Differential Scanning Calorimetry Test
Differential Scanning Calorimetry (DSC) technique is a well-known thermal analysis method that focuses on the difference between the amount of heat required to increase the sample temperature and the measured reference. During the DSC test, sample material and reference material are maintained at the same temperature. Also, the holder temperature increases linearly (as a function of time) during the
DSC test. It is a reliable technique to measure the latent heat of fusion, melting point, and heat capacity for PCMs.
Experimental Setup and Procedures of DSC
For DSC tests, the experimental instrument ТА Q200 was used for the sample materials with weight 0-10mg. To hold the samples, an aluminum pan (Perkin Elmer) was used, while an empty pan was considered as the reference. Q200 chamber was used for heating and cooling of samples at various rates. Data collection was done with a program, ТА Universal Analysis; then, the convenient cycles were chosen for the analysis. Q200 chamber and pans (both reference and sample) are shown in Figure 9.4. By using the experimental setups, different commercial PCMs were tested.
Results and Discussion
As an organic PCM, paraffin wax was used since it is a very common and inexpensive PCM that is widely applied especially for the low-temperature TES systems. In the experiments, 6.9 mg paraffin wax sample (VWR, Singapore) was used. The measurements were done by a weight balance with high precision (accuracy: ±0.0001 g). To reach accurate results, a slow heating rate, 5°C/min, was applied to the sample pan. Figure 9.5 shows that the glass transition of the paraffin wax was seen from 29.09°C to 33.92°C, whereas the melting was observed from 45.01°C to 61.73°C and charged with 128.20J g_l as its latent heat.
As an inorganic PCM, lithium nitrate (LiNO,) was selected for DSC experiments. LiNO, has high latent heat and melting point; therefore, the heating rate was selected as 20°C/min. As shown in Figure 9.6, the freezing range was between 250.25°C and 220.70°C, while the melting range was between 232.21°C and 275.13°C.
Since the melting and freezing periods showed different behaviors, that difference must be considered as well to really apply LiNO,. The latent heat values were observed to be different as well. For the melting and freezing periods, they were 304 and 325 J g_l, respectively. The difference between the specific heats of the solid and liquid phases might be the main reason for the obtained 21J g_l difference.
FIGURE 9.4 DSC test equipment: (a) ТА Q200 and (b) aluminum pans. (Modified and redrawn from Qin (2016).)
FIGURE 9.5 The DSC melting curve of 6.9mg paraffin wax under the heating rate of 5°C/min. (Modified and redrawn from Qin (2016).)
FIGURE 9.6 The DSC curve for 13.6mg LiNO} under heating rate 20°C/min. (Modified
and redrawn from Qin (2016).)
T-history Method Experiment
The method was firstly presented by Zhang and Jiang (1999). It provides an easy way to measure the PCM performance characteristics such as the latent heat of fusion, melting point, degree of sub-cooling, thermal conductivity, and specific heat. It has similarities with the DSC technique from the function angle, but relatively heavier samples, normally more than 10g, are used in the experiments. Another difference between the T-history and DSC techniques is that the T-history method can present thermal property values closer to real PCM performance since real applications use a large amount of PCM, unlike the DSC technique. In the T-history method, the cooling curve is plotted first; then, all the PCM performance characteristics are obtained from the cooling curve by using the lumped capacitance method. For uniform temperature distribution, the test sample should have the Biot number (Bi) value <0.1. Bi depends on tube length, Lc, which is the radius of the test tubes in this experiment, r; convective heat transfer coefficient between the ambient and PCM, /г; and the thermal conductivity, k, as defined in 9.11.
Experimental Setup and Procedure of T-history Method
A mortar was used to grind the samples; then, they were used in the test unit (a test tube), which was inserted into a heating block (aluminum) for melting. A K-type thermocouple was used to measure the temperature increment after its calibration. The calibration provides less error for more accurate results. From 30°C to 90°C, the calibration was done with 15°C intervals in a bath calibrator with thermal oil. The reverse cooling was conducted following the same intervals. After the melting process, the PCM was cooled down to the ambient temperature. The calibration curves of three selected thermocouples are shown in Figure 9.7.
The observed difference was ±0.96°C; therefore, it was accepted that the thermocouples were calibrated in good agreement. During the heating and cooling processes, the cooling curve was observed. Figure 9.8a and b presents the experimental setup for T-history method and the heating/cooling steps of the experiments, respectively.
T-history Analysis for Reference Material: KNO[sub(3)]
As a reference material for T-history experiments, KNO, was selected due to its high melting point at 334°C, and it does not experience any phase transition in the T-history method temperature range. The obtained cooling curve of KNO, is shown in Figure 9.9, and the h value can be calculated through the curve.
For the calculation of h, the heat transfer balance between the convective heat loss and the heat stored by the tube and material were built first, as defined in Eq. (9.12),
FIGURE 9.7 The thermocouples calibration results for T-history experiments. (Modified and redrawn from Qin (2016).)
FIGURE 9.8 T-history method: (a) the experimental setup and (b) the heating/cooling steps
of the experiments. (Modified and redrawn from Qin (2016).)
FIGURE 9.9 Cooling curve for KNO, as reference material. (Modified and redrawn from Qin (2016).)
where Л = f(T, Татьis the area under temperatuie time curve, ifir is the mass <0
of the test tube, cpJ is the specific heat of the test tube, m, is the mass of reference material KNO„ cps is the specific heat of KNO,, T0 is the initial temperature, and Tf is the final temperature. The convective heat transfer area, Д , was calculated by assuming the PCM test tube as cylindrical, which allowed assuming Ac as the area by multiplication of tube diameter and tube height. Following the mathematical procedure, the h value was found as 26.05 W m~2°C. The experiments were conducted three times to present accurate results, and the repeated measurements showed that the results were in good agreement w'ith one another, as shown in Figure 9.10.
The mean h value was found as 26.4 W trr2-°C. With respect to the obtained curves, the h value was also calculated for three repeated runs, and the results are presented in Table 9.5 with the calculated thermal conductivity value, >0.934 W rrr'-K, which was obtained by considering the maximum limit of Bi number as 0.1 (see Eq. (9.11)). The experimental procedure presented for KNO, is able to give the main procedure for the eutectic mixtures (e.g. LiN03-KNO,) that are a type of PCM.