Thermal performance analysis of a large scale water pit heat storage

Abstract

Solar thermal application is an effective approach to achieve carbon neutrality. The large scale water pit heat storage (PTES) is a key component to ensure 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%. In addition, it can be found 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 days, the PTES average temperature increases to 68°C. In which the PTES formed a temperature stratification with 80°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 10m thickness. The average temperature of the PTES gradually decreases to 16°C during the discharge period after the 250th day. The actual charging and discharging energy are 11,868 MWh and 11,250 MWh per year, respectively, and the total heat loss is 1,157 MWh. The storage efficiency is as high as 90.1% because PTES is used for both 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.