| Summary: | Finding a low-cost method to improve the heat transfer rate in thermal energy storage (TES) is essential in the modern world. In the current research, the comparison of inclined systems, which are among the inexpensive methods, with the new combined system employing the auxiliary fluid of water is discussed. Oleic acid is selected as an unsolvable PCM in water to reuse chosen materials at the end of the melting process in more TES cycles. In the combined system, the density difference between water and PCM is utilized to enhance the melting rate of PCM by placing water on the top of PCM to make direct contact. Therefore, water, which has a heavier density than PCM, takes the place of the melted PCM at the bottom of the enclosure during the melting of PCM. At first, the inclined system is examined at seven different inclination angles to identify the energy storage rate. A mathematical model is formulated using the water-PCM system's continuity, momentum, and energy equations. Then, the melting process of the combined system is studied to compare the energy storage rate of combined and inclined systems. Comparing the combined system with the optimal case of inclined systems, the average fluid velocity is 5.44 times higher, which shows the improvement of convection. It was found that the energy storage rate of the combined system (0.299136 kW/kg) is 1.85 times higher than the inclined system (0.161983 kW/kg at the inclination of 30°). As a result, the comprehensive study of the combined system is essential in future research to replace expensive methods.
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