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What is the cheapest material per ton that one could purchase on our Earth in the 21st century? This could also include the cheapest material per ton to harvest so dirt may seem like the obvious answer but because of the easily harvestable dirt per acre it may not be the most optimal solution.

What problem are you trying to solve? Find the most affordable material for "gravitational potential energy storage with solid masses"

What are you trying to accomplish? Have an extremely long term (think stonehenge) energy storage solution.

restrictions/requirements: The material must be gathered together within the circumference of a 10 mile radius far away from large bodies of water. The cost per ton must be considered including transportation costs or logistical concerns.

research: Hydroenergy storage is already being used by pumping lakes to higher ground artifical lakes but a more exotic approach would be required for areas not containing large quantities of water.

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    $\begingroup$ Seawater and air, quite obviously. There is plenty of them and they can be had for free in unlimited quantities. There are no regulations whatsoever for the harvesting of sea water or air; just go and take as much as you want. $\endgroup$ – AlexP Dec 3 '19 at 2:07
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    $\begingroup$ There are many types of materials where people will pay you to take it off of their hands. $\endgroup$ – Michael Kutz Dec 3 '19 at 2:28
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    $\begingroup$ Not a worldbuilding question, and impossible to answer as asked, too. In your room you breathe for free, if you dive you pay for the oxygen bottles. $\endgroup$ – L.Dutch - Reinstate Monica Dec 3 '19 at 3:30
  • $\begingroup$ @L.Dutch-ReinstateMonica so you are saying I get 5 tons of free oxygen per year? but surely I must factor in taxes into the cost. $\endgroup$ – CodeCamper Dec 3 '19 at 3:57
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    $\begingroup$ @L.Dutch - Reinstate Monica: You don't use bottled oxygen in diving, because at higher pressures it can be toxic: en.wikipedia.org/wiki/Oxygen_toxicity For recreational scuba diving, what you use is simply compressed air. (And what you're paying for is having that air compressed to ~3000 psi or so.) For more technical diving, you might use things like a helium-oxygen mix. $\endgroup$ – jamesqf Dec 3 '19 at 4:52
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If I had to guess, it would straight up be water. Some big companies literally pump millions of tons of this stuff out of the ground and then distribute it across the globe to make a very healthy profit.

I found this article which states that "Coca-Cola Amatil" pays 2.40 for a million litres of water. Basically it costs $2.40 for 1000 metric tons of water, which makes it cheaper than dirt.

I'm sure the cost varies a little, but water and sea water are basically free resources.

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  • $\begingroup$ good catch on the article, but at the same time you pay 2-3$ for a small bottle in places with no natural drinkable water. And I think the transportation of such a mass of water can be dangerous comparing to dirt (leakage, water behaviour in non-narrow trasportation path..) $\endgroup$ – Zavael Dec 3 '19 at 12:11
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    $\begingroup$ Just to elaborate, this satisfies the "no where near a source of water" requirement because pumping water does not care how far one end of a pipe is from the other, just that you create the needed pressure difference between the two. Assuming your piping is properly sealed, you can pump water 1000s of horizontal miles just about as easily as you could pump it between two adjoining reservoirs. The upstart cost of a long pipe can be a lot, but past that, it's darn near free. $\endgroup$ – Nosajimiki Dec 3 '19 at 15:52
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Construction / demolition debris.

From comments:

There are many types of materials where people will pay you to take it off of their hands. – Michael Kutz 13 hours ago

Debris and especially demolition debris must be hauled away from the site where it is generated. People are paid to haul it away, rendering the cost of such materials a negative number - you both get the material and you get paid. Mining tailings are another example as described here: https://www.innocentive.com/ar/challenge/9934231

Concrete demolition debris and mining tailings are not biodegradable and will persist for a long period of time as they are, and so either would be suitable for the gravitational potential energy storage application.

The cost of moving materials a distance depends on the distance and transportation means available: cost of barges < trains < trucks < wheelbarrows. The cost of handling and moving a material is the biggest cost, and if you are looking for truly massive quantities of mass, siting your operation near these quantities will save you money.

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Precipitation.

The big advantage here is that it starts off at an (almost) arbitrary altitude, so all you need to do is open the roof of your 10-mile-radius tank and collect away.

Sure, it'll take a while to fill up your high tank. But when you're operating on the time scales of Stonehenge, waiting a couple thousand years isn't a problem.

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Fill dirt can be had for as little as 3-12 dollars per ton.

For comparison, Sand costs 5-15 dollars per ton. When looking at these materials, the dominating factor is going to be transportation costs, not raw material cost.

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    $\begingroup$ Good point about transportation costs, maybe I should modify my question to somehow factor in that component. Of course it is a complicated question depending on the delivery location of the material and the bulk discount of delivery. $\endgroup$ – CodeCamper Dec 3 '19 at 1:29

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