High School Exploration of a Phase Change Material as a Thermal Energy Storage

Lauric acid (LA, content 99%) from ACROS Organics was purchased from Italmar (Thailand) Co., Ltd, and used as is without further purification. Thermocouples with a data logger were used for measuring the temperature. A water bath was used as a heat source.

The students first set up the apparatus as in Fig. 1. LA was used as the PCM. It has the melting point of about 43 °C. The students observed and recorded the temperature changes during the experiment. Five grams of LA was used. A water bath filled with glycerol was used as the heating source. Three thermocouples were used to measure the temperature changes of the experiment: the 1^st thermocouple measured the temperature of the glycerol in the water bath (as the environment temperature); the 2^nd thermocouple measured the temperature of the LA inside the vial; and the 3^rd thermocouple measured the temperature of the air inside the test tube inserted in the vial containing the LA. The temperature changes in the glycerol, the LA, and the air inside the test tube were recorded using a data logger. The data logger started to record the temperature changes before the water bath was turned on and the recording continued until the temperature of the whole system cooled down to room temperature or the three thermocouples gave the same reading. The water bath would be turned on until the LA completely melted.

Figure1.

Diagram of the apparatus used to study phase changes and how PCMs work.

Before the experiment, the students engaged in an experiment to understand the effect of temperature or heat on the kinetic energy of substance based on the addition of a food-coloring pigment in hot water and cold water without stirring. The students observed and compared the movement of the food coloring in hot water and cold water. The difference in the diffusion of the pigments throughout the beakers was used to explain how increasing temperature will increase kinetic energy. This activity aimed to guide the students to think more on the molecular level, which would help explain what happened to the material during the phase change. Thirty–four high school students participated in this study. They were divided randomly into a group of 4–5 students. They were asked to explain the result with the use of evidence to support their claims. The questionnaire (three situations) related to the basic concepts of PCMs and a multiple–choice test containing 21 questions were used to assess the students’ understanding before and after the activities.

As seen from Fig. 2, the food coloring in hot water diffused throughout the water, while that in cold water did so in a much lesser extent. This laboratory demonstration helped the students understand the effect of temperature (or energy) on the molecules in the material. This demonstration helped them to understand that increasing the energy made the molecules move faster (increased kinetic energy) and to intuitively visualize the microscopic level during the phase transition of PCMs.

Figure2.

(A) Student’s illustration of the movement of food coloring in hot water and cold water; and (B) the picture of the food coloring in hot water and cold water after ~ 5 minutes.

Fig. 3 shows an example of the students’ results obtained for the heating and cooling curves of LA. The heating and cooling curves clearly show that the temperature of LA and the temperature inside the test tube with LA were stable at about 43 °C, which is the melting point of LA. The temperature of the glycerol (environment) was maintained at ~46 °C. If the temperature of glycerol was significantly higher than 46 °C, we would not clearly see the effect of PCM to maintain a constant temperature of the air inside the test tube during the phase transition (of the heating curve). The LA started to melt after being heated for about 15 min. The heating and cooling curves of the air inside the test tube were quite similar to the heating and cooling curves of LA. This was because the heat energy inside the test tube (surrounded by the LA) was absorbed and released by the LA, which helped maintain the temperature inside the test tube at about the melting point of LA. In addition, the temperature of the air inside the test tube increased at a slower rate compared to the increase in temperature of the environment (glycerol). This slower-late of increasing temperature during the daytime can help shift peak load to off-peak hours, reducing electricity bills.²⁶ The capability to store and release large amounts of thermal energy of PCMs can provide an effective way to save energy by reducing the electricity required for heating and cooling. The students could observe the phase transition of LA directly during the experiment, which helped them understand how the PCMs work.

Figure3.

Students’ results of the heating and cooling curves.

Multiple choice test. The students’ scores from the pretest and posttest were analyzed using a paired-samples t−test to identify the mean differences between the pre- and post-scores at the significance level of 0.05, as shown in Table 1. Before conducting the paired–samples t–test analysis, the pre– and post-scores of the students were normally distributed as confirmed by the Shapiro–Wilk test (p > 0.05).

Questionnaire. The questionnaire was used to assess the students’ ability to explain situations concerning the concepts of phase change and applications of PCMs. The questions were adapted from the questionnaire for measuring chemical literacy.^27–28 Each situation was followed by 5–7 sentences related to the topic. Three situations were used. The students were asked to determine whether the sentence was true or false, and if the sentence was incorrect, they had to write down the correct answer. The Wilcoxon signed rank test was used to determine whether there was a significant difference between pre– and posttest scores, as shown in Table 2. The percentages of the students’ answers to items 1–3 are shown in Tables 3–5. The Wilcoxon signed ranks test’s results show that the students’ pre- and post-scores were significantly different for all situations (Situation 1, Z = 5.13, p = 0.00; Situation 2, Z = 4.75, p = 0.00; Situation 3, Z = 4.66, p = 0.00; overall scores, Z = 5.09, p = 0.00). The overall mean scores of the pretest and posttest were 5.21 and 13.12, respectively. These results indicated that the students improved their conceptual understanding after participating in the activity.

Table 3 shows that most of the students understand the melting of coconut oil and water. The percentages of students’ correct answers range from 61.8 to 97.1, while the percentages of students’ incorrect answers range from 2.9 to 38.2. Some students were still confused about the temperature change during the phase change.¹⁹ However, the results of items 3 and 6 show a large improvement in the percentages of students' correct answers from 0% of the pretest to 61.8% and 70.6% of the posttest, respectively. In addition, 84.9% of students understood the phase change of coconut oil after they participated in the activity.

Table 4 shows that the percentages of the students’ correct answers range from 8.8 to 97.1, while the percentages of the students’ incorrect answers range from 2.9 to 91.2. The results of items 2 and 3 show that more than 50% of students improved their understanding. These results show that students could connect their understanding of the hands-on activity to explain the application of PCMs. However, some students still have misunderstandings. Examples of students’ misunderstanding are; “(Item 3) PCMs are insulator.”, and “(Item 5) The temperature inside the buildings lower than the melting point and freezing point of PCMs during the daytime and night.”

Table4.

Percentages of students’ answers to pre– and post–questionnaire, situation 2 (n=34)

The highest percentage of students answered item 5 incorrectly in the posttest (91.2%). This might be because some students still misunderstood that the PCM was an insulator which helped maintain the building’s temperature. On the other hand, some students might not carefully read the question, which asked them to consider the case when the temperature was below the melting point of the PCM. Therefore, they thought that the statement was correct.

Table 5 shows that the percentages of the students’ correct answers range between 38.2 and 94.1, while the percentages of the students’ incorrect answers range between 8.8 and 61.8. The highest percentage of students answer item 5 incorrectly in the posttest (61.8%). This item was used to check student’s understanding at the microscopic level, which is abstract. They might think only about the solid particles inside the ice structure. Therefore, they can only collide with other particles around them. These results show that the students still had difficulty imagining about the molecular movement during the phase change. However, the intervention of this study helped students understand better about the concepts of potential energy and kinetic energy of molecules, as shown by the result of item 3 where 0% of students on the pretest correctly answered, but on the post test, 70.6% were correct. The overall results show that most of the students were able to make the connection among the molecular movement, the kinetic energy, and the phase change at the end of learning.