Composite of wood-plastic and micro-encapsulated phase change material (MEPCM) used for thermal energy storage

Composite of wood-plastic and micro-encapsulated phase change material (MEPCM) used for thermal energy storage

Accepted Manuscript Composite of Wood-Plastic and Micro-Encapsulated Phase Change Material (MEPCM) Used for Thermal Energy Storage A. Jamekhorshid, S...

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Accepted Manuscript Composite of Wood-Plastic and Micro-Encapsulated Phase Change Material (MEPCM) Used for Thermal Energy Storage A. Jamekhorshid, S.M. Sadrameli, R. Barzin, M. Farid PII: DOI: Reference:

S1359-4311(16)32264-5 http://dx.doi.org/10.1016/j.applthermaleng.2016.10.037 ATE 9239

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

11 July 2016 27 September 2016 8 October 2016

Please cite this article as: A. Jamekhorshid, S.M. Sadrameli, R. Barzin, M. Farid, Composite of Wood-Plastic and Micro-Encapsulated Phase Change Material (MEPCM) Used for Thermal Energy Storage, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.10.037

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Composite of Wood-Plastic and Micro-Encapsulated Phase Change Material (MEPCM) Used for Thermal Energy Storage A. Jamekhorshida, S. M. Sadramelib*, R. Barzinc, M. Faridc a

Faculty of Oil, Gas and Petrochemical Engineering, Persian Gulf University, Bushehr, Iran b Faculty of Chemical Engineering, Tarbiat Modares University, Tehran, Iran c Department of Chemical and Material Engineering, The University of Auckland, New Zealand

Abstract Application of phase change materials (PCMs) in lightweight building is growing due to the high latent heat of fusion of PCMs and their ability to control temperature by absorbing and releasing heat efficiently. Wood-plastic composites (WPC) are materials used in the interior parts of buildings that have improved properties compared to conventional materials. However, these materials have low energy storage capacity, which can be improved by incorporating PCM in them. Leakage of PCM is a major obstacle to the industrial applications, which can be solved through the use of microencapsulated PCM (MEPCM). This paper presents the performance tests conducted for a composite of wood-plastic-MEPCM for using in buildings for thermal storage. The wood-plastic-MEPCM composites were produced in this project using compression molding and their thermal and mechanical properties were investigated using DSC analysis, cycling test, leakage test, and three point bending analysis. The results showed that there is no leakage of PCM during phase change. The results also indicated that the composite has reasonable thermal properties, but its mechanical properties need to be improved by increasing the pressure during the molding process or by using extrusion method. The produced composites can be used as a building material for thermal energy management of building.

Keywords: Phase change material, PCM, Wood-plastic-MEPCM composite, Microencapsulation.

*

Corresponding author: Tel/Fax: 0098 21 82884902, E-mail: [email protected] (S. M. Sadrameli)

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1. Introduction Nowadays, lightweight constructions require special attention with regards energy use and management. Thermal mass is the ability of building to absorb and store thermal energy. There are different methods of enhancing thermal energy storage capacity in light weight constructions. Latent heat thermal energy storage (LHTES) is the most attractive method since it provide high energy storage density and nearly constant operating temperature, depending on the melting point of phase change material (PCM) [1]. Buildings with incorporated PCMs into their compartments are able to absorb thermal energy during the day and release it during night. This passive heat releasing and absorbing cycle reduces temperature fluctuation inside the building that leads to reduction in energy consumed in heating and air-conditioning. Several researchers have proposed the incorporation of PCMs into different compartments of buildings, e.g. wallboards [2-6], walls [7, 8], floors and ceilings [9-11], and shutters [12]. They used different incorporation methods for PCM including traditional methods such as direct incorporation, immersion, macro-encapsulation, shape-stabilization, and microencapsulation [13]. The use of composites in building structures have experienced extensive growth in recent years, especially polymer based composites [14]. Wood-plastic composites (WPCs) are one of composite that may be used in railing, siding, fencing, window, door frame, and indoor furniture of the buildings. They consist of a polymeric matrix, usually polyethylene, wood particles or flours as filler, and other additives. Compared to wood and plastic, WPCs have improved physical and mechanical properties [15]. WPCs have a low thermal mass and hence incorporating PCMs into their structure would improve significantly their thermal energy storage capacity. However, diffusion and leaking of melted PCM to the surface will always be an issue if direct incorporation of PCM into WPC has been done using traditional methods mentioned earlier. An effective technique of preventing leakage during PCMs melting is microencapsulating them. The polymer or inorganic shell of microencapsulated PCM (MEPCM) provides a non-diffusible barrier for the melted PCM [16].

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Microencapsulation of organic PCMs has been studied by many investigators in recent years aiming to encapsulate different PCMs such as n-octadecane [18-20], n-tetradecane [21], n-pentadecane [22], paraffin waxes [23-24] and n-hexadecane [25]. Therefore, incorporating MEPCM into WPC would leads to a durable composite with high thermal energy capacity. The objective of this study was to prepare a wood-plastic-MEPCM composite and investigate its performance for peak-shaving and for reducing temperature fluctuations in buildings. For this purpose, the composites were prepared using compression molding. DuPontTM Energain® wallboards, which contains about 60 wt% PCM, are commercially available. Therefore, a comparison has been made between the thermal properties of the composite prepared with those of Energain®.

2. Experimental 2.1. Materials The materials used in this study were high density polyethylene (HDPE), wood, and MEPCM. HDPE was purchased from Courtenay Polymer Pty Ltd., New Zealand, in a powder form with the trade name CoteneTM. Pine sawdust was supplied from timber laboratory of the University of Auckland and sieved to obtain particle size below 500 µm. Then it was dried in an oven at 50°C for at least 24 hours. Maleated polyethylene (MPE) was used as coupling agent for improvement the compatibility between the polar wood molecules and non-polar polymeric matrices [17]. MPE powder was obtained from Clariant. MPCM 24D and MPCM 28D were purchased from Microtek Laboratories and Micronal® DS5008X from BASF. The melting point of MPCM 24D, MPCM 28D, and Micronal® DS5008X are 24, 28, and 23 °C, respectively. 2.2. Composite preparation The raw materials were premixed according to the recipe (table 1) and molded into slabs (figure 1) using a hot-compression machine (GARLAND MWEH-9501) at a temperature of 150 °C. After compression, the composite temperature was decreased gradually at room temperature to avoid distortion of the composite surface. Two types of composites were constructed using either PCM or microencapsulated PCM (MEPCM) for purpose of comparison with regards PCM leakage.

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250 mm

150 mm

5 mm

5 mm

Fig. 1. Aluminum molding plate dimensions.

Table 1. Compositions of raw materials in the composite. Composite PCM Type Wood HDPE No. wt.% wt.% 1 MPCM 24D 25 40 2 MPCM 28D 25 37 3 Micronal 25 37 4 MPCM 28D 25 40 5 MPCM 28D 25 35 6 MPCM 28D 25 22 7 40 60 8 PT24 25 22 9 PT24 25 37 10 MPCM 24D 22 40 11 PT29 0 30

MPE wt.% 0 3 3 0 5 3 0 3 3 0 0

PCM wt.% 35 35 35 35 35 50 0 50 35 35 70

2.3. Composite Tests One of the main issues in using PCM with building materials is leakage when it melts. For this purpose leaking test has been performed on the made composite materials using a solvent. Samples of composite has been weighted and kept in the oven for 30 minutes at temperature above the melting point of PCM. Following that the samples were immersed in hexane as a solvent for 30 minutes. The dried samples then weighted and the difference the initial and final weight provide a quantitative measure of PCM leakage. Latent heat storage capacity of the composite was measured by DSC model Shimadzu DSC 60. The heating and cooling temperatures have been altered between 5oC to 45oC with a rate of 2oC/min under atmospheric dry air. Heat cycling test for the samples was performed using the SANYO heating and cooling incubator model MIR-254 with repeated heating and cooling cycles as shown in Fig 2. Two types of samples were prepared for this test. The first type was an aluminum cube contained composite sheets with size of 70×70×5 mm in its interior wall. A 4

thermocouple was used to detect air temperature change inside this cube. The second type was two sheets of composite with size of 70×70×5 mm each, which attached to each other firmly with a thermocouple fixed between them. Finally, for three point bending test, ASTM D-790 standard was employed as a reference method for testing board strength. The length of support span was 64 mm and the cross head speed was calculated and set at 1.3 mm/min.

Fig. 2. Heating and cooling cycle for the heat cycling analyzer.

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Results and discussion

3.1. Preparation of composite In this study, samples of composite from PCM-wood-polymer with dimensions of 150×150×5 mm were prepared for testing as illustrated in Fig. 3.

Fig. 3. Prepared sample of composite wood, plastic, and MEPCM.

3.2. Thermal properties 3.2.1. Heat Storage Capacity Tests The heating characteristic of three prepared samples of composites along with the commercial MEPCMs has been done using DSC and the results are shown in Table 2. The 5

heating capacity of the samples has also been calculated by multiplication of latent heat of pure MEPCM with PCM mass fraction. As seen from the results in all three samples of composites the measured heating capacity is less than the calculated one. This may be due to the nonuniform distribution of MEPCM in the composite, and the extremely small sample used in the DSC analysis. The very large loss of storage capacity of the board containing Micronal suggests that this type of capsules are not suitable for use in such process in which use excessive compression and heat. The shell of this MEPCM is made of PMMA which has lower thermal resistance and higher potential of leaking during the compression molding in comparison to the MEPCM Microtek [18-20]. Table 2. Measured and calculated heating capacities of the different composites. Composite PCM type Measured heating Calculated heating Error % No. capacity, J/g capacity, J/g MPCM 24D 1 26.5 32.4 18.2 MPCM 28D 2 42.8 47.8 10.5 Micronal 3 18.5 30.3 38.9 MPCM 24D 92.5 MPCM 28D 136.7 Micronal 86.5 -

3.2.2. Heat cycling test The result of heat cycling tests for two cubes having composite number 7 (without PCM) and composite number 11 (with PCM) are shown in Fig. 4. The variation of the air temperature surrounding the samples are also illustrated in the figure and mentioned as "Inside temperature". As shown in the figure the internal air temperature of both cubes changes with inside temperature with a time lag which is due to the conductive heat transfer resistance from the cube wall and convective resistance inside the cube. It is also clear from the curves that the temperature of the sample including PCM has a time lag as annotated by point 1 and 2 which proves the influence of PCM on the sample. The time lag is about 30 mins in heating process and 100 mins in cooling.

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Fig. 4. Heat cycling test results for two cubes equipped with different composites.

The heat cycling tests for the composite sample of wood, 25 wt.%, MPE, 3 wt.%, HDPE, 22 wt.%, and PT24, 50 wt.% (composite number 8) and the sample produced by DuPont TM with a brand name of Energain® containing 60 wt.% PCM and heating value of 56.5 J/g is illustrated in Fig. 5. As depicted from the figure the prepared composite shows very good results in comparison with the commercial sample.

Fig. 5. Temperature profiles of a prepared composite and a sample made by DuPontTM .

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3.2.3. Hut Test The standard composite made by DuPont TM has been fixed in a test hut (Fig. 6). The detailed information about the hut may found in [11]. The internal walls and ceiling in one of the huts covered by Energain® samples and its internal temperature has been recorded every 30 seconds. The other hut was used as reference. The results below demonstated the benefit of using PCM in building. It was not possible to use our produced composit as only small size samples were prepared. In section 3.2.2 it was shown that the composite sample produced in this work is superior to that of DuPont TM.

Fig. 6. Two hut rooms located in the University of Auckland, New Zealand.

The temperature distributions of the ambient temperature and room temperature for two weeks period are illustrated in Fig. 7. The results show that in general the internal air temperature of the hut covered by PCM sheets are less than the one without PCM. It is also clear that the temperature fluctuation is less for the hut with PCM. The average temperature of the room with PCM is slightly higher than the one without PCM (19.6oC and 18.5oC, respectively) which is due to the extra conduction resistance of the walls caused by DuPont TM sheets. It is also clear from Fig. 7 that ambient air temperature for the day 11/7/13 was lower than the other days.

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Fig. 7. Temperature variation of ambient and inside rooms with and without PCM.

The temperature variation during three days in which the maximum ambient temperature has increased continuously (20.5, 24.0, and 33.1 °C respectively) is shown in Fig. 8. With regard to this ambient temperature increase, the maximum air temperature of the hut without PCM is increased (23.3, 24.4, and 25.6 °C respectively) while the hut with PCM has experienced much less variations (22.7, 23.2, and 22.8 °C respectively). In the other words, the variations of ambient maximum temperature have little effect on the air temperature of the hut with PCM. So, the amount of PCM used in the hut was enough for this environmental condition. In addition, the maximum room air temperature is affected by the minimum ambient temperature at night. According to the Fig. 9 the minimum air temperature of the hut with PCM at first two nights is not less than 12°C and consequently the maximum air temperature at the next days is increased to 25.7°C and 26.5°C, respectively. The maximum air temperature of the hut with PCM at third day is 22.7°C due to its previous cold night. So, using the passive thermal energy storage systems with PCM have some restrictions. In general, the results of using Energain® panels in the hut prove their effectiveness in reducing the air temperature fluctuation. Consequently, the composite prepared in this work would be an acceptable choice for thermal management in the building due to its superior thermal performance compared with Energain® panels according to the Fig. 5.

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Fig. 8. Temperature variation in three days (11/2/13 till 11/5/13).

Fig. 9. Temperature variation in three days (10/31/13 till 11/3/13).

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3.3 Mechanical Tests Table 3 shows the results of three point bending test for the composites. The reported data for composite number 7 which is made of wood and plastic is named as a reference. Its mechanical properties are less than the reported mechanical properties of a normal woodplastic composite [15, 21]. This is due to the fact that it is made with a low molding pressure and the mixing network of the polymer with wood has not been complete which affects negatively the mechanical properties of the composite [14]. For composite numbers 2 to 6, which are produced from MEPCM, there is a decrease in mechanical properties. The highest decrease (93.4% in maximum flexural strength) is for composite number 6 with 50 wt.% of MEPCM. This is why other samples have been made with 35 wt.% of MEPCM. It is also shown from the results reported in Table 3 that amount of MPE as a coupling agent has a positive effect on the mechanical properties of the mixture as shown in Fig. 10.

Table 3. Results of three points bending test for one of the composites. Composite P (N) EB (MPa) fM (MPa) No. 2 3 4 5 6 7

10.4 5.7 6.7 11.5 1.8 27.6

4.0 2.2 2.6 4.4 0.7 10.6

228.9 140.2 134.0 262.3 22.4 685.6

P: Ultimate load; fM : Maximum flexural strength; EB: Flexural modulus.

Fig. 10. Effect of coupling agent on the flexural strength of the composites.

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3.4 Leaking test The results of leaking test is shown in Fig. 11. Composites 8 and 9 contain free PCM (nonencapsulated) while composites 3 and 10 contain microcapsules. Since PCM percentage in composite 8 is 50 wt.%, it has more leakage in comparison with composite 9 containing 35 wt.% of PCM. Samples 3 and 10 containing MEPCM have lower leakage than other samples and the reported small loss is probably due to moisture loss from the sample. It is observed from the figures that microencapsulation of PCM would positively minimize or eliminate PCM leakage from the composite and this has to be taken into account when using PCM in building materials especially for wood and polymers.

Fig. 11. Results of leaking test with solvent for samples 8, 9, 10 and 3.

4. Conclusions

The results show that the composite of wood-plastic-PCM prepared in this work could be used as a material in the interior parts of buildings for temperature control and thermal management. Leakage is a major obstacle to the industrial applications of wood-plastic-PCM composites which can be tackled by using microencapsulated PCM (MEPCM). The woodplastic-MEPCM composites were produced using compression molding method and their 12

thermal and mechanical properties were investigated using DSC analysis, cycling test, leaking test, and three point bending analysis. The results indicated that the prepared composite has reasonable thermal properties, but its mechanical properties need to be improved by increasing the pressure used during the process or by using the extrusion method instead of compression molding. The results also show that there is almost no leakage during phase change in the composite.

Acknowledgements The authors acknowledge the financial support by the Research Department of Tarbiat Modares University during the project. All the laboratory tests and analysis have been performed in the Chemical and Material Engineering Department, University of Auckland, New Zealand, and their support in this project is also acknowledged.

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Highlights: - A composite of wood –plastic-MEPCM has been produced. - Compression molding has been used for the composite preparation. - Thermal and properties were investigated using DSC analysis and cycling test. - Leakage test has been performed for the encapsulated PCM. - The composites can be used as a building material for thermal energy management.

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