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To be presented at the IEEE Transducers ‘97 Conference, Chicago, IL, June 1997

POWER MEMS AND MICROENGINES

A.H. Epstein, S.D. Senturia, G. Anathasuresh, A. Ayon, K. Breuer, K-S Chen, F.E. Ehrich, G. Gauba, R. Ghodssi, C. Groshenry, S. Jacobson, J.H. Lang, C-C Lin, A. Mehra, J.O. Mur Miranda, S. Nagle,

D.J. Orr, E. Piekos, M.A. Schmidt, G. Shirley, S.M. Spearing, C.S. Tan, Y-S Tzeng, I.A. Waitz Rm. 31-265, Massachusetts Institute of Technology

Cambridge, MA 02139 USA, epstein@mit.edu

SUMMARY

MIT is developing a MEMS-based gas turbine genera- tor. Based on high speed rotating machinery, this 1 cm diameter by 3 mm thick SiC heat engine is designed to pro- duce 10-20 W of electric power while consuming 10 grams/ hr of H2. Later versions may produce up to 100 W using hydrocarbon fuels. The combustor is now operating and an 80 W micro-turbine has been fabricated and is being tested. This engine can be considered the first of a new class of MEMS device, power MEMS, which are heat en- gines operating at power densities similar to those of the best large scale devices made today.

Keywords: Heat engine, turbine, generator. INTRODUCTION

Currently, society finds it most economical to produce electrical power centrally on megascale (with turbines pro- ducing on the order of 100 MW) and then distribute it to individual users. Cooling is likewise centralized whenever possible (building-wide, complex-wide). MEMS may change this model. We propose herein a new class of MEMS devices, power MEMS, characterized by thermal, electri- cal, and mechanical power densities equivalent to those in the best large-sized machines produced today, thus produc- ing powers of 10 to 100 watts in sub-centimeter-sized pack- ages. Based on classical thermodynamic cycles (Brayton, Rankine, etc.), such devices would have significantly dif- ferent behavior than, but equivalent performance to, their more familiar full-sized embodiments. They could see widespread application as mobile power sources, propul- sion engines, and coolers. However, the realization of power MEMS presents new challenges both to micromachining and to the traditional mechanical and electrical engineer- ing disciplines of fluid dynamics, structural mechanics, bearings and rotor dynamics, combustion, and electric ma- chinery design. This paper delineates the technology chal- lenges of power MEMS, suggests promising approaches, and describes the work on-going at MIT in this area.

Thermodynamic Considerations

Thermal power systems encompass a multitude of tech- nical disciplines. The architecture of the overall system is determined by thermodynamics while the design of the system’s components is influenced by fluid and structural

mechanics, and by material, electrical and fabrication con- cerns. The physical constraints on the design of the me- chanical and electrical components are often different at microscale than at more familiar sizes so that the optimal component and system designs are different as well. Most thermodynamic systems in common use today are varia- tions of the Brayton (air), Rankine (vapor), Otto, or Diesel cycles. The Brayton power cycle (gas turbine) was selected for the initial investigation based on relative considerations of power density, simplicity of fabrication, ease of initial demonstration, ultimate efficiency, and thermal anisotropy.

A conventional, macroscopic gas turbine engine consists of a compressor, a combustion chamber, and a turbine (driven by the combustion exhaust) that powers the compressor, and can drive machinery such as an electric generator. The re- sidual enthalpy in the exhaust stream provides thrust. A macroscale gas turbine with a meter diameter air intake gen- erates power on the order of 100 MW. Thus, tens of watts would be produced when such a device is scaled to millimeter size if the power per unit of airflow is maintained. When based on rotating machinery, such power density requires (1) combustor exit temperatures of 1300-1700 K; (2) rotor pe- ripheral speeds of 300-600 m/s and thus rotating structures centrifugally stressed to several hundred MPa (the power den- sity of both fluid and electrical machines scales with the square of the speed, as does the rotor material centrifugal stress); low friction bearings; high geometric tolerances and tight clear- ances between rotating and static parts; and thermal isolation of the hot and cold sections.

These thermodynamic considerations are no different at micro- than at macroscale. But, the physics influencing the design of the components does change with scale, so that the optimal detailed designs can be quite different. Examples include the viscous forces in the fluid (larger at microscale), usable strength of materials (larger), surface area to volume ratios (larger), chemical reaction times (in- variant), realizable electric field strength (higher), and manufacturing constraints (planar geometries).

ENGINE DESIGN

There are many thermodynamic and architectural de- sign choices in a device as complex as a gas turbine en- gine. These involve trade-offs among fabrication difficulty, structural design, heat transfer, fluid mechanics, and elec-

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