Thermoelectric Heat Pipe-based Refrigerator: System Development and
Comparison with Thermoelectric, Absorption and Vapor Compression
Refrigerators
Nandy Putra, Ardiyansyah, Ridho Irwansyah, Wayan Nata Septiadi,
A. Adiwinata, A. Renaldi, K. Benediktus
Heat Transfer Laboratory, Department of Mechanical Engineering, University of Indonesia Kampus UI, Depok, 16424, Indonesia.
nandyputra@eng.ui.ac.id
Keywords: Refrigerator, Thermoelectric Cooler, Absorption, Vapor Compression.
Abstract. Thermoelectric coolers have been widely applied to provide cooling for refrigerators in
addition to conventional absorption and vapor compression systems. To increase heat dissipation in the thermoelectric cooler’s modules, a heat pipe can be installed in the system. The aim of this study is to develop a thermoelectric heat pipe-based (THP) refrigerator, which consists of thermoelectric coolers that are connected by heat pipe modules to enhance heat transfer. A comparative analysis of the THP prototype and conventional refrigerator with vapor compression, absorption and thermoelectric systems is also presented. The prototype system has a faster cooling down time and a higher coefficient of performance than the absorption system but still lower than vapor compression system
Introduction
There are three main types of refrigeration systems for refrigerator applications: vapor compression, absorption and thermoelectric systems. Each of these systems has advantages and disadvantages. The vapor compression system offers a high Coefficient of Performance (COP) value and large cooling capacity, but the use of CFC and HCFC as its refrigerants depletes the ozone layer. The absorption system has an intermediate COP value and may utilize waste or recovered heat but is typically bulky and heavy and involves pressurized parts; thus, it is difficult to maintain. The thermoelectric system is a portable compact device with low noise but has relatively low COP value. Furthermore, this system uses DC power and would require a DC power converter if powered by AC electric sources. The thermoelectric system could also be powered directly using solar energy through the use of photovoltaic cells [1].
Thermoelectric technology has been widely used for both cooling and power generation [2]. Thermoelectric cooling systems have been applied in a wide range of applications, including portable vaccine carrier boxes, surgical devices, cooling systems for electronic equipment, food processing equipment, and military and aerospace instruments [3-11]. Thermoelectric coolers (TECs) require heat sinks to dissipate the energy generated or absorbed at the two junctions. The design and selection of a heat sink is crucial to the overall operation of the system. The heat sink should be designed to minimize the thermal resistance between the hot side of the TECs and the lower temperature reservoir, e.g., the ambient air. The heat transfer between the thermoelectric module’s hot side and heat dissipation device may be further improved by the use of heat pipes [12]. Heat pipes combine the principles of thermal conductivity and phase transition to efficiently manage the heat transfer between two solid interfaces. The aim of using heat pipes is to optimize the heat dissipation process of a heat exchanger. Heat pipes typically consist of a sealed tube with an internal wick and are charged with a refrigerant, such as water, ethanol, methanol or nanofluids. Heat pipes are widely adopted for their high efficiency, cooling capability, reliability and shape flexibility [13-17].
The objective of this research is to develop a thermoelectric heat pipe-based refrigerator, which consists of thermoelectric coolers that are connected by heat pipe modules to enhance heat transfer. A comparative analysis of the THP prototype and conventional refrigerator with vapor compression, absorption and thermoelectric systems is also presented
Advanced Materials Research Vol. 651 (2013) pp 736-744 Online available since 2013/Jan/25 at www.scientific.net
© (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.651.736
Thermoelectric Refrigerator Design
A thermoelectric heat pipe-based refrigerator was designed based on the commercial absorption refrigerator because the refrigerator is commonly used for a refrigerated mini-bar in a hotel. This THP was also experimentally analyzed by comparing its operational characteristics and performance with those of the conventional vapor compression refrigerator, absorption refrigerator and thermoelectric refrigerator without heat pipes. These four types of refrigerators were tested under the same fridge dimensions of 400 x 440 x 560 mm and a refrigerated volume capacity of 40 litre. The two thermoelectric refrigerator designs such as the THP refrigerator and thermoelectric refrigerator without heat pipes, utilize a commercially available thermoelectric module for the cooling system. The dimension of single thermoelectric module is 40 x 40 mm.
Absorption refrigerator
An absorption cycle is a heat-driven thermal cycle in which only thermal energy is exchanged with its surroundings; no appreciable mechanical energy is exchanged. Furthermore, no appreciable conversion of heat to work or work to heat occurs in the cycle [18].The refrigerant vapor leaving the evaporator is absorbed into a liquid solution in the absorber. The pressure of the liquid solution is then raised to the condenser pressure by the pump. The solution is pre-heated in the heat exchanger. By the addition of heat in the generator, refrigerant vapor is driven out of the liquid solution. The absorbent passes back through the heat exchanger and is throttled down to the evaporator pressure in the expansion valve. The generator, condenser and heat exchanger are on the high-pressure side of the system, while the evaporator and absorber are on the low-pressure side [19]. The absorption refrigerator used in this study was a commercially branded refrigerator named KW2001084. The power consumption of this refrigerator was 70 W, and the refrigerated space volume was 40 litre. This refrigerator uses water as the absorbent and ammonia (NH3) as the refrigerant.
Vapor compression refrigerator
A commercial vapor-compression refrigerator manufactured by SANYO (Sanyo SR D50F) was experimentally tested for comparison with other refrigerators. The vapor-compression refrigerator has the same refrigerated space volume and power consumption as the other refrigerators.
Thermoelectric heat pipe-based refrigerator
Legend:
1. Heat sink fan heat pipe 2. Thermoelectric module 3. Cold sink
4. Cold side fan
Figure 1. Thermoelectric heat pipe refrigerator
The THP refrigerator uses cold sink and heat pipe modules to increase both the heat transfer between the thermoelectric module and the air inside the refrigerated space as well as the heat dissipation from the hot side of the thermoelectric module to the ambient air. The THP system uses 6 thermoelectric modules, which are located between the heat pipe and cold sink. The cold sink was attached to the cold side of the thermoelectric module. The cold sink was made of aluminum, and its overall dimensions were 300 x 120 x 35 mm. A commercial heat sink fan heat pipe, the Hyper 212 Plus, which was manufactured by Cooler Master, was attached to the hot side of the thermoelectric module to optimize the heat release. Each Hyper 212 Plus consists of four heat pipes. The THP design is depicted in Figure 1. To minimize the thermal contact between the
thermoelectric module and cold sink, a thin film of Dow Corning 340 thermal silicone paste was used. The cold sink and cold side of the thermoelectric module have the same role as the evaporator in the vapor compression cycle.
Thermoelectric refrigerator without heat pipes
This system is very similar to the THP system, except its excess heat system only uses a heat sink and fan and does not include a heat pipe system. To enhance the heat transfer from the hot side of the thermoelectric module, a 400-x-120-x-35-mm aluminum heat sink was attached to the thermoelectric module, this heat sink was selected because of it was commercially available. The inner and outer fans create a heat pumping system as they promote the forced convection heat transfer in the system. Figure 2 shows the construction of the thermoelectric-heat-sink refrigerator.
Legend:
1. Hot side fan 2. Cover 3. Heat sink
4. Thermoelectric module 5. Cold sink
6. Cold side fan
Figure 2. Thermoelectric refrigerator
Experimental Setup
The thermoelectric refrigerators were tested under varying ambient temperatures, power consumption of the thermoelectric modules and cooling loads. Water was used as the cooling load for the experiment, and the cooling loads used in the experiment were 500 ml, 1.000 ml and 1.500 ml. All of the experiments were conducted within 240 minutes. The experimental setup used to test the thermoelectric refrigerators is depicted in Figure. 3.
Figure 3. Experimental setup
The temperatures of the refrigerators were measured at several points. For the thermoelectric refrigerator, the hot and cold sides of each thermoelectric module, cabin of the refrigerator, water and ambient air temperature were measured. For the absorption refrigerator, the generator inlet and outlet, condenser inlet and outlet, absorber inlet and outlet, evaporator inlet and outlet, cabin, ambient air and water temperature were measured. K-type thermocouples were used as the temperature sensors and connected to a National Instruments data acquisition system. The overall accuracy of the temperature sensor was 0.05 oC. An adjustable DC power supply was used to operate the thermoelectric module and fans.
Legend :
1. Refrigerator 2. DC power supply 3. Thermocouple
4. Data acquisition system 5. Computer
Results and Discussion
Absorption refrigerator
The absorption refrigerator works at 220V of 0.32-ampere electrical circuits AC (alternating current). The refrigerator uses 70 W of power to operate the heater, which serves as a generator in this system. Figure 4 shows the working temperature of the absorption refrigerator at the ambient temperature of 30°C. The absorption system generator can raise the temperature of the refrigerant by 128.65 oC to a temperature of 69.83 °C in a stable condition. After exiting the generator, the saturated vapor refrigerant ammonia temperature was 53.3 °C; when it entered the condenser, the refrigerant experienced the heat release and transformed into liquid phase at 49.19°C. In the evaporator inlet, the refrigerant has a temperature of -10°C after the pressure decreases, and after the heat from the cabin is absorbed, the refrigerant temperature increased to 3.25°C at the outlet of the evaporator. The absorber system is a mixture of ammonia absorption by lowering the temperature of water from 43.71°C to 42.62°C.
Figure 4.Temperature absorption refrigerator
Figure 5. Cabin temperature of the absorption refrigerator
Figure 5 shows the results based on the variation of the environmental temperature; as shown in the figure, the absorption refrigerator cabin temperature is directly proportional to the value of the environmental temperature. The value of the low end of the cabin temperature is 9.84°C under an environmental temperature of 20°C and increased as the environmental temperature increased. Under an environmental temperature of 25°C, the value of the low end of the cabin temperature was 11.73°C, which is 1.89°C greater than that at the ambient temperature conditions of 20°C. Under an environmental temperature of 30°C, the low end of the cabin temperature was 15.15°C or 3.72°C greater than that at an environmental of 25°C because the absorption system is using the heater as the generator system and the environmental temperature is too low to affect the performance of the generator.
Thermoelectric refrigerator without heat pipes
The experiment was conducted by varying several factors, including the input power, ambient temperature, and cooling load.
Figure 6 shows the working temperature of the thermoelectric system with an input power of 72 W at the ambient temperature of 25°C. The cooling occurs quickly as the cold side temperature decreases significantly. The forced convection allows the cabin temperature to decrease quickly. As shown in the figure, the temperature difference between the hot and cold sides is 25°C, with an average temperature of 32.74°C on the hot side and an average temperature of 7.59°C on the cold side.
Figure 7 shows that the ambient temperature affects the obtained cabin temperature. The lowest cabin temperature of 4.11°C was reached when the thermoelectric was run at the ambient temperature of 20°C. The cabin temperature at the ambient temperature of 25°C was 9.63°C, while that at 30°C was 16.06°C. These results indicate that the performance of the thermoelectric refrigerator is affected by the ambient temperature.
Figure 8 shows that the cooling load is one of the factors affecting the degree to which the thermoelectric system can reduce the cabin temperature. The experiment is conducted at the ambient temperature of 30°C with a power input of 72 W. The results show that a lower cabin temperature is reached when the cooling load is lower. The lowest cabin temperature of 14.15°C was reached when no cooling load was applied to the system, and the highest temperature of 17.83°C was reached when the cooling load of 1.500 ml water was applied to the system.
Figure 6. Temperature characteristics of the thermoelectric refrigerator without heat
pipes
Figure 7. Cabin temperature of the thermoelectric refrigerator without heat pipes
Figure 9 shows the temperature conditions in the test cabin of thermoelectric without heat-pipes refrigerator for varying amounts of power. The testing was performed by providing powers of 72, 102 and 252W to the refrigerator, with an environmental temperature of 25°C and a cooling load of 1.000 ml. The lowest cabin temperature was when the 102 W power applied to the refrigerator, meanwhile when the power was 252 W resulting the higher cabin temperature, it happened because of the amount of power being applied to the thermoelectric generate a higher heat on the hot side of the thermoelectric module, which was unable to absorb by the heat sink.
Figure 8. Cabin temperature due to variations in the cooling load
Figure 9. Cabin temperature of TEC Refrigerator due to variations in the power
Thermoelectric heat pipe-based refrigerator
Figure 10 shows the result of the thermoelectric refrigerator driven by a power of 72 W at an ambient temperature of 25°C and a cooling load of 1.000 ml. The graph shows that when the system is activated directly, the thermoelectric temperature changes during the first 25 minutes and the cabin temperature and temperature of the cold side decreased significantly. In this process, a forced convection heat transfer occurs inside of the cabin refrigerators; thus, the cabin room temperature decreases when the temperature of the cold side of the thermoelectric module decreases. This refrigerator takes approximately ± 100 minutes to reach stable conditions, at which time, the refrigerator cabin temperature was within 2-3°C of the temperature of the cold side of the thermoelectric module. The hot side of the thermoelectric module is stable within approximately 30 minutes, following only temporary changes in the environmental temperature afterward.
Figure 11 shows the temperature conditions in the test cabin refrigerator for varying amounts of power. The testing was performed by providing powers of 72, 102 and 252W to the refrigerator, with an environmental temperature of 25°C and a cooling load of 1.000ml. These graphs show that higher amounts of power allow a lower cabin temperature to be reached. The application of heat pipe prove that the decreasing in cabin temperature because of its ability to absorb more heat from the hot side of thermoelectric module.
Figure 10. Temperature characteristics of the thermoelectric refrigerator with heat pipes
Figure 11. Cabin temperature of THP Refrigerator due to variationsin the power
Figure 12 shows the cabin temperature of the refrigerator for varying cooling loads. The tests were performed with 72 W of power and the same environmental temperature of 25°C. The applied loads ranged between 0 and 1.500 ml. As shown in the figure, the cabin temperature reaches the low point after approximately 50 minutes and then stabilizes. However, after the 240 minutes of testing, the cabin temperature at each loading point did not differ significantly, indicating that the number of loading conditions on the refrigerator only affects the time required for the heat transfer process in the cabin. The cabin temperature will reach its lowest position more quickly when less weight is applied.
Figure 13 shows the temperature conditions of the test cabin refrigerator for various ambient temperatures. The testing was performed by providing a power of 72 W and a cooling load of 1.500ml. As shown in the figure, the cabin temperatures at the ambient temperatures of 25 and 20°C differ by up to 30°C. The ends of the cabin temperatures at the ambient temperatures of 20°C and 25°C are within ±1°C, but the temperatures differed by 14.73°C at the ambient cabin temperature of 30°C.
Figure 12. Cabin temperature due to variations in the cooling load
Figure 13. Thermoelectric refrigerator with ambient variations
Performance comparison
The cabin temperatures of the four different types of refrigerators are compared in Figure 14. All of the refrigerators were tested under the same testing conditions, with a 1.000 ml cooling load and an ambient temperature of 25 0C. The comparison shows that the THP refrigerator performs better than
the TEC and absorption refrigerators. The addition of the heat pipe proved to be the main reason for which the performance of the TEC refrigerator improved. The cabin temperature of the THP refrigerator is ±4 0C lower than that of the TEC refrigerator.
The Coefficient of Performance was calculated to compare the performance of the refrigerators (COPR) [21]. Equation 1 was used to calculate the COPR. In this case a 1.000 ml of water was used and ran for 240 minutes in every refrigerator for the calculating the COPR. The properties of water that being used in the calculation are density and heat capacity which value are 1.000 kg/m3 and 4.18 kJ/kg respectively. The data of the initial and final temperatures of water, cabin and ambient were presented in the table 1. Equation 4 was used to calculate the power consumption (Win) of the refrigerators. The power consumption for TEC refrigerator and THP refrigerator was 72 W while the Vapor Compression refrigerator and Absorption refrigerator were 28.6 Watts and 70 Watts respectively. Figure 15 shows the comparison of COP from each refrigerator. The THP refrigerator has a higher COPR value than the absorption and TEC refrigerator but still lower than the vapor compression refrigerator.
Figure 14. Cabin temperatures of the 4
different types of refrigerators Figure 15.COP comparison of refrigerators
Table 1 Data of ambient, water and cabin temperature corresponding with the value of COP
Conclusions
The experimental results showed that the environmental temperature could affect the cabin temperature of the refrigerator. If the ambient temperature increases, the cabin temperature will increase, and vice versa. Furthermore, an increased use of power in the system increases the value of the thermoelectric system’s ∆T, which affects the cabin temperature of the thermoelectric refrigerator. With respect to the cooling time, the thermoelectric system has an advantage over the absorption system because it does not have a delay time for cooling the cabin, while the absorption system has a response time of 20-30minutes. The experimental results show that the vapor compression refrigerator still performs better than the 3 other types of refrigerators. The thermoelectric refrigerator, both with and without heat pipes, performs better than the absorption refrigerator. The application of the heat pipe on the hot side of the thermoelectric module improves the performance of the thermoelectric refrigerator.
Nomenclature
Win : Power consumption of the refrigerator
I : Electrical current
V : Voltage
COPR : Coefficient of performance of the refrigerator
Qcooling : Cooling rate inside the refrigerator
Tf : Water Final Temperature
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Engineering Materials and Application
10.4028/www.scientific.net/AMR.651
Thermoelectric Heat Pipe-Based Refrigerator: System Development and Comparison with Thermoelectric, Absorption and Vapor Compression Refrigerators
10.4028/www.scientific.net/AMR.651.736
DOI References
[1] S.B. Riffat, Gouquan Qiu, Comparative investigation of thermoelectric air-conditioners versus vapour compression and absorption air-conditioners, Applied Thermal Engineering 24 (2004) 1979-(1993). http://dx.doi.org/10.1016/j.applthermaleng.2004.02.010
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http://dx.doi.org/10.1016/j.ijheatmasstransfer.2006.09.037
[4] Nandy Putra, Ardiyansyah, William Sukiyono, David Johansen, Ferdiansyah N. Iskandar, The characterization of cascade thermoelectric cooler in cryosurgery device, Cryogenics 50 (2010) 759-764. http://dx.doi.org/10.1016/j.cryogenics.2010.10.002
[7] Rieyu Chein, Guanming Huang, Thermoelectric cooler application in electronic cooling, Applied Thermal Engineering 24 (2004) 2207-2217.
http://dx.doi.org/10.1016/j.applthermaleng.2004.03.001
[9] A. Hamilton, J. Hut, An electronic cryopore for cryosurgery using heat pipes and thermoelectric coolers : a preliminary report, Journal of Medical Engineering and Technology 12 (1993) 104- 109.
http://dx.doi.org/10.3109/03091909309016215
[10] Hiroki Takeda, Shigenao Maruyama, Junnosuke Okajima, Sestuya Aiba, Atsuki Komya, Development and estimation of novel cryoprobe utilizing the peltier effect for precise and safe surgery, Cryobiology 59 (2009) 275-284.
http://dx.doi.org/10.1016/j.cryobiol.2009.08.004
[11] M.R. Holman, S. J Rowland, Design and development of new surgical instrument utilizing the peltier thermoelectric effect, Journal of Medical Engineering and Technology 21 (1997) 106- 110.
http://dx.doi.org/10.3109/03091909709031155
[12] S. B Riffat, S.A. Omer, Xiaoli Ma, A novel thermoelectric refrigeration system employing heat pipe and phase change material : an experimental investigation, Renewable Energy 23 (2001) 313-323.
http://dx.doi.org/10.1016/S0960-1481(00)00170-1
[15] Y. H Yau, M. Ahmadzadehtalatapeh, A review on the application of horizontal heat pipe heat exchangers in air conditioning system in the tropics, Applied Thermal Engineering 30 (2010) 77-84.
http://dx.doi.org/10.1016/j.applthermaleng.2009.07.011
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http://dx.doi.org/10.1016/j.ijheatmasstransfer.2010.06.037
[20] P.K. Bansal, A. Martin, Comparative Study of vapor compression thermoelectric and absorption refrigerator, International Journal of Energy Research 24 (2000) 93-107.
