29-03-2012, 03:54 PM
micro fabricated gas turbine
4. Main body Micro Fabricated Gas Turbine.docx (Size: 70.11 KB / Downloads: 51)
1. INTRODUCTION
Micro heat engines employing gas turbine components have recently been introduced as a potential means for generating power. The increased power density resulting from the cube-square law, combined with the higher strength-to-density ratio of silicon micro- structures makes such devices an attractive prospect. The realization of power MEMS however, poses new fabrication challenges. While preliminary results do indeed suggest their manufacturability, the requirement for complex uid channels and high aspect ratio structures is complicated by the high temperature operating environment of the engine. The presence of a combustion chamber to convert the chemical energy of a fuel into fluid thermal and kinetic energy necessitates gas temperatures close to the melting point of silicon. Thus, the ability of silicon structures to withstand these working conditions needs to be investigated.
As part of the MIT microengine project an effort is currently underway to develop a micro gas turbine engine capable of producing 10-50W of electrical power in a package less than one cubic centimeter in volume. This paper concentrates on the fabrication aspects of a hydrogen combustor for this engine. The primary contribution of this work is the microfabrication and testing of a device that is capable of running at gas temperatures several hundred degrees above the melting point of silicon, while providing power densities higher than those achieved by large-scale combustors used for power generation or aircraft engine applications. We first begin with a brief review of silicon power MEMS previously reported in the literature. The fabrication and testing of the hydrogen micro combustor is presented next.
2. LITERATURE SURVEY
The earliest example of jet propulsion can be traced as far back as 150 BC to an Egyptian named Hero. Hero invented a toy that rotated on top of a boiling pot due to the reaction effect of hot air or steam exiting several nozzles arranged radially around a wheel. He called this invention an aeolipile.
Around 1500 A.D. Leonardo da Vinci drew a sketch of a device that rotated due to the effect of hot gasses flowing up a chimney. The device was intended to be used to rotate meat being roasted. In 1629 another Italian named Giovanni Branca actually developed a device that used jets of steam to rotate a turbine that in turn was used to operate machinery. This was the first practical application of a steam turbine.
3. THEORY
Microturbines are small combustion turbines approximately the size of a refrigerator with outputs of 25kw to 500kw. 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 a refrigerator. The engine itself is about the size of a beer keg. The most popular models have just one moving part, a shaft with a turbine wheel on one end, a permanent magnet generator on other end, and an air compressor wheel in the middle. This assembly rotates at up to 96,000 rpm. At that speed, traditional oil-lubricated bearings are severely challenged. Accordingly the most popular micro turbine engines use air bearing to float the shaft.
Not only is the turbine turning at high rpm, so is the generator. The generator in turn produces a high frequency electrical output, which is then converted by power electronics unit to grid compatible 400-to-480-volts alternating current, 10-to-60 hertz, 3phase power.
METHODOLOGY
The micro combustor utilises the design of a static gas turbine engine, which means that the combustor has almost the same configuration as the gas turbine engine but without the rotating blades. The designed combustor is fabricated via deep reactive ion etching (DRIE) process; its performance has been investigated via numerical simulation and evaluated via experimental characterisations.
ADVANTAGES
• Higher power density.
• Possibility of a redundant system with increased reliability.
• Small number of moving parts
• Compact size
• Lightweight
• Good efficiencies in cogeneration
• Low emission
• Can utilize waste fuels
• Long maintenance interval
• No vibration
• Less noise than reciprocating engines
• Low weight per unit of power
• Small number of moving parts and thus low maintenance costs
• Low use of lubricating oil or minimal for the case of air bearings
• Multi fuel capabilities and lower emissions.
3.8 DISADVANTAGES
• High overall temperature of the system.
• Low fuel to electricity efficiencies and decreases even more at partial load.
• Loss of power output and efficiency with higher ambient temperature and elevation.
• For some turbines the combustion of very low calorific value gases may not be feasible.
5. CONCLUSION
Microturbines are gas turbines with a power ranging approximately from 10 to 200 kW suitable for the distributed generation of power and heat. Microturbines are compact in size, can burn a variety of fuels, produce low emissions, generate low noise and have low maintenance requirements. On the other hand they have low power-only efficiency, their cost per kW installed is high while at the same time the distributed generation market is still not well developed. The cost of microturbine-based systems for stationary power applications is considerably higher than competing systems commonly based on internal combustion engines.