Thermal Sustainability of Low-temperature Geothermal Energy Systems: System Interactions and Environmental Impacts

In storing heat during summer for use in winter, the ground provides a better source/sink of heat than the outside air in regards to heat pump efficiency, being cooler than the outside air in the summer and warmer in the winter. Due to their good efficiency, the use of geothermal energy is often encouraged; however, two issues arise in the long-term use of ground for thermal purposes: the sustainability and impact of these systems on the environment. Studies show the potential of the geothermal heat exchangers for environmental impacts such as undesirable temperature rises from these systems in temperature-sensitive regions. Furthermore, interference between adjoining installations is being reported, raising issues of sustainability in terms of performance and equitable sharing of natural resources.;The temperature of the soil surrounding the ground heat exchangers (GHEs) and the heat flows in this region are the key factors in determining environmental impacts and their potential thermal interaction. In this study, analytical and numerical models of vertical heat exchangers are presented. First, the effect of system parameters such as borehole spacing on the transient response of two GHEs is described. Second, a numerical finite volume method in a two-dimensional meshed domain is used to evaluate the temperature rise and the heat flows in the soil surrounding borehole systems over the long term. Finally, to examine the effect of temperature rise in the soil surrounding a vertical GHE on the performance of an associated ground heat pump, a reversible heat pump model is coupled to the heat exchanger analytical model via the heat exchanger running fluid temperature. The heat exchanger running fluid temperature, wall temperature and its heat load profile are the main coupling parameters between the three models. The results of the analytical model are compared with ones of a finite volume numerical model.