Thermo-electric refrigeration

INTRODUCTION One cannot refuse the use of refrigeration systems in our life, not only in kitchen but also in shops, industries and commercial purposes. Today’s compression   refrigeration   system   has   given   very   goo d performance. But refrigerants used in these systems are hazardous to the environment and human life, because they react with very useful gas ozone (O3), thereby depleting ozone layer.   Thermo-electric refrigeration system can substitute vapor compression refrigeration system for small-scale applications and act as solution of the problem above discussed. 

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, it obeys the basic laws of thermodynamics. Only the actual system for cooling is different.  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.

Thermo-Electric Refrigeration In Thermo-Electric refrigeration, essentially nothing has changed. The refrigerant in both liquid and vapour 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 and without a refrigerant

Comparison DC power source Compressor Heat Sink Condenser Fins Thermo-Electric Refrigeration System refrigerates without the use of Mechanical Devices and without a Refrigerant

BASIC PRINCIPLES Peltier Effect- when a voltage or DC current is applied to two dissimilar conductors, a circuit can be created that allows for continuous heat transport between the conductor’s junctions. Seebeck Effect- is the reverse of the Peltier Effect. By applying heat to two different conductors a current can be generated. The current is transported through charge carriers (opposite the hole flow or with electron flow). Heat transfer occurs in the direction of charge carrier movement. Applying a current (e- carriers) transports heat from the warmer junction to the cooler junction

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 ( called modules) which are connected by electrical conductors at either end..  The passage of electric current through them 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.  The Coefficient of Performance (COP) of a Peltier module is defined in the same way as for a conventional refrigeration system : 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.

Advantages

No moving parts so maintenance is required less frequently Weight per unit refrigeration is Low Operation in any Place

Precise Temperature Control Ability to Heat and Cool With the Same module Precise Temperature Control Electrically “Quiet” Operation

Environment Friendly Long life No leakage problem Spot Cooling High Reliability No leakage problem Spot Cooling Environment Friendly

Disadvantages

Price Relegated to applications with low heat flux. Not as Efficient, in terms of COP Price

THERMOELECTRIC MATERIALS These are the materials that can be used to convert thermal energy into electrical energy or provide refrigeration directly from electrical energy. Semiconductors are the optimum choice of material to sandwich between two metal conductors because of the ability to control the semiconductors’ charge carriers, as well as, increase the heat pumping ability. e.g. : Bismuth Chalcogenides, Lead telluride etc. Alternatives: Lead telluride and its alloys. Materials based on Nanotechnology. Silicon Germanium.

The most commonly used semiconductor for electronics cooling applications is Bi2Te3 because of its relatively high figure of merit. However, the performance of this material is still relatively low and alternate materials are being investigated with possibly better performance. Alternating thin film layers of Sb2Te3, Bi2Te3,Lead telluride and its alloys, SiGe

APPLICATIONS 1.THERMOELECTRIC GENERATOR 2.TEMPERATURE CONTROL IN MISSILES 3.TELECOMMUNICATION EQUIPMENTS 4.SPACE PROBES 5.COOLING COMPUTERS 6.FOOD INDUSTRY 7.MEDICAL EQUIPMENT 8.DRINKING WATER COOLER

FUTURE OF THERMOELECTRIC REFRIGERATION The use of thermoelectric devices and systems has been limited by their relative low energy conversion efficiency. Present commercial thermoelectric devices operate at about 10% of Carnot efficiency. Today’s thermoelectric devices are particularly useful when the efficiency is a less important issue than small size, low weight, or high reliability It is important to note that the COP of thermoelectric coolers increases significantly with decreasing the temperature lift.

ALTERNATIVE MODERN USE Instead of utilizing a full-fledged thermoelectric cooling system, it is possible to use a thermoelectric heat pump to improve the performance of an existing vapour compression system, so called “hybrid system.” For example, a hybrid vapour compression – thermoelectric cooler systems could use thermoelectric heat pumps to enhance the outlet sub cooling of a condenser, in which thermoelectric heat pumps operate at small ΔT and high COP.

REFERENCES http://mechanicalgarage.blogspot.in/2013/08/thermoelectric- refrigeration.html http://www.nextbigfuture.com/2008/08/thermoelectrics-and- refrigerators.html http://engineersedge.weebly.com/uploads/4/6/8/0/4680709/thermo electric_refrigeration.pdf