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Microturbines 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 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 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. Microturbines also have the advantage of having the majority of their waste heat contained in their relatively high temperature exhaust, whereas the waste heat of recriprocating engines is split between its exhaust and cooling system. [2] 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.
They accept most commercial fuels, such as natural gas, propane, diesel and kerosene. They are also able to produce renewable energy when fueled with biogas from landfills and sewage treatment plants.
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, 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 1990’s 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. According to Professor Epstein current commercial Li-ion rechargeable batteries deliver about 120-150 w-hr/kg. MIT’s millimeter size turbine will deliver 500-700 whr/kg in the near term, rising to 1200-1500 whr/kg in the longer term.
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