Thermal characterization of soils in Aveiro and Lisbon for shallow geothermal applications: A study using transient methods

• First to assess the soil thermal characteristics of natural soils in Portugal. • Impact of water migration on thermal conductivity on long-term sample preservation. • A good agreement was observed between the measurements with the three probes used. • Quartz particles in soil significantly impact the thermal conductivity of soil. Shallow geothermal energy systems are growing as an alternative energy source for building space heating and cooling, with notable contributions to carbon emission reduction. In this context, proper soil thermal characterization is crucial for determining the potential of these systems and evaluating their energy efficiency. This work presents the results of a study of the thermal conductivity of natural soil samples collected from construction sites and boreholes in the cities of Aveiro and Lisbon, Portugal. The evaluation of soil thermal properties was complemented by a comprehensive assessment of soil physical characteristics to establish a broader understanding of their interrelationships, followed by an analysis of the mineralogical composition of the soil. The thermal tests were performed in transient conditions using needle probes and a surface probe and the respective results were compared. Potential sources of error in the various transient probe measurements were examined, including issues such as the impact of water migration due to long-term sample preservation and variations in sample quality. Moreover, the effect of several physical parameters, such as degree of saturation, void ratio and soil density, on the soil thermal conductivity was assessed. As expected, these physical parameters demonstrate a significant influence on thermal characteristics. Additionally, the presence of adequate quartz particles in soil significantly affects the thermal properties of the soil. The study provides initial information for the database regarding the thermal properties of natural Portuguese soil, which can serve as a reference for designing, developing and estimating shallow geothermal systems in these cities.