What should be the energy choice on Mars? The high efficiency, light weight and flexibility of the latest solar cell technology means that photovoltaics could provide all the power needed for a long Mars mission, or even all the power needed for a permanent settlement there, according to a new analysis by scientists at the University of California, Berkeley.

The high efficiency, light weight, and flexibility of the latest solar cell technology means that photovoltaics could provide all the power needed for a long Mars mission, or even all the power needed for a permanent settlement there, according to a new analysis by scientists at the University of California, Berkeley.

Most scientists and engineers thinking about the logistics of living on the surface of the Red Planet have assumed that nuclear power is the best alternative, largely because of its reliability and 24/7 operation. Over the past decade, miniaturized Kilopower nuclear fission reactors have advanced to the point where NASA sees them as a safe, efficient, and abundant energy source and the key to future robotic and human exploration.

Solar power, on the other hand, must be stored for use during the Martian night, which lasts about as long as it does on Earth. And on Mars, the power generated by solar panels can be degraded by the ubiquitous red dust that covers everything. So much so that NASA's Opportunity rover, a nearly 15-year-old rover powered by solar panels, stopped working after a massive dust storm on Mars in 2019.

The new study, published this week (April 27, 2022) in the journal Frontiers in Astronomy and Space Sciences, uses a systems approach to compare the two technologies head-to-head for an extended six-person mission involving a 480-day stay on Mars before returning to Earth. This is the most likely scenario for a mission that extends the time on the surface beyond a 30-day timeframe while reducing the transit time between the two planets.

Astronauts traveling to Mars will need to minimize the weight of the power system they take with them from Earth. If the planned settlement is in the yellow area on this flattened map of Mars, photovoltaics would be the best choice. Also shown are the locations of previous missions that have landed on Mars, including Jezero Crater (top right), which NASA's rover Perseverance is currently exploring. (Graphic work: Anthony Abel and Aaron Berliner, UC Berkeley)

The analysis claimed that if the weight and efficiency of solar panels were taken into account, solar energy was comparable to or better than nuclear in the settlement covering almost half of the Martian surface, as long as this daytime energy produced hydrogen gas used in fuel cells to power the colony at night or during sandstorms.

“Photovoltaic power generation combined with specific energy storage configurations outperforms nuclear fusion reactors (contrary to what has been repeatedly suggested in the literature) over more than 50% of the planet's surface, especially in regions around the equatorial band,” said Aaron Berliner, a UC Berkeley bioengineering PhD student and one of the paper's first authors.

The study brings a new perspective to Mars colonization and provides a roadmap for deciding which technologies to use when planning manned missions to other planets or moons.

"This paper takes a global view of the power technologies that are currently available, how they can be deployed, what use cases are optimal for them, and where they are deficient," he stated. Department of Chemical and Biomolecular Engineering: "If humanity collectively decides that we want to go to Mars, this kind of systems-level approach is necessary to do it safely and minimize the cost in an ethical way. We must have a clear comparison between options when deciding what technologies to use, what places on Mars to go, how to go, and who to bring."

Longer tasks require more power

In the past, NASA's estimates of the power needs of astronauts on Mars have often focused on short-term stays that do not require power-hungry processes to grow food, produce building materials or chemicals. But leaders of NASA and the companies that are now building rockets that could go to Mars, including SpaceX CEO Elon Musk and Blue Origin founder Jeff Bezos, are voicing concern about long-term, off-planet settlements, bigger and larger, and more reliable power sources.

The challenge is that all these materials must be transported from Earth to Mars at a cost of hundreds of thousands of dollars per kilo, making it necessary to make the trips low-weight.

One key need is power for biomanufacturing facilities that use genetically modified microbes to produce food, rocket fuel, plastic materials and chemicals, including medicines. Abel, Berliner and the paper's co-authors are members of the Center for the Utilization of Biological Engineering in Space (CUBES), a multi-university group that makes minor tweaks to microbes using synthetic biology's gene insertion techniques to supply the materials needed for a colony.

But the two researchers found that it was impossible to assess the practicality of many biomanufacturing processes without knowing how much power would be available for an extended mission. So they set out to create a computational model of various power supply scenarios and possible power demands, such as fertilizer production for agriculture like habitat maintenance involving temperature and pressure control, methane production for rocket boosters to return to Earth, and bioplastic production to produce spare parts.

Against a Kilopower nuclear system were photovoltaics with three power storage options. Batteries and two different techniques for producing hydrogen gas from solar energy - electrolysis and photoelectrochemical cells from which energy is obtained directly. In the latter cases, the hydrogen is pressurized and stored for later use in a fuel cell to generate power when solar panels are not available.

Only photovoltaic power with electrolysis (using electricity to split water into hydrogen and oxygen) could compete with nuclear power: On almost half of the planet's surface, it proved more cost-effective per kilogram than nuclear.

The main criterion was weight. The researchers assumed that a rocket carrying a crew to Mars could carry a payload of about 100 tons, excluding fuel, and calculated how much of that payload would have to be dedicated to a power system for use on the planet's surface. A trip to and from Mars would take about 420 days (210 days each way). Surprisingly, they found that the weight of a power system would be less than 10% of the entire payload.

For example, for a landing site near the equator, they calculated that the weight of solar panels plus hydrogen storage would be about 8.3 tons, versus 9.5 tons for a Kilopower nuclear reactor system.

The designed model also specifies how to tune the photovoltaic panels to maximize efficiency for the different conditions that will occur at sites on the Martian surface. For example, latitude affects the intensity of sunlight, while dust and ice in the atmosphere can cause light to be emitted at longer wavelengths.

Developments in Photovoltaics

Photovoltaics are now highly efficient at converting sunlight into electricity, but the best performers are still expensive, Abel said. But the most important innovation is a lightweight and flexible solar panel that facilitates storage on the outbound rocket and reduces the cost of transportation.

“The silicon panels you have on your roof, with steel construction, glass backing, etc., are not going to compete with new and improved nuclear, but the newer lightweight, flexible panels are suddenly going to really, really change that conversation,” Abel said.

He also noted that lighter weight means more panels can be transported to Mars, and a backup can be provided for any panel that fails. While kilowatt nuclear power plants provide more power, fewer are needed, so if one collapses the colony would lose a significant portion of its power.

Berliner, who also has a degree in nuclear engineering, went into the project with a bias towards nuclear power, while Abel, whose undergraduate thesis was about innovations in photovoltaics, was more in favor of solar power.

“I feel that this paper really came out of a healthy scientific and engineering disagreement about the merits of nuclear and solar power, and that it was really just us trying to understand and settle a bet,” Berliner said. “I think I lost based on the configurations we chose to publish this, but of course it's a happy loss.”

Other co-authors of the paper are Mia Mirkovic, a researcher at UC Berkeley in the Berkeley Sensor and Actuator Center; William Collins, professor of earth and planetary science at UC Berkeley and senior scientist at Lawrence Berkeley National Laboratory (Berkeley Lab); Adam Arkin, director of CUBES and the Dean A. Richard Newton Memorial Professor in the UC Berkeley Department of Bioengineering; and Douglas Clark, the Gilbert Newton Lewis Professor in the Department of Chemical and Biomolecular Engineering and dean of the College of Chemistry. Arkin and Clark are also senior faculty members at Berkeley Lab.

The work was funded by graduate research fellowships from NASA (NNX17AJ31G) and the National Science Foundation (DGE1752814).

Levent Aslan

From the Engineering and Technology site...

20 August 2024