Enhancement of heat transfer for thermal energy storage application using stearic acid nanocomposite with multi-walled carbon nanotubes

Highlights

• A nanocomposite made of stearic acid and multi-walled carbon nanotube is prepared for thermal energy storage application.

• Effects of multi-walled carbon nanotube on the thermal performance of the nanocomposite are investigated.

• Multi-walled carbon nanotube enhances the thermal conductivity but weakens the natural convection of stearic acid.

• Discharging/charging rates of stearic acid are increased/decreased by using multi-walled carbon nanotube.

Abstract

A latent heat storage nanocomposite made of stearic acid (SA) and multi-walled carbon nanotube (MWCNT) is prepared for thermal energy storage application. The thermal properties of the SA/MWCNT nanocomposite are characterized by SEM (scanning electron microscopy) and DSC (differential scanning calorimeter) analysis techniques, and the effects of different volume fractions of MWCNT on the heat transfer enhancement and thermal performance of stearic acid are investigated during the charging and discharging phases. The SEM analysis shows that the additive of MWCNT is uniformly distributed in the phase change material of stearic acid, and the DSC analysis reveals that the melting point of SA/MWCNT nanocomposite shifts to a lower temperature during the charging phase and the freezing point shifts to a higher temperature during the discharging phase when compared with the pure stearic acid. The experimental results show that the addition of MWCNT can improve the thermal conductivity of stearic acid effectively, but it also weakens the natural convection of stearic acid in liquid state. In comparison with the pure stearic acid, the charging rate can be decreased by about 50% while the discharging rate can be improved by about 91% respectively by using the SA/5.0% MWCNT nanocomposite. It appears that the MWCNT is a promising candidate for enhancing the heat transfer performance of latent heat thermal energy storage system.

Introduction

Energy storage technology plays an important role in the deployment of energy utilization, especially for renewable energy. It has been considered as one of crucial technologies to address global energy challenges by improving energy-efficiency and achieving energy-saving. Nowadays, most thermal energy storage systems available on the market are based on sensible heat storage using water storage tanks or latent heat storage using phase change materials (PCMs) [1]. In general, the latent heat storage can provide higher energy density and stable operating temperature compared with the sensible heat storage. In recent years, more and more attention has been paid on the latent heat energy storage systems, and various phase change materials have been developed for different practical applications [2], [3], [4].

Low thermal conductivity is the common drawback for many potential phase change materials, and it usually varies between 0.1 and 0.4 W/(m K). Consequently, the performance of a latent heat thermal energy storage system is strongly influenced by the poor thermal conductivity of the PCM employed [5]. Hence, some advanced techniques for heat transfer enhancement need to be developed in order to overcome the common shortcoming for thermal energy storage applications. Among different techniques, enlarging heat exchange area is a simple and effective method to enhance the heat transfer performance of a thermal energy storage system by designing heat exchanger with finned-tube configuration [6]. On the other hand, heat transfer performance can also be enhanced by using composite phase change materials made from PCMs and additives, and the promising method has been widely applied in many latent heat storage systems to improve the thermal efficiency of phase change material [7].

It is well known that the thermal conductivity of a composite is very closely related to the phenomenon of heat transport between the individual components of the composite [8]. Thus, the mechanism of heat transport of a composite can be changed by incorporating an additive with thermal conductivity different from that of the matrix, which would increase or decrease the thermal conductivity of the composite as a whole. According to the fundamental knowledge, the heat transfer performance of a PCM could be enhanced by using some additives with high thermal conductivities, such as graphite matrix [9], [10], carbon fibers [11], [12], metal matrix [13], [14], microencapsulated materials [15], and other matrix [16]. In the past decade, composite phase change materials have been extensively discussed and developed in different thermal energy storage systems.

Carbon nanotube has been regarded as an effective additive for improving the heat transfer of many kinds of materials due to the fact that it has a distinct advantage of ultrahigh thermal conductivity as high as about 3500 W/(m K) [17], which is about an order of magnitude higher than that of copper. The use of carbon nanotube as porous matrix has gained increasing attention because of its light weight and high specific surface area for heat transfer. Wang et al. [18] investigated the heat transfer enhancement of paraffin wax by using multi-walled carbon nanotubes as additive, and found that the thermal conductivity of paraffin wax both in liquid and solid states could be improved by 35–40% using the additive with mass fraction of 2.0%. Carbon nanotubes were proposed by Mei et al. [19] to improve the thermal conductivity of a form-stable composite by absorbing capric acid into halloysite nanotubes and graphite, and the results showed that the heat transfer rate could be enhanced by 1.7–1.8 times using the halloysite nanotubes and graphite as the additives. Recently, Teng et al. [20] compared the thermal performance of modified phase change material of paraffin using direct-synthesis method with MWCNTs and graphite as the additives. They found that the addition of MWCNTs was more effective than graphite in modifying the thermal storage performance of paraffin, and it indicated the great potential of MWCNTs for enhancing the thermal storage characteristic of a PCM.

In this paper, a new latent heat storage nanocomposite is prepared for thermal energy storage applications. Stearic acid (SA) is used as the phase change material and multi-walled carbon nanotube (MWCNT) is employed as the additive to enhance the heat transfer performance of stearic acid. The thermal properties of SA/MWCNT nanocomposite are characterized by scanning electron microscopy (SEM) and differential scanning calorimeter (DSC). Moreover, the heat transfer enhancement and thermal performance of a thermal energy storage system using the SA/MWCNT nanocomposite are investigated during the charging and discharging phases.