The most commonly used method of cooling is with vapor-compression cycles, because it is fairly easy to construct a cooling device employing this method and the cost is low.  In fact, conventional refrigerators use this method of cooling to keep your leftovers and drinks chilled!  Air conditioners also employ a vapor-compression cycle to cool the ambient air temperature in a room.

    Basically, vapor-compression refrigeration employs a heat engine run backwards, so heat energy is taken from a cold reservoir and deposited into a hot reservoir.  By the Second Law of Thermodynamics, heat energy does not spontaneously transfer from a cold to a hot reservoir.  In order to have heat transfer in that direction (and not from from hot to cold, as the system is naturally inclined to do), it is necessary to do work on the system.


            Vapor-Compression Refrigeration Cycle

    This refrigeration cycle is approximately a Rankine cycle run in reverse.  A working fluid (often called the refrigerant) is pushed through the system and undergoes state changes (from liquid to gas and back).  The latent heat of vaporization of the refrigerant is used to transfer large amounts of heat energy, and changes in pressure are used to control when the refrigerant expels or absorbs heat energy.
    However, for a refrigeration cycle that has a hot reservoir at around room temperature (or a bit higher) and a cold reservoir that is desired to be at around 34°F, the boiling point of the refrigerant needs to be fairly low.  Thus, various fluids have been identified as practical refrigerants.  The most common include ammonia, Freon (and other chlorofluorocarbon refrigerants, aka CFCs), and HFC-134a (a non-toxic hydrofluorocarbon).

            Stages of the Vapor-Compression Refrigeration Cycle

    The Vapor-Compression Refrigeration Cycle is comprised of four steps.  The conceptual figure of the process shows the PV changes during each part.


Part 1: Compression
    In this stage, the refrigerant enters the compressor as a gas under low pressure and having a low temperature.  Then, the refrigerant is compressed adiabatically, so the fluid leaves the compressor under high pressure and with a high temperature.

Part 2: Condensation
    The high pressure, high temperature gas releases heat energy and condenses inside the "condenser" portion of the system.  The condenser is in contact with the hot reservoir of the refrigeration system.  (The gas releases heat into the hot reservoir because of the external work added to the gas.)  The refrigerant leaves as a high pressure liquid.

Part 3: Throttling
    The liquid refrigerant is pushed through a throttling valve, which causes it to expand.  As a result, the refrigerant now has low pressure and lower temperature, while still in the liquid phase.  (The throttling valve can be either a thin slit or some sort of plug with holes in it. When the refrigerant is forced through the throttle, its pressure is reduced, causing the liquid to expand.)

Part 4: Evaporation
    The low pressure, low temperature refrigerant enters the evaporator, which is in contact with the cold reservoir.  Because a low pressure is maintained, the refrigerant is able to boil at a low temperature.  So, the liquid absorbs heat from the cold reservoir and evaporates.  The refrigerant leaves the evaporator as a low temperature, low pressure gas and is taken into the compressor again, back at the beginning of the cycle.

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Vapor-Compression Refrigeration

The Peltier Effect and Thermoelectric Cooling

Laser Cooling and Optical Molasses

Magnetic Cooling

References

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