Most of the renewable energy sources, including solar and wind suffer from significant intermittency due to dayight cycles and unpredictable weather patterns. On the other hand increasing share of renewable sources imposes additional stability risks on the power grid. Increased share of solar energy in power generation during noon along with increased power demand during afternoon peak hours generates a significant risk on the stability of power grid. Energy Storage systems are required to enable the renewable energy sources to continuously generate energy for the power grid and enhance the stability of future grid that benefits from more renewable sources. Thermal Energy Storage (TES) is one of the most promising forms of energy storage. Although round trip efficiency is relatively high in thermal storage systems, heat transfer is a well-known problem of most TES systems that use solid state or phase change. Insufficient heat transfer may significantly impact the performance of the TES system. The TES system of this study utilizes molten sulfur as the storage medium. Although thermal conductivity of molten sulfur is relatively low, the sulfur-based TES system benefits from enhanced heat transfer due to the presence of buoyancy-driven flows. In this study, the effect of natural convection on the heat transfer characteristics of a sulfur-based isochoric TES system is studied computationally and theoretically. It turns out that the viscosity of sulfur in the temperature range of this study (250-400 °C) varies by two orders of magnitude. A computational model was developed to investigate the effect of viscosity variations on the buoyancy-driven flow and corresponding charge and discharge times. The computational model is developed using an unsteady Finite Volume Method by a commercially available CFD package. The results of this study show that the heat transfer process in the isochoric TES element is highly impacted by natural convection. The viscous flow of molten sulfur near the boundaries of the isochoric TES element leads to different charge and discharge times. The discharge time is almost two times longer than the charge time due to formation of a viscous layer of elemental sulfur near the heat transfer surface. The viscous layer of sulfur decreases the activity of the buoyancy-driven flow and decreases the heat transfer rate during discharge cycle. The computational model was validated by comparing the results of a representative case with experimental data.
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