the energy efficiency of a conventional thermal power station is defined as saleable energy produced as a percent of the heating value of the fuel consumed. a simple cycle gas turbine achieves energy conversion efficiencies from 20 to 35%. typical coal-based power plants operating at steam pressures of 170 bar and 570 °c run at efficiency of 35 to 38 %, with state-of-the-art fossil fuel plants at 46% efficiency. combined-cycle systems can reach higher values. as with all heat engines, their efficiency is limited, and governed by the laws of thermodynamics.