§ 4.1.3 High Temperature Solar Ovens
Giant solar ovens will introduce entirely new industrial processes undreamed of on Earth. It just isn't feasible to produce the temperatures on Earth which can be easily done with giant solar ovens and containerless processing in zero gravity. Even medium temperatures are easier to produce in space.
Giant solar ovens can be built in zero gravity and with no wind. These can be relatively lightweight structures, e.g., foil mirrors. Zero-gravity containerless processing using very high temperatures will usher in a whole new field of materials science processing and manufacturing impossible on Earth.
Thermal energy is cheap and clean in space.
The McDonnell Douglas Solar Oven
Experiments on Earth (not in space) in processing lunar soil simulants were performed in the early 1990s in a joint research effort by the McDonnell Douglas Space Systems Company (MDSSC), the Aluminum Company of America (ALCOA), and the Space Studies Institute (SSI). (Paper reference.) This was based on a solar oven MDSSC had built for previous research into solar power for producing electricity, a 75 kilowatt thermal solar collector made originally for a 25 kilowatt electric Stirling engine but reapplied to a simple oven for the lunar materials. This solar concentrator can achieve concentration ratios of 10,000 suns (i.e., 1400 Watts/cm2) over a 20 cm (8 inch) wide beam. The device is located at MDSSC's Solar Energy Test Facility in Huntington Beach, California. (MDSSC also developed a 10 megawatt Solar One power tower but that was overkill for lunar materials processing.)
The oven used lunar simulants to produce cast basalt rods, bricks and glass fibers.
Standard ASTM tests found that the rods had compressive strengths of approximately 10,000 p.s.i., which is about two or three times greater than concrete.
One experiment produced a 2 cm thick (approx. 1 inch thick) opaque glass plate of remarkable strength by heating lunar simulant with a moderate intensity of 60 W/cm2. Another welded two bricks together by putting lunar simulant between them and heating it, producing a weld depth of range 1.3 to 1.9 cm. It is thought this process could eventually be used to help produce closed, potentially pressurized structures exclusively from lunar resources (e.g., buried habitats under compression). Further studies were underway on crystallized cast basalt structures and glass composites. (There is quite a lot of experience in cast basalt on Earth, as eastern Europe countries have been producing pipes and other things from melted bulk basalt for decades.)
Together with the Shimizu Corporation (a huge, old Japanese engineering and construction company), MDSSC investigated breaking up rocks by thermal shock on the surface (much like a glass breaks if you pour hot water into a cold glass) for the purpose of enhancing lunar surface mining operations. Tests on rocks from the same quarry as Minnesota Lunar Simulant (MLS) found that the rocks broke up when hit with an intensity of 25 W/cm2, though the relevancy of this work was being studied in view of the potential effects of moisture in the rock on Earth used for these experiments.
Additional work was planned several years ago. If anyone has any information on this additional work, please send a message to . For example, they had planned to implement some design changes to the melt crucible at this facility, and discussions were underway to perform similar research using a solar furnace at the University of Arizona which can achieve comparable solar concentrations over a smaller, 2 cm wide area.
The MDSSC authors call for a so-called Remotely Operated Mobile Solar Concentrator (ROMSC) to be deployed on the lunar surface to produce lunar landing pads, roads and slabs. The mobile solar concentrator could be parked and a processing oven introduced for production of bricks and glasses, and extraction of volatiles. Electricity could be produced by introducing a thermal engine such as a Stirling cycle heat engine. The solar concentrator could become mobile again to weld bricks, and break up ore rocks by thermal shock. (Paper reference.)
Fairly detailed designs for a 984 kg (approx. one ton) mobile ROMSC are given in Magoffin and Stone's published Princeton Conference Paper, together with a report on the above experiments.
En route to the Moon, the solar concentrator could melt spent fuel tanks to provide useful metal products (as was suggested by the authors).
Of course, such experiments are relevant to processing materials in orbital based factories as well.
Other research into solar concentrators
Many solar concentrators have been built for producing both thermal and electrical power, some for Earth-based applications and others as experiments intended for eventual use in space. It is a very basic technology. The main issues are specific designs for processing asteroidal and lunar materials, and tweaking those designs.
Most of the work for developing solar concentrators for use in space has focused on electric power production, e.g., in the 1960s by NASA and the Air Force, and in the 1970s for the SPS design studies (e.g., Lurio's paper review of receivers).
However, the key efforts in this field are in design of specific receivers and processes for converting asteroidal and lunar materials into useful products and feedstocks. The materials can be bulk basalt which is cast into final products as discussed above, or melts for electrolysis, or glass for making fiberglass, or lightweight sintered products, or even vacuum distillation to separate elements from a bulk mix by very high temperature containerless processing.
Notably, it is generally thought that fiber optics can be used to pipe moderately concentrated sunlight into factories for specific thermal operations. Research is needed in this field. The key designs will be those which best couple the cables to the solar flux collectors.