Before you read this, I suggest that you read posts 16.34, 16.35 and 18.29.
In post 16.34, we noted that heat flows spontaneously from a hot object to a cold object. This process is shown in the picture above, where the temperature T1 is greater than the temperature T2, and the arrow shows the direction in which heat flows.
But, in post 18.29, we saw that spontaneous changes can be reversed by doing work on a system. So, by doing work on the system consisting of the hot and cold objects in the picture, we should be able to make heat flow from the cold to the hot object.
A device that does work on a cold object to make heat flow from it to a hotter object is called a heat pump.
We are familiar with heat pumps as refrigerators and air-conditioners. In these applications, heat is pumped from inside a box (like a refrigerator, a car or a house) to the its surroundings, as shown in the picture above. This is achieved by a mechanical device that does work – if you listen to a refrigerator or an air conditioning unit, you can hear that it is working. But you can’t hear hot tea cooling because it occurs spontaneously with no machine involved. (You can hear the machine working because some of its movement is transmitted to the surrounding air and your ear detects the moving air as sound, post 18.13).
So far, I have explained how heat pumps in a very abstract way. I’ve explained why we expect it to be possible to make a heat pump but not how one works. Reverse osmosis reversed a spontaneous process by applying pressure to a solution with a piston. But squeezing a cold object with a piston won’t squeeze its heat out!
So how does a heat pump work? Some gases can be liquified simply by applying a high pressure. Pressure squeezes their molecules closer together until they behave like molecules in any other liquid (post 16.37). Dichlorodifluoromethane (CCl2CF2) is an example of such a gas (see post 16.30 for an explanation of chemical formulae). Gases that can be used in heat pumps are sometimes called refrigerants. When the pressure is released from the liquid, it becomes a gas again.
When the liquid becomes a gas it absorbs heat from its surroundings; this heat is called latent heat of vaporisation (post 16.37). Heat is transferred from the surroundings to give the molecules energy to escape from the liquid surface (post 16.37) – remember that heat is simply the kinetic energy (post 16.21) associated with movement of molecules (post 16.35).
When a gas becomes a liquid, it gives out heat to its surroundings because the molecules in the liquid have less kinetic energy than the molecules in the gas. This is simply he reverse of what happens in liquefaction. The gas molecules transfer their kinetic energy to surrounding molecules.
If we liquify a gas, by compression alone, inside a box, it will absorb heat from the contents of the box. Now we pump the resulting gas outside the box and reduce the pressure. The gas becomes a liquid and gives out heat to the surroundings.
We have made a heat pump! In practice, the refrigerant circulates between the box and its surroundings in a closed loop