Gas Turbines

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 element.)

Energy is released when air is mixed with fuel and ignited in the combustor. Combustion increases the temperature, which in turn increases the pressure (due to ideal gas law) resulting in an increase in the velocity and volume of the gas flow (see gas laws). This is directed through a 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, generators, and even tanks.


Gas turbines for electrical power production
Industrial gas turbines range in size from truck-mounted mobile plants to enormous, complex systems.

The power turbines in the largest industrial gas turbines operate at 3,000 or 3,600 rpm to match the AC power grid frequency and to avoid the need for a reduction gearbox. Such engines require a dedicated building.

They can be particularly efficient — up to 60% — when waste heat from the gas turbine is recovered by a conventional steam turbine in a combined cycle configuration. They can also be run in a cogeneration configuration, where the exhaust is captured to heat steam which is then used to heat buildings or to run airconditioners through a steam turbine.

Simple cycle gas turbines in the power industry require smaller capital investment than combined cycle gas, coal or nuclear plants and can be designed to generate small or large amounts of power. Also, the actual construction process can take as little as several weeks to a few months, compared to years for baseload plants. Their other 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 % thermal efficiency.


Scale jet engines (micro turbines)

Also known as:

* Minature Gas Turbines
* Micro-jets
Many model engineers relish the challenge of re-creating the grand engineering feats of today as tiny working models. Naturally, the idea of re-creating a powerful engine such as the jet, fascinated hobbyists since the very first full size engines were powered up by Hans von Ohain and Frank Whittle back in the 1930s.

Re-creating machines such as engines to a different scale is not easy. The laws of physics governing the behaviour of many machines do not always scale up or down at the same rate as the machine's size (and often not even in a linear way), usually at best causing a dramatic loss of power or efficiency, and at worst causing them not to work at all. An automobile engine, for example, will not work if re-produced in the same shape at the size of a human hand.

With this in mind the pioneer of modern Micro-Jets Kurt Schreckling, produced one of the world's first Micro-Turbines, the FD3/67. This amazing little engine can give out 22 newtons of thrust, and can be built by most mechanically minded people with basic engineering tools, such as a metal lathe.
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Micro turbines

Also known as:

* Turbo alternators
* Gensets
* MicroTurbine® (registered trademark of Capstone Turbine Corporation)
* Turbogenerator® (registered tradename of Honeywell Power Systems, Inc.)

Micro turbines 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.

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.

Micro turbine 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. However, piston engine generators are quicker to respond to changes in output power requirement.

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.

Micro turbine 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 micro turbine efficiencies are 25 to 35 %. When in a combined heat and power cogeneration system, efficiencies of greater than 80 % are commonly achieved.
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Auxiliary power units

Auxiliary power units (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.)