Auxiliary power units

Types of gas turbine engines

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 from the direct impulse of exhaust gases are often called turbojets, whereas those that generate thrust with the addition 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.

Turboprop engines

A turboprop engine is a type of turbine engine which drives an external aircraft propeller using a reduction gear. Turboprop engines are generally used on small subsonic aircraft, but some large military and civil aircraft, such as the Airbus A400M, Lockheed L-188 Electraand Tupolev Tu-95, have also used turboprop power.

Aeroderivative gas turbines

Aeroderivatives are also used in electrical power generation due to their ability to be shut down, and handle load changes more quickly than industrial machines. They are also used in the marine industry to reduce weight. The General Electric LM2500, General Electric LM6000.Rolls-Royce RB211 and Rolls-Royce Avon are 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 scrap yard sales, then operated for display as part of the hobby of engine collecting. 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.

More sophisticated turbojets are also built, where their thrust and light weight are sufficient to power large model aircraft. The Schreckling design constructs the entire engine from raw materials, including the fabrication of a centrifugal compressor wheel from plywood, epoxy and wrapped carbon fibre strands.

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.

Auxiliary power units

APUs are small gas turbines designed to supply auxiliary power to larger, mobile, machines such as an aircraft. They supply:

• compressed air for air conditioning and ventilation,

• compressed air start-up power for larger jet engines,

• mechanical (shaft) power to a gearbox to drive shafted accessories or to start large jet engines, and

• electrical, hydraulic and other power-transmission sources to consuming devices remote from the APU.

Industrial gas turbines for power generation Industrial gas turbines differ from aeronautical designs in that the frames, bearings, and blading are of heavier construction. They are also much more closely integrated with the devices they power — often an electric generator — and the secondary-energy equipment that is used to recover residual energy (largely heat).

They range in size from man-portable mobile plants to enormous, complex systems weighing more than a hundred tonnes housed in block-sized buildings. When the turbine is used solely for shaft power, its thermal efficiency is around the 30% mark. This may cause a problem in which it is cheaper to buy electricity than to burn fuel. Therefore many engines are used in CHP (Combined Heat and Power) configurations that can be small enough to be integrated into portable container configurations.

Gas turbines can be particularly efficient - up to at least 60%— when waste heat from the turbine is recovered by a heat recovery steam generator to power a conventional steam turbine in a combined cycle configuration. 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 the inlet air and increase the power output. technology known as Turbine Inlet Air Cooling.

Another significant advantage is their ability to be turned on and off within minutes, supplying power during peak, or unscheduled, 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 with low fuel costs, a gas turbine power plant may regularly operate most hours of the day. A large single-cycle gas turbine typically produces 100 to 400 megawatts of electric power and has 35-40% thermal efficiency.

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