Microturbines are small combustion turbines approximately the size of a refrigerator with outputs of 25 kW to 500 kW. They evolved from automotive and truck turbochargers, auxiliary power units for airplanes, and small jet engines and are comprised of a compressor, combustor, turbine, alternator, recuperator, and generator.
Microturbines offer a number of potential advantages compared to other technologies for small-scale power generation. These advantages include a small number of moving parts, compact size, light-weight, greater efficiency, lower emissions, lower electricity costs, and opportunities to utilize waste fuels. They have the potential to be located on sites with space limitations for the production of power. Waste heat recovery can be used with these systems to achieve efficiencies greater than 80%.
Turbines are classified by the physical arrangement of the component parts: single shaft or two-shaft, simple cycle or recuperated, inter-cooled, and reheat. The machines generally rotate over 40,000 rpm. Bearing selection, whether the manufacturer uses oil or air, is dependent on usage. A single shaft is the more common design as it is simpler and less expensive to build. Conversely, the split shaft is necessary for machine drive applications, which does not require an inverter to change the frequency of the AC power.
Microturbines can also be classified as simple cycle or recuperated. In a simple cycle, or unrecuperated, turbine, compressed air is mixed with fuel and burned under constant pressure conditions. The resulting hot gas is allowed to expand through a turbine to perform work. Simple cycle microturbines have a 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 transfer 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, in addition, can produce 30-40 percent fuel savings from preheating.
Advanced materials, such as ceramics and thermal barrier coatings, are some of the key enabling technologies to further improve microturbines. Efficiency gains can be achieved with materials like ceramics, which allow a significant increase in engine operating temperature.
Because of their compact size, relatively low capital costs, and expected low operations and maintenance costs, automatic electronic control, microturbines are expected to capture a significant share of the distributed generation market.