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Role of ambient temperature, door opening, thermostat setting position and their combined effect on refrigerator-freezer energy consumption
Energy Conversion and Management 43 (2002) 845–854 www.elsevier.com/locate/enconman
Role of ambient temperature, door opening, thermostat setting position and their combined effect on refrigerator-freezer energy consumption R. Saidur *, H.H. Masjuki, I.A. Choudhury Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 24 October 2000; accepted 19 March 2001
Abstract Refrigerator-freezers energy consumption is greatly affected by room temperature, door opening and thermostat setting position. Two frost free household refrigerator freezers of the same capacity were tested in the laboratory to determine the sensitivity of their energy consumption to various usage conditions. The experiments were conducted to investigate the effect of single variables, such as temperature, thermostat setting positions and door opening, and their combined effect on energy consumption. Our investigation reveals that room temperature has the higher effect on energy consumption, followed by door opening. Thermostat setting position has the lower effect on energy consumption. More detailed tests were performed under different room temperature, thermostat setting position and door opening conditions. With the experimental results, a first order mathematical model has been developed to investigate their combined effect on energy consumption. The test results are discussed and presented. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Door opening; Energy; Refrigerator; Temperature; Thermostat setting
1. Introduction The residential sector of Malaysia consumes about 20% of her total electricity. Despite an economic downturn, energy consumption in this sector continues to grow [1]. Refrigerator freezers are one of the major energy users among household appliances because this appliance is almost common in every house in Malaysia. One study by Agus et al. [2] shows that about 76% of houses are equipped with household refrigerator freezers in Malaysia. The Asia-Pacific Economic *
Corresponding author. Tel.: +60-3-7959-5283; fax: +60-3-7959-5317. E-mail address: [email protected] (R. Saidur).
0196-8904/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 0 1 ) 0 0 0 6 9 - 3
846
R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
Nomenclature b matrix of parameter estimates do door opening E daily energy consumption (W h) EE energy consumption by Model E (W h) ES energy consumption by Model S (W h) room temperature tr thermostat setting position ts x matrix of independent variables transpose of x xT T 1 (x x) inverse of (xT x) y observed daily energy consumption Cooperation (APEC) region produces about 60 million refrigerator freezers out of 100 million refrigerator freezers produced in the world each year [3]. This appliance also has to operate for 24 h, compared to air conditioners and washing machines. Therefore, energy consumption by household refrigerator freezers has recently attracted considerable attention for their efficiency improvement. This efficiency improvement could be established by introducing energy efficiency standards to encourage consumers to use more efficient units. The energy efficiency standards, eventually, will eliminate the least efficient products from the market. To reduce the energy consumption of refrigerator freezers and to design more efficient models, it is important to understand the energy consumption behavior of these devices. In order to do so, a refrigeratorfreezers energy test procedure must be established. Keeping this in mind, the parameters affecting greatly, the refrigerator-freezers energy consumption have been investigated. Refrigerator-freezers energy consumption is dependent on room temperature, door opening, thermostat setting position and relative humidity. Relative humidity has an insignificant effect on energy consumption compared to the other factors [4]. So, we did not consider the effect of relative humidity on energy consumption in this study. Using response surface methodology (RSM) and factorial design of experiment, a first order equation has been developed to observe their combined effect on energy consumption. RSM is a collection of mathematical and statistical techniques, useful for analyzing a problem in which several independent variables influence a dependent variable or response, and the goal is to optimize this response. Many experiments involve a study of the effect of multiple factors. In such a situation, factorial designs are most efficient tools to investigate their combined effect on a response. In this model, a 23 factorial design is used to study the effect of room temperature, thermostat setting position and door opening on energy consumption [5]. 2. Mathematical model for design of experiment If all the variables are assumed measurable, the response surface can be expressed as y ¼ f ðx1 ; x2 ; x3 ; . . . ; xk Þ
ð1Þ
R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
847
The goal is to optimize the response variable y. It is assumed that the independent variables are continuous and controllable by the experimenter with negligible error. The response or dependent variable is assumed to be a random variable. The observed response y as a function of room temperature, thermostat setting positions and door opening can be written as y ¼ f ðx1 ; x2 ; x3 Þ
ð2Þ
If the expected response is denoted by EðyÞ ¼ g, then the surface represented by g ¼ f ðx1 ; x2 ; x3 Þ is called a response surface. It is required to find a suitable approximation for the true functional relationship between y and the set of independent variables, the xi ’s. Usually, a low order polynomial (first order) in some regions of the independent variables is employed. The first order model, k X bi xi þ e ð3Þ y ¼ b0 þ i¼1
is generally utilized in RSM problems. The b parameters of the polynomials are estimated by the method of least squares, and e is a random error. The details of the solution by this matrix approach are explained in Ref. [5]. The matrix approach of solving Eq. (3) has been adopted in the present analysis. y is defined to be an (n 1) vector of observations on y, x to be an (n p) matrix of independent variables, b to be a (p 1) vector of parameters to be estimated and e to be an (n 1) vector of errors. Eq. (3) can be written in matrix form as y ¼ bx þ e
ð4Þ
The least squares estimates of b is the value b which, when substituted into Eq. (3), minimizes e0 e. The normal equation can be expressed as ðxT xÞb ¼ xT y
ð5Þ T
where b is replaced by the b matrix. If (x x) is non-singular, the solution of the normal equation can be written as 1
b ¼ ðxT xÞ xT y
ð6Þ
where xT is the transpose of matrix x and ðxT xÞ1 is the inverse of matrix (xT x). y is defined to be an (n 1) matrix on a logarithmic scale, x to be an (n m) matrix of independent variables, b to be an (n 1) matrix. The solution of this matrix has been performed by using M A T L A B . 3. Experimental set up and instrumentation 3.1. Experiment with single variable The objective of this experiment is to determine the effect of room temperature, thermostat setting position and door opening on the energy consumption of two household refrigerators. The tests were conducted by varying the room temperature, thermostat setting position and door opening independently. While one variable was varied, the other two variables were kept constant. The room temperature was varied from 14°C to 32°C in an environmentally controlled chamber
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located in our laboratory. The fresh food compartment and freezer compartment thermostat setting positions were set fixed at the medium setting. There are 10 thermostat setting positions ranging from 1 to 10 for the fresh food compartment of these two refrigerator freezers. The freezer compartment thermostat positions are minimum, medium and maximum. The thermostat setting is varied by turning the knob to the desired setting position from 1 to 10 for the fresh food compartments to investigate the effect of thermostat setting position on energy consumption. The freezer compartment thermostat settings were set at the minimum position when the fresh food compartment setting position was 1–4. The freezer compartment thermostats were set at the medium position when the fresh food compartment thermostat setting position was 5–7, and the freezer thermostat positions were set at the maximum position when the fresh food thermostat setting position was 8–10. Room temperature was maintained at 26°C in all the tests. The door opening controls are set so that the door remains open for 12 s at an angle of 90°. The total run time was set to be 600 min (10 h) from the commencement of the experiment. Door closing time was a variable parameter in order to maintain 20–75 door openings in the total time mentioned above. Table 1 shows the door opening and closing schedule for the total run time. In this experiment, the door openings were considered for 10 h from the commencement of operation. This was considered a more realistic usage condition in the household environment. On the other hand, Grimes et al. [6] conducted the door openings for 8 h and maintained 24 door openings. Alissi [4] did the same experiment with 16 h of operation and maintained 100 door openings. However, the literature survey says that the usual door opening should be within 40–60 times per day. So, our door opening schedule can be considered as more realistic compared to their studies. Room temperature was maintained at 26°C for all the tests in door opening, and the thermostat setting positions were kept at the medium position for both of the compartments. The technical specifications of these two models have been shown in Table 2. 3.2. Experiment with multivariable This experiment was designed to investigate the combined effect of room temperature, thermostat setting position and door opening on the energy consumption of the refrigerator freezers. 3.2.1. Experimental design and conditions A design consisting of 12 experiments has been used to develop the first order model to investigate the combined effect of room temperature, thermostat setting position and door opening Table 1 Door opening schedule Number of opening
Total run time (min)
Closing times (min)
Door remains open (s)
20 30 40 50 60 75
600 600 600 600 600 600
30 20 15 12 10 8
12 12 12 12 12 12
R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
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Table 2 Technical specifications of Model E and Model S Specifications
Model E
Model S
Freezer capacity (l) Fresh food compartment capacity (l) Power rating (W) Current rating (A) Voltage (V) Frequency (Hz) Defrost system Number of doors
80 220 160 0.9 220 50 Auto defrost 2
80 220 160 0.9 220 50 Auto defrost 2
on energy consumption. Eight experiments represent a factorial design, four experiments represent repetition to estimate the pure error. The experimental design and conditions for the multivariable tests have been shown in Table 3. A factorial design of experiment is one in which all levels of a given factor are combined with all levels of every other factor in the experiment. This design is necessary when interactions between the variables are to be investigated. Furthermore, factorial designs allow the effects of a factor to be estimated at several levels of the other factors, giving conclusions that are valid over a range of experimental conditions. The transforming equations for each of the independent variables are x1 ¼
lnðtr Þ lnð22Þ ; lnð30Þ lnð22Þ
x2 ¼
lnðts Þ lnð5Þ ; lnð10Þ lnð5Þ
x3 ¼
lnðdo Þ lnð45Þ lnð70Þ lnð45Þ
ð7Þ
3.3. Instrumentation A heat pump was used to maintain a steady temperature in the environmentally controlled chamber in order to investigate the effect of room temperature. The temperature variation was controlled by using an Omega type temperature controller. Daily energy consumption was Table 3 Experimental design and coding identification Number
tr (°C)
ts (number)
do (number)
Coding x1
x2
x3
1 2 3 4 5 6 7 8 9 10 11 12
16 30 16 30 16 30 16 30 22 22 22 22
2.5 2.5 10 10 2.5 2.5 10 10 5 5 5 5
30 30 30 30 70 70 70 70 45 45 45 45
1 1 1 1 1 1 1 1 0 0 0 0
1 1 1 1 1 1 1 1 0 0 0 0
1 1 1 1 1 1 1 1 0 0 0 0
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R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
measured by a digital power meter, which was interfaced with a PC. Lab view software was installed in the PC for data storage and analysis. An automated door opening and closing mechanism has been developed. AC motors with gear boxes were mounted on a steel frame, which was built at the top of the refrigerator freezer. Mechanical arms have been prepared in such a way that doors open and close subsequently. The door opening process was controlled using the programmable logic controller (PLC). Programming is achieved using ladder logic. The operating switch, which is an input device, sends signals to the motor to open and close the doors. Total run time, opening time and closing time are inserted into the operating switch, and it works as the experiment requires.
4. Results of single variable on energy consumption 4.1. Effect of room temperature Most of the thermal load on a refrigerator is conduction through the refrigerator wall. ASHRAE [7] shows that about 60–70% of the total refrigerator load comes by conduction through the cabinet walls. This conduction load is proportional to the difference between the ambient temperature and the internal compartment/freezer temperature. The higher the difference, the higher is the load imposed on a refrigerator. For this reason, the temperature of air around a refrigerator is a significant determinant of energy consumption. Since compressor efficiency also declines as the ambient temperature rises, a refrigerator’s electricity use is very sensitive to the ambient temperature. Our experimental data in Fig. 1 shows that energy consumption increases 47 W h for Model E and 53 W h for Model S for each degree increase in room temperature. Meier et al. [8] showed that energy consumption increased 120 W h for each degree increase in temperature. Meier et al. tested the existing old refrigerator freezer in the household environment. All those refrigerator freezers were manufactured before 1993. On the other hand, we tested the latest model from the market. 4.2. Effect of thermostat setting Just like a house, a refrigerator will use less electricity if its thermostat is re-set to a higher (warmer) temperature. Grimes et al. [6] examined the impact of compartment temperature on energy use on a 1977 vintage automatic defrost refrigerator. Energy consumption rose 26% from the warmest acceptable to the coldest possible settings. A more recent study of nine large 1993 vintage US refrigerators [9] found a 6.5% increase in energy consumption for 1°C reduction in freezer temperature. Our data analysis shows that energy consumption increases about 7.8% for each degree reduction in freezer temperature. Fig. 2 shows our experimental data for the two models. 4.3. Effect of door openings When the refrigerator door is opened, warm and moist air exchanges with the cool air inside the refrigerator cabinet. When the door is closed, a mass of air at ambient temperature is trapped in
R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
851
Fig. 1. Variation of energy consumption with room temperature.
Fig. 2. Variation of energy consumption with thermostat setting position.
the compartment. Heat gains during door openings are due to (i) heat/vapor transfer on the interior surfaces of the refrigerator, and (ii) bulk air exchange. The following five categories of loads are associated with door openings, as of Ref. [4]. 1. convective heat transfer from the warm ambient air flowing across the cooler refrigerator surfaces; 2. latent heat transfer with condensation of water vapor from the moist air flowing across the cooler refrigerator surfaces; 3. radiative heat transfer from the surroundings to the interior surfaces; 4. sensible heat transfer from the warm air mass within the cooled space after the door is closed.
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Fig. 3. Variation of energy consumption with door opening.
Parker and Stedman [10] estimated that each door opening causes 9 W h increase of energy consumption. In our experiment, we found 9 W h increase in energy consumption for each door opening for Model E and 12.4 W h increase in energy consumption for Model S for each door opening. Fig. 3 shows the effect of door opening on energy consumption.
5. Experimental results of multivariable effect on energy consumption From the experimental results shown in Table 4, model equations (8) have been developed using Eqs. (3) and (7). The b parameter has been calculated and presented in Table 5. y1 ¼ 3:889 þ 0:714 ln tr þ 0:241 ln ts þ 0:254do ;
ð8Þ
y2 ¼ 3:98 þ 0:773 ln tr þ 0:204ts þ 0:210do
Table 4 Experimental results Trial number
EE (W h)
ES (W h)
1 2 3 4 5 6 7 8 9 10 11 12
976 1610 1415 2213 1178 2140 1690 2438 1503 1513 1477 1479
1008 1788 1513 2327 1261 2251 1832 2577 1607 1616 1588 1543
R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
853
Table 5 Values of b Value of b
For Model E
For Model S
b0 b1 b2 b3
7.382 0.223 0.167 0.112
7.432 0.240 0.143 0.093
The above equations can also be written in the form of energy consumption as a function of room temperature (tr ), thermostat setting position (ts ) and door opening (do ) as follows: EE ¼ 48:86tr0:714 ts0:241 do0:254 ; ES ¼ 53:52tr0:773 ts0:204 do0:210
ð9Þ
Eq. (9) indicates that an increase in room temperature, thermostat setting position and door opening increases the energy consumption. It is also noted that the thermostat setting position and door openings (trivial many) contribute almost a similar magnitude of influence on energy consumption, and the room temperature (vital few) has the higher effect on energy consumption.
6. Conclusions The following conclusions could be made from this study. 1. Room temperature has the higher effect on energy consumption, followed by door opening and thermostat setting position. The door opening and thermostat setting position have very little difference in their relative effect on energy consumption. 2. As Malaysia is going to adopt energy efficiency standards and labels for refrigeration appliances, a more realistic test procedure is needed in order to evaluate their energy consumption behavior. This paper can be considered as an experimental support to develop test standards for household refrigerator freezers in Malaysia, as their energy consumption is greatly dependent on these parameters (i.e. room temperature, thermostat setting position and door opening). 3. The door opening pattern in this study can be considered as more comparable with the actual usage condition that exists in the household environment. 4. The exponent in Eq. (9), which is close to 1, has the higher effect on relative magnitude, and the exponents, which are close zero, have the lower effect on energy consumption.
Acknowledgements The authors would like to acknowledge the Ministry of Science, Technology and Environment, Malaysia for funding the project. The research has been conducted under the IRPA Project no. 02-02-03-0471.
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References [1] Department of electricity and gas supply, Statistics of electricity supply industry in Malaysia, Ministry of Energy, Kuala Lumpur, Malaysia; 1999. p. 19–20. [2] Agus MR, Othman MN, Sim OF. Residential and commercial electricity consumers in Subang Jaya and Bandar Baru Bangi, Kuala Lumpur, Malaysia; 1993. p. 16. [3] Zakaria MY, Farida T. Keynote paper ‘‘Forum on energy performance and labeling standards for refrigeration’’, 27th June, 2000, SIRIM, Grand Blue Hotel, Malaysia. [4] Alissi MS. The effect of ambient temperature, ambient humidity and door openings on household refrigerator energy consumption. MSME Thesis. Purdue University, West Lafayette, Indiana; 1987. p. 134. [5] Montgomery DC. Design and analysis of experiments. 2nd ed. New York: Wiley; 1984. p. 445. [6] Grimes JG, William PEM, Shomaker BL. Effect of usage conditions on household refrigerator-freezer and freezer energy consumption. ASHRAE Trans 1977;83(1):818–28. [7] Household refrigerators and freezers. ASHRAE equipment handbook. 1988. p. 37.4. [8] Meier A, Megowan A, Lit B, Pon B. The New York refrigerator monitoring project: final report. LBL-33708. Lawrence Berkeley Laboratory, CA, 1993. [9] Meier AK. Do refrigerator thermostat setups save energy?. Home Energy 1994;11(3):11. [10] Parker DS, Stedman T. Refrigerator replacement in Florida: a case study. Home Energy 1993;10(1):20–2.
Role of ambient temperature, door opening, thermostat setting position and their combined effect on refrigerator-freezer energy consumption R. Saidur *, H.H. Masjuki, I.A. Choudhury Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 24 October 2000; accepted 19 March 2001
Abstract Refrigerator-freezers energy consumption is greatly affected by room temperature, door opening and thermostat setting position. Two frost free household refrigerator freezers of the same capacity were tested in the laboratory to determine the sensitivity of their energy consumption to various usage conditions. The experiments were conducted to investigate the effect of single variables, such as temperature, thermostat setting positions and door opening, and their combined effect on energy consumption. Our investigation reveals that room temperature has the higher effect on energy consumption, followed by door opening. Thermostat setting position has the lower effect on energy consumption. More detailed tests were performed under different room temperature, thermostat setting position and door opening conditions. With the experimental results, a first order mathematical model has been developed to investigate their combined effect on energy consumption. The test results are discussed and presented. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Door opening; Energy; Refrigerator; Temperature; Thermostat setting
1. Introduction The residential sector of Malaysia consumes about 20% of her total electricity. Despite an economic downturn, energy consumption in this sector continues to grow [1]. Refrigerator freezers are one of the major energy users among household appliances because this appliance is almost common in every house in Malaysia. One study by Agus et al. [2] shows that about 76% of houses are equipped with household refrigerator freezers in Malaysia. The Asia-Pacific Economic *
Corresponding author. Tel.: +60-3-7959-5283; fax: +60-3-7959-5317. E-mail address: [email protected] (R. Saidur).
0196-8904/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 0 1 ) 0 0 0 6 9 - 3
846
R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
Nomenclature b matrix of parameter estimates do door opening E daily energy consumption (W h) EE energy consumption by Model E (W h) ES energy consumption by Model S (W h) room temperature tr thermostat setting position ts x matrix of independent variables transpose of x xT T 1 (x x) inverse of (xT x) y observed daily energy consumption Cooperation (APEC) region produces about 60 million refrigerator freezers out of 100 million refrigerator freezers produced in the world each year [3]. This appliance also has to operate for 24 h, compared to air conditioners and washing machines. Therefore, energy consumption by household refrigerator freezers has recently attracted considerable attention for their efficiency improvement. This efficiency improvement could be established by introducing energy efficiency standards to encourage consumers to use more efficient units. The energy efficiency standards, eventually, will eliminate the least efficient products from the market. To reduce the energy consumption of refrigerator freezers and to design more efficient models, it is important to understand the energy consumption behavior of these devices. In order to do so, a refrigeratorfreezers energy test procedure must be established. Keeping this in mind, the parameters affecting greatly, the refrigerator-freezers energy consumption have been investigated. Refrigerator-freezers energy consumption is dependent on room temperature, door opening, thermostat setting position and relative humidity. Relative humidity has an insignificant effect on energy consumption compared to the other factors [4]. So, we did not consider the effect of relative humidity on energy consumption in this study. Using response surface methodology (RSM) and factorial design of experiment, a first order equation has been developed to observe their combined effect on energy consumption. RSM is a collection of mathematical and statistical techniques, useful for analyzing a problem in which several independent variables influence a dependent variable or response, and the goal is to optimize this response. Many experiments involve a study of the effect of multiple factors. In such a situation, factorial designs are most efficient tools to investigate their combined effect on a response. In this model, a 23 factorial design is used to study the effect of room temperature, thermostat setting position and door opening on energy consumption [5]. 2. Mathematical model for design of experiment If all the variables are assumed measurable, the response surface can be expressed as y ¼ f ðx1 ; x2 ; x3 ; . . . ; xk Þ
ð1Þ
R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
847
The goal is to optimize the response variable y. It is assumed that the independent variables are continuous and controllable by the experimenter with negligible error. The response or dependent variable is assumed to be a random variable. The observed response y as a function of room temperature, thermostat setting positions and door opening can be written as y ¼ f ðx1 ; x2 ; x3 Þ
ð2Þ
If the expected response is denoted by EðyÞ ¼ g, then the surface represented by g ¼ f ðx1 ; x2 ; x3 Þ is called a response surface. It is required to find a suitable approximation for the true functional relationship between y and the set of independent variables, the xi ’s. Usually, a low order polynomial (first order) in some regions of the independent variables is employed. The first order model, k X bi xi þ e ð3Þ y ¼ b0 þ i¼1
is generally utilized in RSM problems. The b parameters of the polynomials are estimated by the method of least squares, and e is a random error. The details of the solution by this matrix approach are explained in Ref. [5]. The matrix approach of solving Eq. (3) has been adopted in the present analysis. y is defined to be an (n 1) vector of observations on y, x to be an (n p) matrix of independent variables, b to be a (p 1) vector of parameters to be estimated and e to be an (n 1) vector of errors. Eq. (3) can be written in matrix form as y ¼ bx þ e
ð4Þ
The least squares estimates of b is the value b which, when substituted into Eq. (3), minimizes e0 e. The normal equation can be expressed as ðxT xÞb ¼ xT y
ð5Þ T
where b is replaced by the b matrix. If (x x) is non-singular, the solution of the normal equation can be written as 1
b ¼ ðxT xÞ xT y
ð6Þ
where xT is the transpose of matrix x and ðxT xÞ1 is the inverse of matrix (xT x). y is defined to be an (n 1) matrix on a logarithmic scale, x to be an (n m) matrix of independent variables, b to be an (n 1) matrix. The solution of this matrix has been performed by using M A T L A B . 3. Experimental set up and instrumentation 3.1. Experiment with single variable The objective of this experiment is to determine the effect of room temperature, thermostat setting position and door opening on the energy consumption of two household refrigerators. The tests were conducted by varying the room temperature, thermostat setting position and door opening independently. While one variable was varied, the other two variables were kept constant. The room temperature was varied from 14°C to 32°C in an environmentally controlled chamber
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R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
located in our laboratory. The fresh food compartment and freezer compartment thermostat setting positions were set fixed at the medium setting. There are 10 thermostat setting positions ranging from 1 to 10 for the fresh food compartment of these two refrigerator freezers. The freezer compartment thermostat positions are minimum, medium and maximum. The thermostat setting is varied by turning the knob to the desired setting position from 1 to 10 for the fresh food compartments to investigate the effect of thermostat setting position on energy consumption. The freezer compartment thermostat settings were set at the minimum position when the fresh food compartment setting position was 1–4. The freezer compartment thermostats were set at the medium position when the fresh food compartment thermostat setting position was 5–7, and the freezer thermostat positions were set at the maximum position when the fresh food thermostat setting position was 8–10. Room temperature was maintained at 26°C in all the tests. The door opening controls are set so that the door remains open for 12 s at an angle of 90°. The total run time was set to be 600 min (10 h) from the commencement of the experiment. Door closing time was a variable parameter in order to maintain 20–75 door openings in the total time mentioned above. Table 1 shows the door opening and closing schedule for the total run time. In this experiment, the door openings were considered for 10 h from the commencement of operation. This was considered a more realistic usage condition in the household environment. On the other hand, Grimes et al. [6] conducted the door openings for 8 h and maintained 24 door openings. Alissi [4] did the same experiment with 16 h of operation and maintained 100 door openings. However, the literature survey says that the usual door opening should be within 40–60 times per day. So, our door opening schedule can be considered as more realistic compared to their studies. Room temperature was maintained at 26°C for all the tests in door opening, and the thermostat setting positions were kept at the medium position for both of the compartments. The technical specifications of these two models have been shown in Table 2. 3.2. Experiment with multivariable This experiment was designed to investigate the combined effect of room temperature, thermostat setting position and door opening on the energy consumption of the refrigerator freezers. 3.2.1. Experimental design and conditions A design consisting of 12 experiments has been used to develop the first order model to investigate the combined effect of room temperature, thermostat setting position and door opening Table 1 Door opening schedule Number of opening
Total run time (min)
Closing times (min)
Door remains open (s)
20 30 40 50 60 75
600 600 600 600 600 600
30 20 15 12 10 8
12 12 12 12 12 12
R. Saidur et al. / Energy Conversion and Management 43 (2002) 845–854
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Table 2 Technical specifications of Model E and Model S Specifications
Model E
Model S
Freezer capacity (l) Fresh food compartment capacity (l) Power rating (W) Current rating (A) Voltage (V) Frequency (Hz) Defrost system Number of doors
80 220 160 0.9 220 50 Auto defrost 2
80 220 160 0.9 220 50 Auto defrost 2
on energy consumption. Eight experiments represent a factorial design, four experiments represent repetition to estimate the pure error. The experimental design and conditions for the multivariable tests have been shown in Table 3. A factorial design of experiment is one in which all levels of a given factor are combined with all levels of every other factor in the experiment. This design is necessary when interactions between the variables are to be investigated. Furthermore, factorial designs allow the effects of a factor to be estimated at several levels of the other factors, giving conclusions that are valid over a range of experimental conditions. The transforming equations for each of the independent variables are x1 ¼
lnðtr Þ lnð22Þ ; lnð30Þ lnð22Þ
x2 ¼
lnðts Þ lnð5Þ ; lnð10Þ lnð5Þ
x3 ¼
lnðdo Þ lnð45Þ lnð70Þ lnð45Þ
ð7Þ
3.3. Instrumentation A heat pump was used to maintain a steady temperature in the environmentally controlled chamber in order to investigate the effect of room temperature. The temperature variation was controlled by using an Omega type temperature controller. Daily energy consumption was Table 3 Experimental design and coding identification Number
tr (°C)
ts (number)
do (number)
Coding x1
x2
x3
1 2 3 4 5 6 7 8 9 10 11 12
16 30 16 30 16 30 16 30 22 22 22 22
2.5 2.5 10 10 2.5 2.5 10 10 5 5 5 5
30 30 30 30 70 70 70 70 45 45 45 45
1 1 1 1 1 1 1 1 0 0 0 0
1 1 1 1 1 1 1 1 0 0 0 0
1 1 1 1 1 1 1 1 0 0 0 0
850
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measured by a digital power meter, which was interfaced with a PC. Lab view software was installed in the PC for data storage and analysis. An automated door opening and closing mechanism has been developed. AC motors with gear boxes were mounted on a steel frame, which was built at the top of the refrigerator freezer. Mechanical arms have been prepared in such a way that doors open and close subsequently. The door opening process was controlled using the programmable logic controller (PLC). Programming is achieved using ladder logic. The operating switch, which is an input device, sends signals to the motor to open and close the doors. Total run time, opening time and closing time are inserted into the operating switch, and it works as the experiment requires.
4. Results of single variable on energy consumption 4.1. Effect of room temperature Most of the thermal load on a refrigerator is conduction through the refrigerator wall. ASHRAE [7] shows that about 60–70% of the total refrigerator load comes by conduction through the cabinet walls. This conduction load is proportional to the difference between the ambient temperature and the internal compartment/freezer temperature. The higher the difference, the higher is the load imposed on a refrigerator. For this reason, the temperature of air around a refrigerator is a significant determinant of energy consumption. Since compressor efficiency also declines as the ambient temperature rises, a refrigerator’s electricity use is very sensitive to the ambient temperature. Our experimental data in Fig. 1 shows that energy consumption increases 47 W h for Model E and 53 W h for Model S for each degree increase in room temperature. Meier et al. [8] showed that energy consumption increased 120 W h for each degree increase in temperature. Meier et al. tested the existing old refrigerator freezer in the household environment. All those refrigerator freezers were manufactured before 1993. On the other hand, we tested the latest model from the market. 4.2. Effect of thermostat setting Just like a house, a refrigerator will use less electricity if its thermostat is re-set to a higher (warmer) temperature. Grimes et al. [6] examined the impact of compartment temperature on energy use on a 1977 vintage automatic defrost refrigerator. Energy consumption rose 26% from the warmest acceptable to the coldest possible settings. A more recent study of nine large 1993 vintage US refrigerators [9] found a 6.5% increase in energy consumption for 1°C reduction in freezer temperature. Our data analysis shows that energy consumption increases about 7.8% for each degree reduction in freezer temperature. Fig. 2 shows our experimental data for the two models. 4.3. Effect of door openings When the refrigerator door is opened, warm and moist air exchanges with the cool air inside the refrigerator cabinet. When the door is closed, a mass of air at ambient temperature is trapped in
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Fig. 1. Variation of energy consumption with room temperature.
Fig. 2. Variation of energy consumption with thermostat setting position.
the compartment. Heat gains during door openings are due to (i) heat/vapor transfer on the interior surfaces of the refrigerator, and (ii) bulk air exchange. The following five categories of loads are associated with door openings, as of Ref. [4]. 1. convective heat transfer from the warm ambient air flowing across the cooler refrigerator surfaces; 2. latent heat transfer with condensation of water vapor from the moist air flowing across the cooler refrigerator surfaces; 3. radiative heat transfer from the surroundings to the interior surfaces; 4. sensible heat transfer from the warm air mass within the cooled space after the door is closed.
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Fig. 3. Variation of energy consumption with door opening.
Parker and Stedman [10] estimated that each door opening causes 9 W h increase of energy consumption. In our experiment, we found 9 W h increase in energy consumption for each door opening for Model E and 12.4 W h increase in energy consumption for Model S for each door opening. Fig. 3 shows the effect of door opening on energy consumption.
5. Experimental results of multivariable effect on energy consumption From the experimental results shown in Table 4, model equations (8) have been developed using Eqs. (3) and (7). The b parameter has been calculated and presented in Table 5. y1 ¼ 3:889 þ 0:714 ln tr þ 0:241 ln ts þ 0:254do ;
ð8Þ
y2 ¼ 3:98 þ 0:773 ln tr þ 0:204ts þ 0:210do
Table 4 Experimental results Trial number
EE (W h)
ES (W h)
1 2 3 4 5 6 7 8 9 10 11 12
976 1610 1415 2213 1178 2140 1690 2438 1503 1513 1477 1479
1008 1788 1513 2327 1261 2251 1832 2577 1607 1616 1588 1543
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Table 5 Values of b Value of b
For Model E
For Model S
b0 b1 b2 b3
7.382 0.223 0.167 0.112
7.432 0.240 0.143 0.093
The above equations can also be written in the form of energy consumption as a function of room temperature (tr ), thermostat setting position (ts ) and door opening (do ) as follows: EE ¼ 48:86tr0:714 ts0:241 do0:254 ; ES ¼ 53:52tr0:773 ts0:204 do0:210
ð9Þ
Eq. (9) indicates that an increase in room temperature, thermostat setting position and door opening increases the energy consumption. It is also noted that the thermostat setting position and door openings (trivial many) contribute almost a similar magnitude of influence on energy consumption, and the room temperature (vital few) has the higher effect on energy consumption.
6. Conclusions The following conclusions could be made from this study. 1. Room temperature has the higher effect on energy consumption, followed by door opening and thermostat setting position. The door opening and thermostat setting position have very little difference in their relative effect on energy consumption. 2. As Malaysia is going to adopt energy efficiency standards and labels for refrigeration appliances, a more realistic test procedure is needed in order to evaluate their energy consumption behavior. This paper can be considered as an experimental support to develop test standards for household refrigerator freezers in Malaysia, as their energy consumption is greatly dependent on these parameters (i.e. room temperature, thermostat setting position and door opening). 3. The door opening pattern in this study can be considered as more comparable with the actual usage condition that exists in the household environment. 4. The exponent in Eq. (9), which is close to 1, has the higher effect on relative magnitude, and the exponents, which are close zero, have the lower effect on energy consumption.
Acknowledgements The authors would like to acknowledge the Ministry of Science, Technology and Environment, Malaysia for funding the project. The research has been conducted under the IRPA Project no. 02-02-03-0471.
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