Influences of the key characteristic parameters on the thermal performance of a water pit seasonal thermal storage

The influence of the key characteristic parameters of a water pit seasonal thermal storage on thermal energy storage capacity in the static mode of operation is investigated in this study. An underground experimental test rig is built, and a numerical model is developed to study the natural convective flow and heat transfer in the water pit thermal storage. Results show that the water temperature close to the tank walls decreasing by the heat losses from the top and sidewalls of the tank, which creates a downward flow along the tank wall. At the center of the tank, a slight upward flow is generated, which lifts the warmer water at the bulk of the tank to a higher level. In this way, the buoyancy-driven flow gradually builds up the thermal stratification in the tank. For the influence of depth, the results indicate that the smaller the depth, the faster the thermal efficiency decreases. For the influence of slope angle, the results show that the smaller the slope angle, the faster the thermal efficiency decreases, the steeper the water pit sidewall, the smaller the maximum velocity and the maximum velocity tends to be stable over time, and the steeper the water pit sidewall, the more significant the temperature stratification in the water pit.