Universidad Politécnica de Madrid. Madrid (Spain)
April 8th, 2024
Summary:
Energy plays an indispensable role in addressing nearly every challenge facing humanity today, with fighting climate change standing out as one of the most crucial ones. Hence, governmental and non-governmental institutions are promoting an energy transition to curtail greenhouse gas emissions, thereby combating the prevailing climate change crisis. This transition must be achieved without compromising the current level of human well-being, ensuring a continuous and cost-effective energy supply. In this scenario, the development of technologically and economically feasible energy storage solutions emerges as a crucial step toward successfully accomplishing the aforementioned energy transition.
Given that approximately 23% of mankind's final energy consumption is dedicated to heating and cooling buildings, the optimal and most efficient method for storing such a substantial amount of thermal energy is through direct thermal energy storage. Among existing technologies, borehole thermal energy storage (BTES) systems not only offer the highest potential energy savings but also stand out due to its versatility. For instance, surplus heat generated from an industrial process can be storage at temperatures ranging from 70C to 80C for direct use in heating the building in winter. Furthermore, there are feasible applications wherein the storage temperature is much closer to the unperturbed ground temperature. In these scenarios, the borehole field acts as a seasonal energy storage that along with a heat pump form a highly efficient heating, ventilation and air conditioning (HVAC) system.
The optimal design of these systems is critical. An excessively large size results in high initial investment costs and impractical payback periods. In contrast, an undersized system implies that, to meet the heating and cooling demands of the building, the heat-carrying liquid requires more extreme temperatures. This invariably results in a decrease in the overall efficiency of the HVAC system. The optimal design of the borehole field requires an accurate forecast of its thermal response over the whole lifespan of the building, of typically 100 years. Unfortunately, these forecasts cannot be obtained directly from detailed numerical simulations of the whole heat transfer problem as nowadays their computational cost makes them unfeasible for engineering purposes. Instead, simplified theoretical models are employed. Most of these theoretical models only take into account heat conduction in ground. However, in many real-world situations the presence of aquifers can highly affect the heat exchange between the boreholes and the ground.
This thesis exploits the presence of large disparities in time and length scales within the heat transfer problem to build simplified, albeit accurate, theoretical models that account for the presence of groundwater flows in the thermal response of geothermal boreholes. These models are rigorously derived mathematically using techniques commonly employed in aerospace engineering and fluid mechanics such as matched asymptotic expansion techniques. The main achievement of this thesis extends beyond enhancing the accuracy of existing models for creeping groundwater flows found in the literature, or the study of new scenarios involving strong groundwater flows. The presented theoretical models also serve to judge and clarify for the first time physical inconsistencies and mathematical-unjustified simplifications found in the state of the art.
Keywords: Geothermal boreholes; Groundwater flows; Thermal response; Peclet number; Thermal resistances; Apparent ground temperature
Citation:
J. Rico (2024), Impact of aquifers on the performance of geothermal boreholes for all peclet numbers. Universidad Politécnica de Madrid. Madrid (Spain).
