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The traditional means to satisfy electrical power requirements for outer planetary space probes is through Radioisotope Thermoelectric Generators (RTGs). Advanced radioisotope systems, using higher efficiency thermal conversion, are being developed to reduce the required plutonium inventory to provide both cost and safety benefits. While reducing the amount of plutonium reduces the health risk associated with an accidental orbital reentry and provides substantial system cost savings, it would be desirable to have a non-nuclear option for deep space missions. However, typical planar photovoltaic (PV) arrays are not effective for space probes traveling beyond Mars (1.5 astronomical units, or AU) due to the decrease in insolation with the square of the distance from the sun.

Solar thermal power systems offer a potential alternative. Progress in advanced lightweight concentrator technology provides a necessary first step toward making solar thermal power for deep space missions a viable option. Companies such as L'Garde, SRS Technologies, ILC Dover, United Applied Technologies, and Harris Corporation are developing concepts for large, lightweight solar concentrators. Figure 1 shows an example of a lightweight concentrator using thin-film, inflatable technology. This advanced concentrator technology offers a factor of five improvement in aerial density (kg/m2) over conventional rigid panel concentrators. The other key elements to a mass competitive solar power system for far-sun missions are high efficiency secondary concentrators and high efficiency, free-piston Stirling convertors.

[Advanced Inflatable Primary Concentrator]
Fig. 1 - Advanced Inflatable Primary Concentrator

Secondary concentrators can provide an increase in the overall geometric concentration ratio as compared to primary concentrators alone. This reduces the diameter of the receiver aperture and the associated infrared cavity losses, thus improving overall efficiency. The use of a secondary concentrator also eases the pointing and surface accuracy requirements of the primary concentrator, making inflatable structures a more feasible option. Typical secondary concentrators are hollow, reflective parabolic cones. Recent studies at Glenn Research Center have investigated the use of a solid, crystalline refractive secondary concentrator for solar thermal propulsion which may provide considerable improvement in throughput efficiency by eliminating reflective losses. The refractive secondary concept, shown in Figure 2, also offers the benefit of directed flux tailoring within the receiver cavity via a unique "flux extractor." Such a device has the potential to greatly improve the energy transfer to the Stirling heater head.

[Refractive Secondary Concentrator]
Fig. 2 - Refractive Secondary Concentrator

Stirling convertors have the potential to provide very high thermal-to-electric conversion efficiency. Stirling Technology Company (STC) in Kennewick, Washington has successfully designed, built, and operated free-piston convertors at 10 watts and 350 watts for terrestrial applications. The 350 watt STC convertor is pictured in Figure 3. STC is also developing a space-rated, 55 watt unit for radioisotope applications designed to provide system conversion efficiencies of greater than 24% (White, 1999). All of these engines share common technology characteristics including flexure bearings and linear alternators. The 10 watt engine has undergone endurance testing to over 50,000 hours in order to demonstrate long life and reliability.

[Free-Piston Stirling Convertor]
Fig. 3 - Free-Piston Stirling Convertor

Table 1 compares system performance of 200 watt solar Stirling power systems at 1.5 AU (Mars), 5.2 AU (Jupiter), and 9.5 AU (Saturn) with several different radioisotope options: ARPS/AMTEC, small RTG, and ARPS/Stirling. The solar power systems utilize a Fresnel primary and refractive/quartz secondary configuration and vary in specific power from just under 3 W/kg for Saturn to almost 11 W/kg for Mars. The radioisotope systems require two units to approach the 200 watt end-of-mission (EOM) requirement resulting in specific power levels between 4 and 6.5 W/kg. A 200 watt solar Stirling for Jupiter has about the same mass as the two ARPS concepts and provides 33% more power at EOM than the ARPS/AMTEC. In reference to the size of the solar collector, the 5.3 m primary concentrator diameter for the Jupiter system is similar to the size of one Tracking and Data Relay Satellite System (TDRSS) antenna.

  Fres/Qtz
Stirling
Fres/Qtz
Stirling
Fres/Qtz
Stirling
ARPS
AMTEC
Small
RTG
ARPS
Stirling
Solar Dist (AU) 1.5 5.2 9.5 Variable, 6 yr design life
Unit Power (BOM,W) ? ? ? 92 106 105
Unit Mass (kg) 18.6 33.6 74.4 14.9 22.6 15.9
No. Units 1 1 1 2 2 2
System Mass (kg) 18.6 33.6 74.4 29.8 45.2 31.8
System Pwr (EOM, W) 200 200 200 150 182 206
System Eff (%) 17.8 18.2 18.1 10.8 6.5 22.2
Sp Power (W/kg) 10.7 5.9 2.7 5.0 4.0 6.5
Primary Dia (m) 1.6 5.3 9.7      

Table 1 - Comparison of Solar Stirling with Radioisotope Power Systems

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Last Updated: 07/31/2002