Thermal storage and loss characteristics of underground water pits of solar district heating systems in Xizang Plateau, China

Large-scale thermal storage systems are crucial for solar district heating systems. Currently, there is less engineering guidance on the heat loss patterns of underground water pits, especially in the special climatic conditions of the Xizang Plateau. Therefore, this study analyzes the heat transfer characteristics of underground water pits in the plateau climate. A cylindrical underground water pit model is constructed, and the heat loss law of underground water pits of the heating season and seasonal heat storage is investigated under multiyear operation. The results show that the heat loss rate of the underground water pit with seasonal heat storage is 0.8 % higher than that of the heating season heat storage for the same volume. The difference in heat loss between the two heat storage modes is small. The heat loss from the top, sides and bottom of an underground water pit accounted for 34.2 %, 36 % and 29.8 % of the heat loss in the first year, respectively. The percentages of heat loss from each surface after ten years of operation were 46.7 %, 36.6 %, and 16.7 %. In addition, the functions of storage volume and heat loss per unit volume of the first year in Xizang were fitted. The relationship equation between heat loss per unit volume of the first year and the annual heat loss per unit volume in the nth year was obtained. This study provides support for the heat loss of underground water pits that operate continuously for many years.