page_title: Microturbine
section_title: Design
hierarchical_section_title: Microturbine / Design
context_page_description: Microturbines are 25 to 500 kilowatt gas turbines evolved from piston engine turbochargers, aircraft auxiliary power units or small jet engines, the size of a refrigerator.
  Early turbines of 30-70 kW grew to 200-250 kW.
context_section_description: They comprise a compressor, combustor, turbine and electric generator on a single shaft or two.
  They can have a recuperator capturing waste heat to improve the compressor efficiency, an intercooler and reheat.
  They rotate at over 40,000 RPM and a common single shaft microturbine rotate usually at 90,000 to 120,000 RPM.
  They often have a single stage radial compressor and a single stage radial turbine.
  Recuperators are difficult to design and manufacture because they operate under high pressure and temperature differentials.
  Advances in electronics allows unattended operation and electronic power switching technology eliminates the need for the generator to be synchronised with the power grid, allowing it to be integrated with the turbine shaft and to double as the starter motor.
  Gas turbines accept most commercial fuels, such as petrol, natural gas, propane, diesel fuel, and kerosene as well as renewable fuels such as E85, biodiesel and biogas.
  Starting on kerosene or diesel can require a more volatile product such as propane gas.
  Microturbines can use micro-combustion.
  Full-size gas turbines often use ball bearings.
  The 1000 °C temperatures and high speeds of microturbines make oil lubrication and ball bearings impractical; they require air bearings or possibly magnetic bearings.
  They may be designed with foil bearings and air-cooling operating without lubricating oil, coolants or other hazardous materials.
  To maximize part-load efficiency, multiple turbines can be started or stopped as needed in an integrated system.Reciprocating engines can react quickly to power requirement changes while microturbines lose more efficiency at low power levels.
  They can have a higher power-to-weight ratio than  piston engines, low emissions and few, or just one, moving part.
  Reciprocating engines can be more efficient, be cheaper overall and typically use simple journal bearings lubricated by motor oil.
  Microturbines can be used for cogeneration and distributed generation as turbo alternators or turbogenerators, or to power hybrid electric vehicles. 
  The majority of the waste heat is contained in the relatively high temperature exhaust making it simpler to capture, while reciprocating engines waste heat is split between its exhaust and cooling system.
  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.
caption: Cutaway of a recuperated microturbine