Microturbines

Illustration showing the operation of a recuperated microturbine.

Compressed air is mixed with fuel and burned in a combustor under constant pressure. The resulting hot gas is allowed to expand through a turbine to perform work. A sheet metal heat exchanger is used to recover some of the heat from the exhaust stream and transfer it to the incoming air stream. The preheated air is then used in the combustion process. The rest of the exhaust stream is passed through a waste-heat recovery device that captures more of the heat for use in industrial processes, space heating, and water heating.

Microturbines are small combustion turbines, approximately the size of a refrigerator, with outputs of 25-500 kilowatt (kW). They evolved from automotive and truck turbochargers, auxiliary power units for airplanes, and small jet engines and are composed of a compressor, a combustor, a turbine, an alternator, a recuperator, and a generator.

Microturbines offer a number of potential advantages over other technologies for small-scale power generation. These include their small number of moving parts, compact size, light weight, greater efficiency, lower emissions, lower electricity costs, and ability to use waste fuels. They can be located on sites with space limitations for the production of power, and waste heat recovery can be used to achieve efficiencies of more than 80%.

Turbines are classified by the physical arrangement of their component parts: single-shaft or two-shaft, simple-cycle or recuperated, inter-cooled, and reheat. The machines generally rotate more than 40,000 rotations per minute (rpm). Bearing selection, whether the manufacturer uses oil or air, is dependent on use. Single-shaft is the more common design because it is simpler and less expensive to build. Conversely, the split shaft is necessary for machine drive applications because it does not require an inverter to change the frequency of the AC power.

Microturbines can also be classified as simple-cycle or recuperated. In simple-cycle, or unrecuperated, turbines, compressed air is mixed with fuel and burned under constant pressure. The resulting hot gas is allowed to expand through a turbine to perform work. Simple-cycle microturbines have lower cost, higher reliability, and more heat available for cogeneration applications than recuperated units. Recuperated units use a sheet metal heat exchanger that recovers some of the heat from an exhaust stream and transfers it to the incoming air stream. The preheated air is then used in the combustion process. If the air is preheated, less fuel is necessary to raise its temperature to the required level at the turbine inlet. Recuperated units have a higher thermal-to-electric ratio than unrecuperated units and can produce 30%-40% fuel savings.

Advanced materials, such as ceramics and thermal barrier coatings, are key enabling technologies to further improve microturbines. Efficiency gains can be achieved with materials such as ceramics, which allow an increase in engine operating temperature.

For information about current DOE efforts in this technology area, please visit the Research and Development page.

For information about DOE accomplishments in this technology area over the last decade, please see Combined Heat and Power: A Decade of Progress, A Vision for the FuturePDF.