Figure 8.1 shows the primary components of a simple-cycle gas turbine. Engineering advancements pioneered the development of the simple-cycle gas turbine in the early 1900s, and turbines began to be used for stationary electric power generation in the later 1930s. Turbines revolutionized airplane propulsion in the 1940s, and in the 1990s through today they have been a popular choice for new power generation plants in the United States. Available in sizes ranging from 500 kilowatts (kW) to 150 megawatts (MW), gas turbines can be used in power-only generation or in combined heat and power (CHP) systems. The most efficient commercial technology for central station power-only generation is the gas turbine-steam turbine combined-cycle plant, with efficiencies approaching 60% (lower heating value, LHV). Simple-cycle gas turbines for power-only generation are available with efficiencies approaching 40% (LHV). Gas turbines have long been used by utilities for peaking capacity; however, with changes in the power industry and advancements in technology, the gas turbine is now being increasingly used for base-load power.