A polyethylene glycol/hydroxyapatite composite phase change material for thermal energy storage

Highlights

• Hydroxyapatite used as supporting material is introduced into phase change material.

• Mass fraction of PEG absorbed by Hydroxyapatite prepared in different hydrothermal temperature is studied in this paper.

• The volume change properties of the composite phase change material are studied in this paper.

• The wetting properties of hydrothermal are favorable for thermal energy in practical application.

Abstract

In this paper, a novel composite form-stable phase change material based on polyethylene glycol (PEG) and hydroxyapatite (HAP) was fabricated by impregnating method. In the composite, PEG was used as phase-change material while HAP was served as supporting matrix, unlike in previous report where it was hardly used as a supporting material. According to Differential Scanning Calorimeter (DSC) results, the satisfactory sample melted at the temperature of 58.2 °C with a phase change enthalpy of 128.9 kJ/kg and solidified at the temperature of 46.9 °C with a phase change enthalpy of 109.2 kJ/kg. PEG blended with HAP prepared in different hydrothermal temperature was studied. The volume change properties and wetting properties were characterized in this study, which are favorable for thermal energy in practical application. All these results indicate that HAP could be a good adsorption material to be applied into thermal energy storage.

Introduction

Materials that can store or release heat energy during their phase change transition process at a nearly constant temperature are defined as phase change materials (PCMs). Due to the high energy density of PCMs, much attention has been paid to them for realizing the control of environmental temperature [1] and matching the energy demand and supply in time and space. The heat storage capacity of PCMs is about 5–14 times higher than the sensible heat storage materials including masonry, rock, liquid and so on [2], [3]. On the basis of different phase change temperature during the phase transition process, PCMs are classified into three categories: low, intermediate and high temperature phase change materials. According to the required temperature range, different PCMs can be chosen in practical application. In conclusion, PCMs have a lot of applications in building energy conservation, solar energy storage and waste heat recovery [4], [5], [6], [7], [8].

Among the various kinds of PCMs, PEG has been widely used in numerous fields due to the desirable features. It is considered as a satisfactory phase change material with the decent phase transition temperature and high heat storage capacity [9], [10], which could be easily adjusted via altering mass fraction and molecular weight. At the same time, PEG owns proper melting behavior, predominant resistance to erosion, great chemical and thermal properties, biological degradation, non-poisonousness, low vapor pressure and low price features [5]. However, PEG in the actual applications faces two enduring problems: liquid leakage during phase transition process and low thermal conductivity. Leakage of PEG also has an interfacial combination problem with surrounding materials and may lead potential danger [11], [12]. Low thermal conductivity of PCMs severely prevents the heat transfer rate, therefore limiting their actual applications.

In order to solve the leakage problem of PCMs, a kind of shape-stable composite phase change materials (CPCMs) via blending PCMs with porous supporting material has captured much interest [13]. Shape-stable CPCMs are generally prepared by impregnating PCMs into porous carrier matrixes. Actually, this shape-stable CPCMs can maintain solid state even when the PCMs are changing from solid to liquid state, which makes liquid PEG easy to control and prevents PEG leakage from harmful interaction with surrounding materials [14], [15].

Meanwhile, numerous carrier matrixes, such as polymer [16] and porous materials, have been widely reported, including silica, diatomite, bentonite, expanded vermiculite, perlite, attapulgite, montmorillonite, expanded graphite, and metal foam [12], [13], [17], [18], [19]. However, the arduous PCMs encapsulation process using polymer supporting materials often increases the prepared costs. In addition, organic shells often cause various problems, such as heat resistance, serious incompatibility problems with surrounding materials and short duration to prevent liquid leakage [20], [21]. Compared with the other supporting materials, the inorganic matrixes have the advantages of fire-retardancy and simple prepared method. Given the desirable virtues of inorganic materials, finding a satisfactory inorganic supporting material for form-stable CPCMs is an excellent way to preventing liquid leakage and enhancing thermal conductivity [22], [23].

At this connection, hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂; HAP) is a well-known inorganic material in biology and chemistry which is close to the component of human bone and teeth [24], [25], [26], [27]. It has been extensively used as an adsorbent for biologically-relevant substances such as proteins and amino acids [28], [29]. Particularly, nanoscale HAP is regarded as one of most important biomaterial adsorbent due to its cheap price, superior compatibility with surrounding materials and higher adsorption capacity. HAP can be used as an adsorbent to absorb heavy metal ion such as Pb²⁺, Cu²⁺ and Cd²⁺, which is attribute to the ion Ca²⁺ by metal cation in solution [25]. At the same time, HAP is considered to be one of the most promising adsorption materials for absorbing protein from mixture. Therefore, it is worthwhile to employ HAP to absorb phase change material and it will be a good opportunity to study the adsorption behavior of phase change materials.

In this paper, preparations and properties of PEG/HAP composite phase change material are reported. To the best of our knowledge, HAP used as a carrier matrix of CPCMs has not been reported to date. HAP is prepared via a simple liquid phase precipitation method in room temperature in this paper, unlikely in the previous reported that the expanded inorganic material (including expanded perlite and expanded vermiculite) and expanded graphite need high temperature expansion, which adds the preparation cost. In addition, the PEG/HAP composite phase change material is fabricated by simple immersion method, which is suitable for mass production. According to Differential Scanning Calorimeter (DSC) results, the satisfactory sample melted at the temperature of 58.2 °C with a phase change enthalpy of 128.9 kJ/kg and solidified at the temperature of 46.9 °C with a phase change enthalpy of 109.2 kJ/kg when the maximum mass fraction of PEG in the composite is 70%. PEG blended with HAP prepared in different hydrothermal temperature is studied. The Specific area and pore volume of samples are analyzed by a N₂ adsorption analyzer. The volume change properties and wetting properties are studied in this paper. The Thermal Conductivity Meter (TCM) results indicate that the thermal conductivity of the composites is 0.162 W/(m K), which presents a good heat-transfer capacity. All these results indicate that HAP could be a good adsorption material to be applied for thermal energy storage.