Effect of Dry Ice Gel PCM on Wall Freezer Temperature Variance

The increasing population growth in cities will cause environmental detriments such as global warming and ozone depletion. The electricity need will increase in the future due to the higher temperature in the cities, leading to the higher usage of cooling equipment such as freezers. The current study proposed a method to improve the freezer's energy efficiency by using phase change material of dry ice gel. Most of previous studies only focused on the impact of position of phase change materials on the temperature of wall freezer while no previous studies consider the detailed methodology of measurement and the impact of phase change material based on dry ice gel to the variance of wall temperature. The sensor thermocouple type T and data logger ADAM 6018+ were used to measure the wall temperature of the freezer. The results showed that the sensor thermocouple was verified to the calibrated thermometer with R 2 = 0.99. The installation of phase change material based on dry ice gel on the wall freezer could maintain the temperature around 1-2 o C while the freezer was turned off for 4, 6, and 8 hours. The efficiency of freezer could be increased from 10% to 30% due to using dry ice gel phase change material. The future study should investigate the various types of phase change materials and the new combination of phase change materials.


Introduction
The problem of global warming is an environmental issue that has emerged since the early days of industrialization (Abram et al., 2016) Global air temperature increased by 0.87°C from before the industrialization period in 1850-1900 to the post-industrial period in 2006-2015.The land on Earth is warmer than the global average temperature, and most areas of the sea are warming at a slower rate than land.As many as 20% to 40% of the global population have experienced an increase in temperature of more than 1.5°C compared to pre-industrial temperatures (IPCC, 2018).Figure 1 shows global warming from 2006 to 2015 compared to the pre-industrial period of 1850 to 1900.(Abram et al., 2016) Population growth in cities and global warming will increase energy requirements for cooling buildings (Tewari et al., 2017;Tuck, & Zaki, 2022).The use of refrigeration equipment such as refrigerators, freezers, and air-conditioners (AC) will be increasingly used due to hotter environmental temperatures and to prevent heat stroke (Imessad et al., 2014;Kubota et al., 2009).Energy consumption, especially electrical energy, will increase in urban areas (Tewari et al., 2017).Electrical energy contributes 26.9% of the world's total energy consumption, and as much as 29.1% of the world's total energy is used by the household sector (United Nations, 2022).This number is expected to continue to increase because the need for refrigeration equipment in households will increase due to economic growth and increasing global temperatures.The need for electrical energy in Indonesia is increasing rapidly due to economic growth resulting from urbanization and industrialization (McNeil et al., 2019).The use of electrical energy will increase carbon dioxide gas emissions by around 1.3% each year, which causes environmental damage such as ozone layer depletion, climate change, and global warming (Yoro & Daramola, 2020).Electrical energy needs in Indonesia can be classified into several sectors: buildings, industry, and transportation.The building sector is divided into two sectors: households and commercial or office buildings.The electricity usage sector in households is higher than in industry and transportation.The need for electricity in households will increase from 49% in 2018 to 58% in the BAU (business as usual) scenario, 60% in the PB (sustainable development) scenario, and 61% in the RK (low carbon) scenario in 2050 (National Energy Council, 2019).Meanwhile, in 2021, households consumed the most electrical energy compared to the industrial and transportation sectors, namely 114 TWh or 44.9% of the total demand for electrical energy (IESR, 2021).
The leading cause of the high use of electrical energy in households is refrigeration, heating, electric motors, and entertainment equipment such as air conditioners, refrigerators, freezers, washing machines, televisions, and induction cookers.Some cooling equipment, such as air conditioners and refrigerators, contribute to the highest electrical energy consumption at home (Biglia et al., 2020;Sena et al., 2021).Indonesia needs a strategy to increase the efficiency of cooling equipment to optimize household electricity needs.One strategy to increase energy efficiency that can be used in cooling equipment such as freezers is using phase change materials (PCM).PCM is an advanced material that can solve problems in refrigeration machines because it can shift the peak load.
PCM is a material that uses latent heat in a phase change process to increase hot or cold storage capacity so as to achieve energy savings in the system (Ghodrati et al., 2022a;Lu et al., 2019).Gin et al. (2011) conducted cold storage research using phase change material (PCM) on vertical cold storage walls.Temperature measurement uses a thermocouple mounted on the center of the cold storage wall.The measurement results show that the wall temperature of the cold storage is -7°C lower than that of the cold storage wall without PCM by 15 o C. Oró et al. (2012) have proven that freezer performance can be improved by attaching sodium nitrate dissolved water type PCM to the shelf of the freezer.Air temperature measurement sensors are installed at each level of the freezer shelves.The M-Packs test package is also used to simulate the thermal mass of food in the freezer.The results showed that the temperature in the freezer was 4-6°C lower by adding PCM.The addition of PCM also helps the cooling temperature lower by 2oC for 3 hours of power failure in the freezer.Defalco (2017) compared the use of the RT5HC paraffin type PCM and the 0M06P PCM type, which is the development of a new type of PCM.The experimental results show that the new type of PCM has a solid temperature of 5°C to 8°C with a latent heat of more than 200 kJ/kg.Energy savings can be achieved by optimizing a chiller of 20-30% and a cold storage capacity that can be designed at 5 kWh.
Taufiqurrahman (2016) analyzed the performance of organic PCM in cold storage.The results showed that the compressor stopped operating without PCM, 0.5 kg PCM, 1 kg PCM, and 2 kg PCM for 17 minutes, 31 minutes, 40 minutes, and 70 minutes, respectively.The cost savings due to the compressor stopping for each month is Rp.237,830.91 for the use of 2 kg of PCM.Ling et al. (2018)investigated the performance of cold storage installed with two types of PCM, namely tridecane and dodecane, with phase change temperatures and enthalpies of -5.4°C and 132 kJ/kg and -9.6°C and 173 kJ/kg, respectively.The results showed that the temperature change on the walls of the cold storage was -15°C to 0°C.PCM has been used to improve work performance and thermal storage in freezers by Abdolmaleki et al. (2020).Freezers were tested without PCM and with PCM installed in the food compartment.Temperature is measured inside the freezer for 24 hours in real-time using a data logger.The use of eutectic type PCM in the freezer has been able to reduce temperature fluctuations by 40.59% with an optimal amount of 2 kg of PCM with a melting temperature of -20 o C. The results of the study show that the use of PCM in the freezer can lower temperatures compared to freezers without PCM.The energy savings that can be achieved are 8.37% with 1.5 kg of PCM used and a melting temperature of -20°C.Ghodrati et al. (2022) characterized the performance of a water and ethylene glycol-type PCM installed in a freezer.The results showed that the use of watertype PCM only used 63% of the energy for 100 minutes.Meanwhile, the freezer will lose 90.9% of its energy for 100 minutes.When the freezer uses ethylene glycol type PCM, only 35.97% of the energy is released for 500 minutes.This study shows that the use of PCM can improve storage performance and energy consumption in freezers.
PCM has been used in various positions, such as evaporators, condensers, cooling racks (Ghodrati et al., 2022), as well as walls of refrigerators and freezers (Omara & Mohammedali, 2020).Modeling using computational fluid dynamics (CFD) with variations in PCM positions on the back, top, and top and rear combinations in the refrigerator shows that the combined PCM position has a lower temperature than the other positions (Du et al., 2018).PCM has been shown to reduce energy consumption in condensers by 10% (Yusufoglu et al., 2015).The use of PCM increases heat transfer in the dimensions of the evaporator, which has been optimized (Marques et al., 2014).
Freezer performance improvement has been made by attaching a eutectic-based PCM to the compartment of the food shelf.Energy savings of 8.37% have been achieved using this method.The temperature is 40.59%lower when PCM is used (Ascione et al., 2019).Research using PCM water and ethylene glycol on the top of the freezer to improve the performance of the system.The research measured temperature, changes in stored energy, and experimental time.Modeling with comsol software is also carried out to simulate the data obtained.The results showed that PCM increases energy storage in the freezer during regular operation and electrical disturbances.Freezers using water PCM lost only 63% of energy after 100 minutes of power failure compared to freezers without PCM.On the other hand, using ethylene glycol resulted in the freezer losing only 35.97% of the system after 500 minutes of use (Gil et al., 2014).
Research on PCM in horizontal freezers with PCM positions on all four walls with the addition of forced air circulation has yet to be carried out by previous studies.Previous research has only focused on attaching PCM to the food shelf (Ascione et al., 2019;Ghodrati et al., 2022;Gil et al., 2014) as well as the condenser and evaporator (Ghodrati et al., 2022b;Yusufoglu et al., 2015).Research on the position of the PCM on the wall has been carried out before, but there were no studies which explained the detailed methodology of measurement and the impact of dry ice gel phase change material to the variance of wall temperature on freezer based on descriptive statistics analysis.

Methodology
The research is initiated by determining the thermocouple type since many types of thermocouple sensors.The thermocouple type T was selected since the measurement on the freezer will be under 0 o C. The calibration was performed by comparing the temperature measurement between calibrated alcohol thermometer and thermocouple sensor for measurement from 30 o C to 90 o C.After the measurement results from the thermocouple sensor were verified based on regression analysis, the research would be continued to set the data logger ADAM 6018+ in installing the thermocouple sensor and attaching phase change material of dry ice gel on the freezer wall.The connection between the computer and the data logger needs to be ensured before the measurement to record the temperature data into the computer.If the connection did not succeed, the setting would be repeated until successfully connected.After the connection between the computer and ADAM 6018+ was successful and the temperature data were recorded into the computer, the measurement would be conducted for 12 hours.Figure 2 shows the research methodology for this research.
Temperature measurement using a T-type thermocouple sensor and ADAM 6018+ data logger needs to be verified to ensure that the measured temperature is valid.The process of verifying the thermocouple sensor measuring instrument and ADAM 6018+ is carried out by comparing the measurement results of a calibrated alcohol and mercury thermometer as shown in Figure 3.The objective of comparison of the two measuring instruments was aimed to ensure that the temperature measurement results obtained by the thermocouple are the same as the temperature results obtained by an alcohol thermometer.The calibration process is carried out in an acid room with an electric stove as a heater for the vessel filled with water, as shown in Figure 4.The ADAM 6018+ data logger is also installed to measure the temperature measured from the thermocouple sensor.The calibration process was carried out for 1.5 hours by taking data for each increase in temperature on an alcohol thermometer by 1 o C. The temperature was measured from 30 o C to 80 o C. The total data compared is 42 data, so it meets the minimum requirements for linear regression, namely 20 data for two variables.The set-up measurements in this research were shown in Figure 5, 6, and 7.The edge of type T thermocouple will be inserted into the terminal of data logger as shown in Figure 5.The positive and negative poles of the thermocouple need to be fully considered since if it is reversed, it would produce readings of temperature with opposite signs.If the measured temperature is positive, it will show negative value, and otherwise.Software ADAM/APAX utility was used as interface among data logger ADAM 6018+, thermocouple sensor and computer as shown in Figure 6.Installing the phase change materials of dry ice gel requires additional tools such as double-sided tape with strong adhesive and aluminum foil tape to ensure that the PCM sticks firmly to the walls of the freezer.Figure 7 shows the PCM type of dry ice gel that has been attached to the freezer wall.

Results and Discussion
Results and discussion explained the verification of sensor thermocouple, measurement of temperature in the freezer cooling machine without phase change material for 12 hours, measuring the wall temperature of the freezer cooler without phase change material by turning off the freezer for 4, 6, 8, and 10 hours, and comparison between the temperature of the freezer walls using PCM and those without PCM.The verification results of the thermocouple sensor measuring instrument and ADAM 6018+ data logger with a calibrated alcohol thermometer are shown in Figure 8. Linear regression shows an R 2 value of 0.9982, which means that the measurement results of the thermocouple sensor have matched the measurement results of a calibrated thermometer.The results of the error analysis test on thermocouple data and thermometer data are shown in table 1.The R 2 value is 0.99, which means that the thermocouple sensor measurement data is the same as the measurement data from the thermometer.The MAPE value is 2.5%, which means the compatibility between the thermocouple sensor data and the thermometer data.The MAE, MSE, and RSME values are very small, meaning that the error values that occur in thermocouple sensor measurements are very small compared to thermometer data.The temperature measurement of the freezer wall, coated with dry ice gel-type phase change material (PCM), was carried out for approximately 12 hours, as shown in Figure 9. Variations in the temperature of the freezer wall installed with PCM are slower to drop to minus temperatures.There are several stages of decreasing the temperature in the freezer using PCM, such as decreasing the PCM temperature (orange colour), changing the PCM phase from gel to frozen (green colour), decreasing temperature to minus (yellow colour) and fluctuations at minus temperature (grey colour).The temperature of the freezer walls that use PCM will be compared with those of the freezer walls that do not use PCM, as shown in Figure 10.Freezers that use PCM will enter minus temperatures slower than freezers that do not use PCM. Figure 11 shows a comparison between the temperature of the freezer walls that use PCM and those that do not use PCM with the freezer turned off for four hours.Freezers that use PCM can maintain temperatures below 0 o C (about -1 o C and -2 o C).Meanwhile, freezers that do not use PCM will immediately rise in temperature above 0 o C to 200 o C to 210 o C, which causes the cold energy stored by the PCM will be slowly released so that the temperature will not exceed 0 o C. When the freezer does not use PCM, the cold energy stored on the walls will be released quickly.Table 2 shows the descriptive statistical analysis results of the average freezer wall temperatures without PCM in four different conditions; namely, the freezer was off for 4, 6, 8, and 10 hours.Standard error and standard deviation show similar values.However, kurtosis, skewness, and maximum and minimum values showed differences caused by the different initial measurement times and initial measurement temperatures resulting in different data distributions.Table 3 compares descriptive statistical analysis on the walls of the freezer installed with dry ice gel type PCM under several conditions; namely, the freezer is turned on all the time, and the freezer is turned off for 4, 6, 8, and 10 hours.The parameter values of the descriptive statistics are quite uniform when the freezer is turned off for several hours, except when the freezer is turned on continuously without turning it off.

CONCLUSION
The procedures of measurement temperatures to investigate the impact of phase change material based on dry ice gel on the variance of temperature on wall freezer had been conducted using descriptive statistical analysis and line graphic.The verification for instrument between the temperature of thermocouple and calibrated thermometer confirmed that the sensor measurement was valid based on the linear regression with R 2 value of 0.99.The installation of dry ice gel showed significant impact to maintain the temperature of wall freezer on 1-2 o C when the freezer was turned off for 4, 6 and 8 hours.The efficiency of freezer should be increased around 20-30% while using dry ice gel phase change material.The future study should consider the various types of phase change materials to the variance of temperature on the wall freezer.

StartFigure 4 .
Figure 4. (a).The Process of Calibrating Measuring Instruments in A Fume Room with An Electric Stove as A Heater, (b).The Process of Installing The ADAM 6018+ Data Logger and Computer During The Calibration Process

Figure 5 .Figure 6 .Figure 7 .
Figure 5. Thermocouple Sensor Attached to The Data Logger with Positive (red) and Negative (white) Poles

Figure 8 .
Figure 8. Calibration Results of Type T and ADAM 6018 Thermocouple Sensors with A Calibrated Alcohol Thermometer

Figure 9 .
Figure 9. Variation of Freezer Wall Temperature with PCM on for 12 Hours

Figure 10 .Figure 11 .
Figure 10.Comparison between The Temperature of The Freezer Walls That Use PCM and Those That Do Not Use PCM with The Freezer Always On

Figure 12 .Figure 13 .
Figure 12.Comparison between The Temperature of The Freezer Walls Using PCM and Those Without Using PCM with The Freezer Turned Off For Six Hours

Table 1 .
Results of Error Analysis of Thermocouple Sensor Data and Thermometer Data

Table 2 .
Comparison of The Average Wall Temperature of The Freezer Without the PCM When Turned Off

Table 3 .
Descriptive Statistical Analysis of The Temperature of The Freezer Wall Installed with Dry Ice Gel Type PCM