Charging of triple-tube heat exchanger PCMs embedded in porous metal foam, nanoparticles and finned surface under different heat transfer fluid inlet temperatures

Latent Heat Storage (LHS) systems are a promising option for thermal energy storage (TES) as they are low-cost to build, present a high energy storage density, and can store energy directly. To solve the low thermal conductivity of phase change materials (PCMs), two different methods, including highly-conductive porous metallic foams and added nanoparticles, offer promising solutions for thermal energy storage for domestic and industrial applications. Besides, these solutions must be also investigated by considering different conditions as well as heat transfer fluids operating temperatures. The main contribution of this study is to appreciate the effects of different inlet heat transfer fluids, that is water, in domestic and industrial applications with a Triplex-Tube Heat Exchanger (TTHX) equipped with the aforementioned thermal conductivity enhancement techniques and with RT35 PCM. Aluminum foams with 10 PPIs and different porosity, as well as CuO nanoparticles with 5% volume fraction, are here chosen. Governing equations are numerically solved with a three-dimensional local thermal non-equilibrium model to predict temperature, liquid fraction percentages, and stored energy. Nanoparticles are included the PCM into a single-phase equivalent medium, while the enthalpy-porosity technique is used to predict the PCM phase change. Results indicate that with both metallic foams with uniform but different porosities, say 0.98, 0.95, and 0.92, and nanoparticles, the melting time decrease can achieve a 9.63%, 65.04, and 69.51%, respectively, when compared with Pure-PCM. Moreover, the melting time for TTHX-NEPCM/Metal Foam with 0.95 porosity reaches a 69.85% reduction compared to the pure PCM if one considers an inlet temperature of 60 °C instead of 40 °C. It is shown also that the heat storage rate is significantly higher for the NEPCM/Metal Foam cases compared with the pure PCM case, especially for lower porosities and for 60 °C inlet temperature. In addition, the heat storage rate for the NEPCM is slightly higher than that of pure PCM, while for HTF with 60 °C (2.39 kW) becomes higher than 50 °C (1.46 kW) and 40 °C (0.582 kW), respectively.