Effect of axial heat transfer and atmospheric conditions on the energy performance of GSHP systems: A simulation-based analysis

A range of analytical and numerical models is available to investigate the thermal behaviour of vertical ground heat exchangers, both in the short- and long-term. However, most of them ignore the thermal effects of weather at ground level that affect the upper part of boreholes. Furthermore, few models look at the integrated simulation of a ground source heat pump system for both the borehole field and heat pump. Consequently, a limited number of applications to real cases are available for designers. This paper shows a study to assess the effects of both axial heat transfer in boreholes and the weather at ground level on the fluid temperature in the boreholes, as well as on the energy efficiency of the heat pump. To this purpose, long-term analysis of two ground source heat pump systems was conducted over ten years by means of a detailed numerical simulation tool. The systems were for two office buildings with unbalanced load profiles in Canada and Italy. These case studies were analyzed as they were being designed, then the influence of the borehole arrangement and borehole length was also investigated. The simulation results show that axial heat transfer affects fluid temperature in the boreholes and the seasonal energy efficiency of the whole system more than the weather. Moreover, when the load profile is unbalanced, neglecting the effect of weather does not always ensure a conservative design of the borehole field.