Light absorptive polymeric form-stable composite phase change material for thermal storage

Journal Pre-proofs Light absorptive polymeric form-stable composite phase change material for thermal storage Shin Yiing Kee, John Lin Onn Wong, Yamuna Munusamy, Kok Seng Ong, Yang Chuan Choong PII: DOI: Reference:

S1359-4311(19)31453-X https://doi.org/10.1016/j.applthermaleng.2019.114673 ATE 114673

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Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

5 March 2019 13 October 2019 10 November 2019

Please cite this article as: S. Yiing Kee, J. Lin Onn Wong, Y. Munusamy, K. Seng Ong, Y. Chuan Choong, Light absorptive polymeric form-stable composite phase change material for thermal storage, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114673

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Light absorptive polymeric form-stable composite phase change material for thermal storage Shin Yiing Kee a, John Lin Onn Wong a, Yamuna Munusamy a,*, Kok Seng Ong b, Yang Chuan Choong b a

Department of PetroChemical Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, 31900 Kampar, Perak, Malaysia b Department of Industrial Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, 31900 Kampar, Perak, Malaysia * Corresponding author. Email-address: [email protected]; Tel.: +605-468-8888 (4555)

Abstract: Light absorptive polymeric form-stable composite phase change tablet (RGO-PARB/MA) was prepared by dip-coating myristic acid (MA) tablets into layers of polymer coating solution with reduced graphene oxide (RGO). RGO serves as light absorptive material to harvest sunlight and converts it to thermal energy to be stored in PCM. Ultraviolet (UV)-Visible absorption analysis showed that RGO absorbs UV and visible light at the absorption peak of 255 nm. In this study, a new in-house experimental setup was also developed to quantitatively measure stored heat energy, solar energy conversion, and storage efficiency in the outdoor environment. RGO-PARB/MA tablet with 1.5 wt% RGO loading had the highest amount of stored heat energy which was 48.91% higher than tablet without RGO. The average solar energy conversion and storage efficiency of 1.5 wt% RGO-PARB/MA tablet was about 21%. The melting temperature and latent heat of 1.5 wt% RGO-PARB/MA tablet were 54.79±0.07 ºC and 119.91±6.67 J/g, respectively. Thermal cycle test shows that the composite phase change material has good thermal durability, withstanding up to 1000 thermal cycles (2.7 years) with changes in thermal properties less than 2%. Keywords: phase change material; composite materials; thermal properties; energy storage; conversion; coating

1. Introduction Thermal energy storage (TES) is used to compensate for the solar fluctuation in a solar heating system. Among all TES, phase change material (PCM) is popular because it has a high thermal storage density with a small temperature variation during the phase change process [1-4]. However, it has disadvantages such as low light absorption, leakage, and corrosive nature, which limit its application in the solar energy field [5-7]. Therefore, the development of stable TES that could also harvest and convert solar energy to heat energy is very important to improve its utilization efficiency [8]. Recently, some researchers focused on the improvement of PCM’s light absorption by adding light absorptive materials such as dye and nanoparticles into PCMs. Light absorptive materials possess strong light absorbance capacity and excellent conversion of light to thermal capability [9-12]. It serves as a photon antenna to capture sunlight and convert it to thermal energy via a non-radiative decay process [13]. Previous study showed that PCM integrated with yellow dye exhibits higher temperature (58 °C) than pure PCM (27 °C) after exposure for 2750 s in light irradiation from the simulated light source [14]. In another research work, PCM doped with titanium black (Ti4O7) exhibit higher temperature (67.9 °C) than pure PCM (49 °C) after exposure for 6600 s under light radiation [15]. However, in all these studies only the increment of temperature was recorded as an indication of light to heat conversion. No testing was carried out to determine the quantitative solar energy conversion and storage efficiency after adding light absorption material in the outdoor environment. Indoor experimental setup with a controlled environment such as the intensity of the artificial light does not provide a realistic assessment on the PCM in solar applications. The exact value of light to heat conversion efficiency in the outdoor environment is crucial for further research development and actual application of light

absorptive PCMs. Thus, a new in-house experimental setup is designed to quantitatively calculate light to heat conversion and storage efficiency. In this study, RGO-PARB/MA tablet was prepared by dip-coating myristic acid (MA) tablets into layers of polymer coating solution blended with reduced graphene oxide (RGO). MA tablet was used as PCM core; Polymer coating was used as supporting material; RGO was used as light absorption materials. Two types of polymer coating solution were used in this study, nitrile butadiene rubber (NBR) and polyacrylic coating (PA). NBR was used as an inner layer because it is elastic to withstand the expansion during the phase change process, while PA is used as an outer layer because it has higher mechanical strength to withstand the pressure due to the volume change of PCM. Both of these coatings will eliminate the leakage of PCM during the phase change process. PA coating solution was blended with RGO to enhance the light absorption of PCM. RGO is efficient in light absorption because of its large optical absorptivity, tunable bandgap and band-position [16-18]. 2. Materials and Methods 2.1. Raw Materials MA, purity ≥ 98.0% was purchased from R&M Chemicals. Triton X-100 was bought from Sigma Aldrich. Nitrile butadiene rubber (NBR) was provided by Synthomer Sdn Bhd. Polyacrylic coating (PA) was supplied by Dr. Chee Swee Yong from the Faculty of Science, Universiti Tunku Abdul Rahman, Malaysia. RGO was synthesized by oxidizing graphite nanofiber via conventional Hummer’s method [19] followed by reduction using hydrazine hydrate [20]. 2.2. Preparation of RGO-PARB/MA Tablet RGO was mixed with Triton X-100 (dispersing agent) before blending with the PA coating solution. The solution was stirred for 1 hour to obtain a homogenous blend. MA powder was compressed into a tablet using hydraulic press model E-Z PressTM supplied by International Crystal Laboratories, USA at the pressure of 4000 psi. The pressed MA tablet was then coated with NBR followed by a PA coating solution blended with RGO. For each layer of coating, the MA tablet was immersed in the coating solution for five seconds. After that, MA tablet was taken out and dried at room temperature for 4 days. The steps were then repeated with the same tablet being flipped upside down. RGO-PARB/MA tablets were prepared in 0, 0.5, 1.0, 1.5 and 2.0 wt % loading of RGO. The samples are shown in Fig. 1.

Fig. 1. RGO-PARB/MA tablet with various RGO loadings (a) 0; (b) 0.5; (c) 1.0; (d) 1.5; (e) 2.0 wt%

Coating mass percentage of RGO-PARB/MA tablet is calculated by using Eq. (1): 𝐶𝑜𝑎𝑡𝑖𝑛𝑔 𝑚𝑎𝑠𝑠 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 =

𝐹𝑖𝑛𝑎𝑙 𝑚𝑎𝑠𝑠 𝑅𝐺𝑂 ― 𝑃𝐴𝑅𝐵/𝑀𝐴 𝑡𝑎𝑏𝑙𝑒𝑡 ― 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑀𝐴 𝐹𝑖𝑛𝑎𝑙 𝑚𝑎𝑠𝑠 𝑅𝐺𝑂 ― 𝑃𝐴𝑅𝐵/𝑀𝐴 𝑡𝑎𝑏𝑙𝑒𝑡

× 100%

(1)

2.3. Characterization and Testings Melting temperature, freezing temperature and specific heat capacity of PCMs were determined by using differential scanning calorimeter (DSC), model DSC1 STARe supplied by Mettler Toledo (M) Sdn Bhd before measuring their solar energy conversion and storage efficiency. Formula to calculate the specific heat capacity of the RGO-PARB/MA tablet from the DSC result had been adapted from previous research paper by Kee et al. [21]. For outdoor solar energy conversion and storage efficiency measurement, RGO-PARB/MA tablets with different RGO loading were placed on a polystyrene sheet and covered with an acrylic transparent box as illustrated in Fig. 2. A pyranometer, model CMP3 supplied by Kipp & Zonen, Netherlands was used to measure the solar irradiance. Temperatures of the composite samples were recorded using thermocouples connected to the data logger, model GL820 supplied by Graphtec, Tokyo, Japan.

Fig. 2. In-house design outdoor experiment setup for solar energy conversion and storage efficiency measurement.

Solar energy (Esolar) is generally absorbed in the top surface of the sample. A portion of the solar energy is converted into heat energy (Eheat) that substantially increases the temperature of the sample (Ts). Sunlight reflected from the top surface and radiative heat transfer to the environment account for heat loss (Eloss) from the sample. Therefore, the rate of energy absorbed by the sample is represented by Eq. (2): 𝐸ℎ𝑒𝑎𝑡 = 𝐸𝑠𝑜𝑙𝑎𝑟 ― 𝐸𝑙𝑜𝑠𝑠 = 𝑚𝑠𝑐𝑠∆𝑇𝑠

(2)

The total solar energy incident on the top surface area of the sample over a period of time is given by Eq. (3): 𝑡

𝐸𝑠𝑜𝑙𝑎𝑟 = ∫𝑡𝑁= 0𝐼𝑠𝑜𝑙𝑎𝑟(𝑡).𝐴𝑑𝑡

(3)

where Isolar is the solar irradiance measured by using the pyranometer (Wm-2) and A (π*D2/4) is the top surface area of the composite sample (m2). Finally, solar energy conversion and storage efficiency (η) can be determined using Eq. (4): 𝜂=

𝐸ℎ𝑒𝑎𝑡 𝐸𝑠𝑜𝑙𝑎𝑟

𝑋 100%

(4)

Thermal cyclic test was carried out using a thermal cyclic system designed by the University of Malaya to determine the thermal reliability of FSPCMs over a period. In each thermal cycle, the tablet samples were put into a sample holder and heated from 30 to 80˚C and then the samples were let to cool to room temperature. This procedure was repeated for 1000 cycles. Each cycle represents a day of heat gain and heat release. The results obtained from the in-house system had been widely accepted and published [22,23]. X-Ray Diffraction (XRD) diffractograms of RGO and the PCMs were recorded using XRD Diffractometer model D5000 supplied by Siemens, Munich, Germany. The characterization was carried out at room temperature in specular reflection mode at the scanning range of 2θ between 5° to 90° using copper Kα radiation (0.1542 nm wavelength). The scanning speed was 1 °/min. Visible light absorption peak of RGO and RGOPARB/MA tablets were determined from UV-Visible Spectrophotometer, model UV-1700 supplied by Shimadzu, Kyoto, Japan. Morphologies of RGO in PA film was recorded by using field emission scanning electron microscope (FESEM), model JSM 6701F from JEOL, Tokyo, Japan, with a magnification of 5000x. Surface topographic images were recorded by using Atomic Force Microscope (AFM) model XE-70 from Park System, Suwon, South Korea, to analysis three-dimensional surface topography of RGO/PA film on a 10 x 10 μm2 area. 3. Results and Discussion 3.1. Performance study of RGO-PARB/MA Table 1 shows that all RGO-PARB/MA tablets had lower latent heat of melting and freezing than pure MA tablet. This is because RGO and coating material had reduced the mass fraction of MA which contributes to the latent heat of the PCM [24]. The melting and freezing point of MA tablet and RGO-PARB/MA tablets are almost similar and confirmed that the coating material and RGO did not affect phase change temperature. The latent heat of melting for 1.5 wt% RGO-PARB/MA tablet is 119.91±6.67 J/g. The latent heat obtained from the present work is comparable with works done by other researchers as shown in Table 2. Therefore, RGOPARB/MA is ideal to be used as a light absorptive PCM. Table 1 Thermal properties of RGO-PARB/MA tablets for various RGO loading. PCM

RGO Loading (wt%)

Coating Mass Percentage (%)

PCM Mass Percentage (%)

Melting Point (0C)

Latent Heat of Melting (J/g)

Freezing Point (0C)

Latent Heat of Freezing (J/g)

MA

0.0

0.00

100.00

55.97 ±

216.04 ±

52.72 ±

216.85 ± 4.54

0.12

4.72

0.27

54.72 ±

131.84 ±

51.56 ±

0.27

5.76

0.58

55.20 ±

118.72 ±

52.42 ±

0.10

4.89

0.15

54.65 ±

121.02 ±

52.18 ±

0.13

5.06

0.20

54.79 ±

119.91 ±

52.38 ±

0.07

6.67

0.24

54.93 ±

121.05 ±

52.08 ±

0.19

6.19

0.06

PARB/MA RGO-

0.0 0.5

37.78 40.12

62.22 59.88

PARB/MA RGO-

1.0

39.79

60.21

PARB/MA RGO-

1.5

39.83

60.17

PARB/MA RGOPARB/MA

2.0

39.23

60.77

130.97 ± 7.40 117.91 ± 5.19 120.76 ± 5.07 119.66 ± 6.60 121.13 ± 6.49

Table 2 Comparison Survey of Thermal Performance for Light Absorptive PCM. No.

PCM/ PCM Composite

Light Absorptive Materials

Melting Point (0C)

Latent Heat of Melting (J/g)

References

1

Methoxypolyethylene glycol (MPEG-750) + polyethylene polyamine

Yellow dye

32.70

103.70

[14]

2

PEG 6000

1,4-bis((2hydroxyethyl) amino) anthracene-9,10dione dye

54.80

78.70

[11]

3

PEG + silicon dioxide (SiO2)

Titanium black (Ti4O7)

59.80

129.80

[15]

4

MA + PARB coating

RGO

54.79  0.07

119.91  6.67

Present study

To verify the light absorption of PCM samples with different RGO loading, the samples were placed under sunlight. Compared with pure PARB/MA tablet (0.0 wt% RGO), the temperature of all RGO-PARB/MA tablet rapidly increase upon exposure to sunlight irradiation. This is because RGO in PARB coating absorb sunlight and transform it into heat through non-radiation thermal decay. Thermally excited molecules dissipate to less energetic vibrational modes via vibrational relaxation and radiationless transition (intersystem crossing and internal conversion) [13]. Figure 3 shows that 1.5 wt% RGO-PARB/MA tablet had the highest increment in temperature compared to pure PARB/MA tablet after exposure to sunlight for 8000 s.

Fig. 3. Temperature of RGO-PARB/MA tablets after exposure to sunlight for 8000 s

The total heat energy stored in RGO-PARB/MA tablets with different RGO loading is shown in Fig 4(a). The total stored heat energy is calculated by Eq. (2) over for 3 sunny days. The RGO-PARB/MA tablet with 1.5 wt% RGO loading stored the highest amount of heat energy among the tablets; 49% higher than the tablet without RGO. The total stored heat energy is lower when the RGO loading is further increased to 2.0 wt%. This

is due to the optimum dispersion of RGO in RGO-PARB/MA pellet at 1.5 wt% of RGO which is further discussed in section 3.2 through the XRD and SEM analysis.

Fig. 4. (a) Total stored heat energy and; (b) average solar energy conversion and storage efficiency of RGO-PARB/MA tablets.

Solar energy conversion and storage efficiency calculated using Eq. (4) for RGO-PARB/MA tablets with different loading of RGO is shown in Fig. 4(b). The efficiency is dependent on the solar energy absorbed by the top surface as well as heat loss to the environment, which is mainly affected by the difference between the tablet temperature and the temperature inside the PMMA container. Therefore, to make a fair comparison of the solar energy conversion and storage efficiency of all tablets, only the results from the first 20 minutes are used in the calculation where the temperature difference is less than 50C. The result shows that the tablet with 1.5 wt% RGO loading has the highest solar energy conversion and storage efficiency. Reliability of 1.5 wt% RGO-PARB/MA tablet after 1000 times repeated thermal cycle were analyzed by DSC as shown in Fig. 5. Table 3 shows that the variances in melting temperature, freezing temperatures, latent heat of melting and freezing for 1.5 wt% RGO-PARB/MA tablet before and after thermal cycle were small and less than 2 %. Therefore, it can be concluded that 1.5 wt% RGO-PARB/MA tablet had good thermal reliability and it can be used for 1000 days (about 2.7 years) without any changes in performance, assuming that it undergoes 1 thermal cycle per day.

Fig. 5. DSC curves of RGO-PARB/MA tablet before and after 1000 thermal cycle Table 3 Thermal properties of RGO-PARB/MA tablet before and after 1000 thermal cycle. Melting Point Latent Heat of Freezing Point Latent Heat of (0C) Melting (J/g) (0C) Freezing (J/g) 54.44  0.09 122.97  4.55 51.98  0.13 123.78  5.24 Before 54.31  0.11 121.69  4.94 51.48  0.14 122.28  5.35 After 0.24  0.09 1.04  0.32 0.96  0.04 1.21  0.16 Changes percentage (%)

3.2 Charaterization of RGO-PARB/MA XRD diffractogram of RGO and PA film at various RGO loading is shown in Fig. 6. The XRD pattern for pure PA film showed no diffraction peaks, which indicated the amorphous nature of PA [25]. While RGO shows a peak at 2θ value of 26.3867°, due to the ordered graphitic structure of RGO sheets. PA film with 0.5, 1.0 and 1.5 wt% of RGO loading exhibit a small peak at 2θ value of 26°. Large reduction of RGO characteristic peaks in PA film at these loadings could be observed. Another broad and less intense peak could be observed at 2θ value of 23 -15˚ for all the three samples. This indicates an expansion of interlayer spacing distance of RGO graphitic sheets from 2.05 Å to 4.50 Å. Thus, RGO is more likely to be intercalated inside the PA matrix rather than exfoliated. The regular and ordered structure of RGO layers had been expanded with the insertion of PA polymeric chains [26]. PA film with 2.0 wt% RGO loading has an intense peak at 2θ value of 26.25052°, indicating that the RGO is not well intercalated. Similar results were also obtained for polymer composites incorporated with other multilayer fillers such as clay at high loading [27]. The multilayer fillers tend to aggregate. PA film with 2.0 wt% of RGO also contains a smaller amount of RGO intercalated which exhibits larger interlayer spacing at 2θ value of 23.6329°. These XRD results indicate that good intercalation of RGO in PA could be achieved at RGO loading of 1.5 wt% and below. Intercalation of RGO will results in a higher surface area of filler for light absorption in PCM.

Fig. 6. XRD diffractogram of RGO on PA film with various RGO loading. Absorption peak of RGO in the aqueous dispersion is at 255 nm but it still absorbed a significant amount of visible light, as shown in Fig. 7. The absorption peak around 260nm is generally regarded as the excitation of p-plasmon of graphitic structure [25]. RGO-PARB/MA tablets with 1.5% RGO loading shows the highest absorbance among all PCM loaded with RGO. XRD results proved that the RGO is well dispersed in PA film, up to 1.5 wt% RGO loading. This provides more surface area for absorbance of UV and visible light. At 2 wt% loading, the dispersion of RGO is poor in PA and this leads to lower surface area for UV and visible light absorption.

Fig. 7. UV-Vis spectra of RGO powder and RGO-PARB/MA tablets.

Cross-section FESEM images of PA with 1.5 and 2.0 wt% RGO loading are shown in Fig. 8 (a) and (b), respectively. The black spot in Fig. 8 represents the RGO and this observation was reported in the literature by Qureshi at al. [28]. PA with 2.0 wt% RGO loading has more and larger aggregate number of multi-layer RGO compared to PA with 1.5 wt% RGO. This indicated that at 2.0 wt% RGO the filler formed agglomerates and not well intercalated in the PA matrix. These images further support the results obtained from XRD diffractogram where the 2ϴ value peak for 2.0 wt% RGO filled PA was similar to the 2ϴ value peak for original RGO. Agglomeration of RGO will reduce the effective surface area for sunlight absorption in PCMs.

Fig. 8. Cross Section FESEM images of PA at (a) 1.5 and (b) 2.0 wt% RGO loading AFM images of PA at 1.5 and 2.0 wt% RGO loading are shown in Fig. 9 (a) and (b), respectively. PA with 2.0 wt% RGO loading has a rougher surface, corrugated with wrinkles, and more agglomeration as compared to PA with 1.5 wt% RGO. This indicated that 2.0 wt% RGO loading is not well dispersed in PA and has more aggregate number of multi-layer RGO.

Fig. 9. AFM images of PA at (a) 1.5 and (b) 2.0 wt% RGO loading

4. Conclusion RGO-PARB/MA tablet was prepared successfully. Absorption wavelength of RGO is at 255 nm. The optimum RGO loading for RGO-PARB/MA tablet was 1.5 wt%, recording the highest UV absorbance, the largest amount of stored heat energy (349.72 J) and the highest solar energy conversion and storage efficiency (21%). Melting temperature and latent heat of melting of the tablets were 54.79 ± 0.07 0C and 119.91 ± 6.67 J/g respectively, which is suitable to be used in TES applications. XRD, FESEM and AFM results also showed that 1.5 wt% of RGO is well dispersed in PA coating film. Produced tablets were thermally reliable as the changes of melting and freezing latent heat after 1000 thermal cycle was less than 2 %.

Conflicts of Interest The authors declare no conflict of interest.

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Graphical abstract

Highlights  A new in house experimental setup for light to heat conversion had been developed.  Stored heat and solar energy conversion efficiency is measured quantitatively.  Improvement of 48.91% higher heat storage was achieved.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.