Urban low-to-medium deep borehole field regeneration with waste heat from energy efficient buildings: a techno-economic study in Nordic climate

Due to large area requirement, ground-source heat pump (GSHP) systems with shallow boreholes are difficult to implement in dense urban areas. To address this limitation, alternative heat sources can be used to reduce heat extraction from ground or to inject regenerative heat to boreholes. This study investigates the techno-economic feasibility of utilizing two commonly available waste heat sources (waste air and wastewater) in urban environment. Passive and heat pump-assisted utilization are studied for apartment and office buildings, with varied borehole depth and two levels of urban density. Long-term GSHP system operation is simulated using iterative heat balance calculation and borehole dimensioning algorithms. The results show significant reduction in required borehole length with waste heat utilization, particularly in shallow borefields, with maximum reductions of 53.9% (apartment building) and 25.8% (office building). The studied waste heat sources are shown to enable a shallow borefield for otherwise insufficient borehole spacing, providing an alternative to deeper boreholes. However, waste heat only available during summer has limited impact on field sizing compared to a seasonally stable heat source. From an economic perspective, the levelized cost of heating could be reduced by 13.5% (apartment building) and 7.3% (office building) compared to baseline without waste heat utilization.