Heat Management

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Nothing is perfectly efficient, not even thermal devices that operate on heat, even in ideal cases. The only exceptions are when you are maximizing heat generation or moving heat around (which can actually exceed 100% efficiency). 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

Most, if not all losses that generate heat, can be traced down to three major things: Friction, electrical resistance, and energy conversion. It's impractical and pretty much nearly impossible to eliminate friction, and unless you have superconductors with zero electrical resistance, we're stuck with plain old copper wiring and semiconductors. As for energy conversion, it turns out that there is only one form of energy conversion which we can make 100% efficient: directly generating heat. We can achieve this with a variety of means, such as with electrical resistors; indeed, your typical home electric heater is a good example of such a perfectly efficient device.

The most famous example of how energy conversion is lossy is Carnot's ideal heat engine . From a thermodynamical perspective, the reason that fundamental inefficiencies and waste heat exist, comes down to the impossibility of absolute zero. Thanks so much, Heisenberg!

Heat transport

We can't destroy heat, so the only thing left to do is move it from where you don't want it to be. The simplest solution to this problem, is that as heat is here considered to be a property of any object - it stands to reason therefore, we can move the object itself around, to move the heat. In fluids, this process is known as advection.

But advection isn't the only way that we can move heat around. The mechanics for heat transport depends on the state of matter in the system we're considering, and while the two are largely coextensive, also the density of matter.

Let's see how heat manifests at the subatomic level. Here, like in the classical picture, heat also takes the form of random oscillations and motion. However, since protons and electrons possess an electromagnetic charge - they generate their own electric fields. Moving electric fields generate moving magnetic fields. These fields intermingle and couple to each other, resulting in the generation of electromagnetic waves - photons. The electron, or proton, loses energy to the photon flying off.

When our system is dominated by a vacuum, like with a gas - these photons can propagate freely. Heat transport here is thus dominated by what we call the mode of "radiation". However, when the density is higher, it's way easier for a photon to hit other objects, such as other atoms and molecules in the system - and these get reabsorbed, transferring heat to the object that was hit. Like the original object that emitted the photon, these atoms and molecules are too, excited, and will reemit another photon. Heat radiation is always present everywhere.

However, as the density of matter increases, so too does the opportunity for atoms to bond increase, and molecules to molecules as well. Two different modes of heat transport exist - heat conduction, and heat convection. In the process of conduction, the manifestation of heat as random particle motion and oscillations, bump into their respective neighbors. In this way, heat is transferred through these collisions, in liquids, gases and solids. However, while in gases and liquids the conduction is dominated by this kind of unorganized mess - solids also transfer heat through collisions of free electrons and the propagation of phonons. Phonons can be thought as quantized soundwaves, just as photons are quanta of light.

Through conduction, systems in thermal disequilibrium (such as if one end is hot, and the other is cold) will equalize as heat spontaneously flows and diffuses from these collisions.

Convection is the combined process of conduction and advection. A typical example is the currents of water in the Earth's oceans. Here, since hotter water is less dense than colder water - it floats on top. As water cools towards the Poles, it sinks downward. The current is then a clockwise (for the Northern Hemisphere) flow, beginning at the Equator - going northward to the Pole, and finally returning back where it started.

A similar process also occurs deep in the Earth, with heat travelling in great and immense loops from the core to the upper mantle - a process which is responsible for volcanism, plate tectonics, hydrothermal vents and so many other phenomena.

Heat pumps

Generally speaking, a heat pump is any device that makes use of work to transfer heat from a cool space, to a hot space. Heat always moves from hot to cold, allowing the system to reach thermal equilibrium - this is the second law of thermodynamics - but we can keep it from reaching equilibrium so long we provide a continuous supply of work.

If we have for example, a spaceship with an engine generating tons of waste heat. We want to reject this heat out into space obviously, but let's say due to our design requirements that the radiator has to run hotter than the engine. Because of the second law of thermodynamics, we can only do this by pumping heat from the engine to the radiator.

With heat generation, we can only achieve a maximum efficiency of 100%. If we measure their, and heat pumps' performance by the ratio of useful heating or cooling provided to work (energy) required - a "Coefficient of Performance" (CoP) - 100% efficiency is represented by a CoP of 1. It turns out that heat pumps are far more efficient than such heat generators, with everyday air conditioners achieving CoPs of 2.5 to 3 (roughly corresponding to 250% or 300% efficiency). This is because heat pumps can bring in additional heat from other sources, rather than just converting work to heat as with say, an electrical resistor.

Thermodynamically, heat pumps are modelled using what's known as "heat pump cycles", or refrigeration cycles. There are many such different ones:

For when the heat comes from outside, not within

Heat Shields

Heat Shields

Credit

Authors: Qalqulserut, Rocketman1999