Heat pump

If Leon opens a window on a hot summer day, it will soon become as hot inside as outside. This is because heat normally flows from where the temperature is high to where it’s low, until the temperature in both places is the same. But there are devices which allow heat to move in the opposite direction - from a cold space to a hot space. Try opening the refrigerator, Leon! When you open the door you will see that even though it is very hot outside, the inside of the fridge remains cold.

How is that possible?! If we look at the back of the fridge we can see a grate with a long coiled metal tube. This tube leads to the inside of the fridge. It’s filled with a fluid called REFRIGERANT. It absorbs the heat from the inside of the fridge and transfers it to the air outside, making the inside of the fridge cool.

The heat flows from hot to cold, so the refrigerant in the tube inside the fridge needs to be colder than its surroundings. Then it somehow has to become hotter than the air in your kitchen to be able to release the heat out. How does it work? It is possible to change the temperature of the refrigerant by increasing and decreasing its pressure. There is a special part which increases the pressure of the refrigerant - a COMPRESSOR.

As the pressure in the compressor rises, the refrigerant’s temperature increases. It becomes a hot gas which then flows through the tube outside the fridge. It’s hotter than the outside, so it releases the heat into the air and cools down. The temperature drops but the pressure is still high, and the gas turns back into a liquid. This process called condensation happens in the grate on the back of the refrigerator.

We call this part a CONDENSER. Next, the liquid passes through another part, which allows the refrigerant to expand. Because the refrigerant expands, the pressure decreases, and the temperature drops as well. The part where this happens is called an EXPANSION VALVE. After the refrigerant goes through the expansion valve, it becomes a mixture of cold liquid and gas.

This mixture flows through the tube inside the refrigerator. There it absorbs the heat from the surroundings and turns into a gas - it evaporates. That’s why this part is called an EVAPORATOR. Then, the gas continues flowing back to the compressor and the whole cycle repeats. Through this cycle, the heat is constantly pumped out from the low to the high temperature.

Devices that can do this are called HEAT PUMPS. Refrigerators are not the only place where we can apply this system. A heat pump can be used to cool down or even heat up our houses! There are different types of heat pumps, which can absorb heat from the air, water or even ground. In summer, a heat pump works just like a fridge - it absorbs heat from the inside of the house and pumps it outside.

In winter, the process can be reversed, and the heat can be absorbed from the outside and pumped into a house. Heat pumps do not produce heat, they just move it from one place to another by condensing and evaporating the refrigerant. They use little energy and are environmentally-friendly. But don’t leave the fridge door open for too long, Leon. Grab some ice cream from the freezer, it will help you cool down too!