Testing of Milliwatt Power Source Components

A milliwatt power s~urcc (MPS) has been developed to satisfy the rcquircments of scvcral potcntial solar system cxploration missions. Thc MPS is a small power source consisting of thrce major componcnts: a spacc qualified heat same (RHU), a thermopile (thermoelectric convcncr or TEC) and a containcc to dircct the RHU heat to the TEC. Thermopiles from Hi-Z Technology, Inc. of San Diego and thc Institute of Thermoclcclrieity of Chemivtsi Ukraine suitable for the MPS were tested and shown to pcrform as cxpcctcd. producing 40 mW of power with a temperature-differcncc of about 170°C. Such lhmopiles were succcssfully life tcstcd for up to a year. A MI'S container designed and built by Swales Aerospace was tested with both a TEC simulator and actual TEC. Thc Swalcs unit, testedunder dynamic vacuum. provided less temperamm diffcrcncc than anticipated. such that the TEC produccd 20 mW of power.with heat input equivalent to a RHU. INTRODUCTION, Several potential.solar system exploration missions are being developedthat require a power: source smaller than tlie current redioisotope thennoelectric generatorCRTG) or planned advanced radioisotope power source (ARF'S). Several systems have been proposed utilizing the standard radio isotope heater unit (RHU) as a heat source. The RHU was developed by DOE for the Galileo and Ulysses missions to maintain specific spacecraft components within normal operating temperatures. The RHU provides 1 W of heat and can be expected to produce about 40 mW of electrical power using a thermoelectric generator. Using a MPS instead of only a RHU will provide electrical power in addition to 96% of the RHU heat. These electrical power systems, generally called milliwatt power sources (MPS) have also been referred to as the "powerstick concept (Chmielewski, 1994). Various MPS systems have been developed at tested in Russia for terrestrial and space applications (Pustovalov, 1999). The MPS is designed to be used in a small planetary rover, small communication transmitter, microspacecraft or an autonomous science package such as a remote weather station, that either uses.coutinuous power at a level of a fraction of a watt or can operate intermittently by using energy from a battery that is continuously charged by a M P S . For example, the proposed Mars Network missions (e.g. PASCAL) could take advantage of MPS's for their mini-meteorological stations. The MPS is designed to produce enough continuous power to operate a sequencer while trickle charging the batteries. The battery power could then be used every couple of weeks 0-7803~7683~81021$17.00 '2002 IEEE 463 21st International Conference on Thermoelectronics (2002) to operate the instruments and transmit the data. Without such a power source, it will be difficult to power such missions for the nearly 20 year duration, particularly in the polar regions. Milliwatt Power Source Description The MPS contains three essential components: the RHU heat source, the thermoelectric converter (TEC), and container. The M P S will probably also require a rechargeable battery. The RHU heat source supplies heat to the thermoelectric converter or thermopile, which produces electricity used to charge the lithium ion battery. The container is essential in that it directs the heat from the RHU into the TEC. The waste heat from the RHU is used to keep the battery warm, as well as other spacecraft components. The energy stored in the battery can be used periodically to operate a science instrument. A small amount of power is also available continuously from the thermopile. Radioisotope Heater Unit (RHU) The RHU, or specifically the light weight RHU, is a standard heat source produced by DOE and designed to provide 1.1 W of thermal power (Tate, 1982) at the start of mission. The unit is a cylinder 32 nun high, 26 mm in diameter and weighs about 40 g. In the unit, the pelletized h238 fuel is surrounded by platinum-alloy capsule, pyrolytic graphite thermal insulation and high technology graphite ablation shell. Over a hundred of these units are used in space such as on the Galileo and Cassini spacecrafts. The PU-238 fuel has a half life of 88 years, providing power for decades as evidenced by the continuing operation of the Voyager spacecrafts. The remaining heat from the RHU in the MF'S is used to keep the battery warm. The electrical imitators of RHU were designed and built at JPL and are used in place if actual RHU's for testing. Thermoelecrric Converter (TEC) or Thermopile The thermoelectric converter, also called thermoelectric module or thermopile has been developed by Hi-Z Technology, Inc. (Elmer, 1999). It uses Bi,Te,-based materials, both nand p-type. The materials are vacuum hot-pressed to provide the strength and machining characteristics required to fabricate the long thin legs for the module. The module consists of 18x18 array of legs. Each leg is 0.381 nun on a side and 22.86 nun long (Fig. 1). Such a solid array should have superior strength and excellent shock and vibration characteristics. A 0.025 mm thick layer of insulating film is used within the interior of the module to electrically separate the legs. The module is expected to have a peak power output of at least 40 mW in a generator using the 1 W RHU as a heat source. The matched load voltage will be 5 volts. The weight of the module is ahout 7 g. The Institute of Thermoelectricity in Chemivtsi, Ukraine is also capable of making such thermopiles using a similar fabrication process. Their designation of this thermopile design is Altec 1050. 464 21st International Conference on Thermoelectronics (2002) FIGURE 1. Thermoclcclric modulc devclopcd by Hi-Z. LcR: Sidc vicw showing apparcnl crack and allached thcrmocauplc, Right: cold end showing inicrconnccu One Hi-Z thermopile was delivered to JPL for testing and is shown in Figure 1. The crack visible in figure I makes that particular thermoelement loose without causing an open circuit during testing as described below. The individual thermoelectric elements and gold inte rconnects are clearly visible FIGURE 2. Thermaelcclric module dcvclopcd by Insfme of Thcrmoclcctncily in Chemivui, Ukramc. Figurc also shows 0.3" thick heaterlthermometcr and TEC simulator above thermopile, and part of A1 fail shielding Five thermopiles were purchased from the Institute of Thermoelectricity in Chemivtsi, Ukraine (designated Ukraine 1 to 5) . These thermopiles are presumably fabricated in a very similar manner, but also contain a black substrate insulator on the hot and cold surfaces as well as a protective coating on the sides (Figure 2). 465 21st international Conference on Thermoelectronics (2002) MPS Container Several versions of the mechanical and thermal design of the M P S have been considered (Borshchevsky, 1997). The Hi-Z design uses aluminized Kapton multi-foil insulation which can operate in vacuum if moisture is properly removed or with a xenon cover gas. The electrical output in vacuum is 40mW while with Xenon it is less than half as much. Spring loaded titanium tie wires are used to hold RHU capsule holder firmly against the TEC both for good thermal contact and to help withstand high shock and vibration loads. A temperature difference of 225 "C was obtained with 1.1W heat source resulting in 40mW electrical output. The Hi-2 conti r has been impact tested to loo0 g's (Allen, 2001). Cover, RHU con'ister