Solar Thermal System for Lunar ISRU Applications: Development and Field Operation at Mauna Kea, HI

Transcription

1 Solar Thermal System for Lunar ISRU Applications: Development and Field Operation at Mauna Kea, HI Takashi Nakamura 1 and Benjamin K. Smith 2 Physical Sciences Inc., Pleasanton, CA This paper discusses the development, deployment and operation of the optical waveguide (OW) solar thermal power system for ISRU applications at the ISRU analog test site on Mauna Kea, HI. In this solar thermal system, solar radiation is collected by the concentrator array which transfers the concentrated solar radiation to the OW transmission line made of low loss optical fibers. The OW transmission line directs the solar radiation to the place of utilization of the solar energy. In this paper applications of solar energy to sintering of native soil and thermo-chemical processing of native soil for oxygen production are discussed. P I. Introduction hysical Sciences Inc. (PSI) has been developing the optical waveguide (OW) solar power system. 1-3 Figure 1 shows a schematic representation of the system. In this system solar radiation is collected by the concentrator array which transfers the concentrated solar radiation to the optical waveguide (OW) transmission line made of low loss optical fibers. The OW transmission line directs the solar radiation to the thermal receiver for thermochemical processing of lunar regolith for oxygen production on the lunar surface. Key features of the proposed system are: 1. Highly concentrated solar radiation (~ ) can be transmitted via the flexible OW transmission line directly to the thermal receiver for oxygen production from lunar regolith; 2. Power scale-up of the system can be achieved by incremental increase of the number of concentrator units; 3. The system can be autonomous, stationary or mobile, and easily transported and deployed on the lunar surface; and 4. The system can be applied to a variety of ISRU processes. This paper discusses development of the ground-based solar thermal power system and its application to (i) surface stabilization of the native soil and (ii) carbothermal oxygen production. Both experiments were conducted at the ISRU analog test, Mauna Kea, HI during January February, Optical Waveguide Cable Lunar Regolith Thermochemical Processing Concentrator Array High Intensity Solar Thermal Power Oxygen Figure 1. Solar Thermal System for Oxygen Production from Lunar Regolith. The experiment for surface stabilization of the native soil was conducted in collaboration with Northern Center for Advanced Technology (NORCAT), Ontario, Canada, and experiment for carbothermal oxygen production was conducted in collaboration with Orbital Technologies Corporation (ORBITEC), Madison, WI. 1 Area Manager, Space Exploration Technologies, Applied Sciences, 6652 Owens Drive, Pleasanton, CA 94588, and AIAA Associate Fellow. 2 Senior Scientist, Applied Sciences, 6652 Owens Drive, Pleasanton, CA 94588, and AIAA Member. 1

2 II. The Ground-based Demonstration System Physical Sciences Inc. (PSI) has developed the solar thermal power system during the past two years (December 07 through April 09) under the SBIR Phase III program supported by NASA/GRC, 4 and in part under the SBIR Phase II program supported by NASA/JSC. 5 This solar power system was intended for demonstration of solar thermal power for processing in-situ resource utilization (ISRU). A full assembly of the solar concentrator array is given in Fig. 2. In this figure, seven concentrators, the frame, tracking system and cart are described. Figure 2. Fully assembled solar concentrator array. The construction of the solar thermal power system was completed and system performance testing was conducted in March Figure 3 shows the integrated system viewed from the front of the concentrator array. Seven reflectors are mounted on a single flat array structure. The array structure tracks the sun within 0.04 degree. Each of the seven reflectors are connected to a single optical fiber cable. Seven optical fibers are connected to a single reactor inlet. Figure 4 shows the back of the concentrator array with the reactor inlet. Figure 3. Seven concentrator mounted on the tracking array. 2

3 Figure 4. Back of the array showing 7 cables connected to the reactor interface. Performance Testing The power output from each concentrator/cable was measured on March 20, The measurement was made using the Coherent PM1K thermopile sensor. The result is given in Table 1. Output from each concentrator/cable train was measured separately by covering the other 6 concentrators. Each concentrator/cable gives similar power output and the system efficiency is 37.8%. Table 1. Power Output from Each Concentrator/Cable Train Measured on March 20, 2009 Reflector #1 #2 #3 #4 #5 #6 #7 Total Cable Power (W) Efficiency Ambient Solar Flux: Γ 0 = 880 W/m 2 Reflector Area: m 2 7 = m 2 Reflector #7 is at the center of the top row in the array as shown in Figure 2. #1 is the left of #7. Figure 5 shows the quartz rod (36 mm diameter) attached to the reactor inlet. The quartz rod transfers the high intensity solar radiation transmitted by seven optical fiber cables onto the regolith inside the reactor. A magnified view of the quartz rod emitting solar power is given in Fig. 6. Figure 7 shows the power delivered by the quartz rod. The data points show the data taken on March 23, 2009 when the ambient solar flux was about 884 W/m 2. With all seven concentrators deployed, the power output from the quartz rod was 703W. This power output gives a system efficiency at 33.3%. Power outputs from the concentrator/cable train and the quartz rod are compared in Table 2. The ambient solar flux for both days is 880W/m 2. The difference between the power output from the cable and that from the Quartz rod was due to absorption within the cooling water film in the boundary between the cable and the quartz. The power loss at the boundary is 92W, about 11% of the cable output. This loss could be reduced substantially by design improvements: minimal boundary gap to reduce absorption by water film; or using index matching liquid for the interface. 3

4 Figure 5. The reactor inlet equipped with the quartz rod. Figure 6. The reactor inlet quartz rod emitting solar power. Figure 7. Solar power output from the cable and the quartz rod. Table 2. Power output with 7-Concentrator Array Cable Output (3/20/09) Quartz Rod Output (3/23/09) Ambient Solar Flux (W/m 2 ) Power (W) System Efficiency (%)

5 In Table 3 efficiency of each component of the system is given. These efficiency data were taken during the Phase III program as we developed and tested each component. The primary concentrator of the system was made of enhanced aluminum whose spectral averaged reflectivity was measured to be The secondary reflector is coated with silver. The spectral averaged reflectivity of the silver for the solar light reflected by the enhanced aluminum is The intercept factor, which is a measure of the concentrator s ability to focus, is This means that 72% of the reflected solar power is focused within the aperture of the optical fiber cable while 28% of the reflected solar power is not utilized for the system. The value of the intercept factor depends on the surface accuracy of the optical system. The imaging quality concentrator system we considered would have high intercept factor (0.9 ~ 0.95) but the cost of the concentrator system was beyond the scope of the SBIR Phase III program. The inlet optics is located at the entrance of the optical fiber cable and funnels the focused solar ray into the optical fiber cable. Photos of the inlet optics are given in Figure 24 later in this paper. The efficiency of the inlet optics for this prototype was measured to be The fiber cable transmission efficiency for the system was measured to be 0.75 and the interface between the optical fiber cable and the reactor optics (quartz rod) was The expected efficiencies of the future system components are presented on the right columns of Table 3. The method of achieving the expected efficiency is described in the Remark column. Table 3. Component Efficiency for the ground-based demonstration system and the future space-based system Ground-based Demonstration System Space-based System Component Efficiency Improvement Measure Efficiency Improvement Measure Primary Conc Enhanced Aluminum Protected Silver Coating Coating Secondary Reflector 0.94 Protected Silver Coating 0.94 Protected Silver Coating Conc. Intercept Factor 0.72 Affordable Reprocating Ni- Substrate: Low Surface Precision 0.90 High Precision Imaging Quality Composite Substrate: High Surface Precision Inlet Optics 0.82 Prototype Optical Surface Machining and Coating 0.94 High Precision Optical Surface Machining and Coating Fiber Cable* 0.75 Low Cost Polymer/Silica 0.90 High Quality Silica/Silica Fiber Cable-Reactor Water Cooled Prototype Interface Optical Interface Total System Field deployed and operated Future System *Include Fresnel reflection loss at inlet and outlet, Cable Length = 5 ~ 10 m Fiber 0.98 Index Matched Precision Optical Interface III. On-site Preparation of the Solar Thermal System at Mauna Kea, HI PSI participants of the ISRU Analog Test (Takashi Nakamura and Benjamin Smith) arrived at the Mauna Kea Analog Test Site on January 27, Solar concentrator array components were transported from the crate staging site to the field site for assembly. Figure 8 shows four images of transporting the solar concentrator array structure by folk lift. 5

6 Figure 8. Assembling solar concentrator array structure. The following day, January 28, we started integration and fine tuning of the system components. Figure 9 shows the main solar concentrator array structure fully assembled and placed on the wooden platform. The platform allows the solar concentrator unit to be moved to the left where it is used for sintering the Tephra and to the right where it is used for Carbothermal oxygen production. Figure 9. Fully assembled solar concentrator system. 6

7 IV. Field Operations of Solar Thermal Power System A. 1: Solar Thermal Sintering of Tephra Solar thermal sintering experiment was conducted in collaboration win Northern center for Advanced Technology Inc. (NORCAT). 6 For this experiment, solar concentrator cable was connected with the sintering optics as shown in Figure 10. The sintering optics connected to the solar concentrator was integrated with the NORCAT X-Y rastering system for initial shakeout. Figure 11 shows the combined system undergoing the initial test. The solar power delivered on the Tephra surface was about 540W. In this initial test, we tested the effect of the standoff distance between the sintering optics and the Tephra surface (heat flux intensity), rastering speed (total thermal power) and rastering pitch. Figure 10. Sintering optics connected to the Solar Concentrator Cable. Figure 11. PSI solar sintering system integrated with NORCAT rastering system undergoing initial shakedown tests (Power to the Tephra surface: 540W). Figure 12 shows melting, not sintering, of Tephra when heat flux is too high. The surface temperature was measured by Raytek MMG5H single color pyrometer (5 µm) as shown in Figure 13. When surface temperature exceeded 1150C, molten slag was formed. There is a very narrow temperature range of 1000C ~ 1100C where Tephra appears to be sintered. Above that temperature, molten slag will form. Sintering experiments went on for the next two days (January 30 and 31). 7

8 Figure 12. Surface melting, not sintering, of Tephra (Surface temperature 1170C). Figure 13. Solar power (~ 540W) applied to Tephra surface and the single color pyrometer (5 µm) for temperature measurement. During the third day we were able to sinter the Tephra surface by choosing the right parameters. Figure 14 shows the sintering process and the sintered Tephra surface. Due to the scheduling for other experiments, we were not able to spend more time on this important subject. The sintered surface shown in Figure 14 is made of only one layer of sintered surface. Additional layers, by piling on the new Tephra, could have made a multi-layer surface with stronger properties. During this tightly scheduled experiment, we were able to demonstrate feasibility of solar sintering of the Tephra surface. Solar sintering is a very effective method in that the heat source is readily available where surface stabilization needs to be implemented. In terms of effectiveness of the operation, the sintering optics we prepared was not optimal. Heat flux is concentrated in a narrow zone as shown in Fig. 15. Considering that we only need 50~70W/cm 2 for sintering, we can make more effective sintering optics which will give more uniform and broad heat flux distribution. 8

9 Figure 14. Solar sintering of the 15 in 15 in Tephra surface (sintering rod power = 540W, surface temp = ~ 1100C; rastering speed = 1 ~ 2.35 mm/sec). Figure 15. Heat flux intensity from the sintering optics. A. 2: Solar Thermal Carbothermal Oxygen Production After the solar sintering test, the solar concentrator was moved from the west end of the platform to the east end of platform for Carbothermal (CT) experiments (see Figure 9). The solar concentrator system was integrated with the ORBITEC CT reactor for the oxygen production experiment. Figure 16 shows the PSI solar concentrator integrated with the ORBITEC CT reactor. 7 The Tephra feed hopper was installed by the CT reactor to feed Tephra. The monitor installed at the CT reactor shows the video image of the Tephra melt during the carbothermal reaction process. The solar power directed to the Tephra melt through the quartz rod and the protective window was 570W when the ambient solar flux was 1080W/m 2. Tephra melt experiments were conducted on February 1, 2, 3, 4, 5, 6, 8, and 9. Figure 17 shows the CT experiment on February 2. Temperature of the Tephra melt was measured by a two-color pyrometer during the melt experiment. The lower plot is the temperature in degree C. The noise is caused by the interference from the solar light reflected on the molten Tephra surface. The reading, therefore, should not be considered as the exact temperature of the Tephra melt. However, it is a reasonably good indication of the thermal state of the Tephra melt. The PSI solar concentrator was developed to create the regolith melt at 1800C, and Figure 17 shows that this 9

10 objective has been achieved. Figure 18 shows the Tephra melt within the ORBITEC CT reactor after the carbothermal reaction experiment conducted on February 6, Figure 17. The melt temperature taken by ORBITEC two color pyrometer. The display Power [Watt] is not correct. It should be Temperature (Centigrade). Figure 18. Tephra melt in the ORBITEC CT reactor. On February 9, 2010, The PSI Solar Concentrator, the ORBITEC CT Reactor and JSC Water Electrolyzer were operated remotely from NASA/JSC, Houston TX. Figure 19 shows the three systems being remotely operated by NASA/JSC Operation Center. 10

11 Figure 19. PSI Solar Concentrator, ORBITEC Carbothermal Reactor and NASA/JSC Water Electrolyzer operated remotely from Houston, TX. III. Solar Concentrator Operation and Performance In this section we discuss operation and performance of the PSI solar concentrator system as it was deployed at the ISRU Analog Test, Mauna Kea, HI. A. Ambient Solar Flux Measurement The ambient solar flux (direct) was measured by the imaging solar flux sensor which cuts off the non-direct solar flux. Figure 20 shows the aperture stop of the solar flux sensor which cuts off the diffuse light around the solar image (left). On a clear day the entire solar image is transmitted through the aperture (right). Figure 20. Imaging solar flux meter for direct solar flux measurement. The ambient solar flux was calculated from the solar power through the aperture stop and the area of the collection lens taking into account the Fresnel loss. Table 4 summarizes daily accounts of the solar flux value. 11

12 Table 4. Daily Solar Flux Summary Ambient Solar Flux Date (W/m 2 ) 1/28/ Clear but overcast Comment 1/29/ ~ 992 Thin high cloud 1/30/ ~ 889 Partially cloudy 1/31/ ~ 1006 Overcast with high cloud 2/1/ ~ 650 Cloudy 2/2/ ~ 1078 Clear at noon time, high cloud towards the end of the day 2/3/ ~ 1026 Clear 2/4/ ~ 1034 Clear 2/5/ ~ 1078 Clear 2/6/ ~ 1060 Clear 2/8/ ~ 1033 Clear 2/9/ ~ 1051 Warm, Clear with thin high cloud B. Power Output Measurement The solar power output from the cable was measured using the Coherent PM1K thermopile sensor having an effective diameter of 50 mm. One problem is that the solar power coming out of the cable termination spreads beyond the diameter of the sensor. Figure 21 shows how the optical fiber cable is arranged at the cable termination. At the center of the termination is the cable coming from concentrator #7. Around #7 cable are six other cables #1 thorough #6. The diameter of the total cable is about 35 mm. The solar power coming out of #7 cable falls within 50 mm of the sensor diameter, while the solar power coming out of cables #1 through #6 leaks out of the sensor diameter. At the pre-ship measurement conducted in March 2009, we opened each mirror successively (single mirror one at a time) and focused the solar ray from each cable at the center of the thermopile sensor. In this way we were able to measure the power output correctly though it took a long time to perform this process (see Table 1). Figure 21. Cable output flange which is the interface with the quartz rod. At the analog test site, Mauna Kea, we were not able to conduct precise measurements. The cable termination shown in Fig. 21 was connected rigidly to the Coherent PM1K thermopile sensor. We opened the cover for each concentrator successively allowing the solar power from cables #1 thorough #6 to leak out of the effective sensor diameter of 50 mm. We assigned the efficiency of the solar concentrator based on the power output from #7 concentrator/cable. 12

13 Power measurement data were taken periodically during the test. Table 4 summarizes the measurement data. The row termed Nominal Cable Power means the power output measured with the detector which, due to geometrical constraints, cannot capture all of the solar power. Figure of Merit is the nominal power divided by the solar power intercepted by the concentrator. We will use this number to assess the dust effect on the concentrator performance. True Cable Output is the cable output value properly measured. For the Hawaii Analog Test data, it is calculated from the efficiency of the #7 concentrator/cable. The Quartz Rod Output is the power output from the quartz rod when it is connected with the cable termination. In reference to Table 5, we notice that the efficiency of the system measured on January 29 is slightly higher than the value measured at the pre-ship test back on March 20, We may ascribe this higher efficiency to the new inlet optics (Aluminum reflective surface on Aluminum substrate). We also notice that the Figure of Merit goes down from on 1/29/10 to on 2/3/10. The Figure of Merit on 2/3/10 through 2/5/10 is lower than that of 1/29/10. The Figure of Merit goes back up to 0.28 after cleaning all the mirrors. Table 5. Performance of the Solar Power System San Ramon, CA Hawaii Analog Test 2010 Date 3/20/09 1/29/10 2/3/10 2/4/10 2/5/10 2/6/10 2/9/10 Solar Flux (W/m 2 ) Nominal Cable Power (W) Figure of Merit True Cable Output (W) System Eff. (%) Quartz Rod Output (W) System Eff. (%) Comments Pre-ship test results. Silver coated S.S. Inlet Optics, New Fiber, Clean Mirrors (865)** 39.2* First test in Hawaii. Al deposited Al Inlet Optics Mirror dusty Mirror dusty * The efficiency based on the cable power output from the #7 concentrator/cable. ** The power output calculated from the #7 concentrator/cable efficiency Mirror dusty (657)** 32.0* C. Dust on the Primary Mirror The test site was very dusty and the wind kicked up the dust regularly. Consequently, the primary mirror and the secondary mirror became covered with dust. Figure 22 shows the dust deposit on the primary concentrator mirrors. We tried the following cleaning methods: blowing with compressed air; and water spray followed by compressed air blowing. These two methods were effective in removing large particles. However, there was always a persistent thin layer of fine particles on the reflective surface. The third method we tried was to wash the reflective surface with water using a fine paint brush, then wiping the surface with soft paper. After the third method, the primary mirrors looked clean as shown in Fig. 23. In reference to Table 5, we notice that the Figure of Merit for 2/6/10 is The Figure of Merit for 2/5/10, immediately before we cleaned all 7 mirrors, was Thus, by cleaning the dust deposited on the primary mirrors, we increased the power by 9.5% Dust cleaned from all mirrors Low flux early in the morning, higher flux (~ 1050) later in the day 13

14 Figure 22. Dust deposit on the primary concentrators. Figure 23. Seven primary concentrators cleared of dust deposit. We did not clean the secondary mirror surface because we thought the dust deposit will be minimal as those mirrors face downward. This conjecture may not be justified and, had we cleaned the secondary mirrors, higher performance might have been obtained. Conclusion of the dust effect investigation is that performance degradation on the order of 10% is expected for conditions such as we encountered in Mauna Kea. 14

15 D. Dust Effect on the Inlet Optics After the Mauna Kea Analog Test, we removed the inlet optics to identify the dust effects on the inlet optics. Figure 24 shows one of the seven inlet optics before (left) and after (right) the analog test. The photo on the right shows significant contamination (deposit or corrosion) which is likely the result of dust deposit on the reflective surface. One plausible explanation is that fine Tephra dust particles may have been melted in the intensive solar flux and deposited on the cold reflective surface. Another possible mechanism is chemical corrosion. We do not have a definite explanation at this point. In any case the contamination of the reflective surface of the inlet optics is most likely caused by the dust at the test site. Figure 24. Aluminum inlet optics before and after the Mauna Kea tests. IV. Summary and Conclusions Two PSI members (T. Nakamura and B. K. Smith) participated in the ISRU Analog Test from January 27 through February 11, PSI conducted two tests: Solar Sintering of Tephra Surface with NORCAT; and Solar Powered Carbothermal (CT) Oxygen Production with ORBITEC. The PSI team, with capable and friendly support by other members from CANADA and the U.S., assembled and operated the PSI solar concentrator system. The solar concentrator system operated as designed, providing solar power to the sintering system and to the CT reactor. The PSI/NORCAT team successfully demonstrated solar sintering of Tephra, paving the way to lunar surface stabilization with solar thermal sintering of lunar regolith. We gained valuable experience and experimental data regarding the solar heat flux (~ 70 W/m 2 ), the surface temperature (1100C) and the rastering speed (1 ~ 2.35 mm/s) for sintering. We successfully prepared a sintered pad (15in 15in). Due to time constraints we were only able to make a single layer sintered surface. For mechanical strength a sintered surface with two or three layers is desirable. With valuable lessons learned during the solar sintering experiment, we are ready to design better sintering optics for the next generation of solar thermal sintering. The PSI/ORBITEC team conducted a series of very successful Carbothermal (CT) oxygen production experiments. The PSI solar concentrator provided solar thermal power necessary for the CT reaction. The solar thermal power delivered to the CT reactor created the Tephra melt at 1700 ~ 1800C, sufficient temperature for CT reaction. With the solar thermal power provided by the PSI solar concentrator, ORBITEC conducted CT reaction experiments, each time producing a Tephra melt: one small melt samples on 2/1/10; three normal melt samples on 2/2/10; two normal melt samples on 2/3/10; four normal melt samples on 2/4/10, three normal melt on 2/5/10, one sample on 2/6/10, two samples on 2/8/10, two samples on 2/09/10. In total ORBITEC conducted 16 CT reaction tests. On February 9, the PSI concentrator, the ORBITEC CT reactor and the JSC Electrolyzer were operated remotely from the command post at JSC, Houston. The PSI team operated the solar concentrator system in field deployment conditions. During the test PSI was able to evaluate the performance of the solar concentrator in adverse environments that are not encountered in laboratory test setting. Solar flux varied in a broad range (450 ~ 1050 W/m 2 ) depending on the weather conditions. Dust effects on key components of the system (primary reflector and inlet optics) were investigated. It was shown that the dust cover on the primary concentrator will lower the power output by about 10%. The dust also affected the inlet optics and decreased system performance by 6% during the experiment. The ability of the solar concentrator system to track the sun was proved to be excellent. We did not encounter any problem with solar tracking. In all, the PSI solar concentrator completed all of its tasks reliably during the term of the Lunar Analog Test, Mauna Kea, HI. 15

16 Acknowledgements The PSI team would like to thank those who helped us in preparation, set up, deployment and operation of the PSI solar concentrator. Collaborations with NORCAT and ORBITEC personnel have been very effective, stimulating and rewarding. Our participation in this ISRU Analog Test, Mauna Kea, HI was made possible by the Phase III SBIR contract administered at NASA/KSC (NNK10EA03P), Mr. Anthony Muscatello, the technical contact. We are grateful for the opportunity given to us to participate in this important event. The Solar Thermal Power System was developed with the Phase III SBIR fund administered by NASA/GRC (NN08CA59C), Dr. Aloysius Hepp, the technical contact. Development of the Solar Thermal Power System was also supported, in part, by the Phase II SBIR fund administered by NASA/JSC (NNJ08JD44C), Mr. Aaron Paz, the technical contact. References 1 Nakamura, T., Case, J. A., and Senior, C. L., Optical Waveguide Solar Energy System for Lunar Material Processing, Final Report, SBIR Phase II (NAS ), August Nakamura, T., Solar Thermal Propulsion System for Small Spacecraft, Phase I Final Report for AFRL, Contract No. F M-0158, PSI-2777/TR-1923, May Nakamura, T. and Van Pelt, A.D., Multi-use solar thermal System for Oxygen Production from Lunar regolith, Final Report, NNJ07JB26C, PSI-6002/TR-2228, July Nakamura, T., and Smith, B.K., Solar Thermal System for Oxygen Production from Lunar Regolith Ground Based Demonstration System, Final Report SBIR Phase III, NNC08CA59C, NASA/GRC, April Nakamura, T., and Smith, B.K., Multi-use Solar Thermal System for Oxygen Production from Lunar regolith, SBIR Phase II, NASA/JSC, NNJ08JD44C, Final Report, November Smith, B, Nakamura, T, Theiss, R. and Boucher, D., Results of Solar Sintering Experiment presented at The Space Resources Roundtable (SRR) and The Planetary and Terrestrial Mining Sciences Symposium (PTMS), Colorado School on Mines, Golden Colorado, June 8-10, Gustafson, R., White, B. and Fidler, M., Analog Field Testing of the Carbothermal Regolith Reduction Processing System, AIAA , AIAA SPACE 2010 Conference and Exposition, Anaheim, California, Aug. 30-2,