Uncertainty and Efficiency in geothermal systems in heterogeneous aquifers

Shallow geothermal systems represent a unique opportunity for heating and cooling of buildings with green energy and low operational costs.

Efficiency of  geothermal system is strictly related to the local subsurface flow field that moves water and energy; given the great spatial variability of hydrological and thermal properties in the subsurface environment a reliable assessment of the geothermal system efficiency requires a probabilistic approach that takes into account the uncertainty on the predictions. 

Homogeneous domain and purely advective flow are typical hypotheses currently adopted in the design of geothermal systems, the aim of our research is to investigate how the variability of thermo-hydrological and engineering parameters impact the different heat transport dynamics and how they result in the GS efficiency.

The study adopt a Lagrangian description of the heat transport based on the travel time evaluation.

As application example we consider an open loop system made by a well doublet placed into a confined heterogeneous aquifer of constant thickness.

The efficiency of the system is evaluated considering lumped parameters, usually adopted in the GS deign, such as the water recirculation ratio or the first breakthrough time and introducing more effective descriptors such as the total breakthrough time curve or the temperature evolution at the abstraction well.

The analysis suggests that the first breakthrough time, the key parameter adopted in the GS design, decreases with heterogeneity, furthermore, the uncertainty associated with early arrivals increases with heterogeneity. Medium heterogeneity, on the other hand, has a very small impact on the recirculation ratio and on the long-term period, while the pumping rate and other geometrical parameters have a strong impact on its value.

Since well screens usually cross a short depth we perform a detailed analysis on the uncertainty related to the ergodicity issue. Results of a single realization can significantly differ from its ergodic counterpart. As a practical consequence, a thermal feedback occurring in a heterogeneous medium could significantly differ from the expected theoretical one.