Refrigeration methods can be broadly classified as Cyclic, Non-cyclic and Thermoelectric methods. In non-cyclic method, refrigeration is done by melting ice. It is used in small-scale refrigeration systems such as portable coolers, and laboratories. Dry ice or solid carbon dioxide is also used as a refrigerant. As dry ice has no liquidity at normal temperature, it sublimes directly from solid state to vapor state at a temperature of -78.5 °C. Dry ice is mostly used and is effective for maintaining products at lower temperatures while subliming. Ice maintains a constant melting point of 0 °C and melts when 333.55kJ.kg of heat is absorbed. Food products maintained at this temperature have a much longer shelf life.
Cyclic refrigeration composes of a refrigeration cycle where heat is taken out from a low-temperature area or source and discarded off to a high-temperature space with the help of external load or work and its opposite, the Thermodynamic power cycle. Second Law of thermodynamics is satisfied during cyclic refrigeration. A cycle shows the changes in a refrigerant as it alternately attracts and rejects heat while circulating through a refrigeration system. Naturally heat flows from high temperature to low temperature. So some external load or work has to be applied to pump heat from a lower temperature source into a higher temperature sink. Insulation is used to lessen the work and energy required to accomplish and sustain lower temperatures in the cooled areas. Absorption heat pumps are also used in a few of the applications.
Cyclic refrigeration is divided into vapor cycle and gas cycle. Vapor cycle system is further classified into vapor compression refrigeration and gas absorption refrigeration. Vapor compression method is commonly used in household refrigerators and also in large scale commercial and industrial systems. Vapor absorption method uses water-ammonia systems primarily, but it lost its applicability after the advent of vapor compression cycle (coefficient of performance being one fifth that of vapor compression cycle). Both differ in the method of raising the pressure of refrigerant vapor. Compressor is replaced by an absorber which then dissolves the refrigerant in an appropriate liquid and a liquid pump that raises the pressure and a generator which drives off the refrigerant vapor from the high pressure liquid on heat addition.
A gas cycle uses a gas to be compressed and expanded, but at the same time doesn’t change its phase. Air is the most often used fluid. The components related to the condenser and evaporator of a vapor compression cycle are the cold and hot gas to gas heat exchangers in a gas cycle. The gas cycle works on the reverse Brayton cycle where as vapor compression method uses reverse Rankine cycle. To achieve the same cooling effect, gas cycle needs a large mass flow rate and is bulky. Due to this bulkiness and lower efficiency than vapor compression, gas cycle is not preferred.

Peltier effect is used in Thermoelectric cooling to create a flux between the junctions of different materials. This is used mostly in portable coolers and camping and for cooling small instruments and electronic components. Magnetocaloric effect which is an intrinsic property of magnets is used in Magnetic refrigeration or adiabatic demagnetization. Paramagnetic salts such as cerium magnesium nitrate are used as refrigerants. By applying a strong magnetic field to the refrigerant, various magnetic dipoles are aligned into a state of lowered entropy. Heat released by the refrigerant is then absorbed by the heat sink. Magnetic field is then switched off and thermal contact is broken with the heat sink which provides the insulation. This increases the heat capacity thus decreasing the temperature of the refrigerant below that of heat sink. Since few magnetic materials exhibit this property, this system is confined to research and cryogenics.
Other methods of refrigeration are Vortex tube used for spot cooling, Air cycle machine used in aircraft, thermoacoustic refrigeration using sound waves.