Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material Ahmet Sarı * , Ali Karaipekli Department of Chemistry, Gaziosmanpas ßa University, 60240 Tokat, Turkey Received 12 December 2005; accepted 8 November 2006 Available online 3 January 2007 Abstract This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain form- stable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using tran- sient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid–liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temper- atures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values cal- culated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Paraffin; Expanded graphite; Form-stable composite PCM; Thermal conductivity; Latent heat thermal energy storage 1. Introduction Energy storage is critical in enhancing the applicability, performance, and reliability of a wide range of energy sys- tems as the discrepancy between energy supply and its demand can be eliminated by use of proper thermal energy storage (TES) methods [1–7]. Of various TES methods, latent heat thermal energy storage (LHTES) which was phase change material (PCM) is one of the most preferred forms because of its high storage density and small temper- ature variation from storage to retrieval [8–10]. Several inorganic and organic PCMs and their mixtures have been investigated as LHTES materials [8–13]. Among the investigated PCMs, paraffins have been widely used for LHTES applications due to their large latent heat and proper thermal characteristics such as little or no super cooling, low vapor pressure, good thermal and chemical stability, and self-nucleating behavior [8,12–15]. In spite of these desirable properties of paraffins, the low thermal conductivity (0.21–0.24 W/m K) is its major draw- back decreasing the rates of heat stored and released during melting and crystallization processes which in turn limits their utility areas [8,12–14]. To overcome the low thermal 1359-4311/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2006.11.004 * Corresponding author. Tel.: +90 356 252 16 16; fax: +90 356 252 15 85. E-mail address: asari@gop.edu.tr (A. Sarı). www.elsevier.com/locate/apthermeng Applied Thermal Engineering 27 (2007) 1271–1277