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Performance analysis of a household refrigerator integrating a PCM heat exchanger
Accepted Manuscript Research Paper Performance analysis of a household refrigerator integrating a PCM heat exchanger R. Elarem, S. Mellouli, E. Abhilash, A. Jemni PII: DOI: Reference:
S1359-4311(17)30789-5 http://dx.doi.org/10.1016/j.applthermaleng.2017.07.113 ATE 10781
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
5 February 2017 13 July 2017 16 July 2017
Please cite this article as: R. Elarem, S. Mellouli, E. Abhilash, A. Jemni, Performance analysis of a household refrigerator integrating a PCM heat exchanger, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/ j.applthermaleng.2017.07.113
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Performance analysis of a household refrigerator integrating a PCM heat exchanger R. Elarem1,2, S. Mellouli1,2,3,*, E. Abhilash3, A. Jemni1 1
Laboratory of Thermal and Energetic Systems Studies (LESTE) at the National School of Engineering of Monastir, University of Monastir, Tunisia.
2
Higher School of Science and Technology of Hammam Sousse, University of Sousse, Tunisia. 3
Mechanical Engineering Department, College of Engineering, University of King Khalid, Abha, Kingdom Saudi Arabia.
*Corresponding author: [email protected] Abstract An experimental investigation was carried out to improve the energy efficiency of a household refrigerator by integrating a Phase Change Material (PCM) to accumulate thermal energy and stabilize the temperature in the refrigerator compartment. A novel design of PCM heat exchanger is proposed in this investigation. The experimental results indicate that by integrating this novel PCM heat exchanger, power consumption is reduced by 12% and the COP is increased by 8% compared to the refrigerator without PCM. In order to identify the best performing designs among various cases of refrigerator compartments placed with PCM, 2D unsteady state CFD simulations were made. For all the cases, the household refrigerator was simulated to study the influence of the PCM emplacement on the temperature and velocity fields. The computational results indicate that using PCM emplacements on the evaporator, walls, and in the racks of the refrigerator compartments has a significant influence to stabilize and homogenize rapidly the temperature (86.66% improvement over the basic configuration ). However, there is a limit beyond which increasing PCM coverage in the racks more than 75% does not lead to any significant improvements.
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Keywords: Household refrigerator, Phase change material, Thermal storage, Heat exchanger, power consumption.
Nomenclature g qR
acceleration due to gravity (9.81 m s-2) net radiative flux (W m-2)
Ra
Rayleigh number
t
time (s)
Tamb
ambient temperature (K)
Tw
wall temperature.
u
velocity in
(m s-1)
v
velocity
(m s-1)
I
radiative intensity in the s direction at the r position. (W m-2 per unit solid angle)
Iin
intensity of incident radiation in s direction (at r position)
n
normal vector
Greek symbol
air thermal conductivity of air (Wm-1 K-1)
thermal expansion coefficient (K-1)
emissivity of the refrigerator wall
air dynamic viscosity (Pa s)
solid angle (sr) 2
1. Introduction Domestic refrigerators and freezers are among the most energy demanding appliances in a household due to their continuous operation [1]. Worldwide, it has been estimated that there are approximately one billion domestic refrigerators in use [2]. Although, their direct greenhouse gas emissions have been greatly reduced by the introduction of hydrocarbon refrigerants, their indirect emissions remain very high due to the ever increasing power consumption of these appliances. A growing global environmental awareness and rapidly increasing cost of electric energy are driving the demand for finding a frugal and viable solution for energy saving. So far, the most common approaches for reducing the electric energy consumption of household refrigerators are as follows: (i) improvement of the thermal insulation of refrigerator compartments, (ii) usage of high efficiency compressor [3], and (iii) the enhancement of the heat transfer in the evaporator and the condenser by extending the area of heat transfer or by adding ventilators [4]. Another novel option is to use thermal latent heat storage by integrating Phase Change Material (PCM) heat exchangers in refrigerator compartments.The integration of PCMs is considered as a potential way to improve the efficiency of the household refrigerators by reducing the number of working cycle of the compressor. This technical approach can be further explained as follows: (i) when a PCM is used inside the cabinet, it absorbs the heat by changing its phase from solid to liquid, (ii) the temperature within the cabinet remains constant until the melting process is finished, which in turn prolongs the off cycle of the compressor. Hence, it will decrease the electrical energy consumption of the refrigerator. Many research works [5-17] have been devoted to the numerical and experimental studies on the household refrigerators which integrates a latent heat storage system. Onyejekwe et al [5] have studied a prototype of a simple latent heat
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container filled with eutectic NaCl/H2O mixture and integrating it in a photovoltaic refrigerator. Wang et al [6-8] have studied experimentally and numerically the influence of adding a PCM between compressor and condenser in a cooling system. They found that a 6-8% improvement of efficiency can be achieved by this modification. Further, Azzouz et al [9,10] have demonstrated experimentally that by integrating a latent heat storage system in household refrigerators, the coefficient of performance (COP) can be increased by 5-15%. Recently, Lu et al [11] have proposed a new combined shelf using heat pipes and PCMs. Their experimental results indicated the improvements in temperature distribution within the cabinets without much reduction in energy consumption. However, the study of Gin et al [12] proves that the presence of PCM during defrosting and door openings can result in lower power consumption. Similar studies to improve the efficiency household refrigerators were carried out by Y.T. Ge et al [13], E. Oro [14],Wen-Long et al [15] and Imran Hossen Khan et al [16] by integrating PCM in evaporator or in condenser. Recently, Sonnenrein et al [17] have evaluated the influence of latent heat storage system on the temperature of a standard wire-and-tube condenser equipped with different PCMs (water, paraffin or copolymer compound). These results also indicated that the application of PCM can lowers the condenser temperature and reduces the power consumption. In recent years, extensive efforts [5-29] have been paid to reduce the energy consumption of household refrigerator by integrating PCMs. Most of these studies on the performance improvement of refrigerators focused mainly on simple configurations of PCM heat exchangers (plate exchanger or box). To the best of the authors knowledge, there are no major studies in the open literature that investigate the effects of PCM and its placements inside the refrigerator to improve the thermal stability and the efficiency. This study presents a household refrigerator equipped with a novel PCM heat
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exchanger. The effects of this novel heat exchanger on power consumption of the household refrigerator has been investigated experimentally. In addition to this, various cases of refrigerator compartments placed with PCM were simulated to identify the best performing design which rapidly stabilizes the temperature in the cabinet while increasing its compressor cutoff time and thereby minimizing the energy consumption of the refrigerator. 2. Experimental study 2.1 Experimental test rig A ready-made vapor compression household refrigerator of 136 liter capacity was selected as a prototype for the experiment. The modified PCM-based refrigerator has a single evaporator cabinet with a single door (Fig. 1a). The major technical specifications of this conventional refrigerator are presented in Table 1. A detailed design of the PCM heat exchanger is shown in Fig. 1b. This novel heat exchanger is an array of twelve U-type tubes occupying the whole evaporator covered with A Plus-ICE PCM of phase change temperature 4 0C. Thermo-physical proprieties of the PCM are presented in Table 2. Linear length of each tube, internal and external diameter of each tube are 0.66 m, 0.007 m and 0.008 m, respectively. The material of the heat exchanger tubes is commercial copper and are in contact with the air in the refrigerator cabinet. The placement of the PCM heat exchanger can be seen in Fig.1a . The experimental set up include a conventional refrigerator, the novel PCM heat exchanger, two pressure gauges, five thermocouples, a data acquisition system, an energy logger and a seek thermal camera. The refrigerator circuit was modified to integrate two pressure gauges. Fig. 2 shows the detailed circuit of the setup and the location of the pressure gauges which are used to measure the evaporation and condensation pressure at the inlet and outlet of the
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compressor. Temperature at various locations (evaporator, condenser, compressor and cabinet) as indicated in Fig.2 are measured using K-type thermocouples of 0.5 mm diameter. The history of energy consumption is recorded using a computer interfaced Energy Logger 4000. This energy meter is of type Voltcraft Energy-Logger 4000 with a power range of 0.1 to 3500 W. The relative measurement error of the considered Energy Logger is approximately 1%. For the visualization of the temperature distribution in the compartments of the refrigerator, a thermal imaging camera was used. This thermal camera has a resolution of 32,136 Pixels to detect temperature ranging from -40 0C to 330 0C.
2.2 Experimental procedure Both the conventional and the refrigerator with PCM was tested under the same operating conditions. The experiments was carried out in a room where the temperature and humidity were maintained constant. An air-conditioning system was used to maintain the ambient air temperature 22 0C and a relative humidity of 50% inside the test room. The power supply, the energy consumption, the temperatures of the refrigerator were measured using data acquisition systems. To further investigate the operating characteristics, the evaporator midpoint temperature, the condenser outlet temperature, and the compressor outlet temperature were also measured. All these data acquisition assumes a steady state condition of the refrigerator.
3. Numerical study 3.1. Various arrangement of PCM heat exchanger In the present study, the effect of the PCM heat exchanger was investigated by
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arranging it in different ways within the refrigerator. Four such arrangements were identified to evaluate the change in efficiency of the refrigerator through simulation studies. These various cases are as follows. Case 1: The heat exchanger occupying the whole evaporator covered with PCM (similar to the PCM heat exchanger as described in Section 2.1) as shown in Fig. 3a. In this case, a direct-contact between the air inside compartment and the PCM is ensured so that the heat in the compartment can be stored in the PCM. Case 2: This configuration of the PCM heat exchanger is similar to that in Case 1, but a rectangular parallelepiped container filled with PCM is added and mounted vertically adjacent to the lateral wall of the refrigerator compartment. The schematic of this configuration is shown in Fig. 3b. Case 3: This configuration of the PCM heat exchanger is similar to that in Case-1, but in addition, three racks are made horizontally within the compartment and are covered with PCM. The PCM heat exchanger is covered only 90% of the rack surfaces in order to permit the circulation of air near the lateral wall (Fig. 3c). Case 4: This configuration of the PCM heat exchanger is a combination of the Case-2 and the Case-3. The schematic of this configuration is shown in Fig. 3d.
3.2.Mathematical formulation To simplify the simulation study, only half the cross-section of the physical domain is simulated (internal dimensions: 0.74×0.21 m) assuming symmetry. The following assumptions are made in the study: i.The physical properties of the medium (air), except the density, are assumed to be constant. ii.
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0 1 Tamb T
Considering these assumptions, the equations governing the heat transfer and the fluid dynamics within the
are given as follows:
3.2.1
u v 0 x y
3.2.2
0
u u u P 2u 2u 0u 0 v 2 2 t x y x x y
where 0 and are the density (kg m-3) and the dynamic viscosity (Pa s) of the air, respectively.
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0
v v v P 2v 2v u 0 v 2 2 0 T T0 g t x y y x y
where P, , T, T0 and g are the pressure (Pa), the thermal expansion coefficient (K-1), the temperature (K), the ambient temperature (K) and the acceleration due to gravity (9.81 ms-2), respectively. 3.2.3
2T 2T T T T 2 2 .q R 0 C P u v x y y t x where C P ,T, and qR are the thermal capacity(J kg-1K-1), the temperature (K), the thermal conductivity of the air (Wm-1 K-1) and the radiative heat flux (Wm-2), respectively. 3.2.4 The radiative transfer equation In order to understand the phenomena occurring in the refrigerator compartments, it is necessary to consider the radiation in the simulation because both the radiation and the natural convection heat transfer coefficients have a significant effect and are of same order of magnitude in refrigerator compartments [18]. The discrete ordinates radiation model was employed to model the radiation between surfaces within the refrigerator compartment. Air within the refrigerator is assumed transparent and absorption and diffusion of radiation by air is neglected. The radiation
heat transfer equation in the s direction is given as follows: .I r , s s 0 where I r , s is the radiative intensity in the s direction at the r position.
The radiative heat flux leaving the refrigerator walls (grey diffuse surfaces) is given as follows:
qout 1 w qin wTw4 9
where, w is the wall emissivity, σ is the Stefane-Boltzmann constant and Tw is the wall temperature. The incident radiative heat flux at each of the refrigerator walls, is calculated using this equation:
qin
I
s .n 0
in
s .nd incident flux
where, I in is the intensity of incident radiation in the s direction (at the r position), n is
the normal vector and is the solid angle.
3.3. Initial and boundary conditions Initially, a uniform air temperature of 25 0C is assumed within the refrigerator compartment. The boundary conditions are defined as follows: (1) symmetry boundary:
T q .n 0, R
uv0
(9)
Where n represents the normal vector to the corresponding wall.
(2)
radiation: opaque walls with a diffuse fraction of 1 and internal emissivity of the walls was assumed to be 0.9. (3) At the interface between air and PCM, conjugate heat transfer is assumed and noslip condition for velocities and coupled temperature are specified. Also, the emissivity is assumed to be 0.15 [13]. 3.4.Computational procedure and model validation The computational domains were created in the commercial software GAMBIT 2.3.16.
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It was also used for meshing, labeling the boundary conditions and determine the computational domain. The aforementioned equations are solved for unsteady state conditions using Fluent 6.3.26.
The equations for momentum, energy and radiation were solved using second order upwind discretization. In order to validate the study, an initial simulation was carried out for refrigerator design described as Case-1. The numerical model for this configuration was validated against experimental data collected from the test rig described in Section 2.1. Fig.4a shows the predicted temperature contours of the refrigerator compartment and Fig.4b shows infrared images of the refrigerator compartment. These infra-red images taken at an ambient temperature of 20 0C indicate the temperature distribution at different stages of the experiment. Fig.4c shows infrared images of the rear view of the refrigerator, indicating the temperature distribution of the condenser at different stages of the cycle: immediately after the start of the compressor and before the end of the compressor runtime when the maximum temperature is reached. Comparing Fig. 4a & Fig. 4b, a close agreement between predicted results and the measured results can be observed. 4. Experimental results 4.1 Evaporator temperature To understand the effect of using PCM the evaporator temperature of refrigerator with and without PCM were compared. As Fig. 5a indicates, for the refrigerator without PCM, the evaporator temperature varied from -16 0C to -3.5 0C and the average evaporator temperature was -10 0C over the cycle. Further, for the refrigerator with PCM, the evaporator temperature was from -12 0C to -3.5 0C and the average evaporator temperature was -8 ᵒC. Obviously, the evaporator temperature of the
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refrigerator with PCM varied in a short range. It indicates that the lowest and the average evaporator temperature of the refrigerator with PCM are higher than that of the refrigerator without PCM by about 4 0C and 2 0C, respectively. This can be explained by the fact that when the PCM is added on the evaporator, it absorbs the heat of the compartment and after that the heat is transferred to the evaporator. Therefore, the heat dissipation load of the evaporator during the on-time could be greatly reduced, which resulted in that the refrigerator could work under a higher evaporation temperature and a higher evaporation pressure compared to the case without PCM. Considering the close relationship between the evaporator temperature and the evaporating temperature, the higher evaporator temperature may correspond to a higher evaporating temperature. Thus, the coefficient of performance of the cooling system of the refrigerator is improved by increasing the evaporating temperature. 4.2 Condenser temperature The comparison between the condenser outlet temperatures for different cases is shown in Fig. 5b. For the refrigerator without PCM, the condenser outlet temperature increased gradually while the compressor was in operation to a highest outlet temperature (about 32.2 0C) until the compressor was stopped. For the refrigerator with PCM, the condenser outlet temperature was higher than the ambient temperature by 1.8 0C; which indicated that maximum of condensation heat was released into the environment during the idle cycle. Therefore, the heat dissipation load of the condensers during the operating cycle could be greatly reduced, indicating that the refrigerator could work under a lower condensation temperature and pressure.
4.3 Compressor temperature The comparison between the compressor outlet temperatures for different cases is shown in Fig. 5c. For the refrigerator without PCM, while the compressor was in idle 12
cycle, the compressor outlet temperature dropped to 45 0C due to the pressure drop inside the condensers, and during the operating cycle the temperature increased gradually to 57 0C. This temperature was much higher than the ambient temperature due to the influences of the environment and the cabinet heat capacity. For the refrigerator with PCM, while the compressor was in idle cycle, the refrigerant within the condensers remained at a higher temperature than the ambient temperature due to the effect of the PCM exchanger heat capacity. When the compressor was in operation, this part of refrigerant was pushed out of the condensers, resulted in a rapid increase of the outlet temperature to a peak temperature of 53 0C. This peak outlet temperature gradually decreased to a value about 43 0C during the idle cycle. Also, from this comparison it was noted that, the period of the cycle was reduced for the refrigerator with PCM; which indicated an impact of energy consumption.
4.4 The effect of PCM on power consumption of refrigerator The comparison of the power consumption between the different cases is shown in Fig. 6. It can be seen that, the maximum power consumption of the refrigerator with PCM was lesser than that of the refrigerator without PCM. Also, from Fig. 6 it can be seen that the total cycle time is 0.5 h and the ratio of operating cycle time to the total cycle time for the refrigerator without PCM is 0.4, however, the corresponding values for refrigerator with PCM is 0.4 h and 0.37 respectively. Further, it can be noticed that the total cycle time and the ratio of operating cycle time to the total cycle time for the refrigerator with PCM is less. This fact can be explained as follows: (i) The refrigerator with PCM was working under a lower condensation temperature, a higher evaporation temperature and a higher subcooling degree contributing to a less energy consumption, (ii) The difference between
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the internal and the external compartment temperatures of the refrigerator with PCM was lower than that of the refrigerator without PCM due to the heat storage effect of the PCM heat exchanger. Therefore, the heat lost through the refrigerator compartment is reduced, resulting in a shorter operating cycle time. Consequently, it can be inferred that the refrigerator with PCM had a greater COP. The COP calculated using the cycle plotted in the P-h diagram of refrigerant R134a (Fig. 7) is presented below.
COPWithoutPCM COPWith PCM
h1 h4 3.48 h2 h1
h1 h4 3.75 h2 h1
(10)
(11)
Hence, the percentage of COP improved by using the PCM heat exchanger is 8% under the operating conditions presented in this study. A higher COP may be achieved by increasing the amount of PCM within the refrigerator compartments or by appropriate emplacement of PCM.
4.5 The effect of PCM on compressor running time at different loads. The thermal load inside the refrigerator compartments was varied by changing the quantity of hot water in bottle placed inside the refrigerator compartments. Fig. 8 shows the percentage of average running time per cycle of the compressor at different thermal loads. From the Fig. 8 it can be found that: (i) the system with PCM significantly reduced the average compressor running time per cycle, which in turn reduced the energy consumption (ii) average compressor running time per cycle is reduced from 2.5 to 5% as compared to the case without PCM, which depends on different thermal loads. (iii) the percentage of average running time per cycle of the compressor is proportional to thermal loads for both the cases, however the slope of the curve is found lower for the refrigerator with PCM heat exchanger. 14
5. Numerical results 5.1. The effect of PCM heat exchangers emplacement The effectiveness of incorporating thermal heat storage system in a household refrigerator was evaluated by comparing the four emplacement cases of the PCM heat exchanger in the refrigerator compartment (Fig. 3). In order to find the best case which rapidly stabilizes the temperature in the refrigerator compartment, modeling and simulation experiments are done. The predicted temperature distributions and the contours of velocity vectors at different instants (60s, 240s and 600s) in the refrigerator compartments are analyzed for each case.
5.1.1 Study on Case 1 The temperature distributions and the velocity vectors obtained from simulation for the Case 1 are shown in Fig. 9. It can be noted that the heat exchanger PCM fitted on the evaporator indicated a stratified temperature profile with a cold zone at the top and a warm zone at the bottom of the refrigerator compartment. After 600 s the maximum temperature of 12 0C is predicted for the bottom right corner and the average temperature in the compartment refrigerator is 9 0C. Initially, the velocity vectors indicated a circular airflow pattern in the top of the compartment and airflow along the walls. At the end, a region of air recirculation in the middle of compartment is observed and the air in the top of the compartment is mostly stagnant. The distribution of the velocity vectors shows a negligible air velocity at the wall of PCM heat exchanger and a maximum air velocity at the center of the refrigerator compartment. Also, an unsteady recirculation has been observed at the middle of the compartment.
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5.1.2 Study on Case 2 The contours of predicted temperatures and the velocity vectors at different instants (60 s, 240 s and 600 s) within the refrigerator compartments for Case-2 is shown in Fig. 10. The temperature field is influenced by the presence of PCM vertically fitted inside the compartment. A lower temperature is observed at the bottom and at the middle of compartment refrigerator compared to that of Case-1.This is due to the fact that the vertically fitted PCM heat exchanger accelerates the air circulation in the central zone of the refrigerator compartment. Also, it can be noted that the presence of the PCM influences the airflow and thus a circular airflow is observed in the refrigerator compartment. Cold air flows downward with an increase in magnitude along the vertical wall of PCM and it reaches a maximum value while approaching the bottom of the compartment. Further, the air flows with a decreasing magnitude upward along the plane of symmetry to the surface of PCM fitted on the evaporator and then flows horizontally towards the vertical PCM wall . The air is nearly stagnant at the top of the refrigerator cabinet.
5.1.3 Study on Case 3 The contours of predicted temperatures and the velocity vectors at different instants (60s, 240s and 600s) in the refrigerator compartments for Case-3 is shown in Fig. 11. Compared to Case-2, a nearly uniform temperature distribution can be observed for this case. Also, the average temperature in the compartment is found to be 4 0C lower than that of Case-2. The velocity vectors for the Case 3, shows small air recirculation loops between the racks covered with PCM. Through the gap of each racks, the air flows downwards along the vertical wall with increasing velocity and attain a maximum value at the bottom cavity. At the same time, within the bottom
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cavity two vortices that circulates in opposite directions are observed due to the temperature difference at the top and bottom. This observation is in agreement with the temperature contours of the air in the compartment, which is shown in Fig. 11.
5.1.4 Study on Case 4 The temperature distribution for the Case -4 (cabinet fitted with PCM racks and a vertical PCM wall ) appeared to be very uniform compared to other cases, which is shown in Fig. 12. In addition to the overall thermal stratification in the compartment, stratification is also observed between the racks due to negligible air flow. The velocity contours indicate that the airflow is a combination of the contours of Case-2 and Case-3. Initially, the airflow between the PCM racks of the compartment had a similar behavior to that observed in Case-3. After some instants, small air loops are found circulating between the PCM racks . The airflow velocity decreases gradually and becomes stagnant in the cavities that are in the middle of the compartment. These cavities have shown better air mixing than that in Case-3 due to the presence of cold vertical PCM wall of the refrigerator. However, air circulation is observed in the bottom cavity due to the warm air located near the non-adiabatic bottom wall. To summarize, compared to other case studies, the vertical PCM wall predominantly affects the airflow by increasing the velocity and the PCM racks significantly enhances the air mixing in the cavities of the compartment. 5.1.5 Comparison between different Cases The performance of various PCM emplacements inside the refrigerator compartment is compared to identify the best performing design which rapidly stabilizes the temperature within the compartment.
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The evolution of the average temperature and the average velocity of air for all the four cases are presented in Fig. 13a and 13b, respectively. Since, the stabilization of the air temperature is controlled by the heat transfer rate from the air to the PCM, time intervals for the reduction of the average air temperature from 25 0C to 10 0C for each case is obtained and compared. From the Fig.13a it can be seen that, to reach the reduction of the temperature, Case-1 took much time (600 s) compared to Case-2, Case- 3 and Case-4 which took 195 s, 90 s and 80 s, respectively. A comparison of the three novel cases (Case-2, Case-3 and Case4) with the basic configuration (Case-1) indicated 67.5%, 85% and 86.66% reduction in duration, respectively. Therefore, Case-4 offers better heat transfer performance to reach the stabilization temperature inside the compartment which in turn reduces the energy consumption.
5.2. Influence of the PCM coverage in the racks In the numerical simulation, the Case-4 is taken to evaluate the influence of the percentage of PCM in the racks on the uniformity of the air temperature within the refrigerator compartment, the simulations are made for various PCM coverage (10%, 50%, 75% and 90%) by assuming same initial and the boundaries conditions. Fig. 14 shows the contours of temperature and velocity of the air within the compartment after 240 s for different PCM coverage. For all the cases, thermal stratification is observed with a cold zone at the middle of the refrigerator compartment and a warm zone at the top and the bottom. In addition, a cold zone is also observed along the vertical PCM wall. When the PCM coverage is increased, the temperature of the air between two racks is lower than that in the bottom cavity. In the top of the compartment, the temperature is relatively uniform for all the
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cases. The temperature field is considerably influenced by the presence racks and the PCM coverage in it. For the racks with 10% PCM coverage, a slightly lower temperature is observed at the bottom and a slightly higher temperature at the middle compared with all other cases. From Fig. 15, it is observed that the presence of racks also influences the air circulation. However, this influence is strong for 10% and 50% PCM coverage because of increased gap between the racks and the vertical PCM wall which facilitates the air flow. Therefore, the PCM racks should be placed far from the PCM vertical wall, in order to enhance the air circulation inside the compartment. The variation of the average temperature and the air velocity within the compartment refrigerator is shown in Fig. 15. Examining this figure, it can be noted that increasing the PCM coverage from 10% to 50% shorten the time interval (for the reduction of the average air temperature from 25 0C to 5 0C) by 42%; whereas, when the PCM coverage is increased from 75% to 90%, there was no significant change in the time interval. Also, it is observed that the airflow velocity can be decreased by increasing PCM coverage to a particular value (75%). Thus, if the compartment with more efficient heat transfer is desirable, increasing PCM coverage more than this value have no significant effect. This can be explained by the fact that the Raleigh number decreases with the increase of PCM coverage and then the convective heat exchange between the air and the PCM decreases. On the other hand, when the PCM coverage increases, the air exchanges heat with a large surface maintained at the phase change temperature. These two opposite phenomena explain the negligible effect of the PCM coverage when it is increased more than 75 %.
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6. Conclusions In this work, a novel design of a PCM heat exchanger was made. Based on this design, an experimental test rig was developed to conduct a series of experimental studies in order to minimize the energy consumption of the household refrigerator. The experimental results show that under the standard test conditions, the power consumption of the household refrigerator with PCM heat exchanger is reduced by 12% and the COP is increased by 8% compared to the refrigerator without PCM. Also, in order to study the influence of the PCM emplacement on the temperature and velocity fields, four configurations were simulated. Using the numerical simulation of air flow and heat transfer within the refrigerator compartments, the following conclusions can be made: 1. To effectively integrate a PCM heat exchanger in a refrigerator compartment, it is necessary to employ PCM emplacements in evaporator, walls and racks of the compartment which helps to rapidly stabilize and homogenize the temperature. This increases the cutoff time of compressor and thereby minimizes the energy consumption of the refrigerator. 2. A comparison of the three novel cases (Case-2, Case-3 and Case-4) with the basic configuration (Case-1) indicated 67.5%, 85% and 86.66% reduction in duration, respectively. Therefore, Case-4 offers better heat transfer performance to reach the stabilization temperature inside the refrigerator compartment. 3. It is found that for the Case-4, a genuine optimization of the PCM coverage in the racks is necessary to ensure the thermal storage. The racks with higher PCM coverage has a favorable performance to achieve the stabilization temperature inside the compartment in shortest time. However, there is a limit beyond which increasing PCM coverage in the racks more that 75% does not lead to any significant reduction
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in time to achieve the uniform temperature. The CFD simulation developed in study can be further extended to understand the influence of operating conditions on the temperature and velocity fields. It is expected that the use of variables such as evaporator temperature (parameter related to the thermostat setting by the consumer), the dimensions of the evaporator (parameter related to design), the emplacement of the thermal load inside the refrigerator compartment etc., in simulation studies will contribute much to understand the use of PCM in household refrigerators. References [1] A.P. Simard, M. Lacroix, Study of the thermal behavior of a latent heat cold storage unit operating under frosting conditions, Energy Convers. Manag. 44 (2003) 1605-1624. [2] S. Ben Amara, O. Laguerre, M.-C. Charrier-Mojtabi, B. Lartigue, D. Flick, PIV measurement of the flow field in a domestic refrigerator model: Comparison with 3D simulations, Int. J. of Refrigeration 31 (2008) 1328-1340. [3] P. Binneberg, E. Kraus, H. Quack, Reduction in power consumption of household refrigerators by using variable speed compressors, Int. Refrigeration and Air Conditioning Conference (2002), Paper 615. Available from: http://docs.lib.purdue. edu/iracc. [4] P. Roth, Energy Saving on the High Pressure Side of a Refrigerating Plant. KI 03/2008, pp. 30-35. [5] D. Onyejekwe , Cold storage using eutectic mixture of NaCl/H2O: an application to photovoltaic compressor vapours freezers, Sol. Wind Tech., 6 (1989) 11-18.
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[14] E. Oro´, L. Miro´, M.M. Farid, L.F. Cabeza, Improving thermal performance of freezers using phase change materials, Int. J. Refrigeration 35 (2012) 984-991. [15] Wen-Long Cheng, Bao-Jun Mei, Yi-Ning Liu, Yong-Hua Huang, Xu-Dong Yuan, A novel household refrigerator with shape-stabilized PCM (Phase Change Material) heat storage condensers: An experimental investigation, Energy 36 (2011) 5797-5804. [16] MD. Imran Hossen Khan Hasan M. M. Afroz, Effect of phase change material on compressor on-off cycling of a household refrigerator, Sc. Tech. Built Env. 21 (2015) 462-468. [17] G. Sonnenrein, A. Elsner, E. Baumhogger, A. Morbach, K. Fieback, J. Vrabec, Reducing the power consumption of household refrigerators through the integration of latent heat storage elements in wire-and-tube condensers, Int. J. of refrigeration 51 (2015) 154-160. [18] O. Laguerre, D. Flick, Heat transfer by natural convection in household refrigerators, J Food Eng. 62 (2004) 79-88. [19] V.T. Lacerda, C. Melo, J.R. Barbosa, P.O.O. Duarte, Measurements of the air flow field in the freezer compartment of a top-mount no-frost refrigerator: the effect of temperature, Int. J. of Refrigeration 28 (2005) 774-783. [20] O. Laguerre, S. Ben Amara, J. Moureh, D. Flick, Numerical simulation of air flow and heat transfer in domestic refrigerators, J. of Food Eng. 81 (2007)144-156. [21] I.S. Lee, S.J. Baek, M.K. Chung, D. Rhee, A study of air flow characteristics in the refrigerator using PIV and computational simulation, J. Flow Visual. Im. Pro. 6 (1999) 333-342.
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[22] M. Cheralathan, R. Velraj, S. Renganarayanan, Performance analysis on industrial refrigeration system integrated with encapsulated PCM-base cool thermal energy storage system, Int. J. Energy Res 31 (2007) 1398-1413.
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[25] A.C. Marques, G.F. Davies, J.A. Evans, G.G. Maidment, I.D. Wood, Theoretical modelling and experimental investigation of a thermal energy storage refrigerator, Energy 55 (2013) 457-465.
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[27] M.I.H. Khan, H.M. Afroz, Diminution of temperature fluctuation inside the cabin of a household refrigerator using phase change material, Recent Adv. Mech. Eng. (IJMECH) 3 (2014) 43-52. [28] M. Berdja, B. Abbad, M. Laidi, F. Yahi, M. Ouali, Numerical simulation of a phase change material (PCM) in a domestic refrigerator powered by photovoltaic energy, ICHMT Digital Library Online (2012). [29] W. Cheng, M. Ding, X. Yuan, B. Han, Analysis of energy saving performance for household refrigerator with thermal storage of condenser and evaporator, J. Energy Conv. Manag. 132 (2017) 180-188.
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Figure 1
25
Figure 2
26
Figure 3
27
Figure 4
28
Figure 5
29
Figure 6
30
Figure 7
31
Figure 8
32
Figure 9
33
Figure 10
34
Figure 11
35
Figure 12
36
Figure 13
37
Figure 14
38
Figure 15
List of Figures Fig. 1: (a) Refrigerator model and (b) the design of the PCM heat exchanger Fig. 2: Photograph of the experimental setup ( the household refrigerator with the PCM heat exchanger) Fig. 3: Various cases of the refrigerator compartment for simulation study Fig. 4: (a) The predicted temperature contours of the refrigerator compartment, (b) the infrared images of the temperature distribution and (c) the rear view of the refrigerator Fig. 5: (a) Evaporator temperature, (b) condenser temperature and (c) compressor temperature Fig. 6: The effect of PCM on the power consumption of refrigerator Fig. 7: The refrigeration cycle of the household refrigerator Fig. 8: The effect of PCM on compressor running time at different loads Fig. 9: (a) The temperature contours and (b) velocity vectors for the Case -1 Fig. 10: (a) The temperature contours and (b) velocity vectors for the Case -2 Fig.11: (a) The temperature contours and (b) velocity vectors for the Case -3 Fig. 12: (a) The temperature contours and (b) velocity vectors for the Case -4 Fig. 13: (a) The average temperature and (b) the average air velocity for the four cases Fig. 14: The temperature contours and the velocity vectors for different PCM coverage percentage in racks Fig. 15: The variation of (a) the average temperature and (b) the air velocity within the compartment refrigerator 39
Dimensions (mm)
H1060 x W550 x D540
Evaporator: Mode of heat transfer
Natural convection
Condenser: Mode of heat transfer
Natural convection
Compressor
Hermetic 220–230 V, 60 Hz
Expansion device
Capillary tube
Refrigerant
R-134a
Table 1: The technical specifications of the conventional refrigerator
40
PCM type
A4 PluseICE (Organic)
Melting temperature °C
Latent heat of fusion kJ/kg
density ρ kg/m3
solid thermal conductivity W/mK
Specific heat Cp kJ/kgK
4
200
766
0.21
2.18
Table 2: Thermo-physical proprieties of the PCM
41
HIGHLIGHTS
- An experimental study was carried out on a refrigerator with PCM heat exchanger. - The power consumption of the novel refrigerator with PCM is found reduced by 12%. - Modeling (2D) and simulation was carried out for various cases of PCM emplacement. - The effect of the PCM emplacement on the temperature and air velocity is studied. - An optimal design is proposed by evaluating stabilization temperature.
42
S1359-4311(17)30789-5 http://dx.doi.org/10.1016/j.applthermaleng.2017.07.113 ATE 10781
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
5 February 2017 13 July 2017 16 July 2017
Please cite this article as: R. Elarem, S. Mellouli, E. Abhilash, A. Jemni, Performance analysis of a household refrigerator integrating a PCM heat exchanger, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/ j.applthermaleng.2017.07.113
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Performance analysis of a household refrigerator integrating a PCM heat exchanger R. Elarem1,2, S. Mellouli1,2,3,*, E. Abhilash3, A. Jemni1 1
Laboratory of Thermal and Energetic Systems Studies (LESTE) at the National School of Engineering of Monastir, University of Monastir, Tunisia.
2
Higher School of Science and Technology of Hammam Sousse, University of Sousse, Tunisia. 3
Mechanical Engineering Department, College of Engineering, University of King Khalid, Abha, Kingdom Saudi Arabia.
*Corresponding author: [email protected] Abstract An experimental investigation was carried out to improve the energy efficiency of a household refrigerator by integrating a Phase Change Material (PCM) to accumulate thermal energy and stabilize the temperature in the refrigerator compartment. A novel design of PCM heat exchanger is proposed in this investigation. The experimental results indicate that by integrating this novel PCM heat exchanger, power consumption is reduced by 12% and the COP is increased by 8% compared to the refrigerator without PCM. In order to identify the best performing designs among various cases of refrigerator compartments placed with PCM, 2D unsteady state CFD simulations were made. For all the cases, the household refrigerator was simulated to study the influence of the PCM emplacement on the temperature and velocity fields. The computational results indicate that using PCM emplacements on the evaporator, walls, and in the racks of the refrigerator compartments has a significant influence to stabilize and homogenize rapidly the temperature (86.66% improvement over the basic configuration ). However, there is a limit beyond which increasing PCM coverage in the racks more than 75% does not lead to any significant improvements.
1
Keywords: Household refrigerator, Phase change material, Thermal storage, Heat exchanger, power consumption.
Nomenclature g qR
acceleration due to gravity (9.81 m s-2) net radiative flux (W m-2)
Ra
Rayleigh number
t
time (s)
Tamb
ambient temperature (K)
Tw
wall temperature.
u
velocity in
(m s-1)
v
velocity
(m s-1)
I
radiative intensity in the s direction at the r position. (W m-2 per unit solid angle)
Iin
intensity of incident radiation in s direction (at r position)
n
normal vector
Greek symbol
air thermal conductivity of air (Wm-1 K-1)
thermal expansion coefficient (K-1)
emissivity of the refrigerator wall
air dynamic viscosity (Pa s)
solid angle (sr) 2
1. Introduction Domestic refrigerators and freezers are among the most energy demanding appliances in a household due to their continuous operation [1]. Worldwide, it has been estimated that there are approximately one billion domestic refrigerators in use [2]. Although, their direct greenhouse gas emissions have been greatly reduced by the introduction of hydrocarbon refrigerants, their indirect emissions remain very high due to the ever increasing power consumption of these appliances. A growing global environmental awareness and rapidly increasing cost of electric energy are driving the demand for finding a frugal and viable solution for energy saving. So far, the most common approaches for reducing the electric energy consumption of household refrigerators are as follows: (i) improvement of the thermal insulation of refrigerator compartments, (ii) usage of high efficiency compressor [3], and (iii) the enhancement of the heat transfer in the evaporator and the condenser by extending the area of heat transfer or by adding ventilators [4]. Another novel option is to use thermal latent heat storage by integrating Phase Change Material (PCM) heat exchangers in refrigerator compartments.The integration of PCMs is considered as a potential way to improve the efficiency of the household refrigerators by reducing the number of working cycle of the compressor. This technical approach can be further explained as follows: (i) when a PCM is used inside the cabinet, it absorbs the heat by changing its phase from solid to liquid, (ii) the temperature within the cabinet remains constant until the melting process is finished, which in turn prolongs the off cycle of the compressor. Hence, it will decrease the electrical energy consumption of the refrigerator. Many research works [5-17] have been devoted to the numerical and experimental studies on the household refrigerators which integrates a latent heat storage system. Onyejekwe et al [5] have studied a prototype of a simple latent heat
3
container filled with eutectic NaCl/H2O mixture and integrating it in a photovoltaic refrigerator. Wang et al [6-8] have studied experimentally and numerically the influence of adding a PCM between compressor and condenser in a cooling system. They found that a 6-8% improvement of efficiency can be achieved by this modification. Further, Azzouz et al [9,10] have demonstrated experimentally that by integrating a latent heat storage system in household refrigerators, the coefficient of performance (COP) can be increased by 5-15%. Recently, Lu et al [11] have proposed a new combined shelf using heat pipes and PCMs. Their experimental results indicated the improvements in temperature distribution within the cabinets without much reduction in energy consumption. However, the study of Gin et al [12] proves that the presence of PCM during defrosting and door openings can result in lower power consumption. Similar studies to improve the efficiency household refrigerators were carried out by Y.T. Ge et al [13], E. Oro [14],Wen-Long et al [15] and Imran Hossen Khan et al [16] by integrating PCM in evaporator or in condenser. Recently, Sonnenrein et al [17] have evaluated the influence of latent heat storage system on the temperature of a standard wire-and-tube condenser equipped with different PCMs (water, paraffin or copolymer compound). These results also indicated that the application of PCM can lowers the condenser temperature and reduces the power consumption. In recent years, extensive efforts [5-29] have been paid to reduce the energy consumption of household refrigerator by integrating PCMs. Most of these studies on the performance improvement of refrigerators focused mainly on simple configurations of PCM heat exchangers (plate exchanger or box). To the best of the authors knowledge, there are no major studies in the open literature that investigate the effects of PCM and its placements inside the refrigerator to improve the thermal stability and the efficiency. This study presents a household refrigerator equipped with a novel PCM heat
4
exchanger. The effects of this novel heat exchanger on power consumption of the household refrigerator has been investigated experimentally. In addition to this, various cases of refrigerator compartments placed with PCM were simulated to identify the best performing design which rapidly stabilizes the temperature in the cabinet while increasing its compressor cutoff time and thereby minimizing the energy consumption of the refrigerator. 2. Experimental study 2.1 Experimental test rig A ready-made vapor compression household refrigerator of 136 liter capacity was selected as a prototype for the experiment. The modified PCM-based refrigerator has a single evaporator cabinet with a single door (Fig. 1a). The major technical specifications of this conventional refrigerator are presented in Table 1. A detailed design of the PCM heat exchanger is shown in Fig. 1b. This novel heat exchanger is an array of twelve U-type tubes occupying the whole evaporator covered with A Plus-ICE PCM of phase change temperature 4 0C. Thermo-physical proprieties of the PCM are presented in Table 2. Linear length of each tube, internal and external diameter of each tube are 0.66 m, 0.007 m and 0.008 m, respectively. The material of the heat exchanger tubes is commercial copper and are in contact with the air in the refrigerator cabinet. The placement of the PCM heat exchanger can be seen in Fig.1a . The experimental set up include a conventional refrigerator, the novel PCM heat exchanger, two pressure gauges, five thermocouples, a data acquisition system, an energy logger and a seek thermal camera. The refrigerator circuit was modified to integrate two pressure gauges. Fig. 2 shows the detailed circuit of the setup and the location of the pressure gauges which are used to measure the evaporation and condensation pressure at the inlet and outlet of the
5
compressor. Temperature at various locations (evaporator, condenser, compressor and cabinet) as indicated in Fig.2 are measured using K-type thermocouples of 0.5 mm diameter. The history of energy consumption is recorded using a computer interfaced Energy Logger 4000. This energy meter is of type Voltcraft Energy-Logger 4000 with a power range of 0.1 to 3500 W. The relative measurement error of the considered Energy Logger is approximately 1%. For the visualization of the temperature distribution in the compartments of the refrigerator, a thermal imaging camera was used. This thermal camera has a resolution of 32,136 Pixels to detect temperature ranging from -40 0C to 330 0C.
2.2 Experimental procedure Both the conventional and the refrigerator with PCM was tested under the same operating conditions. The experiments was carried out in a room where the temperature and humidity were maintained constant. An air-conditioning system was used to maintain the ambient air temperature 22 0C and a relative humidity of 50% inside the test room. The power supply, the energy consumption, the temperatures of the refrigerator were measured using data acquisition systems. To further investigate the operating characteristics, the evaporator midpoint temperature, the condenser outlet temperature, and the compressor outlet temperature were also measured. All these data acquisition assumes a steady state condition of the refrigerator.
3. Numerical study 3.1. Various arrangement of PCM heat exchanger In the present study, the effect of the PCM heat exchanger was investigated by
6
arranging it in different ways within the refrigerator. Four such arrangements were identified to evaluate the change in efficiency of the refrigerator through simulation studies. These various cases are as follows. Case 1: The heat exchanger occupying the whole evaporator covered with PCM (similar to the PCM heat exchanger as described in Section 2.1) as shown in Fig. 3a. In this case, a direct-contact between the air inside compartment and the PCM is ensured so that the heat in the compartment can be stored in the PCM. Case 2: This configuration of the PCM heat exchanger is similar to that in Case 1, but a rectangular parallelepiped container filled with PCM is added and mounted vertically adjacent to the lateral wall of the refrigerator compartment. The schematic of this configuration is shown in Fig. 3b. Case 3: This configuration of the PCM heat exchanger is similar to that in Case-1, but in addition, three racks are made horizontally within the compartment and are covered with PCM. The PCM heat exchanger is covered only 90% of the rack surfaces in order to permit the circulation of air near the lateral wall (Fig. 3c). Case 4: This configuration of the PCM heat exchanger is a combination of the Case-2 and the Case-3. The schematic of this configuration is shown in Fig. 3d.
3.2.Mathematical formulation To simplify the simulation study, only half the cross-section of the physical domain is simulated (internal dimensions: 0.74×0.21 m) assuming symmetry. The following assumptions are made in the study: i.The physical properties of the medium (air), except the density, are assumed to be constant. ii.
7
0 1 Tamb T
Considering these assumptions, the equations governing the heat transfer and the fluid dynamics within the
are given as follows:
3.2.1
u v 0 x y
3.2.2
0
u u u P 2u 2u 0u 0 v 2 2 t x y x x y
where 0 and are the density (kg m-3) and the dynamic viscosity (Pa s) of the air, respectively.
8
0
v v v P 2v 2v u 0 v 2 2 0 T T0 g t x y y x y
where P, , T, T0 and g are the pressure (Pa), the thermal expansion coefficient (K-1), the temperature (K), the ambient temperature (K) and the acceleration due to gravity (9.81 ms-2), respectively. 3.2.3
2T 2T T T T 2 2 .q R 0 C P u v x y y t x where C P ,T, and qR are the thermal capacity(J kg-1K-1), the temperature (K), the thermal conductivity of the air (Wm-1 K-1) and the radiative heat flux (Wm-2), respectively. 3.2.4 The radiative transfer equation In order to understand the phenomena occurring in the refrigerator compartments, it is necessary to consider the radiation in the simulation because both the radiation and the natural convection heat transfer coefficients have a significant effect and are of same order of magnitude in refrigerator compartments [18]. The discrete ordinates radiation model was employed to model the radiation between surfaces within the refrigerator compartment. Air within the refrigerator is assumed transparent and absorption and diffusion of radiation by air is neglected. The radiation
heat transfer equation in the s direction is given as follows: .I r , s s 0 where I r , s is the radiative intensity in the s direction at the r position.
The radiative heat flux leaving the refrigerator walls (grey diffuse surfaces) is given as follows:
qout 1 w qin wTw4 9
where, w is the wall emissivity, σ is the Stefane-Boltzmann constant and Tw is the wall temperature. The incident radiative heat flux at each of the refrigerator walls, is calculated using this equation:
qin
I
s .n 0
in
s .nd incident flux
where, I in is the intensity of incident radiation in the s direction (at the r position), n is
the normal vector and is the solid angle.
3.3. Initial and boundary conditions Initially, a uniform air temperature of 25 0C is assumed within the refrigerator compartment. The boundary conditions are defined as follows: (1) symmetry boundary:
T q .n 0, R
uv0
(9)
Where n represents the normal vector to the corresponding wall.
(2)
radiation: opaque walls with a diffuse fraction of 1 and internal emissivity of the walls was assumed to be 0.9. (3) At the interface between air and PCM, conjugate heat transfer is assumed and noslip condition for velocities and coupled temperature are specified. Also, the emissivity is assumed to be 0.15 [13]. 3.4.Computational procedure and model validation The computational domains were created in the commercial software GAMBIT 2.3.16.
10
It was also used for meshing, labeling the boundary conditions and determine the computational domain. The aforementioned equations are solved for unsteady state conditions using Fluent 6.3.26.
The equations for momentum, energy and radiation were solved using second order upwind discretization. In order to validate the study, an initial simulation was carried out for refrigerator design described as Case-1. The numerical model for this configuration was validated against experimental data collected from the test rig described in Section 2.1. Fig.4a shows the predicted temperature contours of the refrigerator compartment and Fig.4b shows infrared images of the refrigerator compartment. These infra-red images taken at an ambient temperature of 20 0C indicate the temperature distribution at different stages of the experiment. Fig.4c shows infrared images of the rear view of the refrigerator, indicating the temperature distribution of the condenser at different stages of the cycle: immediately after the start of the compressor and before the end of the compressor runtime when the maximum temperature is reached. Comparing Fig. 4a & Fig. 4b, a close agreement between predicted results and the measured results can be observed. 4. Experimental results 4.1 Evaporator temperature To understand the effect of using PCM the evaporator temperature of refrigerator with and without PCM were compared. As Fig. 5a indicates, for the refrigerator without PCM, the evaporator temperature varied from -16 0C to -3.5 0C and the average evaporator temperature was -10 0C over the cycle. Further, for the refrigerator with PCM, the evaporator temperature was from -12 0C to -3.5 0C and the average evaporator temperature was -8 ᵒC. Obviously, the evaporator temperature of the
11
refrigerator with PCM varied in a short range. It indicates that the lowest and the average evaporator temperature of the refrigerator with PCM are higher than that of the refrigerator without PCM by about 4 0C and 2 0C, respectively. This can be explained by the fact that when the PCM is added on the evaporator, it absorbs the heat of the compartment and after that the heat is transferred to the evaporator. Therefore, the heat dissipation load of the evaporator during the on-time could be greatly reduced, which resulted in that the refrigerator could work under a higher evaporation temperature and a higher evaporation pressure compared to the case without PCM. Considering the close relationship between the evaporator temperature and the evaporating temperature, the higher evaporator temperature may correspond to a higher evaporating temperature. Thus, the coefficient of performance of the cooling system of the refrigerator is improved by increasing the evaporating temperature. 4.2 Condenser temperature The comparison between the condenser outlet temperatures for different cases is shown in Fig. 5b. For the refrigerator without PCM, the condenser outlet temperature increased gradually while the compressor was in operation to a highest outlet temperature (about 32.2 0C) until the compressor was stopped. For the refrigerator with PCM, the condenser outlet temperature was higher than the ambient temperature by 1.8 0C; which indicated that maximum of condensation heat was released into the environment during the idle cycle. Therefore, the heat dissipation load of the condensers during the operating cycle could be greatly reduced, indicating that the refrigerator could work under a lower condensation temperature and pressure.
4.3 Compressor temperature The comparison between the compressor outlet temperatures for different cases is shown in Fig. 5c. For the refrigerator without PCM, while the compressor was in idle 12
cycle, the compressor outlet temperature dropped to 45 0C due to the pressure drop inside the condensers, and during the operating cycle the temperature increased gradually to 57 0C. This temperature was much higher than the ambient temperature due to the influences of the environment and the cabinet heat capacity. For the refrigerator with PCM, while the compressor was in idle cycle, the refrigerant within the condensers remained at a higher temperature than the ambient temperature due to the effect of the PCM exchanger heat capacity. When the compressor was in operation, this part of refrigerant was pushed out of the condensers, resulted in a rapid increase of the outlet temperature to a peak temperature of 53 0C. This peak outlet temperature gradually decreased to a value about 43 0C during the idle cycle. Also, from this comparison it was noted that, the period of the cycle was reduced for the refrigerator with PCM; which indicated an impact of energy consumption.
4.4 The effect of PCM on power consumption of refrigerator The comparison of the power consumption between the different cases is shown in Fig. 6. It can be seen that, the maximum power consumption of the refrigerator with PCM was lesser than that of the refrigerator without PCM. Also, from Fig. 6 it can be seen that the total cycle time is 0.5 h and the ratio of operating cycle time to the total cycle time for the refrigerator without PCM is 0.4, however, the corresponding values for refrigerator with PCM is 0.4 h and 0.37 respectively. Further, it can be noticed that the total cycle time and the ratio of operating cycle time to the total cycle time for the refrigerator with PCM is less. This fact can be explained as follows: (i) The refrigerator with PCM was working under a lower condensation temperature, a higher evaporation temperature and a higher subcooling degree contributing to a less energy consumption, (ii) The difference between
13
the internal and the external compartment temperatures of the refrigerator with PCM was lower than that of the refrigerator without PCM due to the heat storage effect of the PCM heat exchanger. Therefore, the heat lost through the refrigerator compartment is reduced, resulting in a shorter operating cycle time. Consequently, it can be inferred that the refrigerator with PCM had a greater COP. The COP calculated using the cycle plotted in the P-h diagram of refrigerant R134a (Fig. 7) is presented below.
COPWithoutPCM COPWith PCM
h1 h4 3.48 h2 h1
h1 h4 3.75 h2 h1
(10)
(11)
Hence, the percentage of COP improved by using the PCM heat exchanger is 8% under the operating conditions presented in this study. A higher COP may be achieved by increasing the amount of PCM within the refrigerator compartments or by appropriate emplacement of PCM.
4.5 The effect of PCM on compressor running time at different loads. The thermal load inside the refrigerator compartments was varied by changing the quantity of hot water in bottle placed inside the refrigerator compartments. Fig. 8 shows the percentage of average running time per cycle of the compressor at different thermal loads. From the Fig. 8 it can be found that: (i) the system with PCM significantly reduced the average compressor running time per cycle, which in turn reduced the energy consumption (ii) average compressor running time per cycle is reduced from 2.5 to 5% as compared to the case without PCM, which depends on different thermal loads. (iii) the percentage of average running time per cycle of the compressor is proportional to thermal loads for both the cases, however the slope of the curve is found lower for the refrigerator with PCM heat exchanger. 14
5. Numerical results 5.1. The effect of PCM heat exchangers emplacement The effectiveness of incorporating thermal heat storage system in a household refrigerator was evaluated by comparing the four emplacement cases of the PCM heat exchanger in the refrigerator compartment (Fig. 3). In order to find the best case which rapidly stabilizes the temperature in the refrigerator compartment, modeling and simulation experiments are done. The predicted temperature distributions and the contours of velocity vectors at different instants (60s, 240s and 600s) in the refrigerator compartments are analyzed for each case.
5.1.1 Study on Case 1 The temperature distributions and the velocity vectors obtained from simulation for the Case 1 are shown in Fig. 9. It can be noted that the heat exchanger PCM fitted on the evaporator indicated a stratified temperature profile with a cold zone at the top and a warm zone at the bottom of the refrigerator compartment. After 600 s the maximum temperature of 12 0C is predicted for the bottom right corner and the average temperature in the compartment refrigerator is 9 0C. Initially, the velocity vectors indicated a circular airflow pattern in the top of the compartment and airflow along the walls. At the end, a region of air recirculation in the middle of compartment is observed and the air in the top of the compartment is mostly stagnant. The distribution of the velocity vectors shows a negligible air velocity at the wall of PCM heat exchanger and a maximum air velocity at the center of the refrigerator compartment. Also, an unsteady recirculation has been observed at the middle of the compartment.
15
5.1.2 Study on Case 2 The contours of predicted temperatures and the velocity vectors at different instants (60 s, 240 s and 600 s) within the refrigerator compartments for Case-2 is shown in Fig. 10. The temperature field is influenced by the presence of PCM vertically fitted inside the compartment. A lower temperature is observed at the bottom and at the middle of compartment refrigerator compared to that of Case-1.This is due to the fact that the vertically fitted PCM heat exchanger accelerates the air circulation in the central zone of the refrigerator compartment. Also, it can be noted that the presence of the PCM influences the airflow and thus a circular airflow is observed in the refrigerator compartment. Cold air flows downward with an increase in magnitude along the vertical wall of PCM and it reaches a maximum value while approaching the bottom of the compartment. Further, the air flows with a decreasing magnitude upward along the plane of symmetry to the surface of PCM fitted on the evaporator and then flows horizontally towards the vertical PCM wall . The air is nearly stagnant at the top of the refrigerator cabinet.
5.1.3 Study on Case 3 The contours of predicted temperatures and the velocity vectors at different instants (60s, 240s and 600s) in the refrigerator compartments for Case-3 is shown in Fig. 11. Compared to Case-2, a nearly uniform temperature distribution can be observed for this case. Also, the average temperature in the compartment is found to be 4 0C lower than that of Case-2. The velocity vectors for the Case 3, shows small air recirculation loops between the racks covered with PCM. Through the gap of each racks, the air flows downwards along the vertical wall with increasing velocity and attain a maximum value at the bottom cavity. At the same time, within the bottom
16
cavity two vortices that circulates in opposite directions are observed due to the temperature difference at the top and bottom. This observation is in agreement with the temperature contours of the air in the compartment, which is shown in Fig. 11.
5.1.4 Study on Case 4 The temperature distribution for the Case -4 (cabinet fitted with PCM racks and a vertical PCM wall ) appeared to be very uniform compared to other cases, which is shown in Fig. 12. In addition to the overall thermal stratification in the compartment, stratification is also observed between the racks due to negligible air flow. The velocity contours indicate that the airflow is a combination of the contours of Case-2 and Case-3. Initially, the airflow between the PCM racks of the compartment had a similar behavior to that observed in Case-3. After some instants, small air loops are found circulating between the PCM racks . The airflow velocity decreases gradually and becomes stagnant in the cavities that are in the middle of the compartment. These cavities have shown better air mixing than that in Case-3 due to the presence of cold vertical PCM wall of the refrigerator. However, air circulation is observed in the bottom cavity due to the warm air located near the non-adiabatic bottom wall. To summarize, compared to other case studies, the vertical PCM wall predominantly affects the airflow by increasing the velocity and the PCM racks significantly enhances the air mixing in the cavities of the compartment. 5.1.5 Comparison between different Cases The performance of various PCM emplacements inside the refrigerator compartment is compared to identify the best performing design which rapidly stabilizes the temperature within the compartment.
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The evolution of the average temperature and the average velocity of air for all the four cases are presented in Fig. 13a and 13b, respectively. Since, the stabilization of the air temperature is controlled by the heat transfer rate from the air to the PCM, time intervals for the reduction of the average air temperature from 25 0C to 10 0C for each case is obtained and compared. From the Fig.13a it can be seen that, to reach the reduction of the temperature, Case-1 took much time (600 s) compared to Case-2, Case- 3 and Case-4 which took 195 s, 90 s and 80 s, respectively. A comparison of the three novel cases (Case-2, Case-3 and Case4) with the basic configuration (Case-1) indicated 67.5%, 85% and 86.66% reduction in duration, respectively. Therefore, Case-4 offers better heat transfer performance to reach the stabilization temperature inside the compartment which in turn reduces the energy consumption.
5.2. Influence of the PCM coverage in the racks In the numerical simulation, the Case-4 is taken to evaluate the influence of the percentage of PCM in the racks on the uniformity of the air temperature within the refrigerator compartment, the simulations are made for various PCM coverage (10%, 50%, 75% and 90%) by assuming same initial and the boundaries conditions. Fig. 14 shows the contours of temperature and velocity of the air within the compartment after 240 s for different PCM coverage. For all the cases, thermal stratification is observed with a cold zone at the middle of the refrigerator compartment and a warm zone at the top and the bottom. In addition, a cold zone is also observed along the vertical PCM wall. When the PCM coverage is increased, the temperature of the air between two racks is lower than that in the bottom cavity. In the top of the compartment, the temperature is relatively uniform for all the
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cases. The temperature field is considerably influenced by the presence racks and the PCM coverage in it. For the racks with 10% PCM coverage, a slightly lower temperature is observed at the bottom and a slightly higher temperature at the middle compared with all other cases. From Fig. 15, it is observed that the presence of racks also influences the air circulation. However, this influence is strong for 10% and 50% PCM coverage because of increased gap between the racks and the vertical PCM wall which facilitates the air flow. Therefore, the PCM racks should be placed far from the PCM vertical wall, in order to enhance the air circulation inside the compartment. The variation of the average temperature and the air velocity within the compartment refrigerator is shown in Fig. 15. Examining this figure, it can be noted that increasing the PCM coverage from 10% to 50% shorten the time interval (for the reduction of the average air temperature from 25 0C to 5 0C) by 42%; whereas, when the PCM coverage is increased from 75% to 90%, there was no significant change in the time interval. Also, it is observed that the airflow velocity can be decreased by increasing PCM coverage to a particular value (75%). Thus, if the compartment with more efficient heat transfer is desirable, increasing PCM coverage more than this value have no significant effect. This can be explained by the fact that the Raleigh number decreases with the increase of PCM coverage and then the convective heat exchange between the air and the PCM decreases. On the other hand, when the PCM coverage increases, the air exchanges heat with a large surface maintained at the phase change temperature. These two opposite phenomena explain the negligible effect of the PCM coverage when it is increased more than 75 %.
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6. Conclusions In this work, a novel design of a PCM heat exchanger was made. Based on this design, an experimental test rig was developed to conduct a series of experimental studies in order to minimize the energy consumption of the household refrigerator. The experimental results show that under the standard test conditions, the power consumption of the household refrigerator with PCM heat exchanger is reduced by 12% and the COP is increased by 8% compared to the refrigerator without PCM. Also, in order to study the influence of the PCM emplacement on the temperature and velocity fields, four configurations were simulated. Using the numerical simulation of air flow and heat transfer within the refrigerator compartments, the following conclusions can be made: 1. To effectively integrate a PCM heat exchanger in a refrigerator compartment, it is necessary to employ PCM emplacements in evaporator, walls and racks of the compartment which helps to rapidly stabilize and homogenize the temperature. This increases the cutoff time of compressor and thereby minimizes the energy consumption of the refrigerator. 2. A comparison of the three novel cases (Case-2, Case-3 and Case-4) with the basic configuration (Case-1) indicated 67.5%, 85% and 86.66% reduction in duration, respectively. Therefore, Case-4 offers better heat transfer performance to reach the stabilization temperature inside the refrigerator compartment. 3. It is found that for the Case-4, a genuine optimization of the PCM coverage in the racks is necessary to ensure the thermal storage. The racks with higher PCM coverage has a favorable performance to achieve the stabilization temperature inside the compartment in shortest time. However, there is a limit beyond which increasing PCM coverage in the racks more that 75% does not lead to any significant reduction
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in time to achieve the uniform temperature. The CFD simulation developed in study can be further extended to understand the influence of operating conditions on the temperature and velocity fields. It is expected that the use of variables such as evaporator temperature (parameter related to the thermostat setting by the consumer), the dimensions of the evaporator (parameter related to design), the emplacement of the thermal load inside the refrigerator compartment etc., in simulation studies will contribute much to understand the use of PCM in household refrigerators. References [1] A.P. Simard, M. Lacroix, Study of the thermal behavior of a latent heat cold storage unit operating under frosting conditions, Energy Convers. Manag. 44 (2003) 1605-1624. [2] S. Ben Amara, O. Laguerre, M.-C. Charrier-Mojtabi, B. Lartigue, D. Flick, PIV measurement of the flow field in a domestic refrigerator model: Comparison with 3D simulations, Int. J. of Refrigeration 31 (2008) 1328-1340. [3] P. Binneberg, E. Kraus, H. Quack, Reduction in power consumption of household refrigerators by using variable speed compressors, Int. Refrigeration and Air Conditioning Conference (2002), Paper 615. Available from: http://docs.lib.purdue. edu/iracc. [4] P. Roth, Energy Saving on the High Pressure Side of a Refrigerating Plant. KI 03/2008, pp. 30-35. [5] D. Onyejekwe , Cold storage using eutectic mixture of NaCl/H2O: an application to photovoltaic compressor vapours freezers, Sol. Wind Tech., 6 (1989) 11-18.
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[6] F. Wang, G. Maidment, J. Missenden, R. Tozer, The novel use of phase change materials in refrigeration plant. Part 1: experimental investigation, Appl. Therm. Eng. 27 (2007) 2893-2901. [7] F. Wang, G. Maidment, J. Missenden, R. Tozer, The novel use of phase change materials in refrigeration plant. Part 2: dynamic simulation model for the combined system, Appl. Therm. Eng. 27 (2005) 2902-2910. [8] F. Wang, G. Maidment, J. Missenden, R. Tozer, The novel use of phase change materials in refrigeration plant. Part 3: PCM for control and energy savings, Appl. Therm. Eng. 27 (2007) 2911-2918. [9] K. Azzouz, D. Leducq, D. Gobin, Performance enhancement of a household refrigerator by addition of latent heat storage, Int. J. Refrigeration 31 (2008) 892-901. [10] K. Azzouz, D. Leducq, D. Gobin, Enhancing the performance of household refrigerators with latent heat storage: an experimental investigation, Int. J. Refrigeration 32 (2009) 1634-1644. [11] Y.L. Lu, W.H. Zhang, P. Yuan, M.D. Xue, Z.G. Qu, W.Q. Tao, Experimental study of heat transfer intensification by using a novel combined shelf in food refrigerated display cabinets (Experimental study of a novel cabinets), Appl. Therm. Eng. 30 (2010) 85-91. [12] B. Gin, M.M. Farid, P.K. Bansal, Effect of door opening and defrost cycle on a freezer with phase change panels, Energy Convers. and Manag. 51 (2010) 2698-2706. [13] Y.T. Ge, S.A. Tassou, A. Hadawey, Simulation of multi-deck medium temperature display cabinets with the integration of CFD and cooling coil models, App. En. 87 (2010) 3178-3188. 22
[14] E. Oro´, L. Miro´, M.M. Farid, L.F. Cabeza, Improving thermal performance of freezers using phase change materials, Int. J. Refrigeration 35 (2012) 984-991. [15] Wen-Long Cheng, Bao-Jun Mei, Yi-Ning Liu, Yong-Hua Huang, Xu-Dong Yuan, A novel household refrigerator with shape-stabilized PCM (Phase Change Material) heat storage condensers: An experimental investigation, Energy 36 (2011) 5797-5804. [16] MD. Imran Hossen Khan Hasan M. M. Afroz, Effect of phase change material on compressor on-off cycling of a household refrigerator, Sc. Tech. Built Env. 21 (2015) 462-468. [17] G. Sonnenrein, A. Elsner, E. Baumhogger, A. Morbach, K. Fieback, J. Vrabec, Reducing the power consumption of household refrigerators through the integration of latent heat storage elements in wire-and-tube condensers, Int. J. of refrigeration 51 (2015) 154-160. [18] O. Laguerre, D. Flick, Heat transfer by natural convection in household refrigerators, J Food Eng. 62 (2004) 79-88. [19] V.T. Lacerda, C. Melo, J.R. Barbosa, P.O.O. Duarte, Measurements of the air flow field in the freezer compartment of a top-mount no-frost refrigerator: the effect of temperature, Int. J. of Refrigeration 28 (2005) 774-783. [20] O. Laguerre, S. Ben Amara, J. Moureh, D. Flick, Numerical simulation of air flow and heat transfer in domestic refrigerators, J. of Food Eng. 81 (2007)144-156. [21] I.S. Lee, S.J. Baek, M.K. Chung, D. Rhee, A study of air flow characteristics in the refrigerator using PIV and computational simulation, J. Flow Visual. Im. Pro. 6 (1999) 333-342.
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[22] M. Cheralathan, R. Velraj, S. Renganarayanan, Performance analysis on industrial refrigeration system integrated with encapsulated PCM-base cool thermal energy storage system, Int. J. Energy Res 31 (2007) 1398-1413.
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Figure 1
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Figure 2
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Figure 3
27
Figure 4
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Figure 5
29
Figure 6
30
Figure 7
31
Figure 8
32
Figure 9
33
Figure 10
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Figure 11
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Figure 12
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Figure 13
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Figure 14
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Figure 15
List of Figures Fig. 1: (a) Refrigerator model and (b) the design of the PCM heat exchanger Fig. 2: Photograph of the experimental setup ( the household refrigerator with the PCM heat exchanger) Fig. 3: Various cases of the refrigerator compartment for simulation study Fig. 4: (a) The predicted temperature contours of the refrigerator compartment, (b) the infrared images of the temperature distribution and (c) the rear view of the refrigerator Fig. 5: (a) Evaporator temperature, (b) condenser temperature and (c) compressor temperature Fig. 6: The effect of PCM on the power consumption of refrigerator Fig. 7: The refrigeration cycle of the household refrigerator Fig. 8: The effect of PCM on compressor running time at different loads Fig. 9: (a) The temperature contours and (b) velocity vectors for the Case -1 Fig. 10: (a) The temperature contours and (b) velocity vectors for the Case -2 Fig.11: (a) The temperature contours and (b) velocity vectors for the Case -3 Fig. 12: (a) The temperature contours and (b) velocity vectors for the Case -4 Fig. 13: (a) The average temperature and (b) the average air velocity for the four cases Fig. 14: The temperature contours and the velocity vectors for different PCM coverage percentage in racks Fig. 15: The variation of (a) the average temperature and (b) the air velocity within the compartment refrigerator 39
Dimensions (mm)
H1060 x W550 x D540
Evaporator: Mode of heat transfer
Natural convection
Condenser: Mode of heat transfer
Natural convection
Compressor
Hermetic 220–230 V, 60 Hz
Expansion device
Capillary tube
Refrigerant
R-134a
Table 1: The technical specifications of the conventional refrigerator
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PCM type
A4 PluseICE (Organic)
Melting temperature °C
Latent heat of fusion kJ/kg
density ρ kg/m3
solid thermal conductivity W/mK
Specific heat Cp kJ/kgK
4
200
766
0.21
2.18
Table 2: Thermo-physical proprieties of the PCM
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HIGHLIGHTS
- An experimental study was carried out on a refrigerator with PCM heat exchanger. - The power consumption of the novel refrigerator with PCM is found reduced by 12%. - Modeling (2D) and simulation was carried out for various cases of PCM emplacement. - The effect of the PCM emplacement on the temperature and air velocity is studied. - An optimal design is proposed by evaluating stabilization temperature.
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