Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage. Essay Example

Designing A TEC

ANALYSIS AND DESIGN OF AN EFFECTIVE THERMOELECTRIC COOLER WITH AN ABILITY OF MAINTAINING A MINIMUM TEMPERATURE OF 10O CELSIUS OF A BEVERAGE.

Contents

Abstract 3

Introduction 4

Materials and Methods: 6

Results and Analysis 10

Results and Discussion 12

Conclusion 18

References 19

Abstract

A thermoelectric cooler (TEC) is a special cooling gadget that employee several cooling principles where heat transfer takes place against temperature gradient. Its performance depends on various factors which are vital to the optimization of its cooling function. This paper seeks to show how a steady temperature of 100C can be achieved in order to design a TEC that can work under such conditions. Three experiments relating to the efficiency of the TEC were carried out with respect to current flow, size of the heat sink, heat sink material and thickness of the heat sink material to obtain the best combination of elements for designing the TEC. The results showed that there is a linear relationship between the flowing current and the cooling efficiency. The relationship with the size of the heat sink produced a parabolic curve while the best heat sink size was obtained at 25cm3. Copper the best results for the heat sink material with a cooling temperature equivalent to 8 Co. Considering these parameters, a simulation combining these parameters was achieved with the main aim of maintaining a steady temperature of 10oC.It was concluded that the best TEC design is the one designed using six layers of copper metal, having a heat sink of 25cm3 and, performing at a current of 0.8A for five minutes when, starting with a beverage at room temperature.

Introduction

A thermoelectric cooler (TEC) is an electronic device based on the semi-conductor concept that works as a minute heat pump. It contains a p-n junction necessary for creating a temperature gradient between the surfaces of the junctions (Tse, 1989). When a DC power source of low-voltage is applied to a thermo-electric module, heat moves through the module from one part to the other. One face of the module is cooled while the other part is heated simultaneously. Thermoelectric coolers are modules that are solid-state heat pumps whose operation is based on the Peltier principle which states that “current passed through a thermoelectric device will transfer heat into or out of the device” (Nolas, Sharp, & Goldsmid, 2001). The Peltier principle provides that current is generated from the temperature difference between the two surfaces at the junction or the temperature difference between created by virtue of the external voltage (Tse, 1989). According to Liptak (2001), the operation of a TEC can be explained by the way electrons move within the copper conductors. Electrons move freely in the copper conductors but the same is not for semi-conductors. The electrons leave the copper semi-conductors and reach the hot-face of the p-type semi-conductors. For them to move through the p-type, they have to fill a whole. After the whole is filled, the electrons drop down to a lower level of energy and heat is released in the process. The holes in the p-type are basically moving from the cold side to the hot side of the TEC module. As the wholes travel between the p-type back into the copper conductors on the cold face, the electrons strike back at a higher level of energy absorbing heat in the process. Subsequently, the copper travel through the copper conductors to the n-type semi-conductor as they move through the n-type heat is absorbed. The electrons are then moved to the hot-face of the n-type, freely moved through the copper conductors and dropped to lower energy levels releasing heat in the process (Tse, 1989). Figure 1 below is an example of a TEC module that can help in understanding its operation.

Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage.

Figure 1: TEC Module

Norman (1997) asserts that the spontaneity of the heat transfer after an external voltage application is to a large extent affected by the compatibility of the Peltier coefficients of the p and n junctions. If there is a large disparity in the compatibility of the Peltier coefficients, the module might not perform its intended functions. TECs are very reliable as in cooling and are therefore applied in different areas such as the medical industry, tourism industry, food industry, scientific laboratories and the beverage production industry. They are also used as dehumidifiers (Liptak, 2001). Provided that they are well installed and used in the correct manner, TECs will always have an advantage over the conventional refrigerators because they are portable, precise (with respect to temperature stability, (±0.05)) and they demand less maintenance. Their cooling effect is friendly to the environment because they don’t release chlorofluorocarbons and they don’t need to be refilled.

Since TECs have been known to be beneficial, it is therefore necessary to design an effective TEC that meets specific conditions. To do this well, the components making up a TEC should be carefully selected and used to the maximum (Phelan, et al, 2002). Phelan, et al, (2002) show routines for designing Peltier coolers designed from other semi-conductor materials using the Seebeck coefficient, thermal conductivity and electrical sensitivity. These models let the SINDA/FLUINT to use the relevant source conditions and adjust the temperatures internally as required in both transient simulations and the steady state.

This paper analyses the findings of a series of experiments done to come up with components required to design a TEC with the optimum current, heat sink size and time taken to reach and maintain the right temperatures. In this case, it is necessary that the material for the heat sink is tested to ascertain the best material for the purpose. =

Materials and Methods:

Experiment 1: Effect of current on temperature difference created by a TEC

Materials: Brass metal, heat sink of 25cm3, current of 0.8A flowing for five minutes and a beverage at room temperature

The assembled TEC was attached to a copper heat sink with the heat sink paste which was then attached to the thermocouple, then to the cold and hot faces of the TEC. The TEC was then connected to an external DC source and the ammeter adjusted to 0.1A. The temperature difference was recorded at this point. The current was then changed to different values and the respective temperature differences recorded. A 3 minute allowance was given to let the temperature stabilize before recording. A graph showing the relationship between temperature difference between the cold surface of the TEC and the heat sink versus current was drawn. The maximum temperature difference was obtained at a current of 1.2A.

Experiment 2: Effect of the size of the heat sink

Materials: Timer, aluminum heat sinks of different sizes, thermocouple, aluminum heat sink paste, dc source and an ammeter.

The heat sink was attached using the paste to the thermocouple and later to the aluminum sink and the cold face of the TEC. A fixed current 1.0 A was used with corresponding temperature difference recorded 3 minutes later. This procedure was repeated for different values of heat sink sizes and values recorded in a table. For precautionary purposes, the TEC and heat sink temperatures were set at room temperatures before the onset of the experiment. A graph showing the relationship between the size of the cold surface versus the volume of the heat sink was drawn.

Experiment 3: Effect of heat sink materials. Materials: Congruent sinks made of different materials, corresponding heat sink Pastes, thermocouple, dc source, timer and an ammeter

The procedure in experiment 2 was repeated here with different materials being used for the heat sinks. . For precautionary purposes, the TEC and heat sink temperatures were set at room temperatures before the onset of the experiment. The material that gave the lowest temperature for the cold face of the TEC was established.

Experiment 4: System design using thermoelectric coolers

Materials: Copper calorimeter, thermometer, an Ammeter, heat sinks, timer, water and ice.

The TEC design was developed using brass material with a capacity of 25cm3. Water of 50 g was placed inside the calorimeter on the cold face of the TEC and the ammeter set to a current of 0.8A. The initial temperature of the water was taken before allowing the current to flow. The final temperature was recorded after five minutes. A repeat of the procedure was done for different thicknesses of copper and after the sink was modified with ice. The values were recorded in a table and the best combination recorded. The water was then placed in a calorimeter to obtain the suitable exposure time for the beverage and temperature differences were recorded after every 5 minutes. The values were also recorded in a table and a graph was drawn to obtain the relationship between time and temperature.

Results and Analysis

The following table shows the effect current has on the temperature differences between the cold and the hot faces of the TEC. The temperature difference increased with increase in the current flowing with the highest temperature difference recorded as 21.6oC when the current flowing is 0.8A. The tabulation method of statistics shows that increase temperature is directly proportional to the increase in current.

Table 1: Effect of current on the temp difference

Current (A)

Temperatures of cold surface (C)

Temperatures of hot surface (C)

Δ in temp (Co)

This gives a linear model that is shown in the graph below when a plot of Temperature difference against the flowing current is drawn. Figure 1, below shows this model:

Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage. 1Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage. 2

Fugure 1 of temperature difference of TEC against Current

The temperature difference between the two faces reaches its maximum of 33.7 oC when the current reaches a maximum of 3.0A. The effect of the size of the sink on temperature of the cold surface is shown in Table 2 below. If the size of the heat is large, the temperature of the cold surface of the TEC is low.

Table 3: Effect of heat sink size on temperature of the cold surface

Sink size (cm3)

Temp of cold surface

The trend of the size of heat sink against temperature change assumed a parabolic shape, having an optimal performance at size 1.8 cm achieving the best temperature of 8.6 as shown in Figure 2 below.

Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage. 3

Figure 2: Relationship between heat sink size against temperature of the cold surface of TEC

The effect of the heat sink material was analyzed and the results recorded in Table 3 below. According to the findings of the experiment, copper was the material that showed the appropriate temperature after 3 minutes of exposure to a temperature of 80C. Coppers rate of heat transfer was considered the best for the design.

Table 3: Effect of temperature on the material

Material

Temp (Co) of cold surface

Aluminum

After getting the best parameters required for maintaining the temperature at 10oC, the results of another experiment sought to show the effect of different thickness of copper since copper was found the best material. 6 coppers with ice gave the best condition that would maintain the temperature at 10oC after five minutes of exposure at a current of 0.8A (Table 4).

Table 4: Appropriate design of the TEC system capable of the meeting the set condition.

Current (A)

Experiment

Mass of water (g)

Initial temp (Ti)

Time (min)

Final temp (TF)

2 coppers

3 coppers

with ice
4 coppers

with ice
6 coppers

After 5 minutes of exposure, the rate of temperature chance between the hot and cold surfaces of the TEC reduced with a margin of 3.5oC after the exposure of 25 minutes, the best temperature of water was obtained at 6.30C and at -6.8oC for the TEC as shown in Table 5 below.

Table 5: showing the effect of exposure time to the temperature of both water and TEC

Time (min)

Current (A)

Temp of water (Co)

Temp of TEC (Co)

A representation of this information on a graph gives a parabolic curve showing the relationship between the time of exposure on the temperature of water. The more the water is exposed to the combined conditions making up the TEC, the more its temperature drops giving the cooling effect that is required of a TEC. Figure 3 Below shows this relationship.

Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage. 4

Figure 3 of the effect of exposure time on the temp of water

Discussion

Selecting the best combination of elements is vital when designing a thermoelectric cooling system. Generally, the temperature of the heat sink increases 10oC beyond the ambient temperatures when an external DC source is connected to the TEC with no heat load on the cold surface. There are different operating voltages for different TECs but the power is directly proportional to the voltage power which also dependent on the temperature difference between the two surfaces. According to the results obtained in experiment 1, a current of 0.8A gives a temperature difference of 21.6oC with a maximum difference being reached at 33.7oC when a current of 3A is flowing. A good temperature difference between the hot and cold surfaces is dependent on the thermal isolation between the two surfaces.

To obtain this there should be reasonable distance between the two surfaces. To do this, the analysis of the effect of the size of the heat sink on the temperature of the cold side of the TEC was done. The relationship between the size of the heat sink and the temperature of the cold surface when a current of 0.8A is running is a parabolic curve as seen in figure 2. The best performance was attained at a size of 25cm3. Such a size of the heat sink increases the distance between the cold and hot surfaces of the TEC allowing for thicker insulation and the bolts used for the assembly would be longer. This also allows for the temperature to be controlled easily. This is due to the large thermal resistance created that is created between the hot and cold surfaces hence the cold surface does not heat up very fast.

The best thermal insulation should also be obtained depending on the thickness of the sink material. From the experiment, an analysis of the effect of the thickness of the sink material on the temperature of the TEC was done after it was concluded that copper was the best material for use. The best results were obtained when the hot and the cold surfaces were separated by a thickness of six coppers. This was able to maintain the temperatures at10oC when a current of 0.8A is flowing. The 6 coppers thermally insulated the cold surface from the hot surface giving the best elements needed for the designing of the TEC.

We have seen that good thermal isolation between the TEC surfaces is vital to the performance of the TEC. The portability and the simplicity of the TECs make them the best choice for applications in cooling systems. One of the main applications of the TEC can be for beverages using the combination of the elements above as brought about in experiment 4. The main elements that can be considered for the design of a TEC are copper metal in six layers for the heat sink of size 25cm3, current of 0.8A flowing for 5 minutes when the beverage to be cooled is at room temperature.

This design could be a modification of the many TECs that have been suggested in previous studies (Plan. Et al). The previous models have shown designs with unstable heat transfers due to heat entering the inside of the TEC and at the same time, a lot of heat leaving the inside. Figure 4 below shows a prototype TEC whose design has been developed from the findings of this study.

Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage. 5

The Beverage being cooled

6 layers of copper

25cm3 Copper

Ammeter0.8A

Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage. 6Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage. 7Analysis and design of an effective thermoelectric cooler with an ability of maintaining a minimum temperature of 10o Celsius of a beverage. 8

This prototype uses a copper heat sink of and can be used with a current of 0.8 Amperes with the beverage to be cooled at ambient temperature of 21 to 23oC.

Conclusion

This TEC system can be adopted for the cooling of beverages at room temperature and has some advantages over the older cooling systems such as limited size, noise and vibrations. There are not side effects because its emission of CO2 gas is zero. This design could be improved on in the future to get a steady transfer of heat and more considerations can be to get more precise relationships with the Peltier s principle. The TEC could be tested for more conservative sources of energy like the solar power to see how it works and if it can be adopted for the use of this power to ensure safety in the environment. If this prototype design can be improved on to produce a fridge, it has the potential for commercialization.

References

Liptak, G. B. (2001)Instrument Engineers’ Handbook: Process control and optimization. New York NY: CRC Press.

Norman, A. A. (1997)Instrumentation for process measurement and control (3rd ed),CRC Press.

Tse, S.F., &Morse, E. I. (1989)Measurement and instrumentation in engineering: principles and basic laboratory experiments. M. Dekker Publisher.

G. Nolas, J. Sharp, and H. Goldsmid, Thermoelectrics: Basic principles and New Materials Developments. New York: Springer, 2001, p. 167.

P. E. Phelan, V. A. Chiriac, and T. Y. Lee, “Current and future miniature refrigeration cooling technologies for high power microelectronics,” IEEE Trans. Compon. Packag. Technol., vol. 25, no. 3, pp. 356–365, Sep. 2002.

Plan, Patricia. Tseng, James, 2007. Yoon, Diana. Steady State Heat Transfer.Team 3. Carnegie Mellon University Transport Process Lab

Acknowledgement

Carrying out this experiment was an actual life experience that added value to my academic work and improved my teamwork skills. I would like to express my gratitude to ……………………… for giving me the capacity to complete this experiment. I want to also appreciate them for providing the equipment and facilities required to do this experiment hence enabling me to achieve the goals of this experiment. I have to furthermore extend my heartfelt gratitude to my partner for his support and wonderful help in completing this experiment.