generating even more electricity and increasing the plant's overall efficiency. The efficiency of these plants can be as great as 55%.
The combined cycle plant takes advantage of the capabilities of the single cycle, in that it can provide peaking power due to its quick dispatch time. In addition, to it is able to provide intermediate power from the steam engine. Some modern combined cycle plants can achieve effiencies slightly over 60%, with dispatch times around half an hour.
With these advantages, the combined cycle typically has an increased cost. The EIA estimated that for a simple cycle plant the cost is about US$389/kW, whereas combined cycle plants are US$500 – 550/kW.
The first part of the plant operates by use of a gas turbine and the details can be explored on its page. Basically it works by compressing air, injecting fuel and igniting the mixture, which expands through the turbine causing it to spin. This turbine is connected to a generator, which then causes electricity to flow. This process creates hot exhaust gases, which are typically dispelled in a simple cycle plant.
The exhaust gases are made to flow towards the next unit, called the heat recovery steam generator (HRSG). The HRSG is essentially a heat exchanger, in which the hot gases boil pre-heat water into steam. The steam then expands through a turbine, generating electricity. Once the steam has passed through, it condenses and is recycled through.
A short video from the Tennessee Valley Authority visualizing the process can be seen below: