Modeling of Transient Energy Loss From a Cylindrical-Shaped Solid Particle Thermal Energy Storage Tank for Central Receiver Applications

The use of solid particles as a heat transfer and thermal energy storage (TES) medium in central receiver systems has received renewed attention in recent years due to the ability of achieving high temperatures and the potential reduction in receiver and TES costs. Performance of TES systems is primarily characterized by the percentage of heat loss they allow over a prescribed period of time. Accurate estimation of this parameter requires special attention to the transient nature of the process of charging the TES bin during solar field operation and discharging during nighttime or at periods where solar field operation is interrupted. In this study, a numerical model is built to simulate the charge-discharge cycle of a small cylindrical-shaped TES bin that is currently under construction. This bin is integrated into the tower of an experimental 300-kW (thermal) central receiver field being built in Riyadh, Saudi Arabia, for solid particle receiver research, most notably on-sun testing of the falling particle receiver concept within the context of a SunShot project. The model utilizes a type of wall construction that had been previously identified as showing favorable structural characteristics and being able to withstand high temperatures. The model takes into account the anticipated charge-discharge particle flow rates, and includes an insulating layer at the ceiling of the bin to minimize heat loss by convection and radiation to the receiver cavity located immediately over the TES bin. Results show that energy loss during the full charge-discharge cycle is 4.9% and 5.9% for a 5-hour and 17-hour discharge period, respectively. While large, these energy loss values are primarily due to the high surface-to-volume ratio of the small TES bin being investigated. Preliminary analysis shows that a utility-scale TES bin using the same concept will have an energy loss of less than 1%.