Numerical Modeling of Borehole Heat Exchangers (BHEs) and its interactions with the surrounding soil

Abstract

The utilization of Borehole Heat Exchanger (BHE) to transfer heat from the shallow subsurface has been a common practice for the Ground Source Heat Pump (GSHP) system. To represent realistic application scenarios for numerical simulations of such systems, saturated and unsaturated conditions as well as heterogeneous soil properties have to be considered. In this context, analytical solutions such as the Moving Finite Line Source (MFLS) model are not flexible enough to capture the full dynamics of the system. Furthermore, application examples with a high density of installed BHEs exist. There, temperature plumes produced by the individual BHEs may start to interact with each other and lead to lower thermal output. To simulate this interaction, a dual continuum approach has been implemented into the open-source finite element simulator OpenGeoSys (OGS, www.opengeosys.org). The model is capable of simulating the temperature evolution both in and around the BHE, with the consideration of saturated and unsaturated groundwater flow processes in the surrounding soil. Instead of imposing Dirichlet or Neumann type of boundary conditions at the location of a BHE, the newly developed model allows the user to specify inflow refrigerant temperature and flow rate as the driving force of heat transport. The extended OGS model was successfully verified by simulating an in-door thermal response test performed by Beier et al. (2011). The modeled BHE wall temperatures match perfectly with the measured values. The simulated outflow temperatures reproduce the same trend as monitored data, although with a small deviation (0.5°C). In the next step, the numerical model will be further applied in real geothermal sites including unsaturated soil layers and groundwater flow field.