A numerical study on the usage of phase change material (PCM) to prolong compressor off period in a beverage cooler
Introduction
Energy consumption due to refrigeration is nearly 30% of the global energy consumed [1]. Although commercial air conditioning systems and cold storage applications for food preservation are responsible for the vast portion of the energy demand of refrigeration, the residential air conditioning systems, household refrigerators and freezers also require great electrical power [2]. Joybari et al. [2] and Sonnenrein et al. [3] reported the technical challenges to improve the performance of refrigeration systems, and according to their suggestions, the works can be grouped into three major topics as (i) enhancement of insulations on the door and casing, (ii) increasing the compressor efficiency, (iii) development of heat exchangers, i.e. condenser and evaporator, with improved heat transfer performance. Joybari et al. [2] stated that the modifications on the compressors or improvements of insulation materials are very costly and they may not be readily applicable. Recent works in the literature [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] reveal that, among the other alternatives, phase change materials (PCMs) possess great practical advantages to improve the performance of the refrigerators. PCMs are widely used in latent heat thermal energy storage systems to charge or discharge great amount of thermal energy in various applications to mismatch the gap between the energy resource and the demand [22]. The works that deal with the usage of PCMs in commercial or household refrigeration units can be divided into following two groups: (i) implementation of the PCM outside the cold space and (ii) implementation of the PCM inside the cold space.
In the first group, PCM is incorporated into the insulation material to store the heat gain from the surroundings and reduce the cooling load [3], [4], [5], [6], [7], [8], [9]. Such an application is useful when the heat gain on the outer surfaces of the refrigerated space is intensive. One of the most appropriate forms of PCM embedded insulation is the refrigerated vehicles. Ahmed et al. [4] applied a paraffin-filled PCM slab on the lateral walls of a refrigerated truck to limit the rate of heat transfer from surroundings. It was stated that implementation of the PCM reduces the peak cooling load by 29.1%. Cheng et al. [5] produced a new shape-stable PCM, and the condenser tubes of a household refrigerator were wrapped by the novel PCM to enhance the heat transfer performance. Results revealed that the novel refrigerator reduced the energy consumption by 12%. The on-off duration of PCM integrated refrigerator was shorter than the conventional refrigerator, and the temperature fluctuations inside the freezer compartment were regulated. Cheng et al. [6] proposed a dynamic model to analyze their novel refrigerator with shape stable PCM [5]. Comparative results have been represented at various ambient temperatures, freezer temperature and melting temperature of the PCM. Tinti et al. [7] developed a novel thermal insulation material by incorporating n-tetradecane (Tm = 5 °C) into polyurethane foam to obtain a reduced thermal conductivity and enhanced the storage capability. The proposed composite PCM is a promising alternative to the conventional insulation materials with its lower thermal conductivity and comprehensive strength. Glouannec et al. [8] experimentally and numerically investigated the influence of multilayer insulation walls on the heat gain of a refrigerated vehicle. Multilayer insulation with multi-foil and aerogel limits the heat transfer by 27%. Sonnenrein et al. [3] incorporated PCM on a condenser of a household refrigerator and conducted experiments to discuss the energy savings of the proposed designs. In comparison with the conventional refrigerator up to 10% of energy saving was obtained by incorporating the PCM. Chen et al. [9] developed a form-stable PCM which was composed of dodecane and hydrophobic fumed silica. The proposed PCM is appropriate for cold stores to preserve the heat gain from surrounding.
In the second group of researches, on the other hand, the PCM is placed inside the refrigerated space to regulate the frequency of the compressor on/off cycles and to balance the overheating problems during defrost or the door opening periods [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Azzouz et al. [10] investigated the influence of the usage of the eutectic salt-water mixture in a unventilated household refrigerator. A dynamic lumped mathematical model was developed to compare the performance of the proposed design (with PCM) with the base cooler (no PCM). Implementation of the PCM improved the COP by 72% and reduced the run-time ratio nearly 50%. Azzouz et al. [11], [12] extended their previous work by varying the melting temperature of the PCM as well as the thickness of the slab. Gin et al. [13] applied PCM on the internal walls of the freezer to examine the influences of the PCM on the thermal behavior during the door opening and defrost period. It was found that PCM limited the excess energy consumption during the door opening period and defrosting, by nearly 7%. Oró et al. [14] investigated PCM embedded vertical commercial freezer during door opening and electrical power outage under loaded and unloaded conditions. The package temperatures inside the refrigerator with PCM were remained 4–6 °C lower than the one without PCM. Marques et al. [15] numerically studied temperature distribution and flow-field inside an unventilated household refrigerator. They placed the PCM on the top and rear surfaces of the fridge. Instead of simulating the PCM domain they have simply defined a constant temperature on the PCM embedded surfaces. Yuan and Cheng [16] developed a dynamic multi-objective optimization algorithm for a refrigerator with PCM, and the best configuration of the fridge was introduced regarding the initial cost as well as the energy consumption per 24 h. Yusufoglu et al. [17] investigated the influence of PCM on two different types household refrigerators. The first one, refrigerator #1, has 130-L volume with free convection and the second one, refrigerator #2, has 350-L volume with forced convection. Experimental test results showed that 8.8% and 9.4% of energy savings were obtained for the refrigerator #1 and #2, respectively. Alzuwaid et al. [18] experimentally investigated a chilled food multi-deck display cabinet to observe the influence of PCM on the cabinet temperature and energy consumption. Khan & Afroz [19] introduced a household refrigerator with two PCMs that have different melting temperatures under various heating loads (0, 5, 10, 20 W) and the influence of PCM on the compressor on-off time and fluctuations of the temperature was observed. Alzuwaid et al. [20] developed a comprehensive 2D mathematical model to simulate heat transfer of a display cabinet with PCM. The commercial solver ANSYS-FLUENT is used with user-defined scripts to control the volumetric heat sink inside the evaporator and the thermostatic controller of the cooler.
In this study, PCM incorporated vertical beverage cooler is investigated numerically, and the influence PCM slab thickness on the cooling performance and indoor temperature uniformity are discussed. In the literature, researchers mostly dealt with heat transfer and fluid flow inside unventilated household refrigerators either 1D or 2D space and carried out the numerical analyses for a fixed temperature or constant cooling load boundary conditions. Most importantly, in the previous works, the PCM slab is placed inside the insulation, which means that PCM is not contacted with the airflow and do not cause any change in the airflow characteristics. In the current VBC model, on the other hand, there is a narrow spacing between the evaporator and the rear wall of the cooler (19 mm). The PCM slabs with various thicknesses are placed on the spacing which is directly contacted with the air and blocks the airflow. Consequently, it is the first time that the airflow and thermal characteristics of a PCM embedded refrigerator are investigated in the same work. A 3D and transient numerical model is developed with forced convection, and the analyses are conducted in commercial CFD solver ANSYS-FLUENT. User-Defined scripts are coded to implement the temperature dependent cooling load, and a pressure jump is applied across the fan. The solution method is validated against the experimental measurements, and parametric results are represented.
Section snippets
Definition of the problem
A commercial vertical beverage cooler (VBC) with a storage volume of 360-L is considered. The schematic of the VBC is given in Fig. 1. The height, width, and depth of the VBC are H = 1.65 m, W = 0.65 m, and D = 0.55 m, respectively. The dimensions that are indicated in the Fig. 1(b) are as follows, W1 = 0.325 m, W2 = 0.225 m, H1 = 1.55 m and H2 = 0.1 m. Due to thermal symmetry across the mid-width of the VBC, the computational domain is reduced into one-half of the actual width (W/2). There is a vertical glass door,
Flow characteristics
In the current VBC, regardless of the compressor on/off periods the circulation fan works continuously. Since the fan is continuously operating, forced convection is the dominant heat transfer mechanism, and the effect of natural convection on the heat transfer becomes negligible. Fig. 2 compares the streamlines inside the coolers without and with PCM slab. Across the inlet slot of the fan casing (see Fig. 2(c)) Eq. (10) is defined. It provides a clockwise circulation within the cooler. The fan
Conclusions
In this study, the influence of PCM integration inside a VBC is investigated numerically. A 3D and transient model is developed in ANSYS-FLUENT, and for the conventional VBA, the results are compared with the experimental findings. Five different thicknesses of PCM slabs are considered, and the results are discussed regarding the velocity field, pressure drop, temperature distribution and the run-time ratio. Key findings of the current work are listed below:
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As the thickness of the PCM slab
Acknowledgments
This research is supported by Republic of Turkey Ministry of Science, Industry, and Technology under grant number: 0780.STZ.2014. The authors also wish to express their deepest gratitude to Klimasan Company Manisa – Turkey for financial support.
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