1. Introduction
With the rapid development of construction industry, more and more lightweight building materials are applied in high-rise buildings. Because of the low thermal capacity of lightweight materials, indoor temperature tends to fluctuate under climate change. Phase change materials (PCMs) with their high heat storage capacity have been considered as potential latent storage materials widely studied in building thermal storage [
1]. PCMs absorb redundant heat in daytime, the stored thermal energy release into indoors at night. Through this thermal energy circulation, the indoor temperature maintains in a relative comfortable temperature range. The traditional PCMs are mainly divided into organic materials and inorganic materials. Several reviews have concluded a large category of candidate materials for latent heat storage [
1,
2,
3,
4]. Organic materials such as fatty acids and their eutectic mixtures have superior properties over inorganic materials like litter super cooling, high latent heat, less volume change, good thermal, and chemical stability after repeated cycles [
5,
6,
7,
8]. In recent years, the studies of mixtures of fatty acids and alcohols have caught more attention [
9,
10,
11,
12]. Zeng [
11] studied thermal properties and thermal conductivity of a series of palmitic acid (PA) and tetradecanol (TD) mixture. The phase change temperature of PA-TD eutectic mixture was 29.00 °C, which can be viewed as a new PCM. Zuo [
12] investigated the thermal properties of lauric acid (LA)/1-tetradecanol binary system by differential scanning calorimeter (DSC). The eutectic melting temperature was 24.33 °C under common building environment temperatures of 10–40 °C.
However, PCM cannot be directly applied in building envelop due to lager leakage and excitant odor of PCM. Many scholars have developed a kind of form-stable PCMs, which encapsulated PCM in polymeric network structures. Form-stable PCMs prevent the leakage of liquid PCM when it is in a state of melting [
8,
13,
14,
15,
16]. Inaba prepared a form-stable PCM with mass percentage of 74 wt % paraffin/26 wt % high-density polyethylene (HDPE), and analyzed thermophysical properties, such as thermal conductivity, latent heat, specific heat, and density. Furthermore, correlation equations between mass percentage and the aforementioned were put forward [
17]. Sari investigated thermal properties of form-stable paraffin/HDPE composites, which consist of two different kinds of paraffin. The maximum mass percentage of each paraffin, in two different PCM composites, was as high as 77%. In addition, with the addition of 3 wt % expanded and exfoliated graphite, the thermal conductivity of two composites increased about 14% and 24%, respectively [
16]. From 2005 to 2009, Cai [
13,
18,
19] published a series of literature concerning preparation, properties analysis of different form-stable PCMs, which consisted of paraffin as latent heat storage material. HDPE as supporting material have been studied for a long time. While HDPE is easy to aging and cracking after long-term recycling, thus, it is not suitable to be applied in buildings, which should be used as long as 30 years. In order to solve this problem, HDPE has to be modified. Ethylene-vinyl acetate (EVA) is a kind of modified polyethylene material with good flexibility, anti-aging, and resistance to environmental stress cracking. A new HDPE-EVA polymer blend including different structures of ethylene polymers could be achieved by a mechanical blending method. This polymer was homogeneous and continuous structure on macroscopic. The molecular micelles of HDPE and EVA uniformly dispersed. This polymer can possess the advantage of HDPE and EVA.
In addition, in order to overcome low thermal conductivity of organic materials as PCM, Liu [
20] summarized several mainstream thermal enhancement techniques for high temperature phase change thermal storage systems. Adding high thermal conductive materials, such as metal [aluminum powder (AP), Ag] and expanded graphite, into PCM produced effective thermal storage materials [
16,
21,
22,
23,
24,
25]. According to thermal storage and release tests of pure hexadecane and aluminum/hexadecane composite, Darkwa concluded that thermal response rate especially heat flux rate was accelerated by introducing AP. Thermal conductivity of aluminum/hexadecane composite was 1.25 W·m
−1·K
−1, which is more than eight times to that of pure hexadecane [
21].
Based upon the predecessors’ studies, three aspects of building thermal storage material should be further researched: latent heat storage materials with suitable phase temperature and high latent heat, supporting material with excellent performance, and application of enhanced heat transfer technology. The eutectic mixtures of fatty acid-alcohol have suitable phase change temperature, high latent heat, lower price, and wide sources from which to be obtained, which can be potential thermal storage materials for building energy storage. However, there was little literature to study them as thermal energy storage materials in form-stable PCMs. The purpose of paper is to study thermal physical and chemical properties of a series of form-stable PCMs, which consist of latent heat storage materials [tetradecanol-decanoic acid (TD-CA) eutectic mixture, tetradecanol-dodecanic acid (TD-LA) eutectic mixture, tetradecanol-tetradecanoic acid (TD-MA) eutectic mixture], and HDPE-EVA alloy as supporting materials. These form-stable PCMs were produced by a fusion method. Following, thermal properties, morphology characterization, and chemical compatibility of form-stable PCMs were determined by DSC, scanning electron microscopy (SEM), and Fourier-transform infrared (FI-IR), respectively. Thermal stability was explored by the accelerated melting-solidification experiment. Furthermore, endothermic and exothermal experiment between these form-stable PCMs and corresponding composites with AP was developed to verify the improvement of heat transfer effect when introducing AP into form-stable PCMs.
4. Conclusions
In this paper, three novel form-stable PCMs based on eutectic mixtures of fatty acid-TD and HDPE-EVA polymer were prepared for thermal storage in building. The thermal properties, surface morphology, thermal reliability, chemical stability, and thermal storage/release performance were determined by DSC, SEM, FT-IR, and melting/solidification cycling test. The maximum mass percentages of PCMs in three form-stable PCMs without leakage were 70 wt % (TD-CA), 65 wt % (TD-LA) and 60 wt % (TD-MA). Compared with maximum mass percentages of three form-stable PCMs, it can be found mass leakage decreased with molecular weight of PCM decreasing. The onset melting/solidification temperature and latent heat of form-stable PCMs were measured as 19.13 °C/13.32 °C/100.50 J/g/99.70 J/g (TD-CA), 24.53 °C/24.92 °C/90.20 J/g/88.70 J/g (TD-LA) and 33.15 °C/30.72 °C/128.60 J/g/125.70 J/g (TD-MA). The results of SEM, FT-IR, and 1000 times thermal cycling tests are showed that three form-stable PCMs had good thermal reliability, chemical stability and thermal stability. AP additive improve the thermal storage/release performance of three form-stable PCMs and the maximum thermal storage/release rates were increased almost 32.09% and 29.96%. Based on the aforementioned results, these three form-stable PCMs can be considered as potential building thermal energy storage material to keep comfort indoor environment and save energy.