Refrigeration is one of those technological miracles of modern living that has totally changed life. Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere for the primary purpose of lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. The term cooling refers generally to any natural or artificial process by which heat is dissipated. Cold is the absence of heat, hence in order to decrease a temperature, one "removes heat", rather than "adding cold." In order to satisfy the Second Law of Thermodynamics, some form of work must be performed to accomplish this. This work is traditionally done by mechanical work but can also be done by other means.
Conventional Refrigeration:
The four basic parts to any refrigerator are: compressor, heat-exchanging pipes, expansion valve and refrigerant. The basic mechanism of a refrigerator works like: the compressor compresses the refrigerant gas which raises the refrigerant's pressure and temperature, so the heat-exchanging coils outside the refrigerator allow the refrigerant to dissipate the heat of pressurization. As it cools, the refrigerant condenses into liquid form and flows through the expansion valve. When it flows through the expansion valve, the liquid refrigerant is allowed to move from a high-pressure zone to a low-pressure zone, so it expands and evaporates. In evaporating, it absorbs heat, making it cold. The coils inside the refrigerator allow the refrigerant to absorb heat, making the inside of the refrigerator cold. The cycle then repeats.
Conventional cooling systems, -- refrigerators or air conditioners - require energy-eating compressors and lots of heating coils, noisy, have been developed beyond for years without any further improvement in coefficient of performance (COP) and rely on the properties of gases to cool and most systems use the change in density of gases at changing pressures to cool. The coolants commonly used are either harmful to people or the environment. It is, thus, expected that refrigerators and other cooling devices may one day lose their compressors, coils of piping, harmful refrigerants and become solid state according to researchers who are investigating other methods of refrigeration. It's possible that one day all the cooling power of a noisy, bulky household refrigerator will be available on a small device that is lightweight and has no moving parts. And the same device, when given a heat source like a car's exhaust pipe, could be used to generate electricity. Different methods of refrigeration under development are briefly described below:
Electric refrigeration:
There are efforts in the development of an electric field (as electricity is more convenient) refrigeration unit and for the future, we can envision a flat panel refrigerator with no more coils, no more compressors, just solid polymer with appropriate heat exchangers. Researchers are working with ferroelectric polymers that exhibit temperature changes at room temperature under an electrical field. These polarpolymers include poly(vinylidene fluoride-trifluoroethylene) and poly(vinylidene fluoride-trifluoroethylene)-chlorofluoroethylene, however there are other polarpolymers that exhibit the same effect. The approach uses the change form disorganized to organized that occurs in some polar-polymers when placed in an electric field. The natural state of these materials is disorganized with the various molecules randomly positioned. When electricity is applied, the molecules become highly ordered and the material gives off heat and becomes colder. When the electricity is turned off, the material reverts to its disordered state and absorbs heat. The researchers report a change in temperature for the material of about 22.6 degrees Fahrenheit. Repeated randomizing and ordering of the material combined with an appropriate heat exchanger could provide a wide range of heating and cooling temperatures. The polymers are flexible and can be used for heating and cooling, so there may be many different possible applications.
Magnetic Refrigeration:
Researchers are also trying to make magnetic fields do the cooling. Magnetic cooling technology exploits the fact that when a magnetic material (like gadolinium), is magnetized, heat is produced as a by-product of entropy. The principle of entropy is that there will always be a constant amount of order/disorder in a substance. When the magnet puts the substance in "order", it has to get rid of the excess disorder - and this becomes heat. Conversely, when the magnetic field is again removed, the substance becomes cold. The heat is transferred to a fluid that is pumped back and forth past the substance inside a cylinder. The end that becomes cold will be located inside the refrigerator and the warm end will be outside. The first milestone in magnetic cooling has been achieved between 5 and 10 degrees of cooling and the figure is currently at 8.7°C - this means that a refrigerator at room temperature (20°C) can be cooled to almost 11°C. Of course, this is not quite enough but the objective of conducting research is to test different materials, varying operating conditions and the strength of the magnetic field. It is probably not realistic to think that magnetic cooling technology will be used in consumers' homes right away.
Thermoelectric Refrigeration:
In conventional cooling devices, heat is carried away by a working fluid, such as a chlorofluorocarbon, which involves the moving parts that cause most equipment breakdowns, environmental damage and bulkiness. In thermoelectric devices, the "working fluid" is electrical current that runs through a junction between differently doped semiconductors and pulls heat away from that junction, producing cooling without any moving parts. Current thermoelectric materials operate at roughly 10 percent of Carnot efficiency, the theoretical maximum allowed by the laws of thermodynamics, compared with about 30 percent for an average household refrigerator. The theory behind thermoelectric devices has been around for more than 40 years, but current materials don't rival the efficiency of compressor-based devices.
Such thermoelectric devices already exist in consumer products like plug-in auto beverage coolers, where energy efficiency is less important than portability and low weight. The challenge facing researchers is to find new materials that could bring the technology to the next level in which the efficiency would rival that of conventional coolants in air conditioners as well as refrigerators. As we increase the efficiency of thermoelectric devices, we create another tool in the arsenal for choosing the most efficient way to do things. Also in the future might be miniature cooling devices directly on computer chips. Researchers know they need a material with low thermal conductivity and high electrical conductivity, which has led them to look at compounds of heavy elements like lead, antimony, bismuth and tellurium. The search is focused on uniform bulk materials, which can be prepared in large amounts by traditional synthetic methods, and on compositionally modulated films, which require expensive nanofabrication.
Thermo acoustic Refrigeration:
Thermoacoustic hot air engines (Sonic heat pump and refrigeration or thermoacoustic heat pump and refrigeration) of which nearly all are thermoacoustic stirling engines is a technology that uses high-amplitude sound waves in a pressurized gas to pump heat from one place to another - or uses a heat temperature difference to induce sound, which can be converted to electricity with high efficiency, with a (piezoelectric) loudspeaker. Working of a thermo-acoustic refrigerator is that first, customized loudspeakers are attached to cylindrical chambers filled with inert, pressurized gases such as xenon and helium. At the opposite end of the tubes are tightly wound "jelly rolls" made of plastic film glued to ordinary fishing line. When the loudspeakers blast sound at 180 decibels, an acoustic wave resonates in the chambers. As gas molecules begin dancing frantically in response to the sound, they are compressed and heated, with temperatures reaching a peak at the thickest point of the acoustic wave. That's where the superhot gas molecules crash into the plastic rolls. After transferring their heat to the stack, the sound wave causes the molecules to expand and cool. Each one of these oscillating molecules acts as a member of a 'bucket brigade,' carrying heat toward the source of the sound. Inside the so called "bucket brigade" a gas parcel is compressed and heated by the sound wave and deposits some of its heat to the stack. The sound wave then expands and cools the gas parcel so that the gas can absorb heat from the stack and cool it. The parcels absorb heat from the cold exchanger and pass it along the stack.
The most efficient thermoacoustic devices built to date have a relative Carnot efficiency approaching 40%, which is comparable with low end domestic vapor compression systems today (high end compressors have efficiencies up to 65%) and heat engines are in most cases superior to automotive internal combustion engines. Skeptics say current thermoacoustic designs are inefficient compared to conventional refrigeration systems but scientists continue to improve his invention, which requires only one moving part in the form of a loudspeaker and therefore may be more dependable than CFC-type refrigerators. Someday, household refrigerators and air conditioners might be powered by loudspeakers blasting sound thousands of times more intense than the sound in concerts.
Dr SS Verma, Dept of Physics, SLIET, Longowal
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