How do I store enormous amounts of mechanical energy?

Question

Imagine a typical steampunk setting. Now, let's make two grand assumptions about it:

1. Steam engines here are MUCH more efficient than those in our world. Yep, we have to violate laws of physics for this one.
2. We can't use any electrical energy. To not touch physics again, let's pretend we self-restricted ourselves from using any kind of electricity.

These do not apply to the entire genre - just for here and for now, we need these two for our thought experiment.

While generating whole lot of a mechanical energy, we can't convert it to the electrical one. This forbids us from a convenient way to manage said energy. Yet, our steampunk world still needs to do it somehow.

Is there any effective way to store this energy now? Be it some "batteries" for long-term storing, or "pipes" for temporal one (while transmitting energy for short distances).

Or maybe we could find another convenient form of energy to convert to, working around it?

Assume we don't want to violate physics anymore - apart from those laws we've violated already, of course.

I just wonder how this whole world would look like - it's not really a topic many steampunk universes raise, even though this is (possibly) a main problem these worlds would have to deal with.

Answer 1

Pneumatics or Springs

If you're just storing energy locally, a flywheel or pumped hydro is your friend, but if you want to move energy around in physical storage "batteries", then your best bet is compressed air.

Pneumatics are already widely employed, but you can turn compressed air directly into mechanical energy pretty readily with an air engine or employ it directly, as in compressed air cars.

The only issue would be the materials science required to store gasses under pressure, but if you have steam engines (and magically effective steam engines, at that), that's probably addressable.

The other option I'm lifting directly from The Windup Girl: kink-springs. In the context of the book, kink-springs are kind of like old-school mainsprings from watches and other clockwork, but capable of much higher energy densities because <insert materials science handwavium>. If you're already playing around with physics, a material that can be a spring that winds tighter and with more energy than any metal we have access to, and which will rebound to its normal shape is more than capable of being a mechanical energy storage battery. The problem, as illustrated in part of that book, is that if you break the spring, all that energy gets released at once.

Answer 2

Massive flywheels, baby!

Although in our world we use them to store electric energy, that needn't be so.

There were even full-size buses propelled by flywheels, they were called gyrobuses, which if anything, is a cool name for a steampunk story.

Answer 3

Gravity Battery

A gravity battery spends energy to lift weights, which stores potential gravitation energy that can be extracted later by lowering the weights. See wikipedia

I believe this is a relatively new invention in our world, but the basic concept is really just a crane, which existed in antiquity.

I originally heard of this from a startup Energy Vault that went viral in the news a few years ago.

You could directly use the mechanical energy of the winches lowering the weights to power nearby devices, but to distribute power in as infrastructure, you would need some kind of mechanical-to-heat transducer, the steam analog of an electric generator.

Answer 4

You don't, You store the fuel for the magic steam engines and just have the steam engines in all sizes.

You don't want to convert then energy then store it, if your engines are that efficient, they are more efficient than ANY storage that can be used after that step. Electricity storage is largely pointless anyway in your world, you don't need to handwave it.

You just store the fuel that runs your super efficient steam engines. Steam cars were a real thing. Steam engines can be made very small, or absolutely massive. The smallest working steam engine will fit on your thumbnail, the largest is over a hundred tons and is larger than a house, you can believably have a steam turbine for whatever you want to run. If they are as magically efficient as you say they could be made even smaller.

mechanical storage in your world is only used to smooth out small scale local instability, such as small flywheels or damper systems.

Steam engines can be run off a variety of fuels, wood, coal, gasoline, kerosene, methane, fuel oil, essentially anything that burns. All nuclear reactors are still steam engines at their core, as are thermal solar plants.

A steam engine does not even need to be run all the time to work all the time, steam cars will run for miles even when the fuel is exhausted just on the steam pressure left in the system.

In your world everything has uses power has a steam engine in it. The ultimate steam punk dream. An anime "Kabaneri of the Iron Fortress" even plays around with a similar idea, everyone walks around with a tiny steam engine on their belt to run basically anything small, even guns are steam powered.

A big upside of this is you only need to handwave once the one you already have the magic steam engines, nothing else needs to be unrealistic.

Answer 5

I suggest you read Wind Up Girl. This is a post-apocalyptic world where the main characters are operating a spring-based energy storage system. Spring-based and flywheel-based present the best energy density short of storing it as chemical energy. Chemical energy is still your best bet. Compressed gas suffers serious issues with energy losses due to heating of gas on compression.

Here are a few useful statistics on energy storage techniques. All numbers list where the values max out. Lithium-Ion presented for comparison. Note that density is per volume. If you want to carry this thing around, you need to measure it by kg.

Method	max (MWh)	Density (Wh/l)	Issues
Pumped Hydro	3000	2	Requires lots of land
Compressed gas	1000	6	Efficiency loss from heating
Molten Salt	150	120	High max temperature
Lithium-ion	100	400	It's what we do
Flywheel	20	80	Frictionless bearings, explosive failure
Kink spring		100	Extremely low density per kg

Addendum: Based on OP, flywheels are out of the question. There are no schemes that I'm aware of that allow you to move power into/out of a flywheel without electricity. There is a method that uses magnets, but the loss to heat is intense.

Addendum2: There are numerous examples of using flywheels for short-term energy storage, connected by continuous variable transmission. CVT energy loss is a function of time, making this a viable way to store energy between, for instance, the time between when a bus stops at a bus stop and when it starts up again.

The cam shaft of a single-cylinder engine is basically a flywheel that stores energy from one internal combustion explosion to deliver it to the next pressurization cycle. Such capacitor-like storage of energy is not only possible, but commonplace. It's also not the thing that the OP is looking for. He wants something he can top off and leave it for days, if not months, and have it available when he needs it. A flywheel that lacks electromagnetic levitation will bleed its energy into heat far too quickly to be used for such purposes.

Answer 6

There are actually a fair amount of examples in the real-world, at least for long-term storage.

I'll start with the exotic Sand Battery, by Polar Night Energy which may be the least possible for you. Their concept is simple: an air circuit is embedded in a silo filled with sand. Heating the water leads to storing heat in the sand, cooling the air leads to extracting heat from the sand. Their goal is to heat the tons of sand to 600°C over the Summer, and have the battery last all Winter long, sustaining an entire small town. That's... a lot of energy.

On the more mechanical side, gravity is a good friend. The Swiss are known to buy electricity from their neighbors when it's cheap and use it to pump water up, then release the water when the price goes up, producing electricity they sell back to their neighbors at a higher pricer.

Multiple start-ups are currently investigating more elaborate systems, which generally involved moving massive weights up and down. Those Gravity Batteries operate on the age old principle of the pendulum clock (1656):

• Energy Vault: building with giant bricks being raised and lowered.
• Gravitricity: weighs are being raised and lowered in a vertical underground shaft, such as former mine shafts.

Others have mentioned flywheels, etc...

Note that those solutions are not necessarily good in terms of density, they are for high-capacity, long-term storage. For vehicles, you'll probably want steam engines, with coal/petroleum/... and for very low-capacity, short-term, then you can turn to small springs/micro-explosions.

Answer 7

Or maybe we could find another convenient form of energy to convert to, working around it?

Chemical energy

This is borderline moving from steampunk into dieselpunk, but might make sense for you.

This is also very commonplace in gasolinepunk settings such as real life.

Methane is a source of energy but it is not the best source of energy. Petrol is safer and easier to transport and use due to gasoline not being a gas (that came out weird).

For this reason there is a process called Gas to Liquids, or GTL for short, which converts methane and other lighter hydrocarbons into gasoline or diesel.

And remember, a tank full of gasoline is nothing more, nothing less than a chemical battery. An internal combustion engine is just the boring way to tap into that stored energy, the cool way being a match and optionally canned oxygen.

Anyway, GTL requires energy, so that may be where steam comes in. Depending on economic factors, or technological limitations, steam might be cheaper for large scale applications such as power plants (specially considering its magically more powerful in your world). So you extract methane from wherever, use steam to convert it into gasoline, and then people can use that gasoline to power portable shaving razors or grandfather clocks or whatever it is that your characters use.

By the way, for comparison, a real world 60Ah car battery can hold around 2.6 megajoules. One kilogram (2.205 pounds) of gasoline in a tank holds around 45.8 megajoules. That's about 17.6 fully charged car batteries in a little less than a 1.5L coca cola bottle (gasoline is lighter than water, at ~743g/L).

Storing energy in this way rather than mechanical means also gives your world a way to have a proper energy market. I love all the other answers, but there is a reason why in our own world we buy a liquid rather than pieces of metal spinning at 3,000 rpm or weights tied to springs to put in our cars and trucks. And the only other economically and physically practical alternative has been ruled out in the question.

Answer 8

Steam engines are very efficient in some unexplained way (since they're already something like 40% efficient as a comment says, this really does need magic physics, but this is allowed).

So a really good way of storing and transporting energy in this world is ... steam. You can store it under pressure in very insulated boilers, transport it along insulated pipes.

Answer 9

Mechanical energy could be used in a compressor / cooler unit without electrical input to produce liquid air or liquid nitrogen. With appropriate insulation this can be stored for a long time.

In fact this method of energy storage is currently under development. it uses modern electric compressors and other electric equipment, but these are not essential to the process and could be dispensed with at some efficiency cost.

To regenerate the mechanical energy the liquid air/nitrogen is warmed via the environment (reverse heat pump) and/or other sources of low grade heat to produce a high pressure gas that is then used to spin a turbine.

https://highviewpower.com/news_announcement/highview-power-unveils-cryobattery-worlds-first-giga-scale-cryogenic-battery/

Answer 10

To make this really, really simple let's stop for just a second to see what's really going on: You want some method (or methods) to successfully store WORK done by another engine previously so that you can harvest later to get more WORK done. The easier it is to harvest this WORK, the better.

There are loads of ways to do this, some more inventive than others. (There are lots of options given above, for examples.) Unfortunately I spent an inordinate amount of my youth studying physics and gaming, and had become quite inventive in how this might be accomplished - for portable and non-portable methods.

My favorite portable method was to build small elastic liquid-filled spheres with a single "port" on them. The port was a bayonet-type port, meaning it wouldn't open unless it was secured to another connector. The only requirement for using these was that the sphere be connected with the port at the lowest point of the arrangement - either charging or discharging. One would connect the sphere to a "charger" which would then force air (or other gas) bubbles into it through the port, letting some liquid/fluid act as a one-way valve, and building pressure throughout the elastic "ball" to charge it up. I used water, but you could use oil, alcohol, even maple syrup - it really doesn't make much of a difference. WORK was done to compress the gas (air or whatever) into the "ball" past the liquid, which was then stored in equilibrium of the elastic sphere and the valve. This made it possible to carry the energy around, stored and ready. Or roll it around, if it's too big to carry handily. (In my game world they were made from a rubber-like elastomer that was incredibly tough and cut/puncture-resistant.) Because most of the battery was just compressed air (or some other gas, whatever you prefer) it didn't really weigh any more charged or discharged - so that was just a function on volume. A fully-charged sphere would naturally be physically larger than an empty one. BUT when someone needed to transfer some stored WORK to their application, it was easily transportable and almost immediately ready (you still had to connect it in the correct orientation for it to work). As a bonus, in case of an uncontrolled fire you could just chuck a few batteries on the flames to extinguish it! (Bet you can't do that with today's electrical batteries!) I said the elastomer was cut and puncture resistant. I never said anything about it being fireproof.

I almost forgot - the reason for the liquid/fluid at the bottom of the sphere was two-fold: initially it acted as a "lock" because it was heavier than the gas which bubbled through it to be trapped inside the sphere, and when the elastic limit of the sphere was reached the fluid would get pushed back in through the valve, making it possible to trigger a disengage (or some other activity) to prevent the sphere from being destroyed by being over-inflated. Safety first, and (try to) make it last.

I'll have to scare up my notes for other examples if you need them. (I'm currently at work - on break - and was curious if this would help.)

Answer 11

Steam Accumulator

Spirax Sarco has a detailed engineering explanation of how they work and ways in which they can be configured. The image below is one example from their page.

Essentially you take your steam source and bubble it through water in another pressure vessel. The vessel stores thermal energy in the water. When the pressure is reduced, that super-heated water flashes back to steam. They are commonly used in real world industry to balance the boiler capacity and the process load. The boiler is operated at a higher pressure than the accumulator and control valves are required for a constant steam output.

Steam accumulators bear the same high temperature pressure vessel safety risks as a boiler, but are very efficient when properly insulated. If you are storing fictitiously large amounts of energy without regards to the vessel cost, the efficiency goes up as the vessel gets larger because volume increases faster than surface area.

And of course, more steam and pipes in a steampunk world couldn't hurt ;-)

Answer 12

I interviewed at place that was building flywheels on magnetic bearings in vacuum as local back-up (electrical) power. As long as we're imagining, could we imagine a concentric turbine setup where power is applied as a super-heated steam jet near the periphery, with the flywheel spinning so fast that air and water vapor are flung out, away from the moving parts. Gotta keep wht wheel in a vacuum. This would require a gigantic radius and head room outside it.

Power could be extracted by admitting a working fluid- air, water, olive oil, to a compression turbine on the other face or outside the power-in turbine's diameter. The compressed or accelerated flow exits the container, energized by the flywheel. Can't slow it down enough to compromise the vacuum. Steam to accelerate would go out the same way, and carry with it high residual energy.

Is this too painfully similar to a perpetual motion setup? Maybe.

Answer 13

Physicists have a name for this: potential energy.

In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.

Use steam to pump water up a steep hill to a reservoir; if the land isn't that steep, build large water towers.

Then, when you need the energy, let it flow down to spin whatever you need to spin and then do other things.

@John is right, though: just make the steam when you need it, since coal and wood are a lot easier to store.