Magnatic refrigeration

      Magnatic refrigeration or adiabatic demagnetization , is a cooling technology based on the magnetocaloric effect , an intrinsic property of magnetic solids. The refrigerant is often a paramagnetic salt , such as cerium magnesium nitrate. The active magnetic dipoles in this case are those of the electron shells of the paramagnetic atoms.
      A strong magnetic field is applied the refrigerant , forcing its various magnetic dipoles to align and putting these degrees of freedom of the refrigerant into a state of lowered entropy. A heat sink then absorbs the heat released by the refrigerant due to its loss of entropy. Thermal contact with the heat sink is then broken so that the system is insulated , and the magnetic field is switched off. This increases the heat capacity of the refrigerant , thus decreasing its temperature below the temperature of the heat sink.
      Because few materials exhibit the needed properties at room temperature , applications have so far been limited to cryogenics and research.

Thermoelectric refrigeration

      Thermoelectric cooling uses the Peltier effect to crate a heat flux between the junction of two different types of materials. This effect is commonly used in camping and portable coolers and for cooling electronic components and small instruments.

Gas cycle

      When the working fluid is a gas that is compressed and expanded but doesn't change phase , the refrigeration cycle is called a gas cycle. Air is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle , components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas gas-to-gas heat exchangers in gas cycles.
      The gas cycle is less efficient than the vapor compression cycle because the gas gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. As such the working fluid does not receive and reject heat at constant temperature. In the gas cycle , the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side. Therefore , for the same cooling load , a gas refrigeration cycle needs a large mass flow rate and is bulky.
      Because of their lower efficiency and larger bulk , air cycle coolers are not often used nowadays in terrestrial cooling devices. However , the air cycle machine is very common on gas turbine-powered jet air craft as cooling and ventilation units , because compressed air is readily available from the engines' compressor sections. Such units also serve the purpose of pressurizing the aircraft.
      

Vapor absorption cycle

      In the early years of the twentieth century , the vapor absorption cycle using water-ammonia systems was popular and widely used. After the development of the vapor compression cycle , the vapor absorption  cycle lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapor compression cycle). Today the vapor absorption cycle is used mainly where fuel for heating is available but electricity is not , such as in recreational vehicles that carry LP gas. It is also used in industrial environments where plentiful waste heat overcomes its inefficiency.
        The absorption cycle is similar to the compression cycle , except for the method of raising the pressure of the refrigerant vapor. In the absorption system , the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid , a liquid pump which raises the pressure and a generator which , heat addition , drives off the refrigerant vapor from the high-pressure liquid. Some work is needed by the liquid pump but , for a given quantity of refrigerant , it is much smaller than needed by the compressor in the vapor compression cycle. In an absorption refrigerator , a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent) and water (refrigerant) and lithium bromide (absorbent).      

Vapor - compression cycle

     The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. Following picture provides a schematic diagram of the components of a typical vapor-compression refrigeration system.

Vapor compression refrigeration

      The thermodynamics of the cycle can be analyzed on a diagram as shown follow. In this cycle , a circulating refrigerant such as Freon enters the compressor as a vapor. The vapor is compressed at constant entropy and exits the compressors superheated. The superheated vapor travel through the condenser which first cools and removes the superheat and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature. The liquid refrigerant goes through the expansion valve (also called a throttle valve) where its pressure abruptly decreases , causing flash evaporation and auto-refrigeration of , typically , less than half of the liquid.

Temperature–Entropy diagram

      That results in a mixture of liquid and vapor at a lower temperature and pressure. The cold liquid-vapor mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapor returns to the compressor to the compressor inlet to complete the thermodynamic cycle.
      The above discussion is based on the ideal vapor-compression refrigeration cycle and does not take into account real-world effects like frictional pressure drop in the system , slight thermodynamic irreversibility during the compression of the refrigerant vapor or non-ideal gas behavior (if any).

Thermodynamic Cycle

      According to the second law of thermodynamics heat cannot spontaneously flow from a colder location to a hotter area ; work is required to achieve this. Work is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. The operating principles of the refrigeration cycle was described mathematically by sadi carnot  in 1824 as a heat engine. A heat pump can be thought of as a heat engine which is operating in reverse.
      Heat pump and refrigeration cycles can be classified as vapor compression , vapor absorption , gas cycle or stirling cycle types.

Heat pump and refrigeration cycle

      Thermodynamic heat pump cycles or refrigeration cycles are the models for heat pumps and refrigerators. A heat pump is a machine or device that moves heat from one location (the 'source') at a lower temperature to another location (the 'sink' or 'heat sink') at a higher temperature using mechanical work or a high-temperature heat source. Thus a heat pump may be thought of a "heater" if the objectives is to warm an area , or a "refrigerator" if the objective is to cool an area ; regardless the operating principles are identical.