Energy density would greatly exceed that of a typical battery system.

NASA's Jet Propulsion Laboratory, Pasadena, California

A proposed microelectromechanical system (MEMS) containing a closed-- Brayton-cycle turbine would serve as a prototype of electric-power generators for special applications in which high energy densities are required and in which, heretofore, batteries have been used. The system would have a volume of about 6 cm^sup 3^ and would operate with a thermal efficiency >30 percent, generating up to 50 W 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, as such, would be comparable to that of a fuel cell.

The working gas for the turbine would be Xe containing small quantities of CO^sub 2^, O^sub 2^, and H^sub 2^O as gaseous lubricants. The gas would be contained in an enclosed circulation system, within which the pressure would typically range between 5 and 50 atm (between 0.5 and 5 MPa). 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 novel heat-transfer and heat-management components. The turbine would be connected to an electric power generator/starter motor.

The system would 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 speed of 400,000 rpm or perhaps more, with bearing-wear rates less than 10^sup -4^x those of silicon bearings and 0.05 to 0.1x those of SiC bearings, and with a coefficient of friction about 0.1x that of Si or SiC bearings. The components of the system would be fabricated by a combination of (1) three-dimensional x-ray lithography and (2) highly 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, similarly to other combustionbased power sources.