In chem last year we learned that there's no such thing as cold, but rather different perceptions of heat: hot, less hot, hotter, etc.. So if you can only transfer heat from one source of matter to another, how do these appliances work?
Natural trend is that heat move from hot location to the less hot another location. In refrigeration systems the reverse happens and the process can only be achieved through external work input to the system (motor - compressor). Thermodynamically, the system is regarded as the reverse of a heat engine. Refrigerant vapour is compressed and sent to a heat exchanger (condenser) where it is cooled and condenses to liquid. That liquid is then driven to a low pressure location (evaporator) where it boils and evaporates absorbing heat from the interior of a refrigerators and hence cooling it and goal is scored there.
heat pumps. with the input of energy (the electricity from your house), they transfer heat up a temperature gradient (that is from low temperature to high temperature). You can learn about heat pumps by taking a thermodynamics class or a thermal physics class. See what your school has available.
Through the use of compression and expansion of a gas that can under go a phase change (ligid to gas, gas to liquid) heat can be liberated or absorded through two heat exchangers. In a refrigeration cycle (AC as well) a gas is compressed and the heat of compression is passed to another substance like air or water. The compressed gas is thus cooled and now condenses to a liquid under hight pressure(this first heat exchanger is called a condenser). THe liquid refrigerant is then passed to a second heat exchanger which is under a lower pressure due to its connection to the suction side of the compressor. This second heat exchanger is operating at a lower pressure and the pressurized liguid is allowed to spray into it. The liquid refrigerant upon entering the lower pressure withing this second evaporator changes back into a gas, during this expansion from liquid back into a gas the refrigerant absorbs heat. The absorbed heat comes from the material surrounding the second heat exchanger(which is called an evaporator). Now depending upon what materials you surround your heat exchangers with it is possible to heat or cool that material which could be another liquid or gas. The common liquid or gas is water or air. So a refrigerator or AC can heat or cool water and or air , liberating heat to one while absorbing heat from the other and using the circulation of refrigerant within the system and the compresser to pump that heat from one to the other. Depending upon which heat exchanger is used as a condenser or evaporator and the location of these exchangers will dictate whether the system heats or cools the inside of a building or refrigerator. A system that has valving to switch and redirect refrigerant so as to either heat or cool a building is caled a heat pump.
