What Are Turbines?

Turbines are rotary mechanical devices that extract energy from a flowing fluid and convert that energy into useful rotational work. The working fluid passes through a series of stationary and rotating blade rows, transferring momentum to the rotor and causing it to spin. The rotor shaft then drives a generator, compressor, pump, or propeller, depending on the application. Turbines operate on either the impulse principle, in which high-velocity fluid jets strike moving blades at atmospheric pressure, or the reaction principle, in which pressure drops continuously across the rotating blades as the fluid expands. Most practical machines combine elements of both designs.

The history of turbines begins with Hero of Alexandria's aeolipile in the first century CE, though modern turbomachinery traces its direct lineage to the steam turbine patents filed independently by Charles Parsons and Carl Gustaf de Laval in the 1880s. Today, turbines account for the overwhelming majority of the world's electrical generation, as described in the Energy Education encyclopedia on turbines, and they are central components in aviation, marine propulsion, and industrial heating systems.

Steam Turbines

Steam turbines convert the thermal energy of high-pressure steam produced in a boiler, nuclear reactor, or heat recovery system into shaft rotation. Steam enters a high-pressure stage through admission valves and expands through successive turbine stages, each consisting of a ring of fixed nozzle vanes followed by a ring of moving blades. As the steam expands across each stage, it drops in pressure and temperature, delivering work to the rotor. Multi-stage designs with dozens of stages extract thermal energy efficiently across a wide pressure range. Reheat cycles, in which partially expanded steam is returned to the boiler for additional heating before entering intermediate-pressure stages, improve thermodynamic efficiency. Steam turbines drive most of the world's nuclear and coal-fired generation, and they appear in industrial cogeneration plants that produce both process heat and electricity from a single fuel input. Steam turbine-generators typically connect to the grid through synchronous generators running at 3,000 or 3,600 revolutions per minute, depending on grid frequency.

Gas Turbines

Gas turbines compress ambient air, mix it with fuel in a combustion chamber, ignite the mixture, and allow the resulting high-temperature, high-pressure combustion gas to expand through the turbine stages. Jet aircraft engines are gas turbines that extract just enough work to drive the compressor and ancillary systems, exhausting the remainder as propulsive thrust. Land-based gas turbines for power generation extract more work from the expanding gas to drive a generator shaft. Simple-cycle gas turbine plants start quickly, making them useful for peaking generation, while combined-cycle plants route the hot exhaust through a heat recovery steam generator to produce steam for an additional steam turbine, raising overall thermal efficiency from roughly 35 to above 60 percent. The U.S. Energy Information Administration's electricity generation data shows that combined-cycle gas turbine plants provide approximately 34 percent of U.S. generation annually. Gas turbines also power large marine vessels and industrial compressor drives in the oil and gas sector.

Hydraulic and Wind Turbines

Hydraulic turbines convert the potential energy of water stored at elevation into shaft rotation. Francis turbines, which handle medium heads and large flow volumes, are the most widely installed hydro design; Kaplan turbines handle low-head, high-flow conditions; and Pelton turbines are suited to high-head, low-flow sites. Wind turbines capture the kinetic energy of moving air through aerodynamically shaped rotor blades, which drive a generator through a gearbox or, in direct-drive designs, through a low-speed permanent-magnet generator. Commercial wind turbines range in rated capacity from under one megawatt for distributed installations to over fifteen megawatts for offshore units. Both hydraulic and wind turbines produce no direct combustion emissions during operation, and both require governing or pitch control systems to regulate output under varying fluid conditions. Recent turbine design is reviewed in the ScienceDirect overview of steam and power turbines covering advances in blade materials, aerodynamics, and thermal coatings.

Applications

Turbines have applications across a wide range of energy and industrial sectors, including:

• Baseload and peaking electricity generation in steam, gas, and combined-cycle power plants
• Hydroelectric generation from run-of-river, reservoir, and pumped-storage facilities
• Wind energy conversion in onshore and offshore wind farms
• Aircraft propulsion in turbofan and turboprop engines
• Marine propulsion in naval vessels and large commercial ships
• Industrial cogeneration supplying both process heat and mechanical or electrical power