Heat transfer improvement in a thermal energy storage system using auxiliary fluid instead of nano-PCM in an inclined enclosure : a comparative study

Modern thermal energy storage (TES) systems rely laboriously on finding a low-cost method to improve heat transfer. In the present analysis, adding CuO nanoparticles and tilting the enclosure simultaneously is compared with a novel approach that employed water as a supplemental fluid to improve the melting process using the density difference between PCM and supplemental fluid. Oleic acid is selected as an immiscible PCM in water, which causes PCM and auxiliary fluid utterly separate at the end of the melting process to be usable in more additional TES cycles. By placing water as a heavier material directly on top of oleic acid, the melted oleic acid is replaced by water at the bottom of the enclosure when it melts because water has a heavier density than oleic acid. At first, adding 1% and 2% of CuO nanoparticles in an enclosure with different inclinations of 0°, 45°, and 90° is studied to identify the energy storage rate. Continuity, momentum, and energy equations are used to formulate a mathematical model of the TES system. In the next step, the melting process of the combined system is analyzed to determine the energy storage rate of the combined system compared to the system, including CuO nanoparticles in the inclined enclosure. Comparing the combined system with the optimal case of nano-PCM in the inclined enclosure, it was found that the energy storage rate in the system using auxiliary fluid is 1.396 times higher.