Solidification of a nano-enhanced phase change material (NePCM) in a double elliptical latent heat storage unit with wavy inner tubes

This work investigates an optimal model of the heat recovery from a double elliptical latent heat storage system using wavy inner tubes and nano-enhanced phase change material (PCM). A discharging rate and time, as well as the PCM thermal distribution in a three-dimensional simulation, are evaluated considering various alignments for both inner and outer ellipse pipes. The use of two inner tubes in two different cases considering straight and wavy tubes is also examined, followed by influences of various concentrations and types of nanoparticles. The inlet temperature and Reynolds number of heat transfer fluid (HTF) are also examined. ANSYS-FLUENT software is used to perform the numerical simulation. RT 35 and Silicon Carbide (SiC) are employed as the PCM and nanomaterials, respectively. Water is also considered as the HTF with the Reynolds number of 900. Different values of 20 and 40 mm are proposed for the horizontal and vertical radii of the outer tube to examine the different orientations of the elliptical tube considering the inner radius ranged from 4.95 to 14 mm. The findings show that the vertical alignment of the inner tube compound in the centre of the horizontal position of the outer tube presents higher efficiency during the solidification process. Using double pipes in both straight and wavy configurations increases the surface area, which enhances the heat transfer rate, accelerating the solidification rate by 1.26 and 1.1, respectively. The PCM compound with SiC nanoparticles with 2% and 4% concentrations accelerates the discharging rate by 2.8 and 4.8 times, respectively, compared with the single straight inner pipe case. Furthermore, it is found that by decreasing the temperature of the heat transfer fluid from 285 K to 280 K, the total solidification time reduces from 48 to 39 min.