Heat pumps

Remember the 2nd law of thermodynamics? If not here it is: Heat does not move from a cold place to a hotter place by itself. On the contrary, it moves from a hot place to a cold one. Let's use the example of an apartment; If left alone heat will flow out of the apartment through walls, windows, roofs.

By investing a little electrical energy, a heat pump can heat your apartment with internal energy from the environment around you.

Step 1: Evaporation - In the evaporator which is placed outside, we have a coolant with a low boiling point. When the environment is warmer than the coolant, heat from the environment travels to the coolant, thus making the coolant evaporate.

Step 2: Compressor - After leaving the evaporation process the coolant is now in gas form, but it still has a temperature equal to the outside. The compressor consists of an electrical motor and a pump(which uses electrical energy). The compressor presses the coolant in to a smaller space, increasing the pressure, and consequently the temperature increases to, let's say 70 degrees Celcius. The hot gas is pumped further on to the condensation.

Step 3: Condensation - This part of the system is inside the house/apartment where the gas is still 70 degrees Celcius. Presumably(and hopefully), that is hotter than the temperature inside your apartment and using the 2nd law of thermodynamics: the heat from the coolant(still gas form) will transfer energy(heat) to the apartment whilst lowering the temperature of the coolant, and once that temperature hits the coolant's boiling point, it becomes liquid again.

Step 4: Valve - By looking at the figure below, the coolant is circulating around in a closed circuit. Before the coolant goes back to the Evaporation process, it passes through a valve. The valve makes sure the pressure is higher on the side of the condensation, rather than on the evaporation side.