Evaluation of different heat extraction strategies for shallow vertical ground-source heat pump systems

Shallow vertical ground-source heat pump systems (GSHPSs) have become a popular alternative to conventional heating systems. Typically more than one vertical ground heat exchanger (GHE) is required along with an increasing heat demand. The higher the number of GHEs, the more a system may benefit from optimal design and operation strategies that focus on costs and efficiencies. However, an optimisation of the heat and fluid flows in these systems, based on discretised models, can be computationally time-consuming and sometimes infeasible. To meet this challenge, one might apply simplified models and identify suitable constraints. In this work an analytical finite line source (FLS) model is compared by RockFlow, a finite element approach. The average absolute difference for a long-term investigation between these approaches is obtained at only approx. 0.2°C, which was evaluated as sufficient. Subsequently, the FLS model is successfully applied to demonstrate the existence of borehole-specific heat flux distributions. For all case studies optimal solutions were found. These results confirmed the useful application of novel optimisation methods. The impact of the GHE specific heat flux distributions on the time-dependent and spatial temperature course in the vicinity of the GHE is impressively shown. The investigation of the soil and heat pump cycle revealed the system efficiency potential and costs. The efficiency improvement potential, caused by different optimal heat flux distributions, was approx. 2%. The best energy extraction improvement was nearly 20%, which equated to a monetary saving of 12%.