Improved heat transfer mathematical model for pyramid shaped seasonal heat storage water pit and the influence of boundary conditions

Seasonal heat storage can make use of urban waste heat for low-carbon heating throughout the year. Previous models for pyramid shaped water pit have limited adaptability and ignore the impact of soil temperature in the early stages of operation on the performance of water pits. This article combines two classic mathematical models to establish a mathematical model that can adapt to various sizes of inverted pyramid shaped water pits. The validation of this model was conducted using experimental data from 60000 m3 water pit of Dronninglund, and the influence of boundary conditions for this module were calculated and discussed. The results showed that the root mean square errors of the simulated top, bottom, and average temperature of the water pit were 3.07 °C, 2.39 °C, and 1.37 °C, with relative deviation ratios of 4.25 %, 2.27 %, and 6.62 %, respectively. Furthermore, it was found that for every 5 °C increase in the set average soil temperature, the storage efficiency of the water pits increases by 0.4 %∼0.5 %. In the case of common top insulation of the water pit, the total heat loss at the top, side walls, and bottom are 66.83 %, 31.8 %, 1.37 %. While considering comprehensive insulation in all directions, the heat loss rate for the top, side walls, and bottom are similar with the former, being 67.84 %, 30.79 %, and 1.37 %, respectively. The proposal and findings of this article provide a basis for simplified optimization planning and design of large-scale seasonal heat storage systems.