Within the next decade, several space agencies and commercial space partners will send crewed missions to the Moon. Unlike the “footprints and flags” missions of the Apollo Era, these missions are aimed at creating a “sustained program of lunar exploration.” In other words, we’re going back to the Moon with the intent to stay, which means that infrastructure needs to be created. This includes spacecraft, landers, habitats, landing and launch pads, transportation, food, water, and power systems. As always, space agencies are looking for ways to leverage local resources to meet these needs.
This process is known as in-situ resource utilization (ISRU), which reduces costs by limiting the number of payloads that need to be launched from Earth. Thanks to new research by a team from the Tallinn University of Technology (TalTech) in Estonia, it may be possible for astronauts to produce solar cells using locally-sources regolith (moon dust) to create a promising material known as pyrite. These findings could be a game-changer for missions in the near future, which include the ESA’s Moon Village, NASA’s Artemis Program, and the Sino-Russian International Lunar Research Station (ILRS).
The research team was led by Kätriin Kristmann, a Ph.D. researcher at TalTech and the communications lead for the Estonian Students Satellite Foundation (ESTCube). She was joined by multiple researchers from TalTech’s Department of Materials and Environmental Technology and Physics Division and Advenit Makaya – an advanced manufacturing engineer with the ESA’s European Space Research and Technology Centre (ESTEC). As they indicated in their study, the Moon has the right elements to create monograin layer (MGL) solar cells using pyrite.
As Kristmann told Universe Today via email, their research addresses a major need for future lunar missions: the ability to generate power in a way that is not dependent on Earth. This means focusing on renewable sources and relying on batteries of fuel cells only as a backup. As Kristmann explained:
“Producing power on the Moon is essential if we aim to establish a permanent settlement there. Using solar energy is one of the most promising candidates, because most of the conventional energy sources available today on Earth are not available on the Moon. The main challenge is to use the resources that are available on site from the lunar regolith to build the energy station and take as little materials from Earth as possible.”
Kristmann and her colleagues at TalTech have been working with pyrite for many years because of its potential as a solar cell substrate (a layer placed on top of a photovoltaic cell to make it more absorbent). The term “monograin layer” refers to the material’s structure, which is fashioned from microcrystalline powders, leading to a lightweight and flexible cell. Pyrite has an atomic structure of one iron atom bonded with two sulfur atoms (FeS2), both of which are abundant on the lunar surface.
Marit Kauk-Kuusik, the head of the photovoltaic materials lab at TalTech and co-author on the study, explained the benefits of the material in an article published by TalTech in January 2022:
“TalTech scientists have been working on monograin layer solar cell technology for terrestrial applications for a couple of decades already. The core innovation is the unique light absorbing layer made of the single-crystalline powder, which contains abundant and low-cost elements. Solar cells based on this technology will bring innovation to the building-integrated solar power field.”
For the purposes of this study, Kristmann and her colleagues designed a solar cell structure that consists of a pyrite absorber layer paired with a layer of graphite nickel oxide (NiO) and transparent conducting oxide (TCO). These would be combined with Schottky diodes made of pyrite and platinum. As Kristmann explained, the inclusion of this material presents many advantages:
“Pyrite is an advantageous material because the energy input that goes into producing a pyrite-based solar panel is remarkably lower than for conventional silicon solar cells. This is because we can proceed at much lower temperatures in the preparation of this material solar cells. Lower temperature means lower energy consumption and lower cost. Consisting of iron and sulfur, pyrite also does not contain any hazardous elements for humans.”
To create the pyrite, the team relied on the liquid salt synthesis method, where they combined iron and sulfur in a potassium iodide solution that was then heated to 740 °C (1364 °F) for one week. The solution was then slowly reduced to a temperature of 575 °C (1067 °F), then rapidly cooled to room temperature, which produced a single-phase pyrite monograin powder suitable for making pyrite MGLs. These tiny crystals can easily be fashioned into MGLs using 3-D printers and technology that space agencies are already planning on sending to the Moon.
“The potassium salt acted as a medium for the creation of single crystalline grains of FeS2, and at the end of the reaction, the salt was removed by washing,” said Kristmann. “This process can easily be adjusted to the Moon environment, as it does not utilize any complicated equipment with difficult to obtain prerequisites such as high purity vacuum chambers or strong lasers or magnetic fields.”
This work builds on previous work by Kristmann and many other researchers in TalTech’s Laboratory of Photovoltaic Materials. For over ten years, Taltech has been researching MGL solar cells with applications for renewable energy here on Earth. In addition to being simple, the production process creates high-efficiency single crystalline solar cells that are flexible and thin, making them highly valuable for the growing renewable energy market. But it is the potential for providing renewable energy for astronauts on the Moon and beyond that drives Kristmann’s team to find better energy solutions.
As a member of the European Space Agency, Estonia is contributing to the organization’s plans for creating a lunar habitat known as the Moon Village. This proposed base will serve as a spiritual successor to the International Space Station, where rotating astronaut crews from around the world will live for months at a time and conduct vital research. According to Dr. Taavi Raadik (Kristmann’s Ph.D. thesis supervisor), the ESA became interested in TalTech’s photovoltaic research about six years ago when they looked into MGL solar cell technology and found it promising.
Dr. Advenit Makaya has worked with Kristmann and her colleagues as part of the collaborative relationship between the ESA and TalTech. For many years, he has worked through ESTEC to support the development of promising advanced materials and processes for space applications. As he explained to Universe Today via email:
“The requirements of power systems to support lunar exploration depend on the volume of activities performed at the surface and the time of the missions. Several space agencies and private companies, at the international level, have indicated plans for long duration missions on the lunar surface. This will require power systems which provide enough energy for the range of activities to be conducted, as well as suitable reliability and resistance to the space environment, to provide power for long duration missions.
“Sustainability becomes therefore essential. Solar power has been the traditional source of power for space missions, so far and it is expected to still provide a large share of the energy required in lunar missions. Having the ability to produce the solar cells from local material could increases the sustainability of the missions and help reduce the dependance on supplies from Earth and the associated costs.”
Initially, their collaborative efforts focused on testing the technology to see if it was suitable for applications in space. Once they had proven that the technology could operate in the extreme cold and vacuum of the Moon, their efforts have since shifted to implementing MGL solar cells to power a future lunar outpost and how they can be manufactured using lunar regolith. The success of this latest study shows that it can be done, which will have drastic implications for future lunar missions. Said Kristmann:
“The implications that a sustainable energy production capability can bring to the Lunar exploration is significant. With a reliable energy production [method], we can focus on other important topics, such as science and infrastructure, when talking about the lunar settlement. Reliable solar energy production would let us explore a new world (lunar or beyond) without pollution, warranting that we as humans have been able to learn from our past and from the challenges that we have faced on our own planet.”
As Dr. Makaya added, there’s also the benefit of enduring self-sufficiency where long-duration missions are concerned. In addition to food, water, and other basic necessities, the International Space Station (ISS) also relies on regular shipments of replacement parts and components. This includes replacement solar cells and the electronics and tools needed to keep them operational. But on the Moon, shipments will be fewer and farther between, which makes the ISRU aspect of MGL solar cells very attractive:
“In the perspective of long-term exploration missions on the lunar surface, including potential large settlements, replacement of solar power hardware will become particularly relevant because of potential degradation in the lunar environment (from dust, radiation, micrometeoroids, large temperature variations). The ability to support these maintenance and replacement needs, by manufacturing solar cells with in-situ resources can reduce the cost of hardware resupply from Earth.
“It can also help expand the range of future missions by allowing to produce new hardware for further exploration activities. For destinations where resupplies from Earth is impractical due to the distance (e.g., Mars), in-situ manufacturing, for instance, for maintenance and repair operations, becomes essential.”
In the long run, this research could also lead to the creation of a solar cell manufacturing industry on the Moon. According to Taavi Raadik (Kristmann’s Ph.D. supervisor and study co-author), the ESA is currently moving forward on a program called SOLARIS that will foster the development of space-based solar power (SBSP) applications. This program will be unveiled at the upcoming ESA Council at Ministerial Level, which will be taking place in November 2022. As Radik described it, this ambitious plan envisions the creation of infrastructure that will provide Earth with abundant renewable energy to fight climate change:
“Idea is to build huge “solar farms” into space that convert solar energy to electricity, the generated power is then transmitted wirelessly in the form of microwaves at 2.45 GHz to dedicated receiver stations on Earth, called ‘rectennas’, which convert the energy back into electricity and feed it into the local grid. To build this kind of structure, we need to ship thousands and thousands of tons into the space that is extremely expensive and unfriendly to the environment!
“We would like to save the environment by polluting it even more at the beginning. A more clever way is to build such kinds of structures on the Moon and ship them back to Earth orbit, the lack of atmosphere and low gravitation makes it much easier. And here comes in our idea to manufacture lightweight solar cells in situ on the Moon, that can be also used for SBSP solutions.
As noted, this research already has extensive applications here on Earth. But by developing production methods where pyrite cells and space-based solar arrays can be manufactured using in the most hostile environments, the technology can mature considerably. In other words, methods that can ensure sustainable living in space (where there is no margin for error) will invariably lead to solutions for sustainable living here at home. As the not-so-old adage goes, “solving for space solves for Earth!”