Nanotech free energy

[Ziggurat]

After running a simulation (because physics is hard), I'm fairly certain this setup would work, for an ideal classical situation. Why this isn't possible in real life seems really hard to understand, but seems to involve the fact that a one-way valve for atoms isn't possible. This paper seems to address the problem for actual, or at least possible, devices.

Short hand answer: temperature is linked intimately with energy scales in quantum mechanics. A given temperature has a characteristic energy for excitations (basically describing probabilities of excitations with that energy occuring). The energy scale for gas atoms must be such that some gas atoms have enough energy to trasfer to the door to excite the door into its open (higher energy) state. After the door opens, we want in to close again. And we want it to stay closed. In order for the door to close and stay closed (as opposed to bouncing open and closed), we need the door to be able to transfer that energy to the wall in a dissipative manner. The wall must have modes of excitation with that energy so that it can absorb the energy of the door, or the door will bounce back open. And here's the problem: if the wall can absorb excitations at that energy, it can also create excitations at that energy. If it is at the same temperature as the gas (which is going to be at the same temperature as your gas), then the probability of the gas exciting the door and the probability of the wall exciting the door are going to be the SAME. Which means that you cannot keep the door from swinging open randomly and letting gas out from inside, and you cannot create a pressure differential. The ONLY way to prevent the door from opening in the absence of an incoming molecule from outside is to siphon off energy from the wall - in other words, to actively cool the wall to a lower temperature, so that energy flows from the door to the wall and not vice versa. But actively maintaining the wall at a lower temperature, despite a continual input of heat, requires energy (just like running your refrigerator). And the energy required to do this will suck up any energy you could get from your pressure pump, even assuming perfect efficiency (and you're just losing energy if there's any inefficiency anywhere).

 

[Merko]

I doubt you can make that arbitrarily weak and yet still have a device that won't simply evaporate when struck by something massive enough to pass through rather than rebounding.

Consider a rubber band. These, as you might know, consist of loops of entangled polymere molecules. When the rubber band is deformed, these molecules do not break (unless we strain it too much), but the bonds change orientation. When we release the band, they regain their lowest energy orientation again.

I think we can see from this example, that the deformation can in fact be very significant, and the bonds still stay intact. In our case, we don't need a large deformation. We can make the required change in angle of the door arbitrarily small, by making the gap below the door only infinitesimally smaller than what is required for gas molecules to pass.

Also, the weaker you make the spring the longer the door is going to stay open.

Which is, for the n:th time, not a problem. It only reduces efficiency, it doesn't cancel the effect.

When I said "one way or another" I was referring to the spring. The components of the spring (and whatever the spring is bonded to) are all going to contribute some inertia to the door.

Sure. But consider my example with the rubber band polymere molecules. These bonds can obviously withstand the pulling of very long chains of polymeres to deformation, even when the bonds themselves are deformed, and without breaking. In our case, we only need it to be able to withstand one single molecule, which is perhaps heavier than the door itself, but not necessarily by a high factor.

Why does the interior of your device have more pressure than the room it is in?

That is indeed the question - the turbine is irrelevant.

The interior has more pressure because the door is biased.

Sum all of the energy in the system. For example, you talk about the molecule losing energy from the door, but then getting heated (speeded up) through thermal distribution through the walls (to quote: "The molecules regain this energy through normal heat transfer."). But then you don't account for the fact that the molecules outside the device are now cooler (slower).

No, they are not. Why would they be cooler? We 'lost' some energy when we hit the door, but that energy is re-distributed back to the gas, because the gas and the box obviously have the same temperature.

ETA: to make my turbine objection clearer. The opposite side (outside of your device) of the turbine is getting bombarded with molecules. The molecules inside your device have to fight against that pressure to exit the box. How? Where does this extra energy come from? Show the math.

I'm not sure what you mean with 'extra energy'. Of course, the turbine will only turn if the pressure inside the box is higher than the pressure outside. That is how turbines work. But we know that if this is the case, they work. So we can disregard the turbine completely, and focus on the question of whether there is a pressure difference to begin with. If there is, then the device works.

Again, assuming the device works, this is how the energy flow goes:

Heat in box (including the door) <-> Heat in gas
This makes sure that the gas both inside and outside the box, and the box itself, have the same temperature.

Then we have:
Heat in gas (aka momentum) -> Momentum in turbine
Momentum in turbine -> Electricity

These are one-way effects (strictly neither one of them is, but there is a net effect in this direction).

Result: We get electricity, but the entire system cools down accordingly.

I'll be the first one to call the matter closed if someone can show that it would break the first law of thermodynamics, but I feel very certain that it does not.

 

[Merko]

Suggestion: Let's use the term "engine" rather than "turbine" to drop all the implementation baggage the latter suggests.

Good idea.

And here's the problem: if the wall can absorb excitations at that energy, it can also create excitations at that energy. If it is at the same temperature as the gas (which is going to be at the same temperature as your gas), then the probability of the gas exciting the door and the probability of the wall exciting the door are going to be the SAME. Which means that you cannot keep the door from swinging open randomly and letting gas out from inside, and you cannot create a pressure differential.

This is covered in point 6 of the OP. The door will swing open randomly, but unless the device works, this will have no net effect.

 

[roger]

I don't believe you can ignore the turbine. It is in the system. Is the turbine biased the same way the door is? Because you have an opening on that side, and molecules are going to be trying to get in from that side as well.

 

[Merko]

I don't believe you can ignore the turbine. It is in the system. Is the turbine biased the same way the door is? Because you have an opening on that side, and molecules are going to be trying to get in from that side as well.

No, turbines are not biased. In that case, they'd be spinning backwards on a regular basis, for no apparent reason.

Anyway, the turbine - ehrm, engine - can be removed from the system, for example by a valve. If the pressure increases, we open the valve, and get electricity from the engine. So again, the engine definitely can be ignored.

In the further interest of sounding like real thermodynamicians, I hereby proclaim that the case where the device works will from hereon be known as the 'appalling conclusion'. Not because thermodynamicians are evil people that don't want to solve the energy crisis, but because like all good scientists they are not willing to let their subjective wishes trump solid, well-founded science.

 

[Ziggurat]

This is covered in point 6 of the OP. The door will swing open randomly, but unless the device works, this will have no net effect.

6. On this nano-sized scale, vibrations in the box would cause the hatch to flip open every now and then, allowing molecules to pass from either side. If there is an overpressure in the box, more molecules would then go out of the box, than inside it, equalising the pressure.

This argument assumes that there is an overpressure. If there is, we should indeed expect an equilibrium pressure to occur when this effect equals the effect of the hatch opening from hits by outside molecules. But for the equalising effect to occur at all, there has to be an overpressure in the first place.

Actually, this doesn't save you, though the reasoning here is getting a little more subtle and not quite so obvious. You've set up your device so that if the door opens because of an impact of an atom from outside, it's more likely to let that gas molecule in than to let a gas molecule inside get out (the door deflect the incoming atom inward). If that's not the case, then the idea is dead in the water regardless of random opening of the door. But there's actually a flip side of setting up the door so that it's more likely to let atoms in than out if they impact from outside. Look at the process: outside atom with high energy impacts door, door excites, door pushes atom from outside to inside, door transfers energy to wall. This is a reversible process: wall excites door, atom from inside is gets hit by closing door and turned into high-energy atom outside. In order for the process of moving atoms through the door to be biased in one direction when the door is opened from atoms from the outside (a requirement for your device though I don't think you specified it), it must ALSO be biased in the OPPOSITE direction when the door is opened by excitations from the wall instead of from high-energy atoms from outside. In other words, any time it opens randomly, it must be more likely to let an atom out than in. So there's no free lunch, and random openings will prevent the creation of any pressure gradient in the first place. The flows will equilibrate even without any gradient whenever the gas and the wall are at the same temperature. The only way to make this a one-way valve, as I already stated, is to cool your aparatus, which will take at least as much energy to do as you could generate with it.

 

[AWPrime]

Anyway, the turbine - ehrm, engine - can be removed from the system, for example by a valve. If the pressure increases, we open the valve, and get electricity from the engine. So again, the engine definitely can be ignored.

I don't think that the door mechanism is capable of producing enough pressure difference to start the turbine.

 

[Ziggurat]

I don't think that the door mechanism is capable of producing enough pressure difference to start the turbine.

That's not the problem. Any sustainable gradient, even a small one, could in principle drive an engine. It wouldn't need to be a turbine either, you could just as easily make the inside chamber a piston (which need not have any issues with backwards leakage or minimum pressure differentials). Merko is right to use the term "engine" generically, because the details of how you extract the potential energy present in a pressure gradient don't matter, the important part is whether or not you can generate such a gradient for free. You cannot, as I explained above, but that is indeed the heart of the matter.

 

[roger]

Anyway, the turbine - ehrm, engine - can be removed from the system, for example by a valve. If the pressure increases, we open the valve, and get electricity from the engine. So again, the engine definitely can be ignored.

Okay. Show your math, please. Specifically, how do you measure when the pressure gets higher? How much does that measurement decrease the pressure gradient? Where does the energy to open the valve come from? How much energy does it take to open a valve when there is a pressure differential across it?

 

[roger]

you could just as easily make the inside chamber a piston (which need not have any issues with backwards leakage or minimum pressure differentials).

How?

A heat engine requires a source of energy, and a cold sink. We have the source of energy, the gas molecules. So, what about the cold sink? If we are drawing energy away from the room with the gas molecules, then we no longer have a closed system. We have an external power source (something heated the gas in the first place) a temperature gradient, and cold sink, and no laws are being broken.

 

[Ben Tilly]

I read a scientific paper a few years ago that illustrated a device to extract energy via the Casimir effect. The device is perfectly consistent with thermodynamics, as the Casimir effect is just a peculiar potential that (measurably) occurs when metal plates are very close together. It's like getting energy from dropping rocks down a hill: sure it's possible, and it's perfectly good available energy, but it's extremely non-renewable.

As you note, the energy is non-renewable. Therefore to get net energy out, the energy it takes to build the device has to be less than the energy you get out of the device.

That's a pretty strong condition.

Cheers,
Ben

 

[Merko]

the important part is whether or not you can generate such a gradient for free. You cannot, as I explained above, but that is indeed the heart of the matter.

Of course it is not 'for free', it cools the system. But you didn't explain how your objection would cancel the bias of the door. We agree that it will swing open by chance, sometimes. How is that a problem? Why would there be a greater propensity for molecules to flow out, than to flow in, during these moments?

Specifically, how do you measure when the pressure gets higher? How much does that measurement decrease the pressure gradient? Where does the energy to open the valve come from? How much energy does it take to open a valve when there is a pressure differential across it?

We don't measure the pressure. The valve can be an automatic control valve, which opens and closes because of the pressure difference itself.
Such valves are very commonly used and obviously completely possible.
If there is no overpressure, the valve will never open, and it can be considered as just another part of the wall. If there is an overpressure, then, again - this proves the device works.

And here is a better article describing our type of valve, the relief valve.

 

[Ziggurat]

How?

A heat engine requires a source of energy, and a cold sink.

A piston need not be a heat engine. In this case, it's a pressure engine: it uses differences in pressure, NOT in heat, to drive the engine. You can USE heat to create pressure, but if you've got the pressure already, there's no need for the heat. Creating pressure gradients for free is just as much a violation of thermodynamics as creating heat gradients (which is why the focus is on figuring out why the molecular door doesn't work the way Merko suggests), but if you have one, you can exploit it for energy just as easily.

 

[Ben Tilly]

Regarding the Casimir effect, I think there are practical limitations- mainly that the most motion you can get out of it is like, three nano-inches; you might get energy if it was fifty miles wide, but where the hell you gonna PUT the damn thing? Heh, I'm spoofing, but you get the idea.

This is far less of a barrier than you might think. For instance, what do you think the surface area of the average adult human is? It turns out to be 6000 square meters! Most of that surface area is, of course, in your lungs. Which use a fractal structure to pack a tremendous amount of area into a small volume.

A device to use the Casimir effect would undoubtably have a similar design taken to an extreme.

Cheers,
Ben

 

[blutoski]

The turbine extracts energy from molecules passing, an open door does not.

Why have a door and a chamber? If the turbine extracts energy from molecules passing, put it in the clear. Adding a chamber and hatch just reduces throughput.

Instead of a chamber, just add a funnel to the turbine.

As mentioned above, though, you're still extracting energy from the molecules as they pass through the turbine, so a closed system will run down as the gas environment cools.

 

[Ziggurat]

Of course it is not 'for free', it cools the system.

That conserves energy, yes. But it's for free in the sense that you're decreasing entropy of the system without increasing entropy anywhere else (which would mean free energy from an economics standpoint).

But you didn't explain how your objection would cancel the bias of the door. We agree that it will swing open by chance, sometimes. How is that a problem? Why would there be a greater propensity for molecules to flow out, than to flow in, during these moments?

Because these quantum-scale processes must be reversible, and the probability for a process to occur in reverse must be equal to the probability of it occuring in the forward direction whenever the system is in thermal equilibrium (ie, we're not cooling or heating part of the system selectively). There's some intuitive logic to why this must be so, and you can find out more if you read up on thermodynamics and "detailed balance", but I don't really want to prove it if I don't have to.

So imagine the following processes:

1) Gas from outside transfers energy to door to open it, gas from outside enters, door closes and transfers energy to wall.
2) Gas from outside transfers energy to door to open it, gas from inside enters, door closes and transfers energy to wall.
3) Wall excites door to open it, gas from inside gets out, door excites escaping gas.
4) Wall excites door to open it, gas from outside gets in, door excites gas already outside.

Process 1 and 4 let gas in, processes 2 and 3 let gas out. Notice that 3 is just the reverse process of 1, and that 4 is just the reverse process of 2. Now, if 1 is more likely to occur than 2 (again, if it's not then it's trivial to show the idea fails), that also means that 3 is more likely to occur than 4. This means, quite simply, that in order for gas to be more likely to enter when the door is opened by gas from outside (p1 > p2), it MUST be more likely for gas to escape when the door opens by excitations in the wall (p3 > p4). The bias between processes 1 and 2 will be canceled by the biases between 3 and 4.

 

[Merko]

If the turbine extracts energy from molecules passing, put it in the clear. Adding a chamber and hatch just reduces throughput.

That obviously doesn't work, because there's no pressure difference, and the turbine never turns. The point of the chamber and hatch is that it (supposedly) creates a pressure difference.

Again, focusing on the engine part of the problem gets us nowhere. The hatch is the thing.

But it's for free in the sense that you're decreasing entropy of the system without increasing entropy anywhere else (which would mean free energy from an economics standpoint).

Right.

Now, if 1 is more likely to occur than 2 (again, if it's not then it's trivial to show the idea fails), that also means that 3 is more likely to occur than 4.

Well, sure, assuming that we don't believe it actually will work, it comes down to one of those two possibilities (with 1 and 2 each never happening as a special case). I think I've made a good case for why 1 is more likely than 2. I see absolutely no mechanism that could possibly make 3 more likely than 4.

Stating that it's impossible because of the second law of thermodynamics is easy, but again, it doesn't explain the problem. It's like claiming the magician doesn't really fly, because we know he can't - but we want to figure out how he does it.

 

[Ziggurat]

I think I've made a good case for why 1 is more likely than 2. I see absolutely no mechanism that could possibly make 3 more likely than 4.

That's just it: whatever mechanism makes 1 more likely than 2 is the SAME mechanism which makes 3 more likely than 4, it's just a time reversal of that same mechanism.

Stating that it's impossible because of the second law of thermodynamics is easy, but again, it doesn't explain the problem.

I'm making a much more specific claim than just saying "thermodynamics". Look into the term "detailed balance". That's what my argument hinges upon.

 

[Ziggurat]

As mentioned above, though, you're still extracting energy from the molecules as they pass through the turbine, so a closed system will run down as the gas environment cools.

The problem, once again, is not that this violates conservation of energy, but that it violates the requirement that entropy not decrease in a closed system. It's a 2nd law violation, not a 1st-law violation.

 

[Merko]

I'm making a much more specific claim than just saying "thermodynamics". Look into the term "detailed balance". That's what my argument hinges upon.

Right, but it appears to me that there is no reason why we must assume a detailed balance to be the case, than to satisfy the second law of thermodynamics.

And again, it does nothing to explain these mechanisms. The exact opposite of a molecule hitting the hatch, opening it, and entering, would be a molecule approaching from inside, the hatch vibrating open, the molecule entering the opening, the hatch vibrating back and hitting the molecule, pushing it on outwards of the box. This appears to be an extremely unlikely sequence of events, unlike the reverse. This is because there is absolutely no causal link between the molecule approaching and the hatch opening, and also no explanation for the propensity of the hatch to close and hit the molecule on its way out.

In other words, it does not appear to me that we have a system with detailed balance.