IMAGE ©NASA Daedalus crater shows the problem the indirect solar receiver solves. Orlando’s team proposes positioning solar concentrating mirrors – heliostats – atop the crater rims, aiming concentrated solar irradiation down into the darkness of the crater, with the light focused onto a solar receiver partly embedded in the icy surface to heat it to release water.
A NASA-funded research team at Georgia Tech has developed a method for extracting water from the Moon to generate the hydrogen and oxygen needed for propulsion fuels for solar system exploration. They describe their experimental work in Thermal extraction of H2O(s) from lunar regolith simulant with concentrated solar irradiation: Experimental analysis, published at Acta Astronautica.
The researchers propose an interesting way to extract water from the potentially water-rich icy regions at the Moon’s pole. These regions are of interest to space agencies because the presence of water, which can be extracted or retrieved, is required for human exploration.
Sunlight can be concentrated using mirrors and is the Moon’s most feasible energy resource, with no atmospheric attenuation and with continuous sunlight for approximately 16 Earth days at the icy poles that space agencies are interested in. Given the total lack of lunar atmosphere, sunlight could be beamed off of mirrors for miles on the moon with virtually no attenuation.
However, these icy regions are shaded by deep craters, so using sunlight to extract water seems improbable.
A solution comes from two NASA funded projects at Georgia Institute of Technology led by Thom Orlando examining different lunar ISRU (In-Situ Resource Utilization) scenarios on the Moon. Co-author Peter Loutzenhiser brought his solar background to both projects.
“Thom has articulated a very precise vision of how to realize the available resources that are on the Moon – called in-situ resource utilization – capturing the resources on the Moon and using them to drive further resource utilization or mining,” Loutzenhiser told SolarPACES in a call from Georgia.
Engineering Sunlight into Shadow
To get solar heat into the icy regolith, Orlando’s team proposes positioning solar concentrating mirrors – heliostats – atop the crater rims, aiming concentrated solar irradiation down into the darkness of the crater, with the light focused onto a solar receiver partly embedded in the icy surface. Each solar receiver would be an absorbing and heat conducting body that absorbs the intense solar flux and redirects it through the icy lunar solar.
Loutzenhiser said these solar receivers, partly embedded in the icy regolith to heat and release the water, would be small and mobile.
“We’re thinking of the solar mirrors (heliostats) positioned at the crater edges and the solar receivers placed in the icy regolith with deep penetration depths,” said Loutzenhiser. “We envision these indirect solar receivers as mobile, placed in the ground, water extracted, moved to a different location, and the scenario repeated.”
The reflecting mirrors atop the crater rims would be simple flat or slightly curved mirrors like those in power tower-type concentrated solar plants. They would have a two-axis motion, with a small PV panel to power the slight movement, to refocus the sunlight to each new melting position.
“We envision mounting these heliostats on the rim of the crater and then fixing them in such a way that they beam the solar irradiation down,” he noted. “The concentrations would be much greater than on Earth due to no attenuation, as the Moon has little to no atmosphere.”
Extracting the water would be straightforward.
“When you heat up water in the lunar soil, expansion occurs, which causes the water vapor to move up through the surface due to pressure gradients that are created,” he explained. “Higher pressures below and a lower pressure above force the water vapor to move through the bed to the surface.”
The experimental setup and findings
To test the idea in their lab-scale experiments on Earth, they used a simulated lunar regolith based on the properties of the lunar sample brought back by the Apollo missions. The solar receiver tube is a copper tube, 65 mm long and 40 mm in diameter, with a solid 10 mm thick end cap painted black to enhance absorptance. The experimental campaign was realized by Tyler Farr during his doctoral work at Georgia Tech with inputs from Brant Jones.
In the experimental setup, the solar receiver is embedded within the packed icy regolith simulant to assess heat transfer and extraction performance.
Simulated concentrated irradiation from a high-flux solar simulator is used to provided irradiation of 37 or 75 kW/m² onto packed beds of icy regolith simulant, similar to heliostat fields. The energy needed for extraction was 24.7–37.1 kJ per gram of water removed.
They ran the experiments in vacuum/cryogenic conditions similar to that on the Moon, measuring the water release via mass change using desiccant traps.
In their tests, they had compared direct irradiation (aiming the concentrated sunlight directly at the icy surface) versus heating the copper tube solar receiver to transfer heat to the ice indirectly.
The experiments found that the indirect solar receiver was more efficient. This partly submerged solar receiver increased water yield by an average of 18.7% compared to direct surface heating, despite slower heating rates.
The data also revealed an inverse relationship between the heating rate and extraction efficiency: Hotter, more rapid heating formed a desiccated insulating layer, diminishing heat transfer deeper into the icy deposits. Lower, slower heating allowed deeper penetration and more consistent extraction.
These Georgia Tech findings offer experimental benchmarks for computational ISRU models and engineering data that can guide the design of future water extraction systems on the Moon.
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