Comparative Study On Thermal Performance Of Horizontal Ground Source Heat Pump Systems With Dirichlet And Robin Boundary Conditions On Ground Surface

The heat transfer of horizontal ground heat exchanger (HGHE) is susceptible to the ground surface boundary condition (BC) due to the relatively shallow buried depth. This study aimed to compare the thermal performance of HGHEs with different ground surface BC by developing a 3D numerical model. Three commonly used HGHEs, namely spiral, slinky, and linear-type HGHE, and three ground surface BCs, namely constant temperature, time varying temperature, and energy balance BC, were considered in the numerical models to compare the shortterm performance of HGHEs with various BCs. The ground source heat pump (GSHP) system model, coupling the heat transfer model of HGHE and the model of heat pump, was further proposed to analyze the long-term heating efficiency of horizontal GSHP systems applied in a residential building with different BCs. Concerning that the influence of ground surface BC may vary with the buried depth of HGHE, a series of sensitivity analyses were lastly performed. The results reveal that there is no affirmatory relationship between the ground surface temperature and time, meaning that the use of sinusoidal or other forms of functions cannot accurately evaluate the ground surface temperature. The predicted fluid temperature and coefficient of performance (COP) of the GSHP system are underestimated by forcing the surface temperature to follow the constant or time-varying temperature, and those underestimations decline with the increase of the buried depth. Neglecting the thermal interaction of HGHE and ground surface results in small variation ranges in fluid temperature and COP. Compared to the spiral and slinky-type GSHP system, the heat exchange performance of linear-type GSHP system are more sensitive to the ground surface BC. This paper underscored the importance of using proper BC in modelling HGHE to accurately evaluate the thermal performance of HGHE.