TRS: a numerical code for the assessment of thermal recycling in Ground Water Heat Pumps

The use of Ground Water Heat Pumps (GWHPs) for the heating and cooling of buildings has been increasing in recent years due to the sustainability and economic convenience of this technology. One of the most critical design issues of these systems is the formation of a thermal plume downstream the reinjection well, which can be captured by the abstraction well, thus impairing the energy efficiency of the system. Some analytical models have been already developed to simulate this phenomenon, but they require an imposed injection well temperature, which should be know a priori. With this assumption, the thermal power exchanged with groundwater diminishes with time, due to the thermal alteration of the extracted water. A numerical code called TRS (Thermal Recycling Simulator) was therefore developed to consider a constant temperature difference between the injection and the abstraction well, and hence a constant thermal load, which is a more realistic modelling assumption for GWHPs. The code is based on the potential flow theory and it has been validated against coupled flow and heat transport simulations with FEFLOW. TRS is freely available at www.polito.it/groundwater/software/TRS.html and works in MATLAB environment. An empirical formula, which describes the time evolution of the extracted water temperature, has also been derived from a series of simulations with TRS. The developed mathematical tools can be used to assess the long-term sustainability of a certain plant setup, to perform sensitivity analyses and for large-scale assessments of the thermal exchange capacity of aquifers.