# Most efficient method of storing energy with water on the moon?

I found a large cavern under my lunar homestead. It is several hundred feet (a couple hundred meters) deep and fairly large. There is a large ice deposit at the bottom. I am able to warm it to temperatures that will keep water liquid through the day/night cycle.

Knowing that lunar day/night cycle is 29.5 Earth day I have half that time when I can collect solar energy, and half that time I will need to rely on stored energy.

On Earth water is used to power things, sometimes a river is dammed for hydroelectric power, in other cases Electrolysis is used to create fuels like Oxyhydrogen. Ultimately all these sources of power are rooted in solar energy.

I have some different options for storing the solar energy from the lunar day in water.

1. I can use a pump to elevate the water to the top of the cavern. At night I let the water fall through a turbine and generate electricity.
2. I can use Electrolysis and separate the H and the O then burn them to create heat make steam to turn a turbine and generate electricity.

There are several combinations and probably some options I have not considered.

Assume I need all my energy returned as electricity, not as mechanical energy (i.e. I am not using a water wheel to mechanically power my air purifier)

With current/modern technology circa 2020 how can I most efficiently store and recover solar energy in water on the moon?

• If you get H2 and O2 then it's far more efficient to use a fuel cell. Commented Jun 25, 2020 at 18:45
• It takes a huge amount of energy just to liquify that ice, and a constant energy source just to keep it liquid against the -20 degree wall surfaces. Also, it is highly unlikely that you would find pockets of ice that close to the surface anywhere but near the poles, as the underground temp elsewhere is about -15 to -20c, and at those temperatures ice will sublimate away rather quickly. It's also not likely that a near-surface void or tube would be gas tight, as the top layers of the moon are heavily broken up and porous. Commented Jun 27, 2020 at 20:52

Method one is used currently on this planet. Pumps move water to a reservoir when electricity rates are lowest and demand is low, absorbing generator excess power. When the grid requires additional power beyond the capability of the current generating systems, the water is allowed to flow, spinning turbine generators in the manner you describe. At that point in the cycle, the rates are higher, making the losses inherent in pumping acceptable, in relation to profit.

The energy losses in electrolysis are likely to be much higher than that of pumping, especially considering the volume of water necessary to generate enough hydrogen and oxygen at a level sufficient to produce steam to drive a turbine. The quantity of loss points are increased in this method as well.

I suspect that this question is better suited to the SE for engineering. How have you been able to conceal your existence on the moon?

• RE: "How have you been able to conceal your existence on the moon?" There have not been visitors in a few decades. existing on the moon with out being seen is not the challenge, it getting deliveries made without the rockets being noticed that is the challenge. Commented Jun 25, 2020 at 16:54
• +1 a good start on an answer, but maybe include some hard numbers and references supporting them. Also consider the impact of lower gravity on the potential energy of the water with height change. Also consider phase shifts at different locations. What if I seperate the H & O at the bottom, and burn it at the top, I can get energy from the same drop of water twice then in cycle (not saying this is best choice, but it is an option) Commented Jun 25, 2020 at 16:59
• @JamesJenkins I keep thinking that there should be some way to use liquid/solid phase change as a way to store energy but I am bad at thermodynamics. Lunar subsurface temperatures are about 255 K near the surface and increase about 1.3 K per meter in depth (from here) so there are various temperatures above and below 273 K available in the walls of your 200 meter deep cavern if one did a little drilling. Who knows, it might be liquid at the bottom, and assuming the cavern is sealed and with evaporation there could even be rain!
– uhoh
Commented Jun 25, 2020 at 21:50
• @JamesJenkins to that end I've just asked Would there be snow, rain, fog or clouds in a deep, sealed, subsurface cavern on the Moon if water were present?
– uhoh
Commented Jun 25, 2020 at 21:58