Thermal performance analysis of a large scale water pit heat storage

Solar thermal application is a practical approach to achieving carbon neutrality. Large-scale water pit heat storage (PTES) is a crucial component in ensuring the stability of large-scale solar thermal plants. Based on the model of Type 1535-1301, this paper performs a 2D simulation investigation of the long-term thermal performance of PTES using Trnsys. The accuracy of this model was verified by comparison with measured data from the solar plant in Dronninglund, Denmark. The results show that the average error between the model and the actual measurements is only 3%. The determination coefficients R2 for temperature and energy are more significant than 0.98. In addition, this can find that the PTES average temperature decreases from 24°C to 12°C before the 80th day due to heat losses. During the charging period from the 80th to 250th day, the PTES average temperature increases to 68°C. The PTES formed a temperature stratification with 85°C at the top and 40°C at the bottom. The sloping soil temperature boundary expands and shows a non-uniform stratification of 70-10°C with 5 m thickness. The average temperature of the PTES gradually decreases to 16°C during the discharge period after the 250th day. The storage efficiency is as high as 90.1% because PTES is used for long-term and short-term energy storage. This paper reveals the dynamic thermal process of PTES throughout the year to provide a reference for designers.