Microturbine

A microturbine (MT) is a small Gas Turbine with similar cycles and components to a heavy gas turbine. The MT power-to-weight ratio is better than a heavy gas turbine because the reduction of turbine diameters causes an increase in shaft rotational speed. Heavy gas turbine generators are too large and too expensive for distributed power applications, so MTs are developed for small-scale power like electrical power generation alone or as combined cooling, heating, and power (CCHP) systems. The MT are 25 to 500 kilowatt gas turbines evolved from piston engine turbochargers, aircraft auxiliary power units (APU) or small jet engines, the size of a refrigerator. Early turbines of 30-70 kW grew to 200-250 kW. 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.