Thermal conductivity and heat transfer performance enhancement of phase change materials (PCM) containing carbon additives for heat storage application

doi:10.1016/j.ijrefrig.2014.02.004

International Journal of Refrigeration

Volume 42, June 2014, Pages 112-120

https://doi.org/10.1016/j.ijrefrig.2014.02.004 Get rights and content

Highlights

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PCMs with carbon additives including MWCNT, Graphite and Graphene are manufactured.
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The thermal conductivity of PCM is enhanced up to 21.5% in the case of Graphene of 0.1 vol%.
•
Graphite is the most promising additive for heat transfer enhancement of stearic acid among three carbon additives.

Abstract

In thermal storage system, a high thermal conductivity of Phase Change Materials (PCM) is required to complement the crystallization phenomenon of the PCM during the discharging process. In this study, PCM with carbon additives, Multi-walled Carbon nanotube, Graphite and Graphene, are manufactured and the thermal conductivity of the PCM is measured by the transient hot-wire method for thermal storage application. The thermal conductivity of the PCM is enhanced by adding the carbon additives, and the effect of Poly Vinyl Pyrrolidone (PVP) as a dispersion stabilizer on the thermal conductivity is evaluated. It is found that the heat transfer rate enhances up to 3.35 times in the case of Graphite at 5.0 vol%. It is finally concluded that Graphite is the most promising candidate for heat transfer enhancement of stearic acid among three carbon additives even though Graphene gives the highest thermal conductivity enhancement.

Introduction

Thermal storage system has been paid attention from the view points of the efficient storage and utilization of thermal energy. The purpose of thermal storage system is to collect the excess heat source, to secure the heat source stably, and to supply it for resolving the time discordance of energy supply and demand by thermal storage. This system is applied in a variety of ways in the related fields (Zalba et al., 2003, Shin et al., 1987, Jesumathy et al., 2012, Chang et al., 1999, Meng et al., 2011, Padhmanabhan et al., 2011, Fumoto et al., 2013). Zalba et al. (2003) reviewed the thermal energy storage with phase change on materials, heat transfer and applications including 150 materials and 45 commercially available phase change materials (PCM). Recently, Osterman et al. (2012) reviewed the PCMs based cooling technologies for free cooling applications, encapsulated PCM, air-conditioning and sorption cooling systems.

During the heat exchange in thermal storage system using the PCM, the latent heat is released from the PCM to the low-temperature tube (discharging process). However, during the discharging process, the crystallization of the PCM starts on the outer wall of the low-temperature tube. Due to the crystallization of the PCM, the heat transfer performance is reduced between the PCM and the tube. The reduction of thermal storage/release rate is just the reflection of the heat transfer performance degradation caused by the low thermal conductivity of the PCM.

The addition of nano particle into the base fluid has been studied and developed to improve the low thermal conductivity of the base fluid (Maxwell, 1873, Choi, 1995). According to the reports, the addition of the nanoparticles with a high thermal conductivity can recover the low thermal conductivity of the PCM. Especially some studies reported about the improvement of the thermal conductivity by adding Multi-walled Carbon nanotube (MWCNT) or Graphite on the PCM. The thermal conductivity of the Paraffin wax including MWCNT composites increased with the mass ratio of MWCNTs (Jifen et al., 2009) and the thermal conductivity of Paraffin wax was increased by impregnating porous graphite matrices. (Andrew et al., 2006) Also, the thermal conductivity of SA increased with increasing mass fraction of expanded graphite and carbon fiber (Ali et al., 2007).

In this study, Stearic Acid (SA) is chosen as the PCM for the thermal storage system application at the working temperature of 71 °C, MWCNT, Graphite and Graphene are used as the additives, and Poly Vinyl Pyrrolidone (PVP) is used as a dispersion stabilizer of carbon additives (Lee et al., 2008a, Lee et al., 2008b). It has been reported that the addition of the carbon additives does not affect the solidification point of the nano PCMs while it does a little the melting point. (Li et al., 2013a). After making the PCMs, the dispersion stability in liquid state is evaluated. The thermal conductivity of the PCMs depending on the concentration of the carbon additives is measured in liquid state using the transient hot-wire method. Also, to verify the effect of the thermal conductivity enhancement on the heat transfer performance, it is applied for the real thermal storage system during the discharging phase.

The objectives of this study are to measure the thermal conductivity of the PCMs and to study the effect of the dispersion stabilizer of carbon additives (MWCNT, Graphite and Graphene) on the thermal conductivity enhancement for thermal energy storage application at a high working temperature of 71 °C. The effect of carbon additives on the heat transfer performance is also confirmed for the thermal storage application.

Section snippets

Preparation of the PCM

The PCMs are prepared by the following four-step method. Fig. 1 shows the schematic diagram of manufacturing procedure for PCMs. First, the Stearic acid (supplied by DAE JUNG Chemical & Metal) is heated to make it in liquid state. Second, the PVP (supplied by Aldrich Chem.) as a dispersion stabilizer is added into fully melted SA and well dispersed using a stirring machine. Third, the carbon nanoparticles are added into the mixture prepared by step 2. Fig. 2 shows the FE-SEM images of the MWCNT

Compatibility of nano PCMs

Fig. 5 shows the comparisons of the dispersion stability for SA + MWCNT with and without PVP. The concentration of MWCNT is 0.001 vol% and the ratio of MWCNT and PVP is set to be 1:5 (Lee et al., 2008a, Lee et al., 2008b). In the case without PVP, MWCNT nanoparticles are coagulated each other and sedimented after 7 days. On the other hand, in the case with PVP, the dispersion of SA + MWCNT is well maintained for 10 days.

Fig. 6 shows the comparisons for the dispersion stability of SA + Graphene

Conclusions

In this study, the PCMs (Stearic acid + carbon additives) were prepared to enhance the thermal conductivity of SA as the PCM. The dispersion stability was evaluated and the thermal conductivity of the PCMs was measured by the transient hot-wire method and compared with the pure SA depending on the concentration of carbon additives. Furthermore, the effect of carbon additives on the heat transfer rate was evaluated. The results from the present study are summarized as follows;

1)
In the case of

Acknowledgements

This research was financially supported by the National Research Foundation (NRF) of Korea under the grant No. 20100029120.

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Highlights

Abstract

Introduction

Section snippets

Preparation of the PCM

Compatibility of nano PCMs

Conclusions

Acknowledgements

Int. J. Heat. Mass Transf.

Energy

Energy Buildings

Int. J. Therm. Sci.

Appl. Therm. Eng.

Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications

Renew. Energ.

Thermal conductivity enhancement of phase change materials using a graphite matrix

Appl. Therm. Eng.

Development of phase change thermal energy storage system

KSBEC

Enhancing thermal conductivity of fluids with nanoparticles

ASME

Study on new ice slurry generator using NaCl aqueous solution at low concentration

Int. J. Air Cond. Refrig.

Thermal conductivity enhancement of nanofluids containing graphene nanosheets

J. Appl. Phys.

Percolation in transparent and conducting carbon nanotube networks

Nano Lett.