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Experimental investigation of the use of PCM in an open display cabinet for energy management purposes
Energy Conversion and Management 198 (2019) 111909
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Experimental investigation of the use of PCM in an open display cabinet for energy management purposes
T
R. Ben-Abdallaha,b, D. Leducqa, H.M. Hoanga, , L. Fournaisona, O. Pateaub, B. Ballot-Miguetb, A. Delahayea ⁎
a b
Irstea, UR FRISE, Refrigeration Process Engineering Research Unit, 1, rue Pierre-Gilles de Gennes, F-92761 Antony, France EDF –R&D, EDF Lab Les Renardières, avenue des Renardières, F-77250 Écuelles, France
ARTICLE INFO
ABSTRACT
Keywords: Display cabinet PCM Heat exchanger Flexibility Energy management
Display cabinets are widely used in supermarkets and represent an important part of their energy consumption. Adding PCM to a refrigerated display cabinet can increase its compressor cutoff time as the cold energy accumulated by PCM can replace the refrigeration system during a certain period of time. This technology can be considered as a solution to increase the electricity flexibility in order to match the demand and the production of a supply network, to manage energy flows on the grid and to boost the use of intermittent renewable energy sources. The present study is focused on the performance of the display cabinet with integrated phase change material (PCM). The PCM is selected according to the temperature range of the application. To enhance the heat transfer and facilitate the PCM melting and freezing, PCM is inserted in a heat exchanger. The experimental results show an important potential of PCM to maintain the air and product temperature when the compressor is off (up to 2 h).
1. Introduction
and economic benefits [10,11]. In this context, the integration of thermal energy storage devices using Phase Change Materials (PCM) in display cabinets presents opportunities for peak shaving and reducing supermarket energy costs. Indeed, the PCM inside display cabinets can replace the cold machine during a certain period of time by using its stored energy. This solution can increase the flexibility in the energy management of the display cabinets and of the whole supermarket if many display cabinets are equipped with TES devices. Moreover, PCM can be a solution to the intermittency of renewable energy sources and encourage supermarkets to become self-sufficient via solar or wind energy. It should be noted that many governments offer incentives to encourage this. Many studies have focused on the application of phase change materials for energy storage in refrigeration equipment such as refrigerators [12]. Azzouz et al. [13] placed PCM sheets on the back side of a refrigerator evaporator. The sheets provided higher heat exchange than the evaporator could provide solely through natural convection. The results obtained using PCM showed a higher evaporation pressure, an increased refrigeration capacity, and a significant performance enhancement. The energy stored in the PCM allows for several hours of continuous operation (between 4 and 8 h, depending on the thermal load) without electrical power supply using a 5-mm-thick slab. Maiorino et al. [14] have attached PCM to the evaporator inside the cabinet of a refrigerator; the PCM integration allows an important
Refrigeration is one of the most energy-consuming processes in the food supply chain. About 35% of the electricity consumed by the food sector is dedicated to refrigeration [1]. Nowadays, supermarkets are an important part of the economy and their prominence continues to increase. They represent around 4% of the annual electricity consumption in industrialized countries [2,3]. In food retail, display cabinets are the most widely used and energy intensive equipment [4,5]. Given the increasing demand for energy and the necessity to protect the environment, the European climate foundation claimed an ambitious objective for 2030 [6]: - At least 40% cut on greenhouse gas emissions - At least 27% share for renewable energy - At least 27% improvement in energy efficiency Considering the energy and environmental context, Thermal Energy Storage (TES) may play a key role in enhancing the renewable energy utilization [7,8], balance the energy production and demand [9], and increase the refrigeration plant efficiency. TES can enhance the system flexibility by storing heat and using it later when needed. Thus, it provides environmental
⁎
Corresponding author. Tel: +33 1 40 96 65 02; fax: +33 1 40 96 60 75. E-mail address: [email protected] (H.M. Hoang).
https://doi.org/10.1016/j.enconman.2019.111909 Received 29 May 2019; Received in revised form 31 July 2019; Accepted 1 August 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
Energy Conversion and Management 198 (2019) 111909
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Nomenclature dt E L m Q t
T
stop store
Duration, s Energy consumption, J Specific latent heat, J kg−1 Mass, kg Power demand of the compressor, W Time, s Temperature, °C
Greek symbols
T
amb avg evap min
Dimensionless temperature Time integration variable, s Temperature rise, °C
Abbreviation
Subscripts a
Compressor stop period Stored energy
BPP
Air Ambient Averaged value Evaporator Minimum value
DAG
PCM RAG VBC
reduction of temperature gradient within the cabinet and fluctuations of product temperature, and an extension of the compressor off time. Energy performance of commercial freezers can also be improved by using PCM. Oró et al. [15] placed stainless-steel-plated PCM at different locations in a freezer. They observed that the use of PCM minimized the temperature rise of the freezer and stored product, which occurred due to frequent door opening and electrical power failure (3 h). A similar conclusion was reported by Gin et al. [16] who evaluated the effectiveness of using PCM panels on the internal walls of a freezer. During defrosting, the temperature increase rate was significantly reduced while the energy consumption was only slightly reduced. PCM is also widely spread in refrigerated trucks. A PCM thermal storage unit can even substitute a conventional refrigeration system [17]. In this study, the PCM storage unit was charged by a refrigeration unit located off the vehicle. This concept has been proved as feasible for on-board refrigeration. In addition, the system has been found to consume less energy and to emit less greenhouse gases. Fioretti et al. [18] investigated the use of PCM in the external layer of a refrigerated container and showed that this novel technology can reduce and displace the heat flux generated by external conditions. Recently, Maderic et al. [19] has carried out an experimental study on a beverage cooler with latent heat storage by an ice bank; important energy consumption reduction (15%) and significant decrease of the refrigeration machine start-ups were observed. An important issue of refrigeration systems is the control of frosting and defrosting which can be greatly impacted by the integration of energy storage device. Adaptive defrost methods to improve defrost efficiency was proposed by Yoon et al. [20]. So far, only few studies concerning the application of PCM in display cabinets for chilled products have been published. The use of PCM is expected to offer some benefits. First, the energy stored in the PCM can replace the cold machine for a period of time in refrigerators, freezers and trucks. Second, depending on its placement, the PCM may provide a better air and product temperature stabilization. In particular, the PCM may attenuate the temperature rise during long periods of compressor inactivity (up to several hours) if energy management strategies (peak shaving, electrical curtailment…) are applied in supermarkets. The PCM may also offer a better homogeneity of product temperature distribution. Indeed, important temperature differences (6 °C) were measured on the shelves of a display cabinet without PCM [21]. Lu et al. [22] have integrated heat pipes and PCM within shelves and showed that the shelf equipped with heat pipes leads to lower food temperature (3–5 °C) and improves the homogeneity of temperature distribution. Sevault et al. [23] focused on a novel PCM (pure water) accumulator for a refrigerated display cabinet using CO2 as a refrigerant. Their work consists in optimizing the PCM accumulator design. They observed the influence of the distance between CO2 coils and air ducts on the charging and discharging of PCM. Mehmet et al. studied numerically the effect of adding PCM on the thermal behavior of a vertical beverage
Back perforated panel Discharge air grid Phase change material Return air grid Vertical beverage cooler
cooler (VBC) during a period when the compressor is turned off [9]. The PCM was inserted in the rear duct of the VBC. The influence of the PCM slab thickness on the display cabinet performance and the temperature homogeneity were investigated. They conclude that as the thickness of the PCM slab increases, the flow rate decreases and the pressure drop increases. Their results showed that a higher quantity of PCM reduced the running time ratio of the cold machine (relating to the cold machine on–off cycle, the running time ratio is the ratio between the running time and the cycle duration). A minimum ratio was obtained for a PCM slab of 6 mm. Alzuwaid et al. [24,25] used PCM charged into two single-panel radiators (0.93 L of water gel PCM in each radiator) at the back channel of an open vertical display cabinet. The tests results showed energy savings, lower cabinet temperatures, and a significant stabilization of product temperatures during defrost (up to 12 min). The purpose of this work is to experimentally study the PCM integration on an open display cabinet to replace the refrigeration machine for a long compressor stop period (1–2 h). Important information is still missing in published works. For example, how the presence of the PCM heat exchanger disturbs the airflow and the operation of the display cabinet. The choice of PCM, its quantity, its container, and its positioning also need to be considered. Moreover, the charging and discharging processes of PCM have not been examined yet. These issues have to be taken into consideration to facilitate the deployment of this technology. This work focuses on the characterization of the PCM behavior during charging and discharging processes and its effect on the thermal and energy performances of the display cabinet. PCM (7 kg) was introduced in a heat exchanger; the impact of the heat exchanger on the air flow rate was also studied. Moreover, the comparison of the air and product temperature rise obtained with a display cabinet without PCM and a modified display cabinet with PCM during a compressor stop of 2 h was discussed. 2. Materials and methods In order to evaluate the impact of PCM addition on the thermal behavior of the display cabinet, three configurations (Fig. 1) were considered: - Configuration 1: display cabinet without heat exchanger - Configuration 2: display cabinet with empty heat exchanger - Configuration 3: display cabinet with heat exchanger filled with PCM Temperature (product, air, and PCM) and air velocity were measured at different positions. The electrical power demand was also measured in order to calculate the daily energy consumption. 2
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Fig. 1. Open display cabinet.
2.1. Display cabinet description
vertically through the heat exchanger to the discharge air grid (DAG), then flows downwards into the return air grid (RAG). Along the rear duct and even along the heat exchanger, part of the airflow circulates horizontally through the perforated panel towards the products. Ambient air infiltrates into the air curtain (about 30% of the air curtain flow rate [21]) and mixes with the top-down flow (Fig. 2). The thermostat probe of the display cabinet is placed in the rear duct on the right side of the 1st shelf (Fig. 2) at the same location as before modification [26].
The display cabinet used in the present study (Fig. 1) is located in a test room in which the ambient temperature is fixed at 16 °C. Its dimensions are 1.3 m in length, 0.9 m in width, and 2.0 m in height. It has five shelves numbered from bottom to top and it is loaded with packages of test product (dimensions of a package: 20x10x5cm, material: methylcellulose, density: 1100 kg.m−3, thermal conductivity: 0.49 W.m−1.K−1, specific heat: 3372 J.kg−1.K−1). For each shelf, the packages were arranged into two sheets of 4 rows and 16 columns to simulate the food products at the front (near the air curtain) and at the back (near the perforated panel BPP, Fig. 2). The ambient conditions and loading configuration were taken from related work [21]. The display cabinet has no doors. For configurations 2 and 3, the heat exchanger is located in the rear air duct (up to 3rd shelf). Cold air blown by one fan comes from the outlet of the evaporator and circulates
2.2. PCM integration The size and the capacity of the PCM heat exchanger were chosen so that it can replace the cold machine for at least 1 h (dtstop= 3600 s) with the compressor turned off. Considering the display cabinet’s average power (Qavg = 500 W), the quantity of energy needing to be stored is: 3
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Fig. 2. Integration of PCM heat exchanger inside the display cabinet.
Estore = Qavg × dtstop = 1.8MJ
The PCM occupies 70% of the internal volume of the heat exchanger to take into account the water volume expansion during the phase change (which is about 10% of the water volume). It’s important to note that the energy accumulated in the metallic parts of the heat exchanger represents only 0.5% of the latent energy stored by the PCM.
(1)
Water is chosen as PCM for this application due to its properties: melting temperature (0 °C) which is appropriate for the conservation of chilled product (temperatures between 0 and 6 °C), high latent heat (LPCM = 334 000 J.kg−1), specific heat (4184 J.kg−1.K−1), large availability, safety, low price, non–toxicity (for food preservation application), and non–flammability. The minimum PCM quantity can be estimated as:
mPCM , min =
Estore = 5.4kg LPCM
2.3. Measurements
(2)
2.3.1. Temperature T-type thermocouples were used for the measurement of:
For this study, the PCM mass was fixed at 7 kg. In order to integrate PCM inside the display cabinet, many positions can be considered: rear duct, discharge air grid, return air grid, under shelves. However, the position should be selected by considering a few important criteria. The first criterion is that the position needs to provide a suitable condition of heat transfer between air and PCM so that the PCM can be charged and discharged completely at a given time. The second criterion is that a sufficient volume should be available to store 7 kg of PCM. Moreover, the ease of installation should also be taken into account. In our case, PCM is introduced inside a finned tube heat exchanger at the rear duct (Fig. 2). The tubes are horizontal while the fins are vertical so that air can pass through the heat exchanger. This configuration offers many advantages:
- temperature of products on the surface and in the core (blue dots, Fig. 3) - air temperature: at the air curtain, air supply (DAG) and return grill (RAG), in the back duct (green crosses) and room temperature (red star) Similar instrumentation can be found in other related works [21,26–28]. The PCM heat exchanger is also equipped with T-type thermocouples (black cross) inserted inside the tubes at 5 levels (Fig. 4). At each level, two positions are instrumented: in the middle and on the right side considering the symmetry of the heat exchanger. Each thermocouple is fixed at the center of the tube by a spacer (Fig. 4). All thermocouples were calibrated and have a precision of 0.3 °C.
- The heat exchanger can receive cold air leaving the evaporator. In previous works [21,26], temperature measurements have been carried out to determine the temperature range in different areas of the display cabinet for different ambient conditions (T = 15, 20 and 25 °C). Results showed that the rear duct is the best zone to introduce PCM (Table 1) as its air temperature range is suitable for water freezing (between −5.5 and 2 °C for an ambient temperature of 15 °C). The air temperature at other positions (discharge air grid, return air grid, under shelves) is too high for the PCM charging process. - An important heat transfer area is provided between air and PCM 25m2 (including the surface of the fins, manufacturer’s data). - There is enough space in the rear duct to install a heat exchanger.
Table 1 Air temperature range (°C) for an open display cabinet (without heat exchanger). Ambient
Position 2 Position 3 Position 4
T = 15 °C
T = 20 °C
min
max
min
max
min
max
−5.5 −3.5 −3
1 1.5 2
−4.5 −3 −2.7
2 2 2.5
−4 −2 −2
3.5 3.5 4.5
(Positions 2, 3 and 4 are presented in Fig. 2) 4
T = 25 °C
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Fig. 3. Modified display cabinet instrumentation.
2.3.2. Air velocity A hot-wire anemometer (Testo 435–4) was used to measure air velocities in the display cabinet. This anemometer is calibrated in the range of 0–2 m.s−1 with an uncertainty between 0.02 m.s−1 and 0.06 m.s−1.
started every 4 h with a duration of 16 min. In order to study the impact of energy management strategies (peak shaving, electrical curtailment…) on the display cabinet functioning, a longer compressor stop (up to 2 h) was also studied. For each configuration, two tests were performed. Each test comprised working periods (3 days with regular defrosts every 4 h) and a 2-h compressor stop at the end of the test. For configuration 3 (display cabinet with PCM), defrosting and compressor stop corresponded to the same period: 1.6 to 2 h. Fig. 5 shows the evolution of the air temperature near the thermostat probe and compressor power evolution. Defrosting periods, starting periodically every 7.8 h, end when the thermostat probe temperature reaches 10 °C, a new working period begins (at 0 h and 7.8 h). Because of the temperature rise related to the defrost, longer compressor cycles (duration 0.7 h, between 0 and 0.7 h, or between 7.8 and 8.5 h in Fig. 5a and 5b) are found just after defrosting periods. Then the air temperature begins to fluctuate around −5 °C (set point temperature). A zoom on compressor on–off cycles is shown in Fig. 5c. The duration of an on–off compressor cycle is about 7.5 min. For this configuration, 6 cycles of PCM charging / discharging were investigated. Good reproducibility was observed between cycles (Fig. 8). PCM charging was done during working period while PCM discharging happened during defrosting.
2.3.3. Power and energy consumption A wattmeter (Digiwatt, precision 2%) was used to measure the electrical power demand of the compressor (Q (t)). The electrical power absorbed by the fan is a constant value (25 W, obtained by previous measurement); this power was not measured in the current study. The compressor energy consumption E (t ) is calculated by integrating the measured power for a duration t.
E (t ) =
=t =0
Q ( )d
(3) 4
For a daily energy consumption of 3.6 10 kJ (set point temperature = −5°C), the uncertainty (standard deviation) is 500 J or 1.4%. 2.4. Display cabinet functioning and setting parameter The set point temperature of the display cabinet was fixed at −5 °C for all experiments. Two operating regimes were investigated: working period (compressor on–off cycles) and compressor stop/defrosting (power = 0) as shown in Fig. 5. The ventilation was on during the compressor stop/defrosting. For configurations 1 and 2 (display cabinet without heat exchanger and display cabinet with empty heat exchanger, respectively), defrosts
2.5. Charging ratio A charging ratio is defined according to the temperature of PCM at 4 positions inside the heat exchanger. The phase change is completed at one position when the PCM temperature is lower than 0 °C (freezing 5
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Fig. 4. PCM heat exchanger instrumentation.
temperature of PCM) and begins to decrease (point C in Fig. 8a); the PCM is then considered to be fully solid. During the charging process, a charging ratio of 25% will be reached when the PCM at position 2 is solid, and so on for position 3 (50%), position 4 (75%), and 5 (100%) (Fig. 4). Conversely, during the discharging process, the PCM is considered to be fully liquid when the temperature begins to increase after the phase change stage (Point F in Fig. 8a).
and at the DAG for the two cases. The obstacle due to the addition of the heat exchanger reduces the air passage section and increases the air velocity at the first and second shelves. Lower air velocities are found at the fourth and fifth shelves and at the DAG. The air flow rate measured at the DAG is 0.0097 m3.s−1 for the modified display cabinet compared to 0.0134 m3.s−1 for the unmodified display cabinet. The thermal behavior is also affected by the heat exchanger’s presence. As observed in Fig. 7, for the modified display cabinet, the package temperatures in the fifth shelf are warmer than those in the unmodified display cabinet because of the reduction of the cold air flow rate from the BPP and from the DAG. The front packages are also at a higher temperature except for the first shelf which can be explained by the diminution of the air curtain flow rate (−28%). The package temperatures at the first shelf are cooler in the modified case because the air flow rate through the BPP is higher at that position. Such observations lead to conclude that it is necessary to modify the display cabinet for PCM integration.
3. Results and discussion 3.1. Influence of the presence of the heat exchanger (empty) on the air flow and thermal behavior Tests have been performed in the display cabinet without the heat exchanger (configuration 1) and with an empty heat exchanger (configuration 2) to evaluate the impact of an obstacle in the rear duct on the airflow and on the product temperature, without the effect of PCM charge/discharge. Fig. 6 presents the air velocity along the rear duct 6
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Fig. 5. Evolution of air temperature and compressor power (configuration 3).
3.2. PCM charging and discharging processes – PCM temperature evolution
phase change begins at the position 1 (just after the evaporator) followed by the second and third positions. Similarly, during the compressor stop, PCM changes phase first at positions 1 and 2 because these two positions are most exposed to the returning air. For the PCM at the positions 3 and 4, a phase change plateau is observed and the temperature increases above 0 °C before the end of the compressor stop. During the test, PCM at position 5 was still at 0 °C at the end of the compressor stop and did not melt completely. The PCM temperature evolution is more affected by the compressor for positions 1 and 2 than other positions because these positions are first to receive the air coming from the outlet of the evaporator (arrows showing air flow direction can be observed in Fig. 8b). Smaller temperature fluctuations related to compressor on–off cycles can be observed in higher positions (3 and 4). In order to observe the progress of the charging process, Table 2 illustrates the evolution of the temperature at different positions in the heat exchanger at several times. The left side of the heat exchanger (not instrumented) is supposed to have the same temperature as the right side (instrumented). Initially (0 min), the PCM temperature was positive (red cases). After 50 min, positions 1 and 2 situated near the evaporator were the first to freeze. Then, at 125 min, position 3 starts to freeze in the middle, followed by the two others (position 4 at 185 min and position 5 at 225 min). These results show that the PCM charge is progressive and the middle is more sensitive to temperature variation than the extremities. This phenomenon is due to the position of the fan
Fig. 8a illustrates the PCM behaviour during a cycle of charge and discharge processes at the position 4 of the PCM heat exchanger. In this cycle, 6 steps can be defined: - A-B, cooling step 1 - PCM in liquid phase: the PCM temperature decreases to the phase change level (near 0 °C). - B-C, cooling step 2 - liquid to solid phase change: a temperature plateau around 0 °C is observed, latent heat is stored by the PCM. At the end of this step, the PCM is completely solid. - C-D, cooling step 3 - PCM in solid phase : the PCM temperature continues to decrease. - D-E, compressor stop step 1 - PCM in solid phase: the PCM temperature increases until it reaches the phase change level around 0 °C. - E-F, compressor stop step 2 - solid to liquid phase change: the PCM temperature stays around 0 °C - F-A : Compressor stop step 3 - PCM is melted, liquid PCM temperature increases. Fig. 8b shows the PCM temperature evolution at different positions. It’s interesing to point out that the charging and discharging of the PCM is done gradually as the temperature in the heat exchanger is not homogenous due to the direction of the airflow. During cooling, the 7
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Fig. 6. Air velocity inside an unmodified display cabinet (left, configuration 1) and a display cabinet with empty heat exchanger (right, configuration 2).
and the airflow pattern inside the display cabinet. The same behaviour is observed for the discharge process (Table 3): the discharge is also progressive and starts from the middle of the tube towards the extremities. Table 4 shows charging and discharging time for different charging
ratios. The necessary time to totally charge the heat exchanger is 225 min (3 h 45 min). However, it takes only 96 min (1 h 36 min) for the PCM to melt completely: the discharging is two times faster than the charging. This result can be explained by a larger temperature difference between the air and the PCM during discharge.
Fig. 7. Air and product temperature of unmodified display cabinet (left, configuration 1) and display cabinet with empty heat exchanger (right, configuration 2).
8
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Fig. 8. Temperature evolution of the PCM (configuration 3).
3.3. PCM effect on the display cabinet behavior during one charging/ discharging cycle
defrost. Configuration 3 – with PCM (Fig. 9b) – shows a longer first cycle (0.7 h) than configuration 1 – without PCM (0.1 h). The configurations having PCM need more time to reach the operating condition. Fig. 9c presents the air temperature evolution at the discharge air grid (DAG, cf. Fig. 2); the configuration with PCM shows a significantly lower temperature (mean value = 1.5 °C) than the configuration without PCM (mean value = 8.4 °C). This effect might compensate the reduction of the flow rate of the air curtain (−28%) due to the presence of the heat exchanger. The effect of the PCM on the air and product temperature during a 2-h compressor stop will be analyzed in Sections 3.4 and 3.5. Fig. 9d shows the evolution of the compressor energy consumption (Eq. (3)) during one PCM charging/discharging cycle. For the configuration with PCM, the compressor needs more power than the configuration without PCM during the charging period. Despite this increase, no significant difference in the energy consumption is
The integration of PCM in the display cabinet might generate inconveniences: 1 - it might cause a time delay for reaching the operating condition, and 2 - as the cold energy produced by the refrigeration machine is used for both maintaining the air and product temperature and charging the PCM, the energy consumption might increase. In order to answer these questions, the display cabinet behavior during one cycle of PCM charging/discharging (configuration 3) is analysed and compared to the results of configuration 1 – without PCM. Fig. 9a and b present the evolution of the compressor power demand; the beginning (t = 0) corresponds to the restarting of the refrigeration machine after a defrost. As mentioned in section 2.4, the first compressor on–off cycle is longer than other cycles because of the temperature rise during the 9
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Table 2 PCM temperature (°C) inside the heat exchanger at different moments during charging.
Table 3 PCM temperature (°C) inside the heat exchanger at different moments during discharging.
Table 4 Charging / discharging time. Charging ratio (%) 0 25 50 75 100 Charging ratio (%) 100 75 50 25 0
observed for the whole cycle because of a long discharging period during which the compressor is stopped. This result means the cold energy stored in the PCM is not lost but kept inside the display cabinet for later use – to replace the refrigeration machine during the compressor stop. In other words, the PCM integration allows deferring the use of cold energy and increases the display cabinet flexibility for energy management applications. Table 5 shows measured energy consumption during 24 h for both the modified (with PCM, configuration 3) and unmodified (without PCM, configuration 1) display cabinet. For the same experimental conditions (temperature set point and ambient temperature), PCM integration does not modify the energy consumption (the difference of energy consumption between the two configurations is less than 2%).
Charging time (min) 0 24 125 183 225 Discharging time (min) 0 25 60 68 96
The reduction of the air temperature related to the PCM integration was also observed by Alzuwaid et al. [24]. In the same study, a reduction of the energy consumption of 5% was observed for a modified display cabinet with PCM. The same authors have highlighted that the energy saving is a function of the operating conditions (ambient, cabinet settings, PCM properties…). Important differences between the current study and the work of Alzuwaid et al. [24] -the PCM quantity 10
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Fig. 9. Evolution of compressor demand power (a, b), air temperature at DAG (c) and compressor energy consumption (d) during one cycle of PCM charging/ discharging (configurations 1 and 3).
be more energy-consuming (Table 5). It should be noted that the display cabinet used in this study is not initially designed to receive such a heat exchanger in the rear duct. As the optimization of the aeraulic circuit could affect the overall performances, optimization works related to PCM integration in display cabinet should aim to improve the energy efficiency and thermal behavior of the display cabinet. Some options that can be considered are: modification of the BPP perforation [29,30], and an increase in fan power to enhance the air flow rate. The actual fan power is 25 W which represents only 5% of the display cabinet’s power demand (compressor and fan: 525 W). The installation of a more powerful fan can enhance the heat transfer rate between the
Table 5 Energy consumption of the display cabinet (for both configurations with and without PCM) during 24 h at ambient temperature of 16 °C. set point temperature = −1 °C
set point temperature = −5 °C
3.0 104 kJ
3.6 104 kJ
and nature in particular - might explain the difference in the obtained results. In order to charge the PCM more quickly, it may be necessary to lower the temperature set point (from −1°C to −5°C) and this tends to 11
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Fig. 10. Air temperature in the rear duct during a PCM discharging process (compressor stop, configuration 3).
Fig. 11. Dimensionless air temperature evolution during a compressor stop, comparison between configuration 1 – without PCM and configuration 3 – with PCM (same positions as Fig. 10).
PCM and air inside the rear duct. Optimization options will be considered in further studies and will not be presented in this paper.
flows upwards. This is due to the PCM melting and simultaneously cooling the air along the PCM heat exchanger. For the second type (Ta4 and Ta5), the air is coming out of the heat exchanger and shows a smaller temperature variation than the first type; even after 100 min, the air temperature remains below 6.5 °C. Fig. 11 represents the comparison between the evolution of the di-
3.4. PCM effect on the air temperature inside the rear duct during a compressor stop
mensionless air temperature
Fig. 10 presents the air temperature evolution inside the rear duct during a PCM discharging process by compressor stop. Dashed curves represent air temperature measured at the heat exchanger level and continuous curves represent air temperature after the PCM heat exchanger in the rear duct. They show two types of thermal behavior. For the first one (Ta1, Ta2 and Ta3), the air temperature decreases as the air
(
a (t )
=
Ta (t ) Tamb
Ta (t = 0) Ta (t = 0)
) of two configura-
tions: configuration 1 (without PCM), and configuration 3 (with PCM), during a compressor stop. Different positions in the rear duct (as in Fig. 10) are considered. For the configuration without PCM, the same behavior is observed at different positions: the air temperature increases rapidly just after the compressor is stopped (0 to 0.1 h). This is
12
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Fig. 12. Evolution of product temperature rise during a compressor stop, comparison between configuration 1 – without PCM and configuration 3 – with PCM.
4. Conclusion
Table 6 Product temperature rise ΔT (°C) during a compressor stop. Position (cf. Fig. 12)
After 1 h without PCM
After 1 h with PCM
After 2 h without PCM
After 2 h with PCM
1f 1b 3f 3b 5f 5b
0.7 1.0 1.0 0.9 0.6 0.8
0.2 0.0 0.5 0.2 0.3 0.4
1.8 2.5 2.5 2.1 1.5 1.9
1.1 0.0 1.2 0.8 0.8 0.7
In this work, the PCM integration in an open display cabinet is studied experimentally. An important quantity of PCM was investigated (7 kg), the container was a finned tube heat exchanger and the PCM was water. The heat exchanger was placed at the rear duct of the display cabinet. First, the thermal and flow behavior of the modified display cabinet (with empty heat exchanger) was compared to the results obtained with the display cabinet before modification (without heat exchanger). The heat exchanger’s presence modified the airflow and the product temperature distribution inside the display cabinet. Then, the PCM charging and discharging processes were characterized. It was found that the PCM charge is two times longer than the PCM discharge. The PCM addition allows limiting the product temperature rise during a compressor stop by about 1 °C after 2 h at an ambient temperature of 16 °C. The temperature rise of the configuration without PCM under the same conditions was about 2 °C. These results confirm the potential of the PCM integration to maintain the product temperature inside display cabinet if energy management strategies (peak shaving, electrical curtailment…) are applied in supermarkets. While this technology can demand more energy consumption because of the need to reduce the set point temperature, solutions to enhance the energy and thermal performances can be considered, such as the modification of the display cabinet design or fan power. These optimization changes will be investigated in further studies.
because the air supply from the RAG is not cooled anymore by the evaporator. After this phase, the temperature rise becomes less important as the temperature reaches the ambient temperature. The same data of Fig. 10 are used for the configuration with PCM. It is interesting to observe how the PCM exchanger is able to dampen the slope change of the temperature curves. The PCM effect on the air temperature inside the rear duct is more significant during the first hour when the charging ratio is greater than 50%. 3.5. PCM effect on the product temperature during a compressor stop Fig. 12 shows a comparison between the temperature rise of products during a compressor stop, ΔT (ΔT = T(t) – T(t = 0), the temperature difference between the temperature at every instant and the temperature at the beginning of the compressor stop), for a modified (with PCM, configuration 3) and an unmodified (without PCM, configuration 1) display cabinet. Six positions are considered relating to the back (b) and front (f) loads on the 1st, 3rd, and 5th shelves. The PCM heat exchanger is able to provide cooling for the packages during a compressor stop; the temperature rise of products is only about 1 °C after 2 h (Table 6). For the configuration without PCM, the temperature rise after 2 h is about 2 °C. For position 1b, ΔT of the configuration with PCM stays around 0 °C during the compressor stop which can be explained by the modification of the cold air flow rate through the BPP due to the heat exchanger’s presence (Cf. 3.1). These results confirm the potential of the PCM integration in display cabinets to maintain the product temperature if energy management strategies (peak shaving, electrical curtailment…) are applied in supermarkets.
References [1] Evans JA, Foster AM, Huet JM, Reinholdt L, Fikiin K, Zilio C, et al. Specific energy consumption values for various refrigerated food cold stores. Energy Build 2014;74:141–51. [2] Gullo P, Hafner A, Banasiak K. Transcritical R744 refrigeration systems for supermarket applications: current status and future perspectives. Int J Refrig 2018;93:269–310. [3] Tassou SA, Ge Y, Hadawey A, Marriott D. Energy consumption and conservation in food retailing. Appl Therm Eng 2011;31:147–56. [4] Evans JA, Scarcelli S, Swain MVL. Temperature and energy performance of refrigerated retail display and commercial catering cabinets under test conditions. Int J Refrig 2007;30:398–408. [5] Tahir A, Bansal PK. MEPR versus EEPR valves in open supermarket refrigerated display cabinets. Appl Therm Eng 2005;25:191–203. [6] 2030 climate & energy framework, https://ec.europa.eu/clima/policies/strategies/ 2030_en, last accessed at April 17th 2019.
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R. Ben-Abdallah, et al. [7] Farid MM, Khudhair AM, Razack SAK, Al-Hallaj S. A review on phase change energy storage: materials and applications. Energy Convers Manage 2004;45:1597–615. [8] Guo S, Liu Q, Sun J, Jin H. A review on the utilization of hybrid renewable energy. Renew Sustain Energy Rev 2018;91:1121–47. [9] Ezan MA, Ozcan Doganay E, Yavuz FE, Tavman IH. A numerical study on the usage of phase change material (PCM) to prolong compressor off period in a beverage cooler. Energy Convers Manage 2017;142:95–106. [10] Alva G, Lin Y, Fang G. An overview of thermal energy storage systems. Energy 2018;144:341–78. [11] Dincer I, Rosen MA. Energetic, environmental and economic aspects of thermal energy storage systems for cooling capacity. Appl Therm Eng 2001;21:1105–17. [12] Belman-Flores JM, Barroso-Maldonado JM, Rodríguez-Muñoz AP, CamachoVázquez G. Enhancements in domestic refrigeration, approaching a sustainable refrigerator – a review. Renew Sustain Energy Rev 2015;51:955–68. [13] Azzouz K, Leducq D, Gobin D. Performance enhancement of a household refrigerator by addition of latent heat storage. Int J Refrig 2008;31:892–901. [14] Maiorino A, Del Duca MG, Mota-Babiloni A, Greco A, Aprea C. The thermal performances of a refrigerator incorporating a phase change material. Int J Refrig 2019;100:255–64. [15] Oró E, Miró L, Farid MM, Cabeza LF. Improving thermal performance of freezers using phase change materials. Int J Refrig 2012;35:984–91. [16] Gin B, Farid MM, Bansal PK. Effect of door opening and defrost cycle on a freezer with phase change panels. Energy Convers Manage 2010;51:2698–706. [17] Liu M, Saman W, Bruno F. Development of a novel refrigeration system for refrigerated trucks incorporating phase change material. Appl Energy 2012;92:336–42. [18] Fioretti R, Principi P, Copertaro B. A refrigerated container envelope with a PCM (Phase Change Material) layer: experimental and theoretical investigation in a representative town in Central Italy. Energy Convers Manage 2016;122:131–41. [19] Maderic D, Pavkovic B, Lenic K. An experimental research on energy efficiency of a beverage cooler with the latent heat storage. Appl Therm Eng 2019;148:270–7. [20] Yoon Y, Jeong H, Lee K-S. Adaptive defrost methods for improving defrosting
efficiency of household refrigerator. Energy Convers Manage 2018;157:511–6. [21] Laguerre O, Hoang MH, Flick D. Heat transfer modelling in a refrigerated display cabinet: the influence of operating conditions. J Food Eng 2012;108:353–64. [22] Lu YL, Zhang WH, Yuan P, Xue MD, Qu ZG, Tao WQ. 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 2010;30:85–91. [23] Sevault A, Banasiak K, Bakken J, Hafner A. 2018. A novel PCM accumulator for refrigerated display cabinet: design and CFD simulations. 12th IIR Conference on Phase-Change Materials and Slurries for Refrigeration and Air Conditioning Orford (Québec), Canada. [24] Alzuwaid F, Ge YT, Tassou SA, Raeisi A, Gowreesunker L. The novel use of phase change materials in a refrigerated display cabinet: an experimental investigation. Appl Therm Eng 2015;75:770–8. [25] Alzuwaid FA, Ge YT, Tassou SA, Sun J. The novel use of phase change materials in an open type refrigerated display cabinet: a theoretical investigation. Appl Energy 2016;180:76–85. [26] Ben-abdallah R, Leducq D, Hoang HM, Pateau O, Ballot-Miguet B, Delahaye A, et al. Modeling and experimental investigation for load temperature prediction at transient conditions of open refrigerated display cabinet using Modelica environment. Int J Refrig 2018;94:102–10. [27] Chaomuang N, Flick D, Denis A, Laguerre O. Experimental analysis of heat transfer and airflow in a closed refrigerated display cabinet. J Food Eng 2019;244:101–14. [28] Chaomuang N, Flick D, Denis A, Laguerre O. Influence of operating conditions on the temperature performance of a closed refrigerated display cabinet. Int J Refrig 2019;103:32–41. [29] Gray I, Luscombe P, McLean L, Sarathy CSP, Sheahen P, Srinivasan K. Improvement of air distribution in refrigerated vertical open front remote supermarket display cases. Int J Refrig 2008;31:902–10. [30] Wu X, Chang Z, Yuan P, Lu Y, Ma Q, Yin X. The optimization and effect of back panel structure on the performance of refrigerated display cabinet. Food Control 2014;40:278–85.
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Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Experimental investigation of the use of PCM in an open display cabinet for energy management purposes
T
R. Ben-Abdallaha,b, D. Leducqa, H.M. Hoanga, , L. Fournaisona, O. Pateaub, B. Ballot-Miguetb, A. Delahayea ⁎
a b
Irstea, UR FRISE, Refrigeration Process Engineering Research Unit, 1, rue Pierre-Gilles de Gennes, F-92761 Antony, France EDF –R&D, EDF Lab Les Renardières, avenue des Renardières, F-77250 Écuelles, France
ARTICLE INFO
ABSTRACT
Keywords: Display cabinet PCM Heat exchanger Flexibility Energy management
Display cabinets are widely used in supermarkets and represent an important part of their energy consumption. Adding PCM to a refrigerated display cabinet can increase its compressor cutoff time as the cold energy accumulated by PCM can replace the refrigeration system during a certain period of time. This technology can be considered as a solution to increase the electricity flexibility in order to match the demand and the production of a supply network, to manage energy flows on the grid and to boost the use of intermittent renewable energy sources. The present study is focused on the performance of the display cabinet with integrated phase change material (PCM). The PCM is selected according to the temperature range of the application. To enhance the heat transfer and facilitate the PCM melting and freezing, PCM is inserted in a heat exchanger. The experimental results show an important potential of PCM to maintain the air and product temperature when the compressor is off (up to 2 h).
1. Introduction
and economic benefits [10,11]. In this context, the integration of thermal energy storage devices using Phase Change Materials (PCM) in display cabinets presents opportunities for peak shaving and reducing supermarket energy costs. Indeed, the PCM inside display cabinets can replace the cold machine during a certain period of time by using its stored energy. This solution can increase the flexibility in the energy management of the display cabinets and of the whole supermarket if many display cabinets are equipped with TES devices. Moreover, PCM can be a solution to the intermittency of renewable energy sources and encourage supermarkets to become self-sufficient via solar or wind energy. It should be noted that many governments offer incentives to encourage this. Many studies have focused on the application of phase change materials for energy storage in refrigeration equipment such as refrigerators [12]. Azzouz et al. [13] placed PCM sheets on the back side of a refrigerator evaporator. The sheets provided higher heat exchange than the evaporator could provide solely through natural convection. The results obtained using PCM showed a higher evaporation pressure, an increased refrigeration capacity, and a significant performance enhancement. The energy stored in the PCM allows for several hours of continuous operation (between 4 and 8 h, depending on the thermal load) without electrical power supply using a 5-mm-thick slab. Maiorino et al. [14] have attached PCM to the evaporator inside the cabinet of a refrigerator; the PCM integration allows an important
Refrigeration is one of the most energy-consuming processes in the food supply chain. About 35% of the electricity consumed by the food sector is dedicated to refrigeration [1]. Nowadays, supermarkets are an important part of the economy and their prominence continues to increase. They represent around 4% of the annual electricity consumption in industrialized countries [2,3]. In food retail, display cabinets are the most widely used and energy intensive equipment [4,5]. Given the increasing demand for energy and the necessity to protect the environment, the European climate foundation claimed an ambitious objective for 2030 [6]: - At least 40% cut on greenhouse gas emissions - At least 27% share for renewable energy - At least 27% improvement in energy efficiency Considering the energy and environmental context, Thermal Energy Storage (TES) may play a key role in enhancing the renewable energy utilization [7,8], balance the energy production and demand [9], and increase the refrigeration plant efficiency. TES can enhance the system flexibility by storing heat and using it later when needed. Thus, it provides environmental
⁎
Corresponding author. Tel: +33 1 40 96 65 02; fax: +33 1 40 96 60 75. E-mail address: [email protected] (H.M. Hoang).
https://doi.org/10.1016/j.enconman.2019.111909 Received 29 May 2019; Received in revised form 31 July 2019; Accepted 1 August 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature dt E L m Q t
T
stop store
Duration, s Energy consumption, J Specific latent heat, J kg−1 Mass, kg Power demand of the compressor, W Time, s Temperature, °C
Greek symbols
T
amb avg evap min
Dimensionless temperature Time integration variable, s Temperature rise, °C
Abbreviation
Subscripts a
Compressor stop period Stored energy
BPP
Air Ambient Averaged value Evaporator Minimum value
DAG
PCM RAG VBC
reduction of temperature gradient within the cabinet and fluctuations of product temperature, and an extension of the compressor off time. Energy performance of commercial freezers can also be improved by using PCM. Oró et al. [15] placed stainless-steel-plated PCM at different locations in a freezer. They observed that the use of PCM minimized the temperature rise of the freezer and stored product, which occurred due to frequent door opening and electrical power failure (3 h). A similar conclusion was reported by Gin et al. [16] who evaluated the effectiveness of using PCM panels on the internal walls of a freezer. During defrosting, the temperature increase rate was significantly reduced while the energy consumption was only slightly reduced. PCM is also widely spread in refrigerated trucks. A PCM thermal storage unit can even substitute a conventional refrigeration system [17]. In this study, the PCM storage unit was charged by a refrigeration unit located off the vehicle. This concept has been proved as feasible for on-board refrigeration. In addition, the system has been found to consume less energy and to emit less greenhouse gases. Fioretti et al. [18] investigated the use of PCM in the external layer of a refrigerated container and showed that this novel technology can reduce and displace the heat flux generated by external conditions. Recently, Maderic et al. [19] has carried out an experimental study on a beverage cooler with latent heat storage by an ice bank; important energy consumption reduction (15%) and significant decrease of the refrigeration machine start-ups were observed. An important issue of refrigeration systems is the control of frosting and defrosting which can be greatly impacted by the integration of energy storage device. Adaptive defrost methods to improve defrost efficiency was proposed by Yoon et al. [20]. So far, only few studies concerning the application of PCM in display cabinets for chilled products have been published. The use of PCM is expected to offer some benefits. First, the energy stored in the PCM can replace the cold machine for a period of time in refrigerators, freezers and trucks. Second, depending on its placement, the PCM may provide a better air and product temperature stabilization. In particular, the PCM may attenuate the temperature rise during long periods of compressor inactivity (up to several hours) if energy management strategies (peak shaving, electrical curtailment…) are applied in supermarkets. The PCM may also offer a better homogeneity of product temperature distribution. Indeed, important temperature differences (6 °C) were measured on the shelves of a display cabinet without PCM [21]. Lu et al. [22] have integrated heat pipes and PCM within shelves and showed that the shelf equipped with heat pipes leads to lower food temperature (3–5 °C) and improves the homogeneity of temperature distribution. Sevault et al. [23] focused on a novel PCM (pure water) accumulator for a refrigerated display cabinet using CO2 as a refrigerant. Their work consists in optimizing the PCM accumulator design. They observed the influence of the distance between CO2 coils and air ducts on the charging and discharging of PCM. Mehmet et al. studied numerically the effect of adding PCM on the thermal behavior of a vertical beverage
Back perforated panel Discharge air grid Phase change material Return air grid Vertical beverage cooler
cooler (VBC) during a period when the compressor is turned off [9]. The PCM was inserted in the rear duct of the VBC. The influence of the PCM slab thickness on the display cabinet performance and the temperature homogeneity were investigated. They conclude that as the thickness of the PCM slab increases, the flow rate decreases and the pressure drop increases. Their results showed that a higher quantity of PCM reduced the running time ratio of the cold machine (relating to the cold machine on–off cycle, the running time ratio is the ratio between the running time and the cycle duration). A minimum ratio was obtained for a PCM slab of 6 mm. Alzuwaid et al. [24,25] used PCM charged into two single-panel radiators (0.93 L of water gel PCM in each radiator) at the back channel of an open vertical display cabinet. The tests results showed energy savings, lower cabinet temperatures, and a significant stabilization of product temperatures during defrost (up to 12 min). The purpose of this work is to experimentally study the PCM integration on an open display cabinet to replace the refrigeration machine for a long compressor stop period (1–2 h). Important information is still missing in published works. For example, how the presence of the PCM heat exchanger disturbs the airflow and the operation of the display cabinet. The choice of PCM, its quantity, its container, and its positioning also need to be considered. Moreover, the charging and discharging processes of PCM have not been examined yet. These issues have to be taken into consideration to facilitate the deployment of this technology. This work focuses on the characterization of the PCM behavior during charging and discharging processes and its effect on the thermal and energy performances of the display cabinet. PCM (7 kg) was introduced in a heat exchanger; the impact of the heat exchanger on the air flow rate was also studied. Moreover, the comparison of the air and product temperature rise obtained with a display cabinet without PCM and a modified display cabinet with PCM during a compressor stop of 2 h was discussed. 2. Materials and methods In order to evaluate the impact of PCM addition on the thermal behavior of the display cabinet, three configurations (Fig. 1) were considered: - Configuration 1: display cabinet without heat exchanger - Configuration 2: display cabinet with empty heat exchanger - Configuration 3: display cabinet with heat exchanger filled with PCM Temperature (product, air, and PCM) and air velocity were measured at different positions. The electrical power demand was also measured in order to calculate the daily energy consumption. 2
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Fig. 1. Open display cabinet.
2.1. Display cabinet description
vertically through the heat exchanger to the discharge air grid (DAG), then flows downwards into the return air grid (RAG). Along the rear duct and even along the heat exchanger, part of the airflow circulates horizontally through the perforated panel towards the products. Ambient air infiltrates into the air curtain (about 30% of the air curtain flow rate [21]) and mixes with the top-down flow (Fig. 2). The thermostat probe of the display cabinet is placed in the rear duct on the right side of the 1st shelf (Fig. 2) at the same location as before modification [26].
The display cabinet used in the present study (Fig. 1) is located in a test room in which the ambient temperature is fixed at 16 °C. Its dimensions are 1.3 m in length, 0.9 m in width, and 2.0 m in height. It has five shelves numbered from bottom to top and it is loaded with packages of test product (dimensions of a package: 20x10x5cm, material: methylcellulose, density: 1100 kg.m−3, thermal conductivity: 0.49 W.m−1.K−1, specific heat: 3372 J.kg−1.K−1). For each shelf, the packages were arranged into two sheets of 4 rows and 16 columns to simulate the food products at the front (near the air curtain) and at the back (near the perforated panel BPP, Fig. 2). The ambient conditions and loading configuration were taken from related work [21]. The display cabinet has no doors. For configurations 2 and 3, the heat exchanger is located in the rear air duct (up to 3rd shelf). Cold air blown by one fan comes from the outlet of the evaporator and circulates
2.2. PCM integration The size and the capacity of the PCM heat exchanger were chosen so that it can replace the cold machine for at least 1 h (dtstop= 3600 s) with the compressor turned off. Considering the display cabinet’s average power (Qavg = 500 W), the quantity of energy needing to be stored is: 3
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Fig. 2. Integration of PCM heat exchanger inside the display cabinet.
Estore = Qavg × dtstop = 1.8MJ
The PCM occupies 70% of the internal volume of the heat exchanger to take into account the water volume expansion during the phase change (which is about 10% of the water volume). It’s important to note that the energy accumulated in the metallic parts of the heat exchanger represents only 0.5% of the latent energy stored by the PCM.
(1)
Water is chosen as PCM for this application due to its properties: melting temperature (0 °C) which is appropriate for the conservation of chilled product (temperatures between 0 and 6 °C), high latent heat (LPCM = 334 000 J.kg−1), specific heat (4184 J.kg−1.K−1), large availability, safety, low price, non–toxicity (for food preservation application), and non–flammability. The minimum PCM quantity can be estimated as:
mPCM , min =
Estore = 5.4kg LPCM
2.3. Measurements
(2)
2.3.1. Temperature T-type thermocouples were used for the measurement of:
For this study, the PCM mass was fixed at 7 kg. In order to integrate PCM inside the display cabinet, many positions can be considered: rear duct, discharge air grid, return air grid, under shelves. However, the position should be selected by considering a few important criteria. The first criterion is that the position needs to provide a suitable condition of heat transfer between air and PCM so that the PCM can be charged and discharged completely at a given time. The second criterion is that a sufficient volume should be available to store 7 kg of PCM. Moreover, the ease of installation should also be taken into account. In our case, PCM is introduced inside a finned tube heat exchanger at the rear duct (Fig. 2). The tubes are horizontal while the fins are vertical so that air can pass through the heat exchanger. This configuration offers many advantages:
- temperature of products on the surface and in the core (blue dots, Fig. 3) - air temperature: at the air curtain, air supply (DAG) and return grill (RAG), in the back duct (green crosses) and room temperature (red star) Similar instrumentation can be found in other related works [21,26–28]. The PCM heat exchanger is also equipped with T-type thermocouples (black cross) inserted inside the tubes at 5 levels (Fig. 4). At each level, two positions are instrumented: in the middle and on the right side considering the symmetry of the heat exchanger. Each thermocouple is fixed at the center of the tube by a spacer (Fig. 4). All thermocouples were calibrated and have a precision of 0.3 °C.
- The heat exchanger can receive cold air leaving the evaporator. In previous works [21,26], temperature measurements have been carried out to determine the temperature range in different areas of the display cabinet for different ambient conditions (T = 15, 20 and 25 °C). Results showed that the rear duct is the best zone to introduce PCM (Table 1) as its air temperature range is suitable for water freezing (between −5.5 and 2 °C for an ambient temperature of 15 °C). The air temperature at other positions (discharge air grid, return air grid, under shelves) is too high for the PCM charging process. - An important heat transfer area is provided between air and PCM 25m2 (including the surface of the fins, manufacturer’s data). - There is enough space in the rear duct to install a heat exchanger.
Table 1 Air temperature range (°C) for an open display cabinet (without heat exchanger). Ambient
Position 2 Position 3 Position 4
T = 15 °C
T = 20 °C
min
max
min
max
min
max
−5.5 −3.5 −3
1 1.5 2
−4.5 −3 −2.7
2 2 2.5
−4 −2 −2
3.5 3.5 4.5
(Positions 2, 3 and 4 are presented in Fig. 2) 4
T = 25 °C
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Fig. 3. Modified display cabinet instrumentation.
2.3.2. Air velocity A hot-wire anemometer (Testo 435–4) was used to measure air velocities in the display cabinet. This anemometer is calibrated in the range of 0–2 m.s−1 with an uncertainty between 0.02 m.s−1 and 0.06 m.s−1.
started every 4 h with a duration of 16 min. In order to study the impact of energy management strategies (peak shaving, electrical curtailment…) on the display cabinet functioning, a longer compressor stop (up to 2 h) was also studied. For each configuration, two tests were performed. Each test comprised working periods (3 days with regular defrosts every 4 h) and a 2-h compressor stop at the end of the test. For configuration 3 (display cabinet with PCM), defrosting and compressor stop corresponded to the same period: 1.6 to 2 h. Fig. 5 shows the evolution of the air temperature near the thermostat probe and compressor power evolution. Defrosting periods, starting periodically every 7.8 h, end when the thermostat probe temperature reaches 10 °C, a new working period begins (at 0 h and 7.8 h). Because of the temperature rise related to the defrost, longer compressor cycles (duration 0.7 h, between 0 and 0.7 h, or between 7.8 and 8.5 h in Fig. 5a and 5b) are found just after defrosting periods. Then the air temperature begins to fluctuate around −5 °C (set point temperature). A zoom on compressor on–off cycles is shown in Fig. 5c. The duration of an on–off compressor cycle is about 7.5 min. For this configuration, 6 cycles of PCM charging / discharging were investigated. Good reproducibility was observed between cycles (Fig. 8). PCM charging was done during working period while PCM discharging happened during defrosting.
2.3.3. Power and energy consumption A wattmeter (Digiwatt, precision 2%) was used to measure the electrical power demand of the compressor (Q (t)). The electrical power absorbed by the fan is a constant value (25 W, obtained by previous measurement); this power was not measured in the current study. The compressor energy consumption E (t ) is calculated by integrating the measured power for a duration t.
E (t ) =
=t =0
Q ( )d
(3) 4
For a daily energy consumption of 3.6 10 kJ (set point temperature = −5°C), the uncertainty (standard deviation) is 500 J or 1.4%. 2.4. Display cabinet functioning and setting parameter The set point temperature of the display cabinet was fixed at −5 °C for all experiments. Two operating regimes were investigated: working period (compressor on–off cycles) and compressor stop/defrosting (power = 0) as shown in Fig. 5. The ventilation was on during the compressor stop/defrosting. For configurations 1 and 2 (display cabinet without heat exchanger and display cabinet with empty heat exchanger, respectively), defrosts
2.5. Charging ratio A charging ratio is defined according to the temperature of PCM at 4 positions inside the heat exchanger. The phase change is completed at one position when the PCM temperature is lower than 0 °C (freezing 5
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Fig. 4. PCM heat exchanger instrumentation.
temperature of PCM) and begins to decrease (point C in Fig. 8a); the PCM is then considered to be fully solid. During the charging process, a charging ratio of 25% will be reached when the PCM at position 2 is solid, and so on for position 3 (50%), position 4 (75%), and 5 (100%) (Fig. 4). Conversely, during the discharging process, the PCM is considered to be fully liquid when the temperature begins to increase after the phase change stage (Point F in Fig. 8a).
and at the DAG for the two cases. The obstacle due to the addition of the heat exchanger reduces the air passage section and increases the air velocity at the first and second shelves. Lower air velocities are found at the fourth and fifth shelves and at the DAG. The air flow rate measured at the DAG is 0.0097 m3.s−1 for the modified display cabinet compared to 0.0134 m3.s−1 for the unmodified display cabinet. The thermal behavior is also affected by the heat exchanger’s presence. As observed in Fig. 7, for the modified display cabinet, the package temperatures in the fifth shelf are warmer than those in the unmodified display cabinet because of the reduction of the cold air flow rate from the BPP and from the DAG. The front packages are also at a higher temperature except for the first shelf which can be explained by the diminution of the air curtain flow rate (−28%). The package temperatures at the first shelf are cooler in the modified case because the air flow rate through the BPP is higher at that position. Such observations lead to conclude that it is necessary to modify the display cabinet for PCM integration.
3. Results and discussion 3.1. Influence of the presence of the heat exchanger (empty) on the air flow and thermal behavior Tests have been performed in the display cabinet without the heat exchanger (configuration 1) and with an empty heat exchanger (configuration 2) to evaluate the impact of an obstacle in the rear duct on the airflow and on the product temperature, without the effect of PCM charge/discharge. Fig. 6 presents the air velocity along the rear duct 6
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Fig. 5. Evolution of air temperature and compressor power (configuration 3).
3.2. PCM charging and discharging processes – PCM temperature evolution
phase change begins at the position 1 (just after the evaporator) followed by the second and third positions. Similarly, during the compressor stop, PCM changes phase first at positions 1 and 2 because these two positions are most exposed to the returning air. For the PCM at the positions 3 and 4, a phase change plateau is observed and the temperature increases above 0 °C before the end of the compressor stop. During the test, PCM at position 5 was still at 0 °C at the end of the compressor stop and did not melt completely. The PCM temperature evolution is more affected by the compressor for positions 1 and 2 than other positions because these positions are first to receive the air coming from the outlet of the evaporator (arrows showing air flow direction can be observed in Fig. 8b). Smaller temperature fluctuations related to compressor on–off cycles can be observed in higher positions (3 and 4). In order to observe the progress of the charging process, Table 2 illustrates the evolution of the temperature at different positions in the heat exchanger at several times. The left side of the heat exchanger (not instrumented) is supposed to have the same temperature as the right side (instrumented). Initially (0 min), the PCM temperature was positive (red cases). After 50 min, positions 1 and 2 situated near the evaporator were the first to freeze. Then, at 125 min, position 3 starts to freeze in the middle, followed by the two others (position 4 at 185 min and position 5 at 225 min). These results show that the PCM charge is progressive and the middle is more sensitive to temperature variation than the extremities. This phenomenon is due to the position of the fan
Fig. 8a illustrates the PCM behaviour during a cycle of charge and discharge processes at the position 4 of the PCM heat exchanger. In this cycle, 6 steps can be defined: - A-B, cooling step 1 - PCM in liquid phase: the PCM temperature decreases to the phase change level (near 0 °C). - B-C, cooling step 2 - liquid to solid phase change: a temperature plateau around 0 °C is observed, latent heat is stored by the PCM. At the end of this step, the PCM is completely solid. - C-D, cooling step 3 - PCM in solid phase : the PCM temperature continues to decrease. - D-E, compressor stop step 1 - PCM in solid phase: the PCM temperature increases until it reaches the phase change level around 0 °C. - E-F, compressor stop step 2 - solid to liquid phase change: the PCM temperature stays around 0 °C - F-A : Compressor stop step 3 - PCM is melted, liquid PCM temperature increases. Fig. 8b shows the PCM temperature evolution at different positions. It’s interesing to point out that the charging and discharging of the PCM is done gradually as the temperature in the heat exchanger is not homogenous due to the direction of the airflow. During cooling, the 7
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Fig. 6. Air velocity inside an unmodified display cabinet (left, configuration 1) and a display cabinet with empty heat exchanger (right, configuration 2).
and the airflow pattern inside the display cabinet. The same behaviour is observed for the discharge process (Table 3): the discharge is also progressive and starts from the middle of the tube towards the extremities. Table 4 shows charging and discharging time for different charging
ratios. The necessary time to totally charge the heat exchanger is 225 min (3 h 45 min). However, it takes only 96 min (1 h 36 min) for the PCM to melt completely: the discharging is two times faster than the charging. This result can be explained by a larger temperature difference between the air and the PCM during discharge.
Fig. 7. Air and product temperature of unmodified display cabinet (left, configuration 1) and display cabinet with empty heat exchanger (right, configuration 2).
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Fig. 8. Temperature evolution of the PCM (configuration 3).
3.3. PCM effect on the display cabinet behavior during one charging/ discharging cycle
defrost. Configuration 3 – with PCM (Fig. 9b) – shows a longer first cycle (0.7 h) than configuration 1 – without PCM (0.1 h). The configurations having PCM need more time to reach the operating condition. Fig. 9c presents the air temperature evolution at the discharge air grid (DAG, cf. Fig. 2); the configuration with PCM shows a significantly lower temperature (mean value = 1.5 °C) than the configuration without PCM (mean value = 8.4 °C). This effect might compensate the reduction of the flow rate of the air curtain (−28%) due to the presence of the heat exchanger. The effect of the PCM on the air and product temperature during a 2-h compressor stop will be analyzed in Sections 3.4 and 3.5. Fig. 9d shows the evolution of the compressor energy consumption (Eq. (3)) during one PCM charging/discharging cycle. For the configuration with PCM, the compressor needs more power than the configuration without PCM during the charging period. Despite this increase, no significant difference in the energy consumption is
The integration of PCM in the display cabinet might generate inconveniences: 1 - it might cause a time delay for reaching the operating condition, and 2 - as the cold energy produced by the refrigeration machine is used for both maintaining the air and product temperature and charging the PCM, the energy consumption might increase. In order to answer these questions, the display cabinet behavior during one cycle of PCM charging/discharging (configuration 3) is analysed and compared to the results of configuration 1 – without PCM. Fig. 9a and b present the evolution of the compressor power demand; the beginning (t = 0) corresponds to the restarting of the refrigeration machine after a defrost. As mentioned in section 2.4, the first compressor on–off cycle is longer than other cycles because of the temperature rise during the 9
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Table 2 PCM temperature (°C) inside the heat exchanger at different moments during charging.
Table 3 PCM temperature (°C) inside the heat exchanger at different moments during discharging.
Table 4 Charging / discharging time. Charging ratio (%) 0 25 50 75 100 Charging ratio (%) 100 75 50 25 0
observed for the whole cycle because of a long discharging period during which the compressor is stopped. This result means the cold energy stored in the PCM is not lost but kept inside the display cabinet for later use – to replace the refrigeration machine during the compressor stop. In other words, the PCM integration allows deferring the use of cold energy and increases the display cabinet flexibility for energy management applications. Table 5 shows measured energy consumption during 24 h for both the modified (with PCM, configuration 3) and unmodified (without PCM, configuration 1) display cabinet. For the same experimental conditions (temperature set point and ambient temperature), PCM integration does not modify the energy consumption (the difference of energy consumption between the two configurations is less than 2%).
Charging time (min) 0 24 125 183 225 Discharging time (min) 0 25 60 68 96
The reduction of the air temperature related to the PCM integration was also observed by Alzuwaid et al. [24]. In the same study, a reduction of the energy consumption of 5% was observed for a modified display cabinet with PCM. The same authors have highlighted that the energy saving is a function of the operating conditions (ambient, cabinet settings, PCM properties…). Important differences between the current study and the work of Alzuwaid et al. [24] -the PCM quantity 10
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Fig. 9. Evolution of compressor demand power (a, b), air temperature at DAG (c) and compressor energy consumption (d) during one cycle of PCM charging/ discharging (configurations 1 and 3).
be more energy-consuming (Table 5). It should be noted that the display cabinet used in this study is not initially designed to receive such a heat exchanger in the rear duct. As the optimization of the aeraulic circuit could affect the overall performances, optimization works related to PCM integration in display cabinet should aim to improve the energy efficiency and thermal behavior of the display cabinet. Some options that can be considered are: modification of the BPP perforation [29,30], and an increase in fan power to enhance the air flow rate. The actual fan power is 25 W which represents only 5% of the display cabinet’s power demand (compressor and fan: 525 W). The installation of a more powerful fan can enhance the heat transfer rate between the
Table 5 Energy consumption of the display cabinet (for both configurations with and without PCM) during 24 h at ambient temperature of 16 °C. set point temperature = −1 °C
set point temperature = −5 °C
3.0 104 kJ
3.6 104 kJ
and nature in particular - might explain the difference in the obtained results. In order to charge the PCM more quickly, it may be necessary to lower the temperature set point (from −1°C to −5°C) and this tends to 11
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Fig. 10. Air temperature in the rear duct during a PCM discharging process (compressor stop, configuration 3).
Fig. 11. Dimensionless air temperature evolution during a compressor stop, comparison between configuration 1 – without PCM and configuration 3 – with PCM (same positions as Fig. 10).
PCM and air inside the rear duct. Optimization options will be considered in further studies and will not be presented in this paper.
flows upwards. This is due to the PCM melting and simultaneously cooling the air along the PCM heat exchanger. For the second type (Ta4 and Ta5), the air is coming out of the heat exchanger and shows a smaller temperature variation than the first type; even after 100 min, the air temperature remains below 6.5 °C. Fig. 11 represents the comparison between the evolution of the di-
3.4. PCM effect on the air temperature inside the rear duct during a compressor stop
mensionless air temperature
Fig. 10 presents the air temperature evolution inside the rear duct during a PCM discharging process by compressor stop. Dashed curves represent air temperature measured at the heat exchanger level and continuous curves represent air temperature after the PCM heat exchanger in the rear duct. They show two types of thermal behavior. For the first one (Ta1, Ta2 and Ta3), the air temperature decreases as the air
(
a (t )
=
Ta (t ) Tamb
Ta (t = 0) Ta (t = 0)
) of two configura-
tions: configuration 1 (without PCM), and configuration 3 (with PCM), during a compressor stop. Different positions in the rear duct (as in Fig. 10) are considered. For the configuration without PCM, the same behavior is observed at different positions: the air temperature increases rapidly just after the compressor is stopped (0 to 0.1 h). This is
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Fig. 12. Evolution of product temperature rise during a compressor stop, comparison between configuration 1 – without PCM and configuration 3 – with PCM.
4. Conclusion
Table 6 Product temperature rise ΔT (°C) during a compressor stop. Position (cf. Fig. 12)
After 1 h without PCM
After 1 h with PCM
After 2 h without PCM
After 2 h with PCM
1f 1b 3f 3b 5f 5b
0.7 1.0 1.0 0.9 0.6 0.8
0.2 0.0 0.5 0.2 0.3 0.4
1.8 2.5 2.5 2.1 1.5 1.9
1.1 0.0 1.2 0.8 0.8 0.7
In this work, the PCM integration in an open display cabinet is studied experimentally. An important quantity of PCM was investigated (7 kg), the container was a finned tube heat exchanger and the PCM was water. The heat exchanger was placed at the rear duct of the display cabinet. First, the thermal and flow behavior of the modified display cabinet (with empty heat exchanger) was compared to the results obtained with the display cabinet before modification (without heat exchanger). The heat exchanger’s presence modified the airflow and the product temperature distribution inside the display cabinet. Then, the PCM charging and discharging processes were characterized. It was found that the PCM charge is two times longer than the PCM discharge. The PCM addition allows limiting the product temperature rise during a compressor stop by about 1 °C after 2 h at an ambient temperature of 16 °C. The temperature rise of the configuration without PCM under the same conditions was about 2 °C. These results confirm the potential of the PCM integration to maintain the product temperature inside display cabinet if energy management strategies (peak shaving, electrical curtailment…) are applied in supermarkets. While this technology can demand more energy consumption because of the need to reduce the set point temperature, solutions to enhance the energy and thermal performances can be considered, such as the modification of the display cabinet design or fan power. These optimization changes will be investigated in further studies.
because the air supply from the RAG is not cooled anymore by the evaporator. After this phase, the temperature rise becomes less important as the temperature reaches the ambient temperature. The same data of Fig. 10 are used for the configuration with PCM. It is interesting to observe how the PCM exchanger is able to dampen the slope change of the temperature curves. The PCM effect on the air temperature inside the rear duct is more significant during the first hour when the charging ratio is greater than 50%. 3.5. PCM effect on the product temperature during a compressor stop Fig. 12 shows a comparison between the temperature rise of products during a compressor stop, ΔT (ΔT = T(t) – T(t = 0), the temperature difference between the temperature at every instant and the temperature at the beginning of the compressor stop), for a modified (with PCM, configuration 3) and an unmodified (without PCM, configuration 1) display cabinet. Six positions are considered relating to the back (b) and front (f) loads on the 1st, 3rd, and 5th shelves. The PCM heat exchanger is able to provide cooling for the packages during a compressor stop; the temperature rise of products is only about 1 °C after 2 h (Table 6). For the configuration without PCM, the temperature rise after 2 h is about 2 °C. For position 1b, ΔT of the configuration with PCM stays around 0 °C during the compressor stop which can be explained by the modification of the cold air flow rate through the BPP due to the heat exchanger’s presence (Cf. 3.1). These results confirm the potential of the PCM integration in display cabinets to maintain the product temperature if energy management strategies (peak shaving, electrical curtailment…) are applied in supermarkets.
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