Numerical modelling of thermal recycling in open-loop shallow geothermal systems: the impact of time-varying flow rates

Shallow open-loop geothermal are being increasingly used to provide affordable and low-carbon heating and cooling of buildings. The design of these plants requires the transient simulation of flow and heat transport in the aquifer to assess the propagation of the so-called thermal plume, i.e. the aquifer volume where groundwater temperature is altered by the reinjection of colder/warmer water. The propagation of thermal plumes can also result in the thermal alteration of the abstracted water, a phenomenon that is known as thermal recycling. Thermal recycling has been addressed in the literature with analytical and numerical models. Recent developments allow to assess thermal recycling with constant flow rates: however, this is not a realistic assumption for a building heating/cooling system, where the flow rate follows the thermal needs of the building and hence it has a seasonal variation. In this thesis, thermal recycling with varying flow rates is addressed. A parametric sweep study was performed through tens of numerical flow and heat transport simulations with the finite-element code FEFLOW. Results were used to calibrate a formula that allows to estimate the maximum thermal alteration at the abstraction well with an acceptable margin of error. This formula can be used for a quick assessment of the technical feasibility of a well doublet configuration, that is, to assess whether the hydraulic properties of the aquifer allow for a certain plant to operate sustainably in the long term.