Natural gas is the most prevalent energy source used for electricity generation in the US. Of the 491 GW of natural gas-fired electric-generating capacity in the country, more than half (280 GW) are combined-cycle systems that include both steam turbines and combustion turbines. The predominant configuration for combined-cycle systems involves two combustion turbines and one steam turbine (2x1).
Natural gas-fired electric power plants utilise either simple-cycle or combined-cycle energy conversion. Simple-cycle systems consume natural gas in a single conversion system, such as a combustion turbine, boiler with a steam turbine, or internal combustion engine. In these systems, energy from the primary conversion exhaust heat is lost and not sent to another turbine. Combined-cycle systems send combustion turbine exhaust heat to a heat recovery steam generator, pressurising steam for one or more steam turbines that generate additional electric power.
Combined-cycle systems tend to be more efficient at converting a unit of natural gas (measured in Btu) to a unit of electricity (kWh). Combined-cycle systems have an average operating heat rate of 7146 Btu/kWh compared with simple-cycle systems, which consume approximately 10 000 Btu to generate 1 kWh, based on 2020 averages. Because of the efficiency gains, combined-cycle systems generally operate at higher capacity factors than simple-cycle systems. As a result, combined-cycle systems generally serve base and intermediate loads to the grid, while simple-cycle systems generally serve peak load.
The EIA's Electric Power Annual survey collected a unit code identifier to group combined-cycle components into power blocks. Power block configurations vary widely, including configurations such as multiple combustion turbines supporting one or more steam turbines. Most combined-cycle power blocks have combustion and steam turbine components each connected with a generator. However, 13 GW of combined-cycle capacity are single-shaft systems in which both the combustion and steam turbines drive a common generator on one shaft.
In terms of generating capacity, the largest natural gas combined-cycle power plant in the US is Florida Power & Light’s West County Energy Center. The West County Energy Center consists of three 3x1 power blocks, or a total of 12 combustion and steam turbines. Each of the three power blocks is rated at 1259 MW, giving the plant 3777 MW of total generating capacity. The West County Energy Center generated 20.3 TWh of electricity in 2021; only five nuclear power plants and one coal-fired plant generated more electricity in 2021.
Combined-cycle power blocks have increased in average capacity since 2015, when a new class of combustion turbines became more common. These newer combustion turbines allow combined-cycle power blocks to have larger capacities and improved efficiencies. The average capacity of a 2x1 power block increased from 500 MW for those built before 2015 to over 700 MW for those built after 2015.
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