A numerical study on the sustainability and efficiency of borehole heat exchanger coupled ground source heat pump systems

To utilize shallow geothermal energy, ground source heat pump systems (GSHPS) are often coupled with borehole heat exchangers (BHE). In recent years, some GSHPSs are experiencing a gradual decrease in BHE outflow temperatures and thus have to be shut down. In this work, a comprehensive numerical model was constructed to include flow and heat transport processes, together with the dynamics of heat pump efficiency. The model parameters are based on local conditions in the Leipzig area. Different scenarios were simulated to observe the evolution of BHE outflow and soil temperatures subject to various factors of influence. In the first year, the recovery of shallow geothermal energy only accounts for about 89% of the energy extracted. Over the following years, outflow and soil temperature will gradually drop until they reach a quasi-steady-state. It was also found that groundwater flow and using BHE for cooling will be beneficial to the energy recovery and efficiency of the heat pump. In comparison to other factors, the soil heat capacity and thermal conductivity are considered to have a minor impact on the sustainability of the system. In contrast, it is very likely that undersized systems are the cause of strong system degradation. (C) 2016 Elsevier Ltd. All rights reserved.