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Modeling an ISRU-based energy storage system for sustainable lunar electricity production

Illustration of a Lunar ISRU Energy Storage and Electrical Generation concept (not to scale). The system utilizes three heat transfer fluid circuits. The collection loop in the receiver tube is heated by sunlight from a field of mirrors during the lunar day and circulates to the thermal mass raising its temperature. Upon lunar nightfall, a discharge loop transfers heat from the thermal reservoir to the Stirling engine for electricity production. While the engine is running a thermal regulation loop dissipates heat through the radiator. Credits: system layout by Mario F. Palos and Ricard González-Cinca, modified to add component labels; lunar landscape by Grok 2

One of the difficulties of designing solar power systems for use on the Moon is the challenge of energy storage during the 14 day lunar night at lower latitudes far from the peaks of eternal light at the poles. Such systems would benefit from technology that leverages in situ resource utilization (ISRU) for this critical function rather then expensively transporting batteries from Earth. It would be ideal to model these systems prior to use via computer simulations to optimize the design before bending metal. In the journal Advances in Space Research, Spanish physicists Mario F. Palos and Ricard González-Cinca explore this approach in a paper that examines an ISRU-based system for energy storage and electricity generation.

The architecture of the proposed system, dubbed Lunar ISRU Energy Storage and Electrical Generation (LIESEG), collects solar energy during the lunar day via a mirror field to concentrate sunlight on a receiving pipe containing a heat transfer fluid (e.g. molten sodium). The heated fluid flows to a thermal mass raising the temperature of the energy reservoir. The resultant stored thermal energy from the reservoir is discharged through a second fluid loop used to drive a Stirling engine for electricity production during both day and night. A third heat rejection loop thermally regulates the system by transferring heat from the cold side of the heat engine to the radiator. This modular design balances efficiency and durability under extreme lunar conditions.

The ISRU implementation angle of the study emphasizes the use of lunar regolith and other local materials to minimize reliance on Earth-based supplies. This not only reduces launch costs but also aligns with long-term sustainability goals for lunar habitats.

To analyze the LIESEG system performance, simulations were carried out using EcosimPro software, a tool used by the European Space Agency in multiple aerospace applications, to assess power output, efficiency, and scalability. A comprehensive theoretical model based on the thermodynamics of the subsystems under lunar conditions was developed to analyze the energy flow and efficiency of the system. The study evaluated the specific power performance (power output divided by launch mass) of the system, highlighting its potential to be superior to other conventional methods like photovoltaic systems or nuclear reactors in terms of mass efficiency and sustainability. It also discusses the influence of key factors like the thermal conductivity of lunar regolith, the size and orientation of solar collectors, and the efficiency of the Stirling engine.

The authors conducted a detailed trade-off analysis of technologies, considering criteria like transportability, installation complexity, operational reliability, scalability, and lifespan. Solar collection and thermal conversion technologies were highlighted as critical components for achieving operational stability.

The proposed LIESEG system offers a promising approach for sustainable energy production on the Moon, potentially reducing reliance on Earth-launched resources and enabling longer, more autonomous missions. The system’s feasibility was demonstrated through computer modeling whose results show LIESEG to be practical for initial lunar missions with lower energy needs, as well as for later advanced bases requiring higher power outputs (up to 100 kWe and beyond). This research shows that a LIESEG system has merit for planning future development of energy infrastructure supporting initial lunar outposts and eventually, permanent settlements on the Moon.

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Solar cell manufacturing using lunar resources

Conceptual rendering of a Blue Alchemist solar cell fabrication facility on the Moon. Credits: Blue Origin

Jeff Bezos’ new initiative called Blue Alchemist made a splash last month boasting that the team had made photovoltaic cells, cover glass and aluminum wire from lunar regolith simulant. This is an impressive accomplishment if they have defined the end-to-end process which (with refinements for flight readiness) would essentially provide a turnkey system to fabricate solar arrays to generate power on the Moon. The announcement claimed that the approach “…can scale indefinitely, eliminating power as a constraint anywhere on the Moon.” Actually, this may not be possible at first for a single installation as surface based solar arrays can only collect sunlight during the lunar day and would have to charge batteries for use during the 14 day lunar night, unless they were located at the Peaks of Eternal Light near the Moon’s south pole. But if scaling up manufacturing is possible, coupled with production of transmission wire as described, a network of solar power stations in lower latitudes could be connected to distribute power where it is needed during the lunar night.

Very few details were revealed about the design outputs of the end products (not surprisingly) in Blue Origin’s announcement, particularly the “working prototype” solar cell. An image of the component was provided but it was unclear if the process fabricated the cells into a solar array or if the energy conversion efficiency was comparable to current state of the art (around 21%). Nor do we know how massive the manufacturing equipment would be, how much periodic maintenance is needed or if humans are required in the process. Still, if a turnkey manufacturing plant could be placed on the Moon and it’s output was solar arrays sourced from in situ materials, it would significantly reduce the costs of lunar settlements by not having to transport the power generation equipment from Earth. This particular process has the added benefit of producing oxygen as a byproduct, a valuable resource for life support and propulsion.

Research into production of solar cells on the Moon from in situ materials is not new. NASA was looking into it as recently as 2005 and there are studies even dating back to 1989. Blue’s process produces iron, silicon, and aluminum via electrolysis of melted regolith, using an electrical current to separate these useful elements from the oxygen to which they are chemically bound. Solar cells are produced by vapor deposition of the silicon. The older studies referenced above proposed similar processes.

It would be interesting to perform an economic analysis comparing the cost of a solar power system supplied from Earth by a company focusing on reducing launch costs (say, SpaceX) with that of a company like Blue Origin that fabricated the equipment from lunar materials. Peter Hague has done just that in a blog post on Planetocracy using his mass value metric.

Hague runs through the numbers comparing SpaceX’s predicted cost per kilogram delivered to the Moon by Starship with that of Blue Origin’s New Glenn. At current estimates the former is 5 times cheaper than the latter. Thus, to best Starship in mass value, Blue Alchemist would have to produce 5kg of solar panels for every 1kg of equipment delivered to the Moon, after which it would be the economic winner. Siting a recent analysis of lunar in situ resource utilization by Francisco J. Guerrero-Gonzalez and Paul Zabel (Technical University of Munich and German Aerospace Center (DLR), respectively) predicting comparable mass output rates, Hague believes this estimate is reasonable.

Perhaps we should not get ahead of ourselves as Blue Origin’s timeline for development of their New Glenn heavy-lift launch vehicle is moving a glacial pace and one wonders if they have the cart before the horse by siphoning off funds for Blue Alchemist. Jeff Bezos is free to spend his money any way he wishes and definitely seems to be in no hurry.

Conceptual illustration of New Glenn heavy-lift launch vehicle on ascent to orbit. Credits: Blue Origin

But SpaceX’s Starship has not made it to space yet either and after we see the first orbital flight, hopefully as early as next week, the company will have to demonstrate reliable reusability with hundreds of flights to achieve economies of scale commensurate with their predicted launch cost of $2M – $10M. As SpaceX has demonstrated with it’s launch vehicle development process it is not a question of if, it is one of when.

Image of full stack Starship at Starbase in Boco Chica, TX. Credits: SpaceX

As both companies refine their approach to space development, will it be the tortoise or the hare that wins the mass value price race for the cheapest approach to power on the Moon? Or will each company end up complementing each other with energy and transportation infrastructure in cislunar space? Either way, it will be exciting to watch.

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Power towers at the Peaks of Eternal Light

Peaks of Eternal Light at the lunar south pole annotated with crater labels. Mosaic of 40 images taken by the ESA SMART-1 spacecraft 2005/2006. Area covers 500 x 150 km. Credits: ESA/SMART-1/AMIE camera team; M. Ellouzi/B. Foing, CC BY-SA 3.0 IGO

As most space settlement enthusiasts know, the Peaks of Eternal Light on the rims of craters in the lunar polar regions hold much promise as the ideal location to place collectors for solar energy to power ice mining operations. At the south pole in particular, these peaks lie within just a few kilometers of large frozen water deposits in the permanently dark shallows. But how much solar power is available? Companies such as Trans Astronautica Corporation will want to know so they can inform plans for their Sun Flower™ collector invention as part of a Lunar Polar Mining Outpost.

In a paper posted this month on the pre-print server arXiv.org, a team of researchers at Harvard University and Technische Universität Berlin present the results of a study to answer this question. Using data from high resolution maps of solar illumination on the ridges of Shackleton crater and others, they determined the total available power from collector towers of various heights if they were placed at these locations.

The study found that the power available depends heavily on the height of the panels above the local surface but could be substantial, from a few megawatts for towers of heights less than 100m up to the gigawatt range for towers of 500m or more. This is sufficient power for mining several thousand tons of water per year from Shackleton crater.