Numerical and experimental study of an underground water pit for seasonal heat storage

Water pit heat storage is an important part of smart district heating systems that integrate various renewable energy sources. This project studied the storage capacity and thermal stratification in a 3000 m³ underground water pit in Huangdicheng, China using a finite difference model of the water pit that was validated by experimental data. The total heat loss from the water pit in the first year was measured to be 98 MWh and the storage efficiency was 62%. Further investigations using the validated model show that approximately 57% of the total heat loss took place through the side wall, 30% through the top and the rest through the bottom of the pit. The heat loss coefficient was largest along the side wall at 0.702 W m⁻²∙^oC⁻¹. Higher charging temperatures create higher temperature differences between the top and bottom of the water pit, i.e. greater thermal stratification. The MIX number increases during most of the charging period and cannot represent the thermal stratification in the water pit during charging while the stratification number more accurately represents the stratification. Therefore, the stratification number is recommended for characterizing stratified water pits.