Connecting deviated and vertical deep boreholes to enhance the extraction of geothermal energy - case study

Recently, an Enhanced U-type Borehole Heat Exchanger (EUBHE) system has been installed in Xi’An, China. The EUBHE system is composed on one vertical and a second deviated borehole. The bottom ends of them are connected at a depth of 2.5 km and form a closed “U-type” loop system. During the heating season, water is circulated through the two boreholes to extract geothermal energy for building heating purpose. In this work, a numerical model was developed based on the OpenGeoSys (OGS) software, in which the boreholes of the EUBHE system are represented by the line elements and the soil/rock surrounding them is discretized with 3D prism elements. With this dual-continuum model, the operation of the EUBHE system can be efficiently simulated, particularly for long-term scenarios. The developed numerical model was verified against analytical solution in a benchmark proposed by Ramey. To simulate the long-term operation of EUBHE, a Direchlet-type boundary was imposed at the inlet of the system. Temperature difference between inlet and outlet were calculated based on the building thermal demand. The impact of parameters of the vertical and deviated boreholes and flow rate of the circulating water are further investigated. Preliminary modelling results showed that the sustainable specific heat extraction rate of the EUBHE system can reach up to 200 W/m. With higher grout and pipe conductivities, the system performance will be improved. The heat extraction efficiency of EUBHE system is higher than the traditional Deep Borehole Heat Exchangers (DBHE). However, electricity consumption from the circulating pump is elevated. The developed numerical model presented in this work can also be utilized for the design and optimization of the EUBHE system.