Abstract
One of the emerging thermal energy storage (TES) concepts is the use of solid particles, which can potentially store thermal energy at temperatures approaching 1000 C. Efforts are underway to prepare on-sun testing of this concept at King Saud University (Riyadh, Saudi Arabia) as a part of the research activities in a Sun Shot project led by Sandia National Laboratories. A thorough study of this concept has been conducted and a prototype has been designed. This concept involves the use of proppants (CARBO Accucast ID5OK) as the storage medium, and a thick, multilayered, cylindrical-shaped TES bin as the storage bin. Due to the complexity of building this first-of-its-kind TES bin, it was necessary to model the thermal performance of this design prior to completing the construction process. For this reason, a numerical model was built for the TES bin which is capable of determining the amount of energy loss. The model takes into account that, during daytime operation, the charging flow rate is higher than the discharging now rate to allow the proppants to accumulate within the TES bin over about 7 hours. Once the charging process is completed, the discharging phase whose duration is about 5 hours --- is also modeled, followed by modeling the cooling-down process of the TES bin for 12 hours to complete a 24-hour cycle. This modeling cycle is based on an assumed initial temperature in the interior of the bin. This paper extends the modeling effort to more than one cycle, such that the initial conditions at the beginning of each cycle are based on information obtained from the previous cycle, rather than on assumed values.
Results show that multi-cycle modeling is important, si cc it shows that the assumed initial temperature may not representative and may lead to inaccurate results. Furthermore, lessons learned from the first cycle of operation, especially excessive air leakage into the TES bin during nighttime depletion, help refine modeling of subsequent cycles. Energy loss at the end of the second cycle was found to be 4.3%. While considered large, this value is primarily due to the high surface-to-volume ratio of the prototype 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%, which conforms to the current best practice, and shows that low-cost TES solutions can be used in conjunction with the falling particle receiver concept. (C) 2015 Published by Elsevier Ltd.