Performance improvement and energy consumption reduction in refrigeration systems using phase change material (PCM)

Applied Thermal Engineering 142 (2018) 723–735

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Performance improvement and energy consumption reduction in refrigeration systems using phase change material (PCM)

T

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Subhanjan Bista, Seyed Ehsan Hosseini , Evan Owens, Garrison Phillips Combustion and Sustainable Energy Laboratory (ComSEL), Department of Mechanical Engineering, Arkansas Tech University, 1811 N Boulder Ave, Russellville, AR 72801, USA

H I GH L IG H T S

review of PCM applications in refrigeration systems is presented. • AEffects of using PCM in the evaporator, condenser, compartment and compressor are evaluated. • Using PCM on refrigerators shows positive effects on reduction of electricity consumption. •

A R T I C LE I N FO

A B S T R A C T

Keywords: Evaporator Condenser Phase change material Refrigerator

This paper presents a review of various research investigations on the application of phase change material (PCM) in refrigeration systems. Application of PCMs mostly in vapor compression refrigeration systems refrigeration systems have illustrated significant effects on the performance of the system, compressor on-off cycle and electricity consumption reduction. Since PCM must be chemically and thermally stable over a large number of freezing/melting cycles to be applicable for thermal energy storage in refrigerators, PCM selection for refrigeration systems is discussed as an important issue. Moreover, influences of some parameters such as PCM thickness and phase change temperature of PCM on the performance of refrigeration systems are reviewed. The advantages and drawbacks of using PCM in the evaporator, condenser, compartment section and compressor are evaluated. Using PCM at the evaporator section minimizes the fluctuation of compartment temperature and provides stable conditions against thermal load variations. Since incorporation of PCM at the evaporator increases the compressor running time initially and raises the condensation temperature, several investigations were performed to incorporate PCM at the condenser section. With an alarming rate of rise in the use of refrigerators, along with their total electrical consumption in today’s world, the application of PCM on refrigerators looks like a viable measure to increase the efficiency of refrigerators and reduce the energy consumption.

1. Introduction Electrical power is the backbone of modernization as nearly all appliances consume electricity to perform certain processes or operations. Due to rapid industrialization and progress in the standard of living, the consumption of electricity, without a doubt, is increasing day by day [1]. Among all the appliances, domestic refrigerators and freezers are the most energy demanding appliances in a household because of their continuous operation [2]. Refrigerator is regarded as one of the most popular household appliances, where the number of domestic refrigerators in the world has been estimated about one billion, which consume a considerable part of supplied electricity [3]. China is

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estimated to have 0.2 billion refrigerators, which consumes around 30–40% of its residential electrical power demand [4]. Enhancement of the household refrigerators’ efficiency plays a crucial role in appeasing the rate of electricity consumption which mostly depends upon the refrigerator’s compressor efficiency, thermal load, ambient temperature, refrigerant used and door openings [5]. The inconsistency between the demands of increasing efficiency and decreasing cost is a dilemma. Performance improvement and enhancement of the energy saving of household refrigerators can be implemented by [6]:

• Employing high efficiency compressors.

Corresponding author. E-mail addresses: [email protected], [email protected] (S.E. Hosseini).

https://doi.org/10.1016/j.applthermaleng.2018.07.068 Received 9 April 2018; Received in revised form 1 June 2018; Accepted 14 July 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.

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Nomenclature

Abbreviation

ρ λ E T Tcold tOFF UA V

COP CFC GHG EP PCM PCT SSPCM

density enthalpy of fusion energy ambient temperature cold compartment temperature compressor OFF time overall heat transfer coefficient volume

• Optimization of the control system by applying advanced circulation. • Upgrading thermal insulation of the system by increasing the •

coefficient of performance chlorofluorocarbon greenhouse gases expanded perlite phase change material phase change temperature shape stabilized PCM

[8]. Because of the extreme necessity to diversify energy sources, the search for energy recycling methods through the utilization of thermal losses from equipment has become fundamental [9,10]. Energy efficiency and standard of eco-friendliness are the two important issues confronting the refrigerator manufacturers. Hence, rigorous evaluation of domestic refrigerators for checking the energy efficiency and its sensitivity to variables such as type of refrigerant, size of compressor, ambient temperatures and type of insulation is necessary [11]. This is where the application of phase change material (PCM) in refrigerators is highlighted to enhance their performance. The heat energy associated with PCM is more like a natural phenomenon and can be called green energy [12]. Because of the high-energy storage density of PCM and the isothermally process of the energy storage, the PCM’s enthalpy of fusion can be employed in different thermal applications. Today, the use of PCM holds the key to one of promising sustainable energy techniques of storing thermal energy. This thermal energy can be used on domestic refrigerators to increase their performance and the

thickness of the insulation or applying advanced thermal insulation materials. Improving the heat-transfer in the condenser and evaporator.

Refrigeration and air-conditioning systems have an important impact on the environment and actively participate to the global warming [7]. Nevertheless, environmental regulations established by the Montreal Protocol has relatively decreased the use of some chlorofluorocarbon refrigerants (CFCs) that strongly attack the atmospheric ozone layer. Additionally, their directly emitted greenhouse gases (GHG) have been mitigated by the hydrocarbon refrigerants. However, their indirect emissions are high due to the ever-increasing energy consumption of these appliances. Special attention to the global environmental issues and rapidly increasing cost of electricity are driving the demand for finding a frugal and viable solution for energy saving

Fig. 1. Schematic diagram of a domestic refrigerator [17]. 724

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quality of food stored inside [13]. Reduction of temperature fluctuation and improvement of system performance is the main goals of using PCMs in refrigeration systems [14]. Experimental results show that temperature fluctuations in the refrigerator's fresh-food compartment during the cooling cycle can be decreases from 4 °C to 0.5 °C [15].

temperature [26]. While studying the recrystallization of ice during storage of ice cream, Donhowe and Hartel [27] controlled the freezer temperature with ± 1.0 °C fluctuation and ± 0.01 °C without fluctuation, within a temperature range of −15 to −5 °C. The authors concluded that the rate of recrystallization increases with both extent of temperature fluctuations and storage temperature. Phimolsiripol et al. [28] investigated the temperature fluctuations influences on bread quality and frozen dough during frozen storage. Through their observations, it was concluded that the loss of weight and quality of stored food was due to temperature fluctuation occurring during freezing. In another experiment conducted by Gormley et al. [29], some quality parameters of selected food products during freezing with and without fluctuating the temperature inside the freezer were studied. It was shown that the food is damaged by thermal stress and effects properties such as texture, color, and causes fat oxidation. Some researchers anticipated that PCM could be the new option of reduction of compressor on–off cycling [7,30–32]. Hence, the mitigation of compressor on–off cycling ultimately decreases temperature fluctuation inside the storage cabinet and maintains an almost stable temperature resulting in better food quality [33,34].

2. Domestic refrigerator Calculations for the net energy consumption of an ordinary 165 L household refrigerator have shown that the specific energy consumption varies between 3.23 and 4.78 KWh/yr./L [16]. The working principle of a refrigeration system is exactly the same as that of an air conditioner. The Fig. 1 illustrates a schematic diagram of the refrigerator which like an air-conditioner, includes evaporator, compressor, condenser and expansion device as the four basic components [17]. As a typical design feature of a refrigerator, the evaporator is usually located in the freezer compartment, which forms the coldest part of the cabinet with a temperature of about −15 °C, while the refrigerant evaporates inside the evaporator tubes at −25 °C [17]. There is a chiller tray, just below the freezer and further below are compartments with progressively higher temperatures. The bottommost compartment is the least cold one and is meant for vegetables. The heavier cold air flows down from the freezer to the bottom of the refrigerator. The lighter warm air rises from the vegetable compartment. Current is usually set up to maintain a temperature between the top and bottom of the refrigerator. The temperature in the freezer is maintained about −15 °C, whilst the average temperature inside of the cabinet is 5 °C. The condenser is usually a wire and tube, or plate and tube type mounted at the back of the refrigerator without any fans. The condensation of the refrigerant vapor is performed with surrounding air which rises above by natural convection as it is heated after receiving the enthalpy of fusion of condensation from the refrigerant. The standard condensing temperature is 55 °C [17].

4. PCM selection for a refrigeration system PCM must be chemically and thermally stable during continuous freezing/melting cycles to be applicable for thermal energy storage in refrigerators. Thus, it has to be ensured that there are no alterations in the chemical structure and thermal properties of the PCM after a longterm period. This is because any degradation of the PCM decreases its enthalpy of fusion as well as possibly effecting the phase change temperature. The temperature of phase transition is an important item for designing a cold storage system. The important characteristics of PCMs like enthalpy of fusion and the temperature of phase transition could be analyzed via Differential Scanning Calorimetry (DSC) analysis by varying heating and cooling rates [33]. There are three types of PCMs used in refrigeration system i.e. organic, inorganic and eutectic. The composition of organic PCMs is carbon based while inorganic PCMs are metallic and hydrated salts. The organic PCMs are chemically stable and have a higher enthalpy of fusion but a lower thermal conductivity. In contrast, inorganic PCMs are cheaper and possess a better thermal conductivity with the only drawback being its corrosive nature. Some of the organic and inorganic PCMs are listed in the Table 1 and Table 2 respectively. Eutectic PCMs are mixture of various PCMs to obtain a required melting point. Some eutectic PCMs used so far in research have been listed in Table 3 [33]. Oró et al. [50] investigated a set of PCMs based on ammonium chloride and water binary system (19.5 wt% NH4Cl), which included various additives and concentrations of additives through design of experiments methodology. Sub-eutectic ammonium chloride (NH4Cl) concentrations of 16 wt%, and super-eutectic concentrations up to 22 wt% were studied. During solidification, some additives such as aluminum fluoride (AlF3) and sodium chloride (NaCl) were added to

3. Mechanism of PCM in a refrigeration system In household refrigerators, the compressor runs in on/off mode. When the compressor works, the refrigerant in the evaporator starts absorbing the heat present in the cabinet. However, during the offmode of the compressor, there is an evident increase in temperature inside the evaporator cabinet. This increase in temperature is due to the heat released by the food inside the refrigerator and also the ambient conditions. Consequently, the compressor has to work to release the heat externally via the condenser. Now, this increase in temperature can be alleviated by encapsulating a PCM in the cabinet [18]. The heat can be absorbed by PCM (called the enthalpy of fusion) by altering its phase, from solid phase to liquid phase, which maintains a constant temperature inside the compartment while the PCM melts. Therefore, for a particular period (i.e. as the PCM changes its phase completely while melting), the temperature inside the cabinet is maintained which in return extends the compressor off cycle [19]. Two types of losses are experienced throughout the compressor on/ off cycle. Firstly, during the on cycle, the thermal load of the heat exchangers is relatively higher than for a normal constantly controlled system. Due to the rise in temperature lift, this effect reduces the thermal efficiency. Secondly, because of the refrigerant displacement following on/off processes of the compressor, there are some energy losses [20]. Based on Coulter and Bullard [21] and Janssen et al. [22] reports, these two types of losses result in 5–37% of energy losses in a refrigeration system. Also, the constant on and off mode of the compressor causes temperature fluctuation, which leads to deterioration in the quality of the food inside the evaporator cabinet [23]. Consequences of these fluctuations are sublimation and recrystallization phenomenon related to the stability of ice crystals in and on the surface of the products [24,25]. The size of crystals in samples stored under fluctuation circumstance is larger than the samples stored in constant

Table 1 List of organic PCMs.

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Material

Melting point (°C)

Enthalpy of fusion (kJ/kg)

Refs.

n-Tetradecane Paraffin (C14) Formic acid n-Pentadecane Paraffin (C15) Caprylic acid Paraffin (C16) Glycerin Lactic acid n-Octadecane

5.5 5.5 7.8 10 10 16 16.7 17.9 26 28–28.1

215 228 247 193.9 205 150 237.1 198.7 184 245

[35,36] [37] [38,39] [40] [37,39] [37,40,41] [37,39] [42] [41] [38]

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aqueous solution as PCM. Water holds the advantages such as good time stability, high enthalpy of fusion and known thermophysical properties. Yet the cooling storage capacity of the system with a eutectic aqueous solution was found to be smaller than water as PCM as it is shown in Fig. 2 [51]. This is primarily due to water’s larger enthalpy of fusion. However, the ability of the eutectic solution to maintain the air in the refrigerated cell at appropriate temperature accounts for its added advantage.

Table 2 List of inorganic PCMs. Materials

Melting point (°C)

Enthalpy of fusion (kJ/kg)

Refs.

Hexadecane + tetradecane (2:3–0:1 by volume) 96% Tetradecane + 4% tetradeconol Lauryl alcohol + caprylic acid (2:3 by quality) 45% capric acid + 55% lauric acid Octadecane + docosane 31% Na2SO4 + 13% NaCl + 16% KCl % + 40% H2O 76% Na2SO4·H2O Mn(NO3)·6H2O + MgCl2·6H2O 50% CaCl2 + 50% MgCl2 + 6H2O

1.7–5.3

148.1–211.5

[47]

5.5 6.2

206.4 173.2

[35] [48]

17–21 25.5–27 4

143 203.8 234

[40,42,49] [38] [41]

9.3 25 25

114.4 130 95

[48] [38] [41,46]

5. PCM thickness The PCM thickness greatly affects the performance of refrigeration system as it is reported that an increase in the amount of PCM (approximately 40%) resulted in a 6% raise in the COP [52]. By increasing PCM thickness, its impact is on decreasing the on–off time ratio increases due to extension of compressor’s off time [7]. Application of thicker PCM incurs higher costs and initially requires higher compressor work for solidifying the PCM; hence, the thickness of PCM should be considered based on the load [14]. Once the PCM has been selected, it is essential to determine the amount of PCM to be used. A minimum volume of PCM is required, which can be calculated accordingly.[53]. The PCM’s stored energy (E) in a compartment is given by:

Table 3 List of eutectic PCMs. Materials

Melting point (°C)

Enthalpy of fusion (kJ/kg)

Refs.

H2O LiClO3·3H2O Na2CrO4·10H2O Na2SO4·10H2O Mn(NO3)2·6H2O CaCl2·6H2O LiNO3·2H2O

0 8 18 21 25.8 29 30

333 155–253 – 198 125.8 190.8 296

[36,42] [40,41,43] [40–42] [44] [45] [41,46] [41]

E = ρVλ

(1)

In this equation the sensible heat variations are neglected. The constants λ and ρ are the PCM’s enthalpy of fusion and density, respectively. The unavoidable heat gain from the surrounding condition by the compartment is:

modify the temperature of phase change while melting. To evaluate thermal reliability of the PCMs in terms of enthalpy of fusion and temperature, thermal cycling tests were implemented, and the results illustrate that the assessed PCMs had acceptable chemical and thermal stability following 100 freezing/melting cycles. Furthermore, thermal properties of PCMs were evaluated using DSC analysis. It was found using carboxymethyl cellulose (CMC) in the formulation of PCM could improve the stability of the PCM during a lifetime of cycles. Moreover, based on the experiments methodology, an equation was developed for each additive (NaCl and AlF3) to estimate the material phase change temperature (PCT) as a function of the concentration of the components in its formulation. Azzouz et al. [51] carried out experiments with water and eutectic

Q = (UA)cold (Tamb−Tcold)

(2)

Where, UA is the overall heat coefficient thermal conductance. The stored energy in PCM is equal to the rate of passing energy from the compartment’s walls during OFF time (tOFF) periods of compressor. Therefore, the minimum volume for PCM is given by [53]:

Vmin =

t OFF [(UA)cold (Tamb −Tcold)] ρVλ

(3)

The amount of PCM is usually more than the computed value from Eq. (3) because heat transfer via the compartment’s walls during the

Fig. 2. Time evolution of the average temperature with eutectic solution and water as PCM [51]. 726

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7. Effects of PCM on evaporation temperature and pressure

compressor off time is compensated by PCM. However, the system has to work longer in order to charge higher amounts of PCM. Besides, the ON time ratio is reduced as the compressor OFF duration is much longer. In addition, since all the thickness does not have the chance to undergo phase change process, the PCM cannot be thicker than a certain amount [51]. At such cases, partial melting/freezing of PCM occurs, which decreases its effectiveness. Thicker PCM not only needs longer compressor work for cold storage but also burdens higher costs and hence, PCM thickness is usually selected according to the load [14]. Marques et al. [54] observed through experiments that the total storage capacity for the PCM refrigerator is between 138 kJ for a 2 mm slab and 345 kJ for a 5 mm slab as illustrated in Fig. 3. The authors used a refrigerator with a heat load of 23.8 W to determine the PCM freezing and melting times for different thicknesses. This assumption corresponded to an ambient temperature of 25 °C. The compressor was assumed to be off during the melting process and running continuously with a cooling capacity of 246 W during the freezing process that corresponded to a −10 °C of evaporating temperature. The melting time increased from 101 min to 252 min as the thickness of PCM slab raised from 2 mm to 5 mm as illustrated in Fig. 3. In Fig. 4, it is shown that the freezing time was observed as 13 mins and 34 min for 2 mm and 5 mm slabs respectively. As such, it was claimed that the melting and freezing times clearly increased as the PCM got thicker i.e. a thicker PCM can be used for more time. Likewise, Azzouz et al. [7] evaluated the significant effect of PCM thickness on the running period and the autonomy of the refrigeration system. The autonomy of the refrigeration system for various scenarios are demonstrated in the Fig. 5 [7]. The figures indicate that 28.5–32.6% of running time was saved and an autonomy of 4.7–8.5 h was achieved. The running time saving is defined as the ratio of the difference between the percentages of running time without/with PCM. As the PCM thickness increases, the capability of storing energy increases significantly and relatively more heat is absorbed. Henceforth, a greater amount of heat is released when the PCM solidifies increasing the autonomy of the system.

Improving the efficiency of the evaporator is the most emphasized area in the performance enhancement of a refrigeration system. Depending upon the thermal load and type of PCM used, nearly 2–5 °C higher evaporating temperature was achieved. This is later discussed in Fig. 12 and Fig. 13. The increase in evaporating temperature raises the COP of the system and decreases the power consumption [56]. During the phase change process, high thermal capacity of PCM (because of its high enthalpy of fusion) maintains the temperature of evaporation higher, which is controlled by PCM’s PCT [51]. Compared to a refrigeration system without PCM, the high evaporation temperature of the system with PCM, burdens higher pressure which eventually leads to a higher COP [57]. Due to higher delta temperature, the lower evaporation temperature causes the PCM with fixed PCT to freeze faster. Eventually, the compressor capacity decreases with lower evaporation temperature and thus requires much longer PCM freezing time. By adjusting the thermostat to a lower setting, the refrigerator requires more energy to conduct extra heat from the cabinet which causes reduction in evaporation temperature [14]. Hence, integration of PCM in the evaporator and increment in the evaporation temperature are effective means of enhancing higher COP. Marques et al. [54] evaluated the impacts of evaporating temperature on the PCM by using the cooling capacity of the 8 cm3 compressor at −10 °C (262 W) and −15 °C (213 W) with an ambient temperature of 25 °C. The analysis was performed using a 5 mm PCM slab. The authors found that the freezing time increased by 26% when the evaporating temperature was reduced from −10 °C to −15 °C. This strong influence of evaporating temperature on PCM freezing time was observed after all the compressor COP increases linearly with evaporating temperature.

8. Effect of PCM on thermal loads Thermal loads have an inverse relation with respect to the performance of refrigeration systems i.e. the increase in thermal load decreases the performance of a refrigerator. While the use of PCM in a refrigeration system can improve the system performance, it is essential to analyze behavior of PCM at different loading conditions. Azzouz et al. [7] stipulated that the COP decreases by raising the thermal load, in spite of the presence of a PCM. This is because escalating the thermal load causes the PCM to melt partially as there is no sufficient time for phase change [51]. The evolution of the COP is displayed in Fig. 8 for different thermal loads. Generally, the COP values of configurations with PCM are higher than the one without PCM. Nonetheless, it was observed that the COP decreased with higher loadings too. In Fig. 8, it can be seen that the increase in COP ranges from 5 to 15 %. This is also due to other including factors such as number of door openings, external temperature and PCT. Khan and Afroz [56] claimed that a significant amount of COP decreases as the thermal load is increased due to the attenuation of the

6. Effects of phase change temperature of PCM The principal purpose of refrigerators is to preserve food and maintain a refrigerated space within a required low temperature. Therefore, when a PCM is used, the refrigeration system performance and the quality of food are directly proportional to the PCT of that PCM. PCT plays an important role since the melting point of the selected PCM must be in the range of the thermostat temperature. Due to its lower energy consumption, PCM with high PCT enhances COP of the system. Nevertheless, it has an adverse effect on the food, as it increases the temperature inside the compartment which results in deteriorating food quality [7]. On the other hand, a very low PCT maintains a very low temperature inside the storage cabinet resulting in better quality of food. To prevent freezing of food, it should be ensured that the cabinet’s temperature is not below zero [19]. Therefore, it is necessary to select an allowable PCT that maintains these two upper and lower extremes. PCT varies accordingly with the selection of PCM [14]. Fig. 6 is an observation of the experiment conducted by Azzouz et al. [55] which shows the effect of PCT on the COP of the refrigerator. According to this figure, as PCT increases, a significant increase of COP can be observed. With the increase in COP, a lower power consumption was deduced by the authors due to the fact that pressure compression ratio reduced. However, it should be made sure that the chosen PCT is not too high. A higher solidification temperature will result in excessive air temperature inside the compartment during melting as shown in Fig. 7. This in turn lowers the COP. Therefore, a compromise must be reached between the two extremes of PCT.

Fig. 3. PCM melting time with respect to the thickness of PCM [54]. 727

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Fig. 8. Effect of thermal loads on COP of the refrigeration system [7]. Fig. 4. PCM freezing time with respect to the thickness of PCM [54].

PCM at various loads for a eutectic solution and water. The Eutectic solution, having a melting point of −5°C stored more enthalpy of fusion than water during the off mode of compressor. This amount of heat, ultimately increased the evaporation temperature and pressure of the evaporator when transferred to the refrigerant by faster conduction method during the ON mode. Furthermore, a higher condensation temperature and pressure means more running time of compressor, which results in higher on/off cycling. 9. Effect of ambient temperature on PCM performance The ambient temperature has significant effects on the performance of PCM in the refrigeration system [14]. System COP reduces in higher ambient temperatures because of the higher condensation pressure and temperature and higher cabinet air temperature [55]. PCM’s charging/ discharging period is affected by the high thermal load, which shortens the melting time and delays the freezing time. This is because the compressor should overcome both charging PCM and the thermal load [54]. The performance of PCM decreases in very low ambient temperatures. The compartmental temperature drops faster, if working in a low thermal load. As a consequence, there is not sufficient time for the PCM to completely solidify, before the compressor stops [7]. Marques et al. [54] evaluated the influences of ambient temperature on the charge and discharge rate for a 5 mm PCM slab. As it is illustrated in Fig. 9, when ambient temperature rose from 20 °C to 30 °C the PCM melting time reduced from 320 min to 208 min which corresponded to a mitigation of 65% in refrigerator autonomy. Fig. 10 shows that the freezing time increased slightly with respect to an increase in ambient temperature, along with an additional 7 min being needed to freeze the PCM at 30 °C as compared to 20 °C.

Fig. 5. Autonomy and running time reduction (Text = 20 °C, zero door openings, Tpcm = −3°C) [7].

Fig. 6. Change in COP with respect to change in phase change temperature [55].

10. PCM application in evaporator The evaporator in a domestic refrigerator, performs according to either forced or free (natural) convective heat transfer [14]. The rate of

Fig. 7. Behavior of the average air temperature for different phase change temperatures [55].

sub-cooling effect of the condenser which occurs at the higher operating pressure and temperature of condenser. An experimental investigation was set up for a 0.00425 m3 with the objective of comparing effects of

Fig. 9. Melting time of a 5 mm PCM with respect to the different ambient temperatures [54]. 728

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enhancement depended on the nature of the PCM and the thermal load. The integration of enthalpy of fusion storage in a refrigerator allows 5–9 h of continuous operation without electrical supply. The cool storage capacity of the system is slightly smaller with a eutectic aqueous solution rather than water as a PCM. However, the ability to maintain the air in the refrigerated cell at proper temperature for eutectic solution accounts for its added advantage. Khan and Afroz [72] performed an experiment on a PCM based refrigerator having a single evaporator and a single door. It was concluded that depending on the thermal loads and PCM, approximately 2–5 °C higher evaporating temperatures were observed as illustrated in Figs. 11 and Fig. 12 for 5 W and 10 W loads, respectively. These higher evaporator temperatures reduce the compressor work, enhance the heat transfer of evaporator and improve COP. Elarem et al. [73] reported a new design of a PCM heat exchanger where an experimental test rig was developed. A series of experimental studies were performed by the authors to minimize the energy consumption of the household refrigerator. A total cycle time of 0.5 h and the ratio of operating cycle time to the total cycle time of 0.4 h can be depicted from the Fig. 13 for refrigerator without PCM. In contrast, the corresponding values for refrigerator with PCM were 0.4 h and 0.37 h respectively. Furthermore, it can be seen that the total cycle time and the ratio of operating cycle time to the total cycle time for the refrigerator with PCM is comparatively smaller than the former. This is due to either of the following items: (1) the refrigerator with PCM worked under a lower condensation temperature or (2) a higher elevation temperature or a higher sub-cooling degree. Moreover, due to the heat storage effect of the PCM heat exchanger, the difference between the internal and external compartment temperatures of the refrigerator with PCM was lower than that of the refrigerator without PCM. Therefore, a shorter operating cycle time is reached as the heat lost through the refrigerator compartment is reduced. The COP was calculated by the P-h diagram in Fig. 14. It can be inferred from the Fig. 14 that the refrigerator with PCM had a greater COP.

Fig. 10. Freezing time of a 5 mm PCM with respect to the various ambient temperatures [54].

heat transfer in a naturally cooled evaporator is low and results in temperature mitigation inside the cabinet. Whereas better temperature stability is expected in the forced convection evaporators. However, forced convection leads to some drawbacks such as food weight loss because of high air circulation and higher rates of energy consumption [58]. The application of PCM at the evaporator can be used to mitigate these drawbacks. To charge the energy storage, the working period of compressor should be extended for a longer time. Although, the working period of the compressor is extended in each cycle to charge the PCM, the overall running-time reduces because of longer compressor OFF time. Due to this phenomenon, the overall electricity consumption is mitigated, the destructive impact of frequent compressor start/stop is prevented, and better food quality is expected. Moreover, the role of PCM in refrigeration system is highlighted in cases of power outages as it has an impact on both food and compartment’s inside temperatures and their augmentation rate during any outages [59]. Compared to the forced-cooled evaporator, the natural cooled evaporator with direct contact to PCM is more advantageous as it increases heat transfer from the evaporator and stores system’s excess cooling capacity in the PCM [57]. In addition, this configuration causes the evaporator to have a higher pressure/temperature during PCM’s phase change process [8]. Consequently, this leads to a higher refrigerant density and therefore, cooling capacity rises. Similar results can be achieved for cases in which the evaporator coils are immersed in PCM [30]. When the coils are immersed in PCM there is a faster heat transfer rate because of the faster nature of convection/conduction processes in PCMs compared to the air natural convection. Because of the PCM’s high thermal inertia, the refrigerant temperature and pressure do not drop as much as in the case without PCM. This consequently, provides a higher mass flowrate of the refrigerant. Nevertheless, if PCT of the evaporator when immersed in a PCM is higher than the setting temperature of the cabinet, then a high thermal resistance could be formed around the evaporator eventually leading the system to more frequent compressor start/stop condition [14,60]. Table4 summarizes the advantages and disadvantages of using PCM in evaporator. The presence of the PCM increases the period of operation of the compressor. The COP rises with the increase in PCT. However, the global working time is decreased due to the autonomy of the refrigerator, as the cooling capacity is ensured by PCM’s melting. The addition of PCM increases COP and the cooling capacity by 74% and 87% respectively [55]. From experimental investigations it was observed that the inlet density of refrigeration was higher at the compressor and possessed a more stable condition against thermal load variations. The authors also discussed about the compressor off and on time ratio for better cooling and enhancement in COP of refrigeration system [7]. Azzouz et al. [51] carried out experiments that resulted in favoring the addition of PCM to the refrigerator with an increase of 10–30% in COP. The level of

COPwithout

COPwith

PCM

PCM

=

=

h1−h4 = 3.48 h2−h1

(4)

h1−h4 = 3.75 h2−h1

(5)

Compared to the refrigeration system without PCM, the experimental results illustrate that power consumption was reduced by 12% and the COP increased by 8% (Fig. 14). The performance of various PCM emplacements inside the refrigerator compartment were compared against one another to identify the best performing design for stabilizing temperatures within the compartment. Subramaniam et al. [32] designed a prototype side by side domestic refrigerator with PCM and dual evaporators. Thermal storage was provided by this prototype refrigerator to maintain food quality and Table 4 Advantages and disadvantages of using PCM at evaporator. Advantages of system performance • Enhancement [7,30,51,56,61] in case of power outages • Supports [34,62,63] of refrigerator noise • Decrease [64–66] compressor overall ON time • Shorter ratio [7,55] of electricity in peak hour • Reduction consumption [67–70] in overall costs of • Decrease Refrigeration system [71]

729

Disadvantages condensation • Higher temperature [57] rate of heat transfer from • Higher condenser to cabinet compression ON time • Longer during cycle [7,55]

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that can maximize energy savings. In addition, a thermoelectric refrigeration system with PCM slab was tested by Riffat et al. [74]. It was claimed that by using encapsulated PCM (ClimSelC7) instead of the conventional heat sink system, further performance improvement of the thermoelectric refrigeration system along with a better cooling storage capability can be achieved. In a similar fashion, Omer et al. [75] experimentally investigated a 150 W thermoelectric refrigeration system and compared the use of encapsulated PCM heat sink with conventional heat sink system. The results illustrated that the system with PCM improved the rate of heat transfer and provided a stable temperature inside the storage cabinet.

11. PCM application in condenser As a heat exchanger, a condenser rejects heat of compression from the refrigerator to the surrounding [17]. Forced-cooled, naturallycooled and hot-wall condensers are three various types of condensers that are employed in refrigeration systems [14]. Minimizing temperature in the condenser is the main reason for using PCM at condenser. However, the number of investigations in using PCM at condenser is relatively much smaller than the wide range of experiments carried out on the evaporator. Using PCM on a condenser extends its heat rejection to compressor OFF time, obtaining a lower condensation temperature by increasing condenser heat transfer [51]. Sonnenrein et al. [76] pointed out that particularly the application of PCM decreases the condenser temperature, which leads to a significantly reduced power consumption. Cheng et al. [4] carried out a different approach to increase the overall heat-transfer of a refrigeration system using Shape Stabilized PCM (SSPCM). SSPCMs are regarded as a special kinds of PCMs; consisting of working and supporting materials [33]. During the process of material phase change, the supporting material remains in solid phase. These SSPCMs are generally made by chemical (sol-gel method and graft copolymerization) and physical (impregnation, blending, adsorbing) methods [77–81]. Generally, SSPCMs are applied for space cooling of buildings. Feng et al. [82] prepared an SSPCM formed from a eutectic mixture of Capric acid–lauric acid encapsulated into porous graphite with the mass fraction of 80.47%. The result revealed that the required time for melting/ freezing was mitigated while the thermal conductivity was increased [83]. Cheng et al. [4] proposed a type of heat storage condensers wrapped

Fig. 11. Effect of PCM on evaporating temperature of the system with load 5 W [72].

Fig. 12. Effect of PCM on evaporating temperature of the system with load 10 W [72].

prolong compressor off time. In this PCM-based refrigerator, the food temperatures were more stable for fresh food and freezer during on/off cycling. More optimization of the gas quantity and capillary of refrigerant would led to an improvement of the new PCM based system

Fig. 13. Effect of PCM on power consumption of the refrigerator [73]. 730

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Fig. 14. P-h diagram of the experimented refrigeration system [73].

Fig. 15. Comparison of condenser midpoint temperature [4].

Fig. 16. Comparison of condenser outlet temperature [4].

with the HCE-SSPCM. The HCE-SSPCM was built with paraffin, high density polyethylene and expanded graphite. The thermal conductivity of the HCE-SSPCM, which happens to be 1.35 W/(m K), is comparatively four times more than that of the conventional SSPCMs [84]. The authors claimed that the new system could significantly improve the overall heat transfer of the refrigeration system due to continuous heat dissipation of the condenser during a complete cycle. As illustrated in Fig. 15 and Fig. 16, compared to the ordinary refrigerator, the condenser midpoint/outlet temperatures reduce by 2.3 °C and 6.5 °C respectively, which led to energy consumption saving by 12% approximately. Furthermore, when the power supplied to both experimented refrigerators was compared, the maximum power supplied to the new proposed refrigerator was distinctly smaller than that of the conventional refrigerator. The following Fig. 17 shows the lower amount of electrical consumption of novel refrigerator.

However, a defect was observed i.e frequent start/stop mode of the compressor, which had an adverse impact on the long-term performance of the compressor [85]. Similar to evaporator, the COP decreased as the surrounding temperature increased or freezer set-point reduced. The experiment also resulted that whenever the PCT increases, the energy consumption illustrates a minimum value at 49 °C, which approximately is the same as the SSPCM’s PCT [86]. The results ultimately confirmed that the proposed refrigerator could save up around 20–26% energy of that of the conventional one [4]. Wang et al. [87] conducted a series of experiments employing various types of PCMs at various locations of a refrigerator as demonstrated in Fig. 18. The heat exchanger referred to as PCMA was installed between the condenser and compressor, PCMB was located between the expansion valve and condenser, while PCMC in between the compressor and evaporator. 731

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Fig. 17. Comparison of power supplies of the refrigerator [4]. Fig. 19. Effect of PCM during power outage [59]. Table 6 Advantages and disadvantages of using PCM in the compartment section. Advantages temperature fluctuations in • Reduces stored items [59] energy consumption [34] • Reduces temperature rise during • Reduces power outages [59,62,67]

Table 5 Advantages and disadvantages of using PCM at condenser.

• • • • •

Higher COP [31,85,87,88] Shorter compressor global ON/OFF ratio [4,85] Lower condensation temperature and pressure [4,31,87] Higher subcooling degree [4,85] Faster stable condition of refrigerator [4]

12. PCM application in compartment section

Disadvantages

• • •

usable space for storing • Reduces items [34,59,62] • PCM selection is more crucial [62]

exchangers were found to be beneficial if placed in A or B positions. As for the case in PCMC, further studying must be done to place a PCM heat exchanger in the C position without the pressure drops. The pros and cons of applying PCM at the condenser have been enlisted in Table 5.

Fig. 18. Experiment setup conducted by Wang et al. [87].

Advantages

Disadvantages

The compartment section of the refrigerator is the area where food and other items are stored. There have been several experiments on applying PCM on this area of the refrigerator. Generally, PCM is applied to the walls or the top and bottom of the compartment section. PCM placed in the compartment section is closer to the food items, maintains the temperature at safe levels, and is beneficial in the case of a power outage. One factor to consider when applying PCM to this section is the space that is taken up. The PCM should not take up a large portion of this area because the room to store items is reduced. The orientation should also be considered when placing PCM in the compartment section. Marques et al. [89] looked into the effect of orientation of a PCM slab placed in the compartment section. In their study, CFD simulations for horizontal, vertical, and a combination of horizontal and vertical placements were carried out. The authors found that horizontal placement of the PCM gave a more homogenous temperature distribution in the compartment than the vertical orientation. However, a combination of horizontal and vertical orientations proved to give the best results with a very homogenous temperature distribution and increased air mixing provided by the horizontal PCM. Marques et al. [54] conducted another study, where they experimented with PCM held in a copper box with a heat exchanger running through it. This box was located near the top of the compartment section. The results indicated that the inclusion of the PCM would allow for 3–5 h of continuous operation without power depending on the thermal load. Gin et al. [67] studied the effect of PCM panels placed in the compartment section of a freezer. CFD modeling was used to analyze

Frequent compressor ON/OFF [4] More refrigerant displacement losses [4,85] Heat accumulation [4]

When PCM is placed between compressor and condenser (PCMA), the PCM acted as an extra condenser. The position of PCM reduced condenser pressure and increased sub-cooling. The inlet temperature of the expansion valve decreased from that of the basic system without a PCM heat exchanger. Overall, this system had an increase in COP of about 6%. Next, PCMB had a reduced temperature before the expansion valve. The COP was increased by 8%. Although the air inlet temperatures at the condenser and evaporator were not the same during the basic and PCMB system tests, the effects were small. The increase in performance can be explained by the lower expansion valve inlet temperature. Last, PCMC lowered the evaporator outlet temperature, which decreased the superheating. The expansion valve was purposely set to cause temperature fluctuations and PCMC was able to smooth out those fluctuations to an extent. Although the lowering in superheating would cause the COP to rise, the pressure drops caused by PCMC cancelled any gains in performance. The authors concluded that the PCM heat 732

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Table 7 Brief summary on all the experiments. Ref.

PCM type Water

Cheng et al. [4] Rahman et al. [8] Abhilash et al. [13] Azzouz et al. [55] Azzouz et al. [7] Azzouz et al. [51] Onyejekwe [53] Marques et al. [54] Gin and Farid [59] Khan et al. [72] Elarem et al. [73] Subramanium et al. [32] Yuan and Cheng [86] Wang et al. [87]

Location Eutectic

Paraffin

Evaporator

✓ ✓

✓

✓ ✓ ✓ ✓ ✓ ✓

PCM

Thermal load

Change temp

Thickness

Ambient temp

Evaporation temp

✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓

✓

✓

✓ ✓

✓ ✓ ✓ ✓ ✓

✓

Compartment

✓ ✓ ✓ ✓ ✓ ✓ ✓

✓ ✓

Condenser

Phase

✓

✓

✓ ✓

✓ ✓

Table 7 gives a summary of various experiments performed by different researchers on the application of PCM in the refrigeration system.

the impact of the PCM on the air temperature rise during power outage, and the models showed that PCM placed in the compartment could be used to limit temperature rise. It was stated that the models could be extended to study the impact of PCM surface area and placement in the compartment. Gin and Farid [59] in another work conducted an experiment where aluminum panels containing 10 mm thick PCM slabs were placed on the walls of a freezer compartment section. In their experiment, they compared the temperature deviation of the stored items with and without the PCM along with effects of PCM during power outage. It was found in the test that without PCM, the stored items’ temperature had a fluctuation of 13 °C, while the test with PCM had a fluctuation of only 5 °C. This is beneficial because they found the quality of the food items that experienced lower temperature fluctuations was better than the items that had larger temperature fluctuations. In the case of a power outage they found that by adding the PCM panels, the compartment temperature was maintained at a lower temperature and led to lower product temperatures, which can be seen in Fig. 19. In another test conducted by Gin, Farid, and Bansal [34], they applied PCM in the compartment section using the same method as the previous experiment. The only exception in this case was that the authors studied the effect PCM has on energy consumption during normal operation, under heat loads caused by door openings, and during defrost. It was found that by adding PCM to the compartment section, the energy usage was reduced. Furthermore, it was claimed that for their system the energy usage was reduced by 8% during defrost cycle and by 7% during door openings. Oro et al. [62] encapsulated a PCM in stainless steel plates and placed them horizontally on shelves in the compartment section of a freezer. They investigated the effect of the PCM on the thermal performance of the freezer during door openings and power failure. In their case they found the PCM could maintain the interior temperature for much longer than 3 h. When they changed the storage temperature, they did not find much benefit but point out the importance of PCM selection and state the PCM phase change temperature should be near storage temperature. Taneja et al. [68] used PCM placed in the compartment section of a domestic refrigerator to help shift electricity usage to off peak hours. They found that by adding the PCM, the average power consumption of the fridge was increased from 76.69 W to 86.55 W. They claimed that the benefit was achieved by allowing the fridge to shift running time to off peak hours giving cost savings in electricity and allowing for renewable energy sources to be better utilized. Some advantages and disadvantages of placing PCM in the compartment section is shown in Table 6. More experiments should be conducted for using PCM in the compartment section of refrigerators to better understand the effect they have on the systems operation.

13. Conclusion The selection of PCM depends on the phase change temperature, thickness and thermal loads. PCMs have significant effects on other parameters of the refrigeration system which have to be equally taken into consideration. PCMs placed at the evaporator section gives slower fluctuation of compartment temperature and more stable conditions against thermal load variations. However, incorporation of PCM at evaporator section increases the compressor running time initially and raises the condensation temperature. Considering these problems, some investigations were performed to incorporate PCM at the condenser section. The results showed a higher COP, lower energy consumption, lower condensation temperature and lower condensation pressure. Nevertheless, compressor on-off problems are more frequent and more refrigerant displacement losses were observed. For avoiding these complexities, some investigations have been based on incorporating PCM inside the food storage compartment. Stable temperatures inside the compartment were observed. However, this system experienced lower COP as compared to the other two positions. Application of PCM on domestic refrigerators at various sections i.e. evaporator, condenser and compartment illustrate that the use of PCM on refrigerators increases its COP with a relatively significant margin. Acknowledgment The authors thank Interdisciplinary Research Center (IRC) at Arkansas Tech University (ATU) for the support through the IRC funding. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.applthermaleng.2018.07. 068. References [1] S.E. Hosseini, M.A. Wahid, Hydrogen production from renewable and sustainable energy resources: promising green energy carrier for clean development, Renew. Sustain. Energy Rev. 57 (2016) 850–866, https://doi.org/10.1016/j.rser.2015.12. 112. [2] P.S. Raveendran, S.J. Sekhar, Performance studies on a domestic refrigerators retrofitted with building-integrated water-cooled condenser, Energy Build. 134 (2017) 1–10, https://doi.org/10.1016/J.ENBUILD.2016.11.013. [3] O. Laguerre, S. Ben Amara, J. Moureh, D. Flick, Numerical simulation of air flow and heat transfer in domestic refrigerators, J. Food Eng. 81 (2007) 144–156,

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