Micropower generation with microgasturbines: A challenge

Abstract

This paper describes the development of a microgasturbine with a rotor diameter of 20 mm. The target electrical power output lies around 1 kW. The total system fits in a cylinder with a diameter of 95 mm and a length of 120 mm. The system contains the same components as a large gasturbine generator: compressor, recuperator, combustion chamber, turbine, and electrical generator. Major challenges are the high rotational speed (500 000 r/min), high turbine inlet temperature (1200 K), and the efficiency of the components. Because of the small dimensions, the flow through compressor and turbine is characterized by relatively low Reynolds numbers. The higher flow losses and inherently lower efficiency require a higher blade tip speed (524 m/s) than for large turbines (300-400 m/s). To minimize wear and frictional losses, the rotor is mounted on aerodynamic bearings. To withstand the high centrifugal stresses, a high-strength steel is used for compressor and shaft. The turbine is made of a Si3N4-TiN ceramic composite to withstand the combination of elevated stress and temperature.

Impact of heat transfer on the performance of micro gas turbines

“…A number of materials have been proposed for micro gas turbines, ranging from steel alloys to ceramics such as silicon nitride or silicon carbide [4,41,31]. As heat transfer through the housing and shaft is found to be moderately influential for the baseline material selection, a trade-off study is performed for a range of materials, as shown in Table 1.…”

Precision manufacturing of key components for an ultra miniature gas turbine unit for power generation

“…An exhaust diffuser is added to create a sub-ambient pressure at the turbine exit, such that more power can be extracted. Though certain problems are introduced due to this design comparing to the use of two separated shafts (a high-speed compressor shaft plus a low-speed generator shaft), the single shaft design is still preferable due to its simplicity (Peirs et al 2007). …”

“…But the physics and mechanics influencing the design of the components do change with scale, so that the optimal detailed designs can be quite different (Peirs et al 2007). The main difference between a small and large gas turbine is the amount of fluid submitted to an almost unchanged thermodynamic cycle.…”

“…The maximum TIT is set by material limits to 1,200 K. The nominal shaft speed is set to 500,000 rpm as models predicted that with the given compressor diameter, a pressure ratio of 3 is achievable. Below this value, efficiency drops sharply, while higher values offer smaller efficiency improvements (Peirs et al 2007). When running at maximum rotation speed and with nominal mass flow of 20 g/s, the overall cycle efficiency when applying the recuperator is around 20% and total output is around 1 kW without taking into account bearing friction, thermal loses, pressure drops, and electrical losses.…”

Experimental characterization of an electrospindle with gas bearings

“…An example of application of such bearings are the smart electrospindles for very high speed [1][2][3], in which the spindle must be supported in stable conditions up to 200-250 krpm [4]. Other application sectors are micro-turbomachines [5] and movable electric appliances [6].…”