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One of the features in my world is a crystal cave. The crystal is mined to be used as a source of electricity. I want this electricity to power up simple electric devices, some sort of light bulb. I don't want complex stuff, because this is set in a medieval world.

What I imagine right now is the (long) crystal is broken into two, which causes the electrons to be divided unevenly and allows for electric current when connected with the light bulb. However, I doubt this process generates enough power to light the bulb, or that it's reliable enough to be used at least 2 hours straight.

What I currently have is a crystal made mostly of silica and impurities, just like crystal on earth. However, you can propose different ingredients, as long as macroscopically it looks like crystal, or a gem (even better!).

How can electricity be harnessed from such a crystal?

The proposed method, cracking the crystal into two pieces, is just what I've imagined so far. If you think such crystals can be used to fuel a steam generator (can boil water, for example), I welcome creative approach to use the crystals.

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  • $\begingroup$ Is it absolutely required that a single type of crystal produces electricity, and not two types which are distinguishable but fairly similar overall (e.g. "Red" crystal and "Blue" crystal)? $\endgroup$ – Kamil Drakari Nov 13 '17 at 20:13
  • $\begingroup$ @KamilDrakari no. It's fine to use different type of crystals. I'm planning to have a rainbow crystal cave, anyway. Cookie points if you can suggest crystals with many different colors. $\endgroup$ – Vylix Nov 13 '17 at 20:15
  • $\begingroup$ Reality Check: And have it light a 60W incandescent light bulb for 2h, or boil any significant amount of water? No; absolutely out of the question, for anything of a size you'd call feasible, unless you move on to chemical reactions, at which point the sky's (no longer) the limit. $\endgroup$ – Mazura Nov 14 '17 at 0:06
  • $\begingroup$ @Mazura eh, I don't care if the crystal chemically react to produce that electricity. But that kinda reassuring, meaning my original thought is right. $\endgroup$ – Vylix Nov 14 '17 at 0:46

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I don't know much about the possible chemical composition but I can give you ideas to generate the electricity.

Thermoelectric effect

enter image description here A thermoelectric material works heating one side of it when another one is cold, then, the difference in temperature between them would produce a constant flow of electricity.
Also, if you give energy to it, one side starts to get cold when another side increase its heat (transfer heat). For more information or how to make calculation relationated with it go to this place

Betavoltaic device

Basically, this device produces energy when it receives beta radiation.
I am not sure if this can be made naturally but for example, there exist diamond batteries that are able to produce a low flow of constant energy for thousands of years. Sadly, over time the radioactive material start losing its power and the battery decrease it voltage over time. I'll quote two paragraph of here:

These radioactive diamond batteries would have a very specific purpose – low power and extremely long life. A standard twenty-gram non-rechargeable AA battery stores about 13,000 Joules and will run out of power in about 24 hours of continuous operation. One diamond with one gram of carbon-14 would produce 15 Joules per day, much less than an AA battery. But the power output of the diamond battery is continuous and doesn’t stop. The radioactive diamond battery would still be putting out 50% power after 5,730 years, which is one half-life of carbon-14 or about as long as human civilization has existed. During this time, the diamond battery would have produced over 20 million Joules. And would produce another 10 million during the next 5,730 years.

Also, you can see this five minutes video.

Piezoelectricity

enter image description here

Elements like quartz have the ability to produce energy in their opposite poles when they are under physical pressure (like the smashed or compress). Also, if you give them the energy they have the opposite effect, they increase in size while they get energy.

Photoelectric effect

Another idea could be to use the photoelectric effect, and produce an electrical current based on the amount of light is received.

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    $\begingroup$ Fun fact, proposed Betavoltaic batteries actually are crystals, just like op wants. +1 $\endgroup$ – Mołot Nov 13 '17 at 21:39
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A piezo electric crystal can turn deformation of the crystal into electricity. It's not very efficient, but you can generate a spark. This is now some gas burner igniters work, and some types of lighters.

A watt of light is a good deal of light -- useful enough to light a room. There's a peace project for developing worlds that amounts to setting up villages with a combination of a small solar cell, a battery and a 1 watt LED. This is sufficient that kids can do homework, adults can work in the house after dark.

For a piezo crystal you have to provide the energy for the light. In a stove lighter, you hit the crystal with a hammer. (It's small, and spring powered)

If your crystal was 10% efficient at converting some form of mechanical energy into electricity and you had perfect LEDs you have to supply 10 watts of power. This is roughly equivalent to lifting a liter (quart) of water a meter (yard) every second. Probably similar to running a spinning wheel, or potters wheel, or churning butter.

A different way, requiring a bit of fantasy: The crystals have the ability to store sunlight. They soak up sunlight in the day, and release it continuously. How long the release takes depends on the thickness of the crystal. A thin crystal lasts only a few minutes, a thick one lasts for days. Charging time is longer for thick crystals.

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  • $\begingroup$ Interesting idea, I was just about to make a similar answer. $\endgroup$ – Ender Look Nov 13 '17 at 18:06
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    $\begingroup$ I have a small plastic dragon statue that is glow-in-the-dark and activated by light, so it'll glow a little bit for several hours after the light goes away. If this could be made more efficient (and not green light), you'd have the "light storing crystal", sort of. $\endgroup$ – computercarguy Nov 14 '17 at 14:39
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I'll go full lateral thought on this one ;)

enter image description here

Magnetite is a crystal. Under some circumstances, it can also be a permanent magnet. With some hand-waving, you could decide that the crystals in your story are quite strong magnets.

With permanent magnets, you can make efficient generators, alternators and dynamos. Here is a very primitive one:

https://catalogue.museogalileo.it/multimedia/PixiisMagnetoelectricMachine.html

Timeline of discoveries, inspired from real facts:

  • A permanent magnet makes a compass. These quickly becomes popular.
  • Someone invents the electrical battery
  • Someone notices current in a wire makes a nearby compass move
  • Induction is discovered
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You can use piezoelectricity:

it is the electric charge that accumulates in certain solid materials (such as crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress.

Quartz is piezoelectric, and quartz is made of silica. If you properly orient a set of crystals, manage to wire them properly and to apply stress in homogeneous way, you can manage to generate some current.

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  • $\begingroup$ Sorry for the edit. Is this powerful enough to lit a bulb? $\endgroup$ – Vylix Nov 13 '17 at 17:14
  • $\begingroup$ It depends on what limit you want to set on series/parallel generators... $\endgroup$ – L.Dutch Nov 13 '17 at 17:18
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    $\begingroup$ This directs me to many interesting application of piezoelectricity. I didn't even know it exists until you mentioned this! However, I feel like this is not a complete answer. Can you expand the idea a bit? $\endgroup$ – Vylix Nov 13 '17 at 17:21
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    $\begingroup$ IRL uses of the piezoelectric effect are limited to oscillators and measurement equipment. The when squeezed or heated, crystals generate a very small voltage (μV or lower). It might be possible to generate power this way. I imagine you have millions of crystals in a large shed connected to a water wheel. The crystals are repeatedly hit with small mallets and a complex maze of wires and rectifiers combine the charges they create into a constant supply of electricity. All to raise a single volt with very low amperage which dimly illuminates an LED. Maybe not what you want. $\endgroup$ – Luke Nov 13 '17 at 17:31
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    $\begingroup$ Piezo converts pressure changes into electric charge. You can light a LED with a small piezo crystal, say the size of a penny, but it will need constant motion, because it only generates power when the pressure (or bend radius) changes. Do you remember these shoes with a few LEDs that would flash when your foot hits the ground? Here's a video: youtube.com/watch?v=ACsy6xSIBm8#t=2m40s anyway the power generated is very small, it would be unusable without LEDs which are much more efficient than incandescent bulbs... $\endgroup$ – peufeu Nov 13 '17 at 19:27
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I would suggest some form of electrostatics. Most modern electrical uses are from moving electrical charges, electrostatics is the science of static (unmoving) electrical charges.

Some of the first discoveries of electricity were from electrostatics, rubbing dissimilar materials together (Amber crystals with a cloth) producing an electric charge.

In the 1800s, a type of experimental electricity generators were electrostatic generators. Some of the most successful were the Holtz and Wimshurst Machines, which used multiple spinning glass platters to generate electricity.

Your crystals could be used in a similar way to produce electricity from motion. Most electrostatic generators produce high voltage with very low current, they are often used to power impressive sparking science demonstrations. So a crystal based rotary generator could be used with a spark discharge to create light.


An alternate technique: you could use salt crystals as the electrolyte in some form of a battery. Being solid crystals would ensure chemical purity prior to a well developed chemical supply industry existing.

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I'm going to suppose that these crystals exist and behave the way you describe and I'm going to address what I think is your primary question.

How can electricity be harnessed from such crystal?

You describe breaking a crystal in half and having one half be positively charged(few electrons) and the other being negatively charged(many electrons). This is very similar to static electricity. To use this as a significant power source, the amount of energy involved must be much greater than when you pet a cat on a dry day. I think it would be most useful to think of the broken crystal as a charged capacitor. Capacitors are difficult(but not impossible) things to use as power sources.

Capacitors tend to want to dissipate all their energy at once. I imagine that if you touch the two halves together then there would be a spark, the charges would equalize, and all your power would be gone. Also, if you broke the crystals with your hands, the current would flow through you body and the crystal halves would again be inert. (This might be bad for a person, it depends on the size of the charge) The charge in the crystals would bu just as happy to dissipate into the ground.

The two halves would also be strongly attracted to each other in much the same way magnets are.

A capacitor with enough power to illuminate a light bulb for a couple of hours would be a dangerous thing. If it did manage to discharge all of its power at once, it could electrocute someone or start a fire.

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  • $\begingroup$ So, shot a crystal at your enemy strong enough to break it and enemy is electrocuted? Neat side effects that sound not be ignored in coherent world. $\endgroup$ – Mołot Nov 13 '17 at 22:14
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In a medieval setting you have a windmill. The crystals can be made to provide electricity by applying pressure or squeezing the crystal,if you will. You can have 'Windmill power stations" whereby you use the power of the turning windmill to operate a hammer or some kind of vice to hit or squeeze the crystals many times per second, and thereby obtain a kind of PWM DC power or Pulse Width Modulated. It is not a clean DC signal, but if you can cause the crystal to be activated several times per second(perhaps a staggerd array of hammers?) you will develop the equivalent of a constant DC current which will make a light filament heat up and glow, even though it is intermittent.

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Basically Traditional Batteries

It doesn't actually take much to make a basic battery in real life. The first "battery" able to provide continuous current was the Voltaic Pile, which was made with Copper, Zinc, and Brine. At a basic level, copper and zinc with brine between them created electricity, and stacking multiple of those together increased the power. Most disposable batteries still use similar principles, albeit more refined.

Replacing "Two different metals" with "two different crystals" probably wouldn't work in the strictest hard-science sense, but it's close enough to be believable in my opinion. A feasible explanation for substantive use at an earlier time could build on that by replacing the brine with a third crystal; if a Red crystal and a Blue crystal grow close together with a Green crystal in between, then an "all-natural" battery might form that would inspire curiosity and experimentation.

On the other hand, if you're looking for real-life elements that would work for this, a different path might be better. Both Copper and Zinc technically exist in crystaline form, but they are very far from the translucent gem-like shapes that come to mind from the word "crystal".

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Radioactivity

First of all, I would like to say that this answer is not 100% science accurate as I don’t masteries all the subjects involved in it. At least it is a try and maybe the detail level needed for your story will be low enough to be very free about little imperfections and mistakes.

So, maybe your crystals could “simply” be radioactive. Your crystal would have to be made up of silica and a lot of impurities like Uranium (U), Thorium (Th) or Radium (Ra), etc. I don’t know if it will be enough for a quartz-like crystal to become radioactive but if it’s not, there is another solution:

Torbenite

The Torbenite (Cu(UO2)2(PO4)2·12 H2O) is a natural radioactive mineral with a pretty green color. I am not sure this a crystal but you have to admit that it kinda looks like what people think about when you told them "crystal".

enter image description here [Source]


Ok, I have my radioactive crystal but what now ?

Generally, getting electricity through radioactivity is always a matter of heating water that will produce steam. This steam is then used to move turbines and so on… This is an over complicated way to produce electricity for your medieval world so is their another method?

According to this article, it seems that there is a way to obtain electricity from some complexes nanomaterials.

Quoting Phil McKenna (emphasis mine):

The materials they are testing would extract up to 20 times more power from radioactive decay than thermoelectric materials, they calculate.

Tests of layered tiles of carbon nanotubes packed with gold and surrounded by lithium hydride are under way. Radioactive particles that slam into the gold push out a shower of high-energy electrons. They pass through carbon nanotubes and pass into the lithium hydride from where they move into electrodes, allowing current to flow.

“You load the material with nuclear energy and unload an electric current,” says Popa-Simil.


Yes, but my medieval civilization doesn’t know how to design nanomaterials!

You are right. But maybe with a little interpretation of science we could archive to design such a material naturally. What we need is to combine carbon nanotubes with gold (and then to add a lithium crystal but it's not the most complicated part).

This paper, is talking about the possibility of finding single-walled carbon nanotubes (SWNT) in nature. One of their reflection is that:

The most likely growth mechanism for SWNTs in nature is the transformation of another carbon structure, or the chemical vapour deposition (CVD) of a carbon feedstock under appropriate conditions (conditions similar to those of arc discharge synthesis could also potentially be created by lightning strikes)

So, what if your civilization designed this material by mistake? An altar on the top of a mountain and an offering to some gods: a river of diamonds lying on a gold plate… Then a storm, a terrible storm and a lightning striking the altar! Under these condition, carbons atoms of diamond get reorder into nanotube shape as gold is mixt up with it…

This artifact is then set with lithium hydride and Torbenite crystal for decoration purpose and there you go: Electricity! (A small amount of course...)


Limits

As I previously said, you have to be free about some science simplifications: the altar part is far from plausible in a real world. Moreover, this lead to a single artefact so you will not be able to create electricity for everybody. Finally, we all know effects of radioactivity on human body (SPOILER : it never end up well), so people working around this artefact will have to be “replaceable” if they touch it regulaly and lick one's finger…

But of course this could lead to a sort of cult as the artefact is a gift from the gods and that it is an honor to be the one that maintain its power by “feeding” it with the green crystal…

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I would like to expand on the piezoelectric answer of @L.Dutch above. You may have an easier time integrating the entire pyro-thermo-piezo electricity triangle. See the link below for an example.

By utilizing the full breadth of this physical mechanism, you can effectively imagine a device which can accomplish whatever task your characters have at hand. Since these devices are made from crystals, you can also exploit their optical properties.

There are a number of existing technologies which make extraordinary use of these materials (for example, pyroelectric accelerators for neutron generation, thermoelectric waste heat generators, etc). So even if your world is based in medieval times, the possibilities are truly only limited to imagination (many of these devices have few moving parts).

It is also important to note that combinations of these crystals can lead to interesting devices. For a simple, real world example, check out the of combining two crystal to create thermocouples via the thermoelectric effect.

Need to boil water and charge a battery? Have the local crystalsmith grind down frensel lens to concentrate solar energy. The sunlight boils the water & induces the pyro electric effect in the crystal, storing charge for night use.

Electro-thermo-elastic formulation for the analysis of smart materials

http://cdn.iopscience.com/images/0964-1726/15/2/021/Full/1463701.jpg

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Piezoelectricity has been mentioned a few times already, but the impracticality of it seems to stem from the constant pressure being very...effortful, one person compared it to churning butter!

But what if you lined the underside of a busy boardwalk/road with crystals, and the wires connected them all to something that could be charged.

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  • $\begingroup$ I’m not sure what you’re suggesting.  That the energy would come from the weight and motion of the pedestrians?  If so, that does not seem to satisfy the spirit of the question. $\endgroup$ – Peregrine Rook May 15 at 4:57

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