The NASA Jet Propulsion Laboratory (Pasadena, CA) announced a proposed micro-electromechanical system containing a closed Brayton cycle turbine that would serve as a prototype of electric-power generators for special applications in which high energy densities are required and, in which, batteries have been used. The system would have a volume of about 6cm³ and would operate with a thermal efficiency >30 percent, generating up to 50W of electrical power. The energy density of the proposed system would be about 10 times that of the best battery-based systems now available, and would be comparable to that of a fuel cell.
The working gas for the turbine would be Xe containing small quantities of CO², O², and H²O as gaseous lubricants. The gas would be contained in an enclosed circulation system, within which the pressure would typically range between 5ATM and 50ATM (between 0.5MPa and 5MPa). The heat for the Brayton cycle could be supplied by any of a number of sources, including a solar concentrator or a combustor burning a hydrocarbon or other fuel. The system would include heat-transfer and heat-management components. The turbine would be connected to an electric power generator/starter motor.
The system would also include a main rotor shaft with gas bearings; the bearing surfaces would be made of a ceramic material coated with nanocrystalline diamond. The shaft could withstand speeds of 400,000rpm or more, with bearing-wear rates less than 10(-4) times those of silicon bearings and 0.05 to 0.1 times those of SiC bearings, and with a coefficient of friction about 0.1 times that of Si or SiC bearings. The components of the system would be fabricated by a combination of three-dimensional x-ray lithography and precise injection molding of diamond-compatible metals and ceramic materials. The materials and fabrication techniques would be suitable for mass production.
The disadvantages of the proposed system are that unlike a battery-based system, it could generate a perceptible amount of sound, and, if it were to burn fuel, then it would also generate exhaust, similar to other combustion-based power sources.