Possible effects of shallow geothermal systems installed at coastal zones

Coastal areas are more densely populated than inland areas and present faster rates of population increase and urbanization. This trend is expected to continue in the coming decades, and thus, the demand of natural resources in coastal areas, such as water and energy resources, increasing the pressure and impact on the environment, superposed to the effects of climate change. Currently, in Europe, the demand for heating in buildings and businesses outnumbers the demand for cooling. However, the latter is gradually catching up due to rising demand for air cooling or refrigeration for industry such as food, technological and medical supplies. The energy required to cool buildings in Europe is expected to increase by more than 70% by 2030, while energy used to heat buildings may decrease by 30% (UE, 2018). Low Temperature Geothermal Energy (LTGE) is most likely the green energy production method for heating and cooling with the highest potential to provide affordable and clean energy and meet the CO₂-emissions reduction goals of the Green Deal. Despite advances on LTGE technologies, the efficiency of these systems remains inherently sensitive to changes in hydrodynamics and the media (e.g., changes in the groundwater thermal regime). Groundwater, on the other hand, is the world's largest freshwater resource, and it is especially important in coastal areas because interactions between aquifer systems and sea water may lead to salinization and resource loss. Because geothermal systems and coastal aquifers interact directly, specially at groundwater discharge areas, it is clear that a better understanding of the potential interactions of geothermal systems with current and prospective coastal aquifer processes is essential for their design and foreseeing potential environmental effects. To address these issues, we model variable-density groundwater coupled with heat transport to simulate the long-term evolution of groundwater salinity and aquifer thermal energy discharge. We find that the heating/cooling-induced water density variations affect the seawater intrusion. Understanding the behavior of the groundwater system is required to ensure sustainable water, energy, and coastal ecosystem management.