Looking at Lunar Gravity Batteries

Providing sufficient power for a lunar research station is an incredibly complicated challenge. Unlike here on Earth the Moon’s ‘days’ are almost a month long, 29.53 days, that means if you want to have a moon base with people living there for any meaningful period you will need a way to always have electricity, without electricity your astronauts are likely to die from cold/heat/lack of oxygen or any number of other awful ways to die. With the moon’s lack of atmosphere there are currently two popular ways to generate electricity, solar panels, and nuclear power. Nuclear power can produce electricity 24 hours a day for years, which is amazing, the downside is how expensive it is to get a large enough nuclear power plant that can fully provide the energy that your lunar base will need. Solar power is much cheaper to deploy on the lunar surface but suffers from availability, with some rare exceptions, solar power on the Lunar surface is only available, at best, about 50%* of the time.

Storing surplus power will be critical for the success of future lunar bases, and I believe gravity batteries will be a key technology for powering Lunar bases.

Gravity batteries can be literally as dumb as a rock, lift up any heavy object and you have increased its potential energy, let that heavy object fall and you have converted that potential energy into kinetic energy. Attach the right kind of generator to that falling object and you can convert that falling energy into electricity. The weakness of many gravity batteries here on Earth is how mass inefficient they are. To store enough electrical energy to power a single American home, you would need to lift 1.6 million kg of stuff 10 meters in the air (3.5 million lbs about 33 feet up). That’s a lot of stuff, and on the moon the numbers get even crazier, with the moon’s weaker gravity you only get about 16% of the energy per unit mass out of your generator as you would on Earth.

So why do I think gravity batteries on the moon make sense? It all comes down to the cost of shipping and repairability. If you want to send a traditional chemical battery to the moon, every piece of the system needs to be shipped up from Earth (at least to my knowledge in 2022, I acknowledge that there might already be and ISRU piece on making lunar batteries more locally). It costs thousands to tens of thousands of dollars per kilogram just to get a payload into low Earth orbit, let alone bringing materials to the moon. The price of shipping materials for a lunar base can add up quickly (I don’t have any meaningful numbers for shipping payloads to the Lunar surface, hence being vague). On the other hand, a gravity battery generator has the potential to require a much smaller shipped mass relative to how much energy you want to store. You need your generator, pully systems, and support materials. The potential energy storage medium aka rocks and sand will certainly be coming from the moon and depending on terrain and configuration of our generator some part of the structural elements could come from local stocks.

[ At this time I am being simple in my outline out of a desire to just get the idea out there/not having the resources (aka time for the most part) to do a more detailed analysis. ]

Quick and dirty back of the envelope calcs

Lithium Ion batteries can store between 100-250 Wh/kg ( https://www.cei.washington.edu/education/science-of-solar/battery-technology/#:~:text=What%20are%20some%20advantages%20of,%2D670%20Wh%2FL). )

To store enough energy to provide our 30 kWh of power for an American home for one day we need about 120 kg of batteries and then some number of kilograms of support materails. (30,000 Wh/250Wh/kg=120 kg)

To store that same amount of power using lunar dust we would need to lift 9.6 million kg of lunar dust about 10 m off the ground (assuming our generator is about 70% efficient). Looking at this value, lithium ion batteries win. But how much actually needs to get shipped from Earth? Not nearly as much. You can find 2 kW electric motors online massing well under 20 kg). Next we will need cables to lower our rocks back down, and bags to hold our rocks so we don’t make a big mess. (this is where the research resources bit will come into play, optimizing the number of bags of material that will store our lunar dust. Fewer bags are technically more mass efficient individually, but mean we need more cables, so imagine hand waving).  Alternatively you could use an old lunar buggy that would be loaded up with material at the top of a hill and drive up and down a local slope. By using the motors as a generator the buggy could produce power on its downward trips and use only a small amount of power to return for a new load.

Broadly speaking if you only want to store a few dozen kilowatt hours of energy storage lithium ion will be preferable. Where gravity batteries might potentially shine is to expand energy storage during a lunar base’ awkward teen years. Where there are local manufacturing abilities, but they lack the precision and chemistry needed for more traditional batteries. Assuming even rudimentary sintering capacity, mechanical trolleys and their storage weights could be produced in bulk, only requiring the motors and controller systems to come from Earth. Heck even electrical cables could be made from refined regolith.

So while I don’t know if we will ever actually see a scenario of gravity batteries for a lunar colony, the concept feels like it warrants more effort than just my back of the envelope guestimations

*there are locations called peaks of eternal light where the sun does in fact “always shine”, but they aren’t that common so you can’t guarantee that your moon base will have access.