What is a heat pump?
The term heat pump is actually a bit of a misnomer. It conjures up a device that somehow forces heat to flow in the opposite direction to the natural heat flow, for example, getting heat to flow from inside a 25°C room to 35°C outside summer heat, or extracting heat from inside a fridge at 2°C and releasing into a 25°C room.
From a black box point of view, this works OK. But if you want to understand how a heat pump works, we need to open up the black box and take a look inside. Here we find that heat itself, is, well, not actually being pumped at all.
Rather, a heat pump is a multistage process that in effect destroys heat at one point, and re-creates it somewhere else¹². By doing so, it creates the illusion of “pumping” heat, while not actually moving heat internally.
Think of an atom that absorbs a photon, bumping an electron up to a higher orbital. The light energy disappears and becomes part of the energy in the atomic structure. Sometime later, the electron drops down again and releases a photon, creating light energy and reducing the energy in the atom’s atomic structure. Now imagine that in between the absorption and release stages, we can (somehow) move the atom from one place to another, say 10m away. This gives the illusion that we are “moving light”, when in fact we were just moving the excited atom.
Heat pumps make use of similar “reversible” processes. In thermodynamics a reversible process is something that in principle can be easily reversed without needing external energy. A great example is evaporation and condensation of water: the (forward) process of evaporation absorbs thermal energy, as the water molecules requires more electrostatic energy to support the gas form³. The reverse process, condensation, takes the surplus electrostatic energy no longer required in the liquid form, turning it back into thermal energy (or colloquially, heat).
There are lots of reversable processes, such as compressing gases, phase changes, stretching a rubber band and of course our friend the Peltier. In a Peltier, the forward process occurs when an electron passes from one type of conductor to another, and that particular conductor requires more energy (in the Fermi levels) to support the electron compared to the original conductor. Thermal energy is then forcibly sucked out of surrounding conductor - literally destroying heat - cooling the conductor. The reverse process occurs when an electron passes back to the original conductor type, which requires less energy. The excess energy is again forced on the surrounding conductor - literally creating heat - which we see as warming.
Human beings, being clever, figured out that if you can separate the forward and reverse effects of a reversible process on a working substance (gas, liquid, electrons, rubber band), and attach reservoirs at the right points to extract heating and cooling, you can in effect move thermal energy from A to B.
Logistically, this can get complicated. We need to make sure that these forward and reverse processes happen at the right places (or time), and also we need to grab the heating and cooling out of the working substance, and transfer it into useful places in the outside world. Otherwise, all that wonderful heat destruction and creation just circulates inside the working substance.
A practical working cycle transferring heat from reservoir A to reservoir B typically requires the following discrete stages:
Transfer heat energy from external reservoir A into the working substance
Convert heat energy into structural energy (forward process)
Move the working substance to B
Convert structural energy into heat energy (reverse process)
Transfer the heat energy from the working substance to external reservoir B
Move the working substance back to A (Note⁴)
Actual heat pumps can vary a lot in construction, stages can be combined or physically separated, with intermediate stages and so on. As we will see in future articles, a Peltier device can be enlightening because it is so simple and the above stages can be seen clearly once explained.
A lot of the confusion around heat pumps arises because the above stages are smeared together into a single event. For example, transferring heat to or from the reservoirs (1 and 5) relies on normal, run-of-the-mill heat conduction, which follows the second law of thermodynamics: heat flows from hot to cold with a finite temperature difference. Yet, these steps are ancillary to the core process (2, 3, 4, 6), and in practice the temperature differences at stages 1 and 5 are small and negligible. The core process itself does not rely on heat conduction, so the second law does not apply. And yet, in discussions about heat pumps, the second law is often quoted as a reason why heat pumps cannot work with a dT = 0, a totally false premise.
Future articles will explore this in more detail, but the key takeaway for this article is to understand that heat pumps at their core rely discrete forward and reverse processes (e.g. evaporation / cooling). Seeing these as two separate events (rather than a vague “reversible process”), and also seeing the working substance (refrigerant, electron) as distinct from the external reservoirs (room air, cooling chamber) is an important first step in understanding heat pumps.
Notes:
¹ Here the term “heat” is used in the colloquial sense, to mean thermal energy. According to the formal definition, “heat” refers to flow of thermal energy due to a temperature difference, and was derived at a time when heat was thought to be an actual fluid called “Caloric”. In the modern world, the term heat is interchangeable with thermal energy, but purists may complain about the use in this article. That said, a lot of misconceptions arise out of the insistence to use the formal definition of heat, when most of the world understands something else
² Purists will also complain about the term “destroy” due to the implication that energy is destroyed. The point here is that the original thermal energy is converted to other forms such as electrostatic potential energy, Fermi potential energy, magnetization energy, dielectric polarization, chemical, elastic strain and so on. In the thermal world, it is gone. Nada. Destroyed.
³ A common misconception is that as when a water molecule jumps into the gas form, it has more thermal energy and so leaves the remaining liquid with less thermal energy. That suggests the overall the thermal energy is the same. This is wrong: after evaporation, there is less thermal energy available, at an incredible rate of 2.26kJ per gram of evaporated water. This thermal energy is converted into electrostatic binding energy in the gas, it is no longer available as thermal energy.
⁴ Not all heat pumps (or heat engines) circulate the same working substance back to the start, it is only necessary that the forward and reverse processes are performed on the same working substance. For example, Peltiers have “fresh” electrons entering and pass through multiple heat pumps (PN junctions) and then leave the system. It is not strictly necessary the exact same electrons loop back and go through the Peltier again, only that the new electrons arriving are in the same starting state as the previous ones.