A computationally efficient numerical model for heat transfer simulation of deep borehole heat exchangers

Being a promising technique for seasonal thermal energy storage, the deep borehole heat exchanger (DBHE) has also provided a new variance of ground-coupled heat pump systems especially for applications in cold-climate regions. Numerical simulations are recognized as the appropriate means in the study on heat transfer around the deep boreholes, which often features time-consuming computations. A model has been established in this study for DBHEs with coaxial tubes, considering coupled heat transfer in the tubes and surrounding subsurface. A software package is developed based on the finite difference method for thermal analysis of the DBHEs, and an algorithm for direct solution of resulted algebraic equation set is used so as to achieve very efficient computation. The model and the numerical algorithm are validated by comparison with simulation results from a reference using the finite element method. Performance of DBHEs is then assessed with parameter analyses. A diagram is presented to show relations of the nominal DBHE capacity to its key parameters such as the borehole depth, geothermal heat flux and thermal conductivity of subsurface, which is helpful in engineering practice. Studies also indicate that a high-resistance inner pipe helps in improving the heat transfer capacity of DBHEs.