ZEMCH 2019 International Conference Proceedings April.2020 | Page 136
A heating process was applied in 5‐mg PCM sample using Differential Scanning Calorimetry
device (DSC‐Q200, TA INSTRUMENTS, New Castle, DE, USA) from 0 °C to 70 °C with rate of 5°C/min
while the liquid nitrogen was used to cool down the DSC pans to 0 °C in preparation for the next run
with new sample. The DSC characterization of the paraffin wax and Salt‐Hydrate‐calcium chloride
hexa‐hydrate are shown in Fig. 2 A and B respectively and the thermo‐physical properties of interest
listed in Table 1.
DSC curve shows that paraffin wax has the latent heat of 158 KJ/kg, while the Salt‐Hydrate‐calcium
chloride hexa‐hydrate has higher value of 213 KJ/kg. The wide melting range start at liquidus
temperature of 27°C, solidus temperature of 33°C for both studied PCMs and melting point of
approximately 32.5°C.
Figure 2. Differential Scanning Calorimetry curves for the (A) paraffine material and (B) Salt‐Hydrate‐
calcium chloride hexa‐hydrate
Table 1. Thermo‐physical properties of PCMs used in the experiments in solid state.
Properties
Melting Point (°C)
Latent Heat (kJ/kg)
Specific Heat Capacity (kJ/kg∙K)
Heat Conductivity (W/m∙K)
Density (kg/L)
Paraffin wax
27‐33
158± 7.5%
2
0.2
0.88
Salt hydrate
27.7‐32.23
213.66
2
0.6
1.5
2.2.2. Materials Selection
There are a wide range of physical, chemical, economic, and thermal properties that will determine
the correct PCM to use [5, 11]. The optimum characteristic of PCM reported to be high heat of fusion,
high thermal conductivity, high specific heat and density, less volume changes on phase transformation
and long‐term reliability during repeated cycling, and no degradation after freeze/melt cycles.
Therefore, several criteria of PCM have been studied and considered in the selection stage including
the drawbacks of super cooling, phase segregation issue, and corrosion.
The comparison of the abovementioned thermo‐physical, kinetic and chemical properties of
selected PCMs: salt hydrates, and paraffin waxes highlight the following facts of:
1. The thermal conductivity of paraffin waxes is (0.2 W/m K), lower than salt hydrates (0.6 W/m K).
2. The heat of fusion of paraffin waxes is (158 kJ/ kg) comparable to salt hydrates (210 kJ/kg).
3. The density of the paraffin waxes is (0.88 kg/m 3 ), lower than that of salt hydrates (1.6 kg/m 3 ).
4. The paraffin waxes show no sub‐cooling while the salt hydrates suffer from sub‐cooling issue.
5. The paraffin waxes are not corrosive to metals while the salt hydrates show corrosion to the metals.
The issues caused by super cooling and phase segregation of salt hydrate during the extraction of
stored heat cause a drop in the efciency of the material and lead to thermal cycling degradation,
limiting the long‐term stability and the useful lifecycle of the material. These issues could limit their
potential in building applications, which require systems of a long lifespan.
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ZEMCH 2019 International Conference l Seoul, Korea