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 try to understand how a heat pump works, it is misleading.
Instead, a heat pump is a multistage process that literally destroys heat at one point, and re-creates it somewhere else. 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 1m away. This could give the illusion that we are “moving light”, when in fact we are just moving the excited atom.
Heat pumps have “reversable” processes similar to the absorption/release of a photon. The forward process transforms heat energy into structural energy in a working substance, which is then moved about in space and/or time (or both), and then a reverse process is performed that converts structural energy back into heat energy.
There are lots of reversable processes, such as compressing gasses, 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, which requires more energy to support the electron. Heat energy is 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.
Logistically, there is a lot more to a heat pump because we need to make sure that these forward and reverse processes happen at the right places (or time), and also we need to grab that heating and cooling and get it out of the working substance, to make use of it 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 steps:
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*
Actual heat pumps can vary a lot and sometimes these steps are combined or have intermediate stages, for example an expansion valve in a compressor cycle (Step 2) may sit physically in third location between the main heat exchange (Step 1) and the compressor (Step 4). In a Peltier, Steps 1+2 occur in the same time/area (ceramic is the reservoir, which is bonded directly to the PN junctions), and similarly 4+5 at the other side, while Steps 3 and 6 electron (or hole) moving through the P and N cubes respectively to get to the other side.
A lot of the confusion around heat pumps arises because the above steps are smeared together into a single event. For example, transferring heat to or from the reservoirs (steps 1 and 5) relies on normal, run of the mill heat conduction, which must, according to the second law of thermodynamics, flow only from hot to cold and require a finite temperature difference. Yet, these steps are ancillary to the core process (steps 2, 3, 4, 6), and in practice the temperature differences are very small and often 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 in confusing and contradictory ways.
There is more to the story, but the key takeaway for this article is that heat pumps are multistage cycle that at it’s core has forward and reverse processes. must contain a working substance,
*Note: Not all heat pumps circulate the same working substance, 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 and return to the power supply. It is not strictly necessary the exact same electrons loop back and go through the same route, only that the new electrons arriving are in the same starting state as the previous ones.