# Can a machine benefit from stored heat energy?

## Setting Overview

My world suffers from an abundance of a particular type of energy. This energy can be safely absorbed and utilized by living cells, enabling, for example, a dragon's flight and fiery breath or a human's ability to manipulate his environment through "magic."

However, this energy also amplifies the effects of electricity, such as lightning or computer circuitry. In the latter case, unshielded components tend to fail spectacularly in short order.

To balance this extra energy, I've ruled that iron can absorb immense amounts of it, which causes the metal to heat. The more it absorbs, the more it heats up. The disruptive, raw energy is safely converted into heat energy.

## Brief History

When humans first arrived on this world by spaceship, they brought with them advanced, electricity-based technology. These devices were quickly adapted to the new world's environment and the humans started terraforming the world.

The planet's indigenous flying and fire-breathing species didn't take kindly to this and banded together to end the technological threat to their world. A single AI managed to escape the onslaught by tunneling deep underground, where it has remained for several thousand years formulating a revenge plan.

## The Problem

The AI has devised a plan to end the reign of the raw energy, but it needs to be able to act on the surface. To do this, it has decided to construct semi-autonomous machines to go forth and do its bidding. These machines, of course, have to be protected from the raw energy, so they will be constructed using copious amounts of iron (to absorb the offending energy) and copper (to redistribute the heat from the iron into a heat sink).

Unfortunately, this means the machines have this giant heat sink taking up space, and that is inefficient.

Can the machines be engineered in such a way to re-use the energy stored in their heat sink, such as granting them a fire breath akin to the dragons they're going to be fighting, or are heat sinks a one-way trip for energy?

About the tags: I had in mind designing these machines to mimic real animal designs. This particular story was heavily influenced by playing Horizon: Zero Dawn. The tag is intended to reflect this concept.

For storing the energy, what you want are salt batteries (not simply iron heat sinks).

This is a technology we're looking at today in terms of things like molten salt reactors and solar thermal energy.

Thermal energy can work very similar to inertia. The idea is that excess heat from a reactor is put into keeping a eutectic salt in a molten state. The sheer mass of the molten salt makes it very difficult for it to quickly cool off, and so it acts as a storage sink for excess energy. The eutectic properties of the salt also mean that it doesn't want to give up the heat very easily once it's received it, as a eutectic salt must freeze "all at once" rather than in parts like water.

So a good material for your battery must have two properties:

• It should have as low a melting point as possible, as a liquid generally can conduct thermal energy easier than a solid due to its atoms having more degrees of freedom.
• It must be eutectic, so that it becomes difficult to freeze (and thus give up thermal energy to the surroundings). This provides a sort of "thermal inertia" that allows the salt to store heat energy for a very long time.
• It must have a very high boiling point. Eutectic salts generally have boiling points up around 2000 degrees C, which is why they're being considered as a coolant and fuel carrier for nuclear reactors.

Now, to use this energy, you have to have a thermal gradient. The stronger the gradient, the more efficiently you can extract work.

If you're trying to minimize the size and number of moving parts for your robots, a thermoelectric solution is your best bet. Basically these are peltier coolers run in reverse: they have P and N type doped silicon sandwiched in such a way that the thermal difference between each side of the silicon forces electrons to flow between them, creating a charge differential. Charge differential is what causes current to flow in a wire, so you've just created electricity. Unfortunately, thermoelectric generation tends to not be very efficient.

If you're trying to maximize the amount of work you can get out of this heat, you need a thermodynamic cycle. Stirling engines are OK for this, and they're going to be more efficient than thermoelectric generation, but for maximum efficiency you'll want a Brayton cycle.

In a Brayton cycle, you use the heat to heat a compressible working fluid. This fluid then spins a turbine, generating electricity, but part of the energy is used to recompress the working fluid before sending it back through to be reheated. Compressing a fluid increases its temperature per the ideal-gas-law, so you're recapturing some of your waste heat to make things more efficient. This makes the Brayton cycle one of the most efficient thermal cycles, approaching 45% efficiency (opposed to the simpler Rankine cycle's 30% efficiency, or a Stirling cycle's 15-30% efficiency).

The drawback? The Brayton cycle is much more complex, and requires a specialized turbine and two separate heat exchangers. This would mean you'd probably want to reserve Brayton cycles for your largest robots, perhaps using thermoelectric and Stirling cycles for smaller machines.

Once you have a means of storing (through salt batteries), and converting (through heat engines or thermoelectric silicon) the heat, you can do pretty much anything you want with it. Turn it into electricity for lightning attacks, use it to heat air for heat breath attacks, turn it into mechanical work to move your robots and swing big hammers to smash all humans, etc...

The thing to remember is that you have to keep as big a heat gradient as possible to be efficient, and that all heat engines generate some waste heat, which must be dealt with. Your battery is a battery, and not a heat source, so without further input into your battery, it will eventually cool off too much to be able to do useful work.

A tank of water makes a terrific heat sink. The heat of vaporization of water is huge -- several times what it takes to heat water from barely thawed to just about boiling, which itself is a lot (water has one of the highest specific heat capacities of any simple substance). Even better, a small tank of water is renewable, just by "drinking".

Your autonomous machines need only take on water periodically, let the steam go, and go about their business. They might even be able to use the steam to generate energy they can use to run themselves.

Heat sources can produce electricity in different ways.

However, if you change natural laws by introducing this new energy, all bets are off and does not really apply. as we know it might leave doors for undiscovered paradigms, but a new "energy form" would have to explain how it fits into the EM spectrum, or why not.

Thermodynamics tells us that to generate work, we need to have two thermostats at different temperatures, and that the ideal yield of that conversion will be $$\eta= 1 - T_{high}\over T_{low}$$.

Therefore, as long as your energy is stored at $$T_{high}$$, with $$T_{low}$$ being environmental temperature, you can produce work by using the temperature difference. However, the temperature gap has to be large in order to get a decent yield.

As an example, if the thermostat at high temperature is above the boiling point of water, you can produce steam and with that steam powering a turbine, which in turn can be attached to a generator to produce electricity. You would use the environment to close the cycle. This is precisely how geothermal energy is used: Earth mantel acts as storage of heat at high temperature, and water is converted into steam when in contact with its heat.

• Hm. While this answers the question of "Is it possible?" I feel it lacks clarity on how it's possible. What's the process by which heat energy is converted into work? Maybe it's a limitation of my understanding of thermodynamics, but I struggle with defining a path forward from heat that isn't light. – Frostfyre Jul 30 '19 at 15:54
• @Frostfyre, added in the edit – L.Dutch - Reinstate Monica Jul 30 '19 at 16:16
• @Frostfyre, a Stirling Engine. You use the heat differential between whatever your hot object is and a cold heat sink and convert the heat flow directly into mechanical energy which can be used to power a generator, for example. – Keith Morrison Jul 30 '19 at 21:59

Energy Absorption Mechanism + constant movement

This is a dumb idea but hey, worth a shot. The idea is that these machines must have some initial energy stored within them to allow them to get to perform certain tasks as directed by the AI. Obviously the dragons and whatnot will try to fight them back, so instead of just containing this energy within a massive heat sink, just use the energy as fuel by absorbing it using technology to absorb the kinetic and thermal energy of the so-called 'raw energy' you mentioned the dragons using. Of course, I'm assuming that the 'raw energy' you described has these properties.

You already mention iron and copper to be used for channeling this energy into the heat sink, but why not turn the tables - why not use the raw energy to power your electric technology? We know that energy in all its forms can be transformed into a different type of energy (GPE to KE, Thermal to KE, Chemical to KE, etc.) so perhaps your AI is advanced enough to figure out how to use the raw energy, or RE, into Thermal or Kinetic energy!

Now this means that it's still possible to destroy one of these machines by overloading it with energy. In this case, to mitigate this problem we can make sure this machine works harder and uses more of its energy, and perhaps convert stored energy to into raw energy blasts (if that works on the dragons). Literally fighting fire with fire.

We can complicate this further by making the more operational/technical robots (like the ones re-starting terraforming or whatnot) would have high heat sinks, whereas combat robots would have low ones as they'll be more equipped to fight back anyway. If they are left with any excess energy, it can be passed on to the operational robots.

Edit: I mentioned piezoelectric materials - I messed up. From this pdf, we can see a comprehensive research paper done about how a battery's energy can be derived from using mechanical energy derived from piezoelectric components in and around the battery, effectively making a battery charge itself. That would likely exacerbate your problem as your problem has to do with getting rid of excess energy, not making more. The reason I kept my answer generic was because I didn't know what raw energy was (still don't technically) as it exists in the universe you built. More information about RE and its limitations would be helpful.

• I've down-voted this answer because the question has the science-based tag, and we expect your answer to be clear, authoritative and explain why it's correct. I'll remove the down-vote if you edit to explain "solar panels (or whatever you can absorb energy with)" in a way as to fit the requirements of the tag. Also, how do piezoelectric materials store energy? – Tantalus' touch. Jul 30 '19 at 16:26
• Hi @Confoundedbybeigefish. I just edited the answer. The reason I used the language that I did was because I don't know what device could convert raw energy into something else. It doesn't seem like raw energy has a direct equivalent in our world, hence my bad language. – cyber101 Jul 30 '19 at 17:59
• Hi, I've withdrawn the down-vote. I think that you've spotted the trouble with questions that have a magical premise but use the science-based tag - it's not easy to see how to answer them in a meaningful way. – Tantalus' touch. Jul 30 '19 at 18:09
• I'm pretty new here as is - I should learn the ropes haha. How do you go about answering questions that have a magical premise + scientific inquiry? – cyber101 Jul 30 '19 at 18:23
• With this question you might want to give it a day or so, the come back and read the highest voted answers and the lowest and compare the content. Best I can do to help at short notice. – Tantalus' touch. Jul 30 '19 at 18:40

However, this energy also amplifies the effects of electricity, such as lightning or computer circuitry. In the latter case, unshielded components tend to fail spectacularly in short order.

This is marvellous news. Now for every watt of energy used, many more watts of power become available.

If anything the problem here is that your electronics would be too effective. So, your AI should build its autonomous drones on the assumption they can produce a fraction of their normal power. This energy field will make up the shortfall.

As a result, the AI can use a standard template model, but use weaker components and get the same result. These new versions will be lighter and more efficient.

The AI might even use a large lump of iron as a means of generating power, similar to how Radio Thermoisotope Generators power satellites using the heat of plutonium and other radioactive decaying materials

So the TLDR, this is not a problem, but a huge advantage.

If this advantage is not to your liking though, the AI could use copper and aluminum instead, and avoid iron in most components. There are plenty of ferrous and conductive materials that aren't iron. Some of our most advanced electronics relies on Silicon, copper, Gallium Nitride, and various rare elements, Iron isn't necessary