Long-term performance of heat exchanger boreholes at different climatic conditions

Ground-source heat pump systems consist of heat exchanger boreholes, embedded with circulation pipes, buried in the ground and connected to a heat pump for heating and cooling of buildings. Due to the changes in seasonal energy demands of the building, sustainability of borehole heat exchangers depends on the seasonal load balance. As the soil can be gradually heated up or cooled down considering the unbalanced thermal loads, long-term performance of heat exchanger boreholes is closely related to maintaining a constant ground temperature as progressively changing temperatures indicate loss of heat exchange efficiency in the long term. In order to address the long-term thermal performance for different unbalanced climatic conditions, ground thermal loads were estimated and representative equivalent half-sine waves of thermal loads from a hypothetical four floor medium size office building are created for 100+ different locations. Total required heat exchanger lengths for each location are then estimated for the proposed building. Findings suggest that for different seasonal energy demands, amplitudes and durations of the sine waves change significantly, consequently suggesting different loop lengths. For unbalanced climates, loop lengths found are considerably higher than balanced cases. Ultimately, numerical analyses were simulated for 30 years of heat exchanger operation to investigate the thermal performance under different climatic conditions.