Heat Flow to Hotter Region

Although internal energy will not spontaneously flow from a cold region to a hot region, it can be forced to do so by doing work on the system. Refrigerators and heat pumps are examples of heat engines which cause energy to be transferred from a cold area to a hot area. Usually this is done with the aid of a phase change, i.e., a refrigerant liquid is forced to evaporate and extract energy from the cold area. Then it is compressed and forced to condense in the hot area, dumping its heat of vaporization into the hot area.

Refrigerator

If you place your hand behind an operating refrigerator, you will notice that this area is warmer than the room temperature. Or you may notice that the refrigerator blows warm air on your feet as you stand in front of it. Yet the interior of the freezing compartment is very cold! The refrigerator is taking energy from the freezing compartment, making it colder, and exhausting that heat to the room, making it warmer.

A refrigerator is a heat engine in which work is done on a refrigerant substance in order to collect energy from a cold region and exhaust it in a higher temperature region, therby further cooling the cold region.

Refrigerators have made use of fluorinated hydrocarbons with trade names like Freon-12, Freon-22, etc. which can be forced to evaporate and then condense by successively lowering and raising the pressure. They can therefore "pump" energy from a cold region to a hotter region by extracting the heat of vaporization from the cold region and dumping it in the hotter region outside the refrigerator. The statements about refrigerators apply to air conditioners and heat pumps, which embody the same principles.

Although this process works very well and has been in place for decades, the bad news about it is that fluorinated hydrocarbons released into the atmosphere are potent agents for the destruction of the ozone in the upper atmosphere. Therefore tighter and tighter restrictions are being placed on their use.