Effects of various carbon nanofillers on the thermal conductivity and energy storage properties of paraffin-based nanocomposite phase change materials

Highlights

• Paraffin-based nanocomposite PCMs were prepared with various carbon nanofillers.

• The decrease in energy storage capacity of GNP-based composite was moderate.

• Phase change temperatures were slightly lowered in the presence of the nanofillers.

• Greater enhancement was achieved with decreasing the size of wire-shaped fillers.

• GNPs caused greatest thermal conductivity enhancement up to 164% at 5 wt.%.

Abstract

The effects of adding various carbon nanofillers on the thermal conductivity and energy storage properties of paraffin-based nanocomposite phase change materials (PCMs) for thermal energy storage were investigated experimentally. These included short and long multi-walled carbon nanotubes, carbon nanofibers, and graphene nanoplatelets (GNPs). For each type of the nanofillers, nanocomposite PCM samples with mass concentrations of 1–5 wt.% at an increment of 1 wt.% were prepared. The thermal conductivity of the samples in solid phase was measured using the transient hot-wire method at elevated temperatures. The energy storage properties, including melting/solidification temperatures and enthalpies, were measured using a differential scanning calorimeter. It was shown that the presence of the nanofillers slightly decreases the phase change enthalpies and has negligible influence on the phase change temperatures. The thermal conductivity of the nanocomposite PCMs was found to increase with raising the loading, while the relative enhancement strongly depends on the size and shape of the nanofillers. Of the four types of carbon nanofillers examined, GNPs were observed to cause greatest relative enhancement up to 164% at the loading of 5 wt.%, due to their two-dimensional planar structure that leads to reduced filler/matrix thermal interface resistance, in contrast to the moderately decreased energy storage capacity of GNP-based nanocomposite PCMs.

Introduction

The intermittent nature of available renewable and sustainable energy sources, solar and wind energy for example, creates a persistent need for efficient energy storage technologies. As a form of energy, thermal energy is directly usable, and is accompanied with the energy conversion processes of almost all kinds of renewable and sustainable energy sources. Hence, storage of thermal energy is of great significance, which has been realized with both sensible and latent heat of select materials [1]. The utilization of solid–liquid phase change materials (PCMs), by taking advantage of their latent heat (of fusion) during melting, is an effective approach to thermal energy storage (TES), which offers higher energy storage density over a much narrower temperature swing (nearly isothermal during phase change) than those of the sensible option. PCM-based TES has been widely incorporated in solar thermal energy [2], buildings [3], and cold storage systems [4], in an effort to improve the system performance.

A great number of materials have been examined and practiced as candidate PCMs that may be roughly classified into two categories, i.e., organic and inorganic [5]. As a widely used organic PCM, paraffin wax is a family of alkane mixtures, featuring high latent heat of fusion, little or negligible supercooling, and desirable thermal and chemical stability [6]. These clearly merit its utilization in TES applications from low to medium temperature range. An undesirable property of paraffin wax, however, is its relatively low thermal conductivity that significantly decelerates the energy charging/discharging rates. To address this concern, attempts have long been made to enhance the effective thermal conductivity of paraffin wax and other common PCMs with low thermal conductivities. Early efforts have been dedicated to incorporation of various forms of stationary metallic structures/inserts, e.g., fins and foams, into PCM-based TES systems, as summarized by Fan and Khodadadi [7]. The inclusion of highly-conductive particles to form composite PCMs has been proposed as an alternative solution. The composites may be considered as custom PCMs with enhanced effective thermal conductivity. Attempts have been initiated with thermal conductivity fillers at micro- to mesoscale, including metallic beads/powders/particles [8], [9], [10], carbon fibers [11], [12], and expanded graphite [13], [14], [15], [16], [17], [18], whereas nanocomposite PCMs, in the presence of ultrafine fillers at nanoscale, have recently been proposed [19] and have since received increasing attention [20]. The nanofillers have included metallic/oxide nanoparticles [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], metallic nanowires [32], [33], carbon nanofibers (CNFs) [34], [35], [36], [37], carbon nanotubes (CNTs) [38], [39], [40], [41], [42], [43], [44], [45], [46], and the emerging graphene/graphite nanoplatelets (GNPs) [47], [48], [49], [50], [51], [52].

Among the various nanofillers examined, carbon nanomaterials have been preferred because they possess extremely high thermal conductivity and relatively low density. The GNPs, owing to their unique two-dimensional planar structure, have been identified to even outperform the other wire-shaped carbon nanomaterials, i.e., CNFs and CNTs, in enhancing the thermal conductivity for engineered suspensions [53]. Although the effects of adding a variety of carbon nanofillers on the thermal conductivity and energy storage properties of nanocomposite PCMs have been investigated extensively in the literature, a direct comparison of their performance remains unavailable.

Hence, this paper aims at comparing the performance of carbon nanofillers of various sizes and shapes in enhancing the thermal conductivity of paraffin-based nanocomposite PCMs. The energy storage properties of the nanocomposite PCMs are also characterized.