A gas turbine is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. (Gas turbine may also refer to just the turbine element.)
Energy is added to the gas stream in the combustor, where air is mixed with fuel and ignited. Combustion increases the temperature, velocity and volume of the gas flow. This is directed through a diffuser (nozzle) over the turbine's blades, spinning the turbine and powering the compressor.
Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships and generators.
Theory of operation
Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.
As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, ceramic, or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Most turbines also try to recover exhaust heat, which otherwise is wasted energy. Recuperators are heat exchangers that pass exhaust heat to the compressed air, prior to combustion. Combined cycle designs pass waste heat to steam turbine systems. And combined heat and power (co-generation) uses waste heat for hot water production.
Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternator-rotor assembly (see image above), not counting the fuel system.
More sophisticated turbines may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers.
The largest gas turbines operate at 3,000 or 3,600 rpm to match the AC power grid. They require a dedicated building. Smaller turbines, with fewer compressor/turbine stages, spin faster. Jet engines operate around 10,000 rpm and microturbines around 100,000 rpm.
Thrust bearings and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-cooled ball bearings. This is giving way to hydrodynamic foil bearings, which have become common place in microturbines and APU’s (auxiliary power units.)
See jet engine page.
Gas turbines for electrical power production
Powerplant gas turbines range in size from truck-mounted mobile plants to enormous, complex systems.
They can be particularly efficient—up to 60 percent—when waste heat from the gas turbine is recovered by a conventional steam turbine in a combined cycle.
Simple cycle gas turbines in the power industry require smaller capital investment than coal or nuclear and can be designed to generate small or large amounts of power. Their main advantage is the ability to be turned on and off within minutes, supplying power during peak demand. Large simple cycle gas turbines may produce several hundred megawatts of power and approach 40 percent thermal efficiency.
Microturbines are becoming wide spread for distributed power and combined heat and power applications. They range from handheld units producing less than a kilowatt to commercial sized systems that produce tens or hundreds of kilowatts.
Also known as:
- Turbo alternators
- MicroTurbine® (registered trademark of Capstone Turbine Corporation)
- Turbogenerator® (registered tradename of Honeywell Power Systems , Inc.)
Part of their success is due to advances in electronics, which allow unattended operation and interfacing with the commercial power grid. Electronic power switching technology eliminates the need for the generator to be synchronized with the power grid. This allows, for example, the generator to be integrated with the turbine shaft, and to double as the starter motor.
Microturbine systems have many advantages over piston engine generators, such as higher power density (with respect to footprint and weight), extremely low emissions and few, or just one, moving part. Those designed with foil bearings and air-cooling operate without oil, coolants or other hazardous materials.
They accept most commercial fuels, such as natural gas, propane, diesel and kerosene. The are also able to produce renewable energy when fueled with biogas from landfills and sewage treatment plants.
Microturbine designs usually consist of a single stage radial compressor, a single stage radial turbine and a recuperator. Recuperators are difficult to design and manufacture because they operate under high pressure and temperature differentials. Exhaust heat can be used for water heating, drying processes or absorption chillers, which create cold for air conditioning from heat energy instead of electric energy.
Typical microturbine efficiencies are 25 to 35 percent. When in a combined heat and power cogeneration system, efficiencies of greater than 80 percent are commonly achieved.
APUs are small gas turbines designed for auxiliary power of larger machines, usually aircraft. They are well suited for supplying compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger jet engines, and electrical and hydraulic power. (These are not to be confused with the auxiliary propulsion units, also abbreviated APUs, aboard the gas-turbine-powered Oliver Hazard Perry-class guided-missile frigates. The Perrys' APUs are large electric motors that provide maneuvering help in close waters, or emergency backup if the gas turbines are not working.)
Gas turbines in vehicles
Gas turbines are used on ships, locomotives, helicopters, and in the M1 Abrams and T-80 tanks. A number of experiments have been conducted with gas turbine powered automobiles, but none have yet proved suitable for production.
In 1950, designer F. R. Bell and Chief Engineer Maurice Wilks from British car manufacturers Rover unveiled the first car powered with a gas turbine engine. The two-seater JET1 had the engine positioned behind the seats, air intake grilles on either side of the car and exhaust outlets on the top of the tail. During tests, the car reached top speeds of 140 km/h, at a turbine speed of 50,000 rpm. The car ran on petrol, paraffin or diesel oil, but fuel consumption problems proved insurmountable for a production car. It is currently on display at the London Science Museum. Rover and the BRM Formula One team joined forces to produce a gas turbine powered coupe, which entered the 1963 24 hours of Le Mans, driven by Graham Hill and Richie Ginther. It averaged 107.8 mph (173 km) and had a top speed of 142 mph (229 km/h). American car manufacturer Chrysler demonstrated several prototype gas turbine-powered cars from the early 1950s through the early 1980s. A history of Chrysler turbine cars. In 1993 General Motors introduced the first commercial gas turbine powered hybrid vehicle—as a limited production run of the EV-1 . A Williams International 40 kW turbine drove an alternator which powered the battery-electric powertrain. The turbine design included a recuperator.
Gas turbines do offer a high-powered engine in a very small and light package, but there remain unchanged three main reasons why small turbines have not succeeded in an automotive application. Firstly, small turbines are fundamentally less fuel-efficient than small piston engines. Secondly, this problem is exacerbated by the requirement for automotive engines to run efficiently at idle and low throttle openings; turbines are notably inefficient at this. Finally, turbines have historically been more expensive to produce than piston engines, though this is partly because piston engines have been mass-produced in huge quantities for decades, while small turbines are rarities. It is also worth noting that a key advantage of jets and turboprops for aeroplane propulsion - their superior performance at high altitude compared to piston engines, particularly naturally-aspirated ones - is irrelevant in automobile applications. Their power-to-weight advantage is far less important. Their use in hybrids reduces the second problem. Capstone currently lists on their website a version of their turbines designed for installation in hybrid vehicles.
Their use in military tanks has been more successful. As well as their production use in the T-80 and Abrams, In the 1950's an FV214 Conqueror Heavy Tank was experimentally fitted with a Parsons 650 hp gas-turbine.
Amateur gas turbines
A popular hobby is to construct a gas turbine from an automotive turbocharger. A combustion chamber is fabricated and plumbed between the compressor and turbine. Like many technology based hobbies, they tend to give rise to manufacturing businesses over time. See external links for resources.
There are several small businesses that produce gas turbines for model planes.
Advances in technology
Gas turbine technology has steadily advanced since its inception and continues to evolve; research is active in producing ever smaller gas turbines. Computer design, specifically CFD and finite element analysis along with material advances, has allowed higher compression ratios and temperatures, more efficient combustion, better cooling of engine parts and reduced emissions. Additionally, compliant foil bearings were commercially introduced to gas turbines in the 1990s. They can withstand over a hundred thousand start/stop cycles and eliminated the need for an oil system.
On another front, microelectronics and power switching technology have enabled commercially viable micro turbines for distributed and vehicle power. An excellent example is the Capstone line of micro turbines, which do not require an oil system and can run unattended for months on end.
Gas turbines are used in many naval vessels, where they are valued for their high power-to-weight ratio and their ships' resulting acceleration and ability to get underway quickly. The first gas-turbine-powered naval vessel was the Royal Navy's Motor Gun Boat MGB 2009 (formerly MGB 509) converted in 1947. The first large, gas-turbine powered ships, were the Royal Navy's Type 81 (Tribal Class) Frigate the first of which (Ashanti) was commissioned in 1961.
The first U.S. gas-turbine powered ships were the U.S. Coast Guard's Hamilton Class High Endurance Cutters the first of which (WHEC-715 Hamilton) commissioned in 1967. Since then, they have powered the U.S. Navy's Perry-class frigates, Spruance-class and Arleigh Burke-class destroyers, and Ticonderoga-class guided missile cruisers. USS Makin Island, a modified Wasp-class amphibious assault ship, is to be the Navy's first amphib powered by gas turbines.
Last updated: 10-20-2005 09:06:10