Efficiency of shallow geothermal systems in heterogeneous media, how to manage uncertainty

Shallow geothermal systems represent a unique opportunity for heating and cooling of buildings with green energy and low operational costs. So far, the geothermal system studies have been built on simplifying assumption such as homogeneous porous medium or purely advective flow. However, solutions based on equivalent conductivity and effective macrodispersion coefficients may not be able to grasp the effects of aquifer heterogeneity on geothermal systems.

The aim of our research is to investigate how thermo-hydrological and engineering parameters impact the different heat transport dynamics and how they result in the GS efficiency. The study considers an open loop GS made by a well doublet placed into a confined heterogeneous aquifer of constant thickness; groundwater is abstracted upstream and, after the heat exchange, it is reinjected downstream with a constant altered temperature. The efficiency of GS is intrinsically related to the behavior of the breakthrough time distribution, i.e. the distribution of the travel time the water particles employ to move from the injection to the extraction well, and the recirculating ratio. By means of accurate numerical simulations that mimic the heat transport through a Lagrangian procedure we identify and explore the effect of hydraulic conductivity heterogeneity, pore scale dispersion, thermal diffusion, pumping rate and geometrical parameters.

The analysis hints that the hydraulic conductivity heterogeneity has a strong impact on the early operational time: due to channeling, the thermal plume travels faster in the highly conductive layers. As a result, the first breakthrough time, the key parameter adopted in the design of GS, decreases with heterogeneity, moreover, the uncertainty associated with early arrivals increases with heterogeneity.

The heterogeneity, as well as dispersion and convection, has a negligible effect on the long-term period.The recirculating ratio depends strongly on the pumping rate and other geometrical parameters.

Given that well screens usually cross a short depth we perform a detailed analysis on the uncertainty related to the ergodicity issue. Result 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.