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Let's say that the world has successfully converted and only depends on renewable resources for our home energy needs. In fact, we have a massive surplus - currently in the form of raw electricity being generated from some science-fiction device.

There is a plan being discussed to save this surplus of energy in some form so that it is there if we ever needed it in the future - and so every place on Earth can have a surplus of electricity they can tap into if they desire.

Using our current technology, what would be the best way to store this energy according to the considerations below?


Considerations:
Note that they are in order of (most important -> nice to have)

  • We are planning very long-term storage. We don't know when, or even if we'll need this storage.
  • The energy should hopefully require as little effort/tools to make use of it as possible. For the case where there is a sudden disaster, we don't know what kind of tools or processes will still be available.
  • Size of the storage. In general, we want to use as little space as possible for this huge amount of energy so transportation and storage is easier.
  • Divisible. It would be ideal to be able to split the energy up if it needed to be.
  • Cost - The lower, the better, in creation and in maintenance. (Note that this is least important, but still a factor)

To take these considerations into account, it would be best if each answer could have sources for:

  • Decay Rate (energy loss) of the storage method over time
  • Basic information on how the raw electricity is manipulated/stored
  • Basic information on what's required to get the energy out of storage
  • Efficiency of the energy returned vs. the energy put into creating the storage
  • Energy Density in terms of volume
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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

  • $\begingroup$ I'm not sure that the OP requirements can be met under the hard science tag. I deleted my answer in consideration of that, once I noticed the need to move the Earth out of the solar system in the OP. $\endgroup$ – Serban Tanasa Aug 21 '15 at 22:29
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    $\begingroup$ @SerbanTanasa By "move planets", I mean move humans from one planet to another. I will edit to clarify. $\endgroup$ – DoubleDouble Aug 21 '15 at 22:38
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    $\begingroup$ edited my answer to account for your clarification. Still not convinced we can answer this under hard-science, though $\endgroup$ – Serban Tanasa Aug 21 '15 at 22:52
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    $\begingroup$ I would store it in a big ball of hydrogen at the center of the solar system. :) $\endgroup$ – Schwern Aug 23 '15 at 7:58
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    $\begingroup$ @DoubleDouble my point is that coal IS a store of energy. As is most other fuels for generators. $\endgroup$ – Aron Aug 25 '15 at 16:02

19 Answers 19

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Use the extra power to convert atmospheric CO2 to hydrocarbons and store it underground. This offers the long term storage requirements and ease of handling specified in the question.

Various Energy Storage Options

  • Batteries are known to lose charge over time, usually a few years. This isn't long enough.
  • Capacitors are also known to slowly discharge over time. This is a faster process than with batteries.
  • Fissile materials can't feasibly be made in useful quantities outside of supernovas.
  • Fusion materials are already plentiful so no storage is needed.
  • Antimatter tends to bump into things and explode when it does. Not to mention the astronomically high production costs.

Benefits of Hydrocarbon Storage

  1. Using the excess energy to remove CO2 from the atmosphere reduces global warming and the associated climate change.
  2. The hydrocarbons can be stored at relatively shallow depths this time whereas they were previously available only at great depths at the start of the 21st century. Getting access to them again should be very easy.
  3. Humanity already knows how to handle hydrocarbon energy sources. Unless it has been dismantled already, there is considerable infrastructure available for the movement and processing of liquid hydrocarbon fuels.
  4. Hydrocarbons are liquids at room temperature and can be stored in plain plastic containers. They are easy to move, store and handle when appropriate safety precautions are followed.
  5. These hydrocarbons can also provide feederstock to the chemicals industry for the creation of all manner of products.
  6. Hydrocarbons are stable over million year spans.

Considerations:

  • Decay Rate: In the absence of an oxidizer or bacterial activity, hydrocarbons don't "break down" as shown by the extreme longevity of deep oil deposits all over the world. The US National Institute of Health performed experiments in 1976 to determine conditions for biodegredation of various hydrocarbon fuels. They found significant increases in hydrocarbon-utilizing bacteria in all test plots. Hydrocarbons in the soil were reduced by 48.5 to 90%.

  • Energy storage/manipulation: Depending on the depth of storage, simple oil derricks can extract the hydrocarbons with metalworking capabilities comparable to Europe/Americas between 1800 and 1850.

  • Energy extraction: Steam engines or simple atmospheric burning are sufficient to extract energy from hydrocarbons. More advanced boilers or fuel cells permit more efficient energy extraction.

  • Efficiency of the energy returned vs. the energy put into creating the storage: In terms of processing the atmospheric CO2 into a liquid form and storing it, those energy costs cannot be recovered from the fuel itself (but with the massive surplus described in the OP, this loss isn't a great concern.) However, the energy stored within the chemical bonds of the hydrocarbons don't decay, so future energy consumers will be able to get all that energy back.

  • Energy Density: The energy density of hydrocarbon fuels ranges from 19.9 MJ/kg for methanol to ~55 MJ/kg for liquefied natural gas.

"Cheating" Answer

Use some/all that excess energy to create the parts required for this science fiction device and store those parts with construction plans in a vault somewhere. If the device is capable of generating that much power, why not just store a copy for use by future civilizations?

  • Decay Rate (energy loss) of the storage method over time: None as long as the device is undamaged during storage. Assuming geological time scales, someplace like Yucca Mountain could safely store all the components of the science fiction device with low probabilities of damage.

  • Basic information on how the raw electricity is manipulated/stored: The device doesn't store energy as much as it generates it later.

  • Basic information on what's required to get the energy out of storage; Please include instructions on how to operate the device. Ideally, the device just has a high voltage outlet and a ground outlet with a big button labeled "Go". Ideally, the device is self-managing so it should "just work".

  • Efficiency of the energy returned vs. the energy put into creating the storage: Greater than 100% as the energy required to manufacture the device is far far less than the energy the device will eventually generate.

  • Energy Density in terms of volume: Incredibly high but not infinite

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  • $\begingroup$ Sources are a bit suspect for a hard-science tag... ;) $\endgroup$ – Samuel Aug 21 '15 at 22:05
  • $\begingroup$ Yeah, the Popular Mechanics one isn't the greatest but the others seem decent. $\endgroup$ – Green Aug 21 '15 at 22:11
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    $\begingroup$ @SerbanTanasa Stop downvoting people on nit-picky things like this. I gave a serious objection to your answer, and now you're using it against others in different ways. $\endgroup$ – HDE 226868 Aug 21 '15 at 22:18
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    $\begingroup$ ... and then, 3000 years later, long after a major apocalypse destroyed all information about the plan, they find the hydrocarbons underground and use them for energy, theorizing they were formed by decomposing plant matter. $\endgroup$ – immibis Aug 22 '15 at 4:34
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    $\begingroup$ Plants are great at producing compounds from which we can synthesize hydrocarbons. Using vegetation as an intermediate makes infinitely more sense than direct conversion from atmosphere. And deep storage is EXACTLY what we should do, miles below the water table. Other than those two points, I'd say this is about right. $\endgroup$ – Sean Boddy Aug 23 '15 at 19:32
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How's this for an idea. One of the most common elements in the crust is Aluminum. With lots of excess power you could refine massive amounts of aluminum (or maybe other metals such as zinc?) and store them. They could later be used as metal, or to generate power via aluminum-air batteries. In general, electorefining lots of metals could be an excellent way to store energy for long term use.

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  • $\begingroup$ You wouldn't have to limit yourself to metals either. They are probably the easiest to store, but even even gases from air separation can be stored for a long time without too much loss. Helium, being a limited resource, might be stored in such a way, as has been done before. $\endgroup$ – Hassassin Aug 22 '15 at 7:57
  • $\begingroup$ It could be difficult to retrieve the energy if the future technology is not up to our own, air aluminum batteries are relatively new. It also takes massive amounts of natural materials and shuts them away so we can't use them for 100's of years, when we could recycle them time and again for centuries. $\endgroup$ – sdrawkcabdear Jan 5 '16 at 20:59
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The simple answer is - there aren't any ways with current technology. Storing energy is not a simple problem.

You can do some pumped storage for example, by moving water from a lake at the bottom of the hill to the top of the hill. Running the water back down through a turbine then generates electricity again. That makes no sense for long term storage though, just wait and rain will fill up the top reservoir.

You could split water into hydrogen and oxygen and then store the resulting hydrogen. That would be a fairly dense energy source and you could create sealed chambers underground and just full them with pressurized hydrogen. Hydrogen is a slippery little sucker though and will tend to escape so you'd most likely need to top up the tanks every now and then. To make this easier you could try and generate hydrocarbons rather than hydrogen. Essentially you start creating oil and pumping it back into the earth's crust!

This article describes some techniques being tried - in particular see the section on exactly this idea. They plan to use electrolysis to create hydrogen and methane and then store it in caverns.

They are also looking at Compressed Air Energy Storage and Pumped Hydro in that article, neither really being ideal for long term storage.

There is a fairly long list of storage methods on Wikipedia.

Just scanning the list you can see that none of these techniques is really suited to long term storage though except for generating hydrocarbons or hydrogen and storing it underground.

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  • $\begingroup$ Yay, hydrocarbons! $\endgroup$ – Green Aug 21 '15 at 21:59
  • $\begingroup$ @Green Yep, I upvoted you :) $\endgroup$ – Tim B Aug 21 '15 at 22:03
  • $\begingroup$ Only realistic answer. $\endgroup$ – Serban Tanasa Aug 21 '15 at 22:20
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    $\begingroup$ I don't think the references given here meet the requirements of the hard-science tag, nor are any of the other requirements met. $\endgroup$ – Samuel Aug 21 '15 at 22:24
  • $\begingroup$ +1 Lift a 5gallon bucket of water a few times... Gravitational potential energy and water make a strong combination! $\endgroup$ – Cort Ammon Aug 22 '15 at 2:46
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Flywheels

The method[3]:

  • Generate electricity normally.
  • Use the electricity to accelerate a flywheel to very high speeds.
  • Capture the kinetic energy of the flywheel when needed.

The benefits

  • Essentially no maintenance is required.[1]
  • The flywheel cannot decay as chemicals in batteries can. While it will eventually spin down due to friction, the timescales of meaningful energy loss are tremendously long.[1]
  • Flywheels can operate in environments where chemically-dependent apparatuses (e.g. batteries) can not.[1]
  • Flywheels can be spun up and spun down very quickly.[2]
  • Efficiencies can be higher than 95%.[3]
  • Flywheels have enormous power density, so you can store more energy in the same amount of space. This also means that they can be easily transported in whatever amount(s) is/are necessary.[3]
  • Flywheels are incredibly safe, containing no hazardous materials, as batteries do.[3]
  • Once flywheels are "discharged", they can be "recharged".

Here's a breakdown of a typical flywheel:


There have been some comments about flywheels running down. My response is that all forms of energy storage lose energy over time in some way. Chemicals in batteries can autodischarge, for example. There is no such thing as 100% efficient energy storage over long timescales.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. The ideal outcome of comments is that they lead to improvements in the post (or a different answer, if the author doesn't accept your corrections/objections/etc). It looks like there's some useful discussion here; please bring whatever's appropriate back to answers on this question. Thanks. $\endgroup$ – Monica Cellio Aug 24 '15 at 0:29
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Build Solar Panels ... in SPACE

Well, when you think about it, no matter what resources we use here on Earth, $\Large\textit{there is this giant}$ $\Large\textit{supersized storage place and}$ $\Large\textit{power station}$ (a large cloud of hydrogen which seems to have caught fire we call the Sun) a few minutes away at the center of our system.

Think about it. The average whole-humanity consumption is about 16TW (in 2010), while the Sun shines about 174,000 TW on Earth alone, and about 3,846,000,000,000,000 TW more into empty space each second. That's an unimaginable amount of power, and virtually every Joule of it is wasted every second.

So the best thing you could do for future generations is to start creating a large and ever expanding cloud of solar panels around the sun, and use that surplus to power a space industry to build more and more and more solar capture technology!

The technology for creating solar panels is already with us. Technologies for mining the asteroid belt (for raw materials, to save us the pain of space launches) are being developed currently by private companies in the Western world. There is nothing technically stopping us from deploying a massive solar array in space, aside from the initial launch costs and the political will to do so.

Space based solar power has numerous advantages over regular solar. It does not take up any valuable ecosystem from the natural wild areas. It does not suffer from intermittent supply due to night or weather. It does not get dusty, for the most part. The power can be transmitted wirelessly (and has been for decades) so with some modifications it could be transmitted from space as well

Once the 'Dyson' cloud of solar panels is built, store antimatter

There are now considerable technological difficulties with antimatter production & storage, but if the technologies for production and storage can be miniaturized and made significantly more efficient (right now we're producing antimatter by smashing atom beams together -- bit like trying to produce gasoline by shooting cannon into a methane chamber) it could be the most volume- and conversion-ratio-wise efficient form of energy storage we know of in terms of space-ship reaction mass. Furthermore, the construction process for the solar encasement will realistically take a few million years, so that gives ample time for science to advance and better magnetic confinement devices to be devised.

Once the Dyson sphere is built, you can store your excess power as antimatter in containment fields, collossal flywheels, whatever. But our descendants will have zillions of times more power than we did before them. In fact, it's irresponsible not to do it. It's the only way we could have enough power to move out of the Solar system if we decide we don't like it here anymore, or the sun (billions of years from now) goes dark.

Storing this energy in space will save you from having to carry this TO space with you. The saying goes that once you're out of the Earth's gravity well, you're energetically speaking halfway to anyplace in the solar system.

Moreover, the exercise of building this massive space project will give mankind the much needed practice with building space-habitats and robust space vehicle, something that again you can only learn by being in space, not by saving every little watt on Earth.

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    $\begingroup$ We have the technology to make solar panels, sure -- but not to create what you're proposing. That's a bit like saying that 9th century China had the ability to go to the moon, since gunpowder can make things go up and fast. Putting aside things like course corrections (which require fuel, which probably wouldn't last a few billion years), there's the small matter of being able to actually use the power. Beaming power from space is under research, but I don't think it's fair to say that it's current technology in any sort of production-ready sense. (I didn't downvote, btw.) $\endgroup$ – yshavit Aug 22 '15 at 0:17
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    $\begingroup$ Do you have a source for the claim "but it is the most volume- and conversion-ratio-wise efficient form of energy storage we know of"? The link you have just explains how scientists trap the created antimatter. $\endgroup$ – DoubleDouble Aug 22 '15 at 0:25
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    $\begingroup$ @SeanBoddy: To get mechanical work out of antimatter, can't you react it with matter and capture the gamma rays as heat? Then you "just" need to build a heat engine that can heat the working fluid with gamma rays. And that doesn't do anything chemically nasty when the molecules of the working fluid have some of their atoms modified by annihilation of a proton here, an electron there... Nuke plants deal with this with heat exchangers to carry heat between the reactor core and the boiler, so annihilation can happen somewhere that's just hot all the time, and doesn't move. $\endgroup$ – Peter Cordes Aug 23 '15 at 4:01
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    $\begingroup$ @Sean: ahhh, yeah if you consider the generating efficiency with current methods. IDK if there's much hope of more efficient antimatter creation, but Serban's unsubstantiated claim about "conversion-ratio-wise efficient" is totally bogus unless he's thinking about some potential sci-fi method that isn't in current use. (Either way, links needed for that in Serban's answer, because it's the biggest stumbling block.) $\endgroup$ – Peter Cordes Aug 23 '15 at 4:17
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    $\begingroup$ @SeanBoddy the point of the answer was that we have this giant continuous energy source called the sun, and that it makes far more sense to focus on capturing as much of that much bigger pie, than it does to store crumbs of our thimble-sized pie. We have a few million years to figure out better storage. $\endgroup$ – Serban Tanasa Aug 23 '15 at 11:31
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Answering this question is problematic. It's tagged "hard science" but it's worried about the Sun "going out without destroying the Earth" and it wants to do this with "current technology" but for "very long-term storage". The possible time and energy scales are wildly divergent. It's supposed to use current tech and hard science, suggesting a few decades, but they're worried about the Sun going out... which isn't going to happen for billions of years. What time scale are we talking about? A few decades or a few billion years?

If the time scale is in the billions of years, the problem is already solved. The initial gravitational energy of the solar system is stored in a big ball of hydrogen at the center of the solar system, it's slowly being released through nuclear fusion. Mission complete.


What about in the short term? How much energy do you want to store? Our current bulk energy storage can handle about 10^11 Watts and the world uses about 10^16 Watts. To store a meaningful amount of energy we've got five orders of magnitude to catch up on. That's a lot. What would it take?

Pumped storage is the most efficient long term energy storage system we have available. You're basically storing gravitational energy by raising water to a high basin. How much would we need to store one year's worth of energy production? 10^16 Watts is 10^16 Joules/second. There's about 10^7 seconds in a year... 10^23 Joules. One Joule is 100 grams raised one meter. One of the largest existing facilities raises water 380m, let's take an average of about 100m. One Joule is 1 gram raised 100 meters.

To store 10^23 Joules, one year's supply, at 100 meters and ignoring efficiency loss (it's very efficient, about 80%) we'd need 10^23 grams. There's about 10^24 grams of flowing, fresh water on the Earth (ie. rivers) and we'd need 10% of it. That's a lot, and that's only for a year's worth of our current energy consumption.

You could use seawater, but that would increase construction costs and reduce efficiency considerably as places close to the sea tend to not be very high above sea level. You could attempt to use something more dense, but it would need to be easily pumped.

The storage plan has problems, and it only holds off the inevitable... Peak Sunlight!


The Earth receives about 10^17 Watts from the Sun. We're already using 10% of that and world energy demand doubles about every 40 years. If current trends hold, in a few generations we'll be using more energy than we receive from the Sun. There is only one long term solution...

Become a Type II Civilization

Instead of worrying about storing energy, which is hard for the energies and time scales involved, let's stop wasting so much. The Sun outputs a whopping 10^26 Watts and the Earth only gets a billionth of that. What a waste! Why worry about squirreling away scraps under the mattress when we could be gathering a billion times more? Instead of storing the scant amount of power we're getting, squirreling it away under the mattress, let's use that power to gather more! Invest! Make the pie higher! Become a Type II Civilization that collects all the energy from the Sun, or as much as we can feasibly get.

Use energy to build and put satellites into solar orbit to collect solar radiation and beam it back to Earth, eventually forming a Dyson bubble (Dyson Spheres being difficult to build and unstable). Let's do some quick calculations.

Now, how fast can we build this sphere? Let's say we can build and launch satellites at a rate which is a percentage of our total power. Need more materials? Use some power to grab an asteroid! This is basically investing in more power generators. We'll benefit from the power of compound interest, more satellites means more power means more satellites!

Our principal is the energy received by the Earth and the "interest rate" is how efficient we are at launching satellites. I'll assume we get really good at building and launching satellites so we're pretty much doing it all the time, so we can use the continuous compound interest formula.

power production = 10^16W * e ^ (interest * years)

Plotting it in Wolfram Alpha shows at a modest 1% annual increase in energy production means in 240 years we'll be producing 10 times more power than the Earth currently receives with power production on an exponential curve towards a Type II civilization in less than 10,000 years.

No new science is required, just a lot of practical engineering plus the will and time to do it. Solar panels, solar sails, ion engines, asteroid mining, gravity tractors, microwave and laser energy transmission... these are all current or near-term technology.

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    $\begingroup$ This answer already exists - though I think you state the intentions more clearly. Note that the purpose of the question is not "How to generate/capture even more energy from the sun", but "How do we store this energy" $\endgroup$ – DoubleDouble Aug 23 '15 at 18:54
  • $\begingroup$ The question also states that the technology level must be today's technology level. $\endgroup$ – HDE 226868 Aug 23 '15 at 19:01
  • $\begingroup$ @DoubleDouble Yeah, I didn't see that answer before I wrote this, my bad. I left mine up because it includes the compound energy production calculations. As I argued in the answer, this is about capturing energy from the Sun for future generations. But instead of just hiding it under the bed, I'm investing it! :) Otherwise my first paragraph holds true, it's ridiculous to store energy when there's so much pouring out of the Sun. And we know that's a reliable energy source for billions of years. $\endgroup$ – Schwern Aug 23 '15 at 20:05
  • $\begingroup$ @HDE226868 This is all possible with today's technology level. Solar panels, microwave energy transmission, asteroid collection and mining, ion drives, solar sails... all available or at the prototype stage. Given the absurdity of the basic requirements, what part of the proposal do you find fantastic? $\endgroup$ – Schwern Aug 23 '15 at 20:08
  • $\begingroup$ @Schwern There's a lot of experimentation needed before many of those become as good as they are needed to be. The part I find fantastic is "Become a Type II Civilization". $\endgroup$ – HDE 226868 Aug 23 '15 at 22:04
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To answer your question, lets actually plan for the final goal. We want to be able to built spaceships that will allow us to evacuate the planet when the current neighborhood becomes undesirable.

The only truly heavy lift technology we currently know how to design (in broad strokes at least) is a pulsed Orion spacecraft. A pulsed Orion drive would work by exploding fission or fusion bombs underneath a pusher plate and is capable of lifting city sized ships into orbit. You need about 800 bombs for low earth Orbit, so lets be conservative and say 2500 bombs for launching a colony ship to Alpha Centauri. The biggest design performed in the original Orion design work would lift 8 million tons, though they considered larger designs they did not flesh out the details. About 20,000 such super Orion ships would be about the right scale to evacuate the earth. So you need about 5 million bombs.

Plutonium 239 has a critical mass of 11 kg, so you need 55 million kg of P-239. Half-life of P-239 is only 24,000 thousand years so you actually need to be storing up U-238 which you can then breed in P-239 when you finally need it.

So, what you really need is 55 million kg of U-238. Global recoverable resources for uranium are estimated at over 5 million tons. We don't need to be storing up uranium for future generations. If we have extra power, we can use it to raise conditions for current inhabitants without worrying about the future needs for escaping the earth.

If you are just a prepper at heart, go ahead and refine and stockpile the uranium. However If you really want to plan ahead, build a big space elevator and start moving the population into space, more resources than will even be available when we limit our resources to just this planet.

Actually storing up energy for the future generations to abandon earth is a waste of time.

Now, let say that we know that people will use up uranium in the interim so we need to create uranium on a large scale using our energy surplus -- don't know why since we a getting all of our energy from renewables. Is such a thing possible? Yes we can transmute lighter elements into uranium by using particle accelerators and proton and/or neutron capture. If you capture excess neutrons, atoms will convert neutrons into protons via beta decay. Neutron capture is much easier since you don't have to overcome electrostatic repulsion of the proton and the atomic core.

Thorium is the common element easiest to convert to uranium since it is closest in mass. There is about 4 times as much thorium available as uranium.

Still want more uranium, lead is the only other reasonable source elements for uranium breeding that is more common than uranium and thorium as well as being somewhat close to the mass of uranium. But there is probably less than twice as much lead as thorium in the crust and it is much harder to convert into uranium.

There is actually a lot more uranium available if you are willing to work harder to get it. Proven uranium mining depends upon current economic viability -- if you are willing to pay a higher cost you could mine a lot more, same goes for thorium and lead too. If you get desperate you could even mine the Moon and Mars for uranium and thorium, as well as the asteroids.

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  • $\begingroup$ We are not necessarily storing energy for evacuation of earth, but for anything future generations might want/need it for. The alternative is letting excess energy go to waste. $\endgroup$ – DoubleDouble Aug 22 '15 at 2:40
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    $\begingroup$ Pre-refining uranium is actually counter productive to the goal, as various passive fission factors start piling up and your fuel starts to use itself faster than its intrinsic half-life. If you stockpile anything, it's the ores, chemicals and machinery to refine the fuel on demand. $\endgroup$ – Sean Boddy Aug 22 '15 at 6:31
  • $\begingroup$ Perhaps perform breeder reactions to convert non-fissile $Th_{232}$ & $U_{238}$ into fissile $U_{233}$ and $Pu_{239}$. Storage of large quantities becomes a problem though. $\endgroup$ – Jim2B Jan 5 '16 at 8:56
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Someone mentioned hydrocarbon storage. This reminds me of a quote by Feynman from his wonderful description of Fire:

...the light and heat that’s coming out [of burning wood] that’s the light and heat of the sun, that went in, so it’s sort of stored sun that’s coming out when you burn a log.

If you think about it really hard, wood is merely stored sunlight - literally stored energy. Wood the following advantages:

  • Wood requires very little effort to use to make fire. We've had the technology to harvest energy from wood from the dawn of civilization. Arguable all that's necessary form extracting fire from wood is knowledge: a wooden rod/dowel to generate heat and wood dust to capture embers.

  • It is extremely divisible. It's wood after all.

  • It is fairly cheap. Left alone, it literally creates more of itself.

There's one main disadvantage: storage. Wood is fairly inefficient when it comes to energy density. However, wood can be converted to liquid forms for storage: either alcohol or liquefied wood gas (basically methane). Both are comparable to petroleum when it comes to energy density. Storing large amounts of either would give future generations the same sort of energy economy we have today with petroleum.

For storing said hydrocarbons (even wood) the ultimate storage would be to bury them deep underground - you'd have practically unlimited space for storage. Future generations would just need to mine them the way we mine petroleum and coal.

Of course, for wood all you need to do for storage is to maintain forests. Forests are not the ultimate storage since they take up a lot of space but they're useful as an easily accessible form of energy. The energy stored in forests should be more than enough to be used to mine the stored high density forms of energy.

There's a secondary advantage to hydrocarbons: plastic. If not used as an energy source, they can be used to make plastics which will give future generations the same economic boost we had when we discovered plastic.

In terms of using said energy for space exploration, both alcohol and methane are viable rocket fuels. In addition, with enough knowledge of chemical engineering, any fuel can be used to generate electricity to power chemical plants to produce more advanced rocket fuels and oxidizers.

So, I'd propose the following strategy:

  1. Plant more trees and stop destroying forests - forests are the ultimate future resource for future generations.

  2. Farm forests in order to convert a percentage of the total number of trees on our planet to high density energy storage: alcohol or methane (or if we really want to push it, even diesel).

  3. Store the fuel produced in three stages: the bulk would simply be buried or pumped into mines so that future generations can in turn mine them the way we mine petroleum, a large amount would be stored in tanks in either remote locations or buried underground and lastly keep remainder as living trees in forests.


Potential problems:

While at first glance this seems ideal for solving both long-term energy storage and global warming, doing this at a large scale may have unforseen negative consequences.

Firstly carbon, like anything else on our planet, is a limited resource. Just as there's a limit to how much petroleum we can pump out of the ground, there's also a limit to how much carbon we can extract from the atmosphere (which is what planting trees does). Though technically, that limit is probably much higher than the limit we have with petroleum. At the very least we can extract the same amount of carbon we pumped into the atmosphere by burning petroleum.

Second, I don't know what would happen if we start sequestering carbon beyond what is natural. Just as releasing carbon into the atmosphere beyond natural levels have significant impact to the environment, so does removing carbon from the atmosphere.

I think that at its limits, if we ever reach it, we would have to start managing carbon balance rather than simply burning it like what we do today or storing it as per the plan I've outlined above. It's hard to imagine we'd ever reach this scale of industrialization. But it's not uncommon for sci-fi universes to have industries large enough for this to be an issue.

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Think wider.... think not only "how can we store energy", but also "What would they want to use the energy for?".

  • Fresh water. Desalination requires energy. Fresh water can be stored indefinitely.

  • Fertilizers. Making them requires energy. These can be easily stored.

  • Metals and other base materials. Breaking and refining metals requires lots of energy. Break them now and store them as ingots. Same for other materials that we use in our daily lives.

  • Hydrogen. Can be used in fuel cells for electricity, for heat, for combustion. Create by electro-hydrolizing sea water, which requires energy.

...and the list goes on. So - again - think wider, not only in terms of storing raw energy only, but also in terms of what we are using the energy for and which of those products are storable.

A quick note on hydrocarbon storage: bad idea. And anyone that cannot figure out why it is a bad idea should be thoroughly ashamed of themselves for missing the entire climate issue. Let us not repeat nature's mistake by — yet again — making hydrocarbon reserves available for humans to use.

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    $\begingroup$ I am thoroughly ashamed of myself. Are you referring to global climate change, possibly caused by burning hydrocarbons? Most methods of creating the hydrocarbons I've come across seem to pull CO2 out of the air - so wouldn't that be reversing, or at least slowing, the problem? $\endgroup$ – DoubleDouble Jan 5 '16 at 17:16
  • $\begingroup$ It would pull CO2 out of the atmosphere, creating a dip or a restoration to more normal levels. yes. But then time will pass and the atmosphere and the climate will settle and reach a tentative equilibrium. Then when the future humans find the reserves, they will upset that equilibrium and tip the scales again, just as we have done now. $\endgroup$ – MichaelK Jan 7 '16 at 7:40
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Store energy by storing the "how-to-do-it" manual.

I know that this is not a popular answer, but maybe your book, or your video or your whatever is documented way to use your fantastic energy source. SAVE your information in a powerful way so that anyone can use it at any time.

So, with current technology, the best way to store energy for future generations is not to put it in a battery (or whatever) but to put it in knowledge on how to refill the battery.

You completed your task with a pen and paper (kind of). Good Q.

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  • $\begingroup$ Richard Feynman encouraged people develop non-digital mean of storing large quantities of information. abc7news.com/archive/6755677 $\endgroup$ – Jim2B Jan 5 '16 at 9:00
  • $\begingroup$ Couldn't this be done regardless of what we do with the extra energy? Meaning, if the electricity powers the means of backup - the backup is not there when the electricity goes down. If the electricity does not power the backup, what are we doing with the electricity? $\endgroup$ – DoubleDouble Jan 5 '16 at 17:22
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Plastics,

to appropriate a famous line from a movie.

Look at fossil fuel: Lignum was created but nothing could eat it. It was the non-biodegradable plastic cup of its day, and it just piled up for 50 million years. Vast quantities were buried and taken in by geologic processes, to become coal seams. When fungi finally evolved a way to eat it, the coal beds were already processed by plain heat into nearly pure carbon and also safely out of reach.

So our landfills are doing today. Plastic cups and such are not eaten by decay processes, and won't be for thousands if not millions of years. We are already burying the stuff. In 200 to 500 million years it will be coal and deep underground.

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Donald Sadoway's Molten Salt Battery.

Donald Sadoway designed and assisted in the construction of protypes of a type of battery for large energy storage based on molten cheap minerals.

The structure, as described on the TEDTalk The missing link to renwable anergy is as follows:


Low density liquid metal (Magnesium)


Molten salt


High density liquid metal (Antimony)


Further research has introduced the use of other metals. A known drawback of the design is that it relies on stability of the structure to allow the layers of liquid to be maintained. A sudden strong movement of the batteries may cause a short circuit. This means that an earthquake could discharge the batteries, or worse.

Donald Sadoway's explanation:

To produce current, Magnesioum loses two electrons to become Magnesioum ion which then migrates across the electrolite and acccepts two electron from the Antimony and then mixes with it to form an alloy.

The electrons go to work in the real world (...) powering our devices.

Now, to charge the battery we connect a source of electricity - could be something like a wind farm - and them we reverse the current. And this forces the Magnesium to de-alloy and return to the upper electrode restoring the initial constitution of the battery.

And the current passage between the electrodes generates enough heat to keep it at temperature.

(...)

Stacking these (batteries) into modules, aggregating the modules into a giant battery that fits into a 40 foot shipping container for placement in the field. And this has (...) capacity of 2 MWh (...) that's enough energy to meet the daily electrical needs of 2000 american households.


From the review Liquid Metal Batteries: Past, Present, and Future

the demonstration of long-life liquid metal batteries still remains; however, based upon similar three-liquid-layer industrial electrochemical systems, such as the Hoopes cell, one might expect continuous operational lifetimes in excess of 20 years to be possible.

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You can create "sitting hydroelectric plants". The idea is to have water reservoirs in high altitude with a connected system to allow slow flow of water down thanks to gravity - opened in case of need. Water would flow down via pipes that conduct water thru turbines to generate electricty. So this is both a source of electricty and water.

Recharging means to pump water up. And can be done at a slow rate over time. So this serves as long term storage (req 1) and it is realatively easy to use (req 2). Although it is big (failing req 3) and it is not divisible (failing req 4). The cost of the structure may go down if a good location is found (req 5).

Decay happens due to deterioration of the material that holds the water. If there are leaks, water will be lost over time. It maybe possible to allow rain water to enter naturally, but care should be taken to not allow water to avaporate out of the container.

Of course, an obviour drawback is that it is taking water out of circualtion. If water treatment improves it is less of a problem, but still it is an added cost to be considered.

Edit: I just noticed this solution was dicarded by another answer.

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  • $\begingroup$ Hydroelectric plants require maintenance. $\endgroup$ – Jim2B Jan 5 '16 at 8:58
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Warning: This could be considered an answer to a differnt question. I'm aware of that.

It has been suggested to store generators instead of storing energy. I have also explored the idea of storing potential energy - that is: moving stuff up.

It is possible to create simple generators that will work by setting them in some relatively high location (such as a branch of a tree) and attach a weigh to them. The generator would allow the weigh to fall slowly, using the motion to generate electricity.

This method wouldn't store energy, nor generate large ammounts. But generators of this kind could be done en masse. Of course, the generators would degrade with use, yet it is possible to choose materials that would allow the machine to last for centuries if stored properly (say, in a vault).


This solution can pass all the requirements:

  • Requirement 1: It can be stored for a long time, by using slow degrading materials and proper storage conditions.
  • Requirement 2: It is easy to use, all it requires is to load a weigh, and it would have some energy output to connect an electrical device.
  • Requirement 3: It is very small - you could have a few in backpack. It can be transported to where it is needed.
  • Requirement 4: It cannot be divided per se. But the solution is a lot of small devices, spread them as needed.
  • Requirement 5: Each unit is cheap, although it will be a considerable cost at large scale... yet there is no need to do a single inversion, they can create a few thousand units per month of a long period of time.

it is expected that the machines would damage after months (up to a few years) of use.

And fail to answer the question: It is not energy storage.


For a modern real life version of a similar solution - although not to the same problem - see GarvityLight. A solution for increase durability should be possible.

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This is probably stretching the hard science tag, but if you could synthesize heavy elements (uranium & plutonium), you can manufacture fuel for nuclear reactors.

Some of those isotopes have extremely long high life and the energy density is tremendous.

The problem is that it will probably be complicated to produce the fuel, and to get the energy, you have to operate a nuclear powerplant.

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The best gift we could give to prosperity, as far as their energy needs go, is to develop ways to live prosperous lives that consume even less energy i.e., making things more energy-efficient.

As far as storage goes, turning atmospheric CO2 into lumps of carbon is a solution that requires only the know-how of doing it. Elemental carbon doesn't rot (AFAWK), and isn't damaged by anything other than fire. Makes bricks of the stuff and put them someplace out-of-the-way, in small enough quantities that a single fire doesn't take out too much of the supply.

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This is my source: http://www.rochester.edu/news/show.php?id=2154 https://www.extremetech.com/extreme/190555-this-nuclear-battery-could-power-your-smartphone-forever-as-long-as-you-dont-value-your-life-or-sperm-count-too-highly

BETAVOLTAIC BATTERIES

Similar to the way solar panels work by catching photons from the sun and turning them into current, the science of betavoltaics uses silicon to capture electrons emitted from a radioactive gas, such as tritium, to form a current. As the electrons strike a special pair of layers called a "p-n junction," a current results. What's held these batteries back is the fact that so little current is generated—much less than a conventional solar cell. Part of the problem is that as particles in the tritium gas decay, half of them shoot out in a direction that misses the silicon altogether. It's analogous to the sun's rays pouring down onto the ground, but most of the rays are emitted from the sun in every direction other than at the Earth. Researchers decided that to catch more of the radioactive decay, it would be best not to use a flat collecting surface of silicon, but one with deep pits.

So in theory you could store power for as long as the full decay of a radiotactive element, at a low cost, so just get something that has a halflife of a few million years and youre fine.

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  • $\begingroup$ Most radioisotopes with half-lives that long are alpha emitters, not beta. But you have a few choices like potassium-40. The trouble is that since they have long half-lives they give off very little energy per unit time, so they're a much worse source of energy than normal betavoltaics (which are already very low energy). $\endgroup$ – Charles Jun 8 '18 at 16:22
  • $\begingroup$ For example, with K-40, you would get 2.9e14 beta disintegrations per mol (40 grams) per year which produces 61 J. In other words, to get 10 cents of power (1 kilowatt hour) in 100 years you'd need over 50 pounds of potassium-40, and if the battery is as proportionately heavy as the prototype it would weigh over half a ton by my calculations. $\endgroup$ – Charles Jun 8 '18 at 16:51
  • $\begingroup$ But yes, it would last ages: after a billion years it would still be running at 58% of its original power, if your service contract took care of everything other than the decay. :) $\endgroup$ – Charles Jun 8 '18 at 16:54
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To expand on what Serban Tenasa touched on: Build generators. Instead of storing energy you store the means of making energy.

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    $\begingroup$ What fuels them? $\endgroup$ – Schwern Aug 23 '15 at 8:01
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There are so many answers here, and they are all over the map. The things being covered all make sense to some point, but they all have varying degrees of problems. They are all variations of a theme, which has actually been pointed out very accurately by Schwern - we wish to take the surplus power capacity, and invest it into something that will gain us something in the future. And other issues aside, Serban Tanasa is pretty much on track with the end goal - we want to build a Dyson mega construct. I believe it is fairly safe to say that the hydrocarbon solution proposed by Green is a crucial first step in the storage and investment strategy on our way towards taking these steps.

But the question is still, how the devil do we store usable energy energy in a usable form that will be useful in the eons to come?

Shipping Clean Water to the Moon

NASA is already very excited by the prospect of ice crystals on the moon. Because of our ability to generate fair amounts of electrical energy with solar power, we could process that ice into usable fuel to drive propulsion out of the solar system more efficiently. Ion engines are great, but if you have fuel to accelerate, the engines are far more powerful.

So, what if we shipped a billion gallons of water or so to the moon?

Overcoming the gravity well of Earth to place a significant quantity of usable water on the moon, or even in orbit, would present probably the most significant energy gain that we could achieve in space at this time. The storage mechanism is a bit non-intuitive, but you are literally storing potential energy in the mass of the water as you transport it away from earth. Landing a basic platform for power and food production on the moon and supplying it with copious amounts of water for fuel and personal use creates the opportunities that we would need to even stand a chance of building a Dyson construct.

After we move water to the moon, we could pay the cost to move other resources. If you want humanity to leave the Earth, they'll need a preexisting sustainable food supply, and that's going to be hard. We know how to grow plants in space, but we need to get started so that when we evacuate, it doesn't kill us just by starvation. The water cycle could be self contained, but you still have to have enough water in circulation in the plant life to sustain all crops.

Energy is only as useful as the things it is ultimately used on, and the creation of this potential will require the combined effort of most of the industry and economy on the planet. It will require the judicious application of almost all of the answers here. But ultimately, the final energy cost will be paid by, and stored in, going up.

EDIT

I have had a lengthy discussion with Schwerv, and I need to refine a few points. Admittedly, this is so expensive in terms of energy that if we could force an icy comet to land on the moon instead, we should do that. If we can field spacefaring robots to go and put asteroids on a workable orbit, we should do that. My point needs to be understood in the context that investing in a moon base as a place for those robots to come and go is a good thing if we can afford it and if it prevents those robots from having to come back to Earth, or from having to be serviced by Earth missions. This is the kind of surplus I was assuming was going to be available in short order.

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  • $\begingroup$ Let's do the numbers. The SLS block 2 will be able to move about 60 tonnes to TLI. A tonne of water is 1000 L which is 264 gallons. So the SLS can move 15,840 gallons of water. You want a billion? That's 63,1312 SLS launches. Start lobbying Congress now. Looking at it another way, \$1000 per pound to LEO is the magic cost efficiency goal everyone is working towards, still decades away. A billion gallons of water is 8 billion pounds or \$8 trillion dollars to get it to LEO. Getting to the Moon is even more. $\endgroup$ – Schwern Aug 24 '15 at 0:25
  • $\begingroup$ We almost entertained antimatter. And there aren't significant sources of liquid water anywhere else nearby. We need it out of the gravity well. Next destination Titan - you'll be able to make the oxygen you need to use atmosphere as fuel. $\endgroup$ – Sean Boddy Aug 24 '15 at 0:29
  • $\begingroup$ The Solar System is full of water. Launching it from deep in Earth's gravity well is a big fat waste of resources. You're better off harvesting it in-situ or grabbing a comet. $\endgroup$ – Schwern Aug 24 '15 at 0:30
  • $\begingroup$ @Schwern, totally understood - and agreed. But solve the problem of getting all of humanity in orbit. Or even a quarter of it. The moon has 38 million square kilometers of surface area is only 384,400 km away. At 8 billion people, that's still only 211 or so people per square kilometer. One way or the other, to get out, we need to establish an orbital platform somewhere that we can actually send big things - from Earth. $\endgroup$ – Sean Boddy Aug 24 '15 at 0:41
  • $\begingroup$ Surface area is not a problem, we have plenty on the Earth. Why go to the sterile Moon? Then there is a scale problem. Getting 400 million tons of people off the planet is not feasible. You can't evacuate Earth. Speaking of scale, back to water. 1 billion gallons sure sounds like a lot, but is it? Israel is just 8 million people (1/1000th of the Earth's population) with some very advanced water conservation and they use 270 billion gallons of water per year for agriculture. $\endgroup$ – Schwern Aug 24 '15 at 1:07

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