Dynamic simulation and techno-economic optimization of deep coaxial borehole heat exchangers in a building energy system

Deep coaxial borehole heat exchangers (CBHEs) have been considered to enable cost and energy savings in building energy systems. However, achieving improved performance requires optimal design and operation of the CBHE. This work presents a case study for a building energy system in Finland, for which a hydraulic diagram was developed to incorporate a number of deep CBHEs (800 m to 2400 m) with multiple operating modes. To evaluate the techno-economic feasibility with optimized operating strategies, a four-step methodology is introduced, combining steady-state modelling, dynamic simulation and system optimization. The systems with a CBHE were found economically feasible compared to a district heating connection, enabling lower annualized total system cost (18.3–26.3%) and operating costs (49.1–53.5%). With deeper CBHEs, the main cost reductions originated from improved heat pump performance, increased operating flexibility and decreased capacity requirements of process components. However, the benefits partly decreased non-linearly with increased CBHE depth. Economic feasibility displayed the highest sensitivity to the price of electricity, with a relatively smaller impact for deeper CBHEs. Furthermore, maximizing the heat pump performance was found economically more significant than achieving a high borehole heat extraction rate.