The Adaptable Gas Turbine
Whether creating electricity or moving planes, this engine continues to inspire innovation
In the past 30 years, advances in non-aviation technology have almost doubled the thermal efficiency of new gas turbine electric power plants. In 2011, the worldwide market for nonaviation gas turbines came in at $16 billion, most of it for new electrical plants. Modern gas turbine combined-cycle power plants produce electric power at levels as high as half a gigawatt, with thermal efficiencies that now exceed the 60 percent mark—almost twice what I learned about as an undergraduate mechanical engineering student.
A combined-cycle gas turbine power plant uses a gas turbine (usually fueled by natural gas) to drive an electrical generator. The hot exhaust is then used to produce steam in a heat exchanger (called a heat recovery steam generator) to supply a steam turbine whose useful work output provides the means to generate more electricity. (If the steam is used instead to heat buildings, the unit would be called a cogeneration plant.) A good efficiency value for modern gas turbines is 40 percent, whereas a steam turbine at typical combined-cycle conditions is about 30 percent. Using the first law of thermodynamics and the definition of thermal efficiency, the combined efficiency of the two is about 58 percent, greater than either of the individual devices alone.
The heart of the combined-cycle plant (or more accurately, the combined power plant, because the thermodynamic cycles aren’t combined) is the gas turbine with its gas exhaust temperature, typically at about 1,000 degrees Fahrenheit (or 538 degrees Celsius), sufficient to produce steam to power the steam turbine. The Siemens 375 megawatt gas turbine shown in the third figure is the center of a new 578-megawatt combined-cycle gas turbine plant in Irsching, Germany. On May 19, 2011, Siemens announced it had reached a thermal efficiency of 60.75 percent, which probably makes it the most efficient heat engine ever operated.