Gas turbine

A gas turbine, also called a combustion 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 component.

Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the combustor, combustion of the fuel increases the temperature. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning the turbine which powers the compressor and, for some turbines, drives their mechanical output. The energy given up to the turbine comes from the reduction in the temperature of the exhaust gas.

Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, generators, and even tanks.

History

150: Hero's Engine (aeolipile) — Apparently, Hero's steam engine was taken to be no more than a toy, and thus its full potential not realized for centuries.
1500: The "Chimney Jack" was drawn by Leonardo da Vinci which was turning a roasting spit. Hot air from a fire rose through a series of fans which connect and turn the roasting spit.
1551: Taqi al-Din invented a steam turbine, which he used to power a self-rotating spit.<ref name=Hassan>Template:Citation/core{{#if:|}}</ref>
1629: Jets of steam rotated a turbine that then rotated driven machinery allowed a stamping mill to be developed by Giovanni Branca.
1678: Ferdinand Verbiest built a model carriage relying on a steam jet for power.
1791: A patent was given to John Barber, an Englishman, for the first true gas turbine. His invention had most of the elements present in the modern day gas turbines. The turbine was designed to power a horseless carriage.
1872: A gas turbine engine was designed by Dr. Franz Stolze, but the engine never ran under its own power.
1894: Sir Charles Parsons patented the idea of propelling a ship with a steam turbine, and built a demonstration vessel, the Turbinia, easily the fastest vessel afloat at the time. This principle of propulsion is still of some use.
1895: Three 4-ton 100 kW Parsons radial flow generators were installed in Cambridge Power Station, and used to power the first electric street lighting scheme in the city.
1903: A Norwegian, Ægidius Elling, was able to build the first gas turbine that was able to produce more power than needed to run its own components, which was considered an achievement in a time when knowledge about aerodynamics was limited. Using rotary compressors and turbines it produced 11 hp (massive for those days). His work was later used by Sir Frank Whittle.
1906: The Armengaud-Lemale turbine engine in France with water-cooled combustion chamber.
1910: Holzwarth impulse turbine (pulse combustion) achieved 150 kilowatts.
1913: Nikola Tesla patents the Tesla turbine based on the Boundary layer effect.
1914: Application for a gas turbine engine filed by Charles Curtis.
1918: One of the leading gas turbine manufacturers of today, General Electric, started their gas turbine division.
1920: The practical theory of gas flow through passages was developed into the more formal (and applicable to turbines) theory of gas flow past airfoils by Dr. A. A. Griffith.
1930: Sir Frank Whittle patented the design for a gas turbine for jet propulsion. His work on gas propulsion relied on the work from all those who had previously worked in the same field and he has himself stated that his invention would be hard to achieve without the works of Ægidius Elling. The first successful use of his engine was in April 1937.
1934: Raúl Pateras de Pescara patented the free-piston engine as a gas generator for gas turbines.
1936: Hans von Ohain and Max Hahn in Germany developed their own patented engine design at the same time that Sir Frank Whittle was developing his design in England.
1939: First gas turbine power generator, designed by Aurel Stodola, for Brown Boveri company.

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.

In practice, friction and turbulence cause:

non-isentropic compression: for a given overall pressure ratio, the compressor delivery temperature is higher than ideal.
non-isentropic expansion: although the turbine temperature drop necessary to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work.
pressure losses in the air intake, combustor and exhaust: reduces the expansion available to provide useful work.

As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, nickel, ceramic, or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Some 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/alternative-rotor assembly (see image above), not counting the fuel system. However, the required precision manufacturing for components and temperature resistant alloys necessary for high efficiency often make the construction of a simple turbine more complicated than piston engines.

More sophisticated turbines (such as those found in modern jet engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers.

As a general rule, the smaller the engine the higher the rotation rate of the shaft(s) needs to be to maintain top speed. Turbine blade top speed determines the maximum pressure that can be gained,this produces the maximum power possible independent of the size of the engine. Jet engines operate around 10,000 rpm and micro turbines 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. These bearings are being surpassed by foil bearings, which have been successfully used in micro turbines and auxiliary power units.

Types of gas turbines

Aeroderivatives and jet engines

Airbreathing jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines. Jet engines that produce thrust primarily from the direct impulse of exhaust gases are often called turbojets, whereas those that generate most of their thrust from the action of a ducted fan are often called turbofans or (rarely) fan-jets.

Gas turbines are also used in many liquid propellant rockets, the gas turbines are used to power a turbopump to permit the use of lightweight, low pressure tanks, which saves considerable dry mass.

Aeroderivatives are also used in electrical power generation due to their ability to startup, shut down, and handle load changes more quickly than industrial machines. They are also used in the marine industry to reduce weight. The GE LM2500 and LM6000 are two common models of this type of machine.

Amateur gas turbines

Increasing numbers of gas turbines are being used or even constructed by amateurs.

In its most straightforward form, these are commercial turbines acquired through military surplus or scrapyard sales, then operated for display as part of the hobby of engine collecting.<ref name="latexiron" >Template:Citation/core{{#if:|}}</ref><ref name="Internal Fire, Proteus" >Template:Citation/core{{#if:|}}</ref> In its most extreme form, amateurs have even rebuilt engines beyond professional repair and then used them to compete for the Land Speed Record.

The simplest form of self-constructed gas turbine employs an automotive turbocharger as the core component. A combustion chamber is fabricated and plumbed between the compressor and turbine sections.<ref name="Scrapheap Challenge, gas turbine go-cart" >Template:Citation/core{{#if:|}}</ref>

More sophisticated turbojets are also built, where their thrust and light weight are sufficient to power large model aircraft.<ref name="Schreckling, Gas Turbines for Model Aircraft" >Template:Citation/core{{#if:|}}</ref> The Schreckling design<ref name="Schreckling, Gas Turbines for Model Aircraft" /> constructs the entire engine from raw materials, including the fabrication of a centrifugal compressor wheel from plywood, epoxy and wrapped carbon fibre strands.

Like many technology based hobbies, they tend to give rise to manufacturing businesses over time. Several small companies now manufacture small turbines and parts for the amateur. Most turbojet-powered model aircraft are now using these commercial and semi-commercial microturbines, rather than a Schreckling-like home-build.<ref name="Kamps" >Template:Citation/core{{#if:|}}</ref>

Auxiliary power units

APUs are small gas turbines designed for auxiliary power of larger machines, such as those inside an aircraft. They supply compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger jet engines, and electrical and hydraulic power.

Industrial gas turbines for power generation

Industrial gas turbines differ from aeroderivative in that the frames, bearings, and blading is of heavier construction. Industrial gas turbines range in size from truck-mounted mobile plants to enormous, complex systems. They can be particularly efficient—up to 60%—when waste heat from the gas turbine is recovered by a heat recovery steam generator to power a conventional steam turbine in a combined cycle configuration.<ref>"Efficiency by the Numbers" by Lee S. Langston</ref><ref>Mechanical Engineering "Power & Energy," June 2004 - "A Year of Turbulence," Feature Article</ref> They can also be run in a cogeneration configuration: the exhaust is used for space or water heating, or drives an absorption chiller for cooling or refrigeration. Such engines require a dedicated enclosure, both to protect the engine from the elements and the operators from the noise.

The construction process for gas turbines can take as little as several weeks to a few months, compared to years for base load power plants. Their other main advantage is the ability to be turned on and off within minutes, supplying power during peak demand. Since single cycle (gas turbine only) power plants are less efficient than combined cycle plants, they are usually used as peaking power plants, which operate anywhere from several hours per day to a few dozen hours per year, depending on the electricity demand and the generating capacity of the region. In areas with a shortage of base load and load following power plant capacity or low fuel costs, a gas turbine power plant may regularly operate during most hours of the day. A large single cycle gas turbine typically produces 100 to 300 megawatts of power and have 35–40% thermal efficiency.<ref name=siemens>Template:Citation/core{{#if:|}}</ref>

Compressed air energy storage

One modern development seeks to improve efficiency in another way, by separating the compressor and the turbine with a compressed air store. In a conventional turbine, up to half the generated power is used driving the compressor. In a compressed air energy storage configuration, power, perhaps from a wind farm or bought on the open market at a time of low demand and low price, is used to drive the compressor, and the compressed air released to operate the turbine when required.

Turboshaft engines

Turboshaft engines are often used to drive compression trains (for example in gas pumping stations or natural gas liquefaction plants) and are used to power almost all modern helicopters. The first shaft bears the compressor and the high speed turbine (often referred to as "Gas Generator" or "N1"), while the second shaft bears the low speed turbine (or "Power Turbine" or "N2"). This arrangement is used to increase speed and power output flexibility.

Radial gas turbines

In 1963, Jan Mowill initiated the development at Kongsberg Våpenfabrikk in Norway. Various successors have made good progress in the refinement of this mechanism. Owing to a configuration that keeps heat away from certain bearings the durability of the machine is improved while the radial turbine is well matched in speed requirement.

Scale jet engines

Also known as miniature gas turbines or micro-jets.

With this in mind the pioneer of modern Micro-Jets, Kurt Schreckling, produced one of the world's first Micro-Turbines, the FD3/67.<ref name="Schreckling, Gas Turbines for Model Aircraft" /> This engine can produce up to 22 newtons of thrust, and can be built by most mechanically minded people with basic engineering tools, such as a metal lathe.<ref name="Schreckling, Gas Turbines for Model Aircraft" />

Microturbines

Also known as:

Turbo alternators
MicroTurbine
Turbogenerator

Microturbines are becoming widespread for distributed power and combined heat and power applications. They are one of the most promising technologies for powering hybrid electric vehicles. They range from hand held units producing less than a kilowatt, to commercial sized systems that produce tens or hundreds of kilowatts. Basic principles of microturbine are based on micro combustion.

Part of their success is due to advances in electronics, which allows 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 the generator to be integrated with the turbine shaft, and to double as the starter motor.

Microturbine systems have many advantages over reciprocating engine generators, such as higher power-to-weight ratio, extremely low emissions and few, or just one, moving part. Advantages are that microturbines may be designed with foil bearings and air-cooling operating without lubricating oil, coolants or other hazardous materials. Microturbines also have a further advantage of having the majority of the waste heat contained in the relatively high temperature exhaust making it simpler to capture, whereas the waste heat of reciprocating engines is split between its exhaust and cooling system.<ref>Prime Movers in CHP - Steam Turbines, Gas Turbines, Reciprocating Engines, Spark Ignition</ref> However, reciprocating engine generators are quicker to respond to changes in output power requirement and are usually slightly more efficient, although the efficiency of microturbines is increasing. Microturbines also lose more efficiency at low power levels than reciprocating engines. When used in vehicles the static efficiency drawback is negated by the superior power-to-weight ratio - the vehicle does not have to move a heavy engine and transmission.

They accept most commercial fuels, such as gasoline, natural gas, propane, diesel, and kerosene as well as renewable fuels such as E85, biodiesel and biogas.

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, space 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%. When in a combined heat and power cogeneration system, efficiencies of greater than 80% are commonly achieved.

MIT started its millimeter size turbine engine project in the middle of the 1990s when Professor of Aeronautics and Astronautics Alan H. Epstein considered the possibility of creating a personal turbine which will be able to meet all the demands of a modern person's electrical needs, just like a large turbine can meet the electricity demands of a small city. Problems have occurred with heat dissipation and high-speed bearing in these new microturbines. Moreover, their expected efficiency is very low 5-6%. According to Professor Epstein current commercial Li-ion rechargeable batteries deliver about 120-150 Wh/kg. MIT's millimeter size turbine will deliver 500-700 Wh/kg in the near term, rising to 1200-1500 Wh/kg in the longer term.<ref>Engine on a Chip - TFOT</ref>

External combustion

Most gas turbines are internal combustion engines but it is also possible to manufacture an external combustion gas turbine which is, effectively, a turbine version of a hot air engine. Those systems are usually indicated as EFGT (Externally Fired Gas Turbine) or IFGT (Indirectly Fired Gas Turbine).

External combustion has been used for the purpose of using pulverized coal or finely ground biomass (such as sawdust) as a fuel. In the indirect system, a heat exchanger is used and only clean air with no combustion products travels through the power turbine. The thermal efficiency is lower in the indirect type of external combustion, however the turbine blades are not subjected to combustion products and much lower quality (and therefore cheaper) fuels are able to be used. Indirectly fired systems are now commercially available. BTOLA, an Australian based company is now marketing 250 kW - 2MW units.

Gas turbines in vehicles

Gas turbines are often used on ships, locomotives, helicopters, tanks, and to a lesser extent, on cars, buses, and motorcycles.

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 more important.

Gas turbines offer a high-powered engine in a very small and light package. However, they are not as responsive and efficient as small piston engines over the wide range of RPMs and powers needed in vehicle applications. In series hybrid vehicles, as the driving electric motors are mechanically detached from the electricity generating engine, the responsiveness problem is eliminated when using a gas turbine to turn the generator as it is run at the optimum speed. The emergence of the continuously variable transmission may also alleviate the responsiveness problem.

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 gas turbine engines are rarities; however, turbines are mass-produced in the closely related form of the turbocharger.

Passenger Road Vehicles (Cars, Bikes, and Buses)

A number of experiments have been conducted with gas turbine powered automobiles, the largest by Chrysler<ref>"History of Chrysler Corporation GAS TURBINE VEHICLES" published by the Engineering Section 1979</ref><ref>"Chrysler Corp., Exner Concept Cars 1940 to 1961" undated, retrieved on 2008-05-11.</ref>. More recently, there has been some interest in the use of turbine engines for hybrid electric cars. For instance, a consortium led by micro gas turbine company Bladon Jets has secured investment from the Technology Strategy Board to develop an Ultra Lightweight Range Extender (ULRE) for next generation electric vehicles. The objective of the consortium, which includes luxury car maker Jaguar Land Rover and leading electrical machine company SR Drives, is to produce the world’s first commercially viable - and environmentally friendly - gas turbine generator designed specifically for automotive applications.<ref name="bladon">BLADON JETS AND JAGUAR LAND ROVER WIN FUNDING FOR GAS TURBINE ELECTRIC VEHICLE PROJECT</ref>

The common turbocharger for gas or diesel engines is also a turbine derivative.

Concept Cars

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 (87 mph), 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 on display at the London Science Museum.

American car manufacturer Chrysler demonstrated several prototype gas turbine-powered cars from the early 1950s through the early 1980s. Chrysler built fifty Chrysler Turbine Cars in 1963 and conducted the only consumer trial of gas turbine-powered cars.<ref>Chrysler turbine information</ref> Their turbines employed unique rotating recuperator that significantly increased efficiency. Chrysler put many cars into the hands of consumers, making this effort a small-scale near-production run.

The original General Motors Firebird was a series of concept cars developed for the 1953, 1956 and 1959 Motorama auto shows, powered by gas turbines.

Toyota demonstrated several gas turbine powered concept cars such as the Century gas turbine hybrid in 1975, the Sports 800 Gas Turbine Hybrid in 1979 and the GTV in 1985. No production vehicles were made. The GT24 engine was exhibited in 1977 without a vehicle.

The fictional Batmobile is often said to be powered by a gas turbine or a jet engine. The 1960s television show vehicle was said to be powered by a turbine engine, with a parachute braking system. For the 1989 Batman film, the production department built a working turbine vehicle for the Batmobile prop.<ref>1989 Batmobile Turbine</ref> Its fuel capacity, however, was reportedly only enough for 15 seconds of use at a time.

In the early 1990s Volvo introduced the Volvo Environmental Concept Car which was a gas turbine powered hybrid car.<ref>Article in Green Car</ref>

In 1993 General Motors introduced the first commercial gas turbine powered hybrid vehicle—as a limited production run of the EV-1 series hybrid. A Williams International 40 kW turbine drove an alternator which powered the battery-electric powertrain. The turbine design included a recuperator. Later on in 2006 GM went into the EcoJet concept car project with Jay Leno.

At the 2010 Paris Motor Show Jaguar demonstrated its Jaguar C-X75 Concept car. This electrically powered supercar has a top speed of 204 mph (328 km/h) and can go from 0 to 62 mph (0 to 100 km/h) in 3.4 seconds. It uses Lithium-ion batteries to power 4 electric motors which combine to produce some 780 bhp. It will do around 100 miles on a single charge of the batteries but in addition it uses a pair of Bladon Micro Gas Turbines to re-charge the batteries extending the range to some 560 miles.<ref>http://www.automoblog.net/2010/10/01/the-electric-cat-jaguar-c-x75-concept-supercar/</ref>

Racing Cars

Rover and the BRM Formula One team joined forces to produce the Rover-BRM, 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/h) and had a top speed of 142 mph (229 km/h). American Ray Heppenstall joined Howmet Corporation and McKee Engineering together to develop their own gas turbine sports car in 1968, the Howmet TX, which ran several American and European events, including two wins, and also participated in the 1968 24 Hours of Le Mans. The cars used Continental gas turbines, which eventually set six FIA land speed records for turbine-powered cars.<ref>Template:Citeweb</ref>

For open wheel racing, 1967's revolutionary STP Oil Treatment Special four-wheel drive turbine-powered special fielded by racing and entrepreneurial legend Andy Granatelli and driven by Parnelli Jones nearly won the Indianapolis 500; the STP Pratt & Whitney powered turbine car was almost a lap ahead of the second place car when a gearbox bearing failed just three laps from the finish line. The next year the STP turbine car won the Indianapolis 500 pole position even though new rules restricted the air intake dramatically. In 1971 Lotus principal Colin Chapman introduced the Lotus 56B F1 car, powered by a Pratt & Whitney gas turbine. Chapman had a reputation of building radical championship-winning cars, but had to abandon the project because there were too many problems with turbo lag.

Buses

The arrival of the Capstone Microturbine has led to several hybrid bus designs, starting with HEV-1 by AVS of Chattanooga, Tennessee in 1999, and closely followed by Ebus and ISE Research in California, and DesignLine Corporation in New Zealand (and later the United States). AVS turbine hybrids were plagued with reliability and quality control problems, resulting in liquidation of AVS in 2003. The most successful design by Designline is now operated in 5 cities in 6 countries, with over 30 buses in operation worldwide, and order for several hundred being delivered to Baltimore , and NYC.

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 and better cooling of engine parts. On the emissions side, the challenge in technology is increasing turbine inlet temperature while reducing peak flame temperature to achieve lower NOx emissions to cope with the latest regulations. 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.

Advantages and disadvantages of gas turbine engines

<ref>how stuff works</ref>

Advantages of gas turbine engines

Very high power-to-weight ratio, compared to reciprocating engines;
Smaller than most reciprocating engines of the same power rating.
Moves in one direction only, with far less vibration than a reciprocating engine.
Fewer moving parts than reciprocating engines.
Low operating pressures.
High operation speeds.
Low lubricating oil cost and consumption.

Disadvantages of gas turbine engines

Cost
Less efficient than reciprocating engines at idle
Longer startup than reciprocating engines
Less responsive to changes in power demand compared to reciprocating engines

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