Myristic acid/polyaniline composites as form stable phase change materials for thermal energy storage

Highlights

• PANI is applied as supporting materials in form-stable PCMs.

• MA/PANI form-stable PCM possesses thermal energy storage capacity of 150.6 J/g.

• Preparation of the form-stable PCM is very simple and convenient.

• The thermal energy storage capacity of the MA/PANI form-stable PCM is tunable.

Abstract

Form-stable phase change materials (PCMs) possess the advantages of direct use without additional encapsulation, making their practical applications more feasible comparing to solid–liquid PCMs. In this paper, a new kind of myristic acid (MA)/polyaniline (PANI) form-stable PCMs were readily prepared by means of surface polymerization method. MA and PANI were applied as thermal energy storage material and supporting material, respectively. Morphology and structure characterization revealed that in the form-stable PCMs, MA particles were wrapped by PANI particles, which in turn were polymerized from aniline. Thermal stability and thermal energy storage properties of the prepared form-stable PCMs were investigated by means of thermogravimetry (TG) and differential scanning calorimetry (DSC). The results indicated that the form-stable PCMs exhibited good thermal stability when their phase change temperature was concerned. The highest loading of MA in the form-stable PCM with good form stability could attain 82 wt%, corresponding to the phase change enthalpy of 150.63 J/g.

Introduction

Solar thermal energy application has attracted great interest due to the energy crisis, high oil prices, environmental concerns, as well as its low cost [1]. However, the fluctuation of solar radiation makes thermal energy storage system indispensable in the solar thermal energy application. Phase change materials (PCMs) are a kind of latent heat energy storage materials and can be applied to store thermal energy which is collected from solar radiation [2], [3]. Solar thermal energy application systems embedded with PCMs show certain advantages over conventional systems where sensible heat storage materials are applied. The main advantage is the high storage density in small temperature intervals. Over the past few decades, extensive efforts have been made to develop PCMs and to apply PCMs for solar thermal energy application systems [2], [3], [4], [5], [6] where heat is stored during the period of sun shining and used when heat is needed.

Since PCMs play a key role in solar thermal energy applications, the developing of new PCMs with high performance is very important. PCMs that store or release thermal energy by changing their phase between solid and liquid states are called solid–liquid PCMs. Solid–liquid PCMs have certain shortages to hinder their application, such as the need of encapsulation and high volume change. Consequently, form-stable PCMs, which possess the advantages of high latent heat, shape-stable and direct use without additional encapsulation, etc. are attractive candidates as thermal energy storage materials [7]. Besides, the thermal conductivity of organic solid–liquid PCMs are normally low, so that thermal conductive fillers [8], [9], [10] are needed to enhance their thermal conductivity. However, based on the view point of chemical thermodynamics, the fillers will precipitate from the solid–liquid PCMs during their long term melting–crystallizing cycles, and hence the thermal conductive enhancement will be lost. If the fillers are fixed in form-stable PCMs, the precipitation of the fillers will be prevented by the supporting materials and thus the thermal conductivity will be maintained.

Kenisarin and Kenisarina [7] have summarized the state of the art in the developing of form-stable PCMs for thermal energy storage. Normally, a form-stable PCM is composed of a solid–liquid PCM and a supporting material. The solid–liquid PCM acts as thermal energy storage material while the supporting material maintains the solid shape of the form-stable PCM when the temperature is higher than the melting point of the solid–liquid PCM. The solid–liquid PCMs in form-stable PCMs are mainly organic compounds, such as paraffin [11], fatty acids [12], fatty alcohol [13], [14], polyethylene glycol [15] and their mixtures [16]. The supporting materials include polymers and inorganic compounds. The solid–liquid PCMs are encapsulated in the voids of the polymers by means of melt blending [17], electrospinning [18] or in situ polymerization [19], or are grafted to the chains of polymers [20], [21], [22], to obtain form-stable PCMs. Inorganic supporting materials are mainly porous materials such as vermiculite [23], halloysite nanotube [24], carbon materials [25], [26] and expanded perlite [27], etc. The solid–liquid PCM are impregnated into the pores of the inorganic supporting materials to obtain form-stable PCMs.

Polyaniline (PANI) is a kind of conductive polymer without solid–liquid transition until it is pyrolysed. PANI possesses the merits of good environment stability, lower cost and easy production and can be prepared in a variety of morphology such as nano/micro-tubes [28], [29] and porous structure [30]. Furthermore, PANI has been applied in smart windows for dynamic daylight and solar energy control in buildings [31]. As a result, it is worthwhile to investigate the possibility of applying PANI as supporting material to prepare form-stable PCMs. Fatty acids possess the merits of high phase change enthalpy and tunable phase change temperature. An additional advantage is that fatty acids are derived from the vegetable and animal oil that provides an assurance of continuous supply. In this work, myristic acid (MA) is selected as solid–liquid PCM and PANI is selected as supporting material. A new kind of MA/PANI form-stable PCMs are prepared and their properties are investigated. The results show that the as-prepared PCMs exhibit good form-stable property and the highest phase change enthalpy could attain 150.63 J/g, indicating that they can be applied in low temperature solar thermal energy applications.