Combined-cycle electric generating systems are combustion and steam turbines that operate in groups commonly referred to as power blocks. Since 2014, the average size of a natural gas-fired combined-cycle power block has increased significantly. The average combined-cycle power block installed between 2002 and 2014 was about 500 megawatts (MW). After 2014, power block capacity increased, reaching an average of 820 MW in 2017. Power blocks have increased in size as the performance of combined-cycle units has continued to improve, and current and projected natural gas prices and supply provide a competitive advantage for the combined-cycle technology. The most common configuration involves two combustion turbines supporting one steam turbine.

EIA captures the capacity, fuel consumption, and electric generation of generating units in its Annual Electric Generator Report. EIA uses an identifier called a unit code to group the component units of combined-cycle power blocks to help understand their design and operational characteristics. Most of the installed capacity of natural gas-fired combined-cycle units comes from power blocks that have capacities of 600 MW to 700 MW. Of the 644 natural gas-fired combined-cycle power blocks installed nationwide, 16% are within this range. In 2017, two-thirds of the power blocks installed that year were 600 MW or higher, helping to drive the increase in average capacity when compared with power blocks installed in earlier years.

Factors such as operating cost and performance affect the sizing of a combined-cycle power block for a particular application. The heat rate of a generator, measured as the amount of British thermal units (Btu) required to generate a kilowatthour of electricity, is the metric most commonly used to represent the efficiency of commercial generators.

The trend toward larger combined-cycle power blocks can largely be explained by the efficiency gains (lower heat rates) available from larger power blocks. For example, the capacity-weighted average heat rate of power blocks less than 500 MW is 6% higher (or less efficient) than that of power blocks larger than 1,000 MW. Larger power blocks also generally have lower per-unit capital costs, making them more attractive investments.

Smaller power blocks still play an important role in providing electricity. Generators have a minimum output level—known as minimum load—below which continued operation creates risks of instability, inefficiency, and high emissions. Smaller blocks, which inherently have lower minimum loads, are commonly associated with greater flexibility because they can operate through low demand periods without having to shut down.

Principal contributor: Glenn McGrath