Effects of various types of graphite on the thermal conductivity and energy storage properties of ternary eutectic fatty acid-based composite as phase change material
© Jebasingh. 2016
Received: 22 June 2015
Accepted: 15 February 2016
Published: 16 March 2016
Energy is the greatest challenge facing the environment. Energy efficiency can be improved by energy storage by management of distribution networks, thereby reducing cost and improving energy usage efficiency. This research investigated the energy efficiency achieved by adding various types of graphite (e.g., flake and amorphous) to organic-based ternary eutectic mixtures like capric acid (CA)–myristic acid (MA)–palmitic acid (PA)-based composite phase change materials (PCMs) under the assistance of ultrasonication to improve thermal properties for thermal energy storage. The graphite was surface modified under a Fresnel lens by using concentration of solar rays, then exfoliation of flake graphite by solar irradiation (xG-F) and exfoliation of amorphous graphite by microwave irradiation (xG-A). For each type of graphite exfoliation, ternary eutectic mixtures with mass concentrations of 5 wt% were prepared. The structure, thermal energy storage properties, and thermal stability of the composite PCM were investigated. Thermal conductivity of the samples in the liquid phase was measured using the transient line source method (KD2Pro). The thermal conductivity was increased by loading xG while energy storage properties were slightly decreased. Furthermore, CA–MA–PA + 5 % xG-F has a slightly modified phase change temperature and enthalpy of melting (T m = 17.5 °C; ΔH m = 143.7 J/g) and freezing (T f = 6.7 °C; ΔH f = 125.5 J/g); this PCM showed higher thermal conductivity of 0.170 W/(m K), representing an increase of up to 114 % relative to the parent material. On the basis of the above results, xG-A was cheaper than xG-F, but they decrease the energy storage capacity according to DSC results obtained at 2 °C/min. CA–MA–PA/xG-F has more potential for use in low temperature energy storage applications.
KeywordsAmorphous graphite Composite PCM Energy storage properties Flake graphite Thermal conductivity
Latent heat thermal energy storage (LHTES) has the advantages of high energy storage density and small temperature variation during the phase change process. LHTES has widely employed in various fields including condensation heat recovery, building energy conservation, temperature-regulating textiles, and solar energy system. Phase change materials (PCMs) used in LHTES are generally categorized as inorganic and organic. Inorganic PCMs are salt hydrates, salts, metals, and alloy, have a high heat of fusion, good thermal conductivity, cheap and nonflammable, but their applications are limited due to corrosive to metals, undergo supercooling and phase decomposition. Organic PCMs can be classified into two major categories: paraffin and non-paraffin materials. Paraffin materials have been widely used owing due to desirable thermal characteristics, such as minimal supercooling, varied phase change temperature, low vapor pressure in the melt, good thermal, chemical stability and self-nucleating behavior (Sharma et al. 2009; Baetens et al. 2010; Regin et al. 2008; Kenisarin 2010). Among the organic PCMs evaluated fatty acids are promising and then paraffin-based materials fatty acid have suitable phase change temperature, high latent heat capacity and easy manufacturing from common vegetable and animal oils (Oró et al. 2012; Yuan et al. 2014a, b; Karaipekli and Sari 2008; Sari and Kaygusuz 2002; Li et al. 2011; Yanping et al. 2011; Karaipekli and Sarı 2010).
In spite of the desirable properties of organic-based fatty acid PCMs, they have the major drawback of low thermal conductivity that reduces the rate of heat storage and extraction during the melting and solidification cycles. Their thermal conductivity was increased up to 10 % by using an organic fatty acid surfactant such as 5 % sodium myristate, 5 % sodium palmitate, or 5 % sodium stearate (Fauzi et al. 2013). Some inorganic materials have high thermal conductivity e.g., graphite and carbon nanomaterials on addition of treated graphite to the PCM increases their thermal properties and thermal conductivity (Ince et al. 2015; Sari et al. 2008; Sari and Karaipekli 2009; Zhang et al. 2013, 2014a, b, c, 2015; Fang et al. 2010; Liu et al. 2014; Yuan et al. 2014a). Exfoliated graphite (xG) fabricated by exfoliation natural graphite has superior properties mechanical, electrical, and thermal properties (Fukushima et al. 2006). Paraffin/exfoliated graphite (XGnP) is used to enhance the thermal conductivity, latent heat, and heat conductivity of composite PCMs (Kim and Drzal 2009). PCMs with low temperature can be used for cold storage applications like transport temperature-sensitive foods, medical applications, refrigeration and biotechnology industries.
In this paper, exfoliated graphite was produced from flake graphite and amorphous graphite by treating with solar irradiation and microwave irradiation respectively. Then employed as loading content to ternary eutectic fatty acid for improving thermal properties. Then structural morphology, thermal stability, and thermal conductivity of the obtained PCMs were analyzed. Thermal storage properties of the composite PCMs were also investigated.
Capric acid (CA, 98 % purity), myristic acid (MA, 98 % purity), and palmitic acid (PA, 98 % purity) of analytical grade were bought from Alfa Aesar and Sigma Aldrich. - 320 Mesh flake graphite Purchased from Alfa Aesar and 60 Mesh amorphous graphite from Loba chemie.
Preparation of exfoliated graphite
Exfoliation of flake (xG-F)
Thermal exfoliation of flake graphite was performed by brief solar irradiation under a Fresnel lens and brief treatment with nitric acid and potassium permanganate (30 min for solar treatment on natural graphite then 3 min for exfoliation of graphite).
Exfoliation of amorphous (xG-A)
Exfoliation of amorphous graphite was done by using a homemade microwave oven and brief treatment with nitric acid and potassium permanganate (150 min for solar treatment but they were able to under goes on 1.15 min for exfoliation under microwave oven due to present of oxides).
Preparation of CA–MA–PA/xG
On the basis of the theoretical mass ratios of CA–MA–PA ternary eutectic mixtures calculated from Eq. (1) (as discussed below), a series of ternary eutectic mixtures were prepared by heating CA–MA–PA (64.8:22.6:12.6) with different CA, MA, and PA contents at a constant temperature of 70 °C, then stirring at 1200 rpm with a magnetic stirrer (2MHL, REMI) to ensure the homogeneity of the mixtures, and slowly cooling to room temperature. The optimum mass ratio of CA–MA–PA to xG-F or xG-A for preparation of CA–MA–PA/xG was 95:5 and obtained under assistance of high speed ultrasonication.
The morphology of the xG-F and xG-A was observed by scanning electron microscopy (SEM, VEGA3 TESCAN) at room temperature. SEM images were obtained with an accelerating voltage of 5 kV and working distance of 12 mm. The thermal energy storage properties of CA–MA–PA and CA–MA–PA/xG were analyzed by differential scanning calorimetry (DSC, 200 F3, Maia, NETZSCH); the melting and heat storage behaviors of the pure PCM and composite PCMs were examined at 2 °C/min heating rate in the range of 0–40 °C under a constant flow of nitrogen. Thermogravimetric analysis (STA 409 PL LUXX, NETZSCH) was carried out to determine the decomposition temperature.
Thermal conductivity analysis
Thermal conductivity of pure CA–MA–PA and CA–MA–PA/xG was determined by using the transient line source method of the KD2Pro thermal conductivity analyzer (Decagon, USA). The sensor used a single needle (KS-1) with a diameter of 1.3 mm and a length of 60 mm.
Results and discussion
Mass ratio of CA–MA–PA ternary eutectic mixtures
Indeed, the mass ratio of CA–MA–PA ternary eutectic mixture 64.8:22.6:12.6 was obtained at T m 17.7 °C with little deviation compared with the theoretical value. The discrepancy is due to the errors of the calculation formula and the effect on PCM’s purity on the mass ratio and phase change temperature of eutectic mixtures. If there are several endothermic peaks in the DSC curve of a PCM, it means that the PCM is not a eutectic mixture. However, in this case the entire composite has a single endothermic peak which shows that the prepared mixture is homogenous. Therefore, the CA–MA–PA ternary eutectic mixture is a good choice for thermal energy storage for low temperature application.
Characterization of eutectic PCM composites
Figure 2a shows a solar-treated exfoliated flake graphite surface with petal-like layered structure. Figure 2b shows a microwave-treated exfoliated amorphous graphite surface with a structure which is different from that of flake graphite. In both cases, the stucture is different to the worm-like structure reported by Wei et al. (2008) for microwave-exfoliated flake graphite surfaces. This result highlights the different effect of the treatment process on the resulting structure.
Thermal properties of phase change materials
Thermal properties of PCM
T onset (°C)
Latent heat (J/g)
T onset (°C)
Latent heat (J/g)
The melting temperature and freezing temperatures of CA–MA–PA/xG were slightly lower than those of CA–MA–PA because xG has higher thermal conductivity that accelerates the heat transfer rate of PCM from the outside to inside and decreases the phase change temperature. However, the composite PCM storage ability of CA–MA–PA/xG is decided by the content of CA–MA–PA, which accounts for 95 wt% in CA–MA–PA/xG composite.
Comparison of latent heat of melting CA–MA–PA/xG with another component in literature
Pure PCM latent heat (J/g)
PCM composite latent heat (J/g)
Sari et al. (2008)
Sari and Karaipekli (2009)
Zhang et al. (2014c)
Zhang et al. (2015)
Zhang et al. (2013)
Liu et al. (2014)
Liu et al. (2014)
Zhang et al. (2014b)
Yuan et al. (2014b)
Thermal conductivity of composite PCM composites
Thermal conductivity of composite PCM
xG loading content
Thermal conductivity W/(m K)
Increase rate (%)
Thermal conductivity W/(m K)
Increase rate (%)
Thermal stability of composite PCM
CA–MA–PA composites loaded with xG-F and xG-A were prepared with the aid of sonication to afford PCMs with high thermal conductivity. The melting temperate, freezing temperature, and latent heats of CA–MA–PA/xG-F 95:5 were 17.5 °C, 6.7 °C, 143.7 J/g, and 125.5 J/g respectively. In comparison, the latent heat of CA–MA–PA/xG composites were lower than those of CA–MA–PA. Exfoliated graphite increased the thermal conductivity of CA–MA–PA, and CA–MA–PA/xG-F 5.0 wt% showed an increase in thermal conductivity by 114 %. TGA tests revealed that the prepared composite PCM has a high thermal stability in the working temperature range. Thermal storage and release rates were significantly increased as a result of the increase in thermal conductivity. The results indicate that composite PCMs have great potential for use in low temperature heating and cooling applications.
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