THERMOELECTRIC REFRIGERATION

1. INTRODUCTION :         Today one cannot refuse the use of refrigeration system in our life, not only in kitchen but also in shops, industries and commercial purposes.Today’s compression   refrigeration   system   has   given   very   good performance. But refrigerants (Chlorofluorocarbons, Hydrochloroflorocarbons) used in this systems are hazardous to the environment and human life, because they react with very useful gas ozone (O3).  Thus depletion of ozone layer is the main problem in front of us.          Thermo-electric refrigeration system can able to substitute vapor compression refrigeration system for small-scale applicant and may result as solution of the problem above discussed. 

COMPARISION WITH CONVENTIONAL COOLING SYSTEM:          Thermo-electric cooling is described as a solid-state method of heat transfer generated primarily through the use of dissimilar semiconductor materials. To understand the cooling method, it is first necessary to know how thermoelectric cooling systems differ from their conventional refrigeration counterparts. Like conventional refrigeration, thermoelectric obey the basic laws of thermodynamics. Both in result and principle, then, thermoelectric cooling has much in common with conventional refrigeration methods - only the actual system for cooling is different.

Perhaps the best way to show the differences in the two refrigeration methods is to describe the systems themselves. In a conventional refrigeration system, the main working parts are the evaporator, condenser, and compressor. The evaporator surface is where the liquid refrigerant boils, changes to vapor and absorbs heat energy. The compressor circulates the refrigerant and applies enough pressure to increase the temperature above ambient level. The condenser helps discharge the absorbed heat into the ambient air. In thermo-electric refrigeration, essentially nothing has changed. The refrigerant in both liquid and vapor form is replaced by two dissimilar conductors. The cold junction (evaporator surface) becomes cold through absorption of energy by the electrons as they pass from one semiconductor to another, instead of energy absorption by the refrigerant as it changes from liquid to vapor. The compressor is replaced by a DC power source which pumps the electrons from one semiconductor to another. A heat sink replaces the conventional condenser

fins, discharging the accumulated heat energy from the system. The difference between the two refrigeration methods, then, is that a thermo-electric cooling system refrigerates without the use of mechanical devices, except in the auxiliary sense, and without refrigerant.2. BASICS OF THERMOELECTRIC REFRIGERATION:               Thermo-electric   refrigeration   system   is   based   upon   the    principle   of thermo-electric effect, which is observed first by Henrich Lenze in 1938. This effect is based on different five laws, which are as stated below-  2.1   Seebeck effect :                            “When two junctions of a pair of two dissimilar metals maintained at different temperatures, there is generation of e.m.f.”              Mr. Seeback   conducted   a   series of   tests   by   varying the temperatures of the junctions of various combinations of set of materials. The e.m.f. Output was found to be:                                            E    T                                            E = ab T                                          Where   ab= Seeback coefficient.

2.2   Peltier effect :                  “If direct current is passed through a pair of dissimilar metals there is heating at one junction, cooling at other depending upon material combinations.”                Q  I                Q=ab. I  Where Q = rate of heating or cooling.               I = current passing through the junction.                = Peltier coefficient.Early in the 19th Century, Jean Peltier discovered that a refrigerating power is obtained by passing current along a circuit containing dissimilar materials.  Heat is absorbed at one junction of the two materials and heat is released at the other junction.  The transfer of heat is caused by the change in electron energy levels when electrons access the conduction band as defined by quantum physics.  The conduction band varies with each material which means that conducting electrons in some materials are at a higher energy level than in other materials.  When electrons pass down a circuit of dissimilar materials, the electrons alternately extract energy or release energy with each change in conduction band.  The desired

refrigerating effect occurs when electrons move to a higher energy level upon change of material.  A reverse effect also occurs where electricity is generated from a circuit of dissimilar materials that are exposed to a temperature differential.  This is the physical principle that forms the basis of the thermocouple and is known as the Seebeck effect.  The Peltier and Seebeck effects are complimentary manifestations of the same physical phenomenon. Thermo-electric power generation is also currently under research as a means of obtaining electrical energy out of waste heat from steelworks and incineration plants, automobile exhausts.  A schematic diagram of the electrical circuit that generates the Peltier effect and the electronic mechanism of refrigeration is shown in Figure 2.

For most electrical conductors, viz. metals, the effect is too weak to be useful.  With metals, a weak Peltier effect is overwhelmed by strong Ohmic heating and there is no net cooling effect. 

2.3    Thomson effect:                         It is reversible thermoelectric phenomenon. “When a current passes through a single conductor having temperature gradient has exhibited.”

2.4   Joulean effect:  “When the electric current passed through a conductor, there is dissipation of electrical energy in the form of heat.”                      According to Joule it is related as                                         qj = I2R                             Where   I= current                                          R= electrical resistance2.5   Conduction effect:                          “If  the  ends   of   any   element   are  maintained  at  different temperatures,   there   is   heat   transfer  from  hot  end  to  cold  end ”  &  it  is related by                                   Qcond = U (Th-Tl)                            Where                                           U = overall conductance                                           Th=high temperature                                            Tl=low temperature*Abram Ioffe and co-workers discovered that some semi-conductors exerted a much stronger Peltier/Seebeck effect and developed thermo-electric power modules that were used to generate electricity in remote

locations.  In 1950, a co-worker of Ioffe, Lazar Stilbans, developed the first recorded design of thermo-electric refrigerator using bismuth telluride and related compounds to achieve a cabinet interior temperature of –2.3 degrees Celsius inside a room temperature of 19 degrees Celsius.3. WORKING OF THERMO-ELECTRICREFRIGERATION:As shown in figure 3 two different materials are connected by battery in which P-type region is connected to the positive terminal and N-type to the negative terminal.                           If a current is passed trough them, the cooling is produced at one junction and heat is produced at other junction. If Th is maintained at ambient temperature, Tc will be lower at ambient temperature. It also to be noted that which of the junction or ends will become cold or hot depends on direction of flow of current.                                            From the analysis of all the effects it has been found thatcoefficient performance of system is –           COP= qc/energy supplied              = [ab I Tl-I2R/2-U (Th-Tl)]/ [ab (Th-Tl) I+I2R]                                 Where ab= Seeback constant                                                I = Current passed                                                R= Electrical resistance                                             U= Overall conductance                                             Th= High temperature                                             Tl= Low temperatureAssumptions made for   COP equation are:

1.     Heat transfer takes place through the semiconductor at the ends only.2.     No energy exchange between the conductors through space separating

them.3.     Properties such as conductivity resistance are invariant with temperature.

Theory of Operation

The semiconductor materials are N and P type, and are so named because either they have more electrons than necessary to complete a perfect molecular lattice structure (N-type) or not enough electrons to complete a lattice structure (P-type). The extra electrons in the N-type material and the holes left in the P-type material are called "carriers" and they are the agents that move the heat energy from the cold to the hot junction.

Heat absorbed at the cold junction is pumped to the hot junction at a rate proportional to carrier current passing through the circuit and the number of couples. Good thermo-electric semiconductormaterials such as bismuth telluride greatly impede conventional heat conduction from hot to cold areas, yet provide an easy flow for the carriers. in addition, these materials have carriers with a capacity for carrying more heat.Heat Sinks

The design of the heat exchanger is a very important aspect of a good thermo-electric system. The upper part of the diagram illustrates the steady-state temperature profile across a typicalthermo-electric device from the load side to the ambient. If the heat sink is not capable of rejecting the required Qs from the given system, the temperature of the entire system will rise and the cold junction temperature will increase. If the thermo-electric current is increased to maintain the load temperature, the COP (Coefficient of Performance) tends to decrease. Thus, a good heat sink contributes to improve COP. Energy may be transferred to or from the thermo-electric system by three basic modules: conduction, convection, and radiation. The values of Qc and Q1 may be easily estimated; their total along with the power input gives Qs, the energy the hot-junction heat sink must dissipate.Refrigeration based on the Peltier effect:

It is obtained by arranging a series of thermo-electric cells in a horizontal array which is then encased in plates made of an electrical insulator.  Each thermo-electric cell consists of a pair of dissimilar semi-conductors which are connected by electrical conductors at either end. The requisite dissimilarity in semi-conductors is obtained not only by using dissimilar materials, but also by using different dopants.  The tablet shaped component that is produced by this means is called a module or Peltier module.  The passage of electric current through the module causes one of the plates to become hot and the other to become cold.  When there is adequate cooling to the heated plate, the opposing plate can reach a low temperature or extra heat on a continuous basis.  Figure 4 shows a schematic diagram of a Peltier module and how it transfers heat.

The Coefficient of Performance (COP) of a Peltier module is defined in the same way as for a conventional refrigeration system,viz.Coefficient of Performance = Rate of heat extraction divided by Electrical Power input.  Critical materials parameters to ensure a high COP are a high thermo-electric coefficient to generate the cooling effect, a high electrical conductivity to suppress Ohmic heating and a low thermal conductivity to prevent much heat being conducted from the hot side of the module to the cold side of the module.Peltier modules are widely used to produce localized cooling in scientific and technical applications such as laser-chip coolers and a portable insulin cold-box.  The advantages of mechanical simplicity and suitability for small-scale applications are the principle reasons for selecting thermo-electric cooling instead of compressor-driven refrigeration for these applications.

Large scale cooling applications such as air-conditioning and refrigeration have been attempted and technically functional systems were developed to cool trains, helicopters and aircraft.  Small volume applications such as the train driver’s cab where convenience and lack of moving parts overweighed considerations of power consumption were found to be appropriate for thermo-electric air conditioning. A refrigerator (without freezer) was developed for hotel bedrooms where the air-conditioning prevents over-heating on hot summer days and the lack of compressor noise is considered a major asset.  This thermo-electric refrigerator is now sold in Japan to hotels as a quiet and non-polluting food storage system for hotel rooms.

The energy efficiency or coefficient of performance of thethermo-electric refrigerator remains a major consideration.  The efficiency of the thermo-electric modules are more sensitive to the temperature difference between hot and cold sides than a corresponding compressor system.  The efficiency of the thermo-electric refrigerator largely depends on the heat transfer system used to transfer heat from the refrigerated cabinet through the comparatively small thermo-electric module and out to the external atmosphere.  A well-designed thermo-electric system can offer a higher coefficient of performance than an adsorption refrigerator.

The thermo-electric system which only needs water or brine for efficient functioning has become a potentially attractive mode of refrigeration.  In this article, the major limitations of current thermo-electric refrigeration systems are discussed together with proposals for future remedy.

Thermo-electric device manufacturers report that a single-stagethermo-electric cooler can achieve temperature differences up to 70°C (126°F) or can transfer a maximum load of 125 watts (426 Btu/h) when the temperature difference is zero. By cascading several devices together, multistage coolers increase cooling capabilities. Greater temperature differences up to 131°C (236°F) can be achieved in this manner.However, there are a number of recent developments that have made the application of thermo-electric cooling and heating more attractive for commercial products. New applications, materials and technology have enhanced those commercial potentials significantly. For example, certain semi-conductors (alloys of tellurium are of particular interest) have thermo-electric properties superior to conductor materials. These characteristics have allowed several niche applications to develop.

4. CURRENT THERMO-ELECTRIC TECHNOLOGYThe simplest system involves air cooling on both the hot and cold

sides; more advanced systems have water cooling on either the hot or cold sides or else on both faces.  The air-air system can be used for air-conditioning where indoor air to be cooled is blown directly onto the cold face of the Peltier module while heat is released directly to outdoor air.  A commercial cooling system involving air-cooling on the hot side and heat transfer to a coolant or test fluid on the cold-side is marketed by several companies.  A system using heat transfer to water on both the cold and hot faces of the module was developed for use in a refrigerator.

The main advantage of air-cooling is simplicity since only fins and a fan are required but the major disadvantage is reduced thermal efficiency.  It is found that the poor thermal conductivity of air causes a high temperature to develop on the hot face and conversely a very low temperature on the cold face for even a moderate level of heat transfer.  For example, if the difference in temperature between the interior of refrigerator and the external atmosphere is 20 degrees Kelvin, then the thermo-electric module would have to operate at approximately 40 degrees Kelvin temperature difference in order for sufficient heat transport to balance the heat leakage into the cabinet.  Each face of the module would need approximately 10 degrees Kelvin temperature difference relative to either the refrigerated

cabinet or the external air before there is sufficient heat transfer by convection in air.  This larger temperature difference causes the coefficient of performance to decline from approximately 1 to 0.5 or less because of reverse thermal conduction in the module.

A liquid-liquid heat transfer system for the Peltier module usually involves a liquid coolant which transfers heat from the module to the air by a radiator.  It is also possible to cool a process fluid directly without using a radiator.  This is a more efficient process but may involve problems of corrosion or blockage inside the heat exchange tubes.  A pump is required to circulate the coolant and the radiator will probably require a fan, thus raising the level of mechanical complexity of the system when compared to direct air cooling.  In most cases, water is used as the coolant because it is readily available, non-corrosive and an efficient medium for heat transfer.  Brine is also used on the cold side of the module in order to prevent blockage by freezing of the coolant.

The main advantage of using water-based cooling systems is that the Peltier module can work at a temperature difference that is far closer to the nominal temperature difference of the system.  This is because the convective heat transfer coefficient between water and a solid interface is much higher than air for comparable flow conditions.  The Peltier module is then able to work at close to its optimum thermodynamic efficiency thus reducing electricity consumption to practicable levels.  Refrigeration is a major source of electricity consumption and there is little purpose to mitigate ozone destruction if in return, the greenhouse effect is intensified by an increase in electricity demand.  European Union (EU) legislation has imposed limits on the amount of electricity that can be consumed annually by an individual refrigerator inside EU countries.  This legislation necessitates either a high coefficient of performance from the refrigerating system or very efficient thermal insulation on the refrigerator cabinet.

The maximum temperature difference between hot and cold side for practical functioning by Peltier modules is approximately 70 degrees Celsius.  Larger temperature differences can be obtained by stacking the Peltier modules where the waste heat from the coldest module is conducted to the cold side of the warmer module.  The disadvantage of this method is the low COP so that it is mostly used for specialized instrumentation applications.

A valuable feature of thermo-electric refrigeration is the ease at which fractional power settings (for example, half-power) can be maintained.  The full power of the thermo-electric system is reserved

for cooling the cabinets from ambient temperature to set temperature while the fractional power setting at steady state is optimized for maximum COP.  A thermo-electric refrigeration system can be set at a power level sufficient to maintain the set temperature indefinitely instead of hunting around a set point, as is the case with a compressor refrigerator.  Typically a compressor driven refrigerator is controlled by a thermostat which only starts up the compressor when the temperature is approximately 3 degrees Kelvin higher than the set-point.  It is possible to reduce this temperature bandwidth but then the compressor must function at reduced efficiency because of frequent operations for short periods of time when the compressor is still warm.  Thermo-electric refrigeration enables food to be held within a narrow temperature range without being exposed to periods of unsuitably high or low temperatures.  This control of temperature minimises low temperature damage (chilling injury) to fruits and vegetables, while suppressing the growth of pathogenic organisms such as salmonella, in stored meats.  Bacterial growth rates have an exponential relationship with temperature, which means that even brief excursions of temperature above the set-point generate a disproportionately large amount of bacterial growth.  Bacteria can degrade the nutrients within the food and release toxins, which may cause illness for the consumer of the food. 

5. DEVELOPMENT OF MATERIALS:Since the beginning of the industrial revolution, humanity has

demanded an ever-increasing supply of energy.TE devices are currently used in automotive seat coolers/heaters

(over 500,000/yr), in portable refrigerators that plug into an automobile’s cigarette lighter, and in chemical and nuclear generators in arctic regions and space probes. Increasing the efficiency of TE materials has been the primary goal of research in the field, and may allow penetration of the economical and environmentally friendly technology. Thermo-electric might then be coupled to any number of heat sources to extract electricity from heat that would otherwise have been dissipated into the environment as waste. Examples of potentially useful heat sources include fuel cells, the steam generator systems inherent in all large power plants, solar collectors, the shaded sides of solar cells, and automotive exhaust. A Japanese collaboration has predicted that gas mileage would be improved by several miles per gallon if the alternator were replaced by an array of TE generators. Generators could also be attached to wood stoves to electrify

remote areas. Proposed uses of efficient TE refrigerators include the cooling of high-temperature superconductor cables that could be used to distribute electric power without loss and the cooling of microchips to enable faster computing and more sensitive detectors. The military is considering the use ofthermoelectric in wireless IR detectors, temperature stabilization of optics, cooling of microprocessors and CCDs, controlling heat signatures, individual man portable micro-climate systems, remote power sources, and air conditioning and waste heat recovery for ships, submarines, land vehicles, and aircraft.

TE materials naturally generate a temperature gradient in the presence of an electromotive force (emf) and they produce an emf in a temperature gradient. While all materials except superconductors possess some TE character, only a few systems are efficient enough to generate interest. These include the lead, bismuth, and antimony chalcogenides, skutterudites such as cobalt triantimonide, bismuth antimony, silicon germanium, boron carbides, and more complex compounds and alloys based on these systems. A TE refrigerator connects two or more pieces of TE material to of voltage source. A generator can be made from the same device is the voltage source is replaced by a load (e.g. a battery charger). Nearly all devices use two different types of materials, one "n-type" and the other "p-type." These pieces must be connected so that they are electrically in series, but thermally in parallel. This situation is illustrated in the figures below.

The figure above is a simplified schematic of a TE cooler. The voltage source moves electrons and holes (think of them as bubbles in a sea of electrons) to the right in the n- and p-type materials, respectively. These charge carriers also carry heat as they travel, picking it up on the left and dumping it on the right. Simultaneously, phonons (vibrations in the atoms of a solid) carry some heat back, detracting from the performance of the device.

Earlier Bismuth and Antimony were used in thermo-electric refrigerator. Latter on various semiconductor materials developed. In today’s status materials like BiTe3/Sb2Te3/Bi2Se3 alloy are being used in Peltier refrigeration.  Further investigations suggest compounds made from elements found in the lower right corner of the periodic table group IIIB to VIB.

Some materials and their figure of merits are as shown in chart.

6.   ADVANTAGES AND DISADVANTAGES:     Thermo-electric coolers are small, compact, lightweight, use solid state construction and have no moving parts, fluids or gases. These characteristics can offer many advantages compared to other heating/cooling technologies. A fan or pump may be needed for heat transfer in larger thermoelectric systems; however, proper choice can limit energy consumption and keep the system quiet. As uses increase and mass production becomes more of a reality, the prices of these devices are expected to decrease, making their applications even more attractive.  How Efficient Are TE's?

The efficiency of a TE device or material is captured by its figure-of-merit:ZT=S2 σ T/κWhere S is the Seebeck coefficient, σ is the electrical conductivity, It is the absolute temperature, and κ is the thermal conductivity. The Seebeck coefficient describes the magnitude of the voltage that develops between the two ends of a device/material held at different temperatures. Typical materials have figures-of-merit near one which make them about 10% efficient. Compressor and steam generator systems, in contrast, operate near 30% efficiency. Researchers have long sought to improve the efficiency of TE materials. To accomplish this, one must increase the Seebeck coefficient (large effective masses, unusual band structures), increase the carrier mobility (covalently bonded solids, quantum wells), and/or decrease the thermal conductivity (large unit cells, large effective masses, increasing disorder to decrease mean free path of phonons). However, difficulties in enhancing ZT arise because these materials properties are not normally independent—increases in S lead to decreases in , and increases in  lead to increases in . New approaches to improving the figure of merit center around attempts to decouple these relationships using super lattice structures, segmentation, quantum confinement, and phonon “rattlers.”Advantages1. Absence of moving parts eliminates vibration problem as well               as regular attendance. Therefore, it can be best suited for system         where vibration is undesirable.    It is lighter in weight per unit mass of refrigeration.

2.     It is most suitable or the production cooling suit.

3.     Since no refrigerant is used, there is no question of toxicity environmental problem and can directly used for air condition.

4.     The load can be easily controlled by means of adjusting the current to meet the situation.

5.     Its design and manufacture is rather much simpler then the other refrigeration systems.

DisadvantagesThe major disadvantage with thermo-electric cooling remains the

low efficiency. New materials with better characteristics need to be developed before thermoelectric cooling can be used in larger air-conditioning and refrigeration systems. Yet, there are already many niche applications where this technology is used effectively.              Overall COP of this system experimentally found to be 0.1 to 0.2.

7. APPLICATIONS:One application in practical use is that of a drinking water cooler.

Although there may be variations by different manufacturers, the author has observed and used one thermo-electrically cooled (and heated) drinking water cooler that dispenses ice water. The thermoelectric module cools a disk at the bottom of a holding tank well. In this well, there is a focus beam that shines across the disk to a light sensor. The 120 volt power supply has a switching circuit that detects the amount of light across the disk. When ice forms on the disk and it builds to a thickness that deflects some of the light, the power supply is switched off. With the current stopped, some of the heat from the module’s hot side heat exchanger will, warm the disk to free the ice by conduction. The remainder of the heat is dissipated to the atmosphere with a small fan. Once the ice breaks free, it will float to the top of the holding tank, the light sensor will turn the power supply back on, and a new block will be produced approximately every hour.

Today, thermo-electric cooling is used in medical and pharmaceutical equipment, spectroscopy systems, and various types of detectors, electronic equipment, portable refrigerators, beverage coolers, chilled food and beverage dispensers, and drinking water coolers. Requiring cooling devices with high reliability that fit into small spaces, powerful integrated circuits in today's personal computers also employ thermoelectric coolers.

These devices are also used to provide temperature control in telecommunication systems and have even been used as a toy in an electronic pen that draws and erases on a thermally-sensitive writing pad. Using solid state heat pumps that utilize the Peltier effect, thermo-electriccooling devices are also under scrutiny for larger spaces such as passenger compartments of idling aircraft parked at the gate.Thermo-electric devices manufacturers are expanding their production lines and are offering custom items designed and built to precise customer specifications. Cooling assemblies are being made now, using a coordinated system of heat exchangers, cold plates and even customer- supplied accessories ready to be plugged into a system by the user.

 

8. FUTURE DEVELOPMENTS:The two main issues in thermo-electric refrigeration are the development of new materials with stronger Peltier effects and the application of these materials to real engineering problems such as refrigeration and control of process heat.  The former issue is primarily the domain of physicists and materials scientists who test a large number of materials looking for crystalline structures which combine high electrical conductivity with low thermal conductivity as well as a strong thermo-electric characteristic.  The latter issue is of greatest concern to mechanical engineering where problems such as heat transfer between the module and cheap manufacture of modules are of concern.  For refrigeration, unlike air-conditioning, the power consumption is relatively small, typically 50 Watts which means that the number of modules and their cost is also small.  This means that the main issue for refrigeration is heat transfer between the module and its external environment.  The level of interest in these engineering problems is intensifying as the efforts of physicists and materials scientists produce thermo-electric materials with usefully high levels of performance.

There has been steady progress in raising the performance of the materials and construction of thermo-electric modules since the first application of bismuth telluride in the 1950’s.  A purified form of bismuth telluride now enables the manufacture of thermo-electric modules with a Coefficient of Performance approximately equal to unity for

temperature differences of 29 degrees Kelvin.  The standard test temperature difference for a refrigerator cabinet is 29 degrees Kelvin where the cabinet interior is set at 3 degrees Celsius and the exterior at 32 degrees Celsius. The thermo-electric module would operate at a higher temperature difference than this because of conduction and convection losses in the thermo-electric refrigeratingsystem.    A high efficiency of the Peltier module is obtained when these secondary temperature losses are reduced to very small values compared to the temperature difference across the Peltier module. Enhancement of the heat transfer between the hot and cold faces ofa Peltier module and the working fluid is still however a major topic of research since the relative power consumption of a Peltier when used in a refrigeration system remains high.  The key factor to improve energy efficiency is efficient heat transfer.  A major problem is the small size of the Peltier modules compared to their heat output which means that a generous heat transfer coefficient is needed to prevent a large temperature difference between the module and the working fluid.  It is fortunate that water is an effective heat transfer since the choice of fluids is greatly limited by considerations of non-toxicity and non-corrosiveness for a domestic refrigerator.  The sensitivity of Peltier module efficiency to temperature difference between hot and cold face means that even a saving of 1 degree in temperature losses can generate a significant increase in the overall Coefficient of Performance.  A fundamental problem is that the same pumps and fans which generate vigorous convective heat transfer and thereby raise the coefficient of performance of the Peltier module, also consume power to lower the overall system efficiency.  The efficiency of the pumps and manifolds should be as high as possible with a balanced distribution of electrical power to the various sub-systems within the refrigerator.

9. CONCLUSION:               From the all above discussion we can predict that thethermo-electric refrigeration is in experimental stage. Though it is so, today it is being used in surgery for cooling the instrument used for extracting the crystalline lens out of    the eye.              There is problem from testing of thermo-electric refrigeratorthat by using the heat pipe, we can achieve heat transfer rate 500 times more than the conventional heat removal aids like fins etc.by evaporating the heat pipe reverse heat transfer which occurs after the shutoff power supply can

be solved So it has been noticed that use of heat pipe will lead to improve the performance of the thermo-electric module and ulmatly the refrigerator.Thermo-electric refrigeration is likely to become a significant form of domestic refrigeration within the medium term because of the need to avoid refrigerating fluids that are hostile to the environment.Precise control of temperature for better food preservation, low noise and a reduced number of moving parts are also significant benefits of thermo-electric refrigeration.The energy consumption of thermo-electric refrigeration can be reduced to moderate levels with further improvements in the heat transfer between the various stages of the refrigerating system.                Last but not least, I feel that though thermo-electric refrigeration system is at experimental stage and have less application today, in future it can become popular, convenient, reliable eco-friendly alternative refrigeration system.