Role of ambient temperature, door opening, thermostat setting position and their combined effect on refrigerator-freezer energy consumption
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 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 refrigerator-freezers 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].
Section snippets
Mathematical model for design of experiment
If all the variables are assumed measurable, the response surface can be expressed asThe 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 asIf the expected
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 located in our laboratory. The
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
Experimental results of multivariable effect on energy consumption
From the experimental results shown in Table 4, model equations (8) have been developed using , . The b parameter has been calculated and presented in Table 5.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:Eq. (9) indicates that an
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.
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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