Improved thermoelectric converter units (TCUs) and radioisotope thermoelectric generators (RTGs) that contain them have been undergoing development for use as small, lightweight sources of electricity at potentials up to 5 V and power at levels up to 40 mW. These RTGs are intended primarily for supplying power to operate electronic equipment in outer space or at remote or uninhabitable locations on Earth; terrestrial applications could include monitoring of nuclear-waste-storage facilities, meteorological monitoring at polar locations, deep sea exploration, and monitoring of geological activity inside volcanic craters and at underground locations.
In general, a thermoelectric generator includes a TCU plus a source of heat. In the case of an RTG, the source of heat is a radioisotope heater unit (RHU). The present RTG design is derived partly from the design of 75-mW radioisotope power systems that were built for the United States government in the late 1970s and early 1980s. The present design also incorporates some of the concepts reported in "Miniature Radioisotope Power Source" (NPO-19339), NASA Tech Briefs, Vol. 19, No. 9 (September 1995), page 60. The RHU in the present design generates thermal power of 1 W and is of a type that has been proven in use aboard spacecraft to warm instruments that are sensitive to cold.
Alternatively, the improved TCUs could be energized by nonradioactive heat sources, including sources of waste heat (e.g., household appliances, power tools, camping equipment), small burners (similar to cigarette lighters) that burn hydrocarbon fuels, or any of a variety of other sources that are inexpensive and readily available. Power supplies based on this concept could be used to charge batteries or to operate low-power electronic equipment under emergency or outdoor conditions, for example. These power supplies offer an important additional advantage in that they could function over wide temperature ranges, including temperatures both above and below the operational temperature ranges of chemical batteries. The improved TCUs could also be used as heat pumps or coolers, especially in electronic equipment; in comparison with conventional thermoelectric coolers, these TCUs would be more durable and could be made smaller.
A TCU is a thermopile made from state-of-the-art thermoelectric materials — in this case, p- and n-doped, vacuum-hot-pressed bismuth telluride. The present developmental TCU is fabricated as a module (see Figure 1) that contains an 18-by-18 square array of alternating p- and n-doped pieces of bismuth telluride separated by pieces of polyimide film. Each element of the array is a square 0.015 in. (0.38 mm) wide. The overall dimensions of the module are 0.291 by 0.291 by 0.9 in. (7.4 by 7.4 by 22.9 mm). The elements of the array are electrically connected in series by gold contact strips on the hot and cold sides. The design hot and cold junction temperatures are 250 and 25 °C, respectively.
Fabrication of the module involves a process of stacking, cutting, and restacking pieces of the p- and n-doped thermoelectric material and polyimide films, followed by placing the final stack in alignment tooling and vacuum hot pressing the final stack to bond the pieces together. Accurate alignment and proper bonding of the array elements in the stack are prerequisite for the next step, in which the gold contact strips are applied by a photo-masking/deposition process used commonly in the electronics industry.
Figure 2 is a simplified cross section of a proposed RTG that would incorporate the developmental TCU. The RHU capsule would be contained in an aluminum capsule holder, which would be mounted in contact with the TCU and pressed against the TCU by tension in four spring-loaded titanium tie wires. The RHU capsule would be spring-loaded to keep it stationary within the holder in the presence of shock and vibration and to minimize thermal resistance between the RHU and the TCU. A thermally conductive electric insulator (made of boron nitride) would be placed between the capsule holder and the hot side of the TCU to prevent electrical short-circuiting of the TCU. Thermal insulation for the hot parts of the RTG would comprise multiple layers of aluminized polyimide film interspersed with layers of ceramic paper. The interior of the RTG could be either evacuated or else filled with xenon.
This work was done by John C. Bass of Hi-Z Technology, Inc., and Alex Borshchevsky of NASA's Jet Propulsion Laboratory for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4 – 8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16556.