# Combined cycle gas plant

Figure 1. Combined cycle gas plant in Iran.[1]

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%[2], but the dispatch times tend to be slower than simple cycle natural gas plants. 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.[3]

## Operation

The first part of the plant operates by use of a gas turbine and the details can be explored on its page here. Essentially, 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 (releasing waste heat).

The exhaust gases are made to flow towards the next unit, called the heat recovery steam generator (HRSG).[4] The HRSG is essentially a heat exchanger, in which the heat from the hot gases is used to 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[5] visualizing the process can be seen below: