Linear Alternator Technologies Used for Free Piston Stirling

Introduction: The NASA Glenn Research Center (GRC) is conducting in-house research on linear alternators (LA’s) to assist in developing free-piston Stirling convertors for radioisotope space power systems. This abstract intends to summarize the existing and potential types of LA’s that may be used with Stirling power convertors. Many of the existing LA types have been tested at NASA GRC and work continues in reseearching and potential testing of advanced concepts. Desirable Characteristics of Stirling Linear Alternators: Stirling technology is being developed for radioisotope power generation for future space science missions. With this aim, the design objectives for linear alternator (LA) advancement are to lower the mass of the LA and increase the lifetime and reliability of the LA at Stirling operating temperatures [1]. An increased efficiency is desirable if it lowers system mass. Other considerations include: 1) Reducing internal losses magnetic hysteresis, eddy currents, and flux leakage; 2.) Minimizing size, moving mass and cost; 3.) Increasing the ease and effectiveness of analysis and fabrication; 4.) Providing adequate cooling to temperature-sensitive components; and 5.) Addressing side-loads and bearing requirements. Types of Linear Alternators: Three primary types of LA’s are recognized [2]: 1. Moving permanent magnet, 2. Moving iron with permanent magnet(s) attached to the stator. 3. Moving coil with permanent magnet(s) attached to the stator, Moving coil and moving magnet styles frequenly have a lower moving mass than the moving iron style. Most free piston Stirling engines that use a linear alternator are relatively low power levels, and therefore use only a single phase. Twoand three-phase power generation is possible in a linear generator, but will not be covered in this context. Moving Magnet LA’s: Most Stirling LA’s are moving permanent magnet type, since this configuration offers the lowest moving mass. A common theme around which variations have been built is a hollow cylindrical magnet (radial polarity) oscillating amid an internal and external stator. The stator holds the cylindrically wound armature coil(s). Figure 1 shows an axial side-cut view which, when rotated about the axis of reciprocation, represents this style of alternator. Figure 1. Sunpower LA with Moving PM. Image courtesy of Sunpower, Inc. [3] Variants of this style have been used by Sunpower, Inc. for the Advanced Stirling Convertors (ASC’s) on test at NASA GRC, as well as Mechanical Technology Incorporated (MTI) and the National Aerospace Laboratory of Japan (NAL). The radial polarity magnet will generate alternating flux directions in the radial laminations of the stator at the ends of the stroke. An alternate style of moving magnet LA is the STAR alternator pioneered by Clever Fellows Innovation Consortium (CFIC), now Q-Drive. The mover has two layers of magnets with opposite polarity (only top layer is shown in Figure 2) which alternately activate the axially stacked laminations, offering flux reversal.