energies
Article Thermal and Energy Evaluation of a Domestic Refrigerator under the Influence of the Thermal Load
Juan M. Belman-Flores 1,*, Diana Pardo-Cely 1, Miguel A. Gómez-Martínez 1, Iván Hernández-Pérez 2 , David A. Rodríguez-Valderrama 1 and Yonathan Heredia-Aricapa 1
1 Engineering Division, Campus Irapuato-Salamanca, University of Guanajuato, C.P. 36885 Salamanca, Mexico; [email protected] (D.P.-C.); [email protected] (M.A.G.-M.); [email protected] (D.A.R.-V.); [email protected] (Y.H.-A.) 2 División Académica de Ingeniería y Arquitectura, Universidad Juárez Autónoma de Tabasco, C.P. 86690 Cunduacán, Mexico; [email protected] * Correspondence: [email protected]; Tel.: +52-464-647-9940
Received: 4 December 2018; Accepted: 22 January 2019; Published: 27 January 2019
Abstract: This study seeks to understand the thermal and energetic behavior of a domestic refrigerator more widely by experimentally evaluating the main effects of the thermal load (food) and the variation of the ambient temperature. To carry out the experiments, the thermal load was classified based on the results of a survey conducted on different consumers in the state of Guanajuato, Mexico. The thermal behavior of both compartments of the refrigerator, the total energy consumption, the power of the compressor in its first on-state, and the coefficient of performance, according to the classification of the thermal loads and the room temperature, were evaluated. Finally, it is verified that the thermal load and the room temperature have a significant influence on the energy performance of the refrigerator.
Keywords: energy consumption; thermal load; domestic refrigeration system
1. Introduction The domestic refrigerator is one of the most popular household appliances because of its use in food preservation. Most of these refrigerators are based on vapor compression technology, and their continuous operation represents a high-energy consumption. Currently, it is claimed that the refrigeration sector (including air conditioning) consumes about 17% of the total electricity used worldwide, where there are currently more than 1.5 billion domestic refrigerators in use [1]. In Mexico, approximately 86% of households have at least one refrigerator, representing more than 28 million domestic refrigerators in use [2]. According to the Trust for Saving Electrical Energy (FIDE, from its Spanish initials), the refrigerator represents around 30% of the total energy consumption in a household [3]. For several decades, there was an imminent growth in the refrigeration industry, which also led to a considerable increase in energy consumption. Thus, these appliances are a point of interest in search of energy improvements. Some methods, such as energy labeling, take into account the efficiency of the product [4], which guarantees to some extent the regulations on energy saving. Thus, the labeling provides a guide to study different mechanisms that can increase a refrigerator’s energy efficiency, such as the design of the main components, thermal insulation, adequate thermal behavior, and use of alternative refrigerants, among others [5]. However, the refrigerators’ energy consumption does not only depend on the technical characteristics of the components, but it also depends on the usage habits of the consumer and the environmental conditions where the appliance is located, as it is specified on the energy label [6]. There are factors such as ambient or room temperature [7], relative humidity [8], and frost formation, [9], among others, that significantly affect the energy performance of a domestic refrigerator.
Energies 2019, 12, 400; doi:10.3390/en12030400 www.mdpi.com/journal/energies Energies 2019, 12, 400 2 of 16
In addition to the above, other factors depend on the usage habits of the consumer [10], who plays a significant role in the cold chain and the proper conservation of food. Among these factors, the following can be mentioned: the frequency in the opening of doors, the position of the thermostat, the amount of food, and the cleaning in the case of external condensers, among others. In the literature, there are works on the study of these factors; for instance, Saidur et al. [11] experimentally evaluated the temperature of the room and other factors, such as the opening of doors and the position of the thermostat, on the energy consumption of a refrigerator. The authors concluded that the temperature of the room affects, to a greater extent, the energy consumption, followed by the opening of doors. Hasanuzzaman et al. [12] analyzed the energy consumption of a domestic refrigerator by varying factors such as the number of door openings, opening duration, cabinet load, thermostat position, and room temperature. The authors found that all factors influence energy consumption, with the most notorious case (with a 40% increase) when the refrigerator operates with open doors compared to when it is used with closed doors. Later, the authors extended their study to analyze factors such as the position of the thermostat, the thermal load, and the ambient temperature on the heat transfer and the energy consumption of the refrigerator. The authors concluded that the largest contributions occurred when the thermal load varied from 0 to 12 kg, with an increase in energy consumption of 58%, and when the ambient temperature changed from 18 ◦C to 30 ◦C, with a 41% increase [13]. Khan et al. [14] presented another work, similar to the previous studies, confirming an increase in energy consumption of up to 30% depending on the frequency of door opening, an increase of 30% when the ambient temperature varied from 20 ◦C to 30 ◦C, and an increase of 59% when the load varied from 0 m3 to 0.007 m3. In the literature, there are also works with a statistical approach based on a series of surveys and related to the usage habits of consumers. For instance, Janjic et al. [15] investigated the conditions, such as temperature, cleanliness, and storage practices, under which food is subjected inside refrigerators. The authors reported that about half of the refrigerators considered in the survey had an incorrect food storage practice. Furthermore, the internal temperature of the refrigerator was considered to be high compared to the recommended temperature for this household appliance. Geppert and Stammiger [16] evaluated the behavior of the consumer in relation to the use of the refrigerator and the main characteristics of these appliances. They analyzed the conditions of the ambient temperature, the internal temperature of the compartment, and the heat sources near the refrigerator, aspects that influence the thermal and energetic performance of the household appliance. Based on the results, the authors made a series of recommendations on energy efficiency, and concluded that there is a lack of information provided to the consumers on this subject. Later, the authors extended their study to experimentally evaluate some of the operational factors that reflect the daily use of refrigerators such as the ambient temperature, the position of the thermostat, and the thermal load influenced by the amount of food. They concluded that the energy consumption is very sensitive to the ambient temperature and, to a lesser extent, the internal temperature of the refrigerator and the thermal load [17]. On the other hand, James et al. [18] made a review of diverse works where they analyzed factors such as the frequency in the opening of doors, the cleaning, the handling and storage of food, and the age of the refrigerator. This compilation was carried out aiming to analyze the thermal behavior of the refrigerator and the cleaning of the food on the impact on the consumers’ health. Thus, it is clear that factors, where the consumer is involved, reflect in a meaningful way the energetic and thermal behavior of the refrigerator, together with the environmental conditions where the appliance is located. The literature review indicates the importance of external factors on the energy consumption of domestic refrigerators, and it also shows that those factors are unrelated to the design of the refrigerator components. One of the factors that affects the energy consumption to a great extent is the room temperature where the appliance is located. Other factors of importance in the energetic operation of the refrigerator are also those related to the usage habits of the consumer. On the other hand, in the studies found in the literature, there is no justification for the thermal load (food) evaluated. Energies 2019, 12, 400 3 of 16 Energies 2019, 12 FOR PEER REVIEW 3
95 evaluated.In addition, In the addition, results presentedthe results withpresented variation with in variation thermal load in thermal focus only load on focus the energyonly on performance the energy 96 performanceof the refrigerator. of the refrigerator. 97 InIn this this paper, paper, with with knowledge knowledge based based on on survey surveyss on on usage usage habits, habits, the the energy energy consumption consumption and and 98 thethe average average temperature temperature in in both both compartments compartments of of a a domestic domestic refrigerator refrigerator are are evaluated evaluated when when the the 99 thermalthermal load load (food) (food) is is varied. varied. Moreover, Moreover, the the effect effect of of room room temperature temperature on on refrigerator refrigerator performance performance 100 isis analyzed, analyzed, with with both both factors factors (thermal (thermal load load and and temperature) temperature) recorded recorded in in the the surveys. Thus, Thus, this this 101 paperpaper provides provides a a basis basis for for a a deeper deeper analysis analysis and and a a better better understanding understanding of of the the energy energy consumption consumption 102 ofof a a refrigerator. refrigerator. This This type type of of study study should should facilitate facilitate recommendations recommendations through through the the manufacturer, manufacturer, 103 fromfrom an an energy energy and and thermal thermal viewpoint, viewpoint, on on how how to to better better use use the the appliance appliance based based on on the the amount amount of of 104 foodfood and, and, in in general, general, on on the the habits habits of of use use that that cause cause great great increases increases in in energy energy consumption consumption and and which which 105 cancan degrade degrade food food quality quality due due to to inappropriate inappropriate temperatures temperatures in in the the compartments. compartments. As As an an additional additional 106 contribution,contribution, this this study study provides provides information information to toconsumers consumers and and manufacturers manufacturers as a as reinforcement a reinforcement to 107 understandto understand how how refrigerator refrigerator usage usage habits habits affect affect thermal thermal conditions conditions and and energy energy consumption consumption and, and, 108 thus,thus, improve improve the the refrigerator refrigerator use use recommendations. recommendations. 109 TheThe rest rest of of the the paper paper is organized is organized as follows: as follows: in Section in Section 2, the 2thermal, the thermal load obtained load obtained from surveys from 110 issurveys shown. is In shown. Section In 3, Sectionthe experimental3, the experimental refrigerat refrigeratoror and the tests and performed the tests performed are presented. are presented. Section 4 111 showsSection the4 shows main the results main resultsof thermal of thermal behavior behavior of compartments of compartments and and energy energy consumption consumption of of the 112 refrigerator.refrigerator. Finally, Finally, Section Section 55 summarizessummarizes thethe mainmain conclusionconclusion ofof thethe study.study.
113 2.2. Presence Presence of of Thermal Thermal Load Load 114 AmongAmong the the different different factors factors influencing influencing the the proper proper performance performance of of a a dome domesticstic refrigerator, refrigerator, the the 115 adequateadequate distribution distribution of of airflow airflow in in the the compartments compartments is is highlighted, highlighted, which which has has an an impact impact on on thermal thermal 116 behaviorbehavior and, and, in turn, affectsaffects energyenergy consumption.consumption. In In this this respect, respect, the the thermal thermal load load (food (food stored stored in thein 117 therefrigerator’s refrigerator’s compartments) compartments) also also influences influences the the thermal thermal behavior, behavior, where where the the consumer consumer plays plays a key a 118 keyrole role in the in the use use of this of this appliance. appliance. 119 TheThe thermal thermal loads loads experimentally experimentally evaluated evaluated in in th thisis work work are are based based on on the the records records of of surveys surveys 120 appliedapplied to to 200 200 random random consumers consumers in in Salamanca, Salamanca, Gu Guanajuato,anajuato, Mexico. Mexico. Along Along with with the the questions questions asked asked 121 toto analyze analyze the the use use of of the the refrigerator refrigerator regarding regarding th thee thermal thermal load, load, and and with with previous previous consent consent of of the the 122 respondents,respondents, visual visual evidence evidence was was collected, collected, as as well well as as the the measurement measurement of of the the amount amount of of food food stored stored 123 inin the the fresh-food fresh-food compartments compartments (crisper (crisper drawers) drawers) and and in in the the freezer. freezer. Figure Figure 1 shows the conditions 124 ofof thermal thermal load load in in both both compartments, compartments, for for which which the the filling filling of of the the refrigerator refrigerator was was classified classified in in four four 125 ranges,ranges, as as shown shown in in the the figure. figure. Th Thee light-blue light-blue color color corresponds corresponds to to the the thermal thermal load load in in the the food food 126 compartment,compartment, whereas whereas the the dark-blue dark-blue color color repres representsents the the amount amount of food in the freezer.
120 51% 100 42% 80 34% 28% 60 26% 40 Refrigerators 9% 20 4% 6% 0 all the time full regularly full frequently keep regularly empty a medium load food compartment freezer 127 Figure 1. Distribution of thermal load in the refrigerator. 128 Figure 1. Distribution of thermal load in the refrigerator.
129 The higher percentages correspond to the consumers frequently keeping their refrigerator at a 130 medium load—51% of consumers (102 refrigerators) for the food compartment and 42% (84
Energies 2019, 12, 400 4 of 16
The higher percentages correspond to the consumers frequently keeping their refrigerator at a medium load—51% of consumers (102 refrigerators) for the food compartment and 42% (84 refrigerators) for the freezer. On the other hand, a low percentage of consumers keep their refrigerator full all the time—9% for the food compartment and 4% for the freezer. According to the statistic shown in Figure1, an average of eight thermal loads were defined in this work (see Table 2). Moreover, during the surveys, the temperature of the room where the refrigerator was located was also measured; this way, an average temperature sample was set during some experimental tests to analyze their effect on the refrigerator’s performance.
3. Experimental Refrigerator According to the surveys, it was observed that a great percentage of consumers have medium-sized refrigerators at home (two-doors and no-frost type). For this reason, a refrigerator meeting most of the features of the survey’s refrigerators was used, as shown in Figure2. The two-door experimental refrigerator had a volume capacity of 0.3 m3 (300 L), separating the fresh-food compartment at the bottom and the frozen-food compartment at the top. The refrigerator was a no-frost type and the heat transfer in the freezer occurred via forced convection. Table1 shows more general features of the experimental refrigerator.
Table 1. General features of the experimental refrigerator.
External Dimensions Width 0.54 m Length 0.67 m Height 1.64 m Net weight 52.4 kg System Characteristics Refrigeration Forced convection Melting element By electrical resistance Defrost Automatic Refrigerant R134a Voltage/Current 127~/60 Hz/1.1 A
3.1. Instrumentation and Measurements The refrigerator was used in this research to evaluate the thermal behavior of the compartments, as well as the energy consumption when the thermal load varied in both compartments according to the surveys. To measure the temperature, 15 J-type thermocouples were used with an uncertainty of measurement of ±0.3 K. Eleven thermocouples were distributed in the food compartment and were located within containers of 0.245 L with a mixture of 50% water and 50% glycol. Four thermocouples were placed in the freezer inside wooden cubes, due to their high capacity of humidity absorption, thus allowing a constant measurement of the temperature. In Figure2a, the distribution of the thermocouples in both compartments is illustrated; moreover, the distribution of the water compartments can be seen, simulating the thermal load for a specific case. On the other hand, to measure the energy consumption, a Fluke 1735 energy logger (Fluke, Everett, WA, USA) calibrated with a measurement error of ±1.5% was utilized. The thermocouples were connected to an NI-9213 card attached to the chassis NI cRIO-9030 (National Instruments (NI), Austin, TX, USA). Via a Universal Serial Bus (USB) connection to a computer, a real-time visualization was possible with the SignalExpress software (National Instruments (NI), Austin, TX, USA) programmed in LabView. The temperature measurement was recorded in intervals of 10 seconds, whereas the measurement of the energy consumption was set in intervals of one minute; the data were stored on a Secure Digital (SD) card. Both the temperature and energy consumption measurements were done simultaneously. Energies 2019, 12, 400 5 of 16 Energies 2019, 12 FOR PEER REVIEW 5
(a) (b)
158 FigureFigure 2. Experimental 2. Experimental test test bench: bench: (a)( temperaturea) temperature distribution; distribution; (b) ( binstrumentation.) instrumentation.
159 3.2.The Proposed thermocouples Tests were connected to an NI-9213 card attached to the chassis NI cRIO-9030 160 (NationalAs mentionedInstruments before, (NI), theAustin, aim ofTX, this USA). study Via was a toUniversal evaluate Serial the effect Bus of(USB) the thermal connection load to (food) a 161 computer,on the thermal a real-time and energy visualization behavior ofwas a domestic possible refrigerator. with the InSignalExpress this sense, the software foods were (National simulated 162 Instrumentswith containers (NI), fullAustin, of water TX, USA) and whose programmed volume capacitiesin LabView. were The 0.3, temperature 1, 1.8, and 4 measurement L. was 163 recordedAccording in intervals to theof 10 information seconds, whereas gathered the in measurement the surveys, of different the energy ranges consumption of thermal was load set were in 164 intervalsclassified of (seeone minute; Figure1 ),the where data thewere total stored variation on a Secure of the averageDigital (SD) thermal card. load Both (food the temperature compartment and and 165 energyfreezer) consumption went from ameasurements minimal load were of 7 kg done (Regularly simultaneously. empty, 5 kg in the fresh-food compartment and 2 kg in the freezer) to a maximum load of 39 kg (All the time full, 27 kg in the fresh-food compartment 166 3.2.and Proposed 12 kg inTests the freezer). Additionally, ambient temperatures of 20 ◦C and 25 ◦C, with a variation in ◦ 167 intervalsAs mentioned of ±0.5 before,C, were the frequently aim of this measured study was in the to evaluate room (giving the effect to surveys) of the wherethermal the load refrigerator (food) 168 onwas the locatedthermal and,and energy in relation behavior to these of a ambient domestic temperatures, refrigerator. In the this loads sense, were the also foods grouped. were simulated The above 169 withdata containers can be observed full of water in Table and2, whose where avolume reference capacities condition were is included, 0.3, 1, 1.8, that and is, 4 whenL. the refrigerator 170 remainsAccording empty. to the information gathered in the surveys, different ranges of thermal load were 171 classified (see Figure 1), where the total variation of the average thermal load (food compartment and Table 2. Thermal loads in both compartments under two conditions of room temperature. 172 freezer) went from a minimal load of 7 kg (Regularly empty, 5 kg in the fresh-food compartment and 173 2 kg in the freezer)Room Temperature to a maximum load of 39 kg (All 20 the◦C time full, 27 kg in the fresh-food 25 ◦C compartment 174 and 12 kg in the freezer). Additionally, ambientFresh-Food temperaturesFreezer of 20 °C andFresh-Food 25 °C, with a variationFreezer in Thermal Load 175 intervals of ±0.5 °C, were frequently measuredCompartment in the (kg) room (giving(kg) to surveys)Compartment where (kg) the refrigerator(kg) 176 was located and,Reference in relation to these ambient temp 0eratures, the 0 loads were also 0 grouped. The 0above 177 data can be observedRegularly in empty Table 2, where a reference 5 condition is included, 2 that is, 11 when the refrigerator 2 178 remainsFrequently empty. keep at medium load 27 2 18 2 179 All the testsRegularly were performed full in the same 27way. Firstly, the 5refrigerator was 18 loaded with a certain 7 All the time full 27 7 27 12 180 amount of food, as shown in Table 2. Once the refrigerator was loaded, the test initiated with the 181 start-up of the refrigerator and at the corresponding room temperature of the load, according to Table 182 2. NoteAll that, the for tests each were test, performed the refrigerator in the sameand the way. thermal Firstly, load the refrigeratorwere at room was temperature. loaded with The a certain test 183 continuedamount ofuntil food, the as thermal shown stability in Table 2was. Once reached the refrigerator in both compartments was loaded, and, the testduring initiated the test, with the the 184 doorsstart-up of the of therefrigerator refrigerator were and kept at the closed. corresponding Also, the damper room temperature (control element) of the load, remained according in the to Tablefifth 2. 185 position,Note that, exactly for each as it test, was the when refrigerator the refrigerator and the thermal left the load factory. were After at room finishing temperature. the test, The the test 186 refrigerator was unplugged and defrosted so that the refrigerator could reach room temperature.
Energies 2019, 12 FOR PEER REVIEW 6
187 Table 2. Thermal loads in both compartments under two conditions of room temperature.
Room temperature 20 °C 25 °C Fresh-Food Fresh-Food Thermal Load Freezer (kg) Compartment (kg) Compartment (kg) Freezer (kg) Energies 2019, 12, 400 6 of 16 Reference 0 0 0 0 Regularly empty 5 2 11 2 Frequently keep at continued until the thermal stability27 was reached in both2 compartments and,18 during the test, the2 doors of themedium refrigerator load were kept closed. Also, the damper (control element) remained in the fifth position, exactlyRegularly as it was full when the refrigerator 27 left the factory. 5 After finishing the 18 test, the refrigerator 7 was unpluggedAll the andtime defrosted full so that the 27 refrigerator could 7 reach room temperature. 27 12
188 4.4. ResultsResults andand DiscussionDiscussion 189 InIn thisthis section,section, thethe mainmain resultsresults comingcoming fromfrom thethe thermalthermal behaviorbehavior ofof bothboth compartmentscompartments ofof thethe 190 refrigeratorrefrigerator areare presented,presented, asas wellwell asas thethe energyenergy consumptionconsumption forfor differentdifferent conditionsconditions ofof thethe thermalthermal 191 loadload andand room temperature. Each Each test test was was done done in in triplicate, triplicate, aiming aiming to toyield yield greater greater reliability reliability in the in 192 theresults, results, which which reflect reflect the the average average of of the the temp temperatureerature and and energy energy measurements. Moreover, thethe 193 presentedpresented resultsresults areare thosethose obtainedobtained whenwhen thethe thermalthermalstability stability was wasachieved achieved in in both both compartments. compartments.
194 4.1.4.1. EffectEffect ofof ThermalThermal LoadLoad onon ThermalThermal BehaviorBehavior ofof thethe CompartmentsCompartments 195 FigureFigure3 3shows shows the the conditions conditions of temperatureof temperature in both in both compartments compartments of the of refrigerator the refrigerator for a room for a ◦ 196 temperatureroom temperature of 20 ± 1of C.20 The± 1 compartment °C. The compartmen temperaturet temperature represents the represents average of the the average thermocouples of the 197 placedthermocouples within them. placed The within horizontal them. The axis horizontal of the figure axis representsof the figure the represents thermal loads,the thermal where loads, 0 kg 198 correspondswhere 0 kg corresponds to an empty refrigeratorto an empty (withoutrefrigerator thermal (without load) thermal in both load) compartments, in both compartments, and 7, 29, 32, andand 199 347, 29, kg 32, correspond and 34 kg statistically correspond to statistically the average to loadthe average (fresh-food load compartment(fresh-food compartment and freezer) and of eachfreezer) of 200 theof each classifications of the classifications of the thermal of the load thermal shown load in Figure shown1 andin Figure Table 21. and The Table light-blue 2. The color light-blue represents color 201 therepresents temperature the temperature of the food compartment, of the food andcompar the dark-bluetment, and color the represents dark-blue the color temperature represents of thethe 202 freezer.temperature In Figure of the3, itfreezer. is observed In Figure that 3, the it is temperature observed that of thethe foodtemperature compartment of the showedfood compartment relatively ◦ 203 smallshowed changes relatively as the small thermal changes load increased,as the thermal remaining load increased, at a maximum remaining difference at a ofmaximum 2 C between difference loads ◦ 204 7of and 2 °C 29 kg.between Furthermore, loads 7 theand freezer 29 kg. experienced Furthermor a maximume, the freezer thermal experienced variability a ofmaximum 4 C between thermal the 205 loadsvariability of 7 and of 4 29 °C kg. between the loads of 7 and 29 kg.
206
207 FigureFigure 3.3. ThermalThermal behaviorbehavior ofof thethe compartmentscompartments atat 2020◦ °C.C.
208 OnOn thethe otherother hand,hand, inin FigureFigure4 4,, the the thermal thermal behavior behavior of of both both compartments compartments at at different different thermal thermal 209 loadsloads andand atat aa roomroom temperature temperature of of 25 25± ± 1 ◦°CC is illustrated. It is worth mentioningmentioning thatthat thesethese thermalthermal 210 loadsloads areare the the most most representative representative for thefor roomthe room temperature temperature measured measured in the surveys.in the surveys. It can be observedIt can be in the figure that both compartments represented a variable thermal condition, without having a clear correspondence between the thermal load and the compartment temperature. The maximum temperature variation in the food compartment was 2.2 ◦C (0 and 20 kg), while, in the freezer, it was 2.7 ◦C (0 and 25 kg). Energies 2019, 12 FOR PEER REVIEW 7
211 observed in the figure that both compartments represented a variable thermal condition, without 212 having a clear correspondence between the thermal load and the compartment temperature. The Energies 2019, 12, 400 7 of 16 213 maximum temperature variation in the food compartment was 2.2 °C (0 and 20 kg), while, in the 214 freezer, it was 2.7 °C (0 and 25 kg). 215 Consistent with these behaviors, it can be confirmedconfirmed that the refrigeratorrefrigerator is capablecapable enough to 216 maintain an operational range of adequate temperaturestemperatures in both compartments,compartments, regardless of the 217 amount of thermal load.load.
218 ◦ 219 Figure 4. Thermal behavior of the compartments atat 2525 °C.C. 4.2. Effect of Thermal Load on Energy Consumption 220 4.2. Effect of Thermal Load on Energy Consumption The cooling capacity of a refrigerator is directly proportional to the cabinet inner thermal load 221 The cooling capacity of a refrigerator is directly proportional to the cabinet inner thermal load (mass), which depends on the food initial temperature, the cabinet temperature, the specific heat, and 222 (mass), which depends on the food initial temperature, the cabinet temperature, the specific heat, and the latent heat of the thermal load (water). Moreover, this mass is heated during the off-state of the 223 the latent heat of the thermal load (water). Moreover, this mass is heated during the off-state of the compressor for cooling again during the on-state. Therefore, the energy consumption must increase as 224 compressor for cooling again during the on-state. Therefore, the energy consumption must increase the thermal load in the refrigerator increases. In this sense, Figure5 shows the total energy consumed 225 as the thermal load in the refrigerator increases. In this sense, Figure 5 shows the total energy by the refrigerator for the different thermal loads. Moreover, the energy behavior is shown for two 226 consumed by the refrigerator for the different thermal loads. Moreover, the energy behavior is shown conditions of room temperature. It can be clearly observed that, when the thermal load increased, the 227 for two conditions of room temperature. It can be clearly observed that, when the thermal load energy consumption also increased; these are similar behaviors found by References [12,14]. In Figure5, 228 increased, the energy consumption also increased; these are similar behaviors found by References it can be noticed that the magnitude of energy behavior at a temperature of 25 ◦C was higher than at a 229 [12,14]. In Figure 5, it can be noticed that the magnitude of energy behavior at a temperature of 25 °C room temperature of 20 ◦C. Here, it is clear that the increase in room temperature caused an increase 230 was higher than at a room temperature of 20 °C. Here, it is clear that the increase in room temperature in the thermal leap between the ambient and the cabinet; thus, a significant amount of heat was 231 caused an increase in the thermal leap between the ambient and the cabinet; thus, a significant transferred via conduction through the refrigerator’s walls. For example, in Figure5 it can be observed 232 amount of heat was transferred via conduction through the refrigerator’s walls. For example, in that, for the reference load (0 kg), there was an increase of 0.4 kWh for a temperature condition ranging 233 Figure 5 it can be observed that, for the reference load (0 kg), there was an increase of 0.4 kWh for a from 20 ◦C to 25 ◦C; on the other hand, for the ambient condition of 20 ◦C, there was an increase 234 temperature condition ranging from 20 °C to 25 °C; on the other hand, for the ambient condition of ranging from 0.4 kWh (0 kg) to 3.5 kWh (34 kg); this values ranged from 0.8 kWh (0 kg) to 4.5 kWh 235 20 °C, there was an increase ranging from 0.4 kWh (0 kg) to 3.5 kWh (34 kg); this values ranged from (39 kg) for the temperature of 25 ◦C. Note that these energy consumptions vary in accordance with 236 0.8 kWh (0 kg) to 4.5 kWh (39 kg) for the temperature of 25 °C. Note that these energy consumptions the thermal stabilization time of each test (see Table3). Based on Figure5, it was concluded that 237 vary in accordance with the thermal stabilization time of each test (see Table 3). Based on Figure 5, it the thermal load represents a strong influence on the refrigerator’s energy consumption. Finally, 238 was concluded that the thermal load represents a strong influence on the refrigerator’s energy it can be said that, as the thermal load increases, so does the evaporation temperature. Therefore, 239 consumption. Finally, it can be said that, as the thermal load increases, so does the evaporation the refrigeration cycle responds according to the evaporation temperature. 240 temperature. Therefore, the refrigeration cycle responds according to the evaporation temperature. In Table3, more information about the refrigerator’s energy behavior is provided. It can be noted that, for the reference test (0 kg), the time estimated to reach thermal stability increased around 4 h for a condition of ambient temperature (room temperature) fluctuating from 20 ◦C to 25 ◦C. This increase caused the switch-on (on-state) percentage of the compressor to rise to 4%, which represents an increase of 0.029 kWh per operating hour. On/off cycles of the compressor clearly evidence the thermal behavior of the refrigerator compartments, which is linked to the temperature control in relation to the position of the damper. It is, therefore, consistent that the time of thermal stability increases as the thermal load increases and due to the increase in ambient temperature. With regard the work cycles shown in the table, a correlation referring to the load increase does not exist. Note Energies 2019, 12, 400 8 of 16
that the percentage switch-on and the cycles decreased as the thermal load increased (e.g., from 29 to 32 kg (20 ◦C) and from 20 to 25 kg (25 ◦C)). For these conditions, the thermal load of the food compartment remained constant, while the freezer load increased in each test (see Table2). Note that, for this particular refrigerator, the compressor regulation work is linked to the temperature of the food compartment and to the temperature of the freezer. Energies 2019, 12 FOR PEER REVIEW 8
241
242 Figure 5. Energy consumption for different thermal loads.
243 In Table 3,Table more 3. informationEnergy behavior about at the different refrigerator’s loads and energy constant behavior ambient is temperature. provided. It can be noted 244 that, forRoom the reference test (0 kg), the timeThermal estimated to reach thermal stability increased around 4 h Thermal Load Total Energy Cycles 245 for aTemperature condition of ambient temperatureStability (room Time temperature)% Switch-On fluctuating from 20 °C to 25 °C. This (kg) (kWh) (24 h) 246 increase caused(◦C) the switch-on (on-state) percentage(h) of the compressor to rise to 4%, which represents 247 an increase of 0.029 kWh per0 operating hour. 8 On/off cycles 32 of the compressor 0.4 clearly evidence 24 the 248 thermal behavior of the refrigerator7 compartments, 21 which 35is linked to the 1.4temperature control 21 in 249 relation to20 the position of 29the damper. It is, 24 therefore, consistent 40 that the time 2.2 of thermal 27 stability 32 33 37 2.8 24 250 increases as the thermal load increases and due to the increase in ambient temperature. With regard 34 38 38 3.6 25 251 the work cycles shown in the table, a correlation referring to the load increase does not exist. Note 0 12 36 0.8 24 252 that the percentage switch-on and the cycles decreased as the thermal load increased (e.g., from 29 to 13 25 39 2.0 34 253 32 kg (2025 °C) and from 2020 to 25 kg (25 °C)). 25 For these conditions, 42 the thermal 2.3 load of 34the food 254 compartment remained constant,25 while the 38freezer load increased 37 in each test 3.3 (see Table 2). Note 26 that, 255 for this particular refrigerator,39 the compressor 46 regulation work 36 is linked to 4.5 the temperature 24 of the 256 food compartment and to the temperature of the freezer. 4.3. Effect of Thermal Load on the First On-State of the Compressor 257 Table 3. Energy behavior at different loads and constant ambient temperature. The stage consuming the most energy in a household refrigerator originated when the foods were stored for theirRoom cooling. Therefore, it is recommendedThermal that this% processTotal be quick to avoid inappropriate Thermal conservation.Temperature For this reason, the first on-stateStability of the compressorSwitch- whenEnergy the refrigeratorCycles (24 ish) started is Load (kg) larger than the(°C) following ones. This occurs whenTime the(h) refrigeratorOn contains(kWh) too much thermal load (food), as shown in Figure6. The figure0 shows the power8 of the compressor32 for0.4 the different24 thermal loads mentioned above. Figure6a corresponds7 to a21 room temperature35 of 20 1.4◦C, and Figure216b corresponds to a temperature20 of 25 ◦C. In both29 figures, it is24 clearly evident40 that the power2.2 input of27 the first on-state was linked to the amount of food32 stored in the33 refrigerator. Some37 studies2.8 mentioned that24 the additional energy consumption originated34 during the food-cooling38 stage38 [17]. This3.6 cooling stage25 is particularly evident during the first on-state0 of the compressor.12 In both36 figures, at0.8 the beginning24 of each cycle, there was a high-power peak due13 to the normal25 behavior of39 the electric2.0 motor. In addition,34 an increase in power was observed25 as the20 load increased;25 this conditions42 the on/off2.3 cycles of34 the compressor, requiring greater power to lower25 the temperature38 for a greater37 quantity3.3 of food. 26 39 46 36 4.5 24
258 4.3. Effect of Thermal Load on the First On-State of the Compressor 259 The stage consuming the most energy in a household refrigerator originated when the foods 260 were stored for their cooling. Therefore, it is recommended that this process be quick to avoid 261 inappropriate conservation. For this reason, the first on-state of the compressor when the refrigerator 262 is started is larger than the following ones. This occurs when the refrigerator contains too much
Energies 2019, 12 FOR PEER REVIEW 9
263 thermal load (food), as shown in Figure 6. The figure shows the power of the compressor for the 264 different thermal loads mentioned above. Figure 6a corresponds to a room temperature of 20 °C, and 265 Figure 6b corresponds to a temperature of 25 °C. In both figures, it is clearly evident that the power 266 input of the first on-state was linked to the amount of food stored in the refrigerator. Some studies 267 mentioned that the additional energy consumption originated during the food-cooling stage [17]. 268 This cooling stage is particularly evident during the first on-state of the compressor. In both figures, 269 at the beginning of each cycle, there was a high-power peak due to the normal behavior of the electric 270 motor. In addition, an increase in power was observed as the load increased; this conditions the on/off 271 cycles of the compressor, requiring greater power to lower the temperature for a greater quantity of Energies 2019, 12, 400 9 of 16 272 food.
(a)
(b)
273 Figure 6. Power during the firstfirst on-state of the compressor: (a) room temperature at 2020 ◦°C;C; (b) room 274 temperature atat 2525 ◦°C.C.
275 As mentionedmentioned before,before, the the ambient ambient temperature temperature affects affects the the refrigerator’s refrigerator’s energy energy behavior behavior to alarge to a 276 extent,large extent, as can as be can seen be inseen Figure in Figures6a,b. For 6a theand specific 6b. For case the specific of the reference case of the load reference (0 kg), thereload was(0 kg), a 277 differencethere was ina difference the average in ofthe the average power of of the approximately power of approximately 11 W, which indicates 11 W, which a power indicates increase a power when ◦ 278 theincrease room when or ambient the room temperature or ambient increased temperature by 5 increaC. Thissed reflects by 5 °C. an This increase reflects of 0.05 an increase kWh in energyof 0.05 279 consumption.kWh in energy Thus, consumption. it is well known Thus, thatit is the well domestic known refrigerator’s that the domestic electricity refrigerator’s consumption electricity is very 280 sensitiveconsumption to the is ambientvery sensitive temperature to the ambient [11]. Another temper aspectature to[11]. observe Another in Figure aspect6 bto is observe that the in thermal Figure 281 load6b is ofthat 39 the kg representedthermal load the of 39 condition kg represent that consumeded the condition the most that energy consumed and whosethe most on-time energy of and the 282 compressorwhose on-time was of approximately the compressor 21 h.was Moreover, approximately in this case,21 h. itMoreover, can be noticed in this that, case, at it around can be 18 noticed hours, 283 athat, defrost at around occurred, 18 indicatedhours, a defrost by the poweroccurred, increase, indicated which by in the turn power caused increase, the compressor which in to turn shut caused down. 284 Finally,the compressor it can be to concluded shut down. that Finally, the on-time it can be of theconcluded compressor that the on theon-time first startof the increased compressor when on the thermal load increased; this behavior was reflected in the total energy consumption of the refrigerator.
4.4. Effect of Ambient Temperature on Thermal Behavior of the Compartments with a Constant Thermal Load In order to expand this study based on survey data, in Figure7, the average temperature of the food compartment for a constant thermal load and under two different conditions of room temperature is illustrated. The fading of the gray color in the bars represents a condition of low room temperature, whereas the discoloration of the blue color represents a higher room temperature. In Figure7, it can be observed that, for a certain load, an increase in the room temperature caused a rise in the temperature of the fresh-food compartment (FF). For example, for the load of 13 kg, the increase from 16 to 25 ◦C in room temperature caused an increase of approximately 1.5 ◦C in the food compartment. Note that the Energies 2019, 12 FOR PEER REVIEW 10
285 first start increased when the thermal load increased; this behavior was reflected in the total energy 286 consumption of the refrigerator.
287 4.4. Effect of Ambient Temperature on Thermal Behavior of the Compartments with a Constant Thermal Load 288 In order to expand this study based on survey data, in Figure 7, the average temperature of the 289 food compartment for a constant thermal load and under two different conditions of room 290 temperature is illustrated. The fading of the gray color in the bars represents a condition of low room 291 temperature, whereas the discoloration of the blue color represents a higher room temperature. In 292 Figure 7, it can be observed that, for a certain load, an increase in the room temperature caused a rise Energies 2019, 12, 400 10 of 16 293 in the temperature of the fresh-food compartment (FF). For example, for the load of 13 kg, the increase 294 from 16 to 25 °C in room temperature caused an increase of approximately 1.5 °C in the food 295 compartment.foregoing is based Note on that the the thermal foregoing stability is based in the on foodthe thermal compartment stability and in the for food a fixed compartment condition of and the 296 fordamper a fixed set condition at 5. For the of thermalthe damper loads set of at 20 5. and For 23 the kg, thermal a very similar loads thermalof 20 and behavior 23 kg, a existed, very similar where 297 thermalthe average behavior temperature existed, of where the compartment the average experienced temperature an of increase the compartment for both room experienced temperatures, an 298 increaseshowing for an both average room thermal temperatures, difference showing of 0.6 ◦ anC inaver bothage cases. thermal Finally, difference for the of thermal 0.6 °C in load both of cases. 30 kg, 299 Finally,it can be for noted the thatthermal the averageload of 30 temperature kg, it can be reached noted inthat the the food average compartment temperature for both reached room in conditions the food 300 compartmentwas almost the for same, both aroundroom conditions 6 ◦C. was almost the same, around 6 °C.
301
302 FigureFigure 7. TemperatureTemperature in the fresh-food compartment ma maintainingintaining the the constant constant thermal thermal load load at at two two 303 ambientambient temperature temperature conditions.
304 WithWith respect to the average thermal behavior of of the the freezer (FZ), Figure 88 showsshows thethe ambientambient 305 temperaturetemperature effect effect for for constant constant thermal thermal loads. loads. It It ca cann be be seen seen that, that, when when a a constant constant thermal thermal load load was was 306 subjectedsubjected to to two two room room temperatures, the the lowest lowest average freezer freezer temperature corresponded corresponded to to the the 307 highesthighest room temperature condition.condition. ForFor example, example, for for the the loads loads of of 13, 13, 20, 20, and and 23 23 kg, kg, a difference a difference in thein 308 thetemperature temperature of 2, of 2.3, 2, and2.3, and 2.6 ◦ 2.6C, respectively,°C, respectively was, obtainedwas obtained in this incompartment, this compartment, observing observing the effect the 309 effectof the of room the temperature room temperature rise. For rise. the loadFor ofthe 30 load kg, asof also 30 seenkg, as in also Figure seen7, the in thermalFigure 7, difference the thermal was 310 differencepractically was very practically small (about very 0.6 small◦C). (about 0.6 °C). 311 BasedBased on on the the reported reported results results in in this this section section and and in inthe the previous previous sections, sections, it can it canbe said be said that, that, for 312 differentfor different thermal thermal loads loads and and different different ambient ambient temp temperatures,eratures, the the refrigerator refrigerator has has the the capacity capacity to to 313 maintainmaintain an an adequate thermal behavior for food preservation. However, However, this this is is achieved achieved with with a a 314 variation in energy consumption according to the ambient condition and the habit of use in the Energiesvariation 2019 in, 12 energy FOR PEER consumption REVIEW according to the ambient condition and the habit of use in the thermal11 315 thermalload management. load management.
316
317 FigureFigure 8. TemperatureTemperature in in the the freezer freezer compartment compartment ma maintainingintaining a a constant constant thermal thermal load load at at two two 318 temperaturetemperature conditions. conditions.
319 4.5. Effect of Ambient Temperature on Energy Consumption with a Constant Thermal Load 320 Figure 9 shows the energy consumption when the refrigerator worked with constant thermal 321 loads and at two room temperature conditions. The fading in gray and blue colors indicates the room 322 temperatures. The energy increase percentage for each thermal load subjected to two conditions of 323 ambient temperature is indicated at the top of the bars. It can be clearly observed that the increase of 324 the ambient temperature caused an increase in energy consumption. For example, the load of 20 kg 325 represented a greater increase in energy consumption, around 72% for an ambient thermal difference 326 of 13 °C (room temperature between 12 °C and 25 °C). For this increase in the ambient temperature, 327 the energy consumption of the refrigerator varied from 1.3 kWh/day to 2.3 kWh/day, a rise of 73 328 Wh/day per °C. 329 In the literature, there are works that reported information about the energy consumption for 330 refrigerators of volumetric capacity different from the one evaluated in this work. The range of 331 thermal load was also different. In order to show the results and compare the orders of magnitude, 332 Hasanuzzaman et al. [12] experimentally concluded that, when the ambient temperature experienced 333 an increase of 12 °C, the energy consumption varied from 2.13 kW/day to 3.64 kW/day, increasing 334 around 126 Wh/day per °C rise in the ambient temperature of a 460-L refrigerator. Masjuki et al. [8] 335 found that, when the ambient temperature increased 15 °C (from 16 to 31 °C), the energy 336 consumption increased from 0.56 kWh/day to 1.12 kWh/day, representing an energy consumption 337 around 40 Wh/day per °C rise in the ambient temperature for a 150-L refrigerator. Meier [19] 338 concluded that in an increase in the ambient temperature of 11 °C (from 17 °C to 28 °C), the energy 339 consumption varied from 1.25 kWh to 2.6 kWh/day. Finally, based on Figure 9, it was concluded that 340 there is a strong influence of the ambient or room temperature on the domestic refrigerator’s energy 341 consumption.
Energies 2019, 12, 400 11 of 16
4.5. Effect of Ambient Temperature on Energy Consumption with a Constant Thermal Load Figure9 shows the energy consumption when the refrigerator worked with constant thermal loads and at two room temperature conditions. The fading in gray and blue colors indicates the room temperatures. The energy increase percentage for each thermal load subjected to two conditions of ambient temperature is indicated at the top of the bars. It can be clearly observed that the increase of the ambient temperature caused an increase in energy consumption. For example, the load of 20 kg represented a greater increase in energy consumption, around 72% for an ambient thermal difference of 13 ◦C (room temperature between 12 ◦C and 25 ◦C). For this increase in the ambient temperature, the energy consumption of the refrigerator varied from 1.3 kWh/day to 2.3 kWh/day, a rise of 73 Wh/day per ◦C. In the literature, there are works that reported information about the energy consumption for refrigerators of volumetric capacity different from the one evaluated in this work. The range of thermal load was also different. In order to show the results and compare the orders of magnitude, Hasanuzzaman et al. [12] experimentally concluded that, when the ambient temperature experienced an increase of 12 ◦C, the energy consumption varied from 2.13 kW/day to 3.64 kW/day, increasing around 126 Wh/day per ◦C rise in the ambient temperature of a 460-L refrigerator. Masjuki et al. [8] found that, when the ambient temperature increased 15 ◦C (from 16 to 31 ◦C), the energy consumption increased from 0.56 kWh/day to 1.12 kWh/day, representing an energy consumption around 40 Wh/day per ◦C rise in the ambient temperature for a 150-L refrigerator. Meier [19] concluded that in an increase in the ambient temperature of 11 ◦C (from 17 ◦C to 28 ◦C), the energy consumption varied from 1.25 kWh to 2.6 kWh/day. Finally, based on Figure9, it was concluded that there is a strong Energiesinfluence 2019 of, 12 the FOR ambient PEER REVIEW or room temperature on the domestic refrigerator’s energy consumption. 12
342 343 FigureFigure 9. 9. EnergyEnergy consumption consumption at at different different co conditionsnditions of of ambient ambient temperature. temperature.
344 OnOn the the other other hand, hand, the the quantity quantity of ofheat heat to tobe beremoved removed can can be estimated be estimated from from knowledge knowledge of the of 345 thermalthe thermal load load(water), (water), including including its initial its initial state at state the atentrance the entrance of the ofcompartments, the compartments, final state, final mass, state, 346 specificmass, specific heat above heat above and below and below freez freezinging temperature, temperature, and and latent latent heat heat [8]. [8 ].Thus, Thus, the the coefficient coefficient of of 347 performanceperformance ( (COPCOP)) can can be be calculated calculated as as follows follows for for heat heat removed removed from from the food compartment: 𝑄 𝑚 𝐶𝑝 𝑇 𝑇 . (1) QFF = mFFCp water(Tamb − TFF ). (1) 348 That for heat removed from the freezer can be calculated as follows: That for heat removed from the freezer can be calculated as follows:
𝑄 𝑄 𝑄 𝑄 ; (2) Q = Q + Q + Q ; (2) FZ FZ1 FZ 2 FZ3 𝑄 𝑚 𝐶𝑝 𝑇 0°𝐶 ; (3) = − ◦ QFZ1 mFZCpwater(Tamb 0 C); (3) 𝑄 𝑚 ℎ ; (4) 𝑄 𝑚 𝐶𝑝 0°𝐶 𝑇 . (5) 349 The capacity of the refrigeration system for the thermal load can be determined from the time 350 set aside for heat removal: 𝐶𝐶 . (6) 351 Therefore, the COP can be estimated as 𝐶𝑂𝑃 . (7) ⁄ 352 Figure 10 illustrates the behavior of the coefficient of performance (COP) of the domestic 353 refrigerator under study. It can be seen that, for a constant load, there was a decrease in COP as the 354 room temperature increased. For example, for a thermal load of 13 kg, there was a decrease in COP 355 of 10% when going from 16 °C to 25 °C. An increase in room temperature reduced the overall 356 refrigeration system COP by increasing the difference between the evaporating and condensing 357 temperature.
Energies 2019, 12, 400 12 of 16
QFZ2 = mFZh f ; (4) ◦ QFZ3 = mFZCp f rost(0 C − TFZ). (5) The capacity of the refrigeration system for the thermal load can be determined from the time set aside for heat removal: Q + Q CC = FF FZ . (6) 3600 t Therefore, the COP can be estimated as
CC COP = . (7) E/t
Figure 10 illustrates the behavior of the coefficient of performance (COP) of the domestic refrigerator under study. It can be seen that, for a constant load, there was a decrease in COP as the room temperature increased. For example, for a thermal load of 13 kg, there was a decrease in COP of ◦ ◦ 10%Energies when 2019 going, 12 FOR from PEER 16 REVIEWC to25 C. An increase in room temperature reduced the overall refrigeration13 system COP by increasing the difference between the evaporating and condensing temperature.
358 359 FigureFigure 10.10. CoefficientCoefficient of performance at at differen differentt conditions conditions of ofambient ambient temperature. temperature.
360 TableTable4 shows4 shows the the energy energy behavior behavior forfor thethe evaluated thermal thermal loads loads in in this this section. section. It can It can be benoted noted 361 thatthat an an increase increase in in ambient ambient oror roomroom temperature al alsoso caused caused a arise rise in in the the thermal thermal stabilization stabilization time time 362 and,and, thus, thus, the the percentage percentage ofof thethe compressor’s start-up start-up increased, increased, resulting resulting in inan anincrease increase in the in thetotal total 363 energyenergy consumption. consumption. InIn otherother words, to maintain the the desired desired temperature, temperature, the the on/off on/off frequency frequency of of 364 thethe compressor compressor (cycles),(cycles), asas wellwell as the starting time, time, increased. increased. This This happened happened because because the theenergy energy 365 consumptionconsumption increased increased withwith thethe rise in room room temper temperature.ature. Another Another existing existing behavior behavior was was that that when when 366 thethe thermal thermal load load rose,rose, andand underunder ambient temperature temperature conditions conditions lower lower than than 20 20°C,◦ C,there there was was an an 367 increaseincrease in in the the number number ofof cyclescycles inin one day; this this means means that that the the on/off on/off cycles cycles of ofthe the compressor compressor were were 368 shortershorter and and more more common. common. This This isis becausebecause thethe changechange raterate of the refrigerator temperature temperature is is linked linked to 369 theto foodthe food thermal thermal capacity. capacity. When When the refrigeratorthe refrigerator is full is full of food, of food, it causes it causes a greater a greater loss loss of energy of energy due to 370 thedue on/off to the cycles. on/off cycles. Therefore, Therefore, energy energy consumption consumption increases increases when when thermal thermal load load increases, increases, to be to able be to 371 able to control the desired temperature. control the desired temperature. In Figure 11, the power of the first start of the compressor is illustrated, for the load variation 372 Table 4. Energy behavior for different thermal loads. shown in Table4. Figure 11a shows the difference in both the starting time and the power for a Room Thermal % ◦ Total◦ thermal loadThermal of 13 kg, which presented an increase from 16 C to 25 C in the ambient temperature. Temperature Stability Switch- Energy Cycles (24 h) This behaviorLoad was (kg) also observed in the other thermal loads subjected to different conditions of room temperature. For example, for(°C) the thermalTime loads (h) of 20 kgOn and 30 kg(kWh) (Figure 11b,d), a greatest increase 16 24 30 1.4 19 in the compressor13 operation time, 5 h approximately, was presented. The thermal load of 30 kg appears to be the condition that prolonged25 this time25 the most for39 the two2.0 conditions of34 room temperature, 12 24 27 1.3 23 representing an increase20 of 0.62 kWh. Hence, the influence of the ambient temperature on the consumed 25 25 42 2.3 34 power by the compressor on the first start was confirmed with these behaviors. 18 22 33 1.9 28 23 22 26 35 2.4 33 20 24 40 2.1 27 30 23 25 44 2.6 23
373 In Figure 11, the power of the first start of the compressor is illustrated, for the load variation 374 shown in Table 4. Figure 11a shows the difference in both the starting time and the power for a 375 thermal load of 13 kg, which presented an increase from 16 °C to 25 °C in the ambient temperature. 376 This behavior was also observed in the other thermal loads subjected to different conditions of room 377 temperature. For example, for the thermal loads of 20 kg and 30 kg (Figures 11b and 11d), a greatest 378 increase in the compressor operation time, 5 h approximately, was presented. The thermal load of 30 379 kg appears to be the condition that prolonged this time the most for the two conditions of room 380 temperature, representing an increase of 0.62 kWh. Hence, the influence of the ambient temperature 381 on the consumed power by the compressor on the first start was confirmed with these behaviors.
Energies 2019, 12, 400 13 of 16
Table 4. Energy behavior for different thermal loads.
Room Thermal Thermal Load Total Energy Cycles Temperature Stability Time % Switch-On (kg) (kWh) (24 h) (◦C) (h) 16 24 30 1.4 19 13 25 25 39 2.0 34 12 24 27 1.3 23 20 25 25 42 2.3 34 18 22 33 1.9 28 23 22 26 35 2.4 33 20 24 40 2.1 27 30 23 25 44 2.6 23 Energies 2019, 12 FOR PEER REVIEW 14
382
383 Figure 11. 11. ConsumedConsumed power power in in the the start start of ofthe the compressor compressor for for different different thermal thermal loads: loads: (a) 13 (a )kg; 13 ( kg;b) 384 20(b) kg; 20 kg;(c) 23 (c) kg; 23 kg;(d) 30 (d) kg. 30 kg.
385 InIn accordance with with the previous sections, the the ex existingisting importance importance of of the the amount of of food with 386 regardsregards to the refrigerator energy consumption was was confirmed confirmed and, and, to a lesser extent, the effect that 387 thisthis causes causes in in the the compartment’s compartment’s thermal thermal behavior. behavior. Th Thisis type type of of refrigerator refrigerator works works within within a a range range of of 388 adequate temperatures for food preservation. 389 According to the surveys done before before this work, only 20% 20% of of consumers consumers purchase purchase a a refrigerator refrigerator 390 with the ideal storage capacity (f (foodood storage capacity). On the the othe otherr hand, hand, 100% 100% of of consumers consumers ignore ignore 391 thethe effect effect of of the the amount amount of of food food and and ambient ambient temper temperatureature on on the the appliance appliance performance. performance. In In this this sense, sense, 392 thisthis type type of of study should facilitate facilitate recommendatio recommendationsns through through the manufacturer from an energy and 393 thermalthermal viewpoint viewpoint on on how how to to better better use use the the appliance appliance based based on on the the amount amount of of food food and, and, in in general, general, 394 on the habits of use that cause great increases in energy consumption and that can degrade the food 395 quality due to inappropriate temperaturestemperatures in the compartments. 396 Finally, based on the experimental data, a mult multipleiple linear regression equation equation was was developed, developed, 397 which shows the combined effect of the ambient temperaturetemperature (Tambamb)) and and the the thermal thermal load load ( (LL)) on on the 398 energy consumption ( E) for this refrigerator. This equation hashas aa correlationcorrelation degreedegree ofof 85%.85%.
𝐸 1.55 0.0395𝑚 0.104𝑇 . (8)
399 5. Conclusions 400 In this work, the effect of the thermal load (food) on energy consumption and the thermal 401 condition of the compartments of a domestic refrigerator was experimentally evaluated. The amount 402 of the evaluated thermal load was based on previous surveys applied to consumers to define the 403 amount of food in both compartments. The main objective of this work was to provide more 404 information about domestic refrigeration in term of the effect of the thermal load and the room 405 temperature on the refrigerator performance. For this, a typical refrigerator of 0.3 m3 was used, where 406 the tests were conducted in triplicate for each assay to achieve reliability. Among the most relevant 407 aspects in this study, the following are mentioned: 408 • A survey was applied to determine the amount of food commonly stored in the refrigerator 409 compartments. This way, the different thermal loads (from a minimal load of 7 kg to a maximum
Energies 2019, 12, 400 14 of 16
E = −1.55 + 0.0395m + 0.104Tamb. (8)
5. Conclusions In this work, the effect of the thermal load (food) on energy consumption and the thermal condition of the compartments of a domestic refrigerator was experimentally evaluated. The amount of the evaluated thermal load was based on previous surveys applied to consumers to define the amount of food in both compartments. The main objective of this work was to provide more information about domestic refrigeration in term of the effect of the thermal load and the room temperature on the refrigerator performance. For this, a typical refrigerator of 0.3 m3 was used, where the tests were conducted in triplicate for each assay to achieve reliability. Among the most relevant aspects in this study, the following are mentioned:
• A survey was applied to determine the amount of food commonly stored in the refrigerator compartments. This way, the different thermal loads (from a minimal load of 7 kg to a maximum of 39 kg) were evaluated in this work, an aspect which was not previously justified in the literature. • The thermal behavior in both compartments of the refrigerator did not show any correspondence with respect to the thermal load increase, while keeping a constant room temperature. • It was observed that the energy consumption had a great effect due to the thermal load increase where, for example, for an ambient condition of 20 ◦C, an increase from 0.4 kWh (0 kg as reference) to 3.5 kWh (34 kg) was observed; this increase was from 0.8 kWh (0 kg) to 4.5 kWh (39 kg) for a temperature of 25 ◦C. • The first on-state of the compressor increased when the thermal load increased; this behavior was reflected in the refrigerator’s total energy consumption. • For a constant thermal load, the increase in room temperature caused an increase in the temperature of the fresh-food compartment. For a load of 13 kg, the increase in room temperature from 16 to 25 ◦C caused a rise of 1.5 ◦C in the food compartment’s average temperature. For the case of the freezer, a maximum decrease of 2.6 ◦C was obtained for a load of 23 kg, whose room temperature increased from 18 to 22 ◦C. • Finally, increases in the refrigerator’s energy consumption were clearly observed for a constant thermal load and an increase in room temperature.
Author Contributions: Conceptualization, J.M.B.-F. and D.P.-C.; methodology, D.A.R.-V. and Y.H.-A.; resources, M.A.G.-M.; analysis of data, I.H.-P., J.M.B.-F. and D.P.-C. wrote the paper. All authors have read and approved the final manuscript. Funding: This research received no external funding. Acknowledgments: We acknowledge the University of Guanajuato for their sponsorship in the realization of this work. Conflicts of Interest: The authors declare no conflict of interest.
Nomenclature
CC Cooling load (kW) COP Coefficient of performance Cp Specific heat (kJ/kgK) E Energy consumption (kWh) hf Latent heat of fusion of the of the water (kJ/kg) m Thermal load (kg) Q Heat removal (kJ) T Temperature (◦C) t Thermal stability time (h) Energies 2019, 12, 400 15 of 16
Subscripts amb Ambient FF Fresh-food compartment FZ Freezer compartment FZ1 Heat removal from the initial temperature to the freezing point of thermal load FZ2 Heat removal to freeze the thermal load FZ3 Heat removal from the freezing point to the final temperature below the freezing point frost Referring to ice water Referring to water
References
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