Performance enhancement of solid/liquid phase-change thermal energy storage systems through the use of a high conductivity porous metal matrix

A detailed experimental study has been carried out to evaluate the heat transfer performance of a solid/liquid phase-change thermal energy storage system that includes porous metal foam. The phase-change material (PCM) and metal foam are contained in a vertically oriented test cylinder that is cooled or heated at its outside boundary, resulting in radially inward freezing or melting, respectively. Detailed quantitative time-dependent volumetric temperature distributions and melt-front motion and shape data were obtained. Under idealized conditions, the phase-change material behaves as a thermal lumped capacitance, providing cooling (or heating) for a wide range of heat transfer rates at a single temperature corresponding to its melting-point temperature. In practice, this temperature exists only at the solid/liquid interface. As the PCM melts or freezes, the interface moves away from the surface of the heat source/sink, and a thermal resistance layer is built up, resulting in a reduced heat transfer rate and/or increased temperature difference between the system to be cooled (or heated) and the PCM. The porous material used in this research (copper foam with porosity of 95%) is intended to minimize the insulating effect of this thermal resistance layer. The phase-change medium was 99% pure eicosane, with a melting temperature of 36.5°C. Results have been generalized to apply to any low-Stefan number PCM.; In the freezing case study, a mathematical model based on a one-dimensional analysis, which considered heat conduction as the only mode of heat transfer was developed. In the melting case study, a heat transfer scale analysis was used in order to help in interpretation of the data and development of heat transfer correlations. In the scale analysis, conduction heat transfer in the solid and natural convection heat transfer in liquid were considered. Comparison of experimental data with scale-analysis predictions of the solid-liquid interface position and temperature distribution was performed.; The effective thermal conductivity of the porous media saturated with solid eicosane was predicted utilizing several models and compared with the measured effective thermal conductivity. The results of this study are discussed in terms of the effectiveness of the metal matrix as a heat transfer enhancement device.