Heat Management

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Nothing is perfectly efficient, not even thermal devices that operate on heat. The only exceptions are when you are maximizing heat generation. These exceptions are: resistive heating (as with home radiators) or when you are moving heat around (you can actually exceed 100% efficiency with these devices, like air conditioners). From an engineering perspective, those device inefficiencies result in heat generation. Heat can also come from the external environment, like if you happen to be piloting a subterrene deep down in the depths of the Earth, or less fantastically, when you are being warmed by the sun's rays.

As said in the article about Heat, heat is a flow of entropy with an associated energy, and neither entropy nor energy can be destroyed. Therefore, the heat must be moved somewhere else, or kept in a place where it won't bother you (insulation - though in practice, nothing is a perfect insulator, and so the heat transfer will occur, just on a very slow timescale).

The cause of it all

This explanation is directly inspired by Veritasium’s excellent video on entropy (“The Most Misunderstood Concept In Physics”)

Imagine that you have an ideal case of a heat engine, consisting of various parts: a chamber, a piston, and a flywheel. The piston is connected to the flywheel, and the working fluid of the engine is a gas, like air. Now, let’s say that we have two big bars, really they can be anything – so long they have a temperature. The first must be hot, and the second must be cold.

When we put a hot bar under the chamber, it will transfer heat as the air comes into contact with the surface of the hot bar. The gas’s temperature increases, things vibrate and fly and bounce around, and expand; just as we learned in the Heat article that the average kinetic energy of a gas’s particles is proportional to its temperature. The hot air’s expansion keeps its temperature constant, while pushing on the piston. The flywheel turns as the piston is pushed upward.

Afterward, we remove the hot bar. The air continues to expand and the temperature begins to decrease, since there is no additional input of heat. In the ideal case, the air eventually cools down to until it is the temperature of the cold bar.

We put the cold bar under.

The flywheel continues to turn, and pushes the piston back down. Since we are increasing the pressure on the air, the total energy that was spread out gets compressed into a small volume. This is because the piston is imparting its own momentum into the gas particles that it comes into contact with. Since all these particles have little mass, what happens? Their velocity increases hugely.

p=mv

Momentum is the product of mass and velocity. When we transfer a slow-moving but large object’s momentum to a small object, it becomes fast-moving. The compressed air heats up.

But since the air is contact in with the cold bar, heat transfer is happening quickly enough that it becomes isothermal. If we didn’t have the cold bar under, the gas would continue to heat up and resist the compression of the piston. When the piston reaches a little before its maximum extent, we remove the cold bar and the air heats up as the piston reaches its maximum, full compression.

Then we put the hot bar under, and repeat the cycle over and over again if we want.

Now, in an ideal heat engine – the process is completely reversible. We could run the engine in reverse, and as long we run the same number of cycles in reverse as those that were done forward, no additional input of energy is needed. There was change, and then we reversed it. Nothing changed.

So what is the efficiency of this engine?

You might say 100%, because it’s fully reversible.

But that’s not what happens, even in the ideal case.

For when the heat comes from outside, not within

Heat Shields

Heat Shields

Credit

Authors: Qalqulserut, Rocketman1999