Ron Warzoha Graduate Student Omar Sanusi Graduate Student Brian McManus Undergraduate Student Amy S. Fleischer 1 Professor e-mail: amy.fleischer@villanova.edu Department of Mechanical Engineering, Villanova University, Villanova, PA 19085 Development of Methods to Fully Saturate Carbon Foam With Paraffin Wax Phase Change Material for Energy Storage In this work, the effect of infiltration method on the saturation rate of paraffin phase change material within graphite foams is experimentally investigated. Graphite foams infiltrated with paraffin have been found to be effective for solar energy storage, but it has been found that it is difficult to completely saturate the foam with paraffin. The effec- tiveness of the fill will have a significant effect on the performance of the system, but the data on fill ratio are difficult to separate from confounding effects such as type of graph- ite or phase change material (PCM) used. This will be the first detailed quantitative study that directly isolates the effect of infiltration method on fill ratio of PCM in graphite foams. In this work, the two most commonly reported methods of infiltration are studied under controlled conditions. In fact, the effect of the infiltration method on the paraffin saturation rate is found to be highly significant. It was found that the more commonly used simple submersion technique is ineffective at filling the voids within the graphite foam. Repeated tests showed that at least 25% of the reported open space within the foam was left unfilled. In contrast, it was found that the use of a vacuum oven lead to a com- plete fill of the foam. These high saturation rates were achieved with significantly shorter dwell times than in previously reported studies and can be of significant use to others working in this area. [DOI: 10.1115/1.4007934] 1 Introduction Energy storage using the latent heat of fusion in phase change materials is of great interest in many applications including solar energy systems. However, most common phase change materials including paraffins and solar salts suffer from low thermal con- ductivity. This low thermal conductivity can inhibit heat penetra- tion into the material leading to ineffective energy storage and discharge. Thus, in order to facilitate an even distribution of heat throughout the PCM and eliminate superheating, a thermal con- ductivity enhancer (TCE) is often added. The TCE should feature a thermal conductivity several orders of magnitude larger than that of the PCM and the capacity to distribute heat evenly. Ning and Wirtz [1] used a pin-fin heat sink as a TCE in solid–solid PCM for high power density applications. It was found that the pin-fin heat sink improved distribution of heat evenly throughout the PCM by separating the PCM into distinct regions with highly conductive boundaries. Embedded finned heat sinks have also been studied for solid–liquid phase change with constant heat flux [2,3] and constant base temperature applications [4], and were found to effectively promote heat flow along the length of the fin and then into the PCM. Metallic honeycombs and foams are also popular candidates for PCM thermal conductivity enhancement because of their high conductivity and surface area. Pal and Joshi [5] studied the effec- tiveness of a metallic honeycomb structure as a TCE for organic PCM. Numerical simulations showed that the honeycomb TCE enhanced the melt front progression despite the suppression of natural convection at the melt front. Experimental work confirmed the results. Other researchers have studied aluminum [2,6] and copper foams [7]. The use of metallic foams results in heat flux along the ligaments and then into the PCM contained within each void. In all cases, the presence of the metallic foam was found to decrease both the melting time and the solidification time. However, this effect was complex and its intensity varied with the porosity of the foam. As foam porosity and pore size decrease, the system is more likely to become conduction dominated as the presence of the less porous foam reduces natural convection in the melt [8]. The suppression of natural convection can lead to tem- perature stratification from source to sink and increase base temperatures. The high thermal conductivity of carbon based materials has also led to their use as TCEs. For example, Py et al. [9] infiltrated liquid paraffin into compressed expanded graphite matrices (CENG) of 50–350 kg/m 3 with pore radii of 0.5 lm. The thermal conductivity of the composite material was measured and used as a basis for a theoretical performance calculation. Pinceman et al. [10] used both flake graphite (d ¼ 400 lm) and powdered graphite (d ¼ 50–500 lm) with high melt temperature hydrated salts. As in Py et al. [9], this paper reported the preparation techniques and materials properties but did not experimentally test the thermal performance. Mills et al. [11] also studied compressed flake graphite, but with 55  C melt temperature paraffin. Sari and Karaipekli [12] used 270 lm compressed graphite powder with infiltrated docosane (T melt ¼ 42  C). A 10 wt. % CENG sample was found to be form-stable and led to a decrease in melt time of 30% although no details about the test or the boundary conditions was provided. In fact, compressed and expanded graphite has been found in several studies to enhance the thermal conductivity of solar salts PCMs [13–15] and paraffins [16]. Numerical simulations of graphite foams embedded with PCMs and heated at a boundary indicate that graphite foam should also create an effective thermal conductivity enhancement [17–19]. Commercially available graphite foams have been tested 1 Corresponding author. Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received February 17, 2011; final manuscript received May 29, 2012; published online November 28, 2012. Assoc. Editor: Rainer Tamme. Journal of Solar Energy Engineering MAY 2013, Vol. 135 / 021007-1 Copyright V C 2013 by ASME Downloaded 24 Jan 2013 to 153.104.2.70. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm