Analysis of operating modes of a ground source heat pump with short helical heat exchangers

This study focuses on different operating modes of a ground source heat pump system in residential buildings. Ground coupling was made using a closed loop system consisting of a helical shaped pipe installed at a shallow depth. Few studies have examined this particular ground heat exchanger. The analysis was carried out using a detailed numerical model capable of considering the geometry of the helical ground heat exchanger as well as the effects of axial thermal conduction and the weather at ground level, variables which cannot be ignored when shallow depths are being investigated. Field measurements were used to validate the model before it was utilized. In addition, the simulation tool considered the entire ground source heat pump system, including both the borehole field and the heat pump. The energy efficiency of the heat pump in three operating modes (continuous daytime, continuous nighttime, and intermittent mode) over a ten year period was analysed. The simulations were performed in two different climatic zones maintaining the daily energy load of the building unmodified. Finally, the effect of the grouting material of the helical ground heat exchanger and of the diameters of both the borehole and the helix on the system’s energy performance was also investigated. Results indicated that the seasonal energy efficiency of the heat pump was approximately the same for the three operating modes and that energy efficiency was nearly constant during the day when the system was operating on an hourly intermittent basis. When the borehole diameter was smaller, the reduction in the heat exchange surface needed to be balanced by a thermally enhanced grouting material.