Scholarly article on topic 'PCMs Heat Transfer Performance Enhancement with Expanded Graphite and its Thermal Stability'

PCMs Heat Transfer Performance Enhancement with Expanded Graphite and its Thermal Stability Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Procedia Engineering
OECD Field of science
Keywords
{"composite PCMs" / "expanding graphite" / "heat fusion" / "thermal charge/discharege" / "thermal stability"}

Abstract of research paper on Materials engineering, author of scientific article — Yi Jin, Qingming Wan, Yulong Ding

Abstract This work focuses on developing a high performance phase change materials (PCMs) adding with a few expanded graphite (EG). A series of composite phase change materials (paraffin) with various EG concentration are formulated in vacuum condition and ambient pressure respectively. The heat fusion of composite PCMs is measured by DSC. A rig for PCMs heat transfer performance test is built in lab. The heat transfer performance test of composites PCMs are carried out. The heat storage and release processed of pure PCMs and composite significantly improved after EG adding. The composited PCMs formulated in vacuum condition also presents better heat transfer performance. It shows composite PCMs can be well mixed in vacuum. Due to the repeated heating and cooling in PCMs application, thermal stability test is carried out in this work. PCMs are putting in water bath with temperature controlling by cyclic phase change to quantify the rate of degradation heat fusion and thermal charge/discharge process. After cycling 60 times, up to 2% heat fusion decrement is observed. Heat transfer performance studies also show that there is little change. In brief, PCMs with EG has high thermal performance for thermal energy storage process.

Academic research paper on topic "PCMs Heat Transfer Performance Enhancement with Expanded Graphite and its Thermal Stability"

(I)

CrossMark

Available online at www.sciencedirect.com

ScienceDirect

Procedía Engineering 102 (2015) 1877- 1884

Procedía Engineering

www.elsevier.com/locate/procedia

The 7th World Congress on Particle Technology (WCPT7)

PCMs heat transfer performance enhancement with expanded graphite and its thermal stability

Yi Jina*, Qingming Wana,c and Yulong Dingb

a Institute of Process Engineering, Chinese Academy of Sciences, Beijing,China bCentre for Energy Storage Research, University of Birmingham, Edgbaston, UK cUniverstiy of Chinese Academy of Sciences, Beijing, China

Abstract

This work focuses on developing a high performance phase change materials (PCMs) adding with a few expanded graphite (EG). A series of composite phase change materials (paraffin) with various EG concentration are formulated in vacuum condition and ambient pressure respectively. The heat fusion of composite PCMs is measured by DSC. A rig for PCMs heat transfer performance test is built in lab. The heat transfer performance test of composites PCMs are carried out. The heat storage and release processed of pure PCMs and composite significantly improved after EG adding. The composited PCMs formulated in vacuum condition also presents better heat transfer performance. It shows composite PCMs can be well mixed in vacuum. Due to the repeated heating and cooling in PCMs application, thermal stability test is carried out in this work. PCMs are putting in water bath with temperature controlling by cyclic phase change to quantify the rate of degradation heat fusion and thermal charge/discharge process. After cycling 60 times, up to 2% heat fusion decrement is observed. Heat transfer performance studies also show that there is little change. In brief, PCMs with EG has high thermal performance for thermal energy storage process. ©2015 TheAuthors.PublishedbyElsevierLtd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Selectionand peer-reviewunder responsibility of Chinese Society of Particuology, Institute of Process Engineering, Chinese Academy of Sciences (CAS)

Keywords: composite PCMs; expanding graphite; heat fusion; thermal charge/discharege; thermal stability

* Corresponding author. Tel.: +86-010-82544917; fax: +86-010-82544814. E-mail address: yjin@ipe.ac.cn

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Selection and peer-review under responsibility of Chinese Society of Particuology, Institute of Process Engineering, Chinese Academy of Sciences (CAS) doi:10.1016/j.proeng.2015.01.326

1. Introduction

Thermal energy storage is an important issue of the energy science and technology. It mainly includes the latent heat storage, chemical reaction heat storage and sensible heat storage[1]. During the latent heat storage system, the phase change materials (PCMs) are regarded as a potential options for thermal energy storage materials because of the high energy storage density, stable thermal performance and small volume changes during the melting and freezing process1-2"4-1. However, the low thermal conductivities of most PCMs decrease the heat storing/releasing rates during the solid-liquid phase change, which limits the PCMs' application. To improve the thermal conductivity of the PCMs, the composite PCMs with high thermal conductivity particle additives have been widely studied recently-5-7-.

The paraffin wax is one of the most preferred PCMs because of its nontoxic, no corrosion and easy production y behaviors-8-. To improve its thermal conductivity, many additives with high thermal conductivity are used to formulate the composite PCMs, such as metal foams[9], Cu nanoparticles[10], graphite matrix-11-, multiwall carbon nanotubes[12], et al.

The expanded graphite(EG) with good adsorption ability and loose structure could not only provide the high thermal conductivity, but also make a good matrix for preparing the composite PCMs[13]. Up to now, several EG-based composite PCMs have been prepared, which in paraffin- , 15], acid[16,17] and N-Octadecane[18] are used as PCMs. The formulation method [19] and the thermal conductivity [20] of those composites are studied in difference EG concentration. The thermal stability of those composites is the most important impact for their application work, but few studies have been carried out.

In this paper, the paraffin wax based composite PCMs with expanded graphite additives were prepared in vacuum condition and at the ambient pressure, respectively. The heat transfer performance during heat storage and release processes was evaluated by a rig built in lab. The thermal stability was analyzed through the thermal cycling tests.

2. Experiment

2.1. EG and composite PCMs preparation

Paraffin wax (PW) with melting point at 60~62 °C was chosen as the phase change materials (PCMs). Expandable Graphite (average diameter: 300 ^m; one single layer thickness: 10~20 ^m; expansion rate: 250 ml/g) was used to formulate the EG through thermal shock at 950 °C in muffle furnace. Fig. 1 (a) and (b) show the SEM images of the expandable graphite and the EG, respectively. From Fig. 1 (a) one can see that the shape of the expandable graphite is similar to the flake graphite with lamellar structure. After expansion, as shown in Fig. 1 (b), the EG with wormlike shape consists of numerous loosely connected graphite nanosheets with a few nanometers in thickness.

Fig. 1 SEM images of the expandable graphite (a) and the EG (b)

The composite PCMs were formulated through the mixing method in vacuum condition and at ambient pressure, respectively. First, the PW in liquid state with different concentration of EG were mixed by ultrasonic method. Then, each mixture sample was divided into two parts. One part was put in the vacuum condition for 32 h, and ultrasound every four hours. The temperature in the vacuum box was kept at 80°C. Another part was put in the thermostatic bath with 80 °C at the ambient pressure for 32 h, and ultrasound every four hours. Table 1 shows the configurations of the composite PCMs samples prepared in this research.

Table 1 Sample configurations of the composite PCMs

Sample Formulation condition

A-1 PW+9.10wt.% EG, prepared at ambient pressure

A-2 PW+6.25wt.% EG, prepared at ambient pressure

A-3 PW+4.76wt.% EG, prepared at ambient pressure

A-4 PW+3.85wt.% EG, prepared at ambient pressure

B-1 PW+9.10wt.% EG, prepared in vacuum condition

B-2 PW+6.25wt.% EG, prepared in vacuum condition

B-3 PW+4.76wt.% EG, prepared in vacuum condition

B-4 PW+3.85wt.% EG, prepared in vacuum condition

2.2. Thermal performance tests

The latent heat of the pure PW and the composite PCMs were obtained by differential scanning calorimeter (STA 449 F3, Netzsch, Germany). The heat transfer performance was characterized through temperature curves during the charging/discharging thermal processes. The charging/discharging processed were finished on the measurement assembled in the lab, as shown in Fig. 2. The thermal stability of the composite PCMs was evaluated by the thermal cycling test. One thermal cycling test contains a melting and freezing process. In this research, the cycling tests of each composite PCMs sample was consecutively performed 60 times.

thermal static bath

data logger

Fig. 2 Schematic of the heat transfer performance measurement

3. Results and discussion

3.1. Composite PCMs and its latent heat

During the composite PCMs preparation, the PW would be adsorbed among the loosely connected graphite nanosheets, and fulfill the gaps. When the EG concentration is high (>4.76 wt.%), the gaps among EG can not be fulfilled by the PW. When the EG concentration decreases to around 4.76 wt.%, the adsorption of PW in EG becomes saturated. If the EG concentration is too low (<4.76 wt.%), the adsorption of PW will be over saturated, and the PW-caking could be observed. Fig. 3 shows the SEM images of the composite PCMs samples. One can see that, when the EG concentration is 4.76 wt.%, the loose and porous structure of the EG in the composited PCMs has been kept, as shown in Fig. 3 (a) and (c). When the EG concentration is higher than 4.76 wt.%, the PW

agglomeration is clear, as shown in Fig. 3 (b) and (d). So no matter the composite PCMs were prepared in the vacuum condition or at the ambient pressure, the critical concentration of the EG in PW was around 4.7 6wt.%.

Fig. 3 SEM images of sample A-3 (a), sample A-4 (b), sample B-3 (c) and sample B-4 (d)

Table 2 shows the latent heat of the pure PW and the composite PCMs with different EG concentration. One can see that with the addition of the EG, the latent heat of the composite PCMs decreased. It is obviously to notice that higher heat fusion can be achieved in the vacuum condition within same EG concentration. The preparation with vacuum condition is regarded as the better method.

Table 2 Latent heat of the composite PCMs

Sample ^Я/kJ-kg-1

PW 184.6

A-4 170.5

A-3 164.6

A-2 161.1

A-1 156.2

B-4 153.6

B-3 149.6

B-2 147

B-1 144.1

3.2. Thermal charge/discharge process

The PCMs thermal conductivity can be improved with EG adding [20]. For the commercial application, its thermal charge and discharge process test is carried out in this part. Fig. 4 shows the temperature curves of discharging processes and charging processes for the pure PW and the composite PCMs. From Fig. 4 (a), one can see that, the heating rate of the composite PCMs is higher than that of the pure PW until the inner temperature of the materials is close to the outside. For the sample A-3 and B-3, it takes about 4 mins when the inner temperature raises from 22 oC to 50 oC, whereas it will take 7 mins for the pure PW. And during the heating processes, a platform is observed on each temperature curve at around 60 oC. From Fig. 4(b), one can see that, the cooling rate of the composite PCMs is higher than that of the pure PW until the internal temperature of the materials is close to the outside. For the sample A-3 and B-3, it takes about 5 mins when the inner temperature decreased from 70 oC to 40 oC. And during the cooling processes, there is also a platform on each temperature curve at around 60 oC, that is similar to the heating processes. This should be related to the phase change of the PW. The high heating/cooling rate during the charging/discharging processes of the composited PCMs is mainly attributed to the high thermal conductivity of the composite PCMs.

Time (min)

PW (b)

Time (min)

Fig. 4 Temperature curves of PW and the composite PCMs during the charging process (a) and the discharging process (b)

3.3. Thermal stability

CÖ fH

Time (min)

CÖ fH

Time (min)

CÖ fH tp a a

Time (min)

Fig. 5 Temperature curves during the thermal cycling tests: (a) the first cycling time; (b) the 31st time; (c) the 60 time

Fig.5 (a)-(c) are the temperature curves as a function of time for the sample A-3 and B-3 during the 1st, 31st and 60th thermal cycling tests, respectively. One can see that, in each figure, the temperature curves of the two samples are almost the same, which means the preparation condition of the composite PCMs has no significant effect on the materials' heating/cooling rate. During the first cycle, it needs about 4 mins when the internal temperature of the samples A-3 and B-3 raise from 30 °C to 60 °C . After 30 cycles, it still needs about 4 mins for the same temperature raising interval. Even during the 60th cycling test, the internal temperature of the composite PCMs can also raise from 30 oC to 60 oC within4 mins. After 60 cycles, the latent heat of the sample A-3 and B-3 were tested by the TG-DSC. For sample A-3, after cycling for 60 times, the latent heat is 162 kJ/kg, that is 1.6% lower than the first test results. For sample B-3, the decrease value is even lower, only 1.1%. So we can conclude that the excellent thermal stability of these composite PCMs prepared at the ambient pressure or in the vacuum condition.

Temperature(oC) Temperature(oC)

Fig 6 heat fusion measurements by DSC (a) sample prepared at ambient (b) sample prepared in the vacuum The composites PCMs heat fusions are tested after 60 thermal cycles. Both of fig 6a and fig 6b show the comparison between the original heat fusion and its heat fusion after 60 thermal cycles. There is a little change for the sample which prepared in the vacuum condition. For the sample prepared at the ambient pressure, the obviously heat fusion increment is observed in DSC curve. it infers that the EG and paraffin separation is occurred after thermal cycles. In the contrary, good stability of composites PCMs formulated in the vacuum condition is found via the heat fusion tests.

4. Conclusions

In this paper, paraffin based composite phase change materials (PCMs) with different expanded graphite (EG) concentration were prepared in the vacuum condition and at the ambient pressure. The heat transfer performance and the thermal stability of these composite PCMs were evaluated. Although the latent heat of the composite PCMs were lower than that of the pure PW, the heat transfer performance was significantly improved after the EG addition. The thermal cycling test results confirmed the thermal stability of the composite PCMs prepared either in the vacuum condition or at the ambient pressure. The work presents that sample prepared in the vacuum condition can achieve well dispersed and good thermal stability.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 21106148), NSFC-EPSRC joint proposal in Grid Storage for Intermittency (No. 51361135702) and Key Technologies R&D Program of China (No. 2012BAA03B03).

References

[1] Zalba B, Marin J M, Cabeza L F, Mehling H. Review on Thermal Energy Storage with Phase Change: Materials, Heat Transfer Analysis and Applications. Applied Thermal Engineering, 2003, 23(3): 251-283

[2] Abhat A. Low Temperature Latent Heat Thermal Energy Storage: Heat Storage Materials. Solar energy, 1983, 30(4): 313-332

[3] Dincer I, Rosen M. Thermal Energy Storage: Systems and Applications. John Wiley & Sons, 2002

[4] Dincer I, Dost S. A Perspective on Thermal Energy Storage Systems for Solar Energy Applications. Int J Energ Res, 1996, 20(6): 547-557

[5] Ye F, Qu J, Zhong J, Wang C, Meng L, Yang J, Ding Y. Research Advances in Phase Change Materials for Thermal Energy Storage. The

Chinese Journal of Process Engineering, 2010, 10(6): 1231-1241

[6] Jegadheeswaran S, Pohekar S D. Performance Enhancement in Latent Heat Thermal Storage System: A Review. Renewable and Sustainable

Energy Reviews, 2009, 13(9): 2225-2244

[7] Fan L, Khodadadi J. Thermal Conductivity Enhancement of Phase Change Materials for Thermal Energy Storage: A Review. Renewable

and Sustainable Energy Reviews, 2011, 15(1): 24-46

[8] He B, Martin V, Setterwall F. Phase Transition Temperature Ranges and Storage Density of Paraffin Wax Phase Change Materials. Energy,

2004, 29(11): 1785-1804

[9] Zhao C-Y, Lu W, Tian Y. Heat Transfer Enhancement for Thermal Energy Storage Using Metal Foams Embedded within Phase Change

Materials (Pcms). Solar energy, 2010, 84(8): 1402-1412

[10] Wu S, Wang H, Xiao S, Zhu D. An Investigation of Melting/Freezing Characteristics of Nanoparticle-Enhanced Phase Change Materials. J

Therm Anal Calorim, 2012, 110(3): 1127-1131

[11] Mills A, Farid M, Selman J, Al-Hallaj S. Thermal Conductivity Enhancement of Phase Change Materials Using a Graphite Matrix. Applied

Thermal Engineering, 2006, 26(14): 1652-1661

[12] Ma B, Li J, Peng Z, Ding Y. Paraffin Based Composite Phase Change Materials for Thermal Energy Storage: Thermal Conductivity

Enhancement. Energy Storage Science and Technology, 2012, 1(2):

[13] Wang, C.Y., Feng, L.L., Li, W., Zheng, J., Tian, W.H., Li, X.G. Shape-stabilized phase change materials based on polyethylene glycol/

porous carbon composite: the influence of the pore structure of thecarbon materials. Solar. Energy Materials and Solar Cells., 2012,105: 21 -26.

[14] Sari A, Karaipekli A. Thermal Conductivity and Latent Heat Thermal Energy Storage Characteristics of Paraffin/Expanded Graphi te

Composite as Phase Change Material. Applied Thermal Engineering, 2007, 27(8): 1271-1277

[15]Zhengguo Zhang , Ni Zhang, Jing Peng, Xiaoming Fang, Xuenong Gao, Yutang Fang, Preparation and thermal energy storage properties of paraffin/expandedgraphite composite phase change material, Applied Energy2012,91 :426 - 431

[16] Sari A, Karaipekli A. Preparation, Thermal Properties and Thermal Reliability of Palmitic Acid/Expanded Graphite Composite as Form-

Stable Pcm for Thermal Energy Storage. Sol Energ Mat Sol C, 2009, 93(5): 571-576

[17]Shuping Wang, Peng Qin, Xiaoming Fang, Zhengguo Zhang, Shuangfeng Wang,Xiaohong Liu, A novel sebacic acid/expanded graphite composite phasechange material for solar thermal medium-temperature applications, Solar Energy. 2014,99 ,283 - 290

[18] Zhang Z, Shi G, Wang S, Fang X, Liu X. Thermal Energy Storage Cement Mortar Containing N-Octadecane/Expanded Graphite Composite

Phase Change Material. Renew Energ, 2013, 50(670-675

[19]Li M, Wu Z, Tan J . Properties of form-stable paraffin/silicon dioxide/expanded graphite phase change composites prepared by sol-gel

method. Applied Energy, 2012, 92:456-461

[20]Jianguo Zhao, Yong Guo, Feng Feng, Qinghua Tong, Wenshan Qv, Haiqing Wang Microstructure and thermal properties of a paraffin/ expanded graphite phase-change composite forthermal storage Renewable Energy, 2011,36(5):1339-1342