1Mtech Scholar & ETC Department RTMNU
JHULELAL INSTITUTE OF TECHNOLOGY – Lonara Nagpur
Air cooling is significantly important in relatively humid countries. So there is a need to bring down the humidity. Different types of cooling systems are available in the market. They can be classified as air cooled, water cooled, refrigerated, and thermoelectric cooled.
Although Thermoelectric (TE) property was discovered about two centuries ago thermoelectric devices have only been commercialized during recent years. The applications of TE vary from small refrigerators and electronics package cooling to Avionic instrumentation illumination control and thermal imaging cameras. Lately a dramatic increase in the applications of TE coolers in the industry has been observed. It includes water chillers, cold plates, portable insulin coolers, portable beverage containers and etc. The research paper is basically focused on how to overcome demerits of HVAC system in industries by thermoelectric cooling system.
Keywords– HVAC, TEC, Peltier effect, Seebeck effect.
Thermoelectric refrigerator sometimes called a thermoelectric cooler module or Peltier cooler is a semi conductor based electric component that functions as a small heat pump. By applying a low voltage direct current (DC) power source to a thermoelectric cooler module, heat will be moved through the module from one side to the other. One module face, therefore, will be cooled while the opposite face simultaneously is heated. Both thermoelectric refrigerators and mechanical refrigerators are governed by the same fundamental laws of thermodynamics and both refrigeration systems; although considerably different in form, function in accordance with the same principles. In a mechanical refrigeration unit, a compressor raises the pressure of a refrigerant and circulates the refrigerant through the system. In the refrigerated chamber, the refrigerant boils and in the process of changing to a vapor, the refrigerant absorbs heat causing the chamber to become cold. The heat absorbed in the chamber is moved to the condenser where it is transferred to the environment from the condensing refrigerant. In a thermoelectric cooling system, a doped semi-conductor material essentially takes the place of the refrigerant, the condenser is replaced by a finned heat sink, and the compressor is replaced by a Direct Current (DC) power source. The application of Direct Current (DC) power to the thermoelectric cooler modules causes electrons to move through the semi-conductor material. At the cold end of the semi-conductor material, heat is absorbed by the electron movement, moved through the material, and expelled at the hot end. Since the hot end of the material is physically attached to a heat sink, the heat is passed from the material to the heat sink and then in turn, transferred to the environment.
Thermoelectric phenomenon was discovered nearly two hundred years ago. Since last sixty years the practical applications from thermoelectric had been exploited.
The first breakthrough that would eventually be used to form the thermoelectric effect was discovered in 1820. Several other breakthroughs in the field were discovered over the next few decades but their relationship was not realized for a full 38 years. William Thomson discovered that heat is absorbed or produced when current flows in material with a certain temperature gradient and that the heat is proportional to both the electric current and the temperature gradient. His publication linked all the discoveries from the preceding decades.
Kryotherm, (2007) Studies based on thermoelectric cooling unit for thermostatic body on refrigerated trucks were conducted by Bulat and Nekhoroshev (2003). In this study a comparison between the thermoelectric cooling units with vapour-compression installations was also made, where it showed that cost price of thermoelectric unit is four-five times cheaper than vapour-compression cooling units. The cooling power obtained for TE cooling was same when compared to compression cooling units.
McStravick, et.al (2009) had invented a medical travel pack with cooling system. The invention has helped people suffering from chronic disease to travel with proper supply of medicine kept at proper temperature. These insulated container using TE modules comprises of a cold plate, heat sink, fan and a temperature sensor.
A. Peltier Effect
In 1834, a French watchmaker and part time physicist, Jean Charles Athanase Peltier found that an electrical current would produce heating or cooling at the junction of two dissimilar metals. In 1838 Lenz showed that depending on the direction of current flow, heat could be either removed from a junction to freeze water into ice, or by reversing the current, heat can be generated to melt ice. The heat absorbed or created at the junction is proportional to the electrical current. The proportionality constant is known as the Peltier coefficient.
If we modify our thermocouple circuit to obtain the configuration shown in fig, it will be possible to observe an opposite
If a voltage (Vin) is applied to terminals Tl and T2 an electrical current (I) will flow in the circuit. As a result of the current flow, a slight cooling effect (Qc) will occur at thermocouple junction A where heat is absorbed and a heating effect (Qh) will occur at junction B where heat is expelled. Note that this effect may be reversed whereby a change in the direction of electric current flow will reverse the direction of heat flow. The Peltier effect can be expressed mathematically as:
Qc or Qh=pxy x I
Where: pxy is the differential Peltier coefficient between the two materials, x and y, in volts I is the electric current flow in amperes Qc, Qh is the rate of cooling and heating, respectively, in watts Joule heating, having a magnitude of I x R (where R is the electrical resistance), also occurs in the conductors as a result of current flow. This Joule heating effect acts in opposition to the Peltier effect and causes a net reduction of the available cooling.
B. Seebeck Effect
In 1821 Thomas Johann Seebeck found that a circuit made from two dissimilar metals, with junctions at different temperatures would deflect a compass magnet. Seebeck initially believed this was due to magnetism induced by the temperature difference. However, it was quickly realized that it was an electrical current that is induced, which by Ampere’s law deflects the magnet. More specifically, the temperature difference produces an electric potential (voltage) which can drive an electric current in a closed circuit. Today, this is known as the Seebeck effect. The voltage produced is proportional to the temperature difference between the two junctions. The proportionality constant () is known as the Seebeck coefficient, and often referred to as the thermoelectric power or thermopower. The Seebeck voltage does not depend on the distribution of temperature along the metals between the junctions. This is the physical basis for a thermocouple, which is used often for temperature measurement.
V = (Th – Tc)
As shown in figure 2 the voltage difference, V, produced across the terminals of an open circuit made from a pair of dissimilar metals, A and B, whose two junctions are held at different temperatures, is directly proportional to the difference between the hot and cold junction temperatures, Th – Tc. To illustrate the Seebeck Effect let us look at a simple thermocouple circuit as shown in fig. The thermocouple conductors are two dissimilar metals denoted as Material x and Material y.
In a typical temperature measurement application as shown in figure 3, thermocouple A is used as a “reference” and is maintained at a relatively cool temperature of Tc. Thermocouple B is used to measure the temperature of interest (Th) which, in this example, is higher than temperature Tc. With heat applied to thermocouple B, a voltage will appear across terminals Tl and T2. This voltage (Vo), known as the Seebeck emf, can be expressed as: Vo = axy x (Th – Tc)
The thermoelectric cooling fan design was performed based on certain mechanical and electrical calculations. The fan’s design was compromised on the availability of parts in the market and budget of the project. As shown in figure 4 the prototype assembly starts with a main fan which is used to blow the ambient air through a circular duct The duct is attached to a blower fan and leads towards a group of four heat sink. The air which is passed through the duct goes into the cluster of four heat sinks which are united together. This heat sinks act as a channel for the air to pass through, there are six TECs that are sandwiched between a long black heat sink. The TECs were installed between the heat sinks using thermal grease, which increases the thermal conductivity by balancing irregular surface of the heat sinks. When the TECs are in operation cold side of the TEC cools down the heat sink channel. Air which is coming out from the channel is chilled air which is lower than the ambient. The cold side heat sinks rests on wooden base. There are two fans fitted on top of the hot side heat sink. They blow air towards the hot heat sink to cool it down when the TECs are in operation. The hot air is channeled away from the user using panels. The whole assembly of the cold side heat sink, hot side heat sink, TECs and the wooden base are fitted tightly with the help of metal clips. These metal clips are tightened together with screws and nuts. The whole assembly is enclosed with sheets or panels.
Assembly of TEC
The ambient air blown from the blower is channelled into goes a group of four heat sinks which acts a rectangular duct as discussed earlier. It was decided to remove maximum amount heat from the point when the air started to enter the first heat sink. Keeping that in mind the first heat sink was installed with two TECs in series and the second one also was installed with another two TECs in series. This will help to remove more heat from of the air when air enters the duct. The third and fourth heat sinks were installed with one TEC each and they were connected in series also. All the two series connected TECs were connected in parallel. Figure 6 illustrates a top view of the connection of TECs as explained above. The arrow indicates direction of air flow.
Each of the TEC will be acting as loads. In other words the layout above can also be termed as three parallel groups of two TECs in series electrically. Figure 7 shows simpler redrawn electrical connection of the TECs.
Total required current and voltage for the all the joined TE modules are 12A and 24V respectively. Therefore a 300W power supply was enough for the cooling system. The electrical power input was greater than cooling power of the TECs and also higher than the calculated Qc.
(300W > 226W > 222W ).
Final testing of TECs for the completed assembly for about 30 mins. Temperature measured at different locations is shown in table 1. Here we can see that as time increases, temperature at outlet decreases. Within 30 minute temperature decreases from 31.5 to 25.1.
Table 1. Time vs Temperature
Thermoelectrics and thermoelectric cooling are being studied exhaustively for the past several years and various conclusions have been conceived regarding the efficient functioning of thermoelectric refrigerators.
Thermoelectric refrigerators are greatly needed, particularly for developing countries, where long life, low maintenance and clean environment are needed. In this aspect thermoelectrics cannot be challenged in spite of the fact that it has some disadvantages like low coefficient of performance and high cost. These contentious issues are the frontal factors hampering the large scale commercialization of thermoelectric cooling devices.
The solution to above problems can only be resolved with the development of new techniques. There is a lot of scope for developing materials specefically suited for TE cooling purpose and these can greatly improve the C.O.P. of these devices. Development of new methods to improve efficiency catering to changes in the basic design of the thermoelectric set up like better heat transfer, miniaturization etc. can give very effective enhancement in the overall performance of thermoelectric refrigerators. Finally, there is a general need for more studies that combine several techniques, exploiting the best of each and using these practically.
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