I.Introduction Althoughthermoelectric (TE) phenomena was discovered more than 150 years ago,thermoelectric devices (TE coolers) have only been applied commercially duringrecent decades. For some time, commercial TECs have been developing in parallelwith two mainstream directions of technical progress – electronics andphotonics, particularly optoelectronics and laser techniques. Lately, adramatic increase in the application of TE solutions in optoelectronic deviceshas been observed, such as diode lasers, super-luminescent diodes (SLD),various photo-detectors, diode pumped solid state lasers (DPSS), charge-coupleddevices (CCDs), focal plane arrays (FPA) and others. The effect of heating orcooling at the junctions of two different conductors exposed to the current wasnamed in honor of the French watchmaker Jean Peltier (1785–1845) who discoveredit in 1834.
It was found that if a current passes through the contacts of twodissimilar conductors in a circuit, a temperature differential appears betweenthem. This briefly described phenomenon is the basis of thermoelectricity andis applied actively in the so-called thermoelectric cooling modules.Thermoelectric devices (thermoelectric modules) can convert electrical energyinto a temperature gradient––this phenomena was discovered by Peltier in 1834.
The application of this cooling or heating effect remained minimal until thedevelopment of semiconductor materials. With the advent of semiconductormaterials came the capability for a wide variety of practical thermoelectricrefrigeration applications. Thermoelectric refrigeration is achieved when adirect current is passed through one or more pairs of n and p-typesemiconductor materials. In the cooling mode, direct current passes from the nto p-type semiconductor material. The temperature of the interconnectingconductor decreases and heat is absorbed from the environment.
This heatabsorption from the environment (cooling) occurs when electrons pass from a lowenergy level in the p-type material through the interconnecting conductor to ahigher energy level in the n-type material. The absorbed heat is transferredthrough the semiconductor materials by electron transport to the other end of thejunction TH and liberated as the electrons return to a lower energy level inthe p-type material. This phenomenon is called the Peltier effect. We studiedworking system of HVAC system and observe temperature and pressure in thissystem.
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Our aim is to introduce the new HVAC system using thermoelectric couplewhich shall overcome all the disadvantages of existing HVAC system. If thissystem comes in present HVAC system, then revolution will occur in theautomotive sector also. With population and pollution increasing at an alarmingrate TEC (thermoelectric couple) system have come to rescue as these areenvironment friendly, compact and affordable. Conventional compressor runcooling devices have many drawbacks pertaining to energy efficiency and the useof CFC refrigerants.
Both these factors indirectly point to the impendingscenario of global warming. As most of the electricity generation relies on thecoal power plants, which add greenhouse gases to the atmosphere is the majorcause of global warming. Although researches are going on, better alternativesfor the CFC refrigerants is still on the hunt. So instead of using conventionalair conditioning systems, other products which can efficiently cool a personare to be devised. By using other efficient cooling mechanisms we can save theelectricity bills and also control the greenhouse gases that are currentlyreleased into the atmosphere. Although Thermoelectric (TE) property wasdiscovered about two centuries ago thermoelectric devices have only been commercializedduring recent years. The applications of TE vary from small refrigerators andelectronics package cooling to Avionic instrumentation illumination control andthermal imaging cameras.
Lately a dramatic increase in the applications of TEcoolers in the industry has been observed. It includes water chillers, coldplates, portable insulin coolers, portable beverage containers and etc.Fig.1. Schematic of thermoelectric module operation 1. (a)cooling mode 1. (b)heating mode II.
Methodology and Experimentation 2.1 Thermoelectric Module: A standard module consists of a number of thermocouplesconnected in series and sandwiched between two ceramic plates (See Figure 3). Byapplying a current to the module one ceramic plate is heated while the other iscooled. The direction of the current determines which plate is cooled. Thenumber and size of the thermocouples as well as the materials used in themanufacturing determine the cooling capacity.
Cooling capacity varies from mill watts to several thousand watts. Different types of TEC modules are single stage, twostage, three stage, four stage, centre hole modules etc. A typical single stageis shown in Figure 2.Fig. 2 A typical single stagethermoelectric module.
Fig. 3 A Classic TE Module AssemblyWhiledesigning a TEC cooling system, the designer must consider the followingparameters:1. Temperature to be maintained for the object that isto be cooled. 2.Heat to be removed from the cooled object.3.Time required to attain the cooling after a DC power is applied.
4.Expected ambient temperature. 5.Space available for the module and hot side heat sink. 6.Expected temperature of hot side heat sink. 7.
Power available for the TEC. 8. Controlling the temperature of the cooledobject if necessary 2.2Parameters of a Thermoelectric Module: Once it is decided that thermoelectric cooler is to beconsidered for cooling system, the next step is to select the thermoelectricmodule or cooler that can satisfy a particular set of requirements. Modules areavailable in great variety of sizes, shapes, operating currents, operatingvoltages and ranges of heat pumping capacity.
The cross section of a TEC isshown in Fig4. The minimum specifications for finding an appropriate TEC by thedesigner must be based on the following parameters.Fig. 4 Cross section of Thermoelectric Module § Coldside temperature (Tc ) § Hotside temperature (Th ) § Operatingtemperature difference, which is the temperature difference between Th andTc . § Amountof heat to be absorbed at the TEC’s cold surface. This can also be termed asheat load. It is represented as (Qc ) and the unit is Watts.
§ Operatingcurrent (I) and operating voltage (V) of the TEC. 2.2.1 Coldside temperature-Ifthe object to be cooled is in direct contact with the cold surface of the TEC,the required temperature can be considered the temperature of the cold side ofTEC (Tc). The aim is to cool the air flowing through the heat sinks.When this type of system is employed the cold side temperature of the TEC isneeded to be several time colder than the ultimate desired temperature of theair. 2.
2.2 Hotside temperature-The hot sidetemperature (Th) is mainly based on the two factors. First parameter isthe temperature of the ambient air in environment to which the heat is beenrejected. Second factor is the efficiency of the heat sink that is in contactwith the hot side of TEC. 2.
2.3 Temperaturedifference-Thetwo temperatures Tc and Th and the difference between them ?T isa very important factor. ?T has to be accurately determined if the coolingsystem is expected to be operating as desired. The following equation shows theactual ?T. Actual ?T is notsame as the system ?T.
Actual ?T is the difference between the hot and coldside of the TEC. On the other hand system ?T is the temperature differencebetween the ambient temperature and temperature of the load to be cooled. 2.
2.4 CoolingLoad- The most difficult and important factorto be accurately calculated for a TEC is the amount of heat to be removed orabsorbed (Qc) by the cold side of the TEC. In this project Qc wascalculated by finding the product of mass flow rate of air, specific heat ofair and temperature difference. Here the temperature difference system ?T inthe difference between the inlet temperature and outlet temperature of thecooling system. The mathematical equation for Qc is as shown below.
2.2.5-ThermoelectricAssembly – Heat Sinks ThermoelectricAssemblies (TEAs) are cooling or heating systems attached to the hot side ofthe TEC to transfer heat by air, liquid or conduction. TEAs which dissipateheat from the hot side use heat exchangers.
TEC requires heat exchangers orheat sinks and will be damaged if operated without one. The two ?T s, actual ?Tand system ?T depend on the heat sinks fitted at the hot sides or cold sides ofTEC. The thermal resistances of the heat sinks could vary the ?T across the TECfor a set ambient temperature and cooling load temperature. Therefore thethermal resistance of the heat sinks could increase the current flowing throughthe TEC. The three basic types of heat sinks are: forced convective, naturalconvective and liquid cooled, where liquid cooled is the most effective. Thetypical allowances for ?T at the hot side heat sink of a TEC are 1. 10 to 15 °Cfor a forced air cooling system with fins – Forced convection 2.
20 to 40 °C for cooling using freeconvection – Natural convection. 3. 2 to 5 °C forcooling using liquid heat exchangers – Liquid cooled.There are severaldifferent types of heat exchangers available in the market. As far this projectis concerned a forced convection type of heat sink was be used based on the ?T.The main heatsink parameter for the selection process is its thermal resistance. Heat sinkresistance can be termed as the measure of the capability of the sink todissipate the applied heat. The equation is as follows.
R isthe thermal resistance (in oC /W or K/W) and Th, the hot sidetemperature and T? ambient temperature respectively. Qh is the heat loadinto the heat sink which is the sum of TEC power Pe and heat absorbed. Thegoal of a heat sink design is to lessen the thermal resistance. It can beattained through exposed surface area of the heat sink. It may also requireforced air or liquid cooling.
2.2.6-PowerSupply and Temperature Control: Powersupply and temperature control are two added items that must be consideredwisely for a successful TE system. TEC is a direct current device. The qualityof the DC current is important. Current and voltage of a TEC can be determinedby the charts provided by the manufacturer.
TEC’s power is the product ofrequired voltage and current. (P = IV).Temperaturecontrol is generally categorized into two groups. One is open loop or manualand the other is closed loop or automatic. For cooling systems normally coldside is used as basis of control.
The controlled temperature is compared to theambient temperature. An on-off or a control using thermostat is the simplestand easiest techniques to control the temperature of a TEC. 2.3-TEC Selection:TheTEC was selected considering few factors such as dimensions, Qc, power supplyand etc. The model of TECs used in this project was manufactured in China byHebei I.T (Shangai) Co. Ltd. The model no.
of the module is TEC1-12706. Theidea was to select a TEC which has a cooling power greater than the calculatedTEC. TEC1-12706 operates with an optimum voltage of 12V. It has maximum voltageof 15.4V. At 12V it draws and maximum DC current of 4 A. The minimum powerrating or the cooling power is 37.
7 W. The maximum power is 48W. It has amaximum operating temperature of 200°C of the TEC are 68 when hot sidetemperature is 25 °C. The charts from the TEC manufacture were alsoanalyzed while choosing the TEC. It had been decided to use 1 module.
2.4-TEC ArrangementThe TEC is sandwiched between two heat sinks whichare fitted with 12V, 0.185A fans. HotSide heat sink: The hot side heat sink usedin the project was a single long one installed on the top side of the TECs. As discussed,thermal resistance of a heat sink is an important factor while designing asystem. Therefore a forced convection heat sink had to be used.
When selecting hot side heat sink for the project other factors such asdimension to fit into the whole assembly, budget and availability were alsotaken to consideration. The heat sink was bought from a local shop and therewas no thermal resistance or datasheets available for the product. Thealternative was to calculate Rt from the resistance of the unfinned area(Rb) and the resistance offered by the fins (Rf). Since both of theseresistances are acting in parallel, total resistance was found using theequation Thecalculated value was 0.
0145K /W. The calculated thermalresistance of the heat sink was lesser than the required. But when consideredthe dimensions of the cooling system the selected heat sink was very apt. IV.
CONCLUSIONSAThermoelectric Air cooling & heating system was designed and built whichcan be used for personal cooling & heating. By using 1 TEC with a DC powersupply through external power supply (dimmer stat) cooling was achieved. Thecooling system is capable of cooling & heating the air when air wascirculating between the heat sinks and it was blown to the object by fans. Comparingthe cost, size, flexibility, and environmental friendliness we conclude that itis better to reliable on TECs than present HVAC with certain modifications andusing better materials for heat sinks.
