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The main challenges in the electromagnetic design are the losses due to the high frequency in the stator core and winding. They are reduced with choosing an appropriate magnetic material and a litz wire winding. 6.2. Power and Control Electronics The power and control electronics need to be able to start the turbine and then switch to generator mode and rectify the generator currents and voltages to a dc output. This is realized with a power stage of six MOSFETs and a controller implemented on a digital signal processor. Since the fundamental frequency already is 8.3 kHz the requirements on current measurement and control are extremely high and the switching frequency exceeds 100 kHz. In addition, the electronics should be small and lightweight. 6.3. Realized Hardware A test bench setup has been realized including all the parts of the electrical system. The individual parts are shown in Figure 7. The weight of the electronics is approximately 20 g and the stator weighs 25 g. 7. SIMULATION TOOLBOX The achievable performance and efficiency of very small thermal machines depends strongly on a good temperature management of the system: Short distances between hot and cold parts with little space for insulation result in high heat flows between parts, smaller pipes have a higher surface-to- volume ratio which increases the heat exchange between parts and the gas flow. In order to get an estimate of the performance of such machines a Simulink library for the simulation of small thermal machines has been developed. The library consists of configurable model blocks for the all basic machine parts (pipes, turbines, shafts, insulation layers, heat exchangers etc.) with connectors for all relevant energy and mass flows. With these components it is possible to easily create Simulink models of different machine designs and permits to quickly estimate the effect of changed component parameters (like changed part dimensions or materials etc.) on the performance of the whole system. Figure 8 shows the temperature distribution in the shaft of a miniaturized single shaft gas turbine machine model for two cases. The only difference between cases 1 and 2 is a change in the machine layout near the compressor which leads to a higher heat flow from the hot shaft (and hot piping) into the gas flow near the compressor. Apart from the welcome effect of a lower shaft temperature this also results in a 20% reduction of the overall system efficiency due to the changed operation points of the compressor and the turbine. Most of the heat is brought into the shaft in the turbine although the losses generated in the bearings and the generator has some influence as well. 8. SUMMARY The design of a mesoscale gas turbine generator imposes significant challenges to all the disciplines involved. Figure 7. and assembled high-speed ball bearings, stator with three phase winding, power and control electronics. Rotor including two permanent-magnets 1000 900 Shaft Temperature Case 1 800 Case 2 700 600 500 400 300 Figure 8. shaft for two slightly different machine layouts. Simulated temperature distribution in the Particularly the small size and the high-speed operation require a special thermal, mechanical and electrical design and demand for ceramic materials and catalytic combustion. An existing turbine and compressor are downscaled and the electrical system is analyzed on a test bench. REFERENCES [1] S. A. Jacobson and A. H. Epstein, “An informal survey of power mems,” ISMME2003, Tsuchiura, Japan, December 1-3, 2003, pp. 513- 520. [2] M. Reinke, J. Mantzaras, R. Bombach, S. Schenker, A. Inauen, “Gas phase chemistry in catalytic combustion of methane/air mixtures over platinum at pressures of 1 to 16 bar,” Combustion and flame 141, 2005), pp. 448-468. [3] M. Schleer and R. S. Abhari, “Influence of geometric scaling on the stability and range of a turbocharger centrifugal compressor,” ASME Turbo Expo 2005, GT 2005-68713. [4] T. Fluri, “Experimental validation of a single stage axial turbine design for low Reynolds Numbers” ETH Zürich Diploma Thesis, 2003. [5] L. Guzzella, “Introduction to modeling and control of internal combustion engine systems,” Springer, 2004. [6] L. Ljung, “Asymptotic behaviour of the extended kalman filter as a parameter estimator for linear systems,” IEEE Transactions on Automatic Control, Vol. AC-24, No. 1, 1979, pp. 36-50. Temperature [K] T Generator T Shaft 1 T Bearing 1 T Shaft 2 T Compressor Rotor T Shaft 3 T Bearing 2 T Shaft 4 T Turbine RotorPDF Image | Ultra-High-Energy-Density Converter for Portable Power
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