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  • Chapter : 4
DSC Analysis of Nano-enhanced Monobasic and Binary Solid-Solid Phase Change Materials for Thermal Storage
K. P. Venkitaraj 1✉,2 Email
S. Suresh 2
B. S. Bibin 1
Jisa Abraham 1
1Department of Mechanical EngineeringCollege of Engineering AdoorAdoorKerala691551India
2Department of Mechanical EngineeringNIT TrichyTiruchirappalliTamil Nadu620015India
Abstract
Solid-Solid PCMs are latent energy storage substance that can absorb, store and release a substantial amount of thermal energy. Since polyalcohols have low thermal properties, nanoparticles are added to enhance these properties. In this study solid-solid NPG, TAM and binary PCM along with different mass percentage (0.1%, 0.5% and 1%) of aluminium oxide (Al2O3) nanoparticle is evenly dispersed using a low energy ball mill. DCS tests were performed on each sample at 10 ℃/min heating rate to investigate its thermal properties. It can be observed that the addition of nanoparticles tends negligible effect on transition temperature and latent heat of enthalpy. The transition temperatures for NPG, TAM and binary PCMs before thermal cycling were 51.86 ℃, 138.5 ℃ and 45.76 ℃, respectively. Also, no significant changes were observed for transition temperatures after thermal cycling.
Keywords
Binary PCM
Differential scanning calorimetry (DSC)
Monobasic PCM
Thermal cycling test
1.
Introduction
Phase change materials (PCMs) are latent heat storage substance that absorb, store and release a substantial amount of thermal energy during phase change processes and have thermal energy storage density greater than that of sensible thermal storage materials which makes it suited for thermal energy storage applications [123]. The principal of PCM is such that, as temperature increases the material changes its phase from solid to liquid by absorbing heat, the reaction being endothermic. Similarly, the phase changes for the material from liquid to solid, when its temperature is decreased. The reaction being exothermic, the PCM desorbs heat. In recent years, latent heat storage systems using phase change materials have gained importance because of their high-energy density and isothermal behaviour during charging and discharging process.
Solid-Solid PCMs have latent energy storage materials that can absorb, store and release a large amount of thermal energy when compared to solid-liquid PCM. Among all PCM solid-solid type are fairly good to use because of its small volume change property, imperceptible sub-cooling, non-toxicity, good thermal efficiency and benign characteristics [4]. The fusion enthalpies of many paraffins are similar to transition enthalpy for solid-solid PCM such as pentaerythritol [PE], pentaglycerine [PG], and neopentylglycol [NPG] [5]. It was studied that the addition of nanoparticles improved the thermal properties of PCM [67]. Thermal cycling tests are to determine whether these thermal exposures will result in migration of the PCM or may affect the thermal properties of the PCM [6]. Binary mixtures of organic compounds have more important in heat storage. The advancement of the binary phase diagram is of significance for thermal energy storage applications [8]. Benson et al. [9] investigated the properties of binary mixture of pentaerythritol and related polyhydric alcohols also Barrio et al. [10] prepared the mixture of pentaerythritol and pentaglycerine and the transition occurring was observed at 168 ℃.
DSC (Differential Scanning Calorimetry) is used to study the thermal characteristics and properties of PCM. In these analyses, the system measures the difference in the amount of heat required to increase the temperature of a material sample (sample pan) and an empty sample (reference pan) as a function of temperature [11]. DSC measures the amount of heat absorbed or released by a sample in comparison with a reference sample. From DSC measurements, heating and cooling curves of PCM are obtained. By analysing these results, the transition temperature and latent heat capacity of PCM can be determined.
This paper studies the thermal characteristics of pure and nano-enhanced NPG, TAM and their mixtures. A comparison between the DSC analysis results is presented in this paper.
2.
Methodology
2.1.
Materials
The materials were purchased from Alfa Aesar with purities of 98% for neopentyl glycol (NPG, C5H12O2) and 99% for Tris(hydroxymethyl)amino ethane (TAM, C4H11NO3). Both are organic solid-solid phase change materials, where the transition takes place at 40–48 ℃ and 168–172 ℃ for NPG and TAM, respectively. Aluminium Oxide (Al2O3) nanoparticles having 99.5% purity with 40–50 nm powder size, used to improve the thermal conductivity of the materials were purchased from Alfa Aesar.
Figure 1 shows the SEM image of NPG, TAM and binary PCM using Jeol JSM 6390LV SEM at magnification 10KX. The microstructure of powdered samples shows that they have loose microstructure and lots of individual lamellae on the surface. The addition of nanoparticles gives a more compact lamellar microstructure to the PCMs.
Fig. 1
SEM images of a NPG b TAM and c Binary PCM
2.2.
Sample Preparation
The nanoparticles (Al2O3) were added to neopentyl glycol and Tris(hydroxymethyl)amino ethane in 0.1, 0.5 and 1% weight fraction. Proper mixing of nano-additives in PCM was ensured with the aid of low-energy lab ball mill (0.5 HP/230 V/50 Hz/300 rpm, Make: VB Ceramics) operated at 200 rpm for 120 min.
Binary PCM was prepared by mixing NPG and TAM in a proportion. Nano-additives were added to the mixture of PCM in different weight fractions (0.1, 0.5 and 1%). Preparation of nano-enhanced was done using low-energy lab ball mill.
2.3.
Methods
2.3.1.
Thermal Cycling Test
A thermal cycling test was done to determine the cycling stability of the PCMs. The samples were subjected to repeated charging and discharging in the thermal cycling unit. The schematic diagram of the thermal cycling experiment setup is shown in Fig. 2. The test unit consists of a hot plate. The PCM sample temperature and hot plate temperatures are recorded with the help of thermocouples connected to a computer-controlled data logger.
Fig. 2
Schematic diagram of thermal cycling test
2.3.2.
Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry (DSC) is a widely used method to characterise the thermal properties of phase change materials. It is a thermo-analytical technique in which the difference in the amount required to increase the temperature of a sample and reference is measured as function of temperature. In this test, the sample and reference are maintained at nearly the same temperature throughout the experiment. Alumina is used as reference sample since it has high-melting temperature. DSC measurements were performed at 10 ℃/min for identifying the transition temperature and latent heat.
The result of a DSC experiment is a curve of heat flux versus temperature or versus time. This curve can be used to calculate the latent heat. The starting, ending and peak temperature of transition can be noted down using the graph that is being obtained.
In case of a solid-solid phase change materials like NPG and TAM, there will be peaks, which represent the solid-solid phase transition peak. The area under the peak indicates the phase transition enthalpy. The onset is the temperature at which phase transition starts. The DSC measurements were carried out by using NETZSCH DSC 204. The instrument had a temperature range of room temperature to max. 150 ℃.
3.
Results and Discussion
3.1.
Transition Temperature and Latent Heat of NPG
The peak in the curve signifies the latent heat of enthalpy of phase change for the PCM. Figure 3 shows the heating curve of NPG and nano-enhanced NPG before thermal cycling.
Fig. 3
Heating curve of NPG and nano-enhanced NPG before thermal cycling
Peak transition temperature of pure NPG before thermal cycling was obtained at 51.86 ℃ and the latent heat of transition was 130.58 J/g. The transition temperature and latent heat are varied with addition of the nanoparticle to the samples. When 0.1 weight percentage alumina was added to NPG, the transition temperature changed to 50.653 ℃ and enthalpy changed from 130.58 to 127.4 J/g. Further increasing of weight percentage of nanoparticle from 0.1 to 0.5%, the temperature of transition varied from 50.653 to 46.35 ℃. The corresponding variation took place in case of latent heat also, which changed to 136.2 J/g. When the weight percentage of nanoparticle reached to 1%, transition temperature is again decreased to 45.85 ℃ and transition enthalpy was about 150.98 J/g.
When the samples undergo thermal cycling, a slight change occurs in transition temperatures and latent heat of enthalpies of PCM samples. Figure 4 shows curves of PCM after thermal cycling.
Fig. 4
Heating curve of NPG and nano-enhanced NPG after thermal cycling
For pure NPG after 100 thermal cycling, the transition was observed as 46.32 ℃ and latent heat of transition was found out as 125.884 J/g. For 0.1% nano-enhanced PCM transition occurred at 46.68 ℃ and the corresponding enthalpy was 102.2 J/g. When the nano-additive increased to 0.5%, the transition temperature after thermal cycling changed to 50.82 ℃ and latent heat of enthalpy changed to 104.4 J/g. With further increase of nano-additive (to 1%), transition occurred at 49.54 ℃ and the enthalpy of transition was 120.48 J/g.
From the thermal cycling test, it was found that the variation in the transition temperatures and latent heat of enthalpies was negligible. From the results, it can be seen that the nano-enhanced PCMs are reliable. Also, the transition occurs in the lower temperature region; therefore, it is used for solar applications.
3.2.
Transition Temperature and Latent Heat of TAM
The DSC curves plotted for samples before and after cycling with different weight percentage of nano-addition 0, 0.1, 0.5 and 1% are shown in Figs. 5 and 6. The onset, peak and endset of crystal structure transition were observed at 134.3, 138.5 and 146.3 ℃ for pure non-cycled TAM. The latent heat corresponding to this solid-solid transition was obtained as 281.8 J/g. For TAM with 0.1% addition of nanoparticles, the values obtained were 138.64 ℃, 134.35 ℃ and 146.69 ℃ for onset, peak and endset, respectively. From the DSC curve for 0.5 weight percentage of nano-enhanced PCM, the onset, peak and offset values of heating curve for samples before thermal cycling were obtained as 134.43 ℃, 139.31 ℃ and 147.6 ℃ respectively and 134.5 ℃, 139.6 ℃ and 148.43 ℃ respectively for TAM with 1% of nano-addition before cycling. It can be seen from the results that the peak value of transition varied from 138.5 to 139.6 ℃ for pure and 1% nano-added samples before cycling indicating that the peak transition value of samples increased with improved addition of nanoparticles.
Fig. 5
Heating curve of TAM and nano-enhanced TAM before thermal cycling
Fig. 6
Heating curve of TAM and nano-enhanced TAM after thermal cycling
The latent heat values of non-cycled samples also varied with addition of nanoparticles. It can be observed from Fig. 5 that the values obtained were 281.8 J/g, 274.39 J/g, 253.15 J/g and 234.8 J/g for 0%, 0.1%, 0.5% and 1% nano-added samples of TAM.
For pure TAM after 100 cycles, the values obtained for crystal structure transition onset, peak and endset are 134.46, 139.34 and 147.73 ℃ which show slightly increased values from non-cycled samples. The onset, peak and endset of crystal structure transition were observed at 134.42 ℃, 140.38 ℃ and 147.2 ℃ respectively for 0.1% Al2O3-added cycled TAM. From the DSC curves for 0.5 weight percentage of nano-enhanced PCM after cycling, the onset, peak and offset values of heating curve were obtained as 134.57 ℃, 141.07 ℃ and 148.2 ℃, respectively and 134.72 ℃, 141.74 ℃ and 141.74 ℃, respectively, for TAM with 1% of nano-addition after cycling. It can be seen from the results that the transition temperature of the samples further increased for each sample when compared with non-cycled samples as a result of constant heating and cooling for 100 cycles.
From Fig. 6, it can be observed that the values obtained for latent heat were 259.16 J/g, 243.13 J/g, 234.48 J/g and 222.23 J/g, respectively, for 0%, 0.1%, 0.5% and 1% nano-added samples of TAM after cycling. Repeated heating and cooling cyclic process on the samples tends to decrease the latent heat values of the samples. It can be seen when comparing the values obtained from non-cycled and cycled samples that the enthalpy values decreased from 281.8 to 259.16 J/g for 0% addition of TAM and similarly for each sample of same weight fraction.
3.3.
Transition Temperature and Latent Heat of Binary PCM
The heating curves of binary PCM obtained from DSC analysis before thermal cycling are shown in Fig. 7.
Fig. 7
Heating curve of binary PCM and nano-enhanced binary PCM before thermal cycling
From the curve of Pure NPG and Pure TAM mixture, the peak transition temperature was identified as 45.76 ℃ and the transition enthalpy was 88.31 J/g. When 0.1% of alumina nanoparticle was added to the mixture, the transition temperature changed to 44.434 ℃ and the corresponding change in transition enthalpy found out. Latent heat of 0.1% nano-enhanced PCM was about 86.8 J/g. Further increasing the nano-concentration to 0.5%, the transition temperature and latent heat of transitions were decreased. For 0.5% nano-addition, the transition was observed at a temperature of 44.34 ℃ and latent heat of transition as 84.18 J/g. When nano-addition was increased to 1 weight percentage, the transition occurred at a temperature of 43.93 ℃ and latent heat of transition was observed as 83.29 J/g.
There was a slight change in the heating curve of PCM sample after thermal cycling. From the curves, it was seen that latent heat of transition and transition temperatures were varied. Figure 8 shows heating curves of binary PCM and nano-enhanced binary PCM after thermal cycling.
Fig. 8
Heating curve of TAM and nano-enhanced TAM after thermal cycling
By analysing the heating curve, it was found that the transition occurred for thermal cycled pure binary PCM at 45.35 ℃ and enthalpy of transition was identified as 88.09 J/g. For 0.1% nano-enhanced binary PCM, transition occurred at temperature of 44.23 ℃ and corresponding transition enthalpy was 71.88 J/g. Further increase of nanoparticle to 0.5 weight percentage, the transition temperature varied from 44.23 to 44.341 ℃ and latent heat of transition changes from 71.88 to 65.43 J/g. When the concentration of alumina nanoparticles reaches to 1%, transition temperature decreased to 43.66 ℃ and transition enthalpy also declined to 57.09 J/g.
After thermal cycling, there was a slight variation in properties of the phase change materials. But the variations were not affecting the stability of the materials. So that, the material was reliable.
4.
Conclusion
In this study, nano-enhanced NPG, TAM and binary PCMs were prepared by adding aluminium oxide using a low-energy ball mill to enhance their thermal properties. Thermal cycling tests were carried out in thermal cycling unit by repeatedly heating and cooling at 10 ℃/min to analyse whether thermal or chemical degradation occurs. Differential scanning calorimetry (DSC) tests were conducted to study the thermal properties such as transition temperature and latent heat of enthalpy.
It was seen from the curves that the transition temperatures of pure NPG, TAM and binary PCMs before thermal cycling were 51.86 ℃, 138.5 ℃ and 45.76 ℃, respectively. The addition of aluminium oxide nanoparticles tends to slightly decrease the transition temperature and latent heat of enthalpy values which can be considered negligible. This is due to by adding nanoparticle to the PCM, thermal conductivity of the modified PCM increased. Thereby accelerate the energy storage and release in faster rate. These cause decline in latent heat of enthalpy and temperature. The enthalpy values for samples after thermal cycling reduced by 3.5% for NPG and 8% for TAM when compared to samples before thermal cycling, while the change in transition temperature remains insignificant after 100 thermal cycles.
Acknowledgements
The authors wish to thank CERD (Centre for Engineering Research and Development) for the financial support provided for this experimental investigation work.
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