1. Introduction
Refrigeration and air-conditioning systems share a remarkably high proportion of energy consumption, especially in building sectors that need air conditioning to maintain temperature [
1]. Reducing the energy consumption of air conditioning and minimizing the use of fossil fuels for power generation are most critical for the environment. One practical solution for improving the performance of air conditioning is to use phase change materials (PCMs), which apply the characteristics of latent heat storage to maintain a stable internal temperature when the phase state changes [
2]. Air conditioning systems using PCMs can save up to 35% on energy and store up to 89 percent of daily cooling load when compared to traditional air conditioning systems [
3]. The practical use of PCMs is to supply the appropriate cooling to the air conditioner’s loading area while reducing the chiller and pump start-up frequency to conserve electricity. The primary premise of energy conservation with PCM cold storage is to shift the power use of an air conditioning system from on-peak (daytime) hours to off-peak hours (during nighttime) [
4]. However, PCMs have many unique characteristics and applications, so the selection criteria and methods of PCMs are critical for energy saving and carbon mitigation in sustainable management.
To solve the problem of material selection, multi-criteria decision making (MCDM) is one of the most popular techniques [
5]. Many researchers propose suitable material selection methods when studying PCMs. In selecting PCMs for construction, Imghoure et al. used the analytic hierarchy process (AHP) to select the optimal PCM from five PCMs and simulated them with a numerical model. The comparison of the results between the two was consistent [
6]. Similarly, Socaciu et al. suggested the AHP method when choosing PCMs for building comfort applications. The optimal choice was made from eight PCMs [
7]. Oluah et al. suggested the utilization of the entropy weight method (EWM) in conjunction with the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) approach for the purpose of choosing the most appropriate phase change material (PCM) [
8]. Xu et al. proposed the combination of AHP and TOPSIS to select PCMs for latent heat storage [
9]. Nicdalde et al. compared different methods when selecting PCMs for vehicle roofs, including AHP, VIseKriterijumska Optimizacija I Kompromisno Resenje (VIKOR), TOPSIS, and COmplex PRoportional ASsessment (COPRAS) [
10]. The results demonstrated that COPRAS and TOPSIS have high levels of correlation.
These studies propose several multi-criteria decision making (MCDM) methods for selecting PCMs in various fields. However, there is no relevant research on selecting cold storage PCMs. Moreover, these studies predominantly use MCDM as the material selection method, but the selection criteria only consider the physical properties of PCMs and the method for criteria selection is not mentioned. Furthermore, some relevant criteria for practical applications, such as toxicity, flammability, cost, and corrosion, do not provide a selection method. Therefore, in addition to establishing a selection model, the purpose of this study is to provide a modified model to enhance the selection of PCMs and be used in selecting materials for actual PCM applications. Consequently, PCMs can be used in air conditioning systems to achieve energy saving and carbon mitigation benefits.
1.1. Phase Change Material (PCM)
A phase change material (PCM) serves as a substance for latent heat storage, capable of capturing and releasing thermal energy while undergoing constant-temperature phase transitions over a wide spectrum of temperatures [
11,
12]. Depending on their phase changes, PCMs can be categorized into solid–liquid, solid–gas, liquid–gas, and solid–solid [
13,
14]. Because other forms of PCMs have technical restrictions such as lesser latent heat capacity and super cooling problems, solid–liquid PCMs are more appropriate for practical usage [
15]. There are different melting point ranges for commercial PCMs on the market. The three most widely used categories are eutectic, inorganic, and organic [
16,
17,
18].
Inorganic phase change materials (PCMs) offer benefits such as a greater heat of fusion, a constant melting temperature, excellent thermal conductivity, and minimal volume alterations during phase transition. They are predominantly employed for PCMs designed for moderate- and lower-temperature applications. However, general salt-type inorganic PCMs are prone to “overcooling” and “phase separation” when recycled [
19]. Organic PCMs are not prone to “overcooling” and “phase separation”. They have the advantages of less corrosiveness, stable performance, and more solid molding. However, they have low thermal conductivity, low material density, volatility, significant loss, insufficient heat storage capacity per unit volume, high price, flammability, and other defects, reducing the efficiency of the heat storage system and limiting its application [
20]. Inorganic or organic PCMs can be converted into organic–inorganic composite PCMs for practical use to address their shortcomings when used alone and achieved the optimal application effect [
21,
22].
1.2. Basic Principle of Phase Change Material
When a PCM melts, it undergoes a transition from a solid state to a liquid state. Throughout the phase transition procedure, the material can absorb a large amount of heat energy at an almost constant temperature. When the PCM freezes and solidifies, the opposite occurs: it releases the heat it absorbs [
23,
24]. Different materials that melt and solidify at different temperatures can absorb different amounts of heat energy.
PCMs are useful because they melt and solidify at a specific pressure and fixed temperature, making them suitable for temperature control in several applications. Compared with sensible heat energy materials, PCMs that melt and absorb heat are more efficient in absorbing heat energy. Accordingly, compared with using materials that do not change phase, the quantity of material required for PCMs to store the same amount of thermal energy is much less.
1.3. Desired Properties of PCMs for Cold Storage
The phase change temperature of PCMs used for cold storage is in the range of 7–14 °C. The primary applications for PCMs are food preservation, material transportation, construction, and air conditioning [
22]. Frigione et al. suggested that when considering possible PCM candidates, some characteristics such as thermophysical, chemical, environmental, and economic properties must be considered [
25]. The desired characteristics from the literature are listed in
Table 1.
1.4. PCM Application for the Cold Storage of Air Conditioning
Said and Hassan investigated a physical model for improving the cooling efficiency of conventional air conditioners by utilizing PCM plates. The model consists of a rectangular duct containing six PCM plates, a centrifugal fan, an electric heater, a variable speed controller, a variable DC power supply, and an AC unit. The PCM plates are coupled with the condenser of the AC unit and use the cold ambient air at night to solidify and store cold energy. During the day, the hot ambient air is cooled by passing over the solidified PCM plates before entering the condenser, thus reducing the condensing temperature and increasing the coefficient of performance (COP) of the AC unit. This study investigated the effect of different PCM plate configurations, inlet air velocities, and temperatures on the charging and discharging processes of the PCM, as well as the performance of the AC unit [
30].
Omara et al. proposed an air conditioning system combined with PCM storage to improve efficiency. This system is comprised of tanks that store PCM and ice-cold substances, refrigeration units, and cooling units. The system functions as follows: Initially, when the return water temperature in the ice tank falls to 8 °C, the PCM tank initiates the storage of thermal energy, causing the PCM to undergo solidification, while the ice tank delivers cooling to fulfill the load demands. Simultaneously, when the return water temperature in the ice tank reaches 14 °C, the PCM tank ceases its cooling storage, and the ice tank is recharged via a heat transfer fluid (HTF). The return water temperature in the ice tank rises with the decreasing building temperature. When the return water temperature reaches 14 °C, and the ice tank alone cannot meet the cooling needs, both the ice tank and the PCM tank start to release cooling to provide for the users. In the third scenario, when the building’s load diminishes, the ice tank exclusively supplies cooling to the building [
3].
Zhai et al. suggested a cold-storage solar air-conditioning system. The main components of this system are solar collectors, an absorption chiller, an air handling unit (AHU), a latent heat storage unit, and a dry cooling system. Solar collectors transform solar radiation into thermal energy to drive the absorption chiller. The absorption chiller generates chilled water for the air handling unit (AHU) to provide space cooling. The latent heat storage unit consists of a heat exchanger filled with PCMs. The PCMs can store the excess heat from the solar collectors during the day and release it to the absorption chiller during the night, thus reducing the cooling load on the dry cooling system so that the dry cooling system can be switched off to save water and energy. The components are linked or connected by pipes, valves, pumps, and controllers, which regulate the flow and temperature of the working fluids (water, refrigerant, and PCM) according to the system operation mode and the cooling demand [
27].
These studies illustrate that PCMs are helpful for peak load shifting and improving the performance of air conditioning systems by using different configurations and switching control. Therefore, PCMs are effective for reducing energy consumption and saving the electricity cost of air conditioning systems.
4. Results and Discussion
4.1. Delphi Results
The results of the Delphi method showed that in addition to the physical characteristics, the cost and environmental impact of PCMs in practical applications must also be considered. Cost is related primarily to the market competitiveness of the cold storage system, and environmental impact is related to whether the material will affect the health of the human body. Therefore, the choice of PCMs is linked to environmental management and sustainable management. The addition of the Delphi method results in selecting PCMs closer to practical applications and provides more complete criteria considerations.
4.2. AHP Results
The AHP weight analysis showed that the phase change temperature is the most critical selection criterion for cold storage systems. Therefore, PCMs that do not meet this criterion should be avoided. The weight distribution of each criterion is depicted in the pie chart of
Figure 2. The weight of phase change temperature is 0.329, and the others in order are heat of fusion, specific heat, thermal conductivity, cost, environmental impact, and material density.
4.3. VIKOR Results
The bar chart provides a visual result of the rankings for the phase change material (PCM) selection based on the VIKOR calculations. As depicted in
Figure 3, the
x-axis represents the material types and ranking orders, and the
y-axis shows the ranking number of materials. Materials are ranked from 1 (the best) to 10 (the worst) based on their overall performance in the VIKOR analysis. From the chart, it is evident that “A9” has the best ranking (lowest
Qj (
v = 0.5) value), indicating it performs the best according to the criteria considered. Oppositely, A6 is ranked 10, which suggests that it is among the worst-performing materials. The order for the other alternatives is SP > S8 > A7 > C7 > S7 > A8 > A6.5 > OM08.
4.4. Discussion
Our proposed model provides a valuable process and method for selecting PCM materials, from criteria selection to material property selection. This model is more practical and realistic than others that do not use the selection method of criteria required for real-world applications. Moreover, it can be used to evaluate new materials in the research and development stage for possible environmental impacts. During the production stage of a system, the material cost can significantly affect the system cost and competitiveness of the company. Therefore, the PCM selected by the model should be included in the list of qualified suppliers, and its performance and economic benefits should be further evaluated in follow-up research to achieve the company’s management, energy-saving, and carbon-reduction goals. Finally, PCMs are the cleanest energy source. The use of fossil fuels can therefore be minimized to save energy and enable companies to achieve cleaner, more sustainable production by applying the selected PCMs to air conditioning systems.
4.4.1. Limitations
Due to the wide variety and temperature ranges of phase change materials, it is not possible to consider all usable PCMs. Therefore, It is important to consider other factors and criteria specific to your application when selecting a phase change material. The model analysis provides valuable insights, but it should be complemented with a broader assessment.
4.4.2. Future Study
For future study, we recommend using this model to select PCMs of different types, temperatures, and fields. Project management and advance quality planning is also suggested for the application of selected PCMs in high-technology companies (
Figure 4) since the industry shares the highest proportion of energy consumption. The process involves four stages: development, design test and verification, pilot run, and system integration. The first stage is the one related to material selection and qualification, which is also the most critical step of early failure detection.
After the selection of the PCM, it must be used to set up a PCM cold storage system. As depicted in
Figure 5, the PCM tank is coupled with a heat exchanger and then connected with air handling units and chillers. The PCM is charged during the nighttime (off-peak time) and releases the cold during the daytime (peak time). Shifting the daytime peak loads to off-peak nighttime periods reduces actual power consumption and, most importantly, avoids daytime punitive electricity rates to reduce annual running costs dramatically. With the reduction in energy consumption, carbon mitigation can also be estimated. Therefore, a cost–benefit analysis should be carried out to ensure the application of phase change materials aligns with sustainable energy goals.
PCM is useful for reductions in energy consumption and carbon mitigation. Therefore, it is suggested to connect the performance of PCMs with the key performance indicators of energy management, environmental management, and sustainability management such as ISO5001 [
39], ISO14001 [
40], and sustainability standards.