Numerical Study on Heat Transfer and Performance of Seasonal Borehole Thermal Energy Storage

Design of the borehole thermal energy storage (BTES) is very important for the seasonal solar thermal storage system. The BTES designed based on the empirical method could lead to some unsatisfied effects in actual operations. This paper simulated the BTES operation in one year to study the effects of soil thermal conductivity, soil volume-specific heat, initial soil temperature and relative surface area of storage on energy efficiency of the BTES. The results show that energy efficiency of the BTES reaches the maximum when soil thermal conductivity is 1.8 W/(m center dot K). According to the results, soil thermal conductivity has more significant effects on the heat extraction per soil volume but has the most negligible impact on energy efficiency of the BTES. At the same time, relative surface area of storage is the most influential factor on energy efficiency of the BTES. Finally, a new BTES design method was proposed which is suitable for the studied parameter range. It is worth noting that the present study focuses on the continuous operation of the system and fixed buried pipe spacing, while the intermittent operation and other buried pipe spacing will be studied in future work.