Experimental Investigations of the Combination of a Heat Pipe with Metal Foam or Foils for Enhancing Heat Transfer during the Melting and Solidification of a Phase Change Material (PCM) for Latent Heat Thermal Energy Storage Applications

Various thermal energy storage (TES) systems including latent heat TES (LHTES), sensible heat TES and chemical TES are reviewed and analyzed with an emphasis on LHTES. LHTES is considered here due its high energy density which results in a significant reduction in the overall system size (volume and mass). However, LHTES systems have been limited in the past by the low thermal conductivity of most phase change materials (PCMs). A detailed experimental investigation is conducted to investigate the innovative combination of a heat pipe (HP) with either metal foam or foils. A cylindrical experimental apparatus is constructed to obtain the liquid fraction and temperature distribution histories, as well as obtain photographic observations during the melting and solidification processes. A relative effectiveness, calculated as the ratio of the liquid fraction for a distinct case to that of a base case, as well as the complete melting and solidification times quantify the relative performance for each configuration. A total of six configurations: HP-Foil-PCM, HP-Foam-PCM, HP-PCM, Rod-PCM, Foam-PCM and Pure PCM, are investigated for a cylindrical PCM enclosure that contains a concentrically-located HP or rod. In a vertical orientation (vertically oriented cylinder) without base heat transfer, improved melting and solidification rates are achieved for the HP-Foil-PCM case with approximately one-third of the foil volume fraction relative to that of the foam in the HP-Foam-PCM case. The HP-Foil-PCM case with a 4.3% foil volume fraction was capable of improving the melting and solidification rates by a factor of 15 and 8, respectively, relative to a Rod-PCM system. When base heat transfer was present, the effect of system orientation (ranging from vertical to horizontal) was shown to be negligible for the HP-Foil-PCM and HP-Foam-PCM configurations when compared to the overall performance of the other configurations. The total melting and solidification times for the HP-Foil-PCM configuration were reduced to 12% and 3%, respectively, of that for a non-enhanced (only base heat transfer) Pure-PCM configuration when the system was oriented both vertically and horizontally