Simulation-based analysis of a ground source heat pump system using super-long flexible heat pipes coupled borehole heat exchanger during heating season

A hybrid ground source heat pump system employing the super-long flexible heat pipes to extract heat from ground is proposed in this work. To evaluate its energy performance and the temperature recovery characteristics near boreholes during space heating season, a comprehensive simulation model was developed considering the dynamic performance of heat pump and the heat transfer of heat pipes and surrounded soil. The results show that the depth of boreholes should be not less 70 m to guarantee a higher long-term energy efficiency. The seasonal coefficient of performance increases with increasing groundwater advection velocity and decreasing building load, with a maximum value up to 4.2 under examined conditions. The short-term variations of ground temperature near boreholes are sensitive to the borehole depth and building load. The overall ground temperature continuously decreases under the condition with no groundwater advection throughout the heating season, but with the presence of groundwater advection, the variation tendency would almost level out after several days of decrease. In addition, it is also revealed that this system has a potential to reduce the ratio of the electrical consumption of circulation pumps to the total consumption, with a reduction of about 7%, showing an advantage of saving energy over the traditional ground-source heat pumps.