Seasonal energy storage remains a critical bottleneck for high-penetration renewable energy systems. Solar-assisted underground thermal energy storage (UTES) offers a solution by capturing excess summer heat in subsurface formations for winter recovery, but identifying suitable sites requires simultaneous evaluation of solar resources and subsurface geology at regional scales.
A research team developed an integrated assessment framework for the Euganean Hills region in Italy, combining geographic information systems (GIS) with laboratory characterization of local rock properties. The methodology measured thermal conductivity and heat storage capacity of 23 rock specimens under both dry and water-saturated conditions, accounting for anisotropy in layered formations. These material properties were then combined with seasonal solar radiation data using a dimensionless suitability index to create a regional screening tool.
The analysis identified carbonate formations as particularly promising, with saturated thermal conductivity exceeding 3.0 W/m·K in compact limestone and dolomite units. When accounting for favorable orientation and water saturation, suitability rankings improved substantially—approximately 30% of mapped areas advanced one classification tier. The Maiolica and Scaglia Rossa formations showed the strongest combination of thermal performance and lateral continuity across the study area.
However, validation measurements at a field site revealed a critical limitation: actual conductivity was 2.5 to 3.5 times higher than laboratory estimates weighted for local geology. This gap reflects contributions from natural fracture networks, groundwater flow, and borehole construction effects that laboratory samples cannot replicate at scale.
The framework successfully prioritizes regions warranting further investigation but cannot predict site-specific performance without field testing. The findings emphasize the importance of uncertainty propagation in renewable energy infrastructure planning. As utilities and developers pursue seasonal storage deployment across Europe, this lithology-resolved assessment approach offers a practical methodology for rapid screening while acknowledging the inherent limitations of laboratory-based characterization in heterogeneous subsurface environments.
