In 1758, founding father of the United States and general scientist, Benjamin Franklin began to investigate the phenomena of cooling. Based on the observation that on a very hot day he stayed cooler by wearing a wet shirt than a dry one, he began conducting experiments based on evaporation, lightly wetting a thermometer with ether and using a pair of bellows to evaporate the substance. He concluded that it was the evaporation of the substance rather than the air from the bellows that chilled the thermometer, which he succeeded in cooling to -14 C in an 18 C room.
Modern physics has built on Benjamin’s initial investigations. Evaporation is an endothermic reaction – thermal energy is absorbed in order to cause the reaction. When the substance condenses, the reaction is exothermic – thermal energy is released.
A modern invention, the air source heat pump, capitalises on this discovery by combining it with another – the relationship between pressure, temperature and state of matter. It is known that by pressurising a gas it can be made to condense despite being far above its usual condensation point (the same can be done in reverse via depressurisation. By strategically inducing a fluid in a closed circuit to evaporate and condense by manipulating the pressure in specific regions of the loop, the fluid can be made to absorb thermal energy at one side (where it is made to evaporate) and relinquish it at the other (when it is made to condense). This is the principle by which an air conditioner works, effectively “pumping” heat from one side to the other.