Thermal energy storage characteristics of sand as filler material for solar thermocline tank

The feasibility of sand at various grain sizes as filler material for solar sensible heat thermal energy storage (TES) was investigated. An indirect contact sensible heat TES experiment setup was built. The TES section is a cylindrical tank, which is embedded with 19 stainless tubes arranged like a hexagonal honeycomb. Air was chosen as the heat transfer fluid to flow inside the tubes and carry heat, while the sand in the tank contacts the outside of tubes. Four types of sand: silver sand, medium sand, filter sand and coarse sand, were selected. The results show that voidage is the determining factor for the TES performance rather than density or grain size. Coarse sand is the best among the four types. For materials such as sand whose basic thermal properties only vary a bit, the change of charge temperature can hardly affect the TES performance. When using air as the heat transfer fluid, the TES efficiency is relatively lower. To enhance heat transfer, a lower air velocity or longer tube length is needed. The simplified 2-D simulation results match well with the experimental results and can be used for the prediction of large scale TES tank performance.