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th MME’02, The 13 Micromechanics Europe Workshop, October 6-8, 2002, Sinaia, Romania the frequency of the generator voltage, is controlled by varying the load. Figure 8 shows the electrical power measured for different supply pressures and speeds. At a pressure of 1 bar, the maximal electrical power is 16 W and is reached at a speed of 100,000 rpm. Measurements show that the air flow and input power depend only on the supply pressure and not on speed or load. Therefore, the input power is the same as in the mechanical test at 1 bar, i.e. 152 W. Figure 9 shows the total efficiency (compressed air to electricity) as a function of speed and for different supply pressures. The maximal total efficiency is 10.5 % and is reached at a speed of 100,000 rpm. VII. SANKEY DIAGRAM The energy flow and the different losses are illustrated in the Sankey diagram shown in figure 10. The diagram is generated for a supply pressure of 1 bar and a speed of 100,000 rpm. This corresponds to the working point at which the Expansion losses 15 W - 9.8 % Leak flow around rotor 4W-2.6% Obstruction losses 1 W - 0.7 % Incidence losses 2 W - 1.3 % Blade profile losses 48 W - 31.6 % Exit losses 52 W - 34.2 % Losses in coupling 2 W - 1.3 % Generator losses 10 W - 6.6 % Legend Fig. 10. Sankey diagram for a supply pressure of 1 bar and a speed of 100,000 rpm. maximal electrical power and maximal total efficiency are reached. Input power, mechanical power, electrical power and the combination of ventilation losses (6) and bearing friction (7) are measured values. This last value (6 + 7) is obtained with a deceleration test of the turbine without generator and without external load. The loss associated with the leak flow around the turbine wheel (2) and the exit losses (8) are calculated from the known air speeds. The expansion losses (1), incidence losses (4) and blade profile losses (5) are calculated using friction and loss coefficients known from large turbines and may be less accurate. The generator losses (10) are derived from the manufacturer’ s data sheets. The obstruction losses (3) and the losses in the coupling (9) are derived as the difference between the calculated and measured values. The major losses are the blade profile losses and the exit losses. The large blade profile losses can be explained by the increased friction in miniature systems (small channels and low Reynolds numbers). The high exit losses can be explained by the low u/c1 ratio (0.25 instead of 0.5 in the optimal case). Additionally, the turbine operates below its optimal speed because the ball bearings limit the speed. Both factors result in higher air speeds at the turbine exit, and thus higher exit losses. VIII. FUTURE WORK The first goal is to increase the efficiency of the turbine, mainly by decreasing the exit losses. This can be reached in two ways: introducing air bearings which allow much higher speeds, or decreasing the speed by using a multiple-stage design. In the long term, a compressor and a combustion chamber will be added to finally come to a microgenerator running on fuel. REFERENCES [1] L. Sitzki et al, Combustion in Microscale Heat- Recirculating Burners, Proc. Third Asia-Pacific Conf. on Combustion, 2001. [2] K. Fu et al, Design and Fabrication of a Silicon-Based MEMS Rotary Engine, Proc. 2001 International Mechanical Engineering Congress and Exposition (IMECE), 2001. [3] A. H. Epstein et al, Shirtbutton-Sized Gas Turbines: The Engineering Challenges of Micro High Speed Rotating Machinery, 8th Int. Symp. on Transport Phenomena and Dynamics of Rotating Machinery, 2000. Input power 152 W - 100 % Mechanical power 28 W - 18.4 % Electrical power 16 W - 10.5 % Ventilation losses + bearing friction 2 W - 1.3 % Measured Calculated Difference from other values 278PDF Image | A MICROTURBINE FOR ELECTRIC POWER GENERATION
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