Figure 1. Minimal Concentration Ratio versus temperature model at different emissivity levels of a solar concentrator. The sink temperature was estimated to be 270 K, but actual lunar sink temperatures are dependent on lunar energy balance. Graph only considers radiation heat transfer from surface. There is a theoretical limit on the concentration ratio for concentrating sunlight. This is due to the fact that the sun is not a point source of light. The sun’s disk has a half angle of 4.653 mrad (8,) when viewed from Earth’s distance. The theoretical limit on concentration ratio is given by 1 over the sine squared of the solar half angle®’, as expressed in Eq. (4)'. Table 2. Approximation of concentrator type and maximum Concentration Ratio (CR) Figure 3. Cassegrainian Light Ray Path. Figure 4. Concentrator with Off-Set Parabolaic Geometry. Figure 5. Diagram of Ground Test Demonstrator at NASA GRC using an offset parabolic solar concentrator. One misgiving introductory optics courses typically give is that the most effective focusing geometry of paralle light rays is one of a circular cross section. In fact, circles create a slight aberration of light rays at the focus; th best geometry is in fact, a parabola. Though the sun’s rays diverge at a small angle, and some concentrators dc account for such aberration, parabolic geometries are sufficient approximations for optimum focusing technology Parallel light rays converge at the parabola’s focus. It is, in fact, why most advanced non-imaging optic: technologies utilize a parabola as the cornerstone of the geometrical design. The offset parabola configuration: then, is a reflecting mirror whose shape represents a portion of a whole paraboloid. If the Cassegrain primary mirror is a paraboloid located at the center, an offset parabola is a portion of the paraboloid’s sidewall, as seen ir Figs 4 and 5. Figure 7. Lens CPC. Figure 6. Hollow-Mirror CPC. Dielectric CPCs are also limited by another restriction; the propagating light is assumed to be totally internally reflected by the concentrator walls. The maximum concentration due to this restriction is shown by Eq. 6: '6 degrees, however, use of a CPC as a primary concentrator would still require some degree of tracking. CPCs are manufactured as both dielectrics (lenses), or as hollowed mirrors. Examples of each type are shown in Figs. 6 and 7 respectively. The largest theoretical CR of all CPC’s, dielectric and mirror-based, is defined by Eq. (5):'° C. Compound Parabolic Concentrators (CPCs) Another advantage to this geometry is that the parabolic mirror can be configured so its entire mirror surface is sunlit, unlike Cassegrains, where some of the sunlight is blocked by structure necessary to keep the secondary mirror in position. Because the parabola is essentially a small section of a much larger paraboloid mirror, the ray lengths of the offset parabola will always be longer than those of a similarly sized Cassegrain mirror. Thus, offset parabolic mirrors will either always have lower CRs, or smaller captured solar flux, than Cassegrains. A mirror design like an offset parabola, however, has more technology opportunities for application. The geometry lends itself to a number of applications such as illuminating the surface to provide ‘thermal wadis’ to store thermal energy to protect rovers and other assets from harsh thermal cycling effects.'° Figure 9a. Model 1.5 x 1.5 m Fresnel Reflector Using 60 Strips. In order to achieve high temperatures for ISRU thermal processes while minimizing re-radiation losses, high concentration ratio solar concentrators are needed. To achieve this reflector configuration in a composite without the need for complex curvature surfaces, DR Technologies developed a point focus Fresnel reflector configuration under an SBIR funded by NASA/GRC. The approach uses an array of simple curvature parabolic strips, each with a small line focus, and each oriented to overlay the line focus into a central focal area, thus simulating a point focus concentrator, as shown in Fig. 9. Figure 9b. Edge view showing individual strips. This choice was based on several significant advantages. The array of simple curvature strips can be fabricated without using an accurately machined mandrel as would be needed for a parabaloidal dish, with each strip cut from a replicated mirror flat and then mounted on an accurately machined structure. The Fresnel reflector has a low Figure 8. The Stretched-Lens Array focuscing incident light onto PV collector. Figure 11. Flux _— distribution predicted by SOLTRACE for the optimized F/d of 1.5. Figure 10. Intercept efficiency as a function of focal length for a 150 cm aperture. ————— ee ————_—_—__ OO OT oe eee ee eee ee LSE NED The concentrator design synthesis used the SOLTRACE code provided by the National Renewable Energy Laboratory. The model drove design trades that varied the width, length and number of Fresnel strips in the concentrator, the focal length, the overall aperture, and then determined the number and distribution of rays falling within a selected aperture size by ray tracing, while considering realistic mirror slope and specularity errors. These trades drove the design parameters to achieve high intercept efficiency with a geometrical concentration ratio of 2500, as shown in Figs. 10. The flux distribution is given in Fig. 11. Figure 12. Implementation of Fresnel mirror on composite rib, and assembled on a positioning tool using a frame that fixed the mirror strip position. Figure 13. Mirror test article and illumination testing using fog machine for visual feedback. Figure 14. Refractive Secondary Concentrator. Figure 15. Cassegrainian Concentrator with Optical Concentrating & Focusing System fF. Secondary Concentrators Figure 16. Simplified diagram of current hydrogen reduction reactor design. Figure 17. Diagram of current Orbitec carbothermal reduction reactor with features identified. If any deposits or particulates accumulated on the Figure 18. Pho’ window, some of the laser energy would be absorbed _ (left) with the C >ausing window heating and eventual failure. A solar _ hardware (right Soncentrator system was later developed by Physical Sciences Inc. (PSI) and integrated with the Carbothermal Regolith Reduction Module. The integrated system was su during the 2010 International Lunar Surface Operations and ISRU Analo Fig. 18. Concentrated solar energy was delivered into the processing chi Fig. 19. Since the temperature of the regolith surface was determined by distance of the quartz rod above the regolith controlled the regolith tempet Figure 18. Photograph of the Solar Concentrator Array (left) with the Carbothermal Regolith Reduction Module hardware (right). the interior of the processing chamber after a carbothermal reduction-processing batch. Note the layer of silica vapor that extends from the processed regolith simulant. When the proper gas flow rate is used, the silica vapor was blown away from the processed regolith Figure 20. Example of silica vapor produced during carbothermal reduction processing of JSC-1A lunar regolith simulant with solar energy. Figure 21. Damge to a protective quartz window due to accumulated silica vapor. Figure 19. Cross-section View of the Processing Chamber within the Carbothermal Regolith Reduction Module. Figure 22. Small glass beads accumulated on bottom of protective quartz window (left) and magnified view (right). Figure 23. Flexible 4 m by 6 m offset parabolic concentrator. Because tradition has linked mirror concentrators and thermal applications, and lens concentrators and PV applications, companies specializing in the manufacture of such technologies have been formed which reinforce such traditional links. However, concentrator options are widely available for multiple functions by different companies: Optiforms, Optical Mechanics, and Hardric Laboratories specialize in the manufacturing of mirrors meant for a spot focus. Minnesota Mining and Manufacturing (3M) manufactures sheets of space-rated silicone for line-focus concentrators of any size, but there are minimal companies who specialize in the creation of point-focus Fresnel lens concentrators.” Figure 26. Percentage of time with solar illumination during lunar winter. The arrows are pointing at the two regions that are illuminated >70% of the time during lunar winter; near the Shackleton Rim. Figure 25. Topographic map of the Moon determined by the Clementine mission of 1999. For locations within the mare regions, either hydrogen reduction or carbothermal reduction can be used as an oxygen production process. Since ilmenite is not found in usable amounts in the highland regions, the carbothermal process would be utilized within these regions. This means higher temperatures and more precise concentrator optics are required. pipes provide isothermal heat transfer into the reactor, which increases the lunar regolith processing efficiency.*’ Figure 28. (a) Demonstration heat pipe test set up and (b) demonstration heat pipe operating at 1050°C using a Kanthal heating element. F. Energy transfer options Figure 27. Optical cable assembly designed and utilized by Physical Sciences Inc. Where AL is the change in the length, a is the thermal expansion coefficient of the material, LZ, is the starting length, and AT is the change in temperature. For a 1 m length of aluminum, the result shows that the length would grow 6.9 mm after the transition from night to day. Consideration would have to be taken to ensure that different parts of the system do not expand differing amounts and cause stresses, breakages, or unwanted gaps.
